Making Mechanical Parts Work
Sliding Parts
As my clockwork spider project develops, one of the goals I’ve had in mind is a final end design that can be assembled by hand, without tools, or additional hardware. 1 This is an ideal to which I hope I can adhere. I just like the idea of printing off a plate of parts, handing them to someone, and letting them assemble all of the bits on the spot. Getting a box of Legos is fun in part because it comes with absolutely everything you need. Without a barrier to assembly, the part out of which the toy is assembled are toys in and of themselves.
If one is to design mechanical parts, I think there are two important considerations. The first consideration is just how close can two objects be designed and printed such that you don’t need too much force to snap them together tightly. If the tolerances are too tight, perhaps the parts can’t be reliably printed or easily fit together by hand. If the tolerance are too loose, the parts won’t stay together. The second consideration is just how much clearance parts need before they can rotate or move against one another smoothly. Too tight and they’ll bind, catch, or seize up. Too loose and they’ll wobble out of place or simply not connect to other critical parts reliably.
After several revisions, I’ve found a “sweet spot” for parts printed with my Thing-O-Matic:
- Interlocking Parts: For a part that needs to fit tightly within another part, I design them so that they have 0.25mm clearance on all sides.
- Moving Parts: For a part that must easily slide within another part, I design them so that they have 0.5mm clearance on all sides.
After some tuning of my profile2 I think I might revise the “moving parts” tolerances to 0.4mm clearance on all sides for a fit with less wibbly wobbly. 3 And if one is to use tools, such as a vise or pliers, and not rely on hand pressure, a clearance of 0.2mm on all sides would probably work for parts that needed to stay locked together.
Do you design mechanical parts that need to fit together? What kinds of tolerances do you use in your designs?
- Photo courtesy of j-ster [↩]
- Again, with Dave’s Profileinator – the only way to tune a profile [↩]
- Or timey wimey. [↩]
| Tagged with | clockwork spider, fit, fitting, interlock, interlocking, mechanical, mechanical parts, resolution, slide, snap, snapping, tolerances | 4 comments |
4 Comments so far
Dna
Heh, at work, the tolerances range from 0.1 mm to 0.025 mm and some tolerances pairs are negative. (eg. 10 mm dia +0.000 – 0.025 on the hole and 10 mm dia +.050 + .100 on the shaft.)
Once they are together, they aren’t coming apart easily.*
Dna
*And no wibbly wobbly.
aubenc
I’m constantly busy with this, almost all my openscad scripts have the parameter “gap”.
When I want to have parts “out of the printer” my values use to be 0.2 and 0.4 because I use to print them with a profile where layer height is set to 0.36, feedrate 32.5 and flowrate 215. This is for printing with makerbot natural ABS and it only works fine 2/3 of the times (filament diameter is mostly 2.87mm but it goes from 2.62 to 2.98).
When I want to be sure to print the parts only once then… I set the gap to 0.1 and I 1st file blobs and oozes and I finish it with sand paper. Normally it takes me something between 2 and 5 min. to have the parts ready. That depends on the complexity of the part and yes, occasionally it may take me up to 2h !! (for a very complex part which needs to be as good as possible)
I don’t think I’ll get to find the magic gap values. Filament consistence, MK4, Z rods and timing belts tension combined with different profiles have prove to be too much variables for me. I’m also a little bit too lazy to modify/create profiles and too much in a hurry to see the part out of the printer.
I do think I save time (and plastic) by finishing the parts by hand and I’m absolutely happy with the outstanding performance of that little amazing cupcake.
MakerBlock
@Aubenc: Even with your MK4, I highly recommend Dave’s Profileinator. People are still working out how to use a MK6 with a Cupcake, but once that’s finished I think you’ll find a huge improvement in terms of resolution, quality and reliability, and a drastically reduced need for finishing work.
However, that’s an excellent consideration. I had only thought about the tolerances I use for parts straight out of the printer without any finishing. I suspect with some very light sanding I could probably get parts to within 0.1mm on all sides as a good tight fit.
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