Hi all,

I recently completed a CNC conversion on the Bolton ZX45 milling machine. I converted to double nut ball screws and am running a Nema 34 1000 stepper in the y and x axis and a Nema 34 1800 or so in the z. I'm running Mach3 software. I am also using a 150lb pulling gas strut mounted vertically. The steppers are mounted as direct drive.

My x and y work flawlessly and repeat to .001 and only needed .003" backlash offset. The Z also does well and repeats within .001 with .005" backlash offset.

The issue is when I'm milling. When milling steel, the z axis will drop and the tool will dig in deeper than I want it to go. I don't seem to have the same issue when milling aluminum. I can take .25" passes with a HSS end mill on aluminum without any issue whatsoever. However, cutting mild steel anything over .050 deep causes the head to drop. And, obviously, Mach3 does not detect it. And it's not a little bit...it's dropped as much as .125 on a 3" pass. I have experimented a bit with feeds and speeds as well. Maybe I need some more direct advice on feeds and speeds?

I have validated that the tool is not moving in the collett and also made sure that the quill is not moving. It is definitely the z-axis on the column moving downward as I taped some masking tape over at the start of a pass with a line on it and you can see the whole thing shuffled down.

So, now, what are some ways to deal with it? I'd think that a 150lb spring plus the larger 1800 stepper would be enough...maybe it's not? I can easily adapt another 150lb gas tension spring on there as I got a good deal on a pair of them. I have to think that the stepper is plenty large? I don't think a Nema 42 will fit without offsetting and adding a pulley.

Any help is appreciated! The first steel part I attempted to make was ruined (a new tool post t-nut for my lathe) and i kept milling on it and other mild steel to see if I could work it out...I haven't...I still get drop and i'm starting to get pretty frustrated. Maybe someone has some advise?

Thanks