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Yesterday i replaced the z axis of my diy printer. Now i have some strange waves on the side of my prints. Are the threaded rods bent or could that be a vibration issue? Or is the coupler too stiff?

enter image description hereenter image description here

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    $\begingroup$ Is your Z axis screw constrained at the top? $\endgroup$ – tjb1 Jul 6 '17 at 16:45
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    $\begingroup$ @tjb1 Sorry for the late reply. Yes the lead screw is fixed with a ball bearing at the top and bottom on both sides. The lead screw has an outer diameter of 9.9 mm and the bearing an inner diameter of 10mm so the coupler is not completely centered. Could that cause the issue? Do i need machined lead screws with exact diameters on both ends? $\endgroup$ – FunnyItsElmo Jul 19 '17 at 8:37
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    $\begingroup$ You could try removing the bearing opposite of the motor. Fixing the screw at both ends causes it to rotate in the middle which may be causing the wobble. $\endgroup$ – tjb1 Jul 19 '17 at 11:01
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    $\begingroup$ Don't overconstrain the Z lead screw. It should be free except at the motor and the lead nut on the carriage. Two points define a line; three define a bent, wobbling line. $\endgroup$ – R.. GitHub STOP HELPING ICE Jul 18 '19 at 12:50
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    $\begingroup$ The rod is moving, that could be a bent rod, or the mounts are not in the right space. You can see it moving at the top, of the Z axis, then you know you have a problem. Solutions are to try new rods, re-measuring your mounting bracks positions and to add a guide at the top of your Z axis. $\endgroup$ – StarWind0 Jul 18 '19 at 16:27
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This looks as if there is a side-to-side force being generated as Z is changed. Ideally, the X or Y position is determined by the smooth rods, which should bear any force, not the threaded rod. The threaded rod should cause no motion except upward and downward.

If the rod is bent, but the rod is lightly constrained, then the top, being the free end, will draw a circle. The bottom is constrained by the motor. The middle is constrained by the nut, so the top would wander. If the threaded rod is perfectly straight, and the stepper motor axis and the nut are perfectly aligned, the top would spin without movement.

If the rod is bent and the nut is very tight, there may be a twisting force exerted by the nut, which could cause movement.

Everything is a tradeoff between tightness and looseness, between precision components and the reality of alignment errors.

You asked if the shaft coupling is too tight. Maybe. Try removing everything except the motor, coupling, and the threaded rod. Does the rod fall exactly where the nut would be? If not, it isn't aligned correctly. Incorrect alignment can be mitigated by a flexible coupling. It looks like you are using a Lovejoy coupling with a plaster spider. Depending on how tightly the spider fits, a Lovejoy coupling can be tight or loose. I see a lot of machines with the one-piece coupling and a spiral-cut joining region. THis might be a good alternative, but they tend to take up more space than a lovejoy coupling.

Because Z-axis motion is pretty slow, it is unlikely to be a z-axis vibration issue. You may have changed the resonant frequencies enough to cause some other problem, but I don't think it is related to the problem in the photograph.

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It is possible that your selected layer height isn't compatible with the pitch of your leadscrew. I believe the reprap website has a layer height calculator based on the pitch of your lead screw. Just another option.

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