K.I.S.S. bowling ball mortar

[Cat Whisperer]

With George's comments soon to follow, let me interpret from my perspective -

keep the line/area of contact wider rather than narrower and
keep the area of contact (trunnion to powder chamber and tube) even.

[Double D]

With George's comments soon to follow, let me interpret from my perspective -

keep the line/area of contact wider rather than narrower and
keep the area of contact (trunnion to powder chamber and tube) even.

So what you are saying there is nothing wrong with what I have, it just would be better  if....

[dominick]

I think the plug can be perfectly flush with the tube whether a flat is cut on the trunnion or not.  The flat just gives a little more surface area and prevents sid loading on the bolts.  All of the high pressure is in the chamber and the outer tube is mostly a guide.

[GGaskill]

All right, post #27 gives the reasons I am saying what I say here.

The only real problem I have with your design is the trunnion interface with the tube (especially) and the breech plug.  With the trunnion bearing only on the tube, the stress will exceed the strength of the metal and you will get deformation, at least on the first few shots.  By then, you will probably be able to draw up the bolts to the point where the breech plug will bear on the trunnion and the deformation will stop.  On the other hand, there is only .005" difference in length and that will go in the first shot and you may get a little deformation the whole length of the trunnion. 

It is generally not good practice to design expecting to have infinite stress (from the line contact) even though the material will spread in response to the stress.  I would put a flat on the breech plug/tube side of the trunnion to eliminate the infinite stress.  It doesn't have to be very wide if the whole mortar base bears on the trunnion.  A .5" flat is only .017" deep and would give you 5 3/8 sq in of surface.  A 1" flat is .068" deep and that would give 10.75 sq in of surface to take the firing load, lowering the stress to 17900 psi.

Actually, I guess I am including reasons here, too.  Simply I would recommend a 1" flat on the upper side of the trunnion and fitting such that tube and plug both bear on the trunnion but are both tight to each other, too.

[Double D]

Now that makes sense George...numbers not theroy. 

I will revise the trunnion drawing for a 1 inch flat.

[Cat Whisperer]

Now that makes sense George...numbers not theroy. 

I will revise the trunnion drawing for a 1 inch flat.

Good move!  Round or with flat will both work, one will last longer. 

[Double D]

Okays here is the whole packet.












This has the trunnion flat.

The Trunnion is attached with two 1"-8tpi socket head cap screw.  The vent liner is made from a 1/2"-13 socket head cap screw drilled  for use with  friction primers.

[jamesfrom180]

I believe the question regarding the trunnion flat are very valid, but I am concerned with the bolts.  As I am an environmental engineer and not completely fluent in materials and related sciences, could someone comment on the possibility of using shouldered bolts and any benefit or detriment to the design due to possibly removing stress risers in the threads and unsupported material? Like wise the vent sleeve intrigues me since mis-alignment seems entirely possible with the discussion of the chamber and barrel assemblages.  Could this itself provide a pinning force to the chamber and clearly be used to provide a redundant means of securement?

OH forgot to add, This is a very good start to a design.  I can not wait to see the finished product and a BB accurately placed 1 mi. downrange. 

[Double D]

The two bolts are there to simply to hold the trunnion on and nothing more.  They don't bear load in firing--well maybe that is not quite correct.  If the trunnion was only supported on its end, I am sure the flex of the 3.75 inch trunnion would put some load on the bolts.  Since the trunnion bar will be fully supported its entire length in its base, flex should be negligible.

The larger bolt size is more for dealing with the stress of bearing weight of the gun while moving.  Shear being a bigger concern than firing in regards to the  bolts.

The trunnion does nothing more than pull the breech plug tight against the seat in the barrel.  The seat in the barrel bears the load.

Drilling the hole for the vent is done as one operation.  The breech plug is put in place and the trunnion bolted on to secure the plug in place then the vent hole is drilled through the barrel and breech plug in one operation.  Tapping is done in same way so the threads align.

I suppose the vent does serve as an additional point of securement. But that is not the  intent and it is minor. 

I am optimistic that we can get this thing to launch a bowling ball 1 mile...accuracy well that is a subjective term.  I am optimistic that I will be able to make a subjective claim of achieving accuracy.... 

[Cat Whisperer]

I believe the question regarding the trunnion flat are very valid, but I am concerned with the bolts.  As I am an environmental engineer and not completely fluent in materials and related sciences, could someone comment on the possibility of using shouldered bolts and any benefit or detriment to the design due to possibly removing stress risers in the threads and unsupported material? Like wise the vent sleeve intrigues me since mis-alignment seems entirely possible with the discussion of the chamber and barrel assemblages.  Could this itself provide a pinning force to the chamber and clearly be used to provide a redundant means of securement?

OH forgot to add, This is a very good start to a design.  I can not wait to see the finished product and a BB accurately placed 1 mi. downrange.

Shoulder bolts going through the trunnion and into the powder chamber would indeed move the much stress (from vibration) from the threaded portion of the bolt.   On the flip side, I've used 1/4-20 bolts holding together trunnion to tube on a dozen or so beer-can caliber mortars with no trouble.  The size indicated is substantial.

Looking at the vent - that is a looooong way to thread it - especially when bridging two disassembleable parts.  Most often a few threads are cut and the looooong portion is clearance drilled.  My concern is if during firing there is any shift that the vent gets damaged - making disassembly difficult.  Light the fuse and find out.

[Double D]

With all this mass tightly bolted together where the heck in this design is this twisting motion coming from and especially one  strong enough to damage a  1/2-13 socket head cap screw.

Of course you are going to have to clearance drill the vent hole. Any good machinist will take one look at the 4 inch x 1/2-13 socket head cap screw and know that--2 inches of thread and 2 inch of solid shaft. 

 

[Cat Whisperer]

An inch or two threaded and clearance above is GOOD. 

[Cat Whisperer]

With all this mass tightly bolted together where the heck in this design is this twisting motion coming from and especially one  strong enough to damage a  1/2-13 socket head cap screw.

Of course you are going to have to clearance drill the vent hole. Any good machinist will take one look at the 4 inch x 1/2-13 socket head cap screw and know that--2 inches of thread and 2 inch of solid shaft.

I have great faith in the flexibilitiy of big tough pieces of steel flexing and moving when under the great stresses of goodly charges of black powder - or 50+ ton presses squeezing brake pads together from hydraulic pressure.  It comes from experience of seeing what should be well build turning out to be underbuilt.  Fact of life - things break.

[jamesfrom180]

Cool that answered my concerns, I again want to say with this design I would have no problem being on the range with it.  A good proofing would be called for but I feel with the chamber design and the energy put into construction, this thing would not be a danger.  I am still questioning the function of being able to disassemble this after some heavy firing, but proofing and time will tell.  Failure of that kind can be tolerated.   

[Cat Whisperer]

Cool that answered my concerns, I again want to say with this design I would have no problem being on the range with it.  A good proofing would be called for but I feel with the chamber design and the energy put into construction, this thing would not be a danger.  I am still questioning the function of being able to disassemble this after some heavy firing, but proofing and time will tell.  Failure of that kind can be tolerated.   

In the field of education, one publishes a masters thesis or a doctoral dissertation.  It allows/promotes peer evaluation - a strength to the profession.

Here we discuss freely the designs and our procedures and techniques - that too brings out the good and bad points.

Safety is the prime consideration, followed closely by keeping things legal. 

Somewhere down the list are manufacturablility, looks, historical accuracy and so forth.

[Double D]

Proof? Why?

The gun is being built to N-SSA cannon construction standards using material exceeding minimum recommended standards.

If loads are developed using the the recommendations for black powder cannons as discussed in Switlik's  The  More Complete Cannoneer: start ridiculously low and work up and do not exceed the maximum recommended loads for that particular size chamber then there is little. 

Besides where are you going to find the standard working pressure for  3.5 inch chambered bowling ball mortar?  SAAMI  recommends loads  of 33% to 44%  greater pressure than standard working pressures, CIP uses 25%  greater pressure.  These of course are  for smokeless powders.  Neither  organization publishes recommended standards for black powder.  CIP does test muzzle loaders with blackpowder but only to 10,000 psi. 



 In the US there is only one arms manufacturer who proofs all their firearms and that is Savage.  The rest of the American manufacturers proof  random samples.  Those random samples are fired with two proof loads and the arms are disassembled and measured for dimensional changes.  The proofed arms are archived for future reference.  If the proofed arm has no significant dimensional, then the design is consider adequate for the cartridge proofed.

CIP-Europe test all guns but only to 125% of working pressure...they have been criticized for this as some consider the pressure to low to adequate disclose design faults.

There is nothing to be gained from proofing this mortar.


 

[GGaskill]

A one mile shot should be adequate proof.

[1Southpaw]

why head all th way o powder chamber ?


My mortar has just a few threads in to the power chamber . the fuse vent is tapered to seal off in the breech  nd hole drilled the rest of the way to powder chamber . less threading  ,good seal , larger vent plug as in 3/8 " bolt vs 1/4".

[Cat Whisperer]

Proof testing should be for new designs - to prove the design.

This design has been proven (thickness rules of thumb, rounding of inside corners, construction from known type of mild steel).

[Double D]

Proof testing should be for new designs - to prove the design.

This design has been proven (thickness rules of thumb, rounding of inside corners, construction from known type of mild steel).

Exactly!!!!   

[Double D]

I thought I would add this to this thread so everyone knows where I stand.  You can click the blue  quote from link and be taken to the original thread and see the entire discussion.

Here is the  response to my letter to ATF

Based on the below letter Bowling ball mortars replicating the Stoke design are no longer off topic on this forum


Full size: http://www.fototime.com/770876464C875AC/orig.jpg


Full size: http://www.fototime.com/715A25C4056EB04/orig.jpg

Although my inquiry to ATF was in regards to bowling ball mortars replicating Stokes mortars it seems from this letter that ATF had no concerns with the replication.  The only two issues they identified as relevant are antique ignition and not designed as a weapon.

Since my mortar  will use antique ignition and  is not designed as a weapon, I think I am on solid ground. 


Now the only real issue for me is to find someone to bore the barrel.  My local machine shop has kind of been reluctant to get involved.  I was talking with the guy yesterday and told him about the ATF letter.  After mentioning the ATF letter I noticed a distinct change in attitude.  He said his only concern was if his boring bar was long enough and rigid enough to do the job. Things are looking better now.

Anybody know where I can find used boring bar that will reach 20 inches deep and make a 8.8 inch bore?

[dominick]

I made my own boring bars.  For a heavy duty one, I used 2" solid bar and bored a cross hole near one end to accept a lathe tool bit, a set screw or bolt in from the top of the 2" bar aligned with the cross hole held the bit tight.  I chucked the bar into the lathe, [with the tool bit hole level] to find center and fabricated a heavy steel mount to the cross slide.  The mount is welded and gussetted right to the bar.  When welding, I placed the ground clamp right on the bar and tack weld only.  Otherwise it can damage the lathe.

[Cat Whisperer]

Douglas -

I have several - glad to loan you one or two.

[Cat Whisperer]

One is an Armstrong, 1-1/2" diameter.  25" from end to cutter bit tip.  Takes 1/2" lathe tool bits.  3 tool-holder ends different angles for holding bits (90, 45 and something else).  Do you need fixturing to hold to cross-slide?  I THINK I've got something that will bolt to Tee slot.

[Double D]

One is an Armstrong, 1-1/2" diameter.  25" from end to cutter bit tip.  Takes 1/2" lathe tool bits.  3 tool-holder ends different angles for holding bits (90, 45 and something else).  Do you need fixturing to hold to cross-slide?  I THINK I've got something that will bolt to Tee slot.

I may take you up on this Tim.  Let me see what the guy at the machine shop says.

[Cat Whisperer]

The 'heads' are marked Armstrong 95-630, -631 and -632.   I've not used it, just allowed it to get rusty.  My bracket for holding a boring bar is NOT 1-1/2".

There is a 'piston' inside each to lock up the lathe cutter.

[Victor3]

I believe the question regarding the trunnion flat are very valid, but I am concerned with the bolts.  As I am an environmental engineer and not completely fluent in materials and related sciences, could someone comment on the possibility of using shouldered bolts and any benefit or detriment to the design due to possibly removing stress risers in the threads and unsupported material? 

 Valid point on threads & stress risers, but quality 1-8 socket head cap screws are so strong you can just about lift an aircraft carrier out of the water with a couple of them. They're weaker in shear, but I can't see as how this mortar design (especially with the flat incorporated) is going to see much force thataway.
 
 Also, most commercial shoulder bolts are rather soft (and weak compared to a SHCS) and are designed more for locating assembled parts together accurately, and thus require tighter position and diameter tolerances in the parts than cap screws normally would (so not as "KISS").
 
 On a side note, I work in the fastener industry and one way we design to reduce the stress riser at the root of an OD thread is via what we call a "modified minor". One design looks something like this...
 
\_/\_/\_/\_/
 
...where the "V" at the root of the thread (minor diameter) is truncated and/or radiused.

[Victor3]

  The Paixhans Monster Mortar was quite an experience.  While making it was fun, we will not be doing another any time soon.

 Now there is a mortar. 
 
 I especially liked the simple yet elegant way you chose to seal the area between the breech plug face and bore with silicone sealant. Any similar design should have something like that for safety's sake in order to keep gunk from building up in that gap where it could produce a hidden ember or corrosion. 

[Double D]

Some progress to report.

I have all the drawings printed out and will be going to see a farmer up at Sweet Grass who has a big lathe his shop.  He is interested in doing the job, but doesn't have the tooling.

Which brings me to tooling.  I have worked a deal with CW for the use of his tooling.  Be careful horse trading with Tim he is brutal! He knew I needed the tooling and he knew he had the tooling and made me empty my pockets.       And I will state this again, Tim  My horse is not a mule, it just has big ears. 

[Double D]

Well I visited with the Farmer and we have a deal!!!

As soon as the tooling gets here I haul it and the steel to Sweet Grass. 

So far under budget and ahead of schedule!!!