chamber plug?

[ds1919]

I have a piece of tubing (12" PIECE OF THICK WALLED COLD DRAWN SEAMLESS STEEL TUBING), 2 1/4" OD, 1" ID.

I need to design a chamber plug, breech, and cascabel. But the tricky part is making the chamber plug, as it is small and may be difficult to machine. Do I really need one for this small diameter bore? Can I use a different metal, like 4140 or stronger? What would be overkill for this project?

thanks, ds1919

ps= I hope this question is okay to ask on this forum..

[Rickk]

A plug welding in place inside the bore would be stronger than simply a plate welded over the end of the tube. This would be for four reasons that I can think of...

the length of a plug is only 1 inch, whereas the length of a plate would be 2 1/4 inches. Thus the plug would be less likely to flex.

A welded plug would be simply a cut off piece of 1 inch solid stock, whereas a plate would be a most likely circle cut from a piece of flat stock. it is as easy to cut the plug longer as shorter, but cutting thick flat stock gets hard.  The end result is that a plug would, or at least could wind up being thicker.

I believe a weld between a plug and the bore would be stronger than a weld between the end of the tube and a plate. My reasoning is that the entire weld of a plug would take the stress at once. On a welded plate, assuming the plate flexes even a small amount,  only the inner part of the weld would take most of the stress. If it started to crack, the streee point would propagate outword a little at a time untill total weld failure woudl occur.

You could cross drill a plug and weld in place a pin, although for a 1 inch bore that may be overkill.

All this assumes that you simply weld a plug in place. If you thread and then weld, then the plug looks even stronger.

I would think that a plug rather than a flat welded breech would leave fewer places for crud to hide as well.

Rick

[ds1919]

thanks for the info. I did find a solid round of high grade hard steel. I plan to cut off a piece and mill it down to the ID of the tube, shape the front of the chamber plug, taper down the back of the plug, press it in, and then add copius amounts of bead into the pocket, thus forming about a 3/4" solid weld (TIG, although I will do some research on the process too). Then smooth, add breech and cascabel.

I thought that this procedure would be too overkill for a 1 inch bore, but after you mentioned back plate flex, and weld deterioration, I now feel comfortable spending the time on a good chamber plug design.


Thanks, ds1919

[Double D]

I suggest you look at the breech plug design found in the N-SSA rules.  This plug is shrink fit  and welded.  The plug is radiused in the bottom to facilitate cleaning and eliminate areas where where sparks or embers can linger.

The shrink fit is supposed to minimize the migration of powder fouling between the wall of the tube and plug, which can start hidden corrosion.  I believe in concept the shrink fit is tighter than a press fit.

You are going to need to band your breech as the walls are less the one caliber thick which is the recommended safe minimum wall thickness.

You will also need a lined vent if you band your breech to provide a continious path so powder fouling doesn't get between the band and barrel and start corrosion.


Here is the link to N-SSA rules.  Search for cannon and barrel liners.

N-SSA National Rules for Cannons

You should also find your self a copy of THE MORE COMPLETE CANNONEER  By M.C. Switlik with selected excerpts from other artillery manuals
The book can be ordered from these two suppliers.
The Complete Cannoneer from Matt Switlik
and
South Bend Replica

Unfortunately the current editions are out of stock right.  We have been told the new addition should be out before the end of the year.

[GGaskill]

The advantage of a thick plug is that it won't significantly flex which prevents stress concentrations in the breech closure.  A relatively thin flat plate will assume a dome-like shape which stretches the plate and the weld and will quickly cause failure. 

[ds1919]

thanks all. I have a few questions for Double D:

1. If my bore is 1", and the wall thickness is 5/8", would not that be thick enough? Are you referring to banding just the breech area?  What about shotguns with very thin walls? I cannot see putting a very large charge in such a small diameter bore, and I thought that the 5/8" wall thickness was strong enough by itself to stand up to any practical charge.

2. Shrink fit: I am thinking that this process is heating up the finished chamber plug and barrel, and then tapping/pressing it in? If so I have done this with valve guides on automobile work....

I do plan to radius the plug for "center" charge release, and for cleaning.

thanks, ds1919

[Terry C.]

When "shrink fitting" you would only heat the tube. The plug should be room temp, or even better, refrigerated.

Doing it this way, you can cut the plug to a diameter that would be an insanely tight (probably impossible) press fit in the tube. The diameter of the tube will expand when heated, and allow the cold plug to be inserted. When the temp between the two equalizes, the tube will seize the plug in a virtual death grip.

Trying to cold-press in a plug with this degree of inteference could cause damage to your plug or tube, or both.

[ds1919]

oh yeah, forgot that I dont need to heat the pllug...

Im thinking 300 degrees in the oven for about 20 minutes or so will get the tube just right?

thanks

[Rickk]

Dry ice for the plug will save you from having to heat the tube excessively... a large temperature delta is what you want. Either hotter tube or colder plug.

The rule of thumb (assuming you want to keep your thumbs) is a wall thickness equal to a bore thickness.  Yes, shotguns are thinner, but they are made by people who theoretically know what they are doing, using steel of known quality and known processes. For the rest of us, a wall thickness equal to a bore diameter keeps us out of trouble.

Rick

[GGaskill]

The thermal expansion rate for steel is about 6 millionths of an inch per inch of diameter per degree Fahrenheit (6.4 to be precise.)  Interference is usually calculated as .001" per inch of diameter plus .001".  One thing you need to consider is the roundness of the inside of your tube.  If it is more than .002" out of round, the plug will not seal the hole, so you should bore the hole until it cleans up, then measure the new hole to size the breech plug.  With only a 1" bore, you will need to make the plug .002" greater in diameter than the hole.  Then calculate how hot you need to get the tube to expand the .002" (or a little more since it won't drop in if exactly the same diameter.)  The actual fitting has to be done quickly because the pieces will be equalizing in temperature as soon as they touch.

[seacoastartillery]

     The following quote was taken from our "Crimean War Behemoth Inspires Golf Ball Mortar" posted back in September of last year.  This paragraph illustrates the importance of doing the fitting quickly, just as George has cautioned us to do.

The Quote:

     "We spent some long hours in the shop finishing this junior size Mallet's mortar, but other than learning a few new (BAD) words from the "Old Salt" as he was machining the base plate, retaining ring and trunnion out of a solid block of 1018 steel, everything went quite well.  After heating the base at 400 deg. F. in the oven for a full hour to provide easy assembly of the larger dia. tube into the smaller dia, retaining ring of the base, Mike slid the two parts together a full 30 deg. out of cync!  After 2 or 3 seconds he realized this and yanked the ring around to the correct alignment.  At this point I heard more of those words my mom forbade me ever to say."

Tracy

[Double D]

thanks all. I have a few questions for Double D:

1. If my bore is 1", and the wall thickness is 5/8", would not that be thick enough? Are you referring to banding just the breech area?  What about shotguns with very thin walls? I cannot see putting a very large charge in such a small diameter bore, and I thought that the 5/8" wall thickness was strong enough by itself to stand up to any practical charge.


thanks, ds1919

This must be one of those times when the  true tone of the writer is not conveyed  properly by the writing in the post. Beca

Lets me ask you some questions?

How many square inches in the wall of a shot gun chamber?

What is the total pressure in the chamber of a shotgun.

How many square inches in the wall of the breech of 1 inch cannon. 

What is the total pressure generated by a maximum charge in a one  inch bore cannon?

Do you know what the maximum charge is for one inch bore?

I am afriad there is no polite way to put  this but did you read anyof the the safety guidlines in the links I posted, especially the N-SSA rules for safe cannon construction?

[Cat Whisperer]

Let me add that one CANNOT depend over the life of the tube for people to be reasonable.

PLAN for 'extra'.  I.e.: USE the rules of thumb that have been proven by MANY people over time.

One has no way of knowing how much a given tube has been stressed before.

AND, when you light the fuse you really should KNOW whether it's a cannon or a bomb.

 

[Victor3]

 One thing you could do to obtain a one caliber thick wall is to machine a powder chamber (basically a blind hole) into your breech plug. A 3/4" diameter x 1" deep chamber would give you one caliber wall thickness where it's needed and plenty of powder volume for a 1" bore cannon.

 Just be sure that there is also at least 3/4" thickness from the bottom of the chamber to the back of the plug, and that you design and install it properly. That means reading up on acceptable and safe designs.

 You also need be sure that the tube, plug and welding rod you use are compatible; 'mystery metal' is not the way to go when building a cannon, especially when multiple components are involved.

[Double D]

The thickness of the Breech plug  must also be at least one caliber.  That's what Victor is saying when he says the back of the breech plu 3/4 inch thick.  The powder chamber he is advocating is 3/4 inch in dimaeter, so wall in all direction need to be at minumum 3/4 inch.  In this case the one caliber is the powder chamber size.

[ds1919]

   Yes Okay, I read the N-SSA file last night. I see the diagrams in 10.1 and 10.2 showing the one caliber specifications. Now I understand, and also from the other postings. I do not wish for this to be a mystery cannon, I want it to be stable.

I do know that the tubing is 4150 now. So I will have a machine shop make a plug that is .002" over.
I will also take into consideration the plug design that Victor3 mentioned.

But before this, I see that I need to do some reading and gathering information, as this is far more advanced and critical than I had thought before (hence, my initial questions on this forum).

[Rickk]

Don't forget to taper the back of the plug, not the bore, for the welding process. That will yield a more secure weld and also leave more of the already thin barrel intact.

[ds1919]

Yes I my plans include the tapering of the back of the plug so a certified TIG welder can drop a fat bead for super strength....

However, the more research I do, and the more information I gather, it seems more practical to scrap this project, and just buy a unit from a reputable builder. This cannon was small for what I wanted, and I thought it was going to be a slam dunk in terms of building it. It is not. It will be expensive, and also requires a lot of energy and focus to make it solid and safe. Now, if this was a 7" solid bronze rod I had started with.......

thank you for all that pointed me in the correct direction, much help, very appreciated!!

ds1919