I spent much of today trying to figure out how I managed to break that mill bit. (and finishing up some parts
I noticed something interesting but I can't explain it.
Note that all discussion below is about the part moving, not the bit - that is after all what actually happens.
I was milling conventionally with the part moving from right to left behind the bit. The bit was a 2 flute 1/2" HSS End Mill. I was cutting about 1/4" at a crack - roughly half of the end mill diameter. The depth was about 20 thou per layer. But changing the depth more or less didn't have any affect on what happened.
Before I started the cut, I took the backlash out of the y axis by pulling the part toward me till it moved an 8th of an inch or so, and then pushing the part away from me till it reached the desired width of cut - about 1/2 of the end mill diameter. To my way of thinking, the bit wants to pull the part toward me so the part must be pushed back toward the rear after using up all the backlash in that direction. At least that's how I thought it was supposed to be done.
However, almost as soon as the cut starts, the part starts to drift toward me even though the y-axis handle has not moved at all. What? And Why?
If I take the y axis backlash out in the opposite direction by pulling the part toward me instead, THE EXACT SAME THING HAPPENS and the part starts to drift toward me.
In my newbie opinion, the latter case is perfectly normal. In fact, it is what I would expect to happen. The cutter is simply pulling the part toward me and is taking up the backlash I provided for it in the process. But the former example totally boggles my mind. There shouldn't be any backlash left to take up! I simply can't understand how that can happen.
Note: It does not matter if the y axis is locked or not. The drift seems to be inevitable.
To get around the problem, I was able to guestimate how much the drift is and compensate for it with a series of 5 thou cuts. The very shallow cut did not cause any appreciable drift regardless of how I dialed out the y-axis.
Recall, this is the mill with a tenth of an inch backlash - approx 90 to 110 thou depending on where you measure it.
Could it be that its not possible to take up that much backlash? Or could it be that the lead screw nut is moving vertically and allowing even more backlash, or perhaps it is on the verge of stripping?
Or could it be that this newbie has a loose screw in his thinking?
