Printing Accuracy

[douglaskeene 0]

Hello I am relatively new to 3d print but i am using Cura to slice my models from sketchup to print on my new anycubic mega x printer. I am trying to print smaller objects of higher precision to coincide with real-world parts, so I am most concerned with settings that will make my 3d output as close to the original sketchup as possible.

 

In my first example, i am printing a 26.0 mm disc with an inset disk at 22mm. Once printed, the outer is 26.0 and the inner is at 21.4mm measured with digital micrometer. THere are many settings which might effect this, should I adjust inner hole expansion with a negative value? I really am most concerned with getting output that approximates the sketchup dimensions.

 

I don't expect it to be perfect, due to warping, shrinkage, expansion, etc., but just to get as close as reasonably possible so that real world parts will mate with what I print!

 

Thank you

Doug

[JeroenR 2]

Hi Doug,

 

I am having exactly the same problem, I am printing a housing for an electric motor. It is essentially a 30x30x45mm block with a cylinder hole with a diameter of 25mm. The print has exactly the correct outer dimensions but the inner cylinder comes out as 24.3mm. The STL file, upon inspection indeed shows 25mm diameter, so it is a slicing or printer settings problem.

 

I have noticed this as well with other cutouts for panel meters, when I measure the size and use that in whatever 3D modeling package, the printed cutout is too small en I have to experimentally adjust until it fits. 
 

I do not understand whether this is a cura of printer settings feature. A friend who is printing the same STL files with PrusaSlicer and Prusa does not suffer from this feature, the models come out as per the dimensions in the STL file.

 

I have not been able to fix it to date and  appreciate insights from people who may have seen this before and know how to fix.

 

Best regards, Jeroen

[gr5 2,173]

This is normal, expected, and common in FFF printing.  I don't know how Prusa slicer gets around this issue but I suspect they have a parameter (on by default?) like this one in Cura: hole horizontal expansion

 

I typically get all my vertical holes about 0.4mm too small.  Regardless if they are 25mm in diameter or 2mm in diameter.

 

The problem is that PLA sticks to itself as it prints, like snot or mucus.  As it prints the inner wall it is stretched like a liquid rubber band (it's tight because the PLA also shrinks as it cools in the first milliseconds out of the nozzle).  This pulls inward and makes vertical holes smaller than desired.

 The best solution by far is to just fix it in CAD.  Note that the outer diameter can shrink also but not as much as the rest of the part supports that outer wall from shrinking.
 

All manufacturing techniques (milling, FFF, SLA, injection molding) require that you fudge things like this in CAD.  Some people who do injection molding don't know this as the "factory" takes care of that step for you.

[JeroenR 2]

I find this a patronising response,  the difference is far greater than 0.4mm and the fact that other (cheaper) slicer/printer combinations do not suffer from this feature is telling. I would have expected and appreciated  from Ultimaker or the moderators a more deliberate and comprehensive response.

 

BR, Jeroen 

[GregValiant 1,251]

@JeroenR if the hole is vertical then have you used the "Horizontal Hole Expansion" setting?

I printed what was essentially a hole chart with holes from 1mm diameter to 30mm diameter.

Horizontal Hole Expansion was required to get the holes to their correct size.  I found that the larger the hole then the smaller the amount of HHE that was required.

Try enabling HHE at 0.3 and print a few layers of your motor housing.  You just need enough of a print for the caliper to get a good measurement.

Be advised though that a hole is a hole and that one doesn't necessarily need to be round.  If you have a vertical hexagon shaped pocket for a nut then HHE will make that shape bigger as well.

The HHE setting is there expressly to compensate for the phenomenon that @gr5 explained.  The "Snot Factor" is real and is dependent on the material, print temperature, and the size of the arc being described by the nozzle.

PrusaSlicer does have a setting for Horizontal Expansion but not a specific setting for Horizontal Hole Expansion.  It might be handled in the background rather than being adjustable by the user.

[gr5 2,173]

I didn't mean to be patronising.  I tried to be informative and helpful.  I explained the phenomenon and gave you two suggested solutions.  Sorry.

[tomnagel 126]

if you are measuring the size of the hole on the bottom surface, you also have the elephant foot that makes the hole smaller.

[JeroenR 2]

Thanks for all the comments and suggestions, very helpful. Indeed the horizontal hole expansion factor is one to the settings that mitigates the problem. What helped me most is to use the Engineering Intent Profile where settings are optimised for precision (Ultimate Support pointed me in that direction: Intent profiles in Ultimaker Cura ). Unfortunately this Cura software option is only enabled when selecting some of the Ultimaker cores and only with Ultimaker filaments, so I had to select other cores, select original Ultimaker filaments and then the Engineering settings became enabled. I saved these settings as a Custom Profile and used this for the print I wanted to make with a different filament. This proved to be almost successful, I had to give it a very small additional HHE adjustment and now the electric motor fits snuggly in its housing. 

 

I learned a lot these past few days and once again thank you for your contributions.

 

BR, Jeroen

Edited April 24, 2022 by JeroenR

[GregValiant 1,251]

Very nice.  Now you just need to cross your fingers and hope that it doesn't get hot when under a load.