Circles ar more like elipses. Fighting with backlash

[edis 0]

I am expierencing some problems with my axes, thought maybe i can get some suggestions here.

For some time now i just can't get my prints to be correct. i think i have some problems with a backslash. When i print square part with a circle hole inside, it's dimensions are different in X-Y axes. I print part with dimensions X50xY25mm and a hole inside of 10mm. The final dimensions are: X49.65, Y19.5. Hole diameter in X is 9.8mm and in Y it's 9.4-9.5mm. Also the circles around holes and fill has some gaps. So i'm guessing it's a backslash problem.Here is my test piece:

Reassembled both axes completely, retightened all pulleys, checked belts. Still nothing. And it seems that the belt tension isn't so important, because if i tighten them normaly i get same results compared to really tight, when it's even to hard for steppers to move.

Anyway, i used dial gauge to measure my backlash. First, for the reference point i measured Z axis. Results are quite interesting...

It looks that i get backlash even with the Z axis, which is moving smoothly. I suppose it's because of the steppers and my ballscrew. E.g. i move stepper forward, then i command to move 1mm back with a feedrate of 50. So instead of 1mm i get only 0.83-0.87mm. Are therse numbers normal?

Next i've measured X axis. The belt i'd say wasn't very tight, you can hear a sound by taping it, but axis moves really easy and smootly, without any bumps. (if i tighten them more, i get runout by eccentric pulleys, then by pushing axis i can feel force difference, like half rotation it's easy and half - hard.) Here are the results:

Here after moving axis forward and then giving a command to move back by 1mm at 50 feedrate, i get movement of 0.74-0.86mm. Also for some reason all my measurements are a little bit smaller than given number, e.g. if i order to move 1mm it moves 0.93-0.99, but for this i'm thinking that my dial indicator wasn't horizontal.

Soo my question is, is this normal for X axis? After this test, i've tried tensioning all belts for X axis, but results were not improved, backlash decreased by 0.02mm, but then it was really hard to move the axis by hand. the movement was like vvvvVVVVvvvvVVVV, the force to move axis also then varies and the motor starts slipping.

i haven't yet measured Y axis, since my dial indicator fitting doesn't fit and my dial indicator is too high to hit the wall of my printer.

 

[owen 18]

Hi

You want your belts tight but definitely not tight enough to require much more force to move the extruder by hand. It doen't look like a backlash problem to me. There doesn't appear to be missing steps and your photo doesn't look that skewed. Maybe try using Daid Marlin builder and reburn your firmware from scratch. Also try printing the 20mm test cube from Thingiverse and see what you get.

Owen

 

[Daid 297]

Your photo shows signs of under-extrusion. The 2 lines of the outside walls are not connecting. So most likely that's the cause of the gap you are seeing.

 

[edis 0]

hmm, that's an interesting thought. I've calibrated my extruder some time ago, will try to do it again.

But that still doesn't explain the X-Y dimensions difference. Or is there a chance that under-extruding can make parts not round? I doubt that..

 

[snowygrouch 1]

To see about oval holes, you need to print symetrical parts. So a square with a hole in the very middle, they

will shrink after printing as they cool, and if the parts is asymetric even if it was printed circular, it can shrink

to be an oval.

So print again with a symetrical test part, and remeasure the circle.

As for the backlash, its inevitable. You have 2 belts in series, compounding any backlash - neither belt

has any automatic tensioner, and its almost impossible to set the short belts up perfectly without printing

extra tensioning devices. So I would say with respect to backlash, its inevitable. You can only minimise it by careful

adjustment of the belt tension.

I have a little set of scales I use (that are supposed to be for checking the weight of your bag for airline travel), I set it - in the middle of one of the long belts and pull down until it reads 1kg.

This gives a deflection of 13mm. So set your long belt tension to give that result and that will be good. The short belts are another matter, but I dont have them anymore so dont know much about optimising them - apart from "pull down hard then do the bolts up again". The standard bolts for the XY steppers only engaged by about 2 threads into the stepper which also doesnt help. Perhaps they are now shipping with longer bolts, I dont know.

Test your belt tension as suggested, then print a symetric test piece...then we will know more.

C.

 

 

[edis 0]

So, i've tested a symetric part. But before that i've checked my extruder it was extruding too much. now after correction my E steps per rotarion is 794.39....Now it really looks much better at the corners. Here are the results

However, the inside diameter should be 30mm. When measuring it varies from 29.6 - 29.9mm. I'll go though my Y axis with dial indicator and check what it shows... This is quite annoying

 

[snowygrouch 1]

Print a much bigger one to get a better idea...

The deviation caused by measuring little bits of fluff and inevitable stringly edges as so on

are a large percentage of your overall diameter at 30. Also would be interesting to see the size of the

external diameter too !

Try 80mm Diameter and it should be a bit more obvious. The first few layers are also subjected to the worst

warping effects. So something a few mm higher will also make it clearer.

Probably if you want much more prescision that that, you will have to look into doing some modifications to the

belt drive system...well thats my opinion.

C.