Casting hollow objects in bronze

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MattMa01
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Casting hollow objects in bronze

Post by MattMa01 » Fri Jul 12, 2013 3:33 pm

I am looking to make some blunderbuss barrels out of naval bronze at some point in the future. I have a question I'd like to ask to anyone who has done some bronze casting, especially casting model cannons. How did you make patterns for a partially or fully hollow object like a cannon barrel?

Harold_V
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Re: Casting hollow objects in bronze

Post by Harold_V » Fri Jul 12, 2013 3:38 pm

Keeping in mind that I am not a foundry person---just someone keenly interested in foundry practice----

Have you studied cores and core prints?

I would imagine you'd have success making a core box, in which you'd create the inside dimensions desired for the bore of the blunderbuss.
The core box would include what are known as core prints, which are the locating features that lend support to the core when it's placed in the mold.

The pattern you'd make for the exterior features of the blunderbuss would include the core prints required.

If all of this is a stranger to you, you'd benefit greatly by doing some reading on foundry practice.

Hope some of this is useful to you.

And, with luck, someone with foundry knowledge will provide better instruction for both of us.

Harold
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jpfalt
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Re: Casting hollow objects in bronze

Post by jpfalt » Fri Jul 12, 2013 4:56 pm

You probably don't want to cast the barrel with a core in it depending on the overall size of the part and whether the breech end will be cast open and then closed with a breech plug. You also don't want to make a thin wall barrel casting. The cast grains in the metal are going to be roughly equal in length, width and thickness. This is not the best configuration for the hoop stress on the barrel when it is pressurized.

An additional problem with a hollow casting is that the core going down the center of the casting will tend to flex, float and drift. The technique usually used to stabilize that core are called chaplets. These are small rods with plates on the rod that nail into the mold. The core rests on the flat plate on the end of the rod. You have to have chaplets all around the core. The weight of the core in the empty mold makes the core want to sag. When full of melted metal, the core wants to float to the top of the mold cavity and metal flowing into the mold tries to push the core sideways. The chaplet is made from the casting material and is sized to partially melt when the mold is poured. This usually doesn't work and you end up with a cold seam running through the casting that has to be cut out and filled with weld metal.

I would recommend that you either cast the barrel solid and bore it, or preferably make it from a wrought bronze material with high impact toughness. This will get you some additional material strength while maintaining the toughness of the material. I would not use a cast bronze hollow bar for the same reasons as the hollow bronze casting.

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steamin10
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Re: Casting hollow objects in bronze

Post by steamin10 » Sat Jul 13, 2013 12:59 am

MattMa01, and JFault: I do not recommend playing with firearm and cannon castings. Period.

Having said that, you are playing with forces that are very definate. End of warning.

Should you be suicidal at various times, go not further. If you want to create unique things and live, read on.

The cannon Barrels I have cast for 50 cal are all cast vertical, breach down and solid. There is a 'head' that tops the muzzle that is 2 barrel diameters tall, and 2 diameters round. The casting sprue comes in from the side at the parting line, near the touch hole area, cut deep in the parting line to provide counterclockwise motion during the pour. A HARD mold is recomended, such as core material, sugar formula, or Silicate (better), so that the speed of the metal does not wash out the sprue channel and leave sand in the casting.

This method of pouring allows crystal formation and even heat /cooling from the bottom up. As the metal shrinks, It will draw metal from the 'Head' described earlier, down the center of the barrel shape. AS the metal freezes out, crystalization may occur down the center of the piece. That is OK, as this is where the majority of the material will be lost in boring. Pouring with the casting on its side, while much easier, will give an eggy casing with unsettling grainyness, or gassiness, on the topside, and in cross section uneven material settling from bottom to top. This is not good, as we must have a consistant metal to even bore straight, let alone the soundness of the metal after being weakened by taking our bore out of a true center. As a basic, the breech of a muzzle loader must be at least 1.5 bore diameters in wall thickness total, as a rough rule of thumb. That means a .50 caliber cannon will have a minimum (1.50 + .50 bore) 2.0 inches of diameter at the breach. (model). I have a stainless barrel that is .50 cal, with a ship mount. Fired with an unpatched .45 ball, it blew the wheels off with 30 grains. It was backed up by a sand bag, but flew in the air anyway. I have a golf ball mortar, that is just a spectacular, chewed out of steel.

Many ways to skin a Jakalope, but use your head, and keep it on your shoulders. Play safe, cause its not.

Copper based gun bronze does not shatter, but is malable and will crack or tear along the bore line, most often. Cheaper cast iron, for gunbarrrels, will fatigue and fracture without warning. Modern replica barrels are now fitted with stainless steel liners for strength. At organized shoots, non lined barrels are generally banned.

Cored bearing material in red brass is a good substitute for cast material, it is spendy (isn't everything?) and will have to have a good breach plug. Otherwise solid stock, bored to caliber is very good. Easy with steels, stick with barrel quality, and save about 60-70% on material for a cannon scaled barrel.

That is as far as I care to go on this home shop Voodoo. I do NOT encourage foolish behavior. Darwin rewards those.

G. Gatskill is your man for info on cut barrels for toy cannon, but not castings. I bid you joy with your toys.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

MattMa01
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Re: Casting hollow objects in bronze

Post by MattMa01 » Sat Jul 13, 2013 1:17 pm

That 1.5 rule, does that go for all muzzleloading barrels or just bronze? I figured that bronze, being softer than steel, would have to be thicker at the breech than a steel barrel. I will get in contact with the person you mentioned about machining the barrels.

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steamin10
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Re: Casting hollow objects in bronze

Post by steamin10 » Sat Jul 13, 2013 11:24 pm

OK.. Rules, are for safty and application. The rule I cite I got from a home built cannon outfit that used to be online. It took into account the vagaries of home building an the innate shortcomings of thinking, and compromising good proactive practice.

Think of the caliber of the (model) gun, .50 cal as an example. The 1.5 rule makes it .750 material at the breech all round the powder chamber. (The powder chamber may be smaller than the bore. Here we get into what is proper gun barrel design, yatta-yata, and I will argue no further. Done in any manner this stuff IS DANGEROUS. So take my input with proper seasoning.) So the concentric circles of bore and material come out to 2 inches. That is for the chamber area. Barrel lightening then takes place in thinning the muzzle and incorparating the thinking of the times. That means a smooth taper, like the coke bottle guns of 1890"s or the earlier Napoeans that incorparated bands between tapers for strength. A major band usually reinforced the trunion areas because they were cast on, and represented a variance in strength. The dynamics of firing a gun put heavier stress on the trunion area, and that thinking was to control it.

Much of old gun building and lore is trial and error, and holds true past the 1870's. Of course engineering had proven numbers, but as meantioned the vagaries of metal content and guesswork led to many problems along the development path.

During the American Conflict of the 1860's smooth bore ordinance rifles and Napoleons were rebored and grooved for rifling for the newer shaped shells. This thinned some barrels beyond their safty margin, and many failed after being reworked. This was for the cheaper iron barrels, as the bronze guns were never worked the same way and remained smooth bores, as the act of firing shaped shell detroyed the lands in the root of the barrel in the rifling tests. It is why the longer range parrot and coke bottle guns, were designed with higher muzzle velocities, as the old Swede that designed the Monitor class of gunboats, argued for longer Iron barrels with more velocity, as the effectiveness of a shaped shell grows by squares. It is a science all itself.

Ok, what you should take from this, is not to re-invent the wheel. Do what someone else is proven to work consistantly and well. There are many proven designs that work well and are easy to reproduce. Doing any less than careful is dangerous.

Sorry to harp, but I cannot stress that enough. Physical damage to humans playing with toys is not acceptable at any level. Sorry to be such a worrying Mary. Getting it right is all the program.

As an aside George Gatskill is noted for running machining classes that machine scaled Cannon barrels in his class. So I take him to be a superior authority, and will set you into books if needed to get things right. He is a moderator on the Gunsmithing threads on this board. I encourage you to seek him out.

EDIT: In reading my drivel, a point for clarity. I mention coke bottle guns, they did not have Coca-cola during the War between the states of course, But refers to the Dahlgren guns of that later period that were shaped with that type of bulbous taper. It may be noted that such blackpowder coastal guns made it well into the 20th Century, with most being cut up in the teens, as breech loaders and nitrate (Nitro) powders gave higher performance demonstrated through the WWI conflict, Exactly as Ericson had predicted to Lincoln so many years ago. It is apparent that Lincoln held sway on the Military to keep the lower velocity guns, as he believed it may hold down the casualty rate. He was forced to decide against killing his fellow Americans, yet win a decidedly horrible war, to reunite the shattered nation. History is full of such moral delemas.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

RONALD
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Re: Casting hollow objects in bronze

Post by RONALD » Fri Jul 19, 2013 8:23 am

It took me a while to find these castings, I had misplaced them.

This canon was an advance project for the students in the Cast Metals classes at the school where I taught Physics.

As you may notice there is a hole in the barrel, that served to support the core print, and was later used for the trunion. Of course, the students also had to add wood parts and other details to make a nice model they could be proud of.

I would not know if any of these were or could be used as the real thing.

Today, there is ZERO technical training at the school, it's all "College Prep"!

I cleaned out the Cast Metals Store Room around 2003, and got a couple of these barrels, saved only one and a set of wheels, the others went into my melts.
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hammermill
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Re: Casting hollow objects in bronze

Post by hammermill » Fri Jul 19, 2013 8:41 am

the school castings would make nice display models....

most larger cannons now have steel bores with bronze exterior to be used at most meets of like minded folks

graybeard north west would be a good site to explore

my aproach to casting would be a solid cast and then machine the bore afterwords

the us navy foundry manuals may well be a good place to look, google book and a few others may have them in the public domane.

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steamin10
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Re: Casting hollow objects in bronze

Post by steamin10 » Fri Jul 19, 2013 10:49 am

I find the castings to be quite nice, from what I can see. The Bore is too big for the material left, and the piercing for the trunion is a no-no to a working gun. This is a generic gun pattern, looks like a gun, but is not a copy of reality that I know of.

On working barrels (models) the trunnions usually are fitted to a ring added outside the barrel so as not to disturb the basic barrel.

The wheel casting is very sharp and clean. Civil war era transport wheels were canted to the outside, and very heavy spokes, 14 in all. A detail that is usually missed. Many museum pieces I have examined are simply wrong in detail, as looking at the tintype pics of guns in battery show Union guns with these details.

Never say never, as the South cut off butter churns for buckets, melted church bells for cannon, and probably used freight wagon wheels, when they had to. Details vary on this kind of stuff because of interpretations and guesswork, or even field repairs.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

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