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astrodenny

Wobbling in Z Axis

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How tight should the Z Axis be when resting within the drive nut? If you were to touch the z axis with your finger how loose should it be when residing in the drive nut?

Or if use the ulticontroller to raise the platform all the way down, then back up, should you see any wobbling or loosness?

 

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Agreed. Ideally the Z-lead screw should be straight and perfectly aligned in the Z-direction. I think the fitment of the lead screw nut and Z axes bearings into the wooden parts of the Z-stage was the most demanding mechanical part of my assembly experience. I messed with the nut for a long time before I could find just the right position to get it to seat accurately. Also, fitting the Z axes bearings was the only place in the build where I had to sand or file any wooden parts, beveling the edges slightly to improve the fit of the bearings into their cavities. Afterwards, the bearings were cradled in good alignment with no slop and no "negative clearance" with the wooden parts (which results in gaps in the wooden parts stackup).

One reason the lead screw is not anchored at the top is probably to permit it to bend and deflect slightly to accommodate slight misalignment and non-linearity. The two thick and well-anchored (top and bottom) axes rods should keep the Z-stage tracking straight and "convince" the lead screw to follow a straight path. Well--maybe it's only one axes that keeps it straight. As I recall, the Z-stage arm on the other side floats.

 

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I seem to be dealing with several issues:

1) The z lead screw definitely does not sit tight within the lead screw nut. There lead screw drive nut is not very tight fitting around the lead screw, it's slightly off in tolerance. With a push of the finger, I can make the lead screw shift within the nut.

2) As I mentioned to Illuminarti in another post, which I have not had time to examine yet, is that the lead screw drive nut does not sit straight between the two laser cut panels which hold it in place. I can push the the nut out of place with enough finger force.

3) With regard to #2, when the bed raises up / down along the lead screw - right about at which the bed passes the x - motor, there is a slight "cracking noise" as if the lead screw drive nut shifts against the wood.

For #1, assuming the tolerances are off for the lead screw drive nut, I have a feeling it needs to be replaced. If so, I'd really like ultimaker to help me on this one.

For #3, I think this has to do with something I messed up once. I forgot to tighten my z end stop while doing a bed level through Cura. The platform ended up hitting against the hot end, causing the bed to slightly bend. I remember hearing a slight crack of wood, but could not find what was happening. My guess is that one of the two small panels which house the nut may have cracked, leading to the lead screw drive nut be able to slightly shift if you apply enough force with your finger tip.

I would love to be able to print replacements for these two panels before I take the bed apart; or perhaps a new design to hold the nut in place. Would either of you happen to know if Ultimaker has the STL drawings for these parts available? I've searched both ultimaker wiki/forums and thingiverse but couldn't find them. The other alternative is to design them myself which I'm trying to avoid (having to take apart the bed, measure the parts, reassemble the bed, print them, unassemble, then reassemble again.

For future reference, does Ultimaker offer any of their part files for download anywhere?

 

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My understanding is that the whole point of the design is that the nut that attaches the z-screw to the bed is supposed to be able to move - exactly so that things can shift in case of slight misalignment, rather than binding solid.

The lasercutter drawings for the Rev 3 UM kit are on Thingiverse here: http://www.thingiverse.com/thing:13571 - that includes the outlines of the panels that are hold the nut in place, I think (although toleranced for lasercutting in plywood, so some modifications may be needed if you were to e.g., print a replacement piece).

 

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Re: 1 &2: Yes--those are the problems I encountered too. The correct position of the lead screw nut is unstable in the wooden holder (it does not naturally seat in the correct position) and I found it difficult to keep it in the correct position while I tightened down the box assembly around it. The lead screw nut always wanted to shift. However, with extensive fiddling (the specifics of which I do not recall) I got it in the right position and it stayed put after the assembly was tightened. It remains in that position to this day. Sorry I don't have any hints for you. Perhaps other will have a better memory for it than I.

Re. parts: I don't think there are any .stl models for the wooden parts because they are laser cut. You can find the .dxf files, however. I think I found them on Thingiverse.

I have a heated build platform and I often run temps of 75C inside a bubble wrap tent when printing ABS (my material of choice). The wooden Z-stage is not perfectly stable in height and level with temperature changes. I have a heated aluminum bed that I made from 5/16" precision Mic6 aluminum tooling plate, which is very flat and very temperature stable. If I were in your shoes, I'd build the entire Z-stage from Mic6, but I have a milling machine and bandsaw too! :) When I have a fair amount of spare time, I plan to do the upgrade, but Mic6 is also pricey.

 

1) The z lead screw definitely does not sit tight within the lead screw nut. There lead screw drive nut is not very tight fitting around the lead screw, it's slightly off in tolerance. With a push of the finger, I can make the lead screw shift within the nut.

2) As I mentioned to Illuminarti in another post, which I have not had time to examine yet, is that the lead screw drive nut does not sit straight between the two laser cut panels which hold it in place. I can push the the nut out of place with enough finger force.

For future reference, does Ultimaker offer any of their part files for download anywhere?

 

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Yes--perhaps side-to side (though my nut is rigidly held but it's also straight). I doubt the UM designers would kill Z-hop capability by letting it move in Z, however!

 

My understanding is that the whole point of the design is that the nut that attaches the z-screw to the bed is supposed to be able to move - exactly so that things can shift in case of slight misalignment, rather than binding solid.

 

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Illuminarti - Thanks for the file, I could not find it for some reason or may have glanced over it. I'll see what I can do with it for replacement panels and take into account they will not be laser cut wood. I suppose too rigid of a z axis would not be as forgiveable during any type of misalignment. If the final design was to allow for slight movement, I can live with that.

calinb- I too had to do some sanding for the Z axis bearings to fit properly. They seem to fit perfect now. The Z nut placement is what scares me the most (shifting during a print) and perhaps I'll have to work on this with replacement panels so it's stable. At this moment, I don't see it shift visually, but when I hear the z axis move to a certain point in height, something creeks which makes me nervous. I just don't want to take it apart until I have the panels printed and ready.

In the near future, I'll be printing with ABS and converting the platform so aluminum would be my first choice although I'll have to source it precut.

Thank you both!

 

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