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Getting Better Dimensional Accuracy - Calibration

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There are a few other topics floating around that discuss accuracy and calibration of the printers, but my issue seems to be above and beyond.  First off, I am running a UM3 with 0.4mm Cores, mostly MatterHackers PRO PLA and PRO Nylon material.  The issue is that my parts are consistently 0.010" (0.25mm) over size on outside dimensions (OD/PIN), and 0.015" on inside dimensions (ID/HOLE).  I have spent a lot of time printing calibration cubes and if I print a single wall cube with 0% infill, my print comes out perfect - as in, +/-0.001 with the occasional 0.005 outlier - but I can live with that, this is a CNC machine.  However, as soon as I add multiple walls, the part grows (consistently) to +0.010 overall.  This is true for 2 walls, 3, 4, and so on.  I can print 6+ walls with the same result.  Since the single wall part prints perfect, I have to assume that my material calibration is correct - right?  Hopefully this describes the issue thoroughly enough.


To combat this issue I have tried:


-->negative horizontal expansion

This *sometimes* works, but some surfaces (typically inner diameters) and angled surfaces do not get adjusted, so they end up being oversized pins and undersized holes if the surface is not perfectly vertical.


--> outer before inner walls

this  helps, but does not seem to do enough


--> adjusting line width

I have 0.4mm cores, which default to 0.35 line width.  lowered it a few 0.01mm increments with no success.  This seems obvious to me since cura just shifts the shell thickness over and you are left with the same "over extrusion"


--> Model parts with 0.010" smaller pins, and 0.015" larger holes

works, but is pretty inefficient.  A lot of the jigs and fixtures I print are typically meant to be machined, but we have been trying to print them in order to save some machine time.  Not everything needs to be made from metal.  If we know this ahead of time, I can make sure the parts are designed with extra clearance, but this is not always so easy.


--> adjust flow rate

This is obviously not a good solution, but I tried it and its pretty tough to dial it in and of course underextrusion results in weaker parts


I want to point out (again) that I understand this is not a CNC machine milling metal and that +/-0.005" is asking a lot of a printer.  however, this parts are consistently oversized, so I feel that there is some tuning or Cura setting that could be changed to compensate for this.  Some part that require extra precision will be post machined with a single pass or something, not a big deal, but I dont want to do this with ever single part - kind of defeats the purpose.  


Sorry for the long post, but I wanted to be thorough.  Any suggestions are greatly appreciated.






Edited by Jakeddesign
added STL file

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So, diameter of the hole is approx. 0.5mm? Cannot help thinking I have translated your scaling factor incorrectly but maybe the hole is really small. I may be misreading you but to me you are saying in your 1st para that the hole has an ID 0.015" larger than the dimension (never heard of that before*|) but then later you talk about modelling the part with 0.015" larger hole.


Interesting post, which is why I wanted to try your model and see what I get - never printed a model with a single wall so you could be right!


Have you checked the calibration of your nozzle? Your post prompted me to recalibrate and my 0.4 nozzle is pushing out 0.45mm width

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I think we are not on the same page regarding scaling, so I have attached a screenshot of Cura with the part scaled to the correct size.  The dimensions should read 57.1x57.1x25.4mm.  The small hold inside of the part is 6.15mm in diameter, but measures 5.77mm.  To be fair, I am not too worried about this particular hole, but I am worried about the larger inner diameter shape on top of the part.  For some reason, the "Horizontal Expansion" option does not adjust this wall, so even though I can get the outside of the part correct, and the "square" hole correct, the upper lip does not compensate for some reason.


I have tested the nozzle by printing a calibration cube with 0% infill and (1) wall.  Then measured with width with calipers.  Width in Cura is set to 0.35mm and I measured almost exactly 0.35mm.


I am printing another test cube right now with the "Outer Wall Line Width" setting changed from 0.35 to 0.25mm just to see what happens.







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Awesome!  The dimension should be 1.852" or 47.04mm.


I am printing a boatload of calibration cubes this afternoon to try and see how exactly some of the settings affect the parts.  So I will post this information later on as well.  The attached "cube" is 1.500"  (38.1mm) square, with a square hole that is 0.600" (15.24mm).




test cube 2.stl

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Lol, I am definitely not having a good week; after all the calibration work, last night I got 2 hours into the print and it stopped extruding! Anyway it was worth it, I ran it again this morning and the results were...

Using two 0.4 walls at 30mm/s and 0.2 layer with 100% fan


OD - target 57.15mm       aborted run   57.15mm   2nd run  57.17mm

ID - target   47.04mm       2nd run  46.85mm (1.844")


So the ID as expected, in fact very good; I work on an error in the range of .2mm to .4mm for IDs so 0.19mm is probably the best I have seen.


Struggling with the cube in Cura15.04 so I will go back to 2.7  ( I went back to 15.04 Wed night to see if my dimensional errors were being caused by 2.7)


So for whatever reason I am not experiencing your problem (using circular geometry in my test). I.E. With multiple walls OD is perfect and ID as expected is smaller.


I will go through your posts again  and see if I can think of anything that could be causing your problem.

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Hi @Jakeddesign,


thanks for sharing your question here, let's see what we can do :)

I'm thinking @gr5 and @ultiarjan may have some good input here as well. 


In your model, is it consistently oversized in both X and Y dimensions, or does your model turn oval?

What version of Cura are you using?

I see you changed your filament diameter to 2.88, however I can't imagine that makes any difference for this.


Could you share your file as a project with your settings, so we can do a print and determine if we should look at the hardware or software?


- what happened when you reduced the outerwall thickness?

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Busy day today, but finally got around to posting.


@yellowshark I ran a PLA part today for better comparison purposes, pictures are below.  My OD numbers are actually pretty close, I measured 0.13mm oversized, but the part was 0.25mm oversized at the bottom. (indicating taper, I suspect this is plastic shrinkage).  The ID was measuring 0.4mm undersized.  I ran (4) walls, 20% infill.  I will try (2) walls on Monday and see if that changes it.  If I recal, I had printed some test cubes a while back that had 1, 2, 3, and 4 walls and I noticed that 1 wall measured perfect, and 2 walls was the same as 4 walls as far as oversized.  But I will try again on Monday.






@SandervG I would say it is more or less consistent, I do notice a slight difference between X and Y of less than 0.005" (.13mm) but maybe this is as good as it can be?  I have checked the belts, on Monday I will measure for square and report back.


I have Cura 3.2.1


Project file attached.


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Well it is good that your ID is smaller than specified; that is to be expected and .400mm may be par for the course, as I said I measured .190mm error which I thought was good! In your first post you said you were 0.015" oversized which really confused me.

Be careful measuring at the bottom as  the bottom layer tends to be squashed down for adhesion and will be larger than specified. How much depends on your machine setup, bed to nozzle distance, but I tend to have my distance quite small so it is always fatter at the bottom. If dimensional accuracy at that location of the part is important I will either back off and risk adhesion for a gain in accuracy or leave it as is and start filing. 


Bottom line is, in my view, that with a nice shiny new machine you should be able to get a lot better than 0.250mm (0.010") accuracy. If I cannot deliver 0.050mm or better I am not happy! (and I will engineer IDs to get that) my 3ntr printer is 4.5 yrs old.


I will  try and play with a variety of walls over the weekend and let you know what I find.

Edited by yellowshark

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Right on, thanks for looking into it.  I definitely suspect I am creeping up the capabilities of FDM printing, it just seems to me that it is consistence enough that software could compensate so I was hoping there was already a setting built in that I had not yet discovered.  I will update if I come across any other clever solutions.

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On 4/13/2018 at 10:51 PM, Jakeddesign said:

I measured 0.13mm oversized, but the part was 0.25mm oversized at the bottom.

What you see here could be a thing called 'elephant feet'; check out this guide for more information :)

Instead of filing, you could make the bottom slightly smaller, this should be pretty consistent and predictable. 


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