Troubleshooting a stuck axis

[HolHorse]

Hi, I was given a cnc-ready Taig mill (year unknown, 2011-2014 most likely, acme leadscrew) a little while ago and am in the process of getting it up and running. I am currently dealing with a stuck Y-axis and would like help and opinions on troubleshooting.

After receiving the mill, first thing I did was rewire the motors quick and dirty, then plugged it into a friend's setup (she has a taig as well, though different year/model) in accordance with the drive & motor datasheets. Everything worked. Motors are wired bipolar series, drive is a geckodrive, control is a custom linuxcnc box. All of the axes drove without issue. Cool. This is where I start to make mistakes.

I noticed there was a ton of play in the Y-axis. Apparently the gib was loose. I tightened it up a bit and figured I'd test it later.

A few days later, I rewired it again with nicer cable, proper strain relief, and fresh connectors. I checked with a voltmeter. Resistance was still correct, no crossed phases, no shorts, correct current-sensing resistor in the DB-9... Looking good. I hook it up to a new linuxcnc computer (same power supply, same geckodrive, different computer & DB25 cable), get the settings in the ballpark, and run it. The X- and Z-axes sound/move just fine, but the Y-axis isn't moving. It looks like binding due to a mechanical part, but my friend insists it's probably a wiring issue that the voltmeter isn't catching.

I've tried the following:
- Reattached to previous computer with all hardware/settings same as when it worked the first time - Y-axis still binds
- Resoldered connections between cable and motor - No improvement
- Loosened gib, tightened gib, loosened it completely - No improvement

The only possible lead I have is that the Y-axis feels a little tight when I try to turn it by hand (whether gib is tight or loose), but I don't have a reference point for what it's supposed to feel like. I can turn the leadscrew by hand and the carriage moves. It feels a bit tighter than the X- and Z-axes, but I'm not sure if it's tight enough to cause this flat-out binding. When I try to jog it at low speed, the lead screw will rotate maybe a few degrees in either direction and I can see it pushing on the Y-motor mount a little bit. When I try at high speed, the carriage and lead screw won't move at all.

Anyone have experience dealing with taig mills? Where's a good starting point for diagnosis?

edit: also, the y-axis is not locked (at least not deliberately by way of the gib lock screw)

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[CL_MotoTech]

Does the stepper run when it's not connected to the axis?

[HolHorse]

That's the next thing I'm going to try. I'll be back at my friend's shop either tomorrow or the day after. More background:

The mill is probably at least 4-5 years old, from what I was told. It looks like it's been run a few times, but otherwise very light use. It sat in a shed in northern california for most of that time. Ways and leadscrews are in good shape, some minimal surface rust on the back of one of the y-ways and on the motor couplings. The oil on the ways and leadscrews looks fine, but I refreshed the oil on the ways just in case. Haven't gotten to greasing the leadscrew yet, or had a chance to look at the nuts.

If the continuity & resistances across each phase look good (measured from the plug of the motor connector) and there's no shorts (either between phases or to the connector or shielding), does that rule out issues with bad connectors or bad soldering? Is it common to have issues with the connector or soldering that would only appear when you apply current?

What else would be worthwhile to look at/try? I swapped the connections between the motors and the drive, issue was still limited to the Y-axis on the mill. I noticed it buzzes a little louder than the rest of the axes when idle.

[Fastest1]

Since the only thing you changed was the pc, I suspect it.
You are using the parallel port?
Many machines had issues when XP went from real time.
From all I have read on Linux it too suffered.
Double check the specs.


Sent from my iPhone using Tapatalk Pro

[HolHorse]

Since the only thing you changed was the pc, I suspect it.
You are using the parallel port?
Many machines had issues when XP went from real time.
From all I have read on Linux it too suffered.
Double check the specs.




Sent from my iPhone using Tapatalk Pro

This problem manifests identically when connected to my friend's control box (which had worked the first time I connected it) , so it's not an issue with the pc, drive, db25, or power supply.

My leading candidates are either the gib, motor, motor coupling, leadscrew, db9 connector, wiring, or splices. I'm not all that experienced with standard machine tool hardware (I know an othermill inside and out), but my gut is pointing to either the gib, the motor, or the coupling.

[HolHorse]

Apparently it's the Y-motor. It's not putting out enough torque. The leadscrew can be driven by hand without significant resistance. The motor spins when disconnected from the coupling, but can be turned by hand without significant resistance when under power. The x-motor has an appropriate amount of resistance when I try to turn the shaft when it's under power.

Not sure what's causing this.
- The resistance across each winding is in the ballpark (2.2 ohms measured, 1.8 ohms ideal)
- no shorts between windings or from a winding to the body.
- The current sensing resistor is set to 1.8 amps (1.77 kohm measured across pins).
- Current sense resistor is not shorted to adjacent pins.
- no resistance when shaft it turned when powered down


Edit: I shorted the phases (one phase at a time, I've only got so many hands~), and rotated the shaft. It does not feel any different from when I do this on the x-motor. There is a definite spike in resistance to turning, and it does not appear to vary depending on which phase I short or what where in the cycle I do it.

I think I'm going to try replacing the db-9 connector, there's a small chance something is shorting when the connector is plugged in vs unplugged.

[HolHorse]

I desoldered the connector and twisted each pair together, then rotated the shaft. There is resistance when I turn slowly, but it slips for maybe a quarter turn at a time when I turn faster/harder. Does this sound like a bad stepper motor?

[awerby]

Steppers rarely go bad. Is this a Geckodrive G540 that you're using? If so, it has a spare driver. Making sure you shut everything down and let the capacitors drain before disconnecting anything, then switch the cables going to the Y axis so they're coming from the unused driver which is for running the A axis. See if the axis behaves any differently when you jog it (using the Home/End keys). If you've already used the A axis for a rotary table, disconnect it and connect to Y instead. If Y starts behaving better, you might have a bad driver, but that can be replaced.

[desktoplathes]

Swap X and Y motor connections at the driver board and see if the problem follows the motor / wire harness or stays with the axis.

[HolHorse]

Swapped connectors, problem follows the motor. Removed motor, it puts out very little torque. Put x-motor on y-axis, it drives just fine.

I spoke to a friend that's a professional machine tool tech, and he thinks it's the motor. Gonna get a new one and see what happens. Replacement has lower inductance and higher current rating, seems more appropriate for this drive anyway from what I've read. Will see how that pans out.

[HolHorse]

Thanks for the troubleshooting advice thus far, all. I'll have questions on tuning next...