How to CAD Model a Welin Interrupted Screw?

Struck me today that modelling the Welin Interrupted Screw as used on the Vickers Bl 8 inch Howitzer (Mal Webber's build) would be a challenge for a 3D CAD package.

My attempt with Fusion360 started well but failed at the last hurdle.

Step 1: - Create breech ring and mark-out 30degree notches

Step 2 - Pull-cut the slots

Step 3 - Multiply slots by 3 with a circular pattern

Step 4 - Model a Real Thread on the innermost surface

Step 5 - Model a Visual Thread on the middle ring. (A visual thread is a representation painted on the surface. It would generate g-code, but to save computer time and memory doesn't actually number crunch and add a real thread to the model.

Although it looks promising at Step 5, Fusion breaks when asked to convert the visual thread into a modelled thread.

Step 6 is mangled!

Can anyone suggest a way of modelling a stepped interrupted screw thread in F360, or show how it can be done in any other 3D CAD package?

Ta

Dave

Edited By SillyOldDuffer on 19/09/2019 20:53:13

I'd probabaly helical sweep each piece of thread 30deg and then do a circular pattern of the sweep to get the 4 parts, then do the same for the next dia

That can wait until tomorrow.

I would do it this way:

Extruding the core for a male plug

Add the segments for the next stage, sweep out the threads.

Add segments for the final stage and sweep out the threads.

Internal would essentially be the same but starting with the largest diameter. This may be harder to visualise which is why I described the male version.

Something like this - the order of parts in the history is important

Edited By Neil Wyatt on 20/09/2019 15:06:48

Thanks Neil, I'll give that a try next. Jasons suggestion works fairly well but the 'threads' produced are straight, rather than screw spiral. (Actually I'm not sure breech threads are spiral cut on a real gun. Maybe they're just locking slots rather than a true interrupted screw. If the obturator provides the gas seal there may be no need to cam the breech into the breech block. Perhaps a trip to Fort Nelson with camera is in order!)

Dave

On the right is an attempt at a breech produced by using a copy of the breech block to cut the outside of a cylinder. Nearly works but I'm having trouble cutting the excess smooth bit off without upsetting the model.

Mine come out spiral as they are done as a helical extrusion so pitch can be set

Shows up more if I double the pitch

Edited By JasonB on 20/09/2019 15:46:05

Darn it, just wasted 20 minutes looking at Youtube. interestingly, as far as I can see the threads are parallel, not helical. Or at least the helix angle is very small. I'd always assumed they were helical.

Modelling it shouldn't be too difficult, athough everyone has missed out the grooves between the thread segments. A more interesting question is how one would machine the part. Hint: I suspect the grooves are there not for functionality but to aid machining.

Andrew

Andrew, I was just about to suggest that as Dave has now been able to draw it that his homework for the weekend should be to make it

Yes a run out groove for the tooling seems logical

Edited By JasonB on 20/09/2019 16:19:11

Ho hum, just dismantled my Windows laptop for the purpose of identifying the hard-drive and memory for upgrades and am currently stuck on Linux. I'll have to log-out and reboot to get Fusion 360 up to see if it does helical scans. Sounds promising.

So far I've only attempted proof of concept models to see if I can produce a breech and breech block that look right. Next step is to make a matching pair that can be jointed to model realistic opening and closing. There's another layer of complexity to come. The breech is turned and swung out on a carrier operated by pulling a lever. First the lever turns the breech to unlock it, then hits a stop causing the motion to transfer to swing the breech out and away from the breech ring. Various ways of doing this of which the Asbury mechanism ended up as favourite. But screw breech guns seem to have used several different mechanisms, and I've not found a good drawing of any of them! As Asbury was invented in the US in 1916 it's unlikely Mel's 1917 British Howitzer uses it.

I guess a machine tool was made specially to cut these threads, perhaps an attachment on the giant lathe used to turn and bore the barrel. I imagine a single point tool on a lathe that only turns 30° per cut and reciprocates on cams, with a very short lead-screw to add pitch. As a machine tool it's more complicated than a plain screw-cutting lathe and must have been a challenge to design itself. Clever geezers those old boys!

Dave

'Strewth, I think you're right and I never knew that before!

In this clip from Sink The Bismarck, you can see the gun-loading process from about 0:16 to about 1:00. HMS Vanguard's gun mountings are representing the catastrophically-fated Hood's, but they were very similar. You can see that the breech plug doesn't appear to retract or advance in the opening or closing rotations.

Denmark Strait

That's interesting! Actually easier to model with a helix as you only need to draw one tooth and extrude it.

Note that the real thing often had some clearance on it to allow it to swing closed in a more compact arrangement.

Neil

I would guess that those threads were produced on a rotary shaper, if such a tool ever existed.

Drawing it is way out of my league ... but Chapter IV of this may be of interest: **LINK**

https://archive.org/details/textbookordnanc00acadgoog/page/n106

MichaelG.

From the link provided by MG it appears that the Welin breech is indeed a screw thread on a helix. Since it is a relatively fine thread and a large diameter the helix angle is small and thus difficult to see. For instance a 1/2" pitch thread of 15" diameter has a helix angle of only 0.6 degrees. I'd been worried about saying the thread was parallel. One of the functions of the breech is to provide a gas tight seal and I didn't see how it could do that without a small axial movement as a thread was tightened.

I've been reading a thread on "practicalmachinist" and I think SoD has it. The consensus was that the thread was single pointed in the conventional way, but with the addition of a cam on the cross slide that stepped it between three pre-determined positions as the work rotated. The slots between threads are to allow the cross slide time to move.

I'm almost tempted to have a go at modelling and making one just for the hell of it. Although I'd use the rotary table on the CNC mill as a sort of vertical lathe rather than mess about with cams.

Andrew

There is a lot of stuff on here about how they were made, and a suggestion that as well as being stepped they were tapered

**LINK**

Edited By duncan webster on 21/09/2019 00:19:17

I noticed the taper in Michael's link too.

.

I saw mention, somewhere, of a Geneva Stop Mechanism

Horology meets Armaments

MichaelG.

.

Ref. Lots of diagrams and animations available on the web

... but here is a decent analysis:

https://www.researchgate.net/publication/319666064_Kinematic_Analysis_and_Design_of_a_Geneva_Stop_Mechanism_Teaching_Aid_for_Intermittent_Motion

... and the Watchmaker's variant:

https://watchmaking.weebly.com/geneva-stop-work.html

Edited By Michael Gilligan on 21/09/2019 07:50:56

Edited By Michael Gilligan on 21/09/2019 08:05:31