Builders have sought alternative means of providing improved damping. One method is the use of a polymer matrix composite made using crushed concrete, granite, or quartz with trade names such as Granitan S103 [Studer n. d.], Mineralit [Emag 1998], and Micro-Granite [Elb 1997]. The materials have significantly greater damping characteristics than steel or even cast iron. They can be cast into almost any shape, do not require aging or annealing, are one third the density of cast iron, and can support rails and slideways if inserts are used to anchor them. They are used either as fillers in weldment structures or as monolithic bases [Drake 2000]. In the latter case, foam cores may be added to improve the weight/strength ratio. With appropriate design, a monolithic polymer structure,
Properties of Polymer Concrete Compared with Cast Iron
ITW Philadelphia’s Polymer Concrete
Compared with Class 40 Cast Iron
Material Property ITW Concrete Class 40 Cast Iron
Compressive Strength (PSI) 20,000 130,000
Tensile Strength (PSI) 2,000 40,000
Compressive Modulus (PSI) 4.2E6 15.0E6
Coefficient of Thermal Expansion (IN/INF) 6.8E-6 6.7E-6
Thermal Conductivity (BTU/FT-HR-F) 91.2 1,300
particularly for a low load-bearing application, can have the same stiffness as cast iron but much greater damping. Perhaps the optimum use of polymers, however, is as used in the Viking centerless grinder [Viking 1998] where epoxy-granite is used as a filler material in a nodular cast-iron base, thus gaining the best of both worlds.
It should be noted when designing machine bases that polymer matrices are not as strong as cast iron and have a much lower thermal conductivity. Arnone [1997] provides the following data for one particular grade of polymer concrete (Table 15.1).
An example of a company that has made significant effort to improve on a standard weldment is Weldon Solutions (York, PA) [Weldon 1991, 1994, 1998]. This machine tool builder makes both standard cylindrical grinders and custom machines with unique base shapes. In earlier efforts to improve damping, they used expansive concrete as filler in a welded steel base. This proved quite effective but added significantly to the weight of the base. More critically the thermal expansion mismatch could cause structural bowing. Weldon, therefore, worked jointly with Dr. A. Slocum of Massachusetts Institute of Technology to develop a more thermally stable system based on internal viscous damping [Hallum 1994, Weldon 1994]. The “Shear Damper” base consists of a series of stiff steel tubes wrapped with a highly viscous polymer tape. The tubes are suspended within the base weldment leaving a 3 to 5 mm gap with the walls. This gap is injected with an epoxy material constraining the viscous layer such that its only movement is in shear that dissipates vibration energy.
The beauty of this method is that it decouples the damping and stiffness functions of the structure and is broad frequency-based unlike, for example, a tuned mass damper. The internal tubes can also act as conduits for coolant or other fluids to control the base temperature (Figure 15.3 and Figure 15.4).
15.1.9 Granite Bases
For grinders where thermal stability is absolutely critical, an alternative approach is to use solid granite as used for inspection tables. Tschudin [n. d.] reports using Granitline, a natural quarried granite, for the bases of high-precision centerless grinders. Buderus [1998] also reports using a natural granite bed for its combination grinding/hard-turning centers that has both higher static stiffness and better damping characteristics than a typical cast-iron base. The use of solid granite illustrates the application of the massive base principle discussed above.