I have a weird situation that I have not encountered before. I am grinding a rotary broach from WH drill rod on my Monoset tool grinder. It has a 5c collet conversion so I use 5C collets in the work head. The collet I am using is a Hardinge 1/2", so no junk there. I have the maching set up with the work head at 92 degrees, so the grinding wheel comes into the the broach from the front, cutting on the side of the CBN wheel, for the most part. When I am grinding I move only the grinding wheel. I do not move the work head at all, except to spin it to work on another face. I have .750" shank on the broach and .625" on that in the collet. When I tighten the collet down the broach feels solid as a rock and I can't move it by hand in any way.
Anyway, at first I was finding that the broach was not being cut the same on each side, in this case a square. I am using a 4 notch index plate, and taking the backlash out the same way each time before I lock the work head spindle prior to grinding a face. The broach sometimes indexes slightly in the opposite direction from the way the wheel turns. I found this very odd since if a parts moves in a collet, it will usually move in the direction of the cutter, as the cutter pulls it that direction. After cutting 4 sides, when I would measure, I'd find that I have .015", or more, taper across the cutting edges of the broach. Now, as I grind on the broach I have found that the broach slowly works its way out of the collet. I do not know if the grinding wheel is pulling it out or if I have a collet failure. I now think this is why the part is indexing wrong.
And even stranger, I am not cutting with the force you'd see from an end mill or indexed cutter, I am grinding, and no more that .005" per pass.
See pics below.
Ideas on what to check or try will be appreciated.
Do you think I have a bad collet?
Thanks,
Chris
Anyway, at first I was finding that the broach was not being cut the same on each side, in this case a square. I am using a 4 notch index plate, and taking the backlash out the same way each time before I lock the work head spindle prior to grinding a face. The broach sometimes indexes slightly in the opposite direction from the way the wheel turns. I found this very odd since if a parts moves in a collet, it will usually move in the direction of the cutter, as the cutter pulls it that direction. After cutting 4 sides, when I would measure, I'd find that I have .015", or more, taper across the cutting edges of the broach. Now, as I grind on the broach I have found that the broach slowly works its way out of the collet. I do not know if the grinding wheel is pulling it out or if I have a collet failure. I now think this is why the part is indexing wrong.
And even stranger, I am not cutting with the force you'd see from an end mill or indexed cutter, I am grinding, and no more that .005" per pass.
See pics below.
Ideas on what to check or try will be appreciated.
Do you think I have a bad collet?
Thanks,
Chris