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zac-b

Accuracy issues when making holes and slots

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I have been printing with my Ultimaker for about a month now and have faced many technical problems along the way however I have managed to make several nice accurate prints. My current issue has come up when trying to print a .250" slot in a extruded rectangle. The hole has been coming out about .020" to small and I feel ive done everything I can to correct it such as using 3 different slicers, measuring the .stl file, remaking the model all together in Solidworks, uploading new firmware, tensioning the belts, looking at the dimensions in the G-Codes themselves and probably a few other things I cant remember.

Specific details bellow

The model was created and exported as an .stl from Solidworks

.stl was sliced in Cura Slic3r and Kisslicer

prints were made from 210 to 225 C

multiple filament extrusion percentages used for perimeter, first layer and fill

Same result every time. .020" small but all other dimensions look perfect

Any help would be greatly appreciated

Zac

 

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Sadly, building things from melted plastic strings isn't very accurate. Plastic shrinks as it cools from a liquid state. What I've had to do when I needed precise holes was to do two or three prints to determine the measurement I need to send to the printer which corresponds to the size I actually need. For example, when I need an 8mm diameter hole, I design it as 8.4mm and send it to the printer. It prints out as 8mm. It was just a trial and error thing to figure out what design measurement to use in order to achieve the required real world measurement. After a bit, I just started remembering the "fake" sizes of things I use often.

 

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If you tell Cura the nozzle is .4mm, then it will print everything .4mm towards the center of the part. If it didn't do this your part would be .4mm too small.

But this is usually not enough correction. If you are printing a circle and you are printing fast, I often get a circle smaller than the movement of the print head. It's like you are stretching the plastic around a circle and it pulls in a bit.

My outer dimensions also tend to be a bit small but not as bad as the inner dimensions.

Anyway I do the same thing as Bill. I print twice. It's wasteful but I don't print the whole part - just the area with the critical dimension. Then I measure accurately and adjust my model. If you tend to have holes of similar sizes or slots or lengths, and if you print at consistent temperature and speed, then you will tend to need a consistent correction factor. Also some people when printing 3mm screw hole print at 3mm knowing it will be too small but then sort of self-tap the screw in there and it comes out great.

Also vertical holes and horizontal holes may have different issues but I've never printed a critically dimensioned horizontal hole.

 

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I also had the same issues. I had parts where the correct outer dimension was not important but the size of the hole was. So I printed with 1.025 overall scale factor. By now I also try to account for the problem by modifying the model.

Part of the reason for the incorrect inner diameters might be the fact that the plastic layers get squashed on top of each other. This might create som plastic overflow towards the inside of the hole.

Shrinkage is also an issue. I have test printed a 10 mm bolt and nut made with the Blender plugin "bolts". The nut would not fit the bolt until I printed it at 1.02 to 1.03 scale.

What I did not check yet is the influence of layer height. Will the same object printed at 0.1 and 0.2 mm show the same inaccuracies?

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That's a good question about the layerheight. I have the feeling that with 0.1 mm, the molten filament is squeezed a little bit to the sides because of the flat nozzle. And with 0.2 mm, the filament has more space under the nozzle so it stays in it's "round" form. Putting a string of really 0.4 mm on your print instead of a squeezed string of >0.4 mm wide.

But this needs to be checked.

 

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There is a very slight difference when the layer height is halved. If you change layer height, definitely do a partial test print and adjust the model.

Also, as gr5 mentioned, the adjustment is different for horizontal holes, because the bridge portion tends to flatten the top a tiny bit and the plastic is shrinking in a different direction. Plus, depending on the size of the hole, your filament, and your print temperature, the amount of flattening changes. So, it's more of an art than a science. Test prints are your best friend. :-P

 

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I too am having this issue, and solved it by doing a lot of test prints.

I feel like CURA is treating my inside holes like outside walls and adding wall thinkness to the inside(making it smaller), is there a way i can tell it the hole needs to have the wall thickness go in the opposite direction? I am new to creating my own files from scratch..just unsing inkscape, blender, and tinkercad, and still learning about how these programs work.

 

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I assume we are talking about vertical cylindrical holes and not horizontal.

I thought the same thing until I looked at the gcode. It's fine. Cura compensates by the radius of your nozzle size so as long as you tell the truth, Cura does the correct thing. So for example if you ask for a 10mm diameter hole and your nozzle is .4mm Cura adds .2mm all around the outside path so that the head moves in a 10.4mm radius circle.

So why is the hole too small?

One factor is the number of lines segments in your circle. If you have the typical 10 segment decagon, each segment is cutting into the interior of the circle a tiny bit. That tiny bit is significant when tollerances are around .1mm for fit versus not fit. That hurts somewhat.

PLA shrinks when it cools. This also causes holes to be smaller.

The "pulling" effect (like pulling on a string) tends to stretch the PLA slightly towards the center of the hole as it pulls the PLA around the rim of the circle. Molten PLA isn't like water - it is a bit sticky and pulls against itself.

 

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