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Mónica

Dimensional accuracy

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hello, 
I have recently been using ultimaker 3 extended and cannot solve dimensional accuracy problems. 
In all the pieces I have an error between 100 and 300 microns.
How can I solve it?

Thank you

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I don't believe you will achieve much better size accuracy than that.  Many of my parts are prototypes for pieces that will be milled from steel.  Trying to replicate the accuracy of milled pieces pushes the limits IMO.  To help, I often print several trial pieces, adjust my scaling independently in X,Y and Z attempting to compensate for uneven shrinkage - while keeping other variables controlled as much as possible.  Sometimes, I edit (scale) the individual features of the model independently.  Uneven shrinkage is hard to predict.  It can differ due to uneven cooling, part shapes of varying section thickness, infill interaction with the shell...  When I have a position or size that must be held, I print slightly large in that area and bring it to size with secondary operations; sanding, filing, milling, drilling.  For rectangular pieces, one of my favorite tools is a piece of polished marble tile with a sheet of 220 sand paper on top.

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Hi, lots of good points there from @mastery. I do think you can achieve a better result than100-300 micron though; it will though depend on the model and its geometry. Also bear in mind that circles will always print smaller and you need to adjust for that in your design. You could consider the following.

 

1. Speed is the killer so slow right down; probably no more than, 30mm/s.

2. Print cool; get your extruder temp.  right down until it under extrudes then add 5 degrees

3. Oh you do not mention the material you are using; I am assuming PLA

4,. Environmental temp.; try to have it warm, at least 20c, 25c could be arguably better 

5. Z-gap; if you really have your 1st layer squashed down for excellent bed adherence then this may well affect your z-axis measurement. It might be possible to adjust the design dimension to compensate. Alternatively you may need to adjust the z-gap distance a bit to give a bigger gap.

6. At end of print, leave the model in situ and close your printer door and cover top and leave you model their until the bed tem has reached the ambient temp. I.e. go away and have a coffee and cake and leave the model to cool down as slowly as possible.

 

I can normally hit better than 50 microns and guarantee 50 microns. Of course for a prototype I will advise the client that if he is happy with 100-150 microns then I will increase the print speed which reduces the price.

 

Edited by yellowshark
Add point on filament
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Oh and also it may be worth trying a different filament; possibly just a different colour from the same manufacturer. I have not done any serious research on filament vs dimensional accuracy but I do know that certain filaments print better than others (from the same manufacturer) and if accuracy is key then I will use one of three filaments from the 20-30 I have tried over the years

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Like yellowshark says, slow down your prints. I recommend manually setting the infill and wall speeds to the same speed for everything; if you let it auto set the speed based on the "print speed" setting it will set variable speeds for different things (inner wall/outer wall/infill). Setting a constant speed helps avoid pressure build up.

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I have got my dimensional accuracy pretty much spot on.
I created a 20mm cube and 20mm cylinder (1mm walls) in fusion 360.

I then printed both and measured the dimensions, the cube was too large and the internal diameter of the cylinder was too small.
Pretty much summed up my fitting issues with multi parts.

I then changed the horizontal expansion setting (inside the Shell menu) to a small negative number and printed again.

Still out, I repeated this and ended up with a -01.6 setting.
I slowed down my print speed to 30mm/sec, 0.15mm layer height.

I then combined the cube and cylinder to form a complex shape to see how the printer handled that and to see if the dimensions were preserved, they were!

 

My parts now print super well and fit perfectly. Attached are some images and the STL files for Cura. You will have to rotate the Cylinder and combined shape so they sit the right way up. The Cylinder is open one end so you can measure the walls accurately. 

Hope this helps.

IMG_1730.jpg

IMG_1731.jpg

IMG_1729.jpg

IMG_1727.jpg

IMG_1726.jpg

20mm_Cube.stl

Combined_Cyl_Cube.stl

Cylinder.stl

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