How to model and print with accurate dimensions?

[kmil81 3]

I’m relatively new to 3D printing and very new to Cura. I’ve modeled some components which have an inner dimension as well as an outer dimension (think a tube / cylinder with a hole in it. I’m finding that the dimensions I model in fusion 360 do not match what the printer is putting out. I suspect that it’s off by approximately half the wall thickness specified in Cura but haven’t conclusively determined if this is the case.

 

What is the best practice for modeling and printing dimensionally accurate components? The two things I can think of are:

1. Dimension the part in fusion 360 with inner dimensions larger than desired and outer dimensions smaller than desired to account for wall thickness 

OR

2. There are some settings in Cura I don’t know about that will account for wall thickness to preserve the model dimensions. 

 

I’ve seen some posts saying that single walled (solid) models come out accurate but double walled (like what I’m trying to do) do not. For reference, I have an Ultimaker 3 using Ultimaker tough PLA for most of my prints. 

 

So what is the right approach to this? Address in the model? Address in Cura? Both?

 

Thank you for your help!

Edited July 4, 2019 by kmil81
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[gr5 2,270]

Cura knows to offset the lines by half the nozzle width.  So that's not it.

 

However vertical holes are always printed too small.  Add around 0.4 or 0.5mm to every vertical hole diameter in CAD.

 

Also other things you can do to improve other dimensions:

Make all printing speeds the same and around 35mm/sec or even 25mm/sec for super accurate.  Disable jerk and acceleration control (this will make ringing worse but will make dimensions more accurate).

 

 

[SandervG 1,521]

Settings in Cura which you might want to experiment with when you're printing with PLA / Tough PLA: 

Line width: 0.4
Wall thickness: 1.2
Top/Bottom thickness: 1.2
Speeds: 40
Jerks: 20
Horizontal expansion: -0.03
walls: 3

[kmil81 3]

Thank you both for your help. Your info dialed in the outer dimensions for me almost immediately. It looks like the key for my inner dimensions appears to be playing with ranges of negative values for horizontal line expansion (and perhaps getting a smaller nozzle since I’m printing very small parts).

[johnse 31]

I’ll echo what @gr5 said: design inner holes with a diameter approximately 0.4mm larger.

 

For example, I am attaching PLA and/or Tough PLA to acrylic sheet cut on my Glowforge with M3 screws. I design in Fusion 360 with diameter 2.8 to account for 0.2mm kerf from the laser and 3.4mm in the PLA part. This is usually a snug fit. I could probably go to 3.5 if I wanted the bolt to slide easily.

 

In general, if I want a printed part to slide inside another, I will design about 0.2mm clearance for a smooth sliding join. If I want a tight fit I might use 0.15mm.

 

most of these parts are under 40mm in longest dimension. Many much smaller.

 

All of these dimensions came from trial and error. Also, first layer squish will make the hole smaller there. Do test measurements on a hole opening further from the bed. It’s easy to open the first layer up to match the rest with a small knife.

 

I’m using 0.4mm nozzle with 0.1 layer height.

 

i do notice that horizontal holes in vertical walls tend to be about 0.1 mm larger than the vertical holes.