Brass Coehorn Progress

[2oakes]

Thanks for the help - things are slowly taking shape.

Yesterday and today I've been cutting the radius - manually, one thou at a time, from an Excel spreadsheet . I've got pretty good at reading the feed dials on the Grizzly! I'm going to stop soon before the remaining stem gets too thin, saw it off and finish the rest off the lathe. Thinking of test firing it and measuring for expansion before I bore out the trunion hole. Any thoughts on a good way to do this?

[GGaskill]

"Thinking of test firing it and measuring for expansion before I bore out the trunion hole. Any thoughts on a good way to do this?"

Two points:  1) without seeing your drawings, the thinnest point is likely to be the bottom of the chamber next to the trunnion cut, so testing before cutting is not going to be a measure of the ultimate reality; 2) assuming you have the prototypically correct tapered breech end, the only way I can see to measure, or at least check, that is to make a tapered gauge that the barrel slips into and you make note of how far it goes in, to be compared with how far it goes post-firing.  You should make the gauge using the same setup of the compound rest as you use to cut the barrel or it will be virtually impossible to make the tapers exactly the same.

[Double D]

As long as all the walls of your powder chamber are the same thickness as the diameter of your powder chamber you will allright. The stronger way to make the trunnion cut is in the Trunnion bar not in the barrel. If you cut the barrel the the one  diameter applies  in the cut.

You don't need to do an expansion test if you follow this rule.  If you get expansion it's to late.  Get to building and stop thinking!

[Michael Az]

Nice looking work. Is it a golf ball? I will be starting on my first one soon. Keep posting photo's!
Michael

[Double D]

Here is some drawings from the West Bros that illustrate the point I am making.  These are drawing from their full size Confederate 24 PDR Coehorn.







The trunnion Bar is cut out not the barrel. 

[Double D]

Here is some drawings from the West Bros that illustrate the point I am making.  These are drawing from their full size Confederate 24 PDR Coehorn.







The trunnion Bar is cut out not the barrel. 

[2oakes]

Nice looking work. Is it a golf ball? I will be starting on my first one soon. Keep posting photo's!
Michael

Thanks Michael. It's my first mortar / cannon too and also my first ever lathe project. This one I kept well under golfball size at about 1.3". The 3" brass round I found for $20 at the local scrap dealer.

[2oakes]

The trunnion Bar is cut out not the barrel. 

-----------------------------------------

Thanks - Thats a neat idea.

Out of interest, is there really a need for the trunnion bar at all?

If I leave the piece behind the breech as a 1.5" diameter cylinder a few inches long (like a hand-gonne), I could bore a similar hole in the wooden sled at the correct angle and secure it with a cross bolt / pin.

I could put on trunnion caps on the sled for cosmetic purposes and really it would be hard to tell the difference.

With its use with blank loads, a decent oak sled should hold up, especially with a stout cross pin through it.

Just thinking out loud.

[Double D]

Becasue you won't use it just for blanks.   

...and what do tyou think the cross pin is, its a trunion!

It's your gun you do what you want.  You can always put a proper trunnion on it later.

[GGaskill]

It is certainly possible to design a small mortar barrel with a flat bottom that sits in a flat bottomed hole drilled at the desired angle of elevation.





But as the bore and shot weight increase, it starts becoming more of a splitting wedge with its concentrated recoil load.  In a Coehorn style block mount, the trunnion probably doubles the amount of surface that engages the recoil force as well as spreading it across virtually the whole width of the mount.

[2oakes]

Took it out back to see what the mortar could do.

.75 inch chamber full of cartridge -

First round a small whoosh sound and a small puff of white smoke...

Then I wrapped the powder in a couple more layers of Al foil. This time a healtier pop and more smoke including a persistent smoke ring.

Not much for the neighbors to see...

Maybe I'll try some FFFg another day

[GGaskill]

If you are making a mortar strictly for noise making, it will work better if you keep the bore small, basically the same diameter as the chamber.  Otherwise it needs a shot to get the pressure up where it will make some noise.