The 10 percent dressing rule

Dear Doc: I use a single-point diamond dresser and typically take a 0.001 ” DOC when dressing 60-mesh wheels. But sometimes I use 240-mesh wheels. Can I take the same depth there?

The Doc Replies: Most grinding handbooks indicate to dress from 0.0002 ” to 0.001 “: 0.0002 ” for a fine dress, 0.001 ” for a sharp dress. However, this doesn’t take into account grit size.

A rule of thumb for a sharp dress is a DOC of 10 percent of the grit diameter. Use a DOC of about 5 percent of the grit diameter for a medium-sharp dress and around 2.5 percent for a dull dress.

The equation for grit diameter is grit diameter (in.) equals 0.6 divided by mesh size, so a 60-mesh wheel has an average grit diameter of 0.010 ” (0.6/60). Therefore, the DOC for a sharp dress (10 percent) of a 60-mesh wheel is 0.001 “, a medium dress (5 percent) is 0.0005 “, and a dull dress (2.5 percent) is 0.00025 “.

For a 240-mesh wheel, a sharp dress is 0.00025 “, medium is 0.000125 “, and dull is 0.00006 “.

Dear Doc: I grind tungsten-carbide shafts with a diamond wheel. The shaft has two diameters, and I sometimes get temperature-induced cracking during grinding where those two diameters meet. What can I do to stop the cracking?

The Doc Replies: Perhaps your problem is not in the grinding but in the part itself. What is the design radius at the circle of contact between the two diameters? If it is 1.0mm, then we’ll have to work on the grinding side. But if the answer is “sharp” or “0.050mm,” then there’s an issue with stress concentrations that are forming because of a small radius.