MACHINING/FINISHING: Diamond Wheel Grinding 101
Most engineers and designers who work with advanced ceramic components know that producing a prototype or small lot production part usually requires some significant grinding to achieve the final shape. As the part is proven out in its intended application, production techniques that produce an almost near-net-shape are then employed, including casting, cold compacting, injection molding, sintering, hot pressing and hot isostatic pressing. These processes, even if they are capable of producing a nearer-net-shape part, typically are not capable of producing a part to tight dimensional tolerances or fine surface finishes. Thus, to meet exacting dimensional requirements, specific surface finishes or other geometrical requirements, final machining is required of the as-fired ceramic.
The geometry, surface integrity, tolerance, spindle power, work chippage and consistency are all parameters that one has to consider before engaging the grinding wheel to the work piece. All of these characteristics determine the retained strength after finishing and, ultimately, the overall cost. Machining of a particular product depends on its material properties and end characteristics and can be accomplished by several methods, such as conventional diamond wheel grinding, creep feed grinding, ultrasonic machining, EDM processing, etc. This article will focus on conventional diamond wheel grinding.
FundamentalsIt is important to recognize that the parameters applicable to grinding other hard materials, such as tungsten carbide/ cobalt grades, are not necessarily applicable to grinding advanced ceramics. Fundamentally, this is due to the differences in the physical properties of tungsten carbide and advanced ceramics.
Tungsten carbide/cobalt material, though brittle, exhibits some degree of plastic deformation when subjected to a high enough stress. Advanced ceramics, on the other hand, show no plastic behavior and are incapable of relieving an applied stress by localized deformation. The applied stress causes microcracks or residual stress, resulting in unpredictable failure.
The grinding process often generates such stress, and the need to control it is far greater in advanced ceramics than in tungsten carbide/cobalt grades. By selecting a wheel containing smaller but sharper abrasive grains, increasing wheel speeds, decreasing the depth of cut and reducing traverse rates, induced stress can be minimized.
Diamond wheel grinding, in an overly simplified way, can be described as removing undesirable portions of material from a part by subjecting it to repeated overlapping contact with a rotating diamond wheel (see Figure 1). To understand the difference between grinding tungsten carbide and advanced ceramics, it is necessary to look at the grinding process and the grinding wheel from a fundamental level.
During the grinding process, the rotating diamond wheel is brought down on the work piece so that the tips of the exposed diamond particles barely touch the surface to be ground. At this starting point, the work piece is subjected to either a reciprocating or a rotating motion, and the wheel is dropped further by an amount equal to the depth of cut (Dc). The process is repeated n times until the desired amount of material equal to n x Dc is removed.
Wheel DetailsA diamond wheel, in a simplified version, consists of a circular metallic or plastic core. The outer rim of this core is composed of a metallic or resinoid layer 1/16 in. or thicker, which contains uniform but randomly distributed diamond particles. The bond system, whether metal, vitrified or resinoid, and the characteristics of the diamond particles regarding friability, shape and other factors, vary among the wheel manufacturers. Table 1 lists the important factors to be considered when selecting a diamond wheel. Despite greater wheel wear, resinoid wheels are commonly used because of their faster stock removal rate at lower tool pressures.
Almost all diamond wheel manufacturers consider specific information regarding their diamond and bond characteristics to be proprietary. However, for any diamond wheel to remain sharp and free cutting throughout the grinding cycle, the bond system has to be abradable enough to keep new particles exposed, but not so abradable that the wheel must be replaced frequently.
In the grinding of tool steels or tungsten carbide/cobalt grades, both the work piece and the swarf particles help dress the grinding wheel during the grinding process. For advanced ceramics grinding, these conditions are just the opposite. The high hardness of the work piece and the small particle size of the swarf (under 5 microns) are not conducive to keeping the diamond wheel open.
Grinding FactorsIf the wheel is fed into the work piece deeper than the exposed diamond length (i.e., if Dc > DL), damage to either the grinding wheel or the work piece will result. In cases where Dc = DL, a considerable amount of heat is going to be generated due to the rubbing that occurs between the work piece and the wheel bonding material. Coolant used for removing excessive heat will also not be very effective due to the collapse of annular space between the wheel and work piece.
Therefore, the ideal situation is when Dc = 1/2 DL and is maintained throughout the grinding range. In almost all types of grinding (reciprocating, cylindrical, centerless, etc.), the feed rate is maintained at the depth of cut per pass.
The greater the DL, the greater the Dc can be, resulting in a higher rate of material removal. The limiting factor, of course, comes from the fact that to increase the DL, coarser grit diamond particles must be used, which influences the surface finish of the part. Table 2 lists common grit sizes and the expected surface finishes attainable.
In a properly dressed diamond wheel, at least 50 to 60% of an exposed diamond particle should protrude beyond the bond surface. That is, DL = approximately 60% of the diamond particle size. One could therefore calculate from Table 2 that for an 80-grit wheel, DL would be .0063 in., and the maximum feed rate should not exceed 0.003 in. per pass. For a 320 grit wheel, DL = .0007 in., and the maximum feed rate should be 0.00035 in. per pass.
The process of grinding requires that the relative behavior between the grinding wheel and the work piece remain constant throughout the grinding cycle. As the exposed diamond particles wear and become dull during the grinding process, they either break into smaller segments, exposing new cutting facets, and/or completely break away from the wheel to expose new diamond particles. This would obviously depend on the number of diamond particles in the bond system and the relative strength of the particles and the bond.
Among the many factors comprising the grinding process is the surface speed of the grinding wheel. This factor gets overlooked in many cases and can affect productivity. Table 3 lists the decrease in surface speed for a given spindle speed. As illustrated, a 38% decrease in surface speed results from changing the wheel diameter from 8 in. to 5 in.