All the issues above are true and in addition you may hit the end stops early if you print something 205mm long in X or Y.
So best to scale in Cura.
There are many other accuracy issues - the biggest issue is with vertical holes that is caused by 3 factors (shrinkage is only 1 of the 3 issues with holes). The best solution in this case is to print twice - after the first print measure everything and then adjust the cad file by the error.
With experience you can get it right the first time in CAD for your particular PLA and print settings. But simply changing the qty of edges in a circle in your CAD will change the size of the hole and there are other issues that you have less control over.
Recommended Posts
anon4321 16
I think the recommended approach would be to use Cura to rescale the object in the XY direction. See "Scaling your object" here: https://www.ultimaker.com/spree/uploads/38/original/Cura_User-Manual_v1.0.pdf
The firmware has a mapping of (stepper) steps per unit distance (mm I guess). However, I've seen in another post that the UM person (Daid or possibly SandervG) recommended not changing them.
If you really want to change them, I think the three ways to do so is:
Through the ulticontroller if you have it
Via the start gcode using (I believe) M92
Modifying the firmware defaults and rebuilding it.
I think the default steps per mm is correct but the difference you see is due to shrinkage. I think the reason why they recommend you don't change the steps/mm is because different materials or the same material from different batches or vendors will shrink a different amount.
Also, the issue with any of the three ways above is that you cause the XY axes to move every so slightly more than Cura expects when it calculated the volume of material to extrude. So you will have an ever so slight underextrusion. When you scale the model, Cura adjusts the extrusion rate as required.
So try scaling the XY axes with a factor of 1.018 or 1.019 and see if the size measures up.
And in case you are wondering, I think the reason why the Z axis is accurate is because as the layers shrink, the layer above is constantly compensates for the shrinkage when it is extruded. Until the last layer which in and of itself doesn't shrink a measurable amount. However, in the XY direction, no such compensation occurs.
Link to post
Share on other sites