3D Printed Curta Part IV

Curta Part Strength at Scale

I ordered parts from Shapeways to check the clearances I determined for moving parts and to see and feel the strength of their plastic. I ordered the digit selector axle and the selector knob. The two parts mate and does some mechanical work since the selector axle turns as the knob is moved up and down it.

The parts came packaged nicely and were in good shape. There was some powder left in the selector knob I had to clear out and the bottom of the selector axle needed a little bit of cleaning to fit on the axle, but the two pieces fit together and functioned well. I did not have a small spring and ball to add to complete the assembly. This was mostly a test of what I could expect from Shapeways for size, fit and strength.
Selector knob and axle assembled.

The held in my hand for scale.

Movement of the shaft and knob.
The parts did mate, but the fit was a little bit looser than I would have liked. The fingers on the knob where it would hold the transmission gear and the small point at the top of the axle were too small and fragile to be useful so even with SLS printing, I would need to scale up to produce useful parts. Not terribly surprising.

I may go back to Shapeways at some point in the future to build a really clean version of the Curta. However since I have to scale up anyway, I am going to produce future tests and parts with my FDM printer for now so that I can get faster iterations. It took eleven days from initial order to received parts. While that is pretty good turnaround for a process that used to take much longer, I can produce multiple revisions in hours on my printer versus one in weeks.

Tolerances in OnShape 

I could go ahead and set up all of the scaling and tolerances I need to print the parts I need, but if I ever got a printer with better accuracy I would need to repeat all of that work. Instead I want to rely on dimensions shared across the parts to define the tolerances. This way I can update the values in one place to have them applied to all of the parts at once.

OnShape does not yet support shared dimensions, but it is a feature that they told me they are working on. I look forward to that update so I can continue my work.

In the meantime, I am back to the idea of using my FDM printer which means supports for overhangs. I would prefer to keep that limited so I am going to revisit the models to consider further alterations that would 3D print more easily.


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