[MunchyMonster]

So I got my DIY MDF machine put together about a year ago and I've been cutting with it for a while now. I've done a few projects, but I have this issue with the x-axis "stalling" from time to time.

I got tired of it today and really started to try and figure out what was going on. Watching the split in the screw clamp I'm using I can see that it always stalls in the same spot. So I went and checked to see if I had any rubbing anywhere on the ACME screw or any of the other hardware on the x-axis. Nope a good 1/8" clearance between anything that moves and anything that is fixed.

So I tried loosening up the motor mount to see if I could detect any wobbling that might indicate that the stepper motor shaft and the x-axis acme screw were not correctly aligned. I don't notice any wobble, but loosening up the motor mount seemed to help with the stalling issue. I ran fine all afternoon and I even did a little bit of cutting.

I went back to it tonight to do a little more cutting and I'm getting the stalling behavior again. Out of frustration I tried stopping the x-axis acme screw by hand. I seriously expected it to just twist out of my hand. To my surprise I was able to stop it pretty easily.

I've got a 400oz/in stepper from probotix on the x-axis, does it seem right that I should be able to stop it with my hand? My machine has a 2'x4' usable cutting area and while I "might" understand that my gantry "may" be a bit large for 1 400oz/in motor to sling around. I find it hard to believe as when I take the motor off I can slide the gantry easily back and forth with light pressure (1 or 2 fingers) and the acme screw turns smooth as silk. And when the x-axis isn't stalling I can get it to slew back and forth on that axis at 300in/min. I've been cutting at 100in/min, but when it starts stalling I can't even "rapid" at 50in/min without a stall messing things up.

Any insight would be greatly appreciated.

Thanks,
MunchyMonster

[ger21]

What pitch screws are you using, and what voltage are you running at?
And what is your acceleration set to?

Are you lubricating the screw?

If you're using a rigid coupler, then alignment is critical, and almost impossible to get perfect on an MDF machine, as the slightest misalignment will cause it to stall.

[MunchyMonster]

I bought all my electronics from Probotix. Not that I'm saying anything good (or bad) about them, I was just lazy and wanted to get all the electronics as a turnkey solution. I bought one of their 3 axis packages with a planet cnc usb controller. Here are the specs on the package I'm using:

3x HT23-400-8 400ozin Stepper Motor
24Volt 6.5Amp Switching Power Supply

My ACME screws were from McMaster-Carr. I'm using:

1018 Carbon Steel Precision Acme Threaded Rod, 1/2"-8 Sz,1"travel/turn, 6'l, Rh Thread,8 Starts

on the x-axis (the longest axis on my machine). I'm pretty sure it isn't the screw as it turns smoothly and has surprisingly little wobble even when I thrash the gantry from side to side by hand and watch it spin.

I've changed my acceleration value from 1 (really slow accel and decel) all the way up to 4 (soft start and stop but responsive acceleration) and it doesn't seem to effect the stalling issue at all.

I've been using 3 in 1 machine oil on the screw and with a fresh dose it really is quite slippery. I can go quite some time without needing a fresh does of oil, but I did indeed oil the machine just yesterday, so I can't believe that is the issue.

As for the coupler, I don't think what I'm using would be considered a rigid coupler. one piece mounts on the stepper motor shaft the other mounts on the acme screw and they are joined by a "rubber spider". I was led to believe that this arrangement would allow for a bit of misalignment between the shaft and the screw. I figured with an MDF machine I would need to take that into account.

[ger21]

I would try increasing the accel to 10-15 and see if it helps.
My guess would be thatyou're seeing resonance. Does it sound rough, or smooth when running? Do you have dual shaft, or single shaft motors? If dual shaft, you can try adding a damper to see if it is resonance.

[MunchyMonster]

Well it wasn't acceleration. I'm going to try running with the higher values and see how bad my machine thrashes when changing directions. Looks like your question about how it sounds has pushed me in the right direction though.

I took the z and the x steppers off their screws and ran them. Lo and behold the x does sound "rougher" than the z. So I ran the y (on its screw) and it sounds like the z. I'm guessing I've got a "bad" stepper on the x. I swapped the steppers on the x and z to see if they sounded the same on their new screws (my z has a higher tpi but a shorter travel). Yep x and y are both silky smooth (300 in/min rapids up from the 100in/min I was getting) and z is rough and stalls. I can use it so long as I crank the z rate WAY down (5in/min).

So is my only choice to get a new stepper? Can I get away with a smaller (aka cheaper) stepper on the z since the higher TPI ought to give it more mechanical advantage?

MunchyMonster