Cut My Own Mould

[nhrifle]

Well fellers, I gave it a shot and cut a mould. NOE hasn't made another run of their 50 BMG mould and I want to cast for it to make my shooting a bit more affordable and just to see if I could make something shootable. My first effort is a bit of a disappointment but it did serve as a good learning experience. I know the mistakes I made and what I need to do differently on my next. I will make this work!

I made the blocks from 6000 series aluminum with 3 alignment pins. The sprue plate was taken from another mould to expedite things, so it is a bit undersize for the blocks, but did serve the purpose quite well. The cutter I made is a 4 flute design, with a .501 bore riding section, .512 driving bands, and three deep lube grooves, and a target weight of 800 grains. I slotted the blocks to fit Lee handles.

All in all, I don't think it was too bad a result for my first grease groove mould. It took me about 4 days off and on to finish, most of the machining being pretty basic stuff. The mould halves don't line up with each other perfectly and fill out wasn't great. The driving band is too thin, and both the bore ride section and the driving bands are undersize by a few thou, .498" and .507"/.509" respectively. I have no doubt I could send these down the bore safely, but I can only imagine the leading I would get! On a happier note, I couldn't be happier with the weight -- 875 grains! These things are huge!


This is my cutter before I milled the flutes. Made from O1 drill rod.


Not a great picture, but a view of the cavity.


Here's the mould assembled on handles.


Finally the mould with a few of the boolits I cast.

This project didn't work out, but I am not giving up yet. I had fun and learned a few things. If nothing else, I made a pretty cool ingot mould! I'm still hoping NOE will be making another run of their BMG moulds though... Swede? Help me out?

[Love Life]

Is the release agent causing the smaller diameter?

[nhrifle]

If it is, only just a little. I tried casting without the release agent, but fill out was horrible. Those were undersize as well. Next mould will have vent lines and hopefully will fill better. I think the reason they are undersize is I didn't figure in shrinkage when I made my cutter. I am trying to find a formula that will get me in the ball park so I can figure how much larger than target diameter the cavity needs to be cut. Perhaps someone on here can chime in and figure how much over cast size I need to go.

[CastingFool]

Go online and do a search on shrink rate of lead. I did and found this.
http://www.muzzleloadingforum.com/fu...hp?tid/234730/

[nhrifle]

I gotta work on my searchfu, never came across that! Thanks for posting CastingFool!

[Doc Highwall]

Check out this site.

http://www.lasc.us/CastBulletNotes.htm

[nhrifle]

More good stuff, thanks! I will have to go over those numbers more carefully, but it looks like I will need to cut the cavity about .0045" oversize to get what I am looking for.

[theperfessor]

Good try, I bet your second try is successful.

If I might offer a suggestion -

Do you have another mold made from the same material? Measure the base band diameter. Then cast some bullets in the mold and measure the same feature on the bullet. Shrinkage is proportional - if it takes 0.003" difference for a .30 caliber bullet to come out the right size, then it will take 0.004" for a .40 caliber mold and 0.005" for a .50 caliber.

(I'm guessing at the numbers, I picked them as an easy way to illustrate my point.)

Remember, it's not just how much the lead alloy shrinks, its also how much the mold cavity expands as the mold heats up to casting temp.

[nhrifle]

Hopefully on my next mould the halves will line up better. That was my fault anyway, I rushed into cutting the cavity without having the blocks squared up properly and using only two alignment pins rather than three. They shifted when I cut them. Oh well, live and learn. Before I go crazy making new parts I'm going to try facing .010" off the inner faces of both blocks and try to recut the cavity, again just to see what happens. Maybe that will work. If not, I am going to proceed with a revised procedure for cutting the new mould. I am going to make three cutters, one for an initial plunge cut into the clamped blocks to establish OAL, a second to open up the bore ride section to .505", and a third to cut the driving bands to .516". Somehow I will make this work.

[TomAM]

The Perfessor is right; Alloy shrinkage is only part of the equation. Aluminum block expansion is substantial at casting temp. Still, you need your cherry to be about .003" larger than desired bullet diameter. You already have the exact answer, though, because you now know how much smaller your boolit is than your cherry.

[Doc Highwall]

Do you have a rotary table and a Bridgport? You could rotate the table truing up the off center part and open the cavity at the same time but not much room for error.

[nhrifle]

Doc, don't I wish. I am working on the Harbor Freight clone of the Seig X2 mini mill and no, no rotary table either. GRRRRRRRRR. So much I could do if I had the tooling. My mill is good and cuts way better than it should, but it does have serious limitations on axis travel. I think by milling the faces of the blocks I can reduce the size of the cavity, leaving the majority of the cavity intact to guide the cutter. May or may not work and I will definitely be making a second one with improved dimensions.

I can't express properly my appreciation of everyone who has posted advice on how to proceed. Keep 'em coming! There's more than one way to skin this cat!

[zuke]

I'm impressed, for a first of it look's nice!

[rattletrap1970]

I didn't notice vent lines. Unless I'm mistaken for proper fill out you're gonna need ample venting.

[dkmulford]

The cutter I made is a 4 flute design, with a .501 bore riding section, .512 driving bands, and three deep lube grooves, and a target weight of 800 grains. I slotted the blocks to fit Lee handles.

I'd be interested in seeing a picture of your cutter.

[Plastikosmd]

Could u start a thead on bmg and bp? Would love to read it

[220swiftfn]

I'll toss in a suggestion of a short section (like 1/8-1/4") of pipe welded to the top of your sprueplate so you don't get voids in the base.....

Dan

[nhrifle]

Here are a couple views of the cutter I used, basically a 4 flute reamer design



Plastikosmd, I assume you are referring to stoking the BMG case with black powder? That's something I have thought about often, and when I get enough to feed my muzzle loaders with some to play with I will probably try it. I think it would be a hoot -- the smoke and fire, almost a mini field gun effect. If anyone has tried it feel free to share your experiences.

[deltaenterprizes]

Did you do any relief to the cutting edges to sharpen it or did you use it with just the flutes cut?

[akraven]

Excellent job especially for a first attempt. I hope to do as well as you did.