Choosing the proper CBN grit size

Dear Doc: I ID-grind the races of 52100 steel bearings with vitrified-bond CBN wheels. I’m dressing the wheel every five parts and feel this is way too much. Is there a way I can grind more parts between dressings?

The Doc Replies: A common mistake when using CBN wheels is to choose a grit size that’s too large and then take drastic actions to cope with it. After dressing with a diamond tool, a CBN wheel, regardless of how it’s dressed, is dull and takes a while to break in and become sharp. Before that happens, the surface finish is very fine but the grinding power is high.

After break-in, users hit the semisteady-state regime, where the wheel is sharp and cuts well, grinding power is low and surface finish is satisfactory. As grinding proceeds, the power drops slowly as the grits sharpen and fall out, and surface finish gradually worsens. Eventually, the finish approaches the roughness limit and you have to dress.

If you choose a grit size that’s too large, the surface finish after break-in is too rough. Grinder operators cope with this by dressing the wheel to be dull. Then, once the wheel breaks in, the surface finish is already approaching the roughness limit and the wheel requires dressing again—way too soon!

Use this rough-and-ready formula to calculate an approximate grit size: grit mesh number = 1,500 ÷ [desired surface finish in microinches Ra to the 0.77 power].

For example, if the required surface finish is 8µin. Ra, you ‘ll need a grit mesh number of 302.5 (1,500 ÷ 80.77), or 300. If 8µin. Ra is your maximum, you ‘ll want to go with a finer mesh, say, 350.

Are you using a grit mesh number that’s smaller (i.e., a larger grit) than recommended? A lot of CBN grinders do. If so, there’s a good chance you’re dressing dull and dressing frequently, staying constantly in the dull region prior to break-in. Instead, choose the right grit size and dress sharp to extend the dressing interval five to 10 times or more before the workpiece surface finish gets too rough.