View Full Version : Forming sheetmetal with gunpowder

08-06-2003, 02:29 PM
Okay, the lil town where I was raised, had a older guy who was into choppers in the 70's. He made gas tanks and formed them with gunpowder. I remember him winning a few motorcycle shows, Any ideal how? he was a recluse, a former club member and kept to himself. He has passed on now taking "how" with him.

His neighbors hated him, I read about him in the papers quite regular like.

Did he blow it into a form? Shaped charges? HOW?

Also, the aircraft that flew before the wright brothers had a gunpowder powered engine, I read that once, don't know for sure. He was sued for causing the livestock in his county to stampede, the only reason history remembers him.

08-06-2003, 02:52 PM
They used to do spittoons that way too. Anything that doesn't spin-form well.

Basically use the powder, primacord, whatever as the "push" and some form of die that will hold together as the form to be duplicated.

Hydroforming would be somewhat like it in effect, but is less spectacular and requires more equipment.

Those guys nobody likes often have plenty of gunpowder around..........

If you do it right, you can blow together two sheets of dissimilar metals, like in coins...or lock one piece in the undercuts of anothe piece, etc, etc.

I think it is generally used where the draw isn't so much, and there is a need for fast action, or a low-tech approach.

Also something to do with the stresses, that tends to alleviate the spring-back that usually would occur. Some things can't be allowed to spring-back

Ragarsed Raglan
08-06-2003, 02:56 PM

Exploforming is the term for it! Has been done for some time with exotic alloys that are hard(!) to work any other way.

I was involved a few years back with an attempt to make a race engine using a steel that was only formable in this way; that and welding was all that you could do with it. The steel was around 20 gauge and was reckoned to be stronger and lighter than HT'd aluminium/magnesium thin wall castings . We were looking at forming cylinder blocks and heads out of this stuff. The object was to get a V12 3litre race engine down to under 200Lbs weight. The company that did the work had a military contract for this method of forming for some wiz bang armour plate.

I believe that Ferrari use this same steel alloy and forming method for their F1 gearboxes.


08-06-2003, 04:03 PM
Blow it into a mold form. Make the form in two pieces with strong bolts holding it together. It has to be open at one/both ends to allow escape of the explosion gases. Do you have indian reservations where you live? Around here they sell fireworks, some of the bigger stuff (M80's) will do quite a bit of forming.

Uncle Sam frowns on explosives now so I don't know who is allowed to legally do this these days.

Using the same form and a bunch of rubber or urethane you can press (with a large hydraulic press) the shape also. The preshaped blank would be put into the form and filled as much as possible with the rubber chunks or solids. Press on the rubber (which is incompressible) and the blank will expand to fit the mold cavity. This process is called "bulging" and is probably done more than explosion forming now.

Whatever method you use, some consideration has to be given to the amount of stretch that your metal can take before tearing. For mild steel maximum stretch percent is in the 25 to 40% range.

Mass production items are done with a combination of drawing (not much thinning) and stretching so the problems of over stretching your material aren't such an issue as with the explosion and bulging methods. Unfortunately, drawing dies get into some big bucks. For example, the dies to do the rear fender on a Harley out of 16ga steel cost nearly 1/2 million bucks.

John Foster
08-06-2003, 06:29 PM
I formed several small cups (about 2" dia.)of annealed copper using the blanks (.22 as I recall)that are for "Ramset" type of stud drivers. Found best results by using a vacuum pump on the mold side of the metal.

Forrest Addy
08-06-2003, 08:43 PM
There's a paper I read years ago "High Energy metal Forming" that discusses the many ways of artfully turning bang into precision parts.

There's some guy in the woods of Louisiana who makes big fan hubs from a welded steel plate open cylinder. There's an outfit not 70 miles from my house that explosively welds aluminum to steel. There's also ways of using a high Joule electrical discharge to form metal.

Many of these processes use shaped charges and water to distribute/focus the shockwave.

Look here:



08-06-2003, 09:23 PM
I think it was mentioned that Uncle Sam really frowns on this sort of thing, as well as the local police.

Realize that a loud boom in your county, could have all of the anti terrorized folks coming to visit you at warp speed.

Plus in many places, causing the explosion is illegal. Trying to do exploforming requires a great deal of knowledge and control. Find an NDT3 tech and ask that person what they would do and use for this process.


08-06-2003, 11:40 PM
It town we had a shaop that built the hatch covers and WT doors for the Marine corps. Helo carriers,the boats had a steel hull from the waterline down and an aluminum superstructure above,the company made the doors and the sills they fit into,to make the union between the steel door frame and the superstructure they used a steel/aluminum sandwich material that was exploded together real expensive.

Have you ever though about hydro-forming?Thats the way they make the aluminum tube frames in the Corvette,also I have seen some "chased"stainless steel cigarette cases that where formed in a die using a hydraulic pump to supply the pressure to expand the metal and press it into the mold.

One other option is a female die cast out of ductle iron and polished bright,cut a blank,then press into die using a rubber block on the press ram,I have seen bulging dies made this way for things like headlight reflectors,they usually make contact on the bottom most part of the piece and then fill the die out from the center there by minimizing thinning.

08-06-2003, 11:55 PM

You would need to check into permits first to do it legally. The process in industry uses high explosives in precise quantities detonated in a mold filled with (normally water) fluid. Hydroforming is almost, but not as precise and "crisp" on the feaatures of the mold as the use of high explosives.

Be a man, beat the crap out of a flat sheet - builds character! http://bbs.homeshopmachinist.net//biggrin.gif

Paul Gauthier
08-07-2003, 12:13 AM

I have heard that some aircraft used a cartridge very similar to a shotgun shell without projectiles. This cartridge was used to start the engine. The cartridge was inserted into a firing mechanism of some sort and when fired would cause the engine to turn and eventually to start. Not sure if this is what you were thinking about but is the only use of gunpowder in an engine that I have ever heard of.

Paul G.

08-07-2003, 12:30 AM
I have been told that old tractor engines used the same method for starting

08-07-2003, 01:08 AM
"Explosive" forming is also done by use of an electric arc from a LARGE capacitor bank, in water. Works like explosive but no permits required.

08-07-2003, 01:34 AM
Electro-Magnetic forming is the "in" forming method now.
I remember my grandad pulling an old car over a stump that was about to be "removed". Did a lot of metal forming on it! The parts that we could find were all in a bowl or S shape. Wish I had that car now. I think it was a 29 Plymouth.

08-07-2003, 02:55 AM
Off the main subject, but, Electroforming, I have saw coins on ebay for sale that have been reduced in size this way. A heavy EMF magnetic pulse. Is that a new alloy, the metal has been compressed somehow. Is it stronger since the metal is more dense?

Re: I asked, I am working on a power hammer that feeds propane, air pressure into a spring loaded piston on a pivot, yep, it has spark plug too. (old Harley twin cam cylinders off a customers bike) gonna drill 2 stroke style exhaust ports, keep the air flowing to purge it. Plans are to fire it to hammer form sheetmetal over a leather sand bag I sewed and filled with sand. No crankshaft, rod hooked directly to pivot.

yeah, Thrud, my lil bitty arm is getting tired, and old.

They say a bloody scottish bloke flew first.

I think it was a old popular mechanics that told of the first aviator who scared cows with his gunpowder powered engine. I am still searching, too stubborn to give up.

[This message has been edited by ibewgypsie (edited 08-07-2003).]

Forrest Addy
08-07-2003, 04:57 AM
"Gun powder engine". Hm. You guys have me baffled.

I'm not saying it can't be done but after spending a few hours sketching and calculating I'm danged if I can see how such a thing develops much usable power or how it can be made light enough to power an airplane.

Excuse me if I speak in non-quantitive terms. I'm not smart enough to give the problem a real analysis even if I did have tables and graphs of projectile propellant performance under varying types of confinements and conbustion pressures.

I see the limitations of monopropellant engines producing mechanical power in terms of endurance and specific output.

Individual gun discharges aren't very energetic in a mechanical sense even though their violence seems overwhelming. The energy rate is high but the pulse is short hence not that much power per discharge.

Someone told me that a 16" Naval rifle discharge develops less mechanical energy than a single turn of a battleship propellor at flank speed.

Another thing is gunpowder (black powder) is a low energy monopropellant. Smokeless propellants typically have about 6 times the energy but still about 15% of combustion involving the same weight of fuel.

One of the Navy's torpedo propulsion systems uses a monopropellant called Otto fuel. This is a liquid propellant containing fuel and oxygen suitable for injection in a specially designed displacement engine. Since it's a monopropellant it requires no atmospheric oxygen. It can be used in submerged vessels. It's ignited by spark in the usual way.

While the system is powerful for its weight the fuel is very expensive and toxic.

As for gun powder engines I can't for the life of me figure out a way to safely stuff gun powder in a cylinder and fire it at high repetition rates, or burn it to drive a turbine or vane motor, or in rockets attached to propellor tips, or direct rocket thrust as in JATO units.

The way I figure it, a 1900 technology airplane powered by storage batteries and electric motors would have superior endurance and specific output than anything powered by black powder or even nitro-cellulose family propellants.

I'd be glad to be proved wrong. Any takers?

Peter S
08-07-2003, 10:01 AM
The gun powder starting system mentioned, one commonly used system was the Coffman starter, used on Napier Sabre Engines. The cartridges were like over-sized shotgun, the Sabre had a indexable five-shot breech, the combustion gases from the cartridge pushed a piston which forced the starter gear to engage, then rotate via a helical spline. Generated about 25 hp, and a vicious start. Was light and needed no power, both handy for a fighter based at forward airstrips (as the Typhoon and Tempest were in Europe, creating havoc in ground attack).

A friend, now dead, recalled how much they hated winding up the inertia starters on, I think, Bristol Pegasus engines - very disappointing if they didn't fire first time.

Another cartridge start I have seen is on the single (large) cylinder Marshall tractor, guy explaining how all he needed to start his tractor was the little hammer he held - sure enough, a light tap on the firing pin, and the motor starts, courtesy of shotgun cartridge. Had provision also for putting a fuse or somesuch in to warm the cylinder a little on cold mornings.

There were other aircraft gas start systems used in 1920's-30's, involving feeding a fuel mix into the cylinders via special piping and valves.

Coffman info in Graham White's book, "Allied Aircraft Piston Engines of WW2", and also Bill Gunston "Piston Aero Engines".

Which doesn't tell you anything about forming sheetmetal...

08-07-2003, 10:10 AM
Remember the original designs for the diesil engine by Herr Diesil himself were designed to operate on coal dust this was oh 1895-1901 right about there somewhere,also several engines have been built with wood flour as a fuel aditive.

The use of gun powder to start an engine is well doucumented,and was and is fairly wide spread in remote instalations,I have seen then in the oil field mainly on engines that run pumpjacks,these use a shell that would make a 12ga. shot shell look small though,the ones I have seen were about 2"od and 6"long,all powder,plus very specific instructions must be followed.
The story of the Flying Scottsman has been told over and over for years,but the guy didn't achieve powered flight,just augmented glide.
Oh, and what were we talking about,I forgot http://bbs.homeshopmachinist.net//biggrin.gif

08-07-2003, 03:13 PM

A propane powered hammer is do-able. You would have to use a solenoid to conrol the propane injection rate. This would be a good Basic Stamp project - shop air could be used to purge the cylinder (another solenoid), and another solenoid could close the exhaust port. Purge, inject, fire!

Some of the "cordless" nail guns are nothing but single cycle butane engines. Porter Cables unit uses an electric fan to draw fresh air into the cylinder, then butane is injected and ignited - BaM! - you've been nailed.

I dunno Dave, the speed of the stoke might be too fast for old farts like you and I... http://bbs.homeshopmachinist.net//biggrin.gif

08-07-2003, 08:05 PM
Hey Thrud, that would be cool (Basic Stamp). Parallax has some really nice stuff now. I hadn't seen any of their stuff in years. I think you could use some of these parts:


to drive automotive injectors for the solenoids. Injectors typically handle 40-70psi., a few even higher. Getting the cycle and pulse width adjusted would be the biggest problem. There's even room for "feedback" and automatic fine tuning.

How would you control heat build-up? Water jacket? This is turning into a BIG hammer.


08-07-2003, 08:54 PM
Ever see how a diesil piledriver works?Might be neat,one hit to form the blank,and another to set the shape?

Peter S
08-07-2003, 08:55 PM

An interesting book that looks at many different Heat Engines, including Guns, is "The Evolution of the Heat Engine" by Ivo Kolin. There are some technical details of "thermal machines", but mostly analyses the thermodynamics involved.

Over 70 "engines" examined, Wankel, Stirling, Diesel, Gas Turbines, Nuclear-powered artificial heart, Apollo 11 (Saturn 5 etc.), V2, James Watt, guns of Nelsons ship's, Lenoir, etc. etc.

Anyway....he looks at various modern guns, and finds, for example, that a modern 76mm howitzer has the same weight/power ratio as a Diesel engine with the same bore size.
With a fire rate of 25 shot/min it develops approx. 322hp

Equivalent power is based on gunpowder charge and rate of fire.
He graphs several different sized guns, the largest being the WW2 German gun "Dora" with an 80 cm bore (31 1/2") and rate of fire about 3 per hour. This develops about 3500 hp. (roughly hp = 45xd, d being bore in cm).

Another interesting comparison, the fuel pumps on the F-1 (1st stage) Saturn 5 rocket develop 55,000 hp, which represents only 1/2000 part of the power of the rocket motor itself. This is the same ratio of power between a Diesel engine and the power absorbed by its fuel injector pump.

08-08-2003, 04:51 AM
Watch the movie "Flight of the Phoenix" It shows Jimmy Stewart using the "shotgun starter. When I saw it I didn't know that that type of starter existed and I thought it was a Hollywood invention.

08-08-2003, 10:15 AM
Later varieties of the shotgun starter were used on jets.

For one, as I recall the British "Canberra" bomber engines used a cartridge starter. I forget the US designation for that plane.

Put out a heck of a smoke cloud when starting, spewing from an exit on the side of the engine. Not explosive, more like a small rocket engine. Lasted quite a while, like 5 or 10 sec, to spin up the engine.

08-08-2003, 01:14 PM
Thrud, all this talk about speed of stroke, are you trying to imtimidate us old farts.

Feel the burn, later that night, think about hiring a young mexican the next day.. ha ha..

Yes, there was a gunpowder engine. search on the internet, the link I posted before spoke of it. Thou, I have not yet found the flying aircraft popular mechanics spoke of yet.

Propane and compressed air, a blow-out exhaust and spring loaded return. My plan, plus spark controlled via limit switch on one position, no cpu needed.

08-10-2003, 01:00 PM

The gunpowder motor was attached to a bicycle in place of the pedals and sprocket. The Philadelphia Inventor claimed the motor, which weighed 3.75 lb. and was eight inches long, would "carry a machine and rider 100 miles." At the engine's forward face, the "exploding chamber" contained a valve that allowed excess gas to escape. Speed was regulated-within limits-by adjusting a thumbscrew in the gunpowder supply chute. This changed the angle of the chute. A rod connected to the rear wheel opened and closed the supply valves as the wheel revolved.

Starting the vehicle was accomplished by opening the valves and exploding a cap within the chamber. Closing the valves caused the machine to come to a gradual halt. The Horseless Carriage, in their January 1897 issue, reports that the engine produced very little smoke or odor, due-"thankfully"-to the small amount of gunpowder exploded at one time. The magazine further notes, "the inventor claims that the action of the motor is so even that riding is much like coasting down a good hill on an ordinary machine." Gunpowder engines were tried but they did not work out.

08-10-2003, 04:04 PM
Sure, it could be done with micro switches. But where is the fun in that? Too easy.... http://bbs.homeshopmachinist.net//biggrin.gif

08-11-2003, 06:07 PM

I finally found the site.

see this:

also this:


08-11-2003, 08:50 PM
to learn more this topic, see Materials and Manufacturing Technology, by Roy A Lindberg, published by Allyn and Bacon, Boston, Ma. 1968, Library of Congress# 68-20573. Seems some popular materials to use are TNT, RDX/TNT, Tetryl, PETN, and Nitroglycerine. Also tells about Ultra High Speed Machining where a 30-06, and later 20mm canon rounds achieved sfpm of 240,000. Tools used were HS steel, Stellite, and ceramics. This is probably not the last word on the subject.

08-12-2003, 02:37 AM
Pretty cool evan..

I am still looking for the flying glider with the gunpowder engine thou. I think it was popular mechanics 80's or?. About 189? somewhere around pa?.... Only reason he was mentioned was he was nearly lynched for flying over and scaring cattle with his poppy-pop engine.

08-13-2003, 08:40 PM
It was George Cayley, one of the leading pioneers of flight. He wrote some interesting articles on flight and engine requirements and the advantages of steam as a lifting gas in airships:
And check out this engine simulation: