Attention must also be paid to the diamond truer design. As CBN is so much harder than conventional abrasive, the truer will wear much more per dress. This makes profiling truers with precisely lapped geometries uneconomic for most applications. Certainly for all flat form dressing applications, a truer with a consumable diamond layer is required.
7.5.6 Impregnated Truers
An obvious solution, based on conventional wheel experience, would be to use an impregnated truer. The difficulty is that these truers are designed such that the matrix wears just enough to keep the diamond exposed. Conventional wheels create a lot of abrasive swarf, which erodes the matrix, but little diamond wear. In contrast, CBN wheels create little swarf and, hence, matrix wear, but cause a much higher degree of diamond attritious wear. Consequently, the matrix must be somewhat less wear resistant for CBN applications but still retain the diamond.
The width of a regular impregnated truer is also a problem in light of the discussion regarding grit fracture before. An impregnated truer will have an effective contact width, bd, many times wider than the diamond size and so will have diamond grains dispersed randomly throughout. Consequently, during dressing, if we picture the situation illustrated in Figure 7.18 but now with a much wider width, some CBN grains may be hit several times while others remain untouched. The first hit will be at the full truing depth and fracture the CBN grit but the following grains will be at a much shallower depth and merely glaze it again. The total number of hits is termed the “collision number.” The effect is very apparent in Figure 7.26, which plots the effect of collision number on normal grinding force under a range of dressing and grinding conditions [Brinksmeier and Qinar 1995]. A single ring of diamond grains is clearly the best option.
It is possible to estimate the equivalent of this single layer, and multiples thereof, for various truer contact widths and diamond grit sizes of impregnated truers as a function of diamond concentration.
Although these guidelines have worked well for general applications, there was a need for a sharper dress for burn-sensitive materials and aggressive removal rates. Several solutions have been developed—some proprietary, several covered by patents.
Most of these consist of a compact or single layer of diamond. For example, Hitchiner [1997] reported an impregnated layer of diamond sandwiched between two steel side plates for support. Although the overall width is 0.080, the actual diamond layer is only about a grain thick. Figure 7.27 shows an X-ray photograph through a truer showing the individual diamonds. The design is suitable primarily for dressing of flat profiles. Winter EP 116668 (Saint-Gobain Abrasives) patented a truer
|
Diamond Size |
Sharp |
Medium |
Dull |
To Dress |
|
D501 |
100 |
150 |
— |
B181-B151 |
|
D301 |
80 |
125 |
160 |
B126-B91 |
|
D181 |
60 |
90 |
120 |
B91-B64 |
|
D126 |
47 |
75 |
95 |
B76-B54 |
|
D91 |
35 |
43 |
72 |
B46 |
|
TABLE 7.5 Recommendations for Impregnated Diamond Truer Compositions |
|
Concentration for 1 mm Contact Width |
Grinding wheel
 |
 |
X-ray photograph of impregnated diamond truer
(Hitchiner 1997)
|
FIGURE 7.28 Needle (“prismatic”) diamond dressing cup design and application.
design based on holding the diamond to the side of a steel support by direct plating. This method allows simple contouring in addition to dressing flat profiles. Finally Norton [1983] (Saint-Gobain Abrasives) developed a method called BPR, or bonded profile roller, using the brazed plated process to produce a truer which is only a diamond grain wide. This maintains a constant tip radius as it wears and can be used for quite complex profiling.
7.5.7 Traverse Rotary Truers Using Needle Diamonds
Finally, there are truers based on the use of needle diamond blocks. These give probably the most consistent, effective dress of any truer design for dressing of flat profiles. For these to function correctly, the diamond must be above the level of the matrix in which it is held. This limits their usable depth to about 0.5 mm without significant fracturing occurring. However, they can be re-exposed two to three times. As with any tool, there is a balance between initial tool cost and overall cost.
Total truing force with these styles of dresser is of the order of 2 to 10 N for typical cylindrical grinding conditions to <2 N for internal grinding. (See Figure 7.28 for needle diamond dressing cup design and application.)