[OpenSCAD] Joining parts

Steven Dick kg4ydw at gmail.com
Fri Jul 19 07:25:05 EDT 2019

I actually designed a part to answer exactly this question.


If you print male and female parts that are exactly the same size,
they are unlikely to fit together.  If you taper one, you can press
them together up to a point at which they get stuck.  Where they get
stuck depends on the accuracy of your machine and to some extent the
type of plastic you are using.  How much they gets stuck depends on
the smoothness of the part and the slipperiness of the plastic.

The idea is that you print this part which has a taper, then you slide
the sides together, and measure where they get stuck and calculate a
tolerance from that.  In retrospect, this part only measures your XY
tolerance;  the Z tolerance will be different.  I suppose you could
try to rotate the part and print it, but if I had designed it for
that, I might not have made it so long and skinny.

Measuring the tolerance from this part only gives you a starting
point.  There are other factors that might change the tolerance over
the life of a printed part.  ABS stretches.  PLA stretches a bit less
but also rubs and changes shape.  Some of the size of the tolerance
comes from wobble in the printer, so if there are bumps in your PLA
part, some of those bumps are smoothed out a bit each time you slide
them together.   TPU does both but not permanently; I thought I could
get away with zero tolerance for TPU but found that isn't true, it
also needs some tolerance to make parts fit.

For sliding parts, my original Makerbot Cupcake with ABS needed
0.25mm.   My Ender 3 with PLA needs a bit more than 0.1mm.

For a friction fit, using the bumps in the Z axis isn't a bad idea,
assuming the part doesn't delaminate under pressure and fall apart
ABS friction fits nicely.  PLA seems to bind to itself less, so I'd
not try a XY friction fit there, as others have mentioned, I'd want to
make a peg that snaps in, although that works well in ABS too.

I'm not sure how legos do it; I would imagine they have an extremely
small dovetail taper or something.  Weather or not they use a taper,
they do use extremely tight tolerances.

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