The tin can lead casting.

[graham-xrf]

The melting of 2.1kG of lead in discard sacrificial half of a lube spray can in a woodburner turned out OK, though with lots of good experience.
That means lots of "don't ever do it that again" all the way to discovering how to deal with shrink voids in the top surface.
Originally from thread --> "Melting Lead Shenanigans", this is what happened.



Lead melts at only 327C (621°F), so putting the can among the wood coals in a living room woodburner was total overkill. Wood coals glow red hot, so are 700C to maybe 800C. Even so, it worked. Upon cooling, there was this big hole in the top surface. At first, I had no idea why.

Fill the hole
I went for a re-melt, this time done lots more conveniently using a regular plumber's butane/propane torch. I mean one of those atop a disposable hand-held fuel cannister. I stood the can on a piece of old fireplace insulation board, and just heated from the side. This time I could more easily stir (with borrowed screwdriver). Some yellow yuk slag and some bits of black stuff came to the top, so I dragged it to the side with a teaspoon, and scooped it out. I kept stirring, but very little more slag came out.



Then I got it that as the lower part cools, it shrinks from the outside. The fix was to let it cool, but keep re-melting from the top to keep the void filled. This, I guess, only happens when the top is wide open, and would not occur in a proper mold with sprues and vents. Clearly the cooled cylinder had shrunk lots more than the can, but it still would not just "drop out". I tore off the can in by twisting it around pliers "sardine can opener" style. It turned out better than I expected.

Upside down view.
A perfect replica of the shape of the bottom of the spray can.
Now we begin exploring what it takes to machine lead. I may need to give it a temporary aluminium disc screwed, glued, or somehow fixed onto one end, to have something that won't just go squish in chuck jaws. There is the need to bore a hole right through it. Drill press? Lathe? Whatever it takes to fit a support mandrel up it, so that one can have the other end held by a tailstock centre



I think the slag drag teaspoon looks well enough to get a wash and be returned to the kitchen before any confessions become necessary!

 

[pdentrem]

The fireplace had superheated the melt and thus formed voids etc upon cooling. By heating with the torch the temperature was lower and therefore less voids. Flame polishing the top surface to allow the shrinking voids in the cast to be filled by the liquid metal at the top is a common process we use in casting our metals. Having a sprue would of solved this issue.
Pierre

 

[benmychree]

Slag on melted lead can be reduced by adding a little powdered rosin, it fluxes the metal and turns most of the slag back into metal.

 

[graham-xrf]

Slag on melted lead can be reduced by adding a little powdered rosin, it fluxes the metal and turns most of the slag back into metal.

That sounds like a better way, leaving what discard slag there is not be partly lead, or lead compounds.

I had been reading about the use of carbon, from a piece of candle thrown in, stirred, to have the impurities and junk bond to, and float to the surface, which is then dragged off as slag. That trick had better be done outside, because the candle fuel vaporizes and suddenly makes a hot, dangerous, smoky event. It tried it, but I had to close the woodburner doors on it immediately.

I do have some rosin flux, along with several other sorts one might use in making solder joints on copper heating pipes. I did consider using it, but I thought it might end up with the can tinned and soldered onto the lead like an old gutter. I had no way of knowing. I need not have worried too much. In the end, I have a roughly cylindrical lump of lead starting material, and I have to figure out how to make a shape a bit like this..



For what it has to do (block X-rays), it seems it pretty much has to be lead. Adventures in that area is stuff for another thread. It has all sorts of potential to provide lead turning chips excitement!

 

[francist]

and I have to figure out how to make a shape a bit like this..

What's the approximate size on that piece? Like basic diameter -- just trying to get a feel for how that part might be produced and it's hard to weigh approaches without knowing rough size.


Edit: eh, forget it. Probably should have thought about it for two seconds more -- it'll be about the diameter of the tin can you used, right? Duh....

 

[benmychree]

I don't think that I would use paste flux, it contains greasy matter that will certainly flame up. Powdered or lump rosin does burn but not flare up, but it does make a lot of pungent smoke. I use it mostly for cleaning Babbit metal before pouring to partially eliminate the loss of some of the alloying elements through oxidation.

 

[graham-xrf]

What's the approximate size on that piece? Like basic diameter -- just trying to get a feel for how that part might be produced and it's hard to weigh approaches without knowing rough size.

Edit: eh, forget it. Probably should have thought about it for two seconds more -- it'll be about the diameter of the tin can you used, right? Duh....

Please don't let go of the thought. It is much appreciated!
So far, choosing the very cautious approach, I am thinking to file one end as best possible, then fix with screws, or glue (or both) to a aluminium disc on that end. This is to have something better to grip in a 4-jaw.

Next would be to drill a central hole. Then fit a mandrel rod for temporary support from the tailstock centre.
Then do the main shape. The 6 little pockets have now become 8, and can be added later. They are not too critical. I might even try a slow drill operation by hand using my long suffering Makita cordless. How that goes will be in another thread. I have never tried anything like this before, and I am still celebrating my first casting.

 

[pdentrem]

That appears to be a complex shape in the rendering. Lead and it’s alloys tend to not machine easily. Grabs the bits, can be distorted by the cutting forces as well.
I would go about this a different way.
Make a near finished piece out of wood, make a two part mould out of body filler. Use thin plywood, cardboard or metal sheet as a container.
Make the mould with the disk in a vertical position.
Once the filler has cured, I would set up the mould upright with a pour channel cut in at the top edge. Not sure on the scale here but likely use a 3/8” at the part end and funnel shaped which will give you a nice opening and a sprue when filled to the top.
Smoke the interior surface.
Clamp together.
Melt enough lead to more than fill the mould and leave some in the ladle.
Pour slowly enough that the lead cools but leaves no voids.
Might take a couple cast close together to get the right finished part due to too fast cooling on the first cast. but at least there would be less material to remove at the end.
This was the way I made lead soldiers over 40 years ago, and the details in the castings were very good.
Pierre

 

[graham-xrf]

@pdentrem
My thanks for the suggestion.
Casting it is plan B. Given the ease with which the melt operation went, and depending on how well the attempt to directly machine it turns out, we might try to go that way. The thing about lead is, if it does snatch, or wreck up, the unfortunate mess and cuttings can be collected up, and re-melted into something not so messed up.

The diameter of the part would be about 60mm (2.4 inches), but I would probably settle for the first clean diameter that I can cut. The raw casting is 66mm diameter (about 2.6 inches). There is enough lead cylinder to get, in theory, two or more parts out of it, but I anticipated this attempt might need re-tries after embarrassing débâcles.

Lead soldiers can be as detailed as the original the mold was made from, and that has attractions. I knew the tin can would be a sloppy floppy container with molten metal in it, but the plan was to leave it so, and avoid anything involving "pouring". That said, if the mold can be made of body filler, the original form could be turned out of plastic, or aluminium.

"Smoke the interior surface", I guess, means something like "hold a candle to it"?
[I still have some candle left over after the 20mm or so got thrown into the overheated lead! ]

A question - The body filler stuff clearly survives enough to keep its shape, regardless getting "smoked", and lead poured onto it. Does the filler resin get smelly fumes toxic at any point?

I admit that one of my motivations was to make these operations happen quietly played down, with minimum opposition from the good lady and others, who already think that the stuff I get up to is crazy nut-bag, likely dangerous and explosions-prone! Like a UK version of Emmet "Doc" Brown from "Back to the Future". Then, they also want to know why I need a Geiger counter!

 

[pdentrem]

The filler will off gas when it gets hot. Kinds of smells like resin burn off when soldering electronics. Nothing to notice for the short time you are likely to use versus my casting 100s of soldier.
You should be melting lead in a vented space in anycase.
Once the mould gets too hot, I would set it aside for a time until I could touch by hand comfortably. It will breakdown over time of course. The more total weight of metal and thicker the section, the slower it cools and the warmer the mould gets. In your case on a short run I foresee no issues.
Yes candle smoke or using graphite powder can work.
Pierre