Backlash on my Taig Mill

I’ve been fighting a small problem for a while and decided to tackle it today (since I’m on vacation until Monday). Small circles weren’t coming out quite round. The reason is what’s known as backlash. When the milling machine changes direction, such as moving to the right and then changing to moving to the left, the knob turns a little bit before the table actually starts to move again. This is usually a result of the lag nut not being completely tight around the screw.

I’m using a program called Mach 3 to drive my milling machine, and it has backlash compensation. But the trick is to figure out how much backlash compensation to enter into the program. A while ago I’d set my Y axis to .003”, but recently discovered that I also had a problem with the X axis. But this time I decided to be systematic about it.

I created a toolpath that cuts small .050” diameter circles with a 1/32” diameter cutter, and then tested different backlash numbers to see what I would get. Here are the results:


None of these are a really nice, clean circle, but after staring at these under a microscope, I decided that a backlash compensation of .002” was the closest to a nice circle. It may be that I can get a better circle after some more tweaking, but I’m going for improvement rather than perfection. This is certainly better than the 0” I had before, which you see in the center of this image.