New mortar, WooHoo!

[Soot]

Just arrived. Golf ball mortar, 4140 steel. 5.0" high, 3.5" base, 3.0" muzzle. I just need to build the sled.

He also sells a thunder mug version, same thing without trunnion pins that i think will make a good start for one of these.

[seacoastartillery]

    Soot,  That's a nice looking mortar.  What size chamber does it have?  What style bed are you thinking of?  If your really interested in making a Mallet's Mortar, then take a look at this thread; we built one last year.  We will try to get some of the missing photos posted later tonight.

     http://www.gboreloaded.com/forums/index.php/topic,125169.0.html

What finish is on your new mortar?  On my screen it looks like brass.

Regards,

Mike and Tracy

[KABAR2]

Congrats on the new mortar!

With the lighting I first thought you has a bronze mortar,

how did he attach the trunnions?

[Soot]

From the builder's site:
EACH SIDE OF MORTAR IS FLAT MILLED THEN BLIND HOLED
AND TAPPED. HALF INCH THREADED BAR IS SCREWED IN. THEN CUSTOM MADE BRONZE
BUSHINGS ARE PRESS FIT IN PLACE AND THEN A CUSTOM 0.750" DIAMETER, ID THREADED
TRUNNION IS SCREWED ON TO THE EXTENDED THREADED ROD. THE TRUNNIONS ARE PLACED
WEIGHT WISE SO THAT THE MORTAR WILL JUST STAY UPRIGHT WHEN FREE MOUNTED. THIS
WAY WHEN YOU MOUNT THE MORTAR ONLY SLIGHT TRUNNION CAP PRESSURE IS NEEDED TO
KEEP IT IN ANY POSITION. OVERALL TRUNNION LENGTH FROM BODY IS 1-1/2". FREE TRUNNION
LENGTH AFTER BRONZE BUSHING IS 1-1/4"
I've never seen this method before but I'm willing to give it a try for $159.00
Is it legal for me to put a link to the builder's site here?

After i got it in my hand's, I remembered the post about the Mallot's Mortar you guys made and looked it up. (search doesn't seem to work on this forum).
I think the simple addition of a plate bolted to the bottom with a trunnion pin possibly welded on and some sort of elevation screw would be pretty easy to do, so I'm gonna go ahead and buy the thunder mug version ASAP. That setup mounted into a solid block of granite would be awesome but possibly very difficult ($$$).
For the bed on this one I was thinking of a pair of triangle plates similar to this fine example, held together with a bottom and front plate and anything else it may need. Since this design isn't based on any historical  gun, I think some it might look pretty cool.

I'm not sure of the chamber size, but I know it will hold at least 400 grains of powder.
I need to research safe load info.
The finish is raw, I looked up prices for having it blued but it's a bit expensive, more that the gun.
Brownelle's sells something called guncoat thats impervious to anything for about $20.00.

[GGaskill]

EACH SIDE OF MORTAR IS FLAT MILLED THEN BLIND HOLED
AND TAPPED. HALF INCH THREADED BAR IS SCREWED IN.

This type of construction concerns me considering where the trunnions are mounted, namely in the walls of the bore.  How deep is the blind hole and how much wall thickness is left?

[Soot]

The trunnions are mounted at the powder chamber, more steel than the bore. I don't have the tools here to disassemble it and measure it. I'll bring them home tomorrow and check it out.
I did find a proper barrel waning label for it though.

[Double D]

I am more concerned about over time those threaded studs shearing off.  Threads are for joining not resisting shear.  The recessed boss is at rear of the trunnion.  There is no support on the front.  The direction of the shear on the trunnion is forward.  Shear on the tube is rearward. The shear stress point is the junction of the trunnion to barrel.

I would at the very least drill out the threads and press fit a steel pin in the hole then press fit the trunnions on Even this might loosen over time. Can bronze be eletro-welded? At minumum the trunnions should be soldered or brazed also.

What George is getting at with his question, is the wall thickness at the bottom of the threaded hole equal to the diameter of the powder chamber. That's the safety factor.



 

[Soot]

I'm going to take everything said here into deep consideration and do a lot of research.
The barrel and trunnion pins are made from 4140 ordinance steel, so it's weldable. Only the bushings pressed onto the trunnions are bronze.
I really didn't think this design was much different than cannons with screw in trunnions, but ultimately shear strength may suffer. I was under the impression that screw in trunnions were acceptable. The wall thickness at the bottom of the threaded hole is 1/2 the chamber diameter. I'm not sure I fully understand the question "is the wall thickness at the bottom of the threaded hole equal to the diameter of the powder chamber".
Where does this safety factor come from and how does it apply to different types of construction and materials?
Has anybody ever done pressure testing in cannons with modern black powders? With a little research and work it may be very possible to obtain these numbers using the CUP units.
I am going to shoot the mortar in it's current configuration, but I will use extra caution and take lots of measurements looking for dimensional changes.
I know this mortar has been built for some time and fired by many with no reported problems, but extra caution cant hurt. CYA.

[Double D]

The problem I see is the trunnion boss has no support and shear forces are directed into the thread. If you could counter sink the full diameter of the trunnion boss into the side of the mortar tube that would give the trunnion support and displace some of the shear force away from the threads and put them into the walls of the countersink and side of the trunnion boss. This will not displace flex 100% in the trunnion thread only reduce it.  Solder and welding will reduce it even more. Welding will direct the shear forces into the weld and away form the thread. Welds are stronger than threads.

You may from time to time see us refer to the N-SSA standards for cannons.  These are the safety standards we point to when it comes to the safe construction of muzzle-loading cannons.  You can find a link to their standards in the sticky at the top of the forum called Safe Loads and Cannon plans. N-SSA safety standards call for a one caliber wall thickness over the breech of muzzle-loading cannons.  The powder chamber of a mortar is considered the bore for this rule.  So if you have a 1 inch diameter powder chamber then the walls of your powder chamber should be no less than 1 inch.

There is nothing wrong with over-building, but there is no reason for short cuts when it come to safety of these guns. One mantra we repeat here often, “Never build one of these as you know you will shoot.  Build it for how the next guy will shoot.”  


Yes in deed modern pressure measurements have been made of Cannon pressures.  Matt Switlik in his book “The More Complete Cannoneer”   has a chapter on just such tests.  One of the things he found was a 40% increase in pressures using an equivalent load of Fg versus the same load in Cannon grade powder.   Very interesting reading!  

In one of the other threads there is a picture of my 1840 Army Howitzer.  It is built to the plans drawn by the late William Green.  Mr. Green’s plans calls for the trunnions to be installed in pockets and the trunnion to have a thread stud much like yours.

I installed my trunnions by screwing them in and then turning them in a bit more with a with a barrel wrench. The trunnions fit very tight in the pocket and the wrench seated the bottom of the trunnion against the bottom of the trunnion pocket.  

I took my gun to a shoot in Northern California.  I was denied entry because my trunnions were not welded in place.   I went over to a local Millwright and cannon maker and he welded my trunnions for me.  When we took the tube out of the carriage the first thing we found was one trunnion was loose in the pocket.  I could turn it with my fingers.   The other trunnion had a gap at the back between trunnion boss and the pocket wall.  The trunnion was bent forward.  I had to use the barrel wrench to get that trunnion out.  We straightened everything out and welded everything in place. A valuable lesson learned for me.

We are not pointing these things out as a criticism.  We are trying to help you make your gun stronger.  CW always likes to say overbuild is good. I can’t argue with  that.