Etch Press Build

[Wal]

Okay, I've been thinking a bit more with regards to the springs under the take-up bearings - an example:



I take it that the primary job of these springs is to pre-load the top roller to remove any slack and prevent it from flopping about when not under a printing load, with a secondary purpose of perhaps aiding the raising of the roller? Or am I missing something? Will I be making a grave error by omitting these springs from my design? I've seen other presses without 'em - but I want to be sure on their purpose and the pros and cons of having them incorporated.

I posed this exact question to a press manufacturer over in the States - he got back to me (and I'm grateful for that) but his answer to the spring question was a rather vague 'yes'. Hmm... yes as in the springs do that job, or yes as in you're making a big mistake by omitting them from your design..? And with regards to getting some general advice on material specs - well - here's what I managed to extract from him:

You have to pick the right material with the proper thickness in the right places.

Hehe. Blood from a stone...

Anyway - back to the question - am I missing something with this spring..? Any mechanical engineers here have any thoughts on the matter..?

Wal.

[m_c]

Going by that photo, the springs only serve to raise the roller up when tension is removed.

If you design so just the screws lift the roller, you're likely to get a bit free movement while going from under tension to lifting the rollers. It just means you need to turn the tension screws a bit further to lift the roller.
By adding suitable springs, it could simplify the screw design, as the spring would hold the end of the screw engaged with the bearing block/carrier.
You could implement it so both the screw and spring do the lifting.

Any method will work, it's just you need something to lift the roller, otherwise it could be a bit of a faff getting everything loaded between the rollers.

[Wal]

Cheers m_c.

Yeah, it would be nice to have a bit of pre-load on the roller to eliminate any rattle when not under load. I may have to go for a spring pushing down, though (also seen in the wild) - the take up bearing I'm using has a curved underside which complicates getting a spring under there - it'll mean a bit more elbow grease when lifting the roller, but if it's too problematic I can always remove it. Thanks again.

Wal.

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[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!

[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.

[Wal]

Heh, cheers guys!

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[JohnHaine]

Wal, a thing of beauty!

You probably know this, but woodcuts are usually done on end-grain blocks, typically maple or boxwood, which can take a fine line. There are also quasi-synthetic versions such as Resingrave. I believe that you can also use MDF at least for test cuts to get the image right, and probably use something like PVA or dry-rot cure resin to stabilise the surface. Lawrence seems to be the preferred supplier.