stone grader calibration

[Ken S]

Stig has presented a fascinating advance in using the stone grader. Instead of just (full) coarse or fine, 220 or 1000 grit, it is possible to to use mid grades, such as 600 grit. I offer a simple, inexpensive technique to measure such midgrits: (Admittedly, this technique is a somewhat controlled approximation.)

Among my collection of machinist tools, I have a small kit of surface samples ranging from straight off a milling or lathe cutter to highly polished. These are factory calibrated and are used to verify that a surface meets the required amount of specified smoothness. This would mean both adequate smoothness for the application of the part and the most cost effective amount of effort expended. The part being fabricated would be compared with the sample. The part should match the desired finish sample.

I propose using a selection of wet or dry abrasive paper grits for comparison. I would cut a strip the width of the grinding wheel from the short dimension of the abrasive paper and cutting this strip in half lengthwise. (These dimensions are not critical.) These strips would then be mounted on thin stiffener backing boards of cardboard, plastic, pressed board or plywood and labelled with the grit. Ideally sample boards would be made from a wide range of grits.

For greater accuracy, I would like to provide samples which were slightly more coarse, equivalent, and slightly finer than the desired grit.

To use, we would feel the grit of the wheel after graded and compare that felt grit with the felt grit of several samples. If the wheel grit was judged too coarse or too fine, the wheel could be carefully and lightly graded a bit more.

This idea occured to me while trying to sleep. I am presently away from home. Why cut the strips in half? Half a strip seems a more convenient size. Used with finger comparison, the strips should not wear out. The second set would be a nice gift for a fellow Tormeker and ship inexpensively, even internationally.

Ken