Granite is a type of igneous rock quarried for its extreme strength, density, durability, and resistance to corrosion. But granite is also very versatile– it’s not just for squares and rectangles! In fact, We confidently work with granite components engineered in shapes, angles, and curves of all variations on a regular basis—with excellent outcomes.
Through our state of the art processing, cut surfaces can be exceptionally flat. These qualities make granite the ideal material to create custom-size and custom-design machine bases and metrology components. Granite is:
■ machinable
■ precisely flat when cut and finished
■ rust resistant
■ durable
■ long lasting
Granite components are also easy to clean. When creating custom designs, be sure to choose granite for its superior benefits.

STANDARDS / HIGH WEAR APPLICATIONS
The granite utilized by ZHHIMG for our standard surface plate products has high quartz content, which provides greater resistance to wear and damage. Our Superior Black colors have low water absorption rates, minimizing the possibility of your precision gauges rusting while setting on the plates. The colors of granite offered by ZHHIMG result in less glare, which means less eyestrain for individuals using the plates. We have chosen our granite types while considering thermal expansion in an effort to keep this aspect minimal.

CUSTOM APPLICATIONS
When your application calls for a plate with custom shapes, threaded inserts, slots or other machining, you’ll want to select a material like Black Jinan Black. This natural material offers superior stiffness, excellent vibration dampening, and improved machinability.

It is important to note that color alone is not an indication of the physical qualities of the stone. In general, granite’s color is directly related to the presence or absence of minerals, which may have no bearing on the qualities that make good surface plate material. There are pink, gray, and black granites that are excellent for surface plates, as well as black, gray, and pink granites that are totally unsuitable for precision applications. The critical characteristics of granite, as they pertain to its use as a surface plate material, have nothing to do with color, and are as follows:
■ Stiffness (deflection under load - indicated by Modulus of Elasticity)
■ Hardness
■ Density
■ Wear resistance
■ Stability
■ Porosity

We have tested many granite materials and compared these material. Finally we get the result, Jinan black granite is the best material we’ve ever know. Indian Black granite and South African granite are similar to Jinan Black Granite, but their physical properties are less than Jinan Black Granite. ZHHIMG will keep looking for more granite material in the world and compare their physical properties.

To talk more about the granite that is right for your project, please contact us info@zhhimg.com.

Different manufacturers use different standards. There are many standards in the world.
DIN Standard, ASME B89.3.7-2013 or Federal Specification GGG-P-463c (Granite Surface Plates) and so on as a basis for their specifications. 

And we can manufacture granite precision inspection plate according to your requirements. Welcome to contact us if you want to know more information about more standards.

Flatness can be considered as all points on the surface being contained within two parallel planes, the base plane and the roof plane. The measurement of distance between the planes is the overall flatness of the surface. This flatness measurement commonly carries a tolerance and may include a grade designation.

For example, the flatness tolerances for three standard grades are defined in the federal specification as determined by the following formula:
■ Laboratory Grade AA = (40 + diagonal squared/25) x .000001" (unilateral)
■ Inspection Grade A = Laboratory Grade AA x 2
■ Tool Room Grade B = Laboratory Grade AA x 4.

For standard sized surface plates, we guarantee flatness tolerances that exceed the requirements of this specification. In addition to flatness, ASME B89.3.7-2013  & Federal Specification GGG-P-463c address topics including: repeat measurement accuracy, material properties of surface plate granites, surface finish, support point location, stiffness, acceptable methods of inspection, installation of threaded inserts, etc.

ZHHIMG granite surface plates and granite inspection plates meet or exceed all of the requirements set forth in this specification. At present, there is no defining specification for granite angle plates, parallels, or master squares. 

And you can find the formulas for other standards in DOWNLOAD.

First, it is important to keep the plate clean. Airborne abrasive dust is usually the greatest source of wear and tear on a plate, as it tends to embed in work pieces and the contact surfaces of gages. Second, cover your plate to protect it from dust and damage. Wear life can be extended by covering the plate when not in use, by rotating the plate periodically so that a single area does not receive excessive use, and by replacing steel contact pads on gauging with carbide pads. Also, avoid setting food or soft drinks on the plate. Note that many soft drinks contain either carbonic or phosphoric acid, which can dissolve the softer minerals and leave small pits in the surface.

This depends on how the plate is being used. If possible, we recommend cleaning the plate at the beginning of the day (or work shift) and again at the end. If the plate becomes soiled, particularly with oily or sticky fluids, it should probably be cleaned immediately.

Clean the plate regularly with liquid or ZHHIMG Waterless surface plate cleaner. The choice of cleaning solutions is important. If a volatile solvent is used (acetone, lacquer thinner, alcohol, etc.) the evaporation will chill the surface, and distort it. In this case, it is necessary to allow the plate to normalize before using it or measurement errors will occur.

The amount of time required for the plate to normalize will vary with the size of the plate, and the amount of chilling. An hour should be sufficient for smaller plates. Two hours may be needed for larger plates. If a water-based cleaner is used, there will also be some evaporative chilling.

The plate will also retain the water, and this could cause rusting of metal parts in contact with the surface. Some cleaners will also leave a sticky residue after they dry, which will attract airborne dust, and actually increase wear, rather than decreasing it.

This depends on the plate usage and environment. We recommend that a new plate or precision granite accessory receive a full recalibration within one year of purchase. If the granite surface plate will see heavy use, it may be advisable to shorten this interval to six months. Monthly inspection for repeat measurement errors using a Electronic level, or similar device will show any developing wear spots and only takes a few minutes to perform. After the results of the first recalibration are determined, the calibration interval may be extended or shortened as allowed or required by your internal quality system.

We can offer service to help you inspect and calibrate your granite surface plate.

There are several possible causes for variations between calibrations:

• The surface was washed with a hot or cold solution prior to calibration, and was not allowed sufficient time to normalize
• The plate is improperly supported
• Temperature change
• Drafts
• Direct sunlight or other radiant heat on the surface of the plate. Be sure that overhead lighting is not heating the surface
• Variations in the vertical temperature gradient between winter and summer (If at all possible, know the vertical gradient temperature at the time the calibration is performed.)
• Plate not allowed sufficient time to normalize after shipment
• Improper use of inspection equipment or use of non-calibrated equipment
• Surface change resulting from wear

How many kinds of holes on precision granite ?

Slots on Precision Granite Components

For many factories, inspection rooms and laboratories, precision granite surface plates are relied on as the basis for accurate measurement. Because every linear measurement depends on an accurate reference surface from which final dimensions are taken, surface plates provide the best reference plane for work inspection and layout prior to machining. They also are ideal bases for making height measurements and gaging surfaces. Further, a high degree of flatness, stability, overall quality and workmanship make them a good choice for mounting sophisticated mechanical, electronic and optical gaging systems. For any of these measurement processes, it is imperative to keep surface plates calibrated.

Repeat Measurements and Flatness

Both flatness and repeat measurements are critical to ensure a precision surface. Flatness can be considered as all points on the surface being contained within two parallel planes, the base plane and the roof plane. The measurement of distance between the planes is the overall flatness of the surface. This flatness measurement commonly carries a tolerance and may include a grade designation.

The flatness tolerances for three standard grades are defined in the federal specification as determined by the following formula:

DIN Standard, GB Standard, ASME Standard, JJS standard... different country with different stand...

More details about standard.

In addition to flatness, repeatability must be ensured. A repeat measurement is a measurement of local flatness areas. It is a measurement taken anywhere on the surface of a plate that will repeat within the stated tolerance. Controlling local area flatness to a tighter tolerance than overall flatness guarantees a gradual change in surface flatness profile, thereby minimizing local errors.

To ensure a surface plate meets both the flatness and repeat measurement specifications, manufacturers of granite surface plates should use Federal Specification GGG-P-463c as a basis for their specifications. This standard addresses repeat measurement accuracy, material properties of surface plate granite, surface finish, support point location, stiffness, acceptable methods of inspection and installation of threaded inserts.

Checking Plate Accuracy

By following a few simple guidelines, an investment in a granite surface plate should last for many years. Depending on plate usage, shop environment and required accuracy, frequency of checking the surface plate accuracy varies. A general rule of thumb is for a new plate to receive a full recalibration within one year of purchase. If the plate is used frequently, it is advisable to shorten this interval to six months.

Before a surface plate has worn beyond specification for overall flatness, it will show worn or wavy posts. Monthly inspection for repeat measurement errors using a repeat reading gage will identify wear spots. A repeat reading gage is a high-precision instrument that detects local error and can be displayed on a high magnification electronic amplifier.

An effective inspection program should include regular checks with an autocollimator, providing actual calibration of overall flatness traceable to National Institute of Standards and Technology (NIST). Comprehensive calibration by the manufacturer or an independent company is necessary from time to time.

Variations Between Calibrations

In some cases, there are variations between surface plate calibrations. Sometimes factors such as surface change resulting from wear, incorrect use of inspection equipment or use of noncalibrated equipment can account for these variations. The two most common factors, however, are temperature and support.

One of the most important variables is temperature. For instance, the surface might have been washed with a hot or cold solution prior to calibration and not allowed sufficient time to normalize. Other causes of temperature change include drafts of cold or hot air, direct sunlight, overhead lighting or other sources of radiant heat on the surface of the plate.

There also can be variations in the vertical temperature gradient between winter and summer. In some cases, the plate is not allowed sufficient time to normalize after shipment. It is a good idea to record the vertical gradient temperature at the time the calibration is performed.

Another common cause for calibration variation is a plate that is improperly supported. A surface plate should be supported at three points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered.

Only three points can rest solidly on anything but a precision surface. Attempting to support the plate at more than three points will cause the plate to receive its support from various combinations of three points, which will not be the same three points on which it was supported during production. This will introduce errors as the plate deflects to conform to the new support arrangement. Consider using steel stands with support beams designed to line up with the proper support points. Stands for this purpose are generally available from the surface plate manufacturer.

If the plate is properly supported, precise leveling is only necessary if an application specifies it. Leveling is not necessary to maintain the accuracy of a properly supported plate.

Extend Plate Life

Following a few guidelines will reduce wear on a granite surface plate and ultimately, extend its life.

First, it is important to keep the plate clean. Airborne abrasive dust is usually the greatest source of wear and tear on a plate, as it tends to embed in workpieces and the contact surfaces of gauges.

It also is important to cover plates to protect it from dust and damage. Wear life can be extended by covering the plate when not in use.

Rotate the plate periodically so that a single area does not receive excessive use. Also, it is recommended to replace steel contact pads on gauging with carbide pads.

Avoid setting food or soft drinks on the plate. Many soft drinks contain either carbonic or phosphoric acid, which can dissolve the softer minerals and leave small pits in the surface.

Where to Relap

When a granite surface plate needs re-surfacing, consider whether to have this service performed on-site or at the calibration facility. It is always preferable to have the plate relapped at the factory or a dedicated facility. If, however, the plate is not too badly worn, generally within 0.001 inch of the required tolerance, it can be resurfaced on-site. If a plate is worn to the point where it is more than 0.001 inch out of tolerance, or if it is badly pitted or nicked, then it should be sent to the factory for grinding prior to relapping.

A calibration facility has the equipment and factory setting providing the optimum conditions for proper plate calibration and rework if necessary.

Great care should be exercised in selecting an on-site calibration and resurfacing technician. Ask for accreditation and verify the equipment that the technician will use has a traceable calibration. Experience also is an important factor, as it takes many years to learn how to correctly lap precision granite.

Critical measurements start with a precision granite surface plate as a baseline. By ensuring a reliable reference by using a properly calibrated surface plate, manufacturers have one of the essential tools for reliable measurements and better quality parts.Q

Checklist for Calibration Variations

1. The surface was washed with a hot or cold solution prior to calibration and was not allowed sufficient time to normalize.

2. The plate is improperly supported.

3. Temperature change.

4. Drafts.

5. Direct sunlight or other radiant heat on the surface of the plate. Be sure that overhead lighting is not heating the surface.

6. Variations in the vertical temperature gradient between winter and summer. If at all possible, know the vertical gradient temperature at the time the calibration is performed.

7. Plate not allowed sufficient time to normalize after shipment.

8. Improper use of inspection equipment or use of noncalibrated equipment.

9. Surface change resulting from wear.

Tech Tips

• Because every linear measurement depends on an accurate reference surface from which final dimensions are taken, surface plates provide the best reference plane for work inspection and layout prior to machining.
• Controlling local area flatness to a tighter tolerance than overall flatness guarantees a gradual change in surface flatness profile, thereby minimizing local errors.
• An effective inspection program should include regular checks with an autocollimator, providing actual calibration of overall flatness traceable to National Inspection Authority.

Among the mineral particles that make up granite, more than 90% are feldspar and quartz, of which feldspar is the most. The feldspar is often white, gray, and flesh-red, and the quartz is mostly colorless or grayish white, which constitute the basic color of the granite. Feldspar and quartz are hard minerals, and it is difficult to move with a steel knife. As for the dark spots in the granite, mainly black mica, there are some other minerals. Although biotite is relatively soft, its ability to resist stress is not weak, and at the same time they have a small amount in granite, often less than 10%. This is the material condition in which granite is particularly strong.

Another reason why granite is strong is that its mineral particles are tightly bound to each other and are embedded in each other. The pores often account for less than 1% of the total volume of the rock. This gives the granite the ability to withstand strong pressures and is not easily penetrated by moisture.

Granite components are made of stone with no rust, acid and alkali resistance, good wear resistance and long service life, no special maintenance. Granite precision components are mostly used in the tooling of the machinery industry. Therefore, they are called granite precision components or granite components. The characteristics of granite precision components are basically the same as those of granite platforms. Introduction to tooling and measurement of granite precision components: Precision machining and micro machining technology are important development directions of machinery manufacturing industry, and they have become an important indicator to measure a high-tech level. The development of cutting-edge technology and the defense industry is inseparable from precision machining and micro-machining technology. Granite components can be smoothly slid in the measurement, without stagnation. Work surface measurement, general scratches do not affect measurement accuracy. Granite components need to be designed and produced according to the requirements of the demand side.

Application field:

As we all know more and more machines and equipment are choosing precision granite components.

Granite components are used for dynamic motion, linear motors, cmm, cnc, laser machine...

welcome to contact us for more information.

Granite measuring devices and granite mechanical components are made of high quality Jinan Black granite. Due to their high precision, long duration, good stability and corrosion resistance, they have been more and more used in product inspection of modern industry and such scientific areas as mechanical aero space and scientific researches.

Advantages

----Twice as hard as cast iron；

----Minimal changes of dimension are due to changes of temperature；

----Free from wringing,so there is not interruption of work；

----Free from burrs or protrusions because of the fine grain structure and insignificant stickness, which ensures the high degree of flatness over a long service life and causes no damage to other parts or instruments；

----Trouble-free operation for use with magnetic materials；

----Long life and rust-free,resulting in low maintenance costs.

The precision granite surface plates are precision lapped to high standard of flatness to achieve accuracy and are used as base for mounting sophisticated mechanical, electronic and optical gauging systems.

Some of the unique features of the granite surface plate:

Uniformity in Hardness;

Accurate under load Conditions;

Vibration Absorbent;

Easy to Clean;

Wrap Resistant;

Low Porosity;

Non-Abrasive;

Non-Magnetic

Advantages of Granite Surface Plate

First, the rock after a long period of natural aging, uniform structure, coefficient minimum, the internal stress completely disappear, not deformed, so the precision is high.

Second, there will be no scratches, not under constant temperature conditions, at room temperature can also maintain the accuracy of temperature measurement.

Third, not magnetization, measurement can be smooth movement, no creaky feeling, not affected by moisture, the plane is fixed.

Four, the rigidity is good, the hardness is high, the abrasion resistance is strong.

Five, not afraid of acid, alkaline liquid erosion, will not rust, do not have to paint oil, not easy to sticky micro-dust, maintenance, easy to maintain, long service life.

Why choose granite base instead of cast iron machine bed?

1. Granite machine base could keep higher precision than cast iron machine base. cast iron machine base is easily affected by temperature and humidity but granite machine base will not;

2. With the same size of granite machine base and cast iron base, the granite machine base is more cost-effective than cast iron;

3. Special granite machine base is more easier to be finished than cast iron machine base.

Granite surfaces plates are key instruments in inspection labs across the country. The calibrated, extremely flat surface of a surface plate enables inspectors to use them as a baseline for part inspections and instrument calibration. Without the stability afforded by surface plates, many of the tightly toleranced parts in various technological and medical fields would be much more difficult, if not impossible, to manufacture correctly. Of course, to use a granite surface block to calibrate and inspect other materials and tools, the accuracy of the granite itself must be assessed. Users can calibrate a granite surface plate to ensure its accuracy.

Clean the granite surface plate prior to calibration. Pour a small amount of surface plate cleaner on a clean, soft cloth and wipe the surface of the granite. Immediately dry the cleaner off the surface plate with a dry cloth. Do not allow the cleaning liquid to air-dry.

Place a repeat measuring gauge on the center of the granite surface plate.

Zero the repeat measuring gauge to the surface of the granite plate.

Move the gauge slowly across the surface of the granite. Watch the gauge's indicator and record the peaks of any height variations as you move the instrument across the plate.

Compare the flatness variation across the surface of the plate with the tolerances for your surface plate, which varies based on the size of the plate and the flatness grade of the granite. Consult federal specification GGG-P-463c (see Resources) to determine if your plate meets flatness requirements for its size and grade. The variation between the highest point on the plate and the lowest point on the plate is its flatness measurement.

Check that the largest depth variations on the surface of the plate fall within the repeatability specifications for a plate of that size and grade. Consult federal specification GGG-P-463c (see Resources) to determine if your plate meets repeatability requirements for its size. Reject the surface plate if even a single point fails repeatability requirements.

Stop using a granite surface plate that fails to meet federal requirements. Return the plate to the manufacturer or to a granite surfacing company to have the block re-polished to meet specifications.

Tip

Perform formal calibrations at least once per year, although granite surface plates seeing heavy use should be calibrated more frequently.

Formal, recordable calibration in manufacturing or inspection environments is often conducted by quality assurance or an outside calibration services vendor, although anyone can use a repeat measuring gauge to informally check a surface plate prior to use.

The Early History of Granite Surface Plates

Before World War II, Manufacturers used Steel Surface Plates for the dimensional inspection of parts.  During World War II the need for steel increased dramatically, and a lot of Steel Surface Plates were melted down. A replacement was needed, and Granite became the material of choice because of its superior metrological properties.

Several advantages of granite over steel became evident. Granite is harder, although more brittle and subject to chipping.  You can lap Granite to much greater flatness and faster than steel. Granite also has the desirable property of a lower thermal expansion compared to steel.  Further, if a steel plate needed repair, it had to be hand scraped by artisans who also applied their skills in machine tool rebuild.

As a side note, some Steel Surface Plates are still in use today.

Metrological Properties of Granite Plates

Granite is an igneous rock formed by volcanic eruptions. By comparison, marble is metamorphosed limestone.  For metrology use, the granite selected should meet specific requirements outlined in Federal Specification GGG-P-463c, from now on called Fed Specs, and specifically, Part 3.1 3.1   Among the Fed Specs,  granite should be a fine to medium-grained texture.

Granite is a hard material, but its hardness varies for several reasons.  An experienced granite plate technician can estimate the hardness by its color which is an indication of its quartz content.  Granite hardness is a property defined in part by the amount of quartz content and lack of mica. The red and pink granites tend to be the hardest, the grays are medium hardness, and the blacks are the softest.

Young’s Modulus of Elasticity is used to express the flexibility or indication of the hardness of the stone.  Pink granite averages 3-5 points on the scale, grays 5-7 points and blacks 7-10 points. The smaller the number, the harder the granite tends to be. The larger the number, the softer and more flexible granite is.  It is important to know the hardness of Granite when choosing a thickness required for tolerance grades and the weight of parts and gauges placed upon it.

In the old days when there were real machinists, known by their trig table booklets in their shirt pockets, black granite was considered to be “The Best.”   The Best defined as the type that gave the most resistance to wear or is harder.  One drawback is that the harder granites tend to chip or ding easier.  Machinists were so convinced that black granite was the best that some manufacturers of pink granite dyed them black.

I have personally witnessed a plate that was dropped off a forklift when moved from storage. The plate hit the floor and split in two revealing the true pink color. Use caution if planning a purchase of black granite out of China.  We recommend you waste your money in some other way. A granite plate can vary in hardness within itself. A streak of quartz can be much harder than the rest of the surface plate. A layer of black gabbro can make an area much softer. A well trained, experienced surface plate repair techs know how to handle these soft areas.

Surface Plate Grades

There are four grades of surface plates. Laboratory grade AA and A, Room Inspection Grade B, and the fourth is Workshop Grade.  Grade’s AA and A are the flattest with a flatness tolerances better than 0.00001 in for a Grade AA plate.  Workshop Grades being the least flat and as the name suggests, they are intended for use in tool rooms.  Where as Grade AA, Grade A and Grade B are intended for use in an inspection or quality control lab.

Proper Testing For Surface Plate Calibration

I have always told my customers that I can pull any 10-year-old out of my church and teach them in just a few days how to test a plate.  It is not hard.  It does require some technique to perform the task quickly, techniques that one learns through time and much repetition.  I should inform you, and I cannot emphasize enough, Fed Spec GGG-P-463c IS NOT a calibration procedure! More on that later.

Calibration of overall flatness (Mean Pane) and Repeatability (localized wear) checks is a must According to the Fed Specs.  The only exception to this is with small plates where repeatability is only required.

Also, and just as critical as the other tests, is the test for thermal gradients. (See Delta T below)

Figure 1

Flatness Testing has 4 approved methods. Electronic levels, autocollimation, laser and a device known as a plane locator.  We only use electronic levels because they are the most accurate and fastest method for several reasons.

Lasers and autocollimators use a very straight beam of light as a reference. One makes a straightness measurement of a granite surface plate by comparing the variation in distance between the surface plate and the light beam.  By taking a straight beam of light, striking it onto a reflector target while moving the reflector target down the surface plate, the distance between the emitted beam and the return beam is a straightness measurement.

Here’s the problem with this method. The target and the source are affected by vibration, ambient temperature, a less than flat or scratched target, contamination in the air, and air movement (currents).  All these contribute additional components of error.  Furthermore, the contribution of operator error from checks with an autocollimator is greater.

An experienced autocollimator user can make very accurate measurements but still faces problems with consistency of the readings especially over longer distances as the reflections tend to widen or become slightly blurred. Also, a less than perfectly flat target and a long day of peering through the lens produces additional errors.

A plane locator device is just silly. This device uses a somewhat straight (as compared to an extremely straight collimated or laser beam of light) as its reference.  Not only does the mechanical device use an indicator normally of only 20 u Inch resolution but the un-straightness of the bar and dissimilar materials add significantly to errors in measurement. In our opinion, although the method is acceptable, no competent lab would ever use a plane locating device as a final inspection instrument.

Electronic levels use gravity as their reference.  Differential electronic levels are not affected by vibration.  They have a resolution as low as .1 arc second and measurements are fast, accurate and there is very little contribution of error from an experienced operator.  Neither Plane Locators nor autocollimators provide computer-generated topographical (Figure 1) or isometric plots (Figure 2) of the surface.

Figure 2

A Proper Flatness of the Surface Test

A proper flatness of the surface test is such an important part of this paper I should have placed it at the beginning.  As stated earlier, Fed Spec. GGG-p-463c IS NOT a calibration method.  It serves as a guide for many aspects of metrology grade granite whose intended buyer is any Federal Government Agency, and that includes the methods of testing and tolerances or grades.  If a contractor claims they adhered to Fed Specs, then the flatness value shall be determined by the Moody Method.

Moody was a fellow from way back in the 50’s who devised a mathematical method to determine overall flatness and account for the orientation of lines tested, whether they be sufficiently close in the same plane. Nothing has changed. Allied Signal tried to improve on the mathematical method but concluded that the differences were so small it was not worth the effort.

If a surface plate contractor uses Electronic Levels or laser, he uses a computer to assist him with the computations.  Without computer assist the technician using autocollimation must calculate the readings by hand.  In reality, they do not. It takes too long and frankly may be too challenging.   In a flatness test using the Moody Method, the technician tests eight lines in a Union Jack configuration for straightness.

The Moody Method

The Moody Method is a mathematical way to determine if the eight lines are on the same plane. Otherwise, you just have 8 straight lines that may or may not be on or near the same plane. Further, a contractor claiming to adherence to Fed Spec, and uses autocollimation,  he must generate eight pages of data.  One page for each line checked to prove his testing, repairing, or both. Otherwise, the contractor has no idea what the real flatness value is.

I am certain if you are one of those that get your plates calibrated by a contractor using autocollimation, you have never seen those pages!  Figure 3  is a sample of just one page of eight necessary to compute the overall flatness.  One indication of that ignorance and malice is if your report has nice rounded numbers.  For example, 200, 400, 650, etc.  A properly computed value is a real number. For example 325.4 u In.  When the contractor uses the Moody Method of computations, and the technician calculates the values manually, you should receive eight pages of computations and an isometric plot. The isometric plot shows the varying heights along the different lines and how much distance separates the selected intersecting points.

Figure 3   (It takes eight pages like this to calculate flatness manually. Be sure to ask why you are not getting this if your contractor uses autocollimation!)

Figure 4

Dimensional Gauge technicians use differential levels (Figure 4) as preferred devices to measure the minute changes in angularity from measurement station to station. The levels have a resolution down to .1 arc seconds (5 u Inches using 4″ sled) are extremely stable, are not affected by vibration,  distances measured, air currents, operator fatigue, air contamination or any of the problems inherent in other devices. Add computer assistance, and the task becomes relatively fast, generating topographical and isometric plots proving the verification and most importantly the repair.

A Proper Repeatability Test

Repeat reading or repeatability is the most important test.  The equipment we use to perform the repeatability test is a repeat reading fixture, an LVDT and an amplifier necessary for high-resolution readings.  We set the LVDT amplifier to a minimum resolution of 10 u Inches or 5 u Inches for high accuracy plates.

Using a mechanical indicator with a resolution of only 20 u Inches is worthless if you are trying to test for a repeatability requirement of 35 u Inches.  Indicators have a 40 u Inches uncertainty!  The repeat reading setup mimics a height gage/part configuration.

Repeatability IS NOT the same as overall flatness (Mean Plane).  I like to think of repeatability in granite viewed as a consistent radius measurement.

Figure 5

If you test for repeatability of a round ball, then you have demonstrated the ball’s radius has not changed. (The ideal profile of a properly repaired plate has a convex crowned shape.)  However, it is evident that the ball is not flat. Well, sort of. On an extremely short distance, it is flat.  Since the majority of inspection work involves a height gage very near to the part, repeatability becomes the most critical property of a granite plate.  It is more important that the overall flatness unless a user is checking the straightness of a long part.

Make certain that your contractor performs a repeat reading test. A plate can have a repeat reading significantly out of tolerance but still pass a flatness test!  Amazingly a lab can get accreditation in testing that does not include a repeat reading test.  A lab that cannot repair or is not very good at repairing prefers to perform flatness testing only. Flatness rarely changes unless you move the plate.

Repeat reading testing is the easiest to test but the most difficult to achieve when lapping. Make certain your contractor can restore repeatability without “dishing” the surface or leaving waves in the surface.

Delta T Test

This test involves measuring the ACTUAL temperature of the stone at its top surface and its bottom surface and computing the difference, Delta T, for reporting on the certificate.

It is important to know the average coefficient of thermal expansion in granite is 3.5 uIn/Inch/degree.   Ambient temperatures and humidity’s effect on a granite plate is negligible. However, a surface plate can go out of tolerance or sometimes improve even if in a .3 – .5 degree F Delta T.  It is necessary to know if the Delta T is within .12 degrees F of where the differential from the last calibration.

It is also important to know that a plates work surface migrates towards heat. If the top temperature is warmer than the bottom, then the top surface rises.  If the bottom is warmer, which is rare, then the top surface sinks. It is not enough for a quality manager or technician to know the plate is flat and repeatable at the time of calibration or repair but what it’s Delta T was at the time of final calibration testing.  In critical situations a user can, by measuring the Delta T himself, determine if a plate has gone out of tolerance due solely to Delta T variations. Fortunately, granite takes many hours or even days to acclimate to an environment.   Minor fluctuations in ambient temperature throughout the day will not effect it. For these reasons, we do not report ambient calibration temperature or humidity because the effects are negligible.

Granite Plate Wear

While granite is harder than steel plates, granite still develops low spots on the surface. Repetitive movement of parts and gages on the surface plate is the greatest source of wear, especially if the same area is constantly in use. Dirt and grinding dust allowed to remain on a plate’s surface speed up the wear process as it gets between parts or gauges and the granite surface.  When moving parts and gages across its surface, abrasive dust is usually the cause of additional wear.  I highly recommended constant cleaning to reduce wear. We have seen wear by on plates caused by daily UPS package deliveries placed on top of plates!  Those localized areas of wear affect the calibration repeatability test readings.  Avoid wear by cleaning regularly.

Granite Plate Cleaning

To keep the plate clean, use a tack cloth to remove grit. Just press very lightly, so you do not leave glue residue. A well-used tack cloth does an excellent job of picking up grinding dust between cleaning. Do not work in the same spot. Move your setup around the plate, distributing the wear. It is OK to use alcohol to clean a plate, but be aware that doing so will temporarily super cool the surface. Water with a small amount of soap is excellent. The commercially available cleaners such as Starrett’s cleaner are also excellent to use, but make sure you get all the soap residue off the surface.

Granite Plate Repair

It should be evident by now the importance of making certain your surface plate contractor performs a competent calibration.   The “Clearing House” type labs that offer “Do it all with one call” programs rarely have a technician that can do repairs.  Even if they do offer repairs, they do not always have a technician that has the experience needed when the surface plate is significantly out of tolerance.

If told a plate can not be repairable because of extreme wear, call us.  Most likely we can make the repair.

Our techs work a one to one and a half year apprenticeship under a Master Surface Plate Technician.  We define a Master Surface Plate Technician as someone who has completed their apprenticeship and has over ten additional years experience in Surface Plate calibration and Repair. We at Dimensional Gauge have three Master Technicians on staff with over 60 years experience combined. One of our Master Technician is available at all times for support and guidance for when difficult situations arise.  All of our technicians have experience in surface plate calibrations of all sizes, from small to very large, varying environmental conditions, different industries, and in major wear problems.

Fed Specs have a specific finish requirement of 16 to 64  Average Arithmetic Roughness (AA).  We prefer a finish in the range of 30-35 AA. There is just enough roughness to ensure parts and gages move smoothly and don’t stick or wring to the surface plate.

When we repair we inspect the plate for proper mounting and levelness. We use a dry lapping method, but in cases of extreme wear requiring substantial granite removal, we wet lap.  Our technicians clean up after themselves, they are thorough, fast and precise. That is important because the cost of granite plate service includes your downtime and lost production.  A competent repair is of paramount importance, and you should never choose a contractor on price or convenience.  Some calibration work demands highly trained individuals. We have that.

Final Calibration Reports

For each surface plate repair and calibration, we provide detailed professional reports. Our reports contain a significant amount of both critical and pertinent information. Fed Spec. requires most of the information we provided.  Excluding those contained in other quality standards such as ISO/IEC-17025,  the minimum Fed. Specs for reports are:

1. Size in Ft. (X’ x X’)

1. Color
2. Style (Refers to no clamp ledges or two or four ledges)
3. Estimated Modulus of Elasticity
4. Mean Plane Tolerance (Determined by Grade/Size)
5. Repeat reading Tolerance (Determined by diagonal length in inches)
6. Mean Plane as Found
7. Mean Plane as left
8. Repeat reading as found
9. Repeat reading as left
10. Delta T (Temperature differential between top and bottom surfaces)

If the technician needs to carry out lapping or repair work to the surface plate, then the certificate of calibration is accompanied by a topographical or isometric plot to prove a valid repair.

A Word Concerning ISO/IEC-17025 Accreditations and the labs that have them

Just because a lab has accreditation in surface plate calibration does not necessarily mean they know what they are doing much less doing it correctly!  Neither does it necessarily imply the lab can repair. The accrediting bodies do not make the distinction between verification or calibration (repair). And I know of one, maybe 2 accrediting bodies that wilL tie A ribbon around my dog if I paid them enough money!  It is a sad fact. I have seen labs get accreditation by performing only one of the three tests required. Moreover, I have seen labs get accreditation with unrealistic uncertainties and get accredited without any proof or demonstration how they calculated the values. It is all unfortunate.

 Summation

You cannot underestimate the role of precision granite plates.  The flat reference that granite plates provide is the foundation upon which you make all other measurements.

You can utilize the most modern, most accurate and most versatile measuring instruments. However,  accurate measurements are hard to ascertain if the reference surface is not flat.  One time, I had a prospective customer say to me “well it is just rock!”  My response, “OK, you are correct, and you definitely cannot justify having experts come in to maintain your surface plates.”

Price is never a good reason to choose surface plate contractors.  Buyers, accountants and a disturbing number of quality engineers  do not always understand that recertifying granite plates is not like recertifying a micrometer, caliper or a DMM.

Some instruments require expertise, not a low price.  After saying that, our rates are very reasonable. Especially for having confidence that we perform the work correctly. We go well beyond ISO-17025  and Federal  Specifications requirements in added value.

Precision Granite Base Can Improve Machine Tool Performances

Requirements are constantly increasing in mechanical engineering in general and in machine tool construction in particular. Achieving maximum precision and performance values without increasing costs are constant challenges to being competitive. The machine tool bed is a decisive factor here. Therefore, more and more machine tool manufacturers are relying on granite. Due to its physical parameters, it offers clear advantages that cannot be achieved with steel or polymer concrete.

Granite is a so-called volcanic deep rock and has a very dense and homogeneous structure with an extremely low coefficient of expansion, low thermal conductivity and high vibration damping.

Below you will discover why the common opinion that granite is mainly only suitable as machine base for high-end coordinate measuring machines is long outdated and why this natural material as a machine tool base is a very advantageous alternative to steel or cast iron even for high-precision machine tools.

We can manufacture granite components for dynamic motion, granite components for linear motors, granite components for ndt, granite components for xray, granite components for cmm, granite components for cnc, granite precision for laser, granite components for aerospace, granite components for precision stages...

High Added Value Without Additional Costs
The increasing use of granite in mechanical engineering is not so much due to the massive increase in the price of steel. Rather, it is because the added value for the machine tool achieved with a machine bed made of granite is possible at very little or no extra cost. This is proven by cost comparisons of well-known machine tool manufacturers in Germany and Europe.

The considerable gain in thermodynamic stability, vibration damping and long-term precision made possible by granite cannot be achieved with a cast iron or steel bed, or only at relatively high cost. For example, thermal errors can account for up to 75% of the total error of a machine, with compensation often attempted for by software – with moderate success. Due to its low thermal conductivity, granite is the better foundation for long-term precision.

With a tolerance of 1 μm, granite easily meets the flatness requirements according to DIN 876 for the degree of accuracy 00. With a value of 6 on the hardness scale 1 to 10, it is extremely hard, and with its specific weight of 2.8g/cm³ it almost reaches the value of aluminium. This also results in additional advantages such as higher feed rates, higher axis accelerations and an extension of the tool life for cutting machine tools. Thus, the change from a cast bed to a granite machine bed moves the machine tool in question into the high-end class in terms of precision and performance – at no extra cost.

Granite’s Improved Ecological Footprint
In contrast to materials such as steel or cast iron, natural stone does not have to be produced with a great deal of energy and using additives. Only relatively small amounts of energy are required for quarrying and surface treatment. This results in a superior ecological footprint, which even at the end of a machine’s life surpasses that of steel as a material. The granite bed can be the basis for a new machine or be used for completely different purposes such as shredding for road construction.

Nor are there any shortages of resources for granite. It is a deep rock formed from magma within the earth’s crust. It has ‘matured’ for millions of years and is available in very large quantities as a natural resource on almost all continents, including all of Europe.

Conclusion: The numerous demonstrable advantages of granite compared to steel or cast iron justify the increasing willingness of mechanical engineers to use this natural material as a foundation for high-precision, high-performance machine tools. Detailed information about granite properties, which are advantageous for machine tools and mechanical engineering, can be found in this further article.

A repeat measurement is a measurement of local flatness areas. The Repeat Measurement specification states that a measurement taken anywhere on the surface of a plate will repeat within the stated tolerance. Controlling local area flatness tighter than overall flatness guarantees a gradual change in surface flatness profile thereby minimizing local errors.

Most manufacturers, including imported brands, adhere to the Federal Specification of overall flatness tolerances but many overlook the repeat measurements. Many of the low value or budget plates available in the market today will not guarantee repeat measurements. A manufacturer who does not guarantee repeat measurements is NOT producing plates that meet the requirements of ASME B89.3.7-2013 or Federal Specification GGG-P-463c, or DIN 876, GB, JJS...

Both are critical to ensure a precision surface for accurate measurements. Flatness specification alone is not sufficient to guarantee measurement accuracy. Take as an example, a 36 X 48 Inspection Grade A surface plate, which meets ONLY the flatness specification of .000300". If the piece being checked bridges several peaks, and the gage being used is in a low spot, the measurement error could be the full tolerance in one area, 000300"! Actually, it can be much higher if the gage is resting on the slope of an incline.

Errors of .000600"-.000800" are possible, depending upon the severity of the slope, and the arm length of the gage being used. If this plate had a Repeat Measurement specification of .000050"F.I.R. then the measurement error would be less than .000050" regardless of where the measurement is taken on the plate. Another problem, which usually arises when an untrained technician attempts to resurface a plate on-site, is the use of Repeat Measurements alone to certify a plate.

The instruments that are used to verify repeatability are NOT designed to check overall flatness. When set to zero on a perfectly curved surface, they will continue to read zero, whether that surface is perfectly flat or perfectly concave or convex 1/2"! They simply verify the uniformity of the surface, not the flatness. Only a plate that meets both the flatness specification AND the repeat measurement specification truly meets the requirements of ASME B89.3.7-2013  or Federal Specification GGG-P-463c.

Ask us about or flatness specification and repeat measurement promise by calling +86 19969991659 or emailing INFO@ZHHIMG.COM

Yes, but they can only be guaranteed for a specific vertical temperature gradient. The effects of thermal expansion on the plate could easily cause a change in accuracy greater than the tolerance if there is a change in the gradient. In some cases, if the tolerance is tight enough, the heat absorbed from overhead lighting can cause enough of a gradient change over several hours.

Granite has a coefficient of thermal expansion of approximately .0000035 inches per inch per 1°F. As an example: A 36" x 48" x 8" surface plate has an accuracy of .000075" (1/2 of Grade AA) at a gradient of 0°F, the top and bottom are the same temperature. If the top of the plate warms up to the point where it is 1°F warmer than the bottom, the accuracy would change to .000275" convex ! Therefore, ordering a plate with a tolerance tighter than Laboratory Grade AA should only be considered if there is adequate climate control.

A surface plate should be supported at 3 points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered. Only 3 points can rest solidly on anything but a precision surface.

The plate should be supported at these points during production, and it should be supported only at these three points while in use. Attempting to support the plate at more than three points will cause the plate to receive its support from various combinations of three points, which will not be the same 3 points on which it was supported during production. This will introduce errors as the plate deflects to conform to the new support arrangement. All zhhimg steel stands have support beams designed to line up with the proper support points.

If the plate is properly supported, precise leveling is only necessary if your application calls for it. Leveling is not necessary to maintain the accuracy of a properly supported plate.

Why Choose Granite for Machine Bases and Metrology Components?

The answer is 'yes' for almost every application. The advantages of granite include: No rust or corrosion, almost immune to warping, no compensating hump when nicked, longer wear life, smoother action, greater precision, virtually non-magnetic, low co-efficient of thermal expansion, and low maintenance cost.

Granite is a type of igneous rock quarried for its extreme strength, density, durability, and resistance to corrosion. But granite is also very versatile– it’s not just for squares and rectangles! In fact, Starrett Tru-Stone confidently works with granite components engineered in shapes, angles, and curves of all variations on a regular basis—with excellent outcomes.

Through our state of the art processing, cut surfaces can be exceptionally flat. These qualities make granite the ideal material to create custom-size and custom-design machine bases and metrology components. Granite is:

machineable
precisely flat when cut and finished
rust resistant
durable
long lasting
Granite components are also easy to clean. When creating custom designs, be sure to choose granite for its superior benefits.

STANDARDS / HIGH WEAR APPLICATIONS
The granite utilized by ZhongHui for our standard surface plate products has high quartz content, which provides greater resistance to wear and damage. Our Superior Black and Crystal Pink colors have low water absorption rates, minimizing the possibility of your precision gages rusting while setting on the plates. The colors of granite offered by ZhongHui result in less glare, which means less eyestrain for individuals using the plates. We have chosen our granite types while considering thermal expansion in an effort to keep this aspect minimal.

CUSTOM APPLICATIONS
When your application calls for a plate with custom shapes, threaded inserts, slots or other machining, you’ll want to select a material like Black Diabase. This natural material offers superior stiffness, excellent vibration dampening, and improved machinability.

Yes, if they are not too badly worn. Our factory setting and equipment allow the optimum conditions for proper plate calibration and rework if necessary. Generally, if a plate is within .001" of the required tolerance, it can be resurfaced on-site. If a plate is worn to the point where it is more than .001" out of tolerance, or if it is badly pitted or nicked, then it will need to be sent to the factory for grinding prior to relapping.

Great care should be exercised in selecting an on-site calibration and resurfacing technician. We urge you to use caution in selecting your calibration service. Ask for accreditation and verify the equipment that the technician will use has a National Inspection Institution traceable calibration. It takes many years to learn how to properly lap precision granite.

ZhongHui provides quick turn-around on calibrations performed in our factory. Send your plates in for calibration if possible. Your quality and reputation depend on the accuracy of your measurement instruments including surface plates!

Our black surface plates have a significantly higher density and are up to three times as stiff. Therefore, a plate made of the black does not need to be as thick as a granite plate of the same size to have equal or greater resistance to deflection. Reduced thickness means less weight and lower shipping costs.

Beware of others who use lower quality black granite in the same thickness. As stated above, properties of granite, like wood or metal, vary by material and color, and is not an accurate predictor of stiffness, hardness, or wear resistance. In fact, many types of black granite and diabase are very soft and not suitable for surface plate applications.

No. The specialized equipment and training necessary to rework these items requires that they be returned to the factory for calibration and rework.

Yes. Ceramic and granite have similar characteristics, and the methods used to calibrate and lap granite can be used with ceramic items as well. Ceramics are more difficult to lap than granite resulting in a higher cost.

Yes, provided that the inserts are recessed below the surface. If steel inserts are flush with, or above the surface plane, they must be spot-faced down before the plate can be lapped. If required, we can provide that service.

Yes. Steel inserts with the desired thread (English or metric) can be epoxy bonded into the plate at the desired locations. ZhongHui uses CNC machines to provide the tightest insert locations within +/- 0.005”. For less critical inserts, our locational tolerance for threaded inserts is ±.060". Other options include steel T-Bars and dovetail slots machined directly into the granite.

Inserts that are properly bonded using high strength epoxy and good workmanship will withstand a great deal of torsional and shear force. In a recent test, using 3/8"-16 threaded inserts, an independent testing laboratory measured the force required to pull an epoxy-bonded insert from a surface plate. Ten plates were tested. Out of these ten, in nine cases, the granite fractured first. The average load at the point of failure was 10,020 lbs. for gray granite and 12,310 lbs. for black. In the single case where an insert pulled free of the plate, the load at the point of failure was 12,990 lbs.! If a work piece forms a bridge across the insert and extreme torque is applied, it is possible to generate enough force to fracture the granite. Partially for this reason, ZhongHui gives guidelines for the maximum safe torque that can be applied the epoxy bonded inserts: https://www.zhhimg.com/standard-thread-inserts-product/

Yes, but only at our factory. At our plant, we can restore almost any plate to 'like-new' condition, usually for less than half the cost of replacing it. Damaged edges can be cosmetically patched, deep grooves, nicks, and pits can be ground out, and the attached supports can be replaced. In addition, we can modify your plate to increase its versatility by adding solid or threaded steel inserts and cutting slots or clamping lips, per your specifications.

Why Choose Granite?
Granite is a type of igneous rock formed in the Earth millions of years ago. The composition of igneous rock contained many minerals such as quartz that is extremely hard and wear-resistant. In addition to hardness and wear resistance granite has approximately half the coefficient of expansion as cast iron. As its volumetric weight is approximately one third that of cast iron, granite is easier to manoeuvre.

For machine bases and metrology components, black granite is the colour most used. Black granite has a higher percentage of quartz than other colours and is, therefore, the hardest wearing.

Granite is cost-effective, and cut surfaces can be exceptionally flat. Not only can it be hand lapped to achieve extremes of accuracy, but re-conditioning can be performed without moving the plate or table off-site. It is entirely a hand lapping operation and generally costs much less than re-conditioning a cast iron alternative.

These qualities make granite the ideal material to create custom-size and custom-design machine bases and metrology components such as the granite surface plate.

ZhongHui produces bespoke granite products that are created to support specific measurement requirements. These bespoke items vary from straight edges to tri squares. Due to the versatile nature of granite, the components can be produced to any size required; they are hard wearing and long-lasting.

Advantages of Granite Surface Plates
The importance of measuring on an even surface was established by British inventor Henry Maudsley in the 1800s. As a machine tool innovator, he determined that consistent production of parts required a solid surface for reliable measurements.

The industrial revolution created a demand for measuring surfaces, so engineering company Crown Windley created manufacturing standards. The standards for surface plates were first set by Crown in 1904 using metal. As the demand and cost for metal increased, alternative materials for the measuring surface were investigated.

In America, monument creator Wallace Herman established that black granite was an excellent surface plate material alternative to metal. As granite is non-magnetic and doesn’t rust, it soon became the preferred measuring surface.

A granite surface plate is an essential investment for laboratories and test facilities. A granite surface plate of 600 x 600 mm can be mounted on a support stand. The stands provide a working height of 34” (0.86m) with five adjustable points for levelling.

For reliable and consistent measurement results, a granite surface plate is crucial. As the surface is a smooth and stable plane, it enables instruments to be carefully manipulated.

The main advantages of granite surface plates are:

• Non-reflective
• Resistant to chemicals and corrosion
• Low coefficient of expansion compared with cart iron so less affected by temperature change
• Naturally rigid and hard-wearing
• The plane of the surface is unaffected if scratched
• Will not rust
• Non-magnetic
• Easy to clean and maintain
• Calibration and resurfacing can be done onsite
• Suitable for drilling for threaded support inserts
• High vibration damping

For many shops, inspection rooms and laboratories, precision granite surface plates are relied on as the basis for accurate measurement. Because every linear measurement depends on an accurate reference surface from which final dimensions are taken, surface plates provide the best reference plane for work inspection and layout prior to machining. They also are ideal bases for making height measurements and gaging surfaces. Further, a high degree of flatness, stability, overall quality and workmanship make them a good choice for mounting sophisticated mechanical, electronic and optical gaging systems. For any of these measurement processes, it is imperative to keep surface plates calibrated.

Repeat Measurements and Flatness
Both flatness and repeat measurements are critical to ensure a precision surface. Flatness can be considered as all points on the surface being contained within two parallel planes, the base plane and the roof plane. The measurement of distance between the planes is the overall flatness of the surface. This flatness measurement commonly carries a tolerance and may include a grade designation.

The flatness tolerances for three standard grades are defined in the federal specification as determined by the following formula:
Laboratory Grade AA = (40 + diagonal² / 25) x 0.000001 inch (unilateral)
Inspection Grade A = Laboratory Grade AA x 2
Tool Room Grade B = Laboratory Grade AA x 4

In addition to flatness, repeatability must be ensured. A repeat measurement is a measurement of local flatness areas. It is a measurement taken anywhere on the surface of a plate that will repeat within the stated tolerance. Controlling local area flatness to a tighter tolerance than overall flatness guarantees a gradual change in surface flatness profile, thereby minimizing local errors.

To ensure a surface plate meets both the flatness and repeat measurement specifications, manufacturers of granite surface plates should use Federal Specification GGG-P-463c as a basis for their specifications. This standard addresses repeat measurement accuracy, material properties of surface plate granites, surface finish, support point location, stiffness, acceptable methods of inspection and installation of threaded inserts.

Before a surface plate has worn beyond specification for overall flatness, it will show worn or wavy posts. Monthly inspection for repeat measurement errors using a repeat reading gage will identify wear spots. A repeat reading gage is a high-precision instrument that detects local error and can be displayed on a high magnification electronic amplifier.

Checking Plate Accuracy
By following a few simple guidelines, an investment in a granite surface plate should last for many years. Depending on plate usage, shop environment and required accuracy, frequency of checking the surface plate accuracy varies. A general rule of thumb is for a new plate to receive a full recalibration within one year of purchase. If the plate is used frequently, it is advisable to shorten this interval to six months.

Before a surface plate has worn beyond specification for overall flatness, it will show worn or wavy posts. Monthly inspection for repeat measurement errors using a repeat reading gage will identify wear spots. A repeat reading gage is a high-precision instrument that detects local error and can be displayed on a high magnification electronic amplifier.

An effective inspection program should include regular checks with an autocollimator, providing actual calibration of overall flatness traceable to National Institute of Standards and Technology (NIST). Comprehensive calibration by the manufacturer or an independent company is necessary from time to time.

Variations Between Calibrations
In some cases, there are variations between surface plate calibrations. Sometimes factors such as surface change resulting from wear, incorrect use of inspection equipment or use of noncalibrated equipment can account for these variations. The two most common factors, however, are temperature and support.

One of the most important variables is temperature. For instance, the surface might have been washed with a hot or cold solution prior to calibration and not allowed sufficient time to normalize. Other causes of temperature change include drafts of cold or hot air, direct sunlight, overhead lighting or other sources of radiant heat on the surface of the plate.

There also can be variations in the vertical temperature gradient between winter and summer. In some cases, the plate is not allowed sufficient time to normalize after shipment. It is a good idea to record the vertical gradient temperature at the time the calibration is performed.

Another common cause for calibration variation is a plate that is improperly supported. A surface plate should be supported at three points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered.

Only three points can rest solidly on anything but a precision surface. Attempting to support the plate at more than three points will cause the plate to receive its support from various combinations of three points, which will not be the same three points on which it was supported during production. This will introduce errors as the plate deflects to conform to the new support arrangement. Consider using steel stands with support beams designed to line up with the proper support points. Stands for this purpose are generally available from the surface plate manufacturer.

If the plate is properly supported, precise leveling is only necessary if an application specifies it. Leveling is not necessary to maintain the accuracy of a properly supported plate.

It is important to keep the plate clean. Airborne abrasive dust is usually the greatest source of wear and tear on a plate, as it tends to embed in workpieces and the contact surfaces of gages. Cover plates to protect them from dust and damage. Wear life can be extended by covering the plate when not in use.

Extend Plate Life
Following a few guidelines will reduce wear on a granite surface plate and ultimately, extend its life.

First, it is important to keep the plate clean. Airborne abrasive dust is usually the greatest source of wear and tear on a plate, as it tends to embed in workpieces and the contact surfaces of gages.

It also is important to cover plates to protect it from dust and damage. Wear life can be extended by covering the plate when not in use.

Rotate the plate periodically so that a single area does not receive excessive use. Also, it is recommended to replace steel contact pads on gaging with carbide pads.

Avoid setting food or soft drinks on the plate. Many soft drinks contain either carbonic or phosphoric acid, which can dissolve the softer minerals and leave small pits in the surface.

Where to Relap
When a granite surface plate needs re-surfacing, consider whether to have this service performed on-site or at the calibration facility. It is always preferable to have the plate relapped at the factory or a dedicated facility. If, however, the plate is not too badly worn, generally within 0.001 inch of the required tolerance, it can be resurfaced on-site. If a plate is worn to the point where it is more than 0.001 inch out of tolerance, or if it is badly pitted or nicked, then it should be sent to the factory for grinding prior to relapping.

A calibration facility has the equipment and factory setting providing the optimum conditions for proper plate calibration and rework if necessary.

Great care should be exercised in selecting an on-site calibration and resurfacing technician. Ask for accreditation and verify the equipment that the technician will use has a NIST-traceable calibration. Experience also is an important factor, as it takes many years to learn how to correctly lap precision granite.

Critical measurements start with a precision granite surface plate as a baseline. By ensuring a reliable reference by using a properly calibrated surface plate, manufacturers have one of the essential tools for reliable measurements and better quality parts.

Checklist for Calibration Variations

1. The surface was washed with a hot or cold solution prior to calibration and was not allowed sufficient time to normalize.
2. The plate is improperly supported.
3. Temperature change.
4. Drafts.
5. Direct sunlight or other radiant heat on the surface of the plate. Be sure that overhead lighting is not heating the surface.
6. Variations in the vertical temperature gradient between winter and summer. If at all possible, know the vertical gradient temperature at the time the calibration is performed.
7. Plate not allowed sufficient time to normalize after shipment.
8. Improper use of inspection equipment or use of noncalibrated equipment.
9. Surface change resulting from wear.

Tech Tips
Because every linear measurement depends on an accurate reference surface from which final dimensions are taken, surface plates provide the best reference plane for work inspection and layout prior to machining.

Controlling local area flatness to a tighter tolerance than overall flatness guarantees a gradual change in surface flatness profile, thereby minimizing local errors.