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.

[Doddy]

From an non mechanically-minded perspective, I'd say if the difference between a 1.5mm and a 1.75mm thread is the difference between the thread working or failing, then you're too close to the edge. I'd go with the tooling that you already have, and if you strip the thread use that newly purchased lathe that you just know that you need to turn a tapped steel insert into the newly reamed hole in the ali pressure plate. Or re-make the whole plate from steel.

Preload?, if ease of turning is an issue make a bigger hand wheel - but I doubt that it is. Finer control?, not really much of a difference to worry about.

[Wal]

>I'd go with the tooling that you already have

Heh - well, I have threaded rod in 1.75 and, yep - you've guessed it - taps in 1.5...!

Wal.

[Doddy]

Ah, one of those classic Homer moments. Doh!

[m_c]

Generally in softer materials, bigger pitch is better.

For this kind of application though, some form of square/acme thread would probably be a more suitable application, but that can get expensive.
I think I'd personally go for finer, rather than courser, as the forces aren't going to be that big in the grand scheme of things. If it does strip, either remake the top bars in steel, or put a helicoil in the aluminium.

[Wal]

Good call on the ACME - the thought had crossed my mind, but I just assumed too expensive and a bit more of a hassle to come by. My assumption regarding the expense was wrong:

https://www.ebay.co.uk/itm/1-2-10-AC...-/382095182523

- that's about half the price of 1m of 1.5 pitch high tensile threaded rod - and I only need around 12", and this tap seems like good value:

https://www.ebay.co.uk/i/35213872052...7750313&crdt=0

If you dig a bit deeper, though, you'll see the rod's readily available in metric, but the taps aren't... Or if they are then they're a couple of hundred quid..!

Right, I'm faffing now. I've ordered a coarse 12mm tap (not in ignorance of your suggestion m_c - I looked at what I already had and what was going to give me the least amount of ball-ache going forward..) - I'll try it. If it's shite then I'll remake it so it's not shite. 👍

Wal.

[JohnHaine]

By the way neither press had the springs originally. I added them to the smaller ones to make it easier for my wife to get stuff between the rollers by reducing the top weight. The big one, which has VERY heavy rollers, doesn't have them but it's still manageable. When you put the plate + paper + blanket on the platen you then have to get it between the rollers and as you can imagine it can get a bit hard to turn the handle.

[JohnHaine]

Wal, I've worked on 2 etching presses for my wife, and I think you are overthinking this. One is a small tabletop one, the other a beast weighing the best part of half a ton that sits on a stand on the floor.

Neither has anything very fancy in the way of bearings, just a u-shaped recess in a lump of steel or iron that a spigot on the end of the roller sits in. On the tabletop press I made a nice pair of CI blocks bored out to fit the spigots that slide on the edges of the slots in the sides, mainly so I could lift the roller using some springs to make it easier to manage, but the previous ones were just brass blocks with the u-shaped cutouts.

There needs to be a steel platen between the rollers that gets wound across supporting the plate and paper. On the smaller press this is 1/8 inch bit of mild steel plate, on the big one its 3/4 inch by 18 inches by 3 ft or so, a real brute.

You absolutely don't need any fancy thread on the pressure screws, an ordinary coarse metric thread would be fine. It doesn't get much use, the pressure isn't very great. Remember that there will be a felt blanket between the rollers and the paper that regulates the pressure and takes up any unevenness.

I'll try to take some photos for you and post them here, any questions just ask.