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astrodenny

Z Axis Drive Nut Assembly Loose

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I've been experiencing "steps" in certain layers of my prints. The print can have a nice quality, then all of a sudden you'll notice layers that either skip or seem to jump. As I narrow the problem down, one thing I noticed that has to be an issue is the drive nut assembly for the Z axis

The two slotted plates which hold the drive nut in place in between the panels - my drive nut is loose and there seems to be some play (http://wiki.ultimaker.com/Ultimaker_rev.4_assembly:_Z-stage)

I can actually wiggle the drive nut a bit. At one point I may have accidentally cracked the wood which may be causing the nut not to stay in place tight. Before I take it apart, I'm wondering if anyone has an stl file for these pieces (see Step 1) so that I can print a replacement ... or perhaps an alternate modification part which will hold the drive nut in place. I only have one printer at the moment (this will change soon :mrgreen:)

The other question I have is regarding the Z axis itself - is the axis normally a bit loose? If I slightly wiggle the axis rod, I'll notice it will move a bit within the drive nut. Is this normal?

 

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The other question I have is regarding the Z axis itself - is the axis normally a bit loose? If I slightly wiggle the axis rod, I'll notice it will move a bit within the drive nut. Is this normal?

 

No. Not normal. I just wiggled my bed, the z screw, everything I could. I pushed the bed up and down at the front and back and side to side and twisted and spun the z screw and tried to move the z screw up and down. Everything is impressively tight. More than I would have thought possible looking at the design.

I remember assembling my bed and nothing fit. I had to file down almost every opening - the openings for the bearings, for the Z nut, the openings to let the plywood into slots. Everything was too tight. It took several hours just to assemble the bed because of all the filing (like sanding but faster) I had to do. I also had to add shims to space out the bearings far enough apart so it could fit onto the vertical rods the correct distance apart. Shims are just little pieces of wood to fill the gap so I could tighten the side arms to the bed piece nice and tight.

 

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gr5 - they don't seem to be in x and y, however I probably should explore this further. I too had to file down a few parts to get things to fit - espescially the print bed. My drive nut does not stay in one place. With enough force I can nudge it in a different direction or make it slightly off center with my finger. At least, this is one of the issues that has me concerned.

At one point while leveling the print bed, I accidentally started the bed leveling wizard in Cura without tightening my z end stop. The bed hit the extruder nozzle and I thought I heard a slight crack of wood in the print bed. I have feeling this is from the two brackets which hold the z nut in place.

 

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I was toying with my Z axis using the ulticontroller. I set my build platform all the way to the bottom. I then pressed auto home to watch the Z raise. As it raises, you know that standard whirring/humming sound it makes from the motor? It used to be a consistent pitch. Very briefly, the sound seems to stutter for a brief second as the bed raises. It's not consistent. I've checked the motor, seems tight. Coupling is tight as well. When the stutter occurs, I noticed the coupling pauses for a fraction of a second as it's rotating. I think as the bed raises, the drive nut shifts or something is preventing it from raising smoothly.

 

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The Z nut should be able to move in the X/Y direction a bit, this is to prevent Z wobble. (The Z axis is not perfectly straight, and thus wobbles a bit, the play in X/Y makes sure this doesn't translate to the bed)

 

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The bed itself does not move at all. The Z axis rod does have some play within the Z nut. The z nut itself does shift a little (when pushing on it). So, when I auto home and the build platform raises, what could that brief stutter be? Shouldn't the sound of the z motor be a smooth whirl? I had posted a seperate topic regarding the Z seam - I get a nasty line showing up on anything circular:

http://umforum.ultimaker.com/index.php?/topic/2304-cant-get-rid-of-this-line-gap-in-circular-prints/

Not sure if this is interrelated?

 

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Yes, the sound should be a continuous, smooth note as the bed rises. Any noise you're hearing could either be directly mechanical (something catching, for instance) or a momentary change in the stepper sound perhaps due to steps being missed. However, aditionally, using the Ulticontroller to move the bed up and down is problematic. The code that converts knob rotation into movement isn't great, and I think it can artificially introduce problems. So, I'd be inclined to use something like pronterface to drive the z-axis for testing - that also lets you control the speed of the movement.

I'd be inclined to home the platform, then move it down, slowly, by 1 cm. Put some blue tape on the back of the printer, and mark the position of the bed on that. Then slowly move the bed down 15cm (hopefully hearing a smooth sound), and mark that position. Measure the distance, and make sure it moved exactly the right distance. Then move the bed up and down the 15cm amount, at various speeds, and see if seems to move smoothly, and end up in the right place. If the movement distance is the right amount, and the bed ends up in the right place, then any noises are mechanical, affecting the smoothness of moving, but not causing skipped steps. If the bed doesn't end up in the right place, then you're skipping steps.

 

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