Losing z-axis zero really fast....

[Sciome]

I just built my first cnc router. I got it up and running on mach3. The table moves x-y under neath a z axis. I cut my road runner with no problem.

When I went to cut the art cam demo, I found a problem. About 4 minutes into the run, the z axis lost it's zero by 0.4 inches... Plunging the router way into the table and causing massive smoke. I estopped and hit return to zero.... And the zero was down 0.4 inches on the z-axis....

I checked the code.... It was fine, a re- ran the program and happened again, but at a different spot... This time it was 0.3ish inches down lower on the z-axis.

I don't think I'm losing steps on the z axis.... It happened to fast.

I reran the code again this time above the work by 2 inches... I kept a eye on the z axis.... It appeared the z axis stepper stalled for a second tring to pick up the spindle.

I have a Nema17 stepper on the z axis. It moves a 3hp Chinese water cooled spindle and mount.

My question is do I need a bigger stepper on the z axis? Or is there another way to fix it? I would hate to build a mount for a nema23 stepper... I just got the other one done.

I was thinking if the stepper is over loaded with the weight of the spindle, could I just rig up a counter weight that pulls the spindle up (over a pulley of course.) I just never heard of anyone doing that.

Any thoughts?

[ger21]

I have a Nema17 stepper on the z axis. It moves a 3hp Chinese water cooled spindle and mount. My question is do I need a bigger stepper on the z axis?

Actually, the real answer is "It depends on how fast you want to go". But the short answer is yes, you need a bigger stepper.
But it may be more complicated than that. The drive and type of screw also come into play. With the wrong drive and screw, it's possible that a larger motor may not help all that much.

I've never seen anyone use a Nema 17 motor to lift a 3HP router. It's just not big enough.

As far as losing position, you can fix it by lowering our acceleration and velocity in motor tuning. Provided the velocity is set OK, a general rule is to decrease acceleration by half. If it still loses position, cut it in half again. Keep doing this until it doesn't lose position.

[aarggh]

My tiny PCB machine has NEMA17's lifting a really small Proxxon (Dremel style) tool, and it's fine for that, but on your machine! How could you do that to the poor little buggers?

I think the top end for any decent NEMA17 is under 60oz/in, and most 3HP spindles have at least a 280-425oz/in stepper lifting them. You've got no hope of getting any work out of that arrangement. Even if you really slow the speed, if your doing any sort of half decent cutting, that's an additional load that will most likely still cause lost steps. As Gerry said, you'll definately need to go to much larger steppers if the design will allow it, or alternately, downsize the spindle dramatically to something much less demanding. If your stuck, a small 1/4" trim router might just be okay, or Proxxon make some decent small millers too, bit pricey but great tools.

cheers,
Ian

[JerryBurks]

Originally Posted by ger21 .... But the short answer is yes, you need a bigger stepper.....

A bigger stepper is probably a reasonable thing to do no matter what. But, before you get disappointed, you should run the same path without the boat anchor attached and make sure that the problem is not somewhere else.

[Sciome]

I took off the boat anchor (ie 3hp spindle), it ran fine. I need to use a Nema 23 stepper. Now I have to build a mount.... Ugh, I thought I was ready to run. Lol

[jsheerin]

You could also add a counterbalance of some type to your z axis. A gas spring or air cylinder connected to a reservoir are typical ways to do that. Depending on how fast you want to go and your screw and motor driver setup, you might need that even with a nema 23 motor.