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Dimensional accuracy

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Posted · Dimensional accuracy

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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Posted · Dimensional accuracy

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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Posted (edited) · Dimensional accuracy

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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Posted · Dimensional accuracy

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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Posted · Dimensional accuracy

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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Posted (edited) · Dimensional accuracy

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 -0.16 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

Edited by LaserBrain
Changed typo on settings numbers
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Posted · Dimensional accuracy

Hi,

I have the same Problem.

what Material do you use ?

how can i put in -1,6 I can only write whole numbers in the field or 0,16 not bigger than 1

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Posted · Dimensional accuracy

Settings are custom once I had changed the Horizontal Expansion settings, but based on normal, speed at 30mm/sec. It's slow but accurate and the print quality is good. 

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Posted · Dimensional accuracy

Great! just what I need. Now I will be able to get rid of all the 0.2mm adjustments in my designs using this info.

Thank you

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Posted · Dimensional accuracy

Did you also manage to get the inner and outer diameter of a tube at the correct size?

Giving the horizontal expansion a negative value increases holes and decreases the outside diameter I discovered.

Now the inner diameter is ok but the outer diameter becomes too small.

How can I independently adjust the inner or outer diameter?

 

Regards, Michel

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Posted · Dimensional accuracy

Hi, perhaps you could try the following settings with (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

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Posted (edited) · Dimensional accuracy
2 hours ago, MichelLin said:

Did you also manage to get the inner and outer diameter of a tube at the correct size?

Giving the horizontal expansion a negative value increases holes and decreases the outside diameter I discovered.

Now the inner diameter is ok but the outer diameter becomes too small.

How can I independently adjust the inner or outer diameter?

 

Regards, Michel

@MichelLin I did, fiddling with the horizontal expansion settings was certainly the main one for me.

The problem for me was that my holes were too small and the outer dimensions too big, just as you described, so it was an instant win for me. Give @SandervG settings a go too and let us all know what you find. 🙂

Edited by LaserBrain

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Posted · Dimensional accuracy

This is a pretty old thread, but perhaps people are still watching it. I have a couple of S5s, and I'm printing really big parts on it that need to mate with parts manufactured on other machines (CNC mill, laser cutting, etc). The problem I am having is that I am getting large enough errors in the X-dim to make parts not fit. I have a part that is supposed to be 295mm in length, but it is really closer to 294mm. As a percent error, maybe this doesn't seem horrible, about 0.33%, but I have bolts that need to attach the printed parts to other parts, and on an M3 bolt, a 1mm misalignment is not workable.

 

Is there a way to perform x,y,z scaling in the printer itself to correct for measured scaling errors, or do I need to manually do this in Cura? It seems like it is really a mechanical issue in the printer, as this would appear to be imprecise math on the motor steps to mm travel. I suppose it is possible that it's print shrinkage or something, which would honestly be more frustrating since that would likely be part and material specific.

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Posted · Dimensional accuracy

Hi Jones4642,

Happy to pick up the thread again. The only way I managed this was to do this in Cura. It’s a bit trial and error, but I used a cube and an open ended cylinder to measure external and internal diameters. Then adjust, print, adjust, print until you’re happy. 
The horizontal expansion settings apply to all polygons in the print, the layer one setting can help reduce elephants foot, also helps when fitting parts. 
it just takes a little time, patience, a reasonable set of callipers to measure with. 
Good luck. 

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Posted · Dimensional accuracy

Thanks for the quick feedback. Does the expansion primarily control wall thickness? From the discussion above it seemed like a method to get the inner and outer dimensions of a part to match desired values better, but not necessarily adjust bulk scaling issues. If I understand it right, if I printed a 1cm square box, and got an outer dimension that was a little too big and an inner dimension that was a little too small, then a slightly negative value of the horizontal expansion could take care of this. However, it's also my impression that if I printed a 200 a 200 cm box, that the actual error (not the percent) error should be about the same as the 1cm box, and the same horizontal expansion adjustment should work.

 

In my case if I print a 1cm part, and measure it to be 0.997 cm, and then print a 295mm part, that measures as 294mm, would horizontal expansion somehow scale up with the part size and fix this? I'm guessing not, and that I really need to multiple x by 1.0033 in Cura to get the right sized part.

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