New Challenge

[Vince Cutajar]

Hi guys

After all the drawings I've done in the last few days, today I found myself not doing anything (except twiddling my fingers). So, I set myself a new challenge to do a drawing and learn how to use a new command (for me). After some failed attempts and a few choice words as I don't have any hair to pull I ended up with this:

I always wanted to find out how to draw this and today I finally learned how to do it.

Vince

[Adrian Harris]

Well done. And easily scaled to fit any sized barrel.

I wouldn't mind having a go at printing that

Adrian.

[Vince Cutajar]

You're welcome to it but this is just a test drawing. No particular diameter and length. Don't even know if the twist angle is correct.

Vince

[Adrian Harris]

To give you an idea, and as I happen to be a Shermanaholic, the US M3 75mm gun had 24 grooves, with a uniform right-hand twist rate of one turn per 25.59 calibres, which translates to near enough one turn per 320mm.

I believe Mark machined the 75mm barrels from aluminium round tube with an ID of 1/2", as that's quite close in scale and a standard item from metal stockists. The outside diameter of the end of the barrel is 18mm, so a 14mm drill would create enough of a pocket. The old M4A3 Sherman has a threaded recess in the end anyway, to take the pepperpot for the blank firer, so that could cleaned up and used. From memory, it's about 25mm deep.

Adrian.

[Vince Cutajar]

Adrian

So that I get it right.

which translates to near enough one turn per 320mm.

The tube has an I.D. of 75mm. It is 320mm long. The groove starts at 12 o clock at one end and finishes at 12 o clock at the other end. Is that correct?

That's just one groove I am showing.

Vince

[Adrian Harris]

Yes, that's the way I understand it.

The whole gun barrel on the real thing is only 37.5 calibres long, so the shell rotates about 1.5 times before exiting the muzzle.

Adrian.

[Vince Cutajar]

I made the drawing of the insert full size. O.D.=85mm. I.D.=75mm. Length of insert=320mm. 24 grooves. Groove width=4.5mm. How does it look?

If I scale it down to 1/6 then the grooves will be 0.75mm wide. Do you think it will work?

Vince

[Adrian Harris]

This is the end of the real thing, so your twist looks too severe to me. Also, the actual grooves are very shallow.

The fine resolution profile on my printer has a 0.1mm layer height, so it ought to be able to cope with 0.75mm.

Just realised, we've mixed the full scale and 1/6th measurements.

In full scale - with an ID of 75mm - a single turn length is 1919.25mm, which is why that looks too twisty.

So in full scale, it's 75mm ID and a twist length of 1919.25mm, or in 1/6th scale it's an ID of 12.5mm and a twist length of 320mm.

Adrian.

[Vince Cutajar]

Re-calculated with new info.

Vince

[Adrian Harris]

That looks very good Vince

Just need to trim it so that it's only the first 25mm or so, rather the the full twist length.

Adrian.