I have been wanting to build a mt howitzer since I started reading this board. I have AOP plans #12 & #25. For my first build I am wanting to try a half scale as I have a 2-5/16 spade drill for the bore and either some 1018, A572 Gr 50, or 4130HT solid round bar drop for the tube. I have two safety issues I'd like some input from those who have been there before, the reduced powder chamber size and shape.
For a half scale the tube dia at the chamber is 3.15 and the only drills I have available are 1-1/4 or 1-1/2. The scaled chamber as drawn would be 1.67 dia x approx 1.6 long with a volume of 3.5 cubic inches and a wall thickness of .74, that's approx 60% of a caliber thickness. If I go 1-1/4 dia x 2.5 long it will get me 3" volume with a wall thickness of .95, approx 75% of a caliber. The only way to get to one caliber wall is to go to 1-1/32 drill which I don't have in the required length. The other option I have come up with is to go 5/8 scale on the outside 3.9375 dia at the chamber and stay 1/2 scale for the bore as I said before I have a drill for that. Now the 1-1/4 drill will give a wall thickness of 1.34, over one caliber. My material selection is limited for the 5/8 scale verses the 1/2 scale but it would give more room to seat the trunions. Does this sound like a better solution than staying to the 1/2 scale based on my material is as stronger as an original bronze would have been?
Now the shape of the bottom of the chamber and the bottom of the bore are going to be as drilled - 130 deg point for bore and 118 deg for chamber, no radiuses or tapers per the original drawings. I have seen mortar drawings on here designed this way and I understand rad's would be better, and for loading powder charges the taper would be better but I am using company tools and the 1-1/4 drill is a twist drill for a specific job we do and not to be modified. If I were using the 1-1/2 drill, it is a spade and we have a bunch of dull inserts I could regrind to any shape I wanted. So, is this a deal breaker or ok?
I have many more questions about full size tubes, projectiles, ignition systems, etc.., maybe another day. I could e-mail my CAD layout if anyone needs it to help answer the above issues.
Thanks,
Steve Seib - cannoneer wantabe