View Full Version : milling a casting for a lead weight mold

01-22-2012, 01:21 PM
I've been asked to mill out a casting which is to be a mold for lead fishing weights. I'm wondering if I have to leave some angle on the sides for the easier removal of the cast weight.

The weight wanted is a 3/4 inch diameter slug of lead about 6 inches long. If I had a round nose 3/4 inch endmill I wouldn't be asking this. But I don't have this animal. The weight doesn't have to be round- it could be roughly square with rounded corners, and I can get this with my 1/4 inch round nose endmill. But that means the sides of the cavity will be straight up and down for about 1/4 inch. I'm thinking that this will make the cast part hard to remove.

I don't want to muck about with trying to angle the mold, since it will be hard enough already to align both parts separately to get the cavities aligned. Do I make myself a 3/4 inch round nose cutter to get this job done? The two halves of the mold are cast aluminum, or some aluminum alloy. It shouldn't be hard to machine, but I haven't tried it yet.

01-22-2012, 01:30 PM
Since you are only making one casting and no precision is required, why not just rotate the vise 1 degree and then put a small shim under two edges as you mill it? That will give you a very small taper in 3 directions and will be fairly repeatable.


01-22-2012, 02:06 PM
Square section weight moulds are usually made with the corners N, S,E & W rather than the flats - no need for draft angles at all then.

For a round section I would fit your alignment pins first, then bore the thing from one end, then fit a sprue cutter a'la bullet moulds, with a reduced pouring hole.

http://inlinethumb39.webshots.com/49894/2561305370105506259S500x500Q85.jpg (http://good-times.webshots.com/photo/2561305370105506259aWuGqh)

Bill McLeod
01-22-2012, 03:57 PM
When I make patterns for the foundry it's a 3 degree taper, the casting likes the same. The guy better watch his lead temp pretty easy to get it to hot and damage or ruin an Alum mold.

01-22-2012, 04:08 PM
Ok. I like the idea of boring from one end, but the mold won't allow me to do this. I'll have to plunge cut the half-rounds and make sure they line up.

I did a test- this material cuts pretty easily. I have a spade bit with a 3/8 shank, 3/4 diameter. I can easily grind a round nose on that, so I'm going to make the cutter. That won't take long anyway, and then I'll have the tool.

More difficult might be machining a partial round groove for the eye to set into. I should have a small 1/4 inch socket that's the same diameter as the eye, and if I put a few cross cuts on the end it should turn into a miniature hole saw. All I need is a groove about .030 deep in each side of the mold- perfectly lined up of course. What I can do there is mill one side, lay the eye into it, then hammer the mold together over the eye. That should leave enough of a mating mark that I can then mill a shallow groove there as well. The mold has to close tightly, pinning the eye in place until the lead is poured.

It does occur to me that I can't go perfectly round on the end of the speed bit, otherwise there'll be nothing at the tip to do any cutting. I'll have to leave it slightly pointed, but that's ok.

Sounds like a couple hours fiddling around on a sunday.

01-22-2012, 08:28 PM
Why not split a piece of 3/4 black pipe longitudinally. It will spring open. Take a pair of vice grips and spring it back closed.close one end and pour from the other. After it solidifies remove the vice grips and it will fall out. I bought some pure Tin that someone just poured in 1/2 of a piece of pipe.

01-23-2012, 01:48 AM
Hey, I like that idea. This one's done now though. Machined like a dream. I will mention to keep the mold temperature down though- I had the same thought. I figured it would be best to pre-heat the mold, but not past say 70C or so, then let it cool to that between pours as well. I'm also assuming there's supposed to be some release agent used when casting- soot from a candle I think I read somewhere?

01-23-2012, 01:56 AM
There's a couple of tricks worth knowing with al. moulds and lead.

First, fold up a bit of witing paper and dip it into the molten lead, when it goes brown, but does not burst into flame, the temp is about right.

Preheat the mould by dipping a corner of it in the molten lead, when the lead no longer freezes onto it, it is up to heat.

I have been using al. moulds with lead and lead alloys for many many years for both bullet and sinker making and have had no problems - I've certainly never seen the melt get anywhere near hot enough to damage a mould.

01-23-2012, 09:23 AM
i went and looked out some of my lead molds , there are spherical down rigger ball mould and cylindrical wieght molds with out any draft , and they all cast fine, so you should be fine .

i have to heat all the molds with a large burner and get them good and hot before they produce good castings, ie i am handling everything with asbestos gloves, the heat will soak through leather welding gloves .

i judge the lead temperature with a white pine stick, it starts to char like mentioned with the paper.

i have never put any release agent on the molds, aluminium has a good layer of oxide on it as do the iron molds