Staying In Shape
Taper— discussed in a previous blog— always means that the walls of the bore are straight; they’re just not parallel. When we have shape problems the walls of the bore are neither straight nor parallel. They are typically either bowed out (barrel shaped) or bowed in (hourglass shaped).
Don’t forget when we say “barrel shaped” or “hour-glass” shaped, these are just figures of speech indicating the general direction of the out of shape condition. It is not something that you are likely to see with your naked eye.
For barrel-shaped parts there are two common solutions. The best bet is to extend the oscillation stroke so that the wheel moves a little further out at each end of the bore. This is similar to what you do to correct inward taper at one end of the part. Since the inward trending condition of the wall occurs at both ends, the oscillation stroke needs to be increased at both ends as well. This is the most common solution for barrel-shaped parts.
If this doesn’t fix the barrel shape, then the cause of the problem may be insufficient coolant reaching the center of the bore. This can result in thermal expansion of the wall at the center of the bore. This unwanted expansion drives the wall into the wheel, causing more material to be removed in the center than at the ends. In this case you can do a couple of things. First, check that the coolant application is adequate; adjustment of your nozzles and/or pressure may be required. Second, sharpen the wheel with more aggressive dress parameters. The more efficient cutting action of a sharper wheel reduces the incidence of heat build up at the center of the part. As a result, the walls remain straight and parallel. (For advice on how to select the best dressing parameters, please refer to our post “How to be a Smart Dresser”.)
For the hourglass out of shape condition there is one typical solution, to reduce the in and out position of the oscillation stroke. The hourglass out of shape condition results when you are oscillating the wheel too far beyond the ends of the bore. Therefore, the wheel will be spending more time grinding at the end positions than along the interior of the bore. This will cause the outward bowed hourglass shape. Reducing the extension of the stroke at both ends will often correct the problem.
Finally, there is an additional out-of-shape problem that results in neither an hourglass nor barrel shape. It’s a weird condition where the bore is out of shape at one or several locations along the bore. In many instances we have found this to be caused by heat generated when a wheel that it slightly too big for the job randomly restricts the flow of coolant at one or more locations and, again, thermal expansion results in uneven material removal. Here the solution is to dress the wheel down so that the wheel is smaller in diameter and the coolant flows more freely.
A less common but equally puzzling cause of the random shape issue results from an overabundance of coolant, which causes a hydroplaning effect. This too can often be solved by turning down the coolant pressure and/or making the wheel smaller in diameter.
There is a chart on our “Tools / Calculators” page that is designed to assist you further with common solutions to shape problems by adjusting the oscillation stroke.
Please follow THIS LINK to download your own copy.
Getting and staying in shape is a relatively simple matter. Unfortunately, beauty is sometimes only skin deep. So in our next excursion into the 10 Most Common Bore Grinding Problems, we will address problems relating to Surface Finish.