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Bad inside surface quality

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Hi everyone!

Im experiencing some troubles with my UM1

First off, i have holes which is oval, for an ø49mm part i get diameters between 48.3-49mm. I think that is a bit to much? anyone have experience here? My short belts are tight, and the long belts are also ok. They have only done like 30hours of printing.

Next off i get decent surface quality all around the outside, whilst in the hole down the middle i get this "laddering" or steps, as if the layers are badly shifted, what can be the reasons for this?

I think these problems are related somehow, but since my belts are tight, i dont think its backlash.

ive added the following pictures to show the problem.

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IMG_2504.thumb.JPG.3842b027a5df8fbe1e9ca2141c80d06c.JPG

Edited by Guest

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Hi,

What are your print settings?

There's a shrinking factor for PLA that makes dimensions less accurate than the designed model. This factor depends on the brand for PLA it can go from 1% to 5%, for ABS it can go up to 10% this could explain the difference you have in dimensions

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Sure as sure! ive taking shrinking into account when i designed the part, usually i go for about 1,5-2% increse in dimension to compensate for shrinking.

The problem is that it is not uniform, and the same error dont show up on the outside as on the inside?

ive been watching the print for some time, and a think it maybe has something to do with the "rotation" of print direction. The outside is always printed in one direction, while the direction changes on the inside.

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ive been watching the print for some time, and a think it maybe has something to do with the "rotation" of print direction. The outside is always printed in one direction, while the direction changes on the inside.

 

That was my first though: "I bet the difference between these layers is that some are clockwise and some are counter clockwise".

I think it's play. You can get play/backlash from too loose cables but also from too tight cables which can cause friction. Really any high friction can cause play. Play means when you command the head to move and then stop, the head stops a bit short - again either friction is so high that there is still a lot of tension on the belt even when it stops, or the belt is loose enough that the head doesn't move far enough. Other things being loose can cause play - a loose nozzle within the print head for example. Try pushing the nozzle around a bit when it's cold and power is off and see if it moves before the head starts moving.

Pluck the 6 belts (6, NOT 4 if you only found 4 belts keep looking). Listen to the pitch. It should be a very low pitch but it should have an audible pitch - so > 40 hertz. Usually it's the short belts meaning you might have to length the slots for the motors. Also consider locating some clothespins - you can remove the spring off the clothespin and that makes a great tensioner - that's what the UM2 uses hidden inside the blocks.

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Oh - and as for high friction - push the head around - it should be easy to do with the smallest finger on each hand placed on the side blocks.

Usually excess friction comes from endcaps too tight. The second most common cause is long belts too tight.

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If i grab the nozzle tip, it has a bit of play, aswell as i small click noise, comming from one of the linear ballbearings, so i have a feeling that the problem may be related to this.

My long belts wont produce a note anything like that, so they must be to slack, adding further to the problem. How do you apply the clothespin springs? i collect them when my plastic clothespins break, so got a bunch in spare!

All my axles got a bit of endplay, so how can the endcaps be to tight? I got sort of an ocilating feeling when pushing the head around, which seems a bit odd.

I unmounted the axles a while back to replace the drivebelts, and all axles rolls properly on a glassplate.

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It's actually not shrinking, but the fact that the nozzle drags the filament string inwards when making circles. The smaller the circle, the more this effect occurs. *

It's quite possible that clockwise and counter-clockwise circles drag differently, which leads to the rippled surface.

The outside surface is clean, so it is not a "mechanical" problem with the printer.

There is also another effect, which I believe to be a bug in Cura or Marlin. I get these "partially shifted" layers, too. It can't be a mechanical problem, because it never affects a full layer, but always specific parts of the print over several layers. I wanted to try another slicer to see if the result would be better, but I've been to busy to do that until now o.O

*: A thought that just came to mind: Maybe the effect can be reduced by slightly increasing flow. The dragging suggests a slight amount of underextrusion...

Edited by Guest

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The outside surface is clean, so it is not a "mechanical" problem with the printer.

 

I disagree.  If you have play in your system and you draw a circle and approach from the inside you will get identical circles every time (slightly elliptical if play is only on one axis - either way the circles will be smaller than commanded). And each "circle" will be on top of the one below it perfectly regardless if you go clockwise or counter clockwise and where you approach the circle as long as you approach radially.

However if you aproach the circle from the outside then you won't get round circles - you will get these "shelves" and if you approach from different directions or rotate clockwise versus counter clockwise in combination you get different "shelves" on the inner circles.  It's exactly as expected. In fact if you approach from outside radially and then make a complete circle you won't end up where you started the circle.

Edited by Guest
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If i grab the nozzle tip, it has a bit of play, aswell as i small click noise, comming from one of the linear ballbearings, so i have a feeling that the problem may be related to this.

 

Well look at those "lines" inside your part and how deep they are (not very - maybe 1/4 mm?) and if the play in the nozzle is the same amount then that's probably it.  I've never felt what you describe - there was someone else who had a similar problem - nozzle was loose.  He was able to fix by just rebuilding the head and tightening up a few things.  The bearings in the head are linear only so they use ball bearings and maybe you lost some.  You could order new ones.

The bearings in the blocks however are simpler - just a tube - no ball bearings.  Because they have to allow rotational *and* linear motion.

Edited by Guest

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