Vertical screw movement

Hi,
I just got to my first print which is printing out the axes compensation test pieces and noticed that as the printer zig-zags to infill the various parts, the vertical screw moved from side to side correspondingly due to some play somewhere, probably in the screw housing. Would there be some benefit to adding a piece to the top of the vertical pillar, first to constrain movement of the top of the screw and secondly to stop it poking me in the eye.

Does this screw movement have a noticeable effect on accuracy?

Alan

Hi Alan,

the top is loose by design, since if the rod is bent (which it most probably is) and constrained at top and bottom, it would pull the X axis around causing wobbles in the print and putting strain on the gears (which may stall the Z motor) and the X-Axis mount. The chances are it does have some effect (causing banding in the z direction), but having the top loose minimises this.

My machine also had a slightly eccentric driven gear (the one the rod sits in) and that enhanced the wobble on mine - I printed a new set and it's much better now ( I have less noticeable z-banding)

Ray

Edited 1 time(s). Last edit at 01/17/2014 02:14PM by rayhicks.

Ray, that's interesting! I also found the z-rod was wobbling. Watching it, it was clear that the problem was not that the rod was bent, it was that the bottom of the rod was not concentric with the bearing. I assumed I had fettled the z-driven gear unevenly to make it fit. So I printed a new z-driven gear, and used a hammer to put a nut in it instead of fettling it. The new one wobbles just as much as the old one did.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

@DC42 and Ray I have this issue and would like to resolve it. My threaded rod was bent on arrival but I straightened it so it was 'good'.
A mechanical engineer friend said that this will be expected with standard threaded rod and nut manufacture.

Question. " does it improve with larger diameter rods?"

@Alan, droftarts strongly indicated the top of the rod should not be constrained in another post.

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Ormerod #007 (shaken but not stirred!)

hmm dc42, revision time for me ... I initially had trouble getting the bearing into my newly printed gear, and drove it on using a brass tube and a hammer - holding the tube as vertical as I could and hitting it softly (I doubt if the bearing would withstand too much axial shock), and the banding did diminish along with the wiggle.

This afternoon I printed Matt's belt clamps and did some major dismantling of the Y stage, and thought I may as well check out the Z while I was at it (it was after answering Alan above). Everything went back together pretty well, and I started printing a new X carriage (ironically mine failed in a similar way to victors' the day after I offered to print him a new one!) and the banding came back, along with a more pronounced wiggle (I noticed banding on the print first), the rotation of the new gear now also looks eccentric, but this time I had just pushed it in since it was a fairly easy fit.

It might be that the original wasn't actually eccentric, but that the bearing's sides are too short to easily get a good axial alignment and the gear is actually precessing around the bearing's axis (tail wagging the dog)- I'll try some truing tomorrow, probably with a brass tube again and see if it fixes it.

Ray

@ treth, Ian has suggested using stainless steel studding (apparntley it's manufactured to better tolerance than the bright zinc plated stuff we have particularly in pitch tolerance), using a sacrificial brass nut, which would wear to even out irregularities in the rod, and having an elongated nut would all help. I'll go for a delrin or brass nut as and when I do my mechanical schemes!

I've got a range of studding ranging from M2 to M5, and yes the smaller diameters have more pitch variation (but I guess it's because they have smaller pitches and the machinery that makes them has limited accuracy). There's a whole world of thread specs to discover if you're really interested - but getting the specs of the stuff you buy is pretty hard unless you're buying by the mile from a manufacturer sadly

Ray