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

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.

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