Solved a mechanical problem on the CNC today and find it interesting enough that I thought I'd share it here.
I'd been getting bump outs at inside corners of 3D carved text lately. I've been using a 60° insert bit for a long time and it does a great job. But on the last project these little bump outs at the inside corners began to show up. It doesn't ruin the carvings but rather just looks like a bit more fancy font. Still though, most annoying.
I first checked for backlash on x,y and z. I had .001 on x and z and .0015 on Y. I would consider that within tolerance on just about any CNC router. That testing was done yesterday so I had a chance to sleep (or not sleep) on the issue before going back to it again this morning.
Thinking more about it I became convinced it was a Z-axis problem even though I was getting those excellent backlash reading. My theory was that as the bit traveled up and outward into those corners it wasn't raising up enough. As such then instead of a sharp inside corner I was going to see a rounded cut from a little further up the 60° v-bit, which hadn't raised all the way up to the tip of the bit.
Yesterday's test only showed .001 backlash when I was using .001 step moves. But this morning I went to .01 moves and that's when something caught my eye. I'd made indicator marks on the belt and pulleys driving the Z-axis lead screw to see if they stayed true in the back and forth. In watching that I caught some metal movement out of the corner of my eye. Turns out the upper mounting block on the vertical Z-axis lead screw was lose. Very loose. One bolt had come out completely and the 2nd of the 2 bolts was half way out. So when you reversed the lead screw direction that top mounting block was free to move up and down, thus reducing the actual travel of the Z-axis. .001 steps weren't enough to force shift the metal block but moves at .01and larger did. When I went to .5" moves in Z you could clearly see the block shift about 1/32".
I tightened it all up and added some Locktite to the two bolts and the machine is running beautifully again.
I just thought this might help someone else down the road so thought I'd take the time to write it out.
BH Davis