Z motors getting out of sync

I've noticed that whenever my machine finishes a long print and homes out again, the z-screw pointers are not pointed in exactlty opposite directions, and when I check the level it's generally out by several thousandths of an inch (I'm using a gauge with .001" increments). I am generally having to re-level after every print that lasts more than an hour or so. I had originally adjusted the Melzi trimpots to the specified voltage, but when I discovered this problem with the Z I tried adjusting it both higher and lower with no apparent change in the (mechanical) results. None of my motors get hot, BTW, just warm, with the Z motors running coolest because they move the least. I do notice my right-hand Z screw wobbles more than the left - it seems to be slightly bent, though I don't know if that would make a difference in this case or not.

Edited 3 time(s). Last edit at 11/23/2014 10:15PM by Rich K..

Do your threaded rods have a metric or imperial thread pitch?

The rods that nophead supplies in his kit have a 1mm pitch so the movement corresponds nicely to metric values specified in GCode. If you have something different and have not corrected for it in the Z steps/mm constant then you will get a cumulative error.

This, of course, assumes that your motors are not skipping steps or slipping at the Z-coupling but perhaps I should not assume that. Worth checking.

Regards,
Neil Darlow

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I try to write with consideration for all nationalities. Please let me know if something is unclear.
Printing with Mendel90 from fedora 25 using Cura, FreeCAD, MeshLab, OpenSCAD, Skeinforge and Slic3r tools.

Try sending the axis up and down a few times and see when they get out of sync. If the motors don't have enough torque it will be on the upwards movement.

Note that if anything motors run cooler when they move compared to being stationary and energised.

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[www.hydraraptor.blogspot.com]

I built a "sturdy", which uses 8 x 1.25mm threaded rods, and my firmware is adjusted accordingly. The motors are getting out of sync with each other.

Quote
neildarlow
Do your threaded rods have a metric or imperial thread pitch?

The rods that nophead supplies in his kit have a 1mm pitch so the movement corresponds nicely to metric values specified in GCode. If you have something different and have not corrected for it in the Z steps/mm constant then you will get a cumulative error.

This, of course, assumes that your motors are not skipping steps or slipping at the Z-coupling but perhaps I should not assume that. Worth checking.

Regards,
Neil Darlow

Did that, and they seemed to stay in sync. I did notice a short while ago, however, that my 4mm borosilicate glass is quite warped - in fact, the upper surface has a very large flake out of it (I ended up flipping it over and putting blue tape on it - I was coating it with Elmer's glue stick - but it looks like I will have to replace it soon...), which I found stuck to the bottom of the tool tray I printed out (in PLA) for the machine. I wonder if the warpage of the glass might have caused enough upward pressure to make the motors skip steps?

For that matter, what do the "jerk" settings do, and would adjusting the "Z jerk" possibly have an appreciable effect?

Quote
nophead
Try sending the axis up and down a few times and see when they get out of sync. If the motors don't have enough torque it will be on the upwards movement.

Note that if anything motors run cooler when they move compared to being stationary and energised.

These are the specs on the motors I'm using on all 3 axes, BTW. Should have adequate torque, I would think.

1, Technical Specifications

Part No.: Wantai 42BYGHW609

Frame Size: NEMA17

Step Angle: 1.8 degree

Voltage: 3.4 V

Current: 1.7 A/phase

Resistance: 2.0 Ohm/phase

Inductance: 3 mH/phase

Holding torque: 4000g-cm

Rotor inertia: 54 g-cm2

Number of wire leads: 4

Weight: 0.24 kg

Length: 40mm

Shaft diameter: 5.0mm

You can't put upwards pressure on the motors because the nuts aren't captive in that direction.

If they don't get out of sync moving up and down I don't see why they would during a build unless the speed in Pronterface is set lower than the firmware limit.

Do the motors stay energised during the build? If the firmware is configured to disable them they might get out of sync.

Jerk defines the instantaneous start speed before it accelerates and also the speed it decelerates to before doing a dead stop. Yes it has an effect if set too high.

Edited 1 time(s). Last edit at 11/25/2014 07:56AM by nophead.

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[www.hydraraptor.blogspot.com]

I don't think the firmware is configured to disable the motors. I am using Neil Darlow's version of Marlin, as I am almost exclusively running the machine with my Panelolu2. Pretty much have to now, since my laptop died this past weekend. I am using Slic3r to create my G-code, and have not actually run ANY prints using Pronterface, only using it to enter manual commands during set-ups (like M114), which I can't do from the P2.