Precision Machining is a process to remove material from a workpiece during holding close tolerance finishes. The precision machine has many types, including milling, turning and electrical discharge machining. A precision machine today is generally controlled using a Computer Numerical Controls (CNC).
Almost all metal products use precision machining, as do many other materials such as plastic and wood. These machines are operated by specialized and trained machinists. In order for the cutting tool to do its job, it must be moved in directions specified to make the correct cut. This primary motion is called the "cutting speed." The workpiece can also be moved, known as the secondary motion of "feed." Together, these motions and the sharpness of the cutting tool allow the precision machine to operate.
Quality precision machining requires the ability to follow extremely specific blueprints made by CAD (computer aided design) or CAM (computer aided manufacturing) programs like AutoCAD and TurboCAD. The software can help produce the complex, 3-dimensional diagrams or outlines needed in order to manufacturer a tool, machine or object. These blueprints must be adhered to with great detail to ensure that a product retains its integrity. While most precision machining companies work with some form of CAD/CAM programs, they still work often with hand-drawn sketches in the initial phases of a design.
Precision machining is used on a number of materials including steel, bronze, graphite, glass and plastics to name a few. Depending on the size of the project and the materials to be used, various precision machining tools will be used. Any combination of lathes, milling machines, drill presses, saws and grinders, and even high-speed robotics may be used. The aerospace industry may use high velocity machining, while a woodwork tool-making industry might use photo-chemical etching and milling processes. The churning out of a run, or a specific quantity of any particular item, can number in the thousands, or be just a few. Precision machining often requires the programming of CNC devices which means they are computer numerically controlled. The CNC device allows for exact dimensions to be followed throughout the run of a product.
Milling is the machining process of using rotary cutters to remove material from a workpiece by advancing (or feeding) the cutter into the workpiece at a certain direction. The cutter may also be held at an angle relative to the axis of the tool. Milling covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes for machining custom parts to precise tolerances.
Milling can be done with a wide range of machine tools. The original class of machine tools for milling was the milling machine (often called a mill). After the advent of computer numerical control (CNC), milling machines evolved into machining centers: milling machines augmented by automatic tool changers, tool magazines or carousels, CNC capability, coolant systems, and enclosures. Milling centers are generally classified as vertical machining centers (VMCs) or horizontal machining centers (HMCs).
The integration of milling into turning environments, and vice versa, begun with live tooling for lathes and the occasional use of mills for turning operations. This led to a new class of machine tools, multitasking machines (MTMs), which are purpose-built to facilitate milling and turning within the same work envelope.
For design engineers, R&D teams, and manufacturers that depend on part sourcing, precision CNC machining allows for the creation of complex parts without additional processing. In fact, precision CNC machining often makes it possible for finished parts to be made on a single machine.
The machining process removes material and uses a wide range of cutting tools to create the final, and often highly complex, design of a part. The level of precision is enhanced through the use of computer numerical control (CNC), which is used to automate the control of the machining tools.
The role of "CNC" in precision machining
Using coded programming instructions, precision CNC machining allows a workpiece to be cut and shaped to specifications without manual intervention by a machine operator.
Taking a computer aided design (CAD) model provided by a customer, an expert machinist uses computer aided manufacturing software (CAM) to create the instructions for machining the part. Based on the CAD model, the software determines what tool paths are needed and generates the programming code that tells the machine:
■ What the correct RPMs and feed rates are
■ When and where to move the tool and/or workpiece
■ How deep to cut
■ When to apply coolant
■ Any other factors related to speed, feed rate, and coordination
A CNC controller then uses the programming code to control, automate, and monitor the movements of the machine.
Today, CNC is a built-in feature of a wide range of equipment, from lathes, mills, and routers to wire EDM (electrical discharge machining), laser, and plasma cutting machines. In addition to automating the machining process and enhancing precision, CNC eliminates manual tasks and frees machinists to oversee multiple machines running at the same time.
In addition, once a tool path has been designed and a machine is programmed, it can run a part any number of times. This provides a high level of precision and repeatability, which in turn makes the process highly cost effective and scalable.
Materials that are machined
Some metals that are commonly machined include aluminum, brass, bronze, copper, steel, titanium, and zinc. In addition, wood, foam, fiberglass, and plastics such as polypropylene can also be machined.
In fact, just about any material can be used with precision CNC machining — of course, depending on the application and its requirements.
Some advantages of precision CNC machining
For many of the small parts and components that are used in a wide range of manufactured products, precision CNC machining is often the fabrication method of choice.
As is true of virtually all cutting and machining methods, different materials behave differently, and the size and shape of a component also have a big impact on the process. However, in general the process of precision CNC machining offers advantages over other machining methods.
That is because CNC machining is capable of delivering:
■ A high degree of part complexity
■ Tight tolerances, typically ranging from ±0.0002" (±0.00508 mm) to ±0.0005" (±0.0127 mm)
■ Exceptionally smooth surface finishes, including custom finishes
■ Repeatability, even at high volumes
While a skilled machinist can use a manual lathe to make a quality part in quantities of 10 or 100, what happens when you need 1,000 parts? 10,000 parts? 100,000 or a million parts?
With precision CNC machining, you can get the scalability and speed needed for this type of high-volume production. In addition, the high repeatability of precision CNC machining gives you parts that are all the same from start to finish, no matter how many parts you are producing.
There are some very specialized methods of CNC machining, including wire EDM (electrical discharge machining), additive machining, and 3D laser printing. For example, wire EDM uses conductive materials — typically metals -— and electrical discharges to erode a workpiece into intricate shapes.
However, here we will focus on the milling and turning processes — two subtractive methods that are widely available and frequently used for precision CNC machining.
Milling vs. turning
Milling is a machining process that uses a rotating, cylindrical cutting tool to remove material and create shapes. Milling equipment, known as a mill or a machining center, accomplishes a universe of complex part geometries on some of the largest objects machined metal.
An important characteristic of milling is that the workpiece remains stationary while the cutting tool spins. In other words, on a mill, the rotating cutting tool moves around the workpiece, which remains fixed in place on a bed.
Turning is the process of cutting or shaping a workpiece on equipment called a lathe. Typically, the lathe spins the workpiece on a vertical or horizontal axis while a fixed cutting tool (which may or may not be spinning) moves along the programmed axis.
The tool cannot physically go around the part. The material rotates, allowing the tool to perform the programmed operations. (There is a subset of lathes in which the tools spin around a spool-fed wire, however, that is not covered here.)
In turning, unlike milling, the workpiece spins. The part stock turns on the lathe’s spindle and the cutting tool is brought into contact with the workpiece.
Manual vs. CNC machining
While both mills and lathes are available in manual models, CNC machines are more appropriate for purposes of small parts manufacturing — offering scalability and repeatability for applications requiring high volume production of tight tolerance parts.
In addition to offering simple 2-axis machines in which the tool moves in the X and Z axes, precision CNC equipment include multi-axis models in which the workpiece can also move. This is in contrast to a lathe where the workpiece is limited to spinning and the tools will move to create the desired geometry.
These multi-axis configurations allow for the production of more complex geometries in a single operation, without requiring additional work by the machine operator. This not only makes it easier to produce complex parts, but also reduces or eliminates the chance of operator error.
In addition, the use of high-pressure coolant with precision CNC machining ensures that chips do not get into the works, even when utilizing a machine with a vertically oriented spindle.
CNC mills
Different milling machines vary in their sizes, axis configurations, feed rates, cutting speed, the milling feed direction, and other characteristics.
However, in general, CNC mills all utilize a rotating spindle to cut away unwanted material. They are used to cut hard metals such as steel and titanium but can also be used with materials such as plastic and aluminum.
CNC mills are built for repeatability and can be used for everything from prototyping to high volume production. High-end precision CNC mills are often used for tight tolerance work such as milling fine dies and molds.
While CNC milling can deliver quick turnaround, as-milled finishing creates parts with visible tool marks. It may also produce parts with some sharp edges and burrs, so additional processes may be required if edges and burrs are unacceptable for those features.
Of course, deburring tools programmed into the sequence will deburr, although usually achieving 90% of the finished requirement at most, leaving some features for final hand finishing.
As for surface finish, there are tools that will produce not only an acceptable surface finish, but also a mirror-like finish on portions of the work product.
Types of CNC mills
The two basic types of milling machines are known as vertical machining centers and horizontal machining centers, where the primary difference is in the orientation of the machine spindle.
A vertical machining center is a mill in which the spindle axis is aligned in a Z-axis direction. These vertical machines can be further divided into two types:
■Bed mills, in which the spindle moves parallel to its own axis while the table moves perpendicular to the axis of the spindle
■Turret mills, in which the spindle is stationary and the table is moved so that it is always perpendicular and parallel to the axis of spindle during the cutting operation
In a horizontal machining center, the mill’s spindle axis is aligned in a Y-axis direction. The horizontal structure means these mills tend to take up more space on the machine shop floor; they are also generally heavier in weight and more powerful than vertical machines.
A horizontal mill is often used when a better surface finish is required; that’s because the orientation of the spindle means the cutting chips naturally fall away and are easily removed. (As an added benefit, efficient chip removal helps to increase tool life.)
In general, vertical machining centers are more prevalent because they can be as powerful as horizontal machining centers and can handle very small parts. In addition, vertical centers have a smaller footprint than horizontal machining centers.
Multi-axis CNC mills
Precision CNC mill centers are available with multiple axes. A 3-axis mill utilizes the X, Y, and Z axes for a wide variety of work. With a 4-axis mill, the machine can rotate on a vertical and horizontal axis and move the workpiece to allow for more continuous machining.
A 5-axis mill has three traditional axes and two additional rotary axes, enabling the workpiece to be rotated as the spindle head moves around it. This enables five sides of a workpiece to be machined without removing the workpiece and resetting the machine.
CNC lathes
A lathe — also called a turning center — has one or more spindles, and X and Z axes. The machine is used to rotate a workpiece on its axis to perform various cutting and shaping operations, applying a wide range of tools to the workpiece.
CNC lathes, which are also called live action tooling lathes, are ideal for creating symmetrical cylindrical or spherical parts. Like CNC mills, CNC lathes can handle smaller operations such prototyping but can also be set up for high repeatability, supporting high volume production.
CNC lathes can also be set up for relatively hands-free production, which makes them widely used in the automotive, electronics, aerospace, robotics, and medical device industries.
How a CNC lathe works
With a CNC lathe, a blank bar of stock material is loaded into the chuck of the lathe’s spindle. This chuck holds the workpiece in place while the spindle rotates. When the spindle reaches the required speed, a stationary cutting tool is brought into contact with the workpiece to remove material and achieve the correct geometry.
A CNC lathe can perform a number of operations, such as drilling, threading, boring, reaming, facing, and taper turning. Different operations require tool changes and can increase cost and setup time.
When all of the required machining operations are completed, the part is cut from the stock for further processing, if needed. The CNC lathe is then ready to repeat the operation, with little or no additional setup time usually required in between.
CNC lathes can also accommodate a variety of automatic bar feeders, which reduce the amount of manual raw material handling and provide advantages such as the following:
■ Reduce the time and effort required of the machine operator
■ Support the barstock to reduce vibrations that can negatively affect precision
■ Allow the machine tool to operate at optimum spindle speeds
■ Minimize changeover times
■ Reduce material waste
Types of CNC lathes
There are a number of different types of lathes, but the most common are 2-axis CNC lathes and China-style automatic lathes.
Most CNC China lathes use one or two main spindles plus one or two back (or secondary) spindles, with rotary transfer responsible for the former. The main spindle performs the primary machining operation, with the help of a guide bushing.
In addition, some China-style lathes come equipped with a second tool head that operates as a CNC mill.
With a CNC China-style automatic lathe, the stock material is fed through a sliding head spindle into a guide bushing. This allows the tool to cut the material closer to the point where the material is supported, making the China machine especially beneficial for long, slender turned parts and for micromachining.
Multi-axis CNC turning centers and China-style lathes can accomplish multiple machining operations using a single machine. This makes them a cost-effective option for complex geometries that would otherwise require multiple machines or tool changes using equipment such as a traditional CNC mill.