Planning a sodacan mortar seeking comments on my tube design

[garyD]

Hello folks;

I'm laying out the design of a soda can mortar and have put up a copy of the tube drawing for comments.  

http://brooksiderod.tripod.com/pdf/scm.pdf

Would ya'll take a look and advise if any significant blunders exist.  I am not entirely sure of the proper size of the powder chamber so would apprecate your feed back.  Tube will be turned from steel.  Trunion bolted on.  

Gary

[GGaskill]

An interesting mix of fractional and decimal dimensions.  (-:

Powder chamber seems a little small; I would go 1x1 or 1dia x 1.25dp.  Otherwise OK.

Where are the bolts going?

[garyD]

Thanks for the reply.  I have revised the sketch with a larger powder chamber.  The bolt will hold on the trunion bar.  Threaded on axis of the tube.  Fixed the dimensioning too.

Gary

[GGaskill]

Suggest using two bolts as far apart as possible.  Putting one on center will make a thin point  between the bottom of the chamber and the top of the bolt hole.

Could also make the bore deeper (an inch easily.)

[CU_Cannon]

Not a bad design.  Here is the link to my designs.  I can also send you cad drawings if you would like.

Cannon and Mortar Plans

The bear can mortar is scaled directly from ordinance drawings of the 24 pdr.

[Cat Whisperer]

Gary -

I had CNC built about 10 mortars very similar to what you've drawn a few years back from 4140.  From that experience, are my thoughts.

The 1x1 powder chamber is OK, but a little larger is more usefull.

By (as George mentioned) using 2 bolts as far apart (which I did on mine) you can put the powder chamber further back.

Make the barrel a little longer on the inside, lengthen the tube if you have to.  A lot of folks want to do more into the range it gives you over just one beer-can-tall depth.

Good rounding on the inside corners.

Do a fillet or round on all the outside corners too - it greatly improves the looks!  I did that on the last run of golf-ball mortars I made and they look MUCH better just because of that touch.

Send me your email (in a PM) & I'll send a scan of the drawings as-made that I did.

[Cat Whisperer]

CU -

I wish I'd had the foresight to make mine according to the form of the 24 pounder -  there are some advantages to having it histerically correct!

[Kuntrykouple]

It looks fine i am in agreement with the poweder camber being enlarged. but i would prolly Move the center ring up about an inch or so to place it more over the powder chamber and where your load will be where it is going to give it added strength

Exterior Form .-- Cannons are in general of a truncated conical form, with a cylindrical opening along the axis to a certain depth. The strongest part surrounds the seat of the charge

 Chapter 2 of the Artillerist Manual

[Artilleryman]

On the Federal 24 pdr coehorn there is a shoulder on the breech end that ties into the trunnion.  They are a real pain to do.  Does anyone do them?  One of the purposes of this part was to cut an angle in the block mount inlet for this part to rest on providing the 45 degree angle.

[Kuntrykouple]

Does anyone do them?

I don't and i have conteplated the idea of trying to reproduce them  but i think  it would take  considerable more time than its worth in  just setting up the mill with the rotary table and the amount of  cuts  needed to make the curve and tolerances would have to be pretty close to make it look good

[GGaskill]

I've done them (rimbases) on the ones I have cast but it is difficult to do them on a machined tube.  I welded them on my beercan mortar but they really should be flush at the top.  One could fill in the space between the rimbase and the barrel with epoxy filler to make them look solid.



1.5" bronze Coehorn




Beer can mortar

[Artilleryman]

Looks good to me.

[Powder keg]

I like it!!! Make sure you show us some construction picts, if you can.

Later, Wes

[Double D]

Forget the bolt on trunnion weld it! It's stronger and save having to weld later when you break one of th ose bolts off.

[GGaskill]

Make sure you show us some construction picts, if you can.

:-)  The beercan mortar was done many years ago, long before GraybeardOutdoors and digital cameras.  Furthermore, it is not done the best way.  But I will speculate some on how to do it better.

The cast one is obviously done by putting the feature on the pattern.  The fabricated one could be done better by milling flats on the sides where the rimbases should go such that the flat is deep enough for the rimbase to be flush in the middle but proud of the surface at the corners.  That was the plan on the golf ball mortar shown below; you can see how little material is removed from a properly shaped Coehorn breech.  The trunnion end of the rimbase is simply cut away using an end mill or a fly cutter to the appropriate length and diameter.  Then weld in place.  I would put a short bead in both top side corners (leave the top free of weld for best appearance) and a full (but small) bead between the trunnion and bottom of the rimbase.  Then mix some epoxy and a filler and fill the voids behind.  Paint covers all.



Regarding attaching the trunnion, if you CAN weld, I would prefer welding to bolting.

[CU_Cannon]

I am planning on doing that on my 12 pdr.  I makes it look more authentic.  IÂ’ll post some pictures when I get to that point.

[garyD]

Thanks for all the replys guys.  It's appreciated.  As I get into making chips, I'll post pics.

Gary

[garyD]

Good morning Guys;

FYI latest drawing revision is up.

http://brooksiderod.tripod.com/pdf/scm.pdf

Thanks again to all who offered comments.

Gary

[Double D]

Forget the bolt and weld that trunnion on.  Bolt is a weak point.  Your trunnion is going to be bear all the weight of the tube and the force of the recoil.  Drilling two holes through the trunnion bar weakens it. To much chance of  breaking.

Plus under force of recoil you are going to be placing a lot strees on the bolts.  If you break one you won't  might not notice it until the secoond one breaks.

Back to the moving van...

[garyD]

Looks like the jury is in.  I can mig the trunion on.  After welding I'll dress it some with a acorn bit.  No need to stress relieve, right?

Gary

[GGaskill]

Since you are still in the revision stage of design, you might consider CU_Cannon's Coehorn drawing (see below.)  Seems like people aren't really concerned about making accurate models at first but later on they wish they had.  But forget the bolts.  Click the image for a full size version.

[Cat Whisperer]

....
 Seems like people aren't really concerned about making accurate models at first but later on they wish they had.  
....

Let me just add, seems like people aren't really concerned about making accurate models at first but later on they wish they had.

[CU_Cannon]

I went with bolts because the steel used is 4150.  Every thing I found said that it was hard to weld.   I had enough work in it that I didn't want to experiment.

[GGaskill]

A good reason for bolts.  Here is a 4.5" scaled version (52%) of a Federal bronze Coehorn.  You might want to reduce the bore and chamber size (don't run the chamber so deep) for whatever projectile you intend to use. The taper is 13 degrees included angle.



Click for a larger version.