Design Thoughts

Hey guys, I'm currently designing a printer and would like some opinions on a matter.

Is it best to minimize the number of (different) parts, or to use different parts where they are most suitable.

For instance, designing a new printer I could make all the bolts M3, since that is the only 'nessecary' size, since that's what the motors are mounted with. Idlers, bearings etc all come in sizes to match this.
In this case, if you used m3 for everything you'd end up with some fairly weak joints, use it on an idler and that idler bolt could get bent since the bolt is quite thing.

Or you could choose an 'optimal' part for each joint/task, and thus end up with more (different/unique) parts. The issue here is that it means you need more different types of parts on hand to repair it if it breaks, rather than a 'one part fits all', and assembly gets more complicated as you have to search for a different bolt/bearing/nut for everything.

As a designer or someone who's put together a few printers, do you have a preference? If something breaks, think about it not just from the perspective of a person who has a drawer for every bolt under the sun, but also the average consumer who may not have many if any spares available, and would need to source the parts for repair. During assembly, how many types of bolts to sort through for assembly is too many?

So far, I'd need at most M3, M4, and M8. I can cut this down to M3 and M8 (at some added $$ expense), or purely M3 (at the expense of relying on M3 bolts for the joints, I'm afraid they might end up bent from printing forces).

Does three bolt types (and three nut types to go with it) sound reasonable to people? Also, if someone could chip in on the matter of M3s getting bent, do you imagine they'd get bent if used for a pivot joint (with a bearing, of course)? Think scara arms.

Thanks.

I try to minimize the number of different screw lengths i need, but not to the point where it is causing any problems. If i can make it fit i use the same size, if not i add it.
I would never try to restrict myself to only M3 or M4, that makes no sense and is only asking for trouble.
I try to design as many printed parts of the printer identical, mainly the holders for bushings etc. For the X-Bridge of my current design i made the left and right end identical, you just have to turn one by 180° to use it on the other side.

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I replied to your other thread but this is a pretty good topic. I think and tell people to reduce BOM as much as possible, but looking back personal my projects always seem to end up with a lot more part numbers than I'd have liked.

Design intent matters a lot, a one-off for personal use can make some concessions but if you are releasing instructions or kits you really want to be efficient.

I'm not too concerned about number of diameters. Something like M8x16, M4x10, M3x6 is a pretty tight BOM. On the other hand something like M3x6, M3x8, M3x10 and M3x12 would be absolutely awful because users will constantly mix up the lengths.

Its hard to provide guidance on M3 bending without context, but substituting a single M3 for M8 doesn't seem smart at first glance. Permanent deformation of the screw isn't your only worry, if its cantilevered you will lose rigidity (which will turn into large displacement at the end of the scara arm).