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donmilne

Circles no longer round (DM version)

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I'm aware that there's another topic with a similar title, but I believe my problem is different so I've added the (DM Version) to my topic.

Anyway, a while back (right before the old forum went offline - bad timing!), I had a mishap with my printer. Without noticing it I had managed to have the printer sitting on a loop of filament. When this pulled tight (Robert's feeder - pulls plenty tight!) the printer made an almighty rata-tat-tat noise. At first I didn't diagnose the issue - I aborted the print and tried again... rata-tata-TAT-TAT-TAT-TAT!

Pulleys were pulled loose. I tightened them up, but ever since then the printer can't do circles properly anymore - they come out slightly elliptical, with the major axis on the topleft to botright diagonal.

I'm assuming that the rod corners were pulled out of square, and that I should have squared them up (somehow?) before tightening up. Does anyone recognize this, and have suggestions how to fix the problem?

Edited by Guest

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That really loud noise happens to many people and has happened to me a few times.  When you first hear it you think "Oh No!  I broke it!".  But the machine is very tough and should be able to do that without damaging anything.  However it is possible you jumped a tooth on a long belt.  It's also possible the rods are a bit loose and sliding back and forth.

There are many possible reasons for getting non-round circles.  First of all is it a UMO or UM2?  Please describe what machines you own in your forum profile.

On the UMO it's common to have this problem if long belts are too tight as they cause excessive friction and you get backlash.

To test for rods that are sliding on UM2, print something and put a finger on either end (doesn't matter which end) while it's printing anything.  Does the rod slide in and out?  If so you can loosen a pulley and push the pulley outwards more to hold the rod from sliding.  For alignment you want the belt lined directly above or below the rod.  It's easiest to test this by pushing the print head towards the side you care about such that the block is almost to the pulley.  The block will position the belt in the perfect spot.  Loosen and retighten the pulley if it's not lined up.

There are other things that could be causing this. At least tell us which printer you have.  And check for high friction by pushing the head around with power off.

Edited by Guest

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I already described my printer in my UM profile. Did these not carry over from the old forums?  Anyway, it's a UM2.  I'll update the new profile.

When you say "rods sliding in and out", as far as I know it's always done that, by a few mm. Is it not supposed to?

I have to say that I'm wary about those pulleys. ISTM that access to the grub screws is poor, and the grub screw heads look likely to get reamed.

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Is there an approved way to get clearer access to the pulley ends? On the UM2 they're hidden under a rim which seems to be part of the main structure, requiring major surgery of the frame if I want to remove it. I.e. it looks like I have to remove the left and right sides first, then the top

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No! Don't remove anything! Certainly not the sides! You can take the whole upper stage of the UM apart without taking any sides off. In fact during assembly the sides are assembled first I believe.

I can get to the grub screws on all of the upper pulleys just by pushing the head around until a set screw is facing straight out between the belts. Make sure you have the correct hex tool. You don't want to damage anything. They are all 2mm. I recommend you get a screwdriver style hex wrench with a ball end but I have ones without the ball end. I have adjusted these pulleys both with the screwdriver type tool and the L shaped tool.

To get to the pulleys on the motors you have to remove the metal covers - 1 screw for each cover half way up one of the edges.

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Okay - UM2. Well non-round holes are rare on UM2 because it has a belt tensioner built into each of the 4 sliding blocks. To get the tension to spread evenly you might have to loosen the pulleys on a particular belt - but only if the tension is very uneven. Even then I've printed for days with a belt that was loose on one side and quality was still excellent.

I guess I'd like to see a photo of the circles you are talking about - particularly on the top layer - bottom layers have non-round holes for other reasons.

Feel both short belt tension and long belt tension. You want the short belts probably a little bit tighter than the long belts. If the long belts are too tight you can cause excessive friction but I've never heard of this in a UM2.

Push the head around - even push it by the nozzle to see if there is some play in the head itself (nozzle somehow loose?) There shouldn't be any play - any looseness will get translated into an equal amount of distance/error in your circles.

Also see if friction is high. You should be able to use the smallest finger on each hand pushing on the side blocks at the same time and move the head that way. Friction should be exactly the same for X versus Y axis.

To tighten short belts you have to loosen the 4 screws holding the motor, then push down hard on the motor while tightening the screws up again. Very easy to do.

Having a rod slide by 1mm won't affect anything except the very corner of your bed and only slightly. It won't make round holes squarish at all. Having a loose screw on a pulley also won't cause round holes to be square - this instead causes parts to "shift" as you move up the layers such that printing a robot will be a crooked or tilted/leaning robot.

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Hi George. I thought I'd provide some closure on this. It has taken me a long time to get around to it, but a couple of months ago I ordered a screwdriver type hex bit set (with ball ends), and this weekend I finally got around to trying it!  The problem turned out to be (I think) a spacer tube on the end of the rod that stops the rod sliding too far and falling out of its bearing! This spacer had been knocked out of position when I had my accident. I was able to loosen a few grubscrews, push everything back and tighten it all up again.

I tried making myself a couple of little clip-on jigs to help align the axes, but I'm not sure that helped a lot - just pushing the spacer hard against the endstop seems to be what made the difference.

Whatever I did, I must have done something right, because I just printed off a test piece (the piece that I previously noticed was elliptical), and it now comes out perfectly circular - well ok, it's 33.6mm at max by 33.0mm at the min, so within the margin of error expected of the .4 nozzle. And the sides were perfectly plumb too.

The only problem I have now is that it makes the loud squeaks on the y-axis when printing. I'm assuming I went overboard on that spacer and now it's probably rubbing on something. Ah well, a problem for another time!

Oh p.s. you previously asked for a photo of the problem, but I doubt that would have helped. The elliptical shape was subtle, I doubt you'd have seen anything in a photo. It's only when you tried to use the part that you notice it's not quite round and the sides weren't quite straight.

Edited by Guest

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Maybe you went a little too tight.

The 4 rods in the upper corners of the printer should be oiled with any light petroleum oil such as 3 in 1 or sewing machine oil. The 4 rods through the head shouldn't be oiled.

Push the head around with power off before and after adding oil. Also put something (newspaper/rag) on the bed so it doesn't get oil on it while doing this.

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Um.. why shouldn't the rods through the head be oiled? I've already given all the rods a drip or two of sewing machine oil in a bid to cure the squeaks (with little if any success) - and that included the two rods you mention. There was no effect good or bad that I noticed.

Is this a temperature concern perhaps? If so I'll watch out, but nothing alarming happened, e.g. no smoke.

ps. I have the UM2+ upgrade on order, so I expect I'll soon be replacing the head and its rods anyway.

Edited by Guest

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I'm really not an expert.  But people have claimed before that oil gummed up their vertical linear bearings.  The rods through the head only move in one direction, they don't rotate.  So that bearing has 4 rows of ball bearings in it.  Because the balls roll against steel on either side (the rod on one side, the insides of the bearing on the other) you can have high friction and they still roll smoothly.  In fact for a car - it's considered *good* to have the wheels not slip on the road - they roll nicely with very little friction slowing the car down.

But I supposed those ball bearings do touch each other on the sides - each bearing is rolling such that where two balls touch the direction is opposite.  I assume.  Unless there is a smaller ball in between them.  Which I doubt.  So a little oil there might help.

The only bad thing about oil is if helps the dust get stuck inside the bearings.  Without the oil the dust would tend to slide right out of the bearing as easily as it gets inside.

So don't worry about it.  I used to oil mine also for years and no problems yet.  Worst case - take it all apart and clean the hell out of those ball bearings with wd-40, then clean it completely dray before reassembling.

In contrast - the upper 4 rods in the upper corners - they pass through the black blocks in the upper edges and those are simpler bearings - no balls - just a metal tube. That definitely needs oil. These rods rotate in the bearing *and* slide in the bearing so linear bearings will not work there.

Edited by Guest

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The think it's that linear bearings have oil inside when they are new. Like the 4x I bought from misumi for the bed. I suppose they have the 'right' amount? I don't know, I oil also the x/y hotend shafts since the start.

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