Are metric and imperial bolts and screws interchangeable on printed parts?

I printed the imperial parts to match the conduit available here in the US, but I find imperial bolt sizes to be confusing. Can I use metric stuff on the imperial part set? I have converted a lot of the sizes of bolts to metric and they are virtually the same. For instance, 5/16 inch is 7.9375 mm. Pretty damn close.

I think it is really the standard to do all engineering with metric. I think this project could have saved itself a lot of effort by going metric from the start.

Metric bolts are hard to find in the US.

I used those sizes intentionally because they convert well and didn’t have to make multiple versions just to change bolt holes. M8 and M3.5 fit very well. I know M3.5 isn’t the most common metric size but if my local US hardware store has them I assume you can get them everywhere. This is used world wide and I want to make it as easy as it can be. I actually do all my designing in metric, but metric hardware around here is at least triple the price of imperial.

So when I made/released this I didn’t know it would be so well received. It was just a weekend project for me. Redesigns are being made to better accommodate the rest of the world, not just myself.

From an Australian perspective,

  • Hardware is expensive, online is the way to go for orders of this size (more than a few items, but not enough to justify boxes of 100/500). Actually, everything is expensive here!
  • The assembly bolts / nuts were easier to find in 6/32 than 3.5mm; 3.5 is an unusual size. 3mm and 4mm are widely available locally, 3.5 not at all at a reasonable price. But 6/32 easy to find from a local, but online, RC / drone parts supplier.
  • 8mm bolts on my build (which was carefully scaled by measuring printed parts at 1.0, then 1.015, then 1.016 scale) tapped into the holes, rather than sliding freely. They were completely threaded bolts. This is not ideal, as the bolts should allow movement of the parts. I’m thinking of replacing these at some point, in which case I might go for the English measurements rather than metric, just for that extra 0.06mm space. But the bolt style is probably at fault, rather than the size.

Also in Australia. Yes M3 and M4 bolts easier to find. I just used M4 and when required drilled out the M3.5 holes. Or (in the case of the Z axis nut trap) redesigned the part for M4.

Yes the M8 bolts were hard to find in the lengths listed, most I couldn’t find in Bunnings (or the now closing down Masters). So I just ended up ordering the complete set of bolts in length and qauntity on line from an Australia Distributor. They arrived in a reasonable time frame.

Important to note: The bolts need to be the length listed. Not 5mm longer or short. The exact length. the tollerances and clearances are very tight.

As for the M8 (in the Model F) tapping. Yes. they did. but as far as I can tell with my build it didnt degrade the placement and tension/alignment of the bearings. All mine worked perfectly, and the ZXY come out perfectly square (to the limits of my obeservation and testing).

Actually the bearings were the easiest to come by. I found them both at a bearing supply shop, and they also pointed me to the fact that any skateboard shop carries the same size bearing usually in packs of 8 (to replace all the bearings on a board).