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11 Questions View all

Z axis occasionally gets jammed, inexplicably gets unjammed

Once in a while, when I raising or lowering the Z axis, it'll appear that the Z axis will get stuck. When the bed is travelling it'll suddenly freeze and I'll hear a loud hammering sound. This isn't a constant occurrence but has happened enough to be a pain and has caused a few prints to fail to start because the bed didn't fully raise at the beginning of the print.

I've lifted the BoXZY onto its side to check if the motor is obstructed and have confirmed that there's nothing jamming the motor or the drive belt. When the axis is jammed the motor will twitch and jerk when it's stuck, which I assume is the source of the hammering sound. The Z axis looks clean and lightly oiled, so there's no obvious reason why it is binding up.

Speed doesn't seem to be a huge factor in when sticking occurs; I've run the Z axis up and down at 4800 mm/s and at 50 mm/s and it's stuck at both speeds. However, direction does seem to be more of a factor - I believe that I've seen the Z axis stick more frequently when travelling up than travelling down.

To make things more interesting, when this sticking does occur, after a while it'll magically become unstuck and will behave normally. I'll try raising and lowering the Z axis while it's stuck, and at some point it'll become unbound and behave totally normally.

Is this a known issue, and is there anything I can do to diagnose what's going on?

Can I try lubricating the Z axis, and if so what grease should I use?

Edit: I just had another print fail at the start; I sliced it with Cura engine and after the extruder was zeroed and heated up, the X axis was suddenly raised and skipped a bit. When the print started the filament didn't make contact with the bed and "oozed" down to the bed haphazardly. I restarted the print and it didn't occur. The related G code is at https://goo.gl/xplJGb.

Answered! View the answer I have this problem too

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Does it stick at the same spot?

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For me its intermittent between top and bottom. It sounds like a grinding sound and the plate looks like its struggling to move. And then as the grinding stops, the plate jumps quickly to catch up to where it should have been had it not been slowed down. A friend saw it happened and thought it might have some to do with ball bearings moving out of place. Turning the device in its side and then back up right made the issue go away.

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@Ken, it doesn't stick at the same spot at the same time but there are problematic areas. When I've used Cura it lowers the bed a bit while heating the extruder and then jerks it up when the print start; that jerking motion has a tendency to skip. Beyond that it may stick a little bit in the middle of the range, but nothing really common.

@Daniel, I've had similar symptoms and I think that after I tilted the BoXZY on its side and back it got better, but it still happens on occasion.

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A new machine should NOT need additional lubrication of the ball screw mechanism. It is pre-loaded with a substantial amount of grease at the factory.

It might be helpful to determine whether this is a mechanical binding, or some odd behavior by Cura and/or the controller. Next time it happens, do NOT turn the machine on its side - just immediately turn the power supply key to OFF. Then reach in and manually rotate the ball screw by hand, up some and down some. If it is still jammed, it may be mechanical. If it's NOT jammed, that would point to a software problem that J3 may be able to address.

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@Marty, thanks for the suggestion. I think that I can rule a software glitch out as Repetier, Cura, and Slic3r have all demonstrated this behavior. I also tried stopping the BoXZY right after a jam but it wasn't able to rotate the Z axis ballscrew - it looked like the motor had the ballscrew locked into place. It could be that the Z axis was bound up, but I was unable to budge the motor at all.

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2 Answers

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First thing to determine is whether this is a electronic issue, or a mechanical one.

MECHANICAL CHECK: With the machine unplugged and off, tilt it backwards and pull the belt underneath to raiser and lower the Z axis (you will not harm anything by pulling the belt, just watch your fingers). Make sure you do not crush the limit switches while doing this (by moving past their activation point). You should be able to move it by hand through the full range of movement, though it will take a little effort. If it gets rigidly and abruptly stuck at a point, there an issue with the ballscrew (verify you didn't max out the axis). It's very unlikely it's an internal ball-nut issue, so we'll put that concern aside for now.

If the axis moves well Skip to "Electronics check".

Solutions:

a) If there is significant and abrupt resistance somewhere in the movement this usually results from a scratch on the ballscrew. Even the tiniest mark on the screw will do it. A scratch or gouge can be safely sanded way with 180-320 grit sand paper until the area is level with or slightly lower than the surrounding material (do not sand a much larger area than where the mark is present, using the creased area in the sand paper can help you pinpoint the scratch only). If you scratch a larger area you can clean this up with very fine grit sand paper and a little elbow grease). The bearings rides on many rows of the ballscrew, so a repaired scratch or two will not affect the accuracy.

b) If the ballscrew is flawless:

1. loosen the clamp that is covering the center of the ballnut (under the platform, below the thick aluminum piece) until it's ready to fall off (loosen both sides evenly and rotate between loosening them), tighten it just enough so it doesn't fall off.

2. Loosen the bearing clamps at the top and bottom of the Z ballscrew until you see the head of the bolt stop touching the flat portion they sit on.

3. Try to move it manually again

4. If that frees it up, tighten the bearing clamps top and bottom just until the head of the bolt is touching the clamp again and no more.

5. Evenly retighten the two bolts on the ballnut clamp. The gap should be the same on both sides and you should rotate between each bolt to keep the gap similar as you tighten. Only light hand pressure should be used to retighten.

5. If the problem returns after tightening everything, they are too tight.

ELECTRONICS CHECK: If you find no major issues moving the axis by hand, the next thing to check is the circuit board. You may have a heatsink that has either come off the driver or is being blocked from cooling by internal wiring or junk in the filter mesh. You may also have a bad driver (these are tested and run so it's likely a cooling issue, though they are known to fail from time to time). A bad driver or overheating driver will create the exact symptoms you've described.

1. Pull your back cover off carefully, look to see if the wires were directly over the top of the driver as you pull it back (they will be nearby, but should not be sandwiched over it). Visually verify you see a heatsink on every driver. These are little red circuit boards. Check to make sure the little trimpots on the driver for the Z axis match the X and Y axis trimpots (match the Z to be the same as the X and Y if it is not already; if improperly set this causes overheating as well)

a) If the heatsink has broken off, you'll need to buy some thermal tape and clean off the existing paste to reinstall it. If it's lost you can easily find a replacement on ebay.

b) If all of these things look in order, you should replace the driver. You can find some great quality replacements here: https://www.pololu.com/product/2981. You'll need to purchase a heatsink as well.

1. Set the trimpot on the driver, it should match the driver currently installed (the flat on the pot relative to the two little metal dots; you'll discover what I mean if this is necessary).

2.If any pins are clipped on the driver you remove, clip the same pins on the new driver before reinstalling (make sure not to install the driver upside down. There are labels on the pins if you can't remember the correct direction)

3. Reinstall your back cover. Put the bottom end to the machine first and use your fingers to push the wires away from the heatsinks as you move the top of the cover toward the machine. Make sure you are confident they are not covering the heatsinks before tightening the cover back on.

(If the trimpot is incorrectly set you will continue to have similar problems)

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Thanks for the comprehensive answer; I'll run though this as soon as I can!

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This was it; the bottom bearing clamp was far too tight. I ran through the steps to adjust them and it started working perfectly!

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I'm really glad to hear that!

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You might unmount the motor or remove its tooth belt, in order to explore if you can turn the spindle by hand. This must be possible. If it is not or if you find regions (heights) where it's heavier or gets stuck, then you should explore the two passive guiding rods. You could then loose the fixation of them and find out if they are the reason. Thus reducing step-by-step all possible sources of friction until you get to the real origin.

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Adrien Thebo will be eternally grateful.
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