[OpenSCAD] Joining parts

Revar Desmera revarbat at gmail.com
Fri Jul 19 17:48:01 EDT 2019

A while back I designed a part for testing XY printer slop as well.

https://github.com/revarbat/snappy-reprap/blob/master/STLs/slop_calibrator_parts.stl <https://github.com/revarbat/snappy-reprap/blob/master/STLs/slop_calibrator_parts.stl>

The value it finds is the extra spacing I had to give between mating faces to have a snug fit.  I double it for a sliding fit.

- Revar

> On Jul 19, 2019, at 4:25 AM, Steven Dick <kg4ydw at gmail.com> wrote:
> I actually designed a part to answer exactly this question.
> https://www.thingiverse.com/thing:3579313
> 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
> afterwards.
> 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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