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mikey

Calibrating U2

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Is there a way to tune the U2 to create more dimension-ally accurate prints or is it set from the factory? I think my x axis was a little off when running the 5mm test print

 

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How little is "a little"? Generally Ultimakers are pretty damn accurate mechanically when set up correctly. However, shrinkage of the parts can cause issues if you need super accurate parts. Different parts will shrink differently depending on their shape, what plastic is used, temperature, infill etc etc so if you need super precise parts you'll need to tweak the parts in CAD to get there (holes for example are notorious for becoming smaller than designed).

This isn't unique to this type of printer, it happens in the big boy world of injection molding as well. I visited a place like that about a year ago and the boss showed me a part where they had to set the bend of the part slightly too big so that it would become correct as the part cooled down after being ejected. Crazy stuff :)

 

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How little is "a little"? Generally Ultimakers are pretty damn accurate mechanically when set up correctly. However, shrinkage of the parts can cause issues if you need super accurate parts. Different parts will shrink differently depending on their shape, what plastic is used, temperature, infill etc etc so if you need super precise parts you'll need to tweak the parts in CAD to get there (holes for example are notorious for becoming smaller than designed).

This isn't unique to this type of printer, it happens in the big boy world of injection molding as well. I visited a place like that about a year ago and the boss showed me a part where they had to set the bend of the part slightly too big so that it would become correct as the part cooled down after being ejected. Crazy stuff :)

 

I will have to run another test to get some better numbers then, because I had some temperature and under extrusion issues at the bottom of the print. But ignoring that I assume its not possible to change anything to improve it after I find my temperature of the filament

 

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It's possible to change the steps-per-mm values for the axes, but it shouldn't be necessary. Different geometries might print/shrink slightly differently, but the for any one model, the results should be very consistent. As Robert said, generally you need to tweak the source files to account for the behavior of the printer. Small holes tent to be the worst parts for poor size accuracy. The overall outer dimensions of your parts should come out pretty close to perfect - within a few tenths of a mm, I generally find.

 

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I don't know the way UM2 uses to perform the calibration. What I noticed is that even if the calibration seems perfect, the print starts with the extruded too close to the glass. I need to perform several calibrations before to have the right distance. But, more annoying, after a perfect print I often have the same issue in the next one.

Do you know if the first step of the calibration (1mm) is taken into account or is it just a preliminary adjustment?

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It's just a preliminary adjustment, I believe. When you complete the leveling process, the head is assumed to be 0.1mm (the thickness of a typical sheet of paper) from the bed.

That said, first layer shouldn't be that much of an issue - be sure and set the first layer height in Cura to maybe 0.2 or 0.3mm; that will give a thicker first layer, and make any slight imperfections in the bed height much less of an issue.

 

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PLA shrinks as it cools. Longer pieces shrink more. The absolute minimum shrinkage will be .3% so you could I suppose recalibrate all the X,Y steps/mm to account for this but you will find it won't work. It might work if you only print cubes or cuboids (box like shapes) but if you print for example two boxes connected by a skinny bridge, the bridge won't be strong enough to pull the side boxes together and you will get less shrinkage at the bridge.

In general I just guess the shrinkage and compensate in the design. If it's a critical dimension I just print the part twice. After printing it the first time I measure everything and then compensate the model by the amount that the result was off by. For example if a surface is .5mm too far towards the center I move it out by .5mm.

 

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Yup PLA does shrink! I noticed this the other day. I printed a box, using colourFabb's PLA/PHA for the box and Faberdashery's PLA for the lid. On completion and fitting together, the lid had noticeably shrunk at the perimeter.

 

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