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Making FDM printed dimensions match CAD dimensions


Adam324
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Posted (edited) · Making FDM printed dimensions match CAD dimensions

I have been racking my brain for months trying to deal my prints always being about 0.2 mm outside to outside or so bigger and hole diameters around the same smaller than what the CAD/STL dimensions are.  This doesn't always matter but for designs that need to fit real world dimensions it can be an issue.  Ideally all parts printed would be the correct dimension for holes and outside dimensions as designed in fusion 360 and allowing the design to be used for 3d printing and other real world uses without needing to compensate for slight printing over-extrusion on the outer walls and skewing your design sizes.

 

Most people seem to calibrate a 100 mm by 100 mm outside to outside dimension object.  That is not ideal though as hole dimensions will be smaller when printed as the outside to outside measurement assumes that a line is exactly 0.4 mm when printed on the outside wall which is not correct in my testing and experience.

 

To calibrate the X and Y dimensions, I print a 90 degree L shaped design that is 4 mm wide by 110 mm in X and 110 mm in Y length.  I designed some lines on top of the surface that are exactly 100 mm apart in both X and Y dimensions.  For a perfectly calibrated FDM printer, If you were to measure from outside to outside of those raised bumps, you would be measuring slightly over 100 mm because the measurement should be 100 mm to 100 mm center to center.  I don't expect a 100 mm by 100 mm cube to be 100 mm to 100 mm outside to outside because that assumes that wall lines are exactly 0.4 mm when printed which isn't the case with FDM printing.  The outside wall line doesn't have any filament to push up against on the outer side so it expands more than 0.4 mm.  You could try and adjust the line width to be exactly 0.4 mm but that would not work well at all.  You wouldn't fill the voids between filament lines enough to have good adhesion and the lines would be more visible.  From everything I read, It seems that it is normal to have a single 0.4 mm line width to be bigger than that.  Because of this I calibrate to be 100 mm center to center of the lines on a 110 mm length print.

 

I have tried to compensate using a few methods for the line width on the outside of the outer wall being slightly bigger but none of them work for all scenarios or they require extra design adjustments to make up for it.

 

1. Designing all my stuff with a around 0.2 mm larger holes and 0.2 mm outside to outside (0.1 mm per side) smaller size for non hole outside faces.

 

2. For already designed STL files that don't have FDM tolerances built in, scale the part size bigger if the holes are smaller than actual dimensions.  For outside dimensions that need to be the correct precise size, I scale the part smaller or do a lot of sanding which sometimes is not possible if the design is very complex.  If you need both holes and outside dimensions to be accurate you are in trouble as you have to pick on or the other and do sanding if you scale it bigger or drilling out the holes.

 

Cura's Horizontal Expansion can do the same thing as scaling the size of it in the slicer but either of those do one of 2 things... either fix holes and expand the outside dimensions or...  fix the outside dimensions and make holes smaller.

 

I finally found the setting in Cura that does exactly what I need.  It moves the outside wall (realize that the inside of a circle is also an outside wall) toward the inside wall by a certain amount.  This allows me to have an exactly calibrated 100 mm to 100 mm line (center to center) and adjust for the line being a little over 0.4 mm causing the outside wall to be over-extruded by somewhere around 0.6 to 0.1 mm per side (Depending on your printer and filament size setting accuracy and flow settings).

 

The setting is called 'Outer wall inset'.  You give it a positive value to move the wall inward toward the inner wall by this amount.  I set it to around 0.1 mm on my prints to get the outside dimensions and hole dimensions to be exactly how I designed them in Fusion 360.

 

For most prints from thingiverse I leave that feature disabled because most designers have tolerance built into prints to make them fit together.  There are some that are not though like some sockets I found on thingiverse.  For my own future designs, I am designing them to the actual real world dimensions that I want and then use the 'Outer wall inset' to adjust for line width being slightly bigger on the outside wall's outside facing surface.

 

Hopefully this makes sense to someone as I am not the best at trying to explain things...  I was very excited to find this setting though because I have searched on this issue for a long time and never found anything mentioning using this setting for this purpose.  The only thing I found on 'Outer wall inset' seemed to indicate it is used to make up for setting a line width smaller than the nozzle size (which Cura automatically adjusts) to get the outer wall to overlap the inner wall more.

 

Then again... I am relatively new to 3d printing (about a year or so)... I have certainly been wrong many times in my life and been in many situations thinking I understand something when I really don't :).

Edited by Adam324
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  • 3 years later...
Posted · Making FDM printed dimensions match CAD dimensions

Thank you for posting this! I was using Horizontal Expansion compensation until now (nearly 4 years) but that would make my thin walls disappear, even with the 'Print Thin Walls' checked. Now I can have the best of both worlds.

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