Mystery metal

A friend of mine gave me a broken casting with the idea that I might be able to repair it. I warned him that the chances were low, but I'd give it a shot. Its the blade guide from an old scroll saw. And he actually has two broken parts (both appear to be made of the same stuff). One is an optional jig which he doesn't need and can be used for 'destructive testing'.
The parts are cast, non-magnetic, have a dull gray finish on the outside and a grainy appearance at the break. I touched the part to a grinder (no sparks) and it grinds to a shiny finish. It scratches (with my pocket knife) about as easily as a chunk of 6061 aluminum I've got lying around. So I figure its some alloy of aluminum.
Now for the fun part: How to repair it. I've got a propane torch (for soldering small parts, plumbing, etc) and an oxyacetylene rig. What's the 'best' method for repairing something like this?
I've heard of Alumaweld. Is it worth trying? Any hints or advice?

From your description, I'd be it is what I've always heard called "pot metal". It has a very low melting point. My one attempt to use some of the zinc based aluminum repair rod did not work. I have had very good results with the rod on aluminum, but not on the pot metal. I suspect the melting point on the pot metal is lower than even the repair rod.
You might make one attempt with JB weld. I've been amazed at some of the things that it has fixed. The next step would be make the part out of an alternate material. Sometimes a bit of cutting and drilling can make the tool usable again.

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Second that. Use JBweld. If you can drill and pin as a re-inforcement, that would help too.
Karl

DanG wrote:
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A combination of JB weld and a couple knurled (for the JB weld to grip) alignment pins in holes strategically drilled in the parts would likely get it back together and rigid enough for it's intended use.

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Sounds like zinc or zamac (alloy)
Use it for a pattern and cast a new one?
Another thought would be to ram casting sand around it and heat the whole thing up in a kiln to > 750F and see if it will reflow.
Just guessing on the suggestion above.
wes

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They sell Alumiweld: http://www.alumiweld.com/h2uaw.html Zinc alloys with aluminum and I've heard of people having success with it, but haven't used it myself.

On 11/25/2010 8:13 AM, Paul Hovnanian P.E. wrote:
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it's pot metal - there is a special rod for that, oxcyacetelyne torch, carbon rich flame, follow instructions cAREFULLY, do not overheat. i've welded it successfully, and there are people who do so professionally. If you only have a propane torch, take it to someone with proper equipment.
Or machine a new blade guide from aluminum

On Thu, 25 Nov 2010 08:13:06 -0800, "Paul Hovnanian P.E."
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Your object is probably pot metal or zamac, which is an alloy of zinc and aluminum. Alumaweld and other such "miracle rods" are essentially pot metal too, and they work even better on pot metal than they do on aluminum.
However, pot metal is about impossible to weld without something to contain the object being welded because it goes from solid to very runny liquid. There is a ceramic putty available that is good for this, and casting sand will also work. You might think of the process as casting in place, though it is also welding because molten filler metal fuses with the parent metal or parent metal is fused to flow together. The sand or putty helps ensure that it flows together in a predictable shape rather than forming a splat on the floor.

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... undoubtedly of pot metal (a zinc-rich aluminum alloy).
I've drilled perforations near the edges, applied epoxy (try to find the industrial stuff, Marine-Tex or JB Weld for this job; little blisterpaks in the grocery store have inferior goods) and used twists of wire to hold the irregular parts (thread the wire through the holes, twist tight with pliers).
You might also consider using a repair plate, just a bit of aluminum shaped to the surfaces, with epoxy and pop rivets. Plates on both sides makes a stronger joint.

"Paul Hovnanian P.E." <> wrote in message
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Blade guide from a scroll saw? It is _most_ likely cast aluminum. Funny thing, a guy just brought me one of these the other day but won't get a chance to work on it till tomorrow, Friday. I've seen other parts similar to this and they were weldable with the TIG process. JBWeld has zero chance of success, same for any epoxy. That blade guide is under a lot of stress in action! Alumaweld is also a waste of time IMHO. If the puddle goes 'black' under the arc then I would say 'potmetal' but that's unlikely. Cast aluminum has been around for a long time. The fractured surface of 'potmetal' is rather fine grained and cast alum has a rough grainy appearance. So, TIG weld it and be done. phil k.

On Thu, 25 Nov 2010 19:58:15 -0500, "Phil Kangas"
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Another test is copper sulfate solution. Pot metal will turn black, aluminum will be unaffected. Copper sulfate is readily available for controlling aphids.
If it is ally and you don't have TIG, there are some good aluminum brazing materials available at http://www.tinmantech.com/html/aluminum_aero_braze.php
Welding aluminum with a torch and a good flux is certainly possible, but learning to do it takes a fair amount of practice.

Don Foreman wrote:
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I'll try that before diving in with the torch. Thanks.

Paul Hovnanian P.E. wrote:
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google "welco 52"

in message
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If it does turn out to be 'potmetal' then measure it and make a new one from scratch ...... phil k.

You can expect a lot of speculation, after all, you're the only one that's seen the actual part.
I've used those "miracle" aluminum/zinc alloy repair rods for several decades, and I'll continue to use it any time I encounter a good use for it.
I've used numerous brands, and they all essentially result in the same type of repair, with the same process. I used a propane torch before I bought a MAPP torch, and the only thing that differs is the time it takes to heat the workpiece to the working temperature.
A sparkplug hole in a big, honkin' Homelite chainsaw that not only had the original threads chewed away, but also had the fuctup hole enlarged by a self-tapping steel thread repair part, which the owner also used until those threads were gone. Result was a hole that was irregular and about 3/16"+ oversize. I didn't even remove the integral head/cylinder which would've cost a big piece of pocket change.
I've used that rod to fabricate strong boat accessories from 1/8" hardware store aluminum stock, and they endured such abuse as repeatedly bumping a 5 lb downrigger weight on the bottom, trolling in unknown water.
Some repairs aren't cost effective, time-wise (the rod doesn't cost much), and making a replacement part is more practical. That's not for others to make an evaluation of.. it's up to you.
I'll generally put more time into fixing my own stuff because I know the tool/item will be used properly.. for others, you have no idea of who will end up using it, or what for.

On Thu, 25 Nov 2010 23:34:58 -0500, "Wild_Bill"
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I'd agree. A few pins and JB Weld won't hold a fraction of what the aluminum braze stuff will hold.

I haven't found many uses for JB or other liquid epoxies. They are basically plastic compounds with little structural strength. These products can sometimes be used where the epoxy forms a cast around (or fills) a broken part, but are generally just a temporary fix, IMO. For filler, such as filling unneeded existing holes in wood, I prefer to use the quick setting epoxy putty product, which just requires a little kneading to mix it.
One use for JB was mounting a claw hammer head on a hickory handle without a wedge. The hammer was a new Stanley and the handle broke just inside the head during the first attempt to pull a nail. I assumed that there was a misalignment issue during manufacturing that stressed the head end of the handle to the point of a partial fracture. The hammer was put aside, and several years later, I thought I'd reshape the end of the handle and reset the head with JB, which has held up, but I've got better hammers for carpentry-type tasks, so the short (about 1" shorter) Stanley doesn't get used much.
Strength specs for the aluminum repair rods are available from a variety of sources, so one can fairly easily compare them to other options. I believe the repair rods method of joining aluminum is far superior to other options such as pop rivets, for example.
I tried making a nut for an acme table screw (one of those Chinese X-Y tables), by making a steel sheetmetal form with the screw passing thru it, but I neglected to coat the screw with soot, and the nut was too tight on the screw. I later decided that new acme screw stock and tapped brass nuts would be a better solution.
I presume that other small parts could be cast with the rod fairly easily in open molds, for model or replica parts, for example.

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I would make a drawing showing the critical dimensions in case the repair process destroys or deforms it.
jsw

Paul Hovnanian P.E. wrote:
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Don't know how you're going to fix yours , but this discussion has me digging out my old tabletop bandsaw and machining a new lower blade guide for it . Some things a portaband , 4X6 , and scroll saw just won't do . Like curves in 3/4" lumber .