By Tom Cappadona
Bore-Grind ID Taper is a problem frequently encountered after wheel dressing. CBN is notorious for this, because CBN does not like to be dressed. Right after dress, you may notice the part becoming smaller or larger as successive measurements are taken further into the bore. A lot of times this problem has to do with whether the wheel is cutting freely or whether it is closed down by the dress and is pushing instead of cutting, thus causing deflection of the quill/spindle.
We can look at this in a number of different ways. Typically with CBN grinding there’s some number of parts between dress cycles. If you’re finding taper in your IDs, the first thing we suggest is to see whether the result is the same result in all pieces between dress cycles. Usually it’s not. With dressing, the part taper may swing one way or the other for the first few parts after dress, and then find it’s “natural state”.
If that’s the case, we need to look and see what’s happening to the grinding wheel during dressing and adjust the dressing parameters to get the wheel to the “natural state” on the first piece after dress. It’s what I like to call smoothing out the “saw tooth” effect. Often this involves making the dress cycle more aggressive to sharpen the wheel better. The mission is always the same … get the part to look the same immediately after dress as it did just prior to dressing.
Occasionally when investigating these problems we find that the taper is not consistently present over the full bore, but just in an area consistent with the length of the grinding wheel. Sometimes this can be corrected simply by dressing a reverse taper into the wheel. Often this does the trick and the problem goes away.
I have run into situations where taper occurs as a result of a long traverse grind on hard materials where high forces are required to penetrate the part’s surface with the wheel. Since a long quill is required to cover the bore, the result is often taper from quill defection. One of the ways I’ve found to eliminate some of the grind pressure is to lift the wheel off the part just prior to sparkout, an amount equal to the taper, thus allowing the quill/spindle to relax, and then come back in to “saw off” the excess material as contact will be made first with the “high spots”. It’s an extra move but you can use it almost as a spark out or a very fine finish in-feed where you’re not putting a lot of pressure back into the quill, wheel, and spindle. Doing this relieves everything and you can get that taper out much more quickly than if you try to just sit there and oscillate back and forth. That approach usually doesn’t work at all.
Sometimes taper or shape in long bores, where the bore is significantly longer than the wheel, can also be influenced by the oscillation stroke positions, specifically the “in” position (where the wheel stops at the bottom of the bore), and the “out” position (where the wheel stops at the top of the bore). The chart to the right can be used to assist in adjusting the oscillation stroke positions to address common shape problems (click the image to make larger…).
It’s also a good idea to inspect the coolant setup. Not enough coolant can result in heat build-up and problems with size and taper. In some cases, in particular very small and long bores, too much coolant can actually cause taper problems from a hydroplaning effect when the coolant is not able to escape from the part properly (grinding wheel riding on a film of coolant). This is especially a problem when coolant can be supplied from both the grinding wheel side and through the part.
Finally, you might have to consider if your machine might be causing the taper problem. If the taper is consistent for all parts between dresses, and modifications to the dress or grinding process does not seem to solve the problem, then it could be an equipment problem. Maybe it can be fixed by adjusting the table back to parallel with the axis movement. We’ve also seen situations where slides have gone bad. Instead of the wheel running in and out of the bore smoothly, little hiccups will occur causing taper problems. You can identify these hiccups by moving the part to a different location on the table. That will cause these hiccups to occur at a different location on the part and you will know what is causing the problem.
Most of the time, one of these measures will fix your taper problem. If not, don’t despair– there has to be a cause for this effect. Ask a trusted supplier to give you a helping hand.