View Full Version : Duplicating a pot metal part
12-11-2005, 03:55 AM
I've a question about the practicality of duplicating a pot-metal auto part in brass or bronze. I've had no experience with metal casting but lots with soldering (I repair projection television sets).
As a hobby I'm restoring a 1985 Porsche 928 and the rear hatch lock body, which is pot metal, is cracked. I understand there is no really satisfactory way to repair pot metal but a replacement for the lock is $455. Yikes!
One of my customers suggested to me that it would be possible to reproduce the original in brass with accurate dimensions, as the original is a casting and doesn't appear to have been bored or milled or otherwise machined.
I realize that there's likely no advantage in price to produce just one but I belong to a 928 owner's group and failure of this part is characteristic, so perhaps it would be practical to cast 5 or 10. It seems to me that the mold is the key, perhaps the original can be used to make an impression in some material to create a mold.
I welcome any comments, especially if this is a fool's errand.
If it can stand being shrunk about 10% (it really only shrinks 3% tops but you'll probably file or sand off a bit, too), go for it. 8)
Alternately, you can add some thickness to the pattern (paint, bondo, etc.) to enlarge it to compensate.
12-11-2005, 04:23 PM
The nice thing about die casting is the final piece would end up with exact hole sizes and location without final machining of the casting which is required when produced as a sand casting, in most cases. The down side is the zinc alloys used to produce the part doesn't age well and will swell and crumble in time.
I often get jobs of reproducing a die cast part from an early car that has broken or swollen to the point it won't work. In some cases it's quicker to fabricate the piece from solid stock rather then repairing the original to used as a pattern and then cast and machine. This is done more often when only one piece is required. In some cases, I will cast a brass block in the basic shape and dimensions to save on having to cut up a large billet.
When using an original as a pattern that is under 3" you can often just rap the pattern in the sand to make it larger. Rapping is the term used in casting in which you move the pattern side to side in the sand not to wrap it with some type of material. I have heard of dipping the part in hot wax to get an increase in pattern size but I have never tried it. I have used tape and bondo to build up a pattern.
You could if there is enough call to justify make an over sized pattern so that several could then be reproduced with the correct dimensions. Do your research first. I once made extras of a part for this rare make of car only to find out only about 6 still existed so now I'm stuck with more parts then cars to put them on.
The other way to make this part is with lost wax but unless it has some unusual shape that wouldn't allow it to be sand cast then this might be the way you would have to go but it involves more and will cost more also. I reserve this method for hood ornaments due to the high details required plus the wax pattern can be re-detailed before placing it in the investment. Bob
At $455 per, there will definitely be a price advantage to casting this.
Repair the part with superglue, dip it in molten wax to enlarge it, clean out any screwholes or details that need to be preserved, and make a sand mold. Fill with your favorite alloy. You could recast it in pot metal, or make aluminum ones, which may be easier than brass. Or you could use brass, or something else entirely. You should have only a small amount of finish work, and a piece that's very close to the right size.
12-12-2005, 02:13 AM
Thanks very much for the replies.
It sounds like sand-casting the lock body would be the way to go, but this part is hollow and would need to have the mold project into the interior of the lock body. The part consists of a flat plate, about 1-1/2 x 3 inches with a box section about 1-1/4 x 1/2 inch with a square opening near one end of the long side through which the plastic latch emerges.
I guessed brass as a material as it is non-corrosive, can be obtained in an alloy which is hard enough and has a much lower melting temperature than steel.
I think I need to read a good textbook on metalcasting in sand molds. Any recommended books? Also what equipment is needed besides a crucible, a suitable heat source and the mold itself? I have basic shop tools such as a drill press, bench and die grinders, band saw but no gas welding gear.
Thank you for your suggestions.
12-12-2005, 04:14 AM
ac4aq contact me at my e-mail address and maybe I can help you out with your project. Bob
12-12-2005, 09:54 AM
I would not even fool with a sandmold for something this small. I would try reproducing the part out of wax and pour it with vanishing wax, or even a styrofoam that will melt well. I would offer to attempt it for you, but I am in the middle of rebuilding a furnace and the cold weather is not cooperating. I am sure somebody on here will be more than happy to pour it for you on the next melt though!!
As long as there aren't any undercuts, you can sandcast it easily enough. The reason I suggested sandcasting is because you can use the existing part for a pattern, almost as-is. Bronze would work well, but aluminum and potmetal are easier to melt, and should work just fine too.
You'll need a crucible of some sort and a way to lift it (for a one-off in aluminum or potmetal, a soup can and a pair of pliers are fine, but you'll need a more elaborate rig for brass), a heat source in some sort of furnace, molding sand in a suitable flask (a wood flask is fine), and a place to do all this: dry, well-ventilated, and free of flammable materials.
A good scrounger can assemble all this for under $20 for aluminum capability, and under $50 for brass. The difference is that brass demands a refractory-lined furnace, and a decent crucible/lifting rig. For people that don't have access to a welder, that means a commercial refractory crucible.
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