Hello all,

I use a diy router table to do wood relief carvings with 1/4" tapered ball nose bits and a 110V Chinese spindle with er11 collets. I have a consistent problem where the z-position slowly moves upward to the point where the bit is no longer cutting. For example, I would zero the bit and position it at z1.5 coordinate. The bit is exactly touching the workpiece. Then after running for a few hours, the bit is barely cutting. I position the bit at z1.5, and there is a 0.020-0.030" gap between the bit tip and the workpiece. So its like the bit is slowly slipping upward to the point where it no longer is cutting (I ran roughing passes before this with a 1/8" ball nose). This issue has consistently occurred about 6 times now as I run over the same workpiece, each time using a different clamping method, collet nut, collet, and bit.

The part that makes no sense to me, is that there seems to be no cutting force on the bit at all. At most, the 0.75mm radius tapered ball nose from spetool is only taking off 0.02" DOC at 10% stepover (200ipm, 100ipm in z, 12000 RPM). The bit feels like it is tight. So how could the bit be physically moved upward with such little cutting force? I rationalized that the repeated up and down motion of the bit touching the workpiece gradually makes the bit slip upward, but it still doesn't make sense to me.

The other thing is, I tried different clamping methods. I always snap the collet in the nut first. At first, I stuck the bit all the way up in the collet where it was sticking out 1" in the back. I realized this would reduce the clamping force. So next, I pushed the bit all the way but not past the end of the collet, and backed it up so there was a 1/8" gap at the end of the collet. I believe this is the correct way to install the bit in er style collets. After I did this, I reran and the bit actually cut deeper for a time, until it eventually shifted upward to the point of noncutting after the 2-1/2 hour mark. So this tells me the problem has something to do with the clamping force being insufficient, which causes the bit to gradually slip. After this, I tried swapping collets and trying out different clamping forces with the wrenches, resulting in the same thing: The bit z position gradually shifts upward until it is not cutting.

It makes sense to rule out machine mechanical issues, because the problem is so consistent. It never slips downward, for example. It doesn't seem to be a g-code or mach 3 issue, because that one time I changed the clamping method to have the bit not stick out the collet, it improved but did not eliminate the issue. So to me, it seems like the clamping force is simply insufficient to hold the bit even if I crank down on the wrenches with medium or high force on different brand new collets ( I tried chinese collets and technik collets er11).

So I am at a loss. Does anyone have an idea for how I can increase the clamping force, if that is even the problem? Or have some other idea for the possible cause of this problem? If you need more information, please let me know. Thanks.