HSM May/June 1988 - The Surface grinder (Part4)

[carrdo]

Some things one has to learn the hard way and I seem to be particularly prone to that. It's called the school of hard knocks.

To start, if you want to sharpen the end of any smaller HSS end mill (say less than 1/2" dia.), using a surface grinder with a straight aluminum oxide vitrified bond wheel, then the wheel must have a sharp edge and not a rounded corner, so basically the wheel(s) one uses for end mill sharpening cannot be used for any other purpose. One needs to first true the wheel with a diamond on both the periphery of the wheel and on the side nearest you. For this you need a stout sliding bar (to hold the diamond) which allows it to slide vertically without shake. One only has to do this on the first 1/2" of the wheel or so. Just remove enough side material to produce the sharp edge.

Secondly, truing and dressing a wheel are different operations entirely and have completely different purposes. Truing makes the wheel perfectly concentric to the spindle and aids in balancing the wheel. But truing with a diamond also dulls the grains of the wheel so it does not cut as freely and openly as it should (this results in generating heat also as the dull grains of the wheel do not shear the metal cleanly but rub to a greater extent). To dress an aluminum oxide vitrified wheel after truing it, one can use a commercial dressing stick (such as a Norbide or a coarse silicon carbide stick) which is lightly presented to the wheel by hand. The purpose of the dressing stick is to remove some of the vitrified bond to expose fresh cutting grains.

Do I dress a grinding wheel after truing it? Not always even though I have all of the various dressing sticks for both diamond and aluminum oxide wheels. It takes a bit of "feel" and experience to dress a grinding wheel by hand and to do it properly and I do not want to advocate people sticking their bare hands around a 3600 RPM grinding wheel. As well, I don't have a brake type dresser (not in the budget) which professional commercial shops use.

So why the sharp corner?

It is not to have the wheel contact the next tooth of the cutter under any circumstances because if there is any contact with the wheel it will be on the cutting edge (lip) of the next tooth which will destroy the cutting edge. A rounded edge on the wheel means one has to move the wheel further and further towards the center of the cutter (and with smaller cutters there is not a lot of end relief or "gashing" on the cutter - more on this later) to permit this especially if one has to "eyeball" the position of the wheel in relation to "have I yet sharpened along the full length of the lip (the cutting edge) of the end mill". I have destroyed many, many cutters by going just a little bit too far and this applies even with a two lipped cutter whose cutting edges are spaced 180 degrees apart. One just can't see what is happening under the wheel.

It gets more interesting but that is enough for now.