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.

To know when you’ve reached the semisteady-state regime, monitor the power. This is done with a device that logs power vs. time, such as my Grindometer, or with the load meter on the machine. What you should see is high initial power followed by a rapid decrease in power. That’s followed by a shift to a slow, steady, gradual decrease in power. If you dress before this shift, it means you’re never getting into the semisteady-state regime.

If you want to get really clever about things, do the following: Once you’re in the semisteady-state regime and the surface finish starts creeping up toward the roughness limit, increase the wheel speed by 40 percent or decrease the workpiece speed by 50 percent. This will decrease the grit penetration depth, making the wheel “act harder,” and buy you time before dressing is needed.

Dear Doc: I hold tight tolerances on large parts and am worried about temperature variation negatively impacting a hand gage’s ability to measure accurately. I asked the gage manufacturer about the effect of temperature, but he hemmed and hawed. Is there a way to measure this?

The Doc Replies: Yes. Stick your gage in the refrigerator for an hour and then measure your reference plug with it. Do the same for the gage in 60° F, 80° F and 100° F rooms. The data points should be fairly linear and you can plot the measured dimension vs. temperature. Fit a line through it in Excel and get the slope of the line. This will be the gage variation per degree Fahrenheit.

It’ll put you in the ballpark of how temperature variation is affecting gage accuracy. Keep in mind that the part is also growing as the temperature rises. The ideal situation is to keep the gage, reference plug, air temperature, coolant and workpiece all at the same temperature. CTE

About the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. He’ll be giving his Carbide Master-Grinder Clinic on April 9-11, hosted by Rollomatic in Mundelein, Ill.