Patternmaking for Bronze Casting

[Bob Perkins]

Hi Everyone,

In the spirit of DIY and maybe cost..

I'm thinking of casting my own windshield brackets for my runabout. At the same time - I may take the bronze casting class at WBS to feed my insanity.

I'm guessing that step one is to make a replica of windshield supports out of ??? (Fill in the blank)

Wood, plastic - are there special considerations .. maybe some of the home metal casting folks will chime in..

Thanks,
Bob

[Gary E]

I have no idea what your brackets look like but if you go to a small foundry in your area they can show you several methods to make castings and the pattern used for each. Since you will most likely not melt the metal yourself this foundry can be of valuable assistance in saving you time and money.

Let your fingers do some walking thru the yellow pages.

If you cant find the right foundry, start calling a few machine shops and ask them who they use for low volume castings.

[ 12-15-2005, 01:57 PM: Message edited by: Gary E ]

[Dave Fleming]

If you can access USENET check out the metalheads on rec.crafts.metalworking.

A mob of folks over there are into backyard casting including making their own cupolas(sp).

Our hosts, IIRC, ran a series on Pattermaking in the past.

Check out Astrigal Press or Linsays on the web for books on Patternmaking.

[Old Bingey]

I saw a pair on a home-built runabout down at the Apalachicola boat show that a man had fabricated by brazing, filing and sanding. I think he farmed out the milling of the groove for the glass but he did the rest himself. They were beautiful.

[Bob Quick]

Bob
My best friend and I have been doing casting for about 8 years now. Mostly brass parts for blackpowder rifles and now some boat parts in bronze. To date we have cast butt plates, trigger guards, lock plates nose caps, some home decorative stuff, cleats and oar locks.
We build patterns out of wood, usually birch or holly, cast them, then if we like them we will cast a permanent pattern from aluminum. We use that so we are not likely to use the pattern on a gun or boat.
We use water bonded sand and hand ram. Built our own propane furnace for up to a #6 crucible.
If you where out here you would be welcome to come and help on our next pour, hopefully over Christmas break. We would even teach you how to pound sand.

Bob

[Sailman58]

I took the first beonze casting class and recommend it to anyone with an interest in the process. In our class we also built our furnaces, but I understand that the class is all casting now. Sam does give directions to build your furnace at home though. Most of our pattern were made from wood, but some of us did use existing objects like oarlocks for patterns. If you are interested in the class it is offered again this year, but call them early on January 2 to ensure that you can get into the class. The phones open at 8 AM and you will be lucky to get through on your first dialing any time that day!

Ron

[Bob Smalser]

Contact Sam Johnson thru the Center for Wooden Boats in Seattle.

He has a book out...."Bronze Casting for the Boatbuilder"....that shows how to set up an inexpensive kiln from a 5-gallon bucket.

He gives worthwhile classes on casting occasionally at CWB's Lake Union facility.

[Ken Hutchins]

Bob, Dennis Brass Foundry in Amesbury, there are lots more foundrys in the area. Patterns can be most any fine grained wood, don't forget to allow for shrinkage and the sides must be tapered to permit removing from the sand.

[Bob Cleek]

Is it possible to use the original parts as patterns? If they are properly porportioned, you may be able to do so without the shrinkage factor being a problem. I'd consider that before making patterns. As your foundryman.

[Bob Perkins]

Thanks everyone!

A bunch of great ideas as usual. I've been to WoodenBoat School once and I'm 90% sure I'll be taking the Bronze casting class this year.

I'll make up some samples out of wood ..

Or how about a wood/clay pattern? I think they make a clay that gets hard in the oven..

Also - If they are nice - I will make aluminum patterns from the wood one. There are no originals (new construction)

I'll look into the locals and do some more research.

You guys are great,
Thanks,
Bob

[maa. melee]

Might I suggest lost wax or lost foam casting? Sure you lose the template but you dont have to worry about tapering sides, unmoldable forms, etc. Plus it leaves a fine finish with little to no inclusions.

Editted to add: Lost foam/wax probably isnt worth the effort for simple windshield channels, but for something alittle more compliated, like a broach or an 19th century cannon....

[ 12-15-2005, 03:13 PM: Message edited by: maa. melee ]

[Bruce Hooke]

You can use almost anything for the pattern as long as you get the shape right. If you plan to make a lot of castings from the original you do need to consider how durable your pattern will be.

Make sure you understand how the mold will be made from the part BEFORE you start making your pattern! It would be very easy, and a real shame, to get a pattern made and discover that it is not useable. It is not uncommon for the pattern to be split up in some way. For example, if one side of the object is not simply a flat surface (and if you could withdrawn the pattern from the sand via that surface) you may well end up with each half of the object on opposite sides of a board so that the sand molds can be placed on each side of said board to create the mold.

In short, you really should either talk to a foundry, take a class, or at least read up a bit BEFORE you start making a pattern.

Remember that if you want to make an aluminum master from your original master and then make your molds from the aluminum master you need to allow for the shrinkage at both steps.

[Bob Perkins]

I'm going to make something that looks like these I think:
http://www.glen-l.com/picboards/picboard13/pic667g14.jpg

So the ends are curved, The centerpost should be relatively simple.

Hopefully they are not too complicated. I have some reading to do.

Thanks folks.

[Bruce Hooke]

Bob,

I just noticed that you are in the Boston area. I just thought I'd pass along the information that if you find that you would like to work with a foundry rather than do everything yourself there are a couple of good foundaries down here in Providence. Drop me a line at the email address in my profile if you are interested in more details. For that matter, one of the foundries also offers classes...

- Bruce

[kc8pql]

Just to give you an idea, here are the split patterns I made for my rudder fittings. They are straight grained (patternmakers) mahagony. The finished parts were cast at Port Townsand Foundry.

http://tinypic.com/inbyvr.jpg

http://tinypic.com/inbzn9.jpg

[Bob Perkins]

So if I'm reading the pictures currect:

You use the split pattern (1/2 in each side of a sand box) and create the mold halves.

Then remove them and join the halves for the pour.
ahhh.. very interesting..

Nice rudder hardware BTW.

Once of the reasons I'm looking at DIY is that I'd just like to learn how to do it. I'm sure I can buy acceptable brackets for less than the class. But that would be less fun.

Plus - you never know when you'll need to make something smile.gif

Bob

[Ken Hutchins]

Bob, If you want to take a close look at some patterns and get some first hand info, time for another visit. I have a couple of patterns made and a few more that I have to make, patterns are a winter project and it's winter.

[seo]

My suggestion is that it's very likely that an existing part can be used as a pattern if it's of a shape that can be rammed into the mold in one piece. In other words, the part has to be a simple shape that tapers all in one direction. It's likely that windshield brackets can be cast in a one-part mold, which means that an existing part can be used without damaging the part that's used as a pattern.
A foundry guy can tell at a glance if this can be done. The molded part will be slightly smaller than the original that you're copying. Bronze/brass shrinks as it cools. With many parts this is no big deal. With others where fit is critical, it's a big deal.
Patterns were traditionally made out of white pine, which is easy to shape and doesn't show grain. This is often called "sugar pine," and a special lumber yard (Like Condon in New York) might have it.
I've also seen patterns made out of bondo, which is cheap, pretty easy to work, and fairly strong.
There are a couple of job foundries down in Providence RI that I've heard do a pretty straightforward job without a lot of arts-craftsy.
If you have any relationship with a college that has a big studio art department, they may very well have a foundry for pouring sculpture. RISD, Pratt, like that.
seo

[kc8pql]

Originally posted by seo:

Bronze/brass shrinks as it cools. With many parts this is no big deal. With others where fit is critical, it's a big deal.Shrinkage for bronze is 3/16" per foot.

[Ken Hutchins]

Bob ebay
brackets (http://cgi.ebay.com/ebaymotors/Brass-Windshield-Brackets-Chris-Craft_W0QQcmdZViewItemQQcategoryZ26455QQitemZ45974 56 193QQrdZ1)

[ 12-15-2005, 08:56 PM: Message edited by: Ken Hutchins ]

[Bob Cleek]

It looks like patterns for the side wing supports would have to be split. You will have to put the pattern into the sand and ram the sand around it. THEN you have to take the pattern out, leaving the void that will be filled with molten bronze. If you can't "pull" the pattern straight out without destroying the shape of where it was, you'll have to split the pattern. There's a lot to designing patterns that will work. Those pretty compound curves with the open space in the middle will probably take somebody with some patternmaking experience to figure out, but, hey... go for it!

[ 12-15-2005, 09:11 PM: Message edited by: Bob Cleek ]

[Bruce Hooke]

I'm thinking that lost wax casting might in fact be the best way to go at the side brackets. They look to me like a really difficult shape to do in a sand mold. The only conceivable way I can think of is to have the top of the bracket (as installed on the boat) pointed down into the sand, and make the triangular hole in the bracket with a large core inserted into the mold, but it would be a really difficult pattern to make. And, the above approach does not take account the bottom of the bracket where it meets the deck.

The basic approach to lost wax would be to make an original, make a plaster cast of it that can be disassembled so that the original can be removed, then pour wax into the plaster mold and then remove the wax copy and use it to make the actual casting mold out of which you melt the wax before pouring the bronze. So, for each casting you use up a wax template, but that is not a big deal when you are just talking about making one of each bracket. Remember, you are going to have to make a seperate pattern for each side (port and starboard).

It would be really useful to actually inspect a set of these brackets because with a cast object in hand it is usually possible to pick out pretty well how it was made.

[kc8pql]

You can shortcut the lost wax process a bit for one off parts by sculpting the pattern directly in wax.

[JimConlin]

Bob-
I had a hard time figuring out the nooks and crannies of the parts in your photo. Could you post a simple sketch?

I'm still looking for my patternmaking books. They're around here somewhere.

Jim

[Bruce Hooke]

Originally posted by kc8pql:
You can shortcut the lost wax process a bit for one off parts by sculpting the pattern directly in wax.True, but then it anything goes wrong with the pour you have to make the pattern over again. So, it comes down questions like how hard the pattern is to make, how hard it is to make a plaster or rubber mold of the pattern, and how confident you are in getting a good pour on the first try...

[PeterSibley]

Assuming that the glass to be fitted is flat ,(is it?),I'd make up a pattern,either split or solid if its only for a one off and take a bit of trouble to make up a core box for the glass groove .Its easier that way to get a really accurate opening without draft.I've been able to make them to the .5 mm ,1/50"accuracy cast...thats probably going to be good enuff.

A good book would be a help and classes would be VERY worthwhile.Its heaps of fun smile.gif One of the (very)old issues of WB has a very good explanatory article on patternmaking.............reckon it might be this one:

Hardware:/patternmaking procedures for casting/Michael Podmaniczky, 39:58

[bgarvey]

Hi-

I'm a mechanical engineer based in Halifax, N.S. with an old Hanna-designed Carol ketch that I'm slowly resurrecting. I have recently acquired a rapid prototyping machine that can output casting ready mold blanks of any shape/complexity - I've been experimenting a bit with this at our local art college foundry, and have successfully made a lovely new custom cast bronze bulwark knee/integrated stancion base that more suits the vintage of the vessel than any of the ssteel stuff on the market.

It's easy as pie with this method - the old patternmakers look at the pattern and just shake their heads, mumbling about progress and redundancy... not that that's a good thing - but the technology is incredible, and easily accessed. I can show you more if anyone's interested.

... been a watcher on the forums here for a while - nice to actually post something finally!

- bg

[Bob Perkins]

There sure are a bunch of smart folks here.

Thanks for all the ideas. I think what I'll do is make up set of windshield brackets in wood/bondo and get them as good as I can. At minimum they will be a reference point to start from.

Then I might attempt the to make the plaster molds from them (reusable of course). That way - I'll either figure out I need split patterns or go for the lost wax method (Which it seems may be the way to go..)

All of you have made this a lot less daunting. Ken, Bob, Bruce, Jim and all .. This thread should make it to the FAQ!.

Regards,
Bob

My Current project is at:
http://www.imagestation.com/album/?id=4291051329

ps. bg - Post away - many of us love to hear about the new stuff too.

[JimConlin]

Michael Podmnaniczsky taught a WB school course in Patternmaking in about 1982. I enjoyed it thoroughly.

[outofthenorm]

Great thread!

BG - You should definitely post a thread on your new technology. We need to know!

- Norm

[Paul Stohlman]

Back in the day when Patternmaking was a vibrant industry, we (Patternmakers) led everyone to believe that was the apex of the woodworkers art. In truth simple patterns like the ones you are hoping to make just need basic woodworking skills, but there are some considerations to make them succesfully.

There is no need to go to lost wax. If the pattern is to be used once, the approach is slightly different than a pattern destined for production. You can even use the original casting and compensate for shrinkage so the new one is the same size.

Send me a personal e-mail, and we can exchange snail mail addresses. Once I know what you want based on a sketch, or whatever, I would be happy to send you a detailed drawing of what you need to do to produce a successful pattern. A picture is worth a thousand words, and this forum isn't the best vehicle for that type of exchange.

[Sailman58]

Hey Bob Smalser,

Sam Johnson is the instructor for the class at WoodenBoat School. If there weren't 3 or 4 other classes I would like to take next year I would take his over again just to get his feedback.
I didn't know that he had a book out. Do you have any more details? I would like to get a copy.

Ron

[preston]

He has a small book that he hands out in his classes. "Bronze Casting for the Boatbuilder." About 80 pages. A chunk he wrote, plus copies of several useful articles, data sheets, etc.

Preston

[Wherry Jack]

I would recommend that you talk with a foundry to find sources of materials. Each alloy will melt at a different temperature. They may have a source of silicon bronze or you might melt down some old marine castings. Patterns can easily be made of wax and then invested in plaster from an art or dental supply store. Another solution would be to make the patterns out of styrefoam glue pieces together with white glue smooth the surface with wax and pack damp sand around it. You will want to sprue and vent your patterns for both methods or if you make the patterns let a foundryman add the required sprues and vents. By using wax or styrefoam for your patterns you won't have to worry about relief or finding a dividing line to allow you to remove the pattern from the mold.

In 1970 I worked in the Peace Corps in Malaysia helping the Malays with brass foundry work using very primative equipment. Their castings were made with nickle bronze a white colored bronze and they used the lost wax process in molds created from clay reinforced with rice hulls. I taught them sand casting. With the lost wax process they could cast 3/16" 24" diameter trays that were impossible to cast with sand casting.

The furnace consisted of a hole in the ground aboult 3 feet deep and 2 x 2 feet square with 2" pipe comming in the bottom of the hole (they used bamboo you could use pvc)this came out of the ground 6' away from the furnace and was attached to a bellows you can use a vacume cleaner blower. An air chamber was created in the bottom of the furnace by punching a bunch of holes in a piece of sheet metal covered with 1/2" of clay to protect it from fire and setting it on some bricks. 1' of charcoal was dumped in the hole and then the crucible was set on top of the charcoal then more charcoal on top of every thing else inculing in the crucible (it acted as a flux)the molds were then put into the furnace and then more charcoal the whole thing was then covered with sheet metal. It was then fired up. After about an hour the cover is lifted. The molds are now red hot and the wax has vaporised. The crucible with the molten bronze in now lifted out. The Malay craftsmen judged the temperature by the looks of the molten bronze. The slag was then scraped off of the crucible and then a peice of old green Coke bottle was thrown in the mix as a flux (I have seen these in antique shops). Using a tongs to pick up the crucible the bronze is pooured into the red hot mold. When it has cooled you crack open the mold and take out the finished casting. :cool:

[Gold Rock]

Paul's right. I'll offer this anecdote merely for illustrative puropses. A fellow brought me a pair of brackets, similar in function to yours, that held the windshield in place on a sports car. They were very organically shaped; all compound curves, plus a number of machined surfaces and bosses drilled and tapped to accept the hardware for the window crank mechanism. Could I duplicate them? Well, no, not exactly. Why? Because molten metal shrinks essentially uniformly as it cools. If I use your parts as patterns, at best I would give you pieces that are smaller than the originals. Would slightly smaller brackets work on your car? Dunno. The complexity of the shapes meant a prohibitively expensive tooling charge if I made a pattern from scratch, assuming that my skills could even duplicate accurately enough such an intricate thing. He went off scratching his head thoughtfully. I haven't heard back from him.
The brackets in your photo, however, don't look terribly complicated. Chances are a founder could butter them up and put in a core print for the glass slot. They'll still be slightly smaller than the originals, but from the looks of it, I doubt that'd be a problem. The skills to prepare the parts for molding are not beyond average skill if you know what to do. Check out the American Foundry Society on line. They have a wealth of information on all things 'casting', pattern making included. Good luck.