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Narrow fit parts: issues


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Posted · Narrow fit parts: issues

I have an Ultimaker3, it prints nice but when I print some parts which must be combined later (narrow fit!) then these parts didn't fit at all.

Parts are allways too big.


Print PETG with 40mm/s, 245'C nozzle, 80'C bed.


Bedleveling calibrated.

Also tried calibrating the steps (X,Y, Z and extruder).  Used all kind of filament, Ultimaker brand or whatever: same results..


As long you print something which doesn't need to be printed very accurate then the printer is ok.


Is there somebody who have this same problem and nows how to fix it ?



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    Posted · Narrow fit parts: issues

    There has to be a clearance between the parts.


    If you slide a part into a 20MM hole the part going into it would have to be around 19.5MM in diameter and it would fit pretty snug, may need to be 0.4 or 0.3 MM smaller than the part it goes into but it does have to have a little bit of clearance.


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    Posted · Narrow fit parts: issues

    Layer lines, blobs, strings, hairs, ringing and thickening at corners, always cause inaccuracies. I usually make a couple of small test models to try which tolerances work best for a particular model feature. Depending on the required fit, I usually take between 0.1 and 0.5mm tolerance for my UM2-printers. Printing slow and in thin layers also helps.

    For example for inserting M4 nuts (which have a diameter of 7.0mm between flats for steel, and 7.2mm for nylon, in my inventory) into a 3D-printed cage, I made this test model with 7.2, 7.3, 7.4 and 7.5mm hex cages. Then I tried what worked best, and I used that for the real design. Steel nuts are hard to force into too small holes in a PLA model, but for nylon nuts it is easier.




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    Posted · Narrow fit parts: issues
    15 hours ago, tinkergnome said:


    @geert_2  - you can modify the shape of the pocket a bit, makes assembling much easier and distributes forces a bit better:




    Thanks, good idea. I have seen the concept before (metrinch wrenches in shops), but never tried it in 3D-printing.


    Do you have a rule of thumb, or a formula, for designing the optimal curves? Or do you do it just visually and based on experience? This isn't a standard shape in my 3D-editor DesignSpark Mechanical, so I will have to draw it manually.


    This concept of "recessed corners" (I don't know a better description) could surely help to reduce the accuracy problems in corners due to ringing, thickening and rounding effects. Also in other designs. Maybe the original poster can also use this concept for a better fit?


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    Posted · Narrow fit parts: issues
    6 hours ago, geert_2 said:

    Do you have a rule of thumb, or a formula, for designing the optimal curves?


    I made a parametric sketch and used a radius that is equal to the wrench size (because it "looks right" 😏).  It turned out that it scales quite well for usual sizes of threaded nuts.


    I hope one can see it:





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    Posted (edited) · Narrow fit parts: issues

    Thanks for all your comments 😀


    After calibration of the x,y,z and e-steps i get now the accurate sizes 😀


    I found out that the z-steps are ok, but the others needs to be calibrated for every new rol of filament!!


    The steps can be changed in the firmware (.sjon file). 

    New problem: after I got a firmware update, this option is not available anymore, or maybe the setting file is moved to another place. (I will look for a fix this weekend)


    Anyway: I have created a calibration part and a howto on: www.thingiverse.com/askmurphy 


    If you like, print the test object and check your step-calibration (you can use the calibration spreadsheet on the above page) and show your results here.


    The Ultimaker3 is a nice printer but for accurate prints it’s not good at all if you don’t calibrate the printer. The 2.85mm filament is one of the main reasons, the print accurate error is much higher then with a 1.75mm filament..


    If you ask Ultimaker about this: you get default answers, useless.


    If you are interested, i can place another calibration part here, narrow fit parts..

    Edited by knutselsmurf
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