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Getting the right dimensions

Posted by LordGG 
Getting the right dimensions
April 18, 2014 08:39AM
Hi all,

I am playing with my Mendel90 Sturdy since a few weeks now. I have printed many calibration objects such as towers, thin walls etc. Print quality is great.

2 important parameters : I use Slic3r and I print PLA.

When I finished building the printer, I tried to print a 20 mm cube. I was surprised to get a perfectly dimensioned cube. A few days later, after calibration process, I printed again the 20 mm cube, Z size is OK, but for X and Y, it is not anymore :/ I have now something like 21.20 mm on Y axis and 21.50 mm on X axis. I think the cause of the change (from the first cube I printed) is because I had to tension better the belts : X and Y axis’ belts were not tensioned enough. It increased print quality but somehow, changed scaling.

I have been reading the forum and found that people were saying that they could get right dimensioned prints only with SkeInforge. I decided to give a try, using “0.3 PLA” profile from nophead. At the beginning, the extruder was not working, I figured out “Activate dimension” was not ticked. Then, I started to print. Sadly, I have spent nights wasting plastic, after a few layers, corners lifted and the printed part came off the bed. I have never faced that problem with Slic3r or KISSlicer. However, I have noticed that GCode generated by SkeInforge never turns on the extruder fan. Could it explain the unsticking issue ?

Anyway, I gave up with SkeInforge (for the moment), and I decided to try KISSlicer. First print was OK. I tried the “Scale by X” function, hoping to solve my dimensions problem. I could get perfect X and Y dimensions… but the Z axis’ scale was also affected sad smiley

Then, I have tried Netfabb, since I read it can scale an object (STL format) on X, Y and Z dimensions independently.

I could finally print a cube with the right dimensions.

What do you think of this process ? Is it reliable to use Netfabb to do the rescaling for printing parts that need precision, like Wade’s extruder gears ? How do you print the parts with perfect dimensions ? Are you all using SkeInforge ? Have you been facing the same issues ?

Thanks for reading, 'hope I wasn't too long winking smiley
Re: Getting the right dimensions
April 18, 2014 09:15AM
I use Slic3r almost exclusively and my parts are normally within 0.1 or 0.2 of the given dimensions. One issue I discovered when I first calibrated my printer is that I over tightened the belts. They shouldn't be loose but they also don't need to be so tight that you could strum them like a guitar. This can cause the motor to skip.

If it's not the belts, the next thing I would try is setting the extrusion width in Slic3r. The setting is under Print Settings > Advanced. Go to the URL below and plug in your layer height, nozzle size, etc and one of the values it'll give you is the expected extrusion width.


Here's a couple widths just to get you started:

0.4 nozzle at 0.2mm layer height = 0.48mm extrusion width
0.35 nozzle at 0.2mm layer height = 0.45mm extrusion width
Re: Getting the right dimensions
April 18, 2014 02:07PM
Hi lions3,

Thx for your reply ! I have untightened both belts and it is much better : 19.9 mm on X instead of 20, and 20.6 mm on Y. The Y axis' belt is indeed a little bit more tensioned, I'll try to loose it just a little.

I spent so much time trying to solve this issue with softwares while I just needed to untight 2 screws, haha grinning smiley

Again, thanks for your help smiling smiley
Re: Getting the right dimensions
April 19, 2014 02:51PM
If SkeInforge didn't have the Dimension tab enabled and didn't run the fan then you are not using the settings I provide.

Re: Getting the right dimensions
April 22, 2014 05:02AM

@nophead > It is weird, I have checked again, I was using your profile named "PLA0.3". But I agree, when I read the file "dimension.csv", I can see the "Activate Dimension True", so the problem is on my side... I was using "latest" version of SkeInforge. I have started everything again, this time using the "Windows portable version" of SkeInforge "sfwin43-nopsyco.zip". The dimension tab was activated, skirt was here too... I thought it was all fine, but the extruder fan is still not working sad smiley Maybe I'll give a try with "sfwin43.zip" version, but I am not convinced.

For now, I can't get better dimensions than 19.9mm x 20.6mm (instead of 20x20), without "cheating" using Netfabb. I am still investigating.
Re: Getting the right dimensions
April 22, 2014 09:25AM
You can get the version of Skeinforge that we distribute from my Github: [github.com]. That is the only version that will match the setting we distribute as the fan control has been extended.

What sort of belts are you using? It seems belt tension affects your dimensions far more than normal. The ones we supply have steel wires in them so don't stretch much. You can correct for it by changing the steps per mm in the firmware. You have to separate the error caused by flow rate from the error caused by the travel and shrinkage. You can do that by printing an L shape and using simultaneous equations.

Re: Getting the right dimensions
April 23, 2014 05:07AM
@nophead > It works much better with the right version of Skeinforge winking smiley The dimensions were not perfect, but the print was clean, with the extruder fan on.

I am using the same kind of belt as the one you supply, I think.
For Y axis :
For X axis :
They also have steel wires in them.

Last night, I succeeded printing with the right dimensions. The 20*20*10 box came out perfect. With bigger objects (about 10cm), I lose a bit of precision (about 0.5 mm). It is not 100% right, but it is still much better than it was. I have measured the actual distance the axis move and changed the steps per mm in the firmware. Z was fine, I just changed X (80 => 80.6) and Y (80 => 78).

I saw in other posts of you, that the choice of pulleys could explain some issues. One thing I omitted to tell is that I use metal pulleys. They are from Ebay/Germany, but I can't be sure they are not actually cheap pulleys from HK. I bought these :
Re: Getting the right dimensions
April 23, 2014 11:55AM
The kits I supply have T2.5 PU and steel belts but I run machines with T5 steel and Kevlar wires as well.

The pulley doesn't affect the steps per mm if it has the right number of teeth. Poor quality ones do cause backlash though which can make the outside dimensions off.

Re: Getting the right dimensions
April 23, 2014 08:20PM
I had changed the X axis belt, I have just replaced the Y axis belt as well, which had suspicious torsion marks. It has strongly increased the precision over Y axis. I suspect my first belts were poor quality (the new ones are also from eBay, but different seller). I have slightly tweaked the steps per millimeter in the firmware, it is all good now !

Thanks for your help smiling smiley
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