What are the benefits of the mineral casting marble bed machining center?

What are the benefits of the mineral casting marble bed machining center?
Mineral castings (man-made granite aka resin concrete) have been widely accepted in the machine tool industry for over 30 years as a structural material.

According to statistics, in Europe, one out of every 10 machine tools uses mineral castings as the bed. However, the use of inappropriate experience, incomplete or incorrect information can lead to suspicion and prejudice against Mineral Castings. Therefore, when making new equipment, it is necessary to analyze the advantages and disadvantages of mineral castings and compare them with other materials.

The base of construction machinery is generally divided into cast iron, mineral casting (polymer and/or reactive resin concrete), steel/welded structure (grouting/non-grouting) and natural stone (such as granite). Each material has its own characteristics, and there is no perfect structural material. Only by examining the advantages and disadvantages of the material according to the specific structural requirements, can the ideal structural material be selected.

The two important functions of structural materials—guarantee the geometry, position and energy absorption of components, respectively put forward performance requirements (static, dynamic and thermal performance), functional/structural requirements (accuracy, weight, wall thickness, ease of guide rails) for materials installation, media circulation system, logistics) and cost requirements (price, quantity, availability, system characteristics).
I. Performance requirements for structural materials

1. Static characteristics

The criterion for measuring the static properties of a base is usually the stiffness of the material—minimum deformation under load, rather than high strength. For static elastic deformation, mineral castings can be thought of as isotropic homogeneous materials obeying Hooke’s law.

The density and elastic modulus of mineral castings are respectively 1/3 of those of cast iron. Since mineral castings and cast irons have the same specific stiffness, under the same weight, the rigidity of iron castings and mineral castings is the same without considering the influence of shape. In many cases, the design wall thickness of mineral castings is usually 3 times that of iron castings, and this design will not cause any problems in terms of mechanical properties of the product or casting. Mineral castings are suitable for working in static environments that carry pressure (eg beds, supports, columns) and are not suitable as thin-walled and/or small frames (eg tables, pallets, tool changers, carriages, spindle supports). The weight of structural parts is usually limited by the equipment of mineral casting manufacturers, and mineral casting products above 15 tons are generally rare.

2. Dynamic characteristics

The greater the rotational speed and/or acceleration of the shaft, the more important the dynamic performance of the machine is. Rapid positioning, rapid tool replacement, and high-speed feed continuously strengthen mechanical resonance and dynamic excitation of machine structural parts. In addition to the dimensional design of the component, the deflection, mass distribution, and dynamic stiffness of the component are greatly affected by the damping properties of the material.

The use of mineral castings offers a good solution to these problems. Because it absorbs vibrations 10 times better than traditional cast iron, it can greatly reduce the amplitude and natural frequency.

In machining operations such as machining, it can bring higher precision, better surface quality, and longer tool life. At the same time, in terms of noise impact, the mineral castings also performed well through the comparison and verification of the bases, transmission castings and accessories of different materials for large engines and centrifuges. According to the impact sound analysis, the mineral casting can achieve a local reduction of 20% in the sound pressure level.

3. Thermal properties

Experts estimate that about 80% of machine tool deviations are caused by thermal effects. Process interruptions such as internal or external heat sources, preheating, changing workpieces, etc. are all causes of thermal deformation. In order to be able to select the best material, it is necessary to clarify the material requirements. The high specific heat and low thermal conductivity allow mineral castings to have good thermal inertia to transient temperature influences (such as changing workpieces) and ambient temperature fluctuations. If rapid preheating is required like a metal bed or the bed temperature is prohibited, heating or cooling devices can be directly cast into the mineral casting to control the temperature. Using this kind of temperature compensation device can reduce the deformation caused by the influence of temperature, which helps to improve the accuracy at a reasonable cost.

II. Functional and structural requirements

Integrity is a distinguishing feature that distinguishes mineral castings from other materials. The maximum casting temperature for mineral castings is 45°C, and together with high-precision molds and tooling, parts and mineral castings can be cast together.

Advanced re-casting techniques can also be used on mineral casting blanks, resulting in precise mounting and rail surfaces that do not require machining. Like other base materials, mineral castings are subject to specific structural design rules. Wall thickness, load-bearing accessories, rib inserts, loading and unloading methods are all different from other materials to a certain extent, and need to be considered in advance during design.

III. Cost requirements

While it is important to consider from a technical point of view, cost-effectiveness is increasingly showing its importance. Using mineral castings allows engineers to save significant production and operating costs. In addition to saving on machining costs, casting, final assembly, and increasing logistics costs (warehousing and transport) are all reduced accordingly. Considering the high-level function of mineral castings, it should be viewed as a whole project. In fact, it is more reasonable to make a price comparison when the base is installed or pre-installed. The relatively high initial cost is the cost of mineral casting molds and tooling, but this cost can be diluted in long-term use (500-1000 pieces/steel mold), and the annual consumption is about 10-15 pieces.

IV. Scope of use

As a structural material, mineral castings are constantly replacing traditional structural materials, and the key to its rapid development lies in mineral casting, molds, and stable bonding structures. At present, mineral castings have been widely used in many machine tool fields such as grinding machines and high-speed machining. Grinding machine manufacturers have been pioneers in the machine tool sector using mineral castings for machine beds. For example, world-renowned companies such as ABA z&b, Bahmler, Jung, Mikrosa, Schaudt, Stude, etc. have always benefited from the damping, thermal inertia and integrity of mineral castings to obtain high precision and excellent surface quality in the grinding process.

With ever-increasing dynamic loads, mineral castings are also increasingly favored by world-leading companies in the field of tool grinders. The mineral casting bed has excellent rigidity and can well eliminate the force caused by the acceleration of the linear motor. At the same time, the organic combination of good vibration absorption performance and linear motor can greatly improve the surface quality of the workpiece and the service life of the grinding wheel.