Etch Press Build

[Wal]

I'm almost definitely over-thinking this - but there's no harm in knowing the correct way of doing something...

I spent yesterday making the knobs for the pressure adjustment spindles. They're out of what I believe to be 2014 (the larger plate from which it was cut sat in a shed for 30-40-years, so it's nice to see something getting made from it..!)





I was assuming I'd be using a standard 1.75 coarse pitch tap, but when I double checked my M12 tap, I found it to have a pitch of 1.5. This got me thinking about which thread-form would actually be most suitable for the application...



The diagram should make things reasonably clear - but basically the knobs attach to a threaded rod which connects to the take-up bearing uniand allows for adjustment of the roller height and the resulting pressure that this puts on the plate/paper which is being printed.

As you can imagine - there will be a fair bit of shear force acting on the threads which are engaged in the aluminium pressure plate and this is where I'd like to know a bit more about which thread-form would do a better job a withstanding this force. I've done a bit of (non-technical) reading on the matter, and Wikipedia says:

Coarse threads are more resistant to stripping and cross threading because they have greater flank engagement. Coarse threads install much faster as they require fewer turns per unit length. Finer threads are stronger as they have a larger stress area for the same diameter thread. Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment. Finer threads develop greater preload with less tightening torque.

Which leaves me a bit confused - Yep - I want something that's more resistant to stripping so 1.75, then? Oh hold on, finer threads are stronger (?!), allow for finer adjustment and develop greater preload with less effort - excellent - so a finer pitch, then..? But wait - I've also read that a finer pitch is more susceptible to stripping and galling in 'softer' materials, like aluminium. Bugger, so back to the coarse pitch of 1.75, then?

Hehe. What would you guys do? And this is before the suitability of materials in even considered - high tensile vs. stainless vs. aluminium... 😂

Wal.