Unlike the late night talk show hosts who read off “Top Ten Lists” starting at # 1O and working backwards until they reach the top, we are going to start with item #1 — taper. Variations of this problem are, by far, the ones most frequently encountered in bore grinding. We don’t think it’s fair to make you wait while we reel off solutions to a bunch of other problems until we finally get to the ones you are most likely to need. So here goes . . .
You have a taper problem when your bore is not as straight as it needs to be. The bore diameter becomes progressively larger or smaller towards one or both ends. In some cases the taper occurs at both ends resulting in an “hourglass” or “barrel” shaped longitudinal profile.
If you are having taper problems here are the best things to try, first one at a time, then in a variety of combinations until you hit the taper-free sweet spot.
Solution # 1: Change Your Oscillation Stroke
If your bore is small (tight) on one end or both it is likely that the ends are not seeing enough of the wheel, while the center of the bore is seeing all of it on each pass (two times during each stroke). The solution may be as simple as increasing the length of the stroke, so that it extends further into, or just beyond, one or both ends of the bore. This insures that every area of the bore gets an equal amount of time exposure to the grinding wheel. Conversely, if the center of the bore is small but the ends are large, do the opposite by reducing the stroke on one or both ends so the wheel spends more time in the center. There is a nice graphic on our tools page to illustrate how oscillation can affect the shape. Follow THIS LINK to see it.
Solution # 2: Dress A Reverse Taper Into The Wheel
What if the bore is relatively shallow and the wheel covers its full length, but you are still getting taper? How does that happen? You would think that every area of the bore would be getting equal exposure to the wheel’s surface. Unfortunately, this is where the reality of “deflection” comes into play. The pressure of the wheel on the part tends to deflect the quill & wheel away from the surface of the bore. So the surface of the wheel is never entirely parallel to the surface it is grinding. This slight deviation from parallel may cause unacceptable taper in the bore where the quill/wheel is bent slightly away from it.
Sometimes, an easy way to make this problem disappear is to reverse dress a compensating taper into the wheel. The wheel will then have a shape that is the opposite of the tapered bore. This may be all you need to do.
Solution # 3: Increase Spark-Outs
Spark-outs are a period of time where the wheel is oscillating but not feeding– typically consuming 2-5 seconds at the end of a 20-second bore grinding operation. So spark-outs can occupy up to 25% of the machine cycle. Some manufacturers cast a greedy eye upon those 2-5 seconds, thinking of them as useless air-cutting time that could be slashed for the sake of productivity improvements. Whether spark-outs were minimized intentionally or not, ones that are too short can result in taper problems. Here’s why:
During the spark-out, all the forces that have accumulated during the grinding cycle are allowed to relax. If the wheel was deflected during grinding it will relax back to parallel during spark-out and grind those places that were tending to taper. So increasing the spark-out period a second or two is often a fast and easy solution for correcting this sort of taper problem.
Solution # 4: Retract and Regrind:
Sometimes spark-outs can take a long time to slowly relax all of the forces that have accumulated in the spindle and you need to keep cycles as short as possible. In this case, another way to approach the situation above is to back off the amount of the taper and then come back in and regrind — sort of a mini grind cycle. When you back off the part, the wheel is no longer in contact, so everything can straighten out faster. Then you can go back in and grind the tapered area quickly without adding as much time to your cycle.
Solution # 5: Sharpen The Wheel To Make It More Aggressive.
Since the most common cause of taper in a part is deflection in the quill, the sharper your wheel, the less the quill is going to deflect. So sometimes just having a sharper wheel will mean less deflection and less taper. You get a sharper wheel by using more aggressive dressing parameters.
You won’t always benefit from having a sharper wheel, but more often than not, you will. As a matter of fact, this solution is one that can be applied to seven of the ten bore grinding problems cited in this blog. To learn more about getting a sharper wheel refer to our tutorial “How To Be A Smart Dresser.” (Click Here)
Solution # 6: Make a Taper Adjustment on Your Machine
Many machines allow the user to make a taper adjustment by swiveling the work head and/or the grinding spindle to compensate for taper. This type of adjustment can be made on most machines but the ease with which this can be done varies considerably. With newer CNC machines these adjustments can often be made in seconds via the control. Some high-end equipment will even compensate for taper automatically. People with these machines have probably skipped this discussion of taper problems.
With older, less automated equipment you will have to stop everything and haul out your wrenches, loosen and tighten bolts here and there to tweak your taper compensation. This is not an exact process. So it may take several times to get it right. In addition, if you do eliminate taper with trial and error mechanical adjustments to the machine you may be misaligned for the next part numbers in your grinding queue and have to go back to the adjustment process once again.
This is why mechanical adjustments to older machines should only be the method of last resort for correcting taper. It is far better to get to the root cause of taper by using some of the best practices listed above.