Question about size issues

Hoonse · February 17, 2017

Hello ladies and gentleman.

I have an ultimaker original and i have ro say it is a incredible good machine

When i have purchased ir when there was bo heated bed commercialy available

And so i build my own.

The printed parts wege +-0.1mm but mostly spot on.

After updating the firmware the homebuild heated bed didnt work anymore

I had a hard time until i uploaded the marlin firmware with custom heatbed support.

The original firmware didnt work because of a thermistor error. (The value of the thermistor wasnt the

Value of the original thermoator so it didnt work.)

Since then the print dimensions arent accurate anymore. Outer dimensions are 0.5mm larger in x and y. It dont depends if the part has 10mm or 100mm dimensions. The part is always 0.5mm bigger.

On inner diameters the dimension is always 0.7mm smaller.

Allso it dont depends if the hole is 2mm or 20mm.

Now my question: can i somewhere set something either in the firmware or in cura so that my prints arw accurate again?

I am using pla as a magterial and the printbed is a custom build heated bed.

I dont think tht the problem has to do with shrinking because the outer dimensions are bigger and the inner dimensions are too small.

I really hope someone can help me with this issue to ger my beloved printer

Back ro his old form.

Best wishes

Hoonse

gr5 · February 17, 2017

Well this all sounds normal. The bottom layer is probably squished a bit so the bottom 1 to 3 layers are probably indeed .25mm larger on each side or .5mm larger. This is usually good because the part will stick well if you squish it in. However if dimensions must be more accurate on the bottom layer then you can move the heated bed a little farther from the nozzle until it's just right. Just be warned that your part might come off the bed during the print. Especially large parts.

Now the second issue is unrelated. vertical holes always come out a bit small. Usually about 0.5mm for me. Possibly 0.7mm - not certain. This happens with PLA on every printer out there. It is caused by 3 factors: circles are modeled as polygons *inside* the circle - it's a cad thing - material shrinkage - but 90% of the problem is caused because the filament comes out of the nozzle like snot. It sticks to itself and is shrinking in the first few milliseconds such that it's like a liquid rubber band. As you place a layer down in a circle the pulling of the liquid pulls inwards.

I have tried many things to fix this and by far the best is to simply add 0.5mm to all vertical holes in CAD (horizontal holes don't have the same issue). The latest cura I think even has some kind of new correction factor somewhere that addresses this but will also make your overall part a bit bigger I think. So best to do it in cad.

Hoonse · February 18, 2017

Hello

Thank you for the fast answer.

Its ok that this is nowmal but at first the printer was 100% accurate. This is very strange.

Is there a setting in the firmware or cura where i can set a value so the printer can correct that issue? The troblem is whrn you want to use standard parts like screws or bearings and they dont fit?

Best wishes

Hoonse

gr5 · February 18, 2017

Yeah I got that part and ignored it. I have seen these issues always on all FFF printers (I've used a dozen or so). I don't believe you didn't have these issues before. You maybe never noticed the vertical hole issue as that's always going to be there. Or maybe you printed another material like ABS. The other issue is probably because you had to relevel after you changed from your old bed to the new bed. And like I said - just fix leveling and that issue is gone -- for most parts -- unless they are too big and warp off the bed.