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Milling a bullet mold question?

Vettepilot

Plastic
Joined
May 9, 2020
Background: Bridgeport mill, older manual machine. Bed just trammelled, machine seems tight and very repeatable. I just used it to machine out two 80% lowers with no problems and perfect results. Now working on a Lee Precision aluminum two cavity mold.

The mold was a 9mm that I wanted to mill out to cast 50 caliber round nose bullets, .800" long. I got everything all set up and indicated/dialed in, and started work with a new .500" round nosed milling bit. I set moderate tension on the quill lock. (Moveable, but with drag.) I then came up .100" with the bed and then down with the quill slowly but steadily to a zero stop. Raise the quill, brush the bit, blow out the chips, lubricate the work/bit. Raise the bed another .100", come down with the quill to zero stop, and repeat until reaching .800" depth. I have found this to be a good technique, especially for smooth holes in aluminum.

So the cavities in the mold came out really great, with a very smooth finish that I wouldn't even need to lap. BUT: Both mold cavities measure exactly .4995" perpendicular to the parting line, but .504" at the parting line. Mold was VERY securely clamped tight together for the milling process, and besides, the oversize is perpendicular to that axis.

What went wrong??

Thanks!
Vettepilot
 
Hmmm ...

Maybe the head is trammed square to the table, but the knee isn't traveling parallel to the quill. Or there's play in the system, knee, and cross slide, that are letting it rock side to side a bit? I'm making an assumption the parting line on the mold was in line with the table.

'Knee rock' is a problem with machines like this. The weight of the table being cranked left to right deflects the other 2 axis, resulting in dimensional problems. the higher the work above the table the worse. I used to drill a hundred plus holes in some large heatsinks, that were held in a vise with tall jaws. And I think maybe the vise was on a swivel base as well. So the face of the heatsink was maybe 8" above the table. End to end, 17-18" extrusion, maybe, I'd get position errors that could be measured with a scale. And the mating parts had to be drilled oversize to get things to fit together. We eventually bought a better mill.
 
I think that you would have been better off cutting under-size and boring it out to hit your diameter. It is very easy for a .5 mill bit (or drill) to cut over-size and almost expected, at least in my less than optimal setup. :o

FWIW

-Ron
 
Moving the knee in mid process is going to allow some workpiece movement versus the spindle axis. I would have locked the knee and both table axes before starting the cut. Enlarging the 9mm hole to .500 is not too much to make in one plunge cut with lube. No pecking needed.

RWO
 
I've also seen cyclic movement with an indicator on an angle plate, cranking the knee up. Like its rocking on the dovetail instead of moving smoothly. Which it likely is.

Also a huge difference switching from up to down.

People much better with mills than me have told me you have to know the machine, how much it shifts when moving the knee, and locking it.

Its a really convenient design for universal use. As is the turret and angle adjustments on the head. And it all compromises the machine for precision work.

Another thought, if you've got the work in the center of the table, and crank it off to the side to tram it, the play in the ways is going to put it out of square. And there are 3 of them adding up.
 
I've always understood that both halves had to be cut simultaneously by feeding them into a "cherry" using a work holder that feeds each half into the cutter the exact same amount.

However, if the bullets will be sized after casting (always a good idea) the only part that will be slightly eccentric are the grease grooves.

Edit: I found this video of the process. Perhaps it will help.

YouTube
 
I think a tiny bit of error in the ball can make the sharp OD take size oversized at start.
One way to avoid would be to pre drill (15/32) and then drill with a drill bit that measures at or under .500 to drill to a depth past the ball EM’s nose, to near depth.. . A drill with a pre drill acts almost as tight as a reamer only a few tenths over micrometer size.
The ball would not Pull the EM off center at start with doing this.

Yes, this for axial feeding a ball nose EM into a part.

Try this on a piece of scrap to see how it works...let us know if you make size.
 
Well, some added info to clarify. The work/mold was quite firmly locked down, with the parting line perpendicular to the mill bed. The oversize is along the parting line axis, which was perpendicular to the clamping force. In other words; the mill vise was clamping the mold blocks together. Then the mill vise was bolted to the bed with the parting line 90 degrees to the length of the bed. Both mold cavities are precisely the same dimensions now, measuring .4995 side to side, and .5040" along the parting line. Hope I am explaining it properly...

It's quite puzzling. If/when I get a chance, I'll find a scrap block of aluminum to do some testing on, but my only guess is that I probably should have used the quill or the knee; not both. I thought I was doing it the best possible way...

I'll likely go ahead and cast a few bullets, and see how they turn out after being sized. These are going to be powder coated, thus no need for lube grooves nor the need to do it with a "cherry".

Thanks!
Vettepilot
 

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The oversize is along the parting line axis, which was perpendicular to the clamping force. In other words; the mill vise was clamping the mold blocks together.

I don't quite understand this. You say the parting line was perpendicular to the clamping force and then say the vise was clamping the blocks together. I don't see how this is possible. To clamp the blocks together (in the casting position), the parting line would be parallel to the vise jaws.

If in fact you did clamp with the parting line perpendicular to the jaws, with "The work/mold was quite firmly locked down," you may have distorted the blocks such that the parting line edges were closer together while clamped (and cut) but sprung apart when unclamped.

I would try the process again with only the minimum needed clamping force to limit work piece distortion.
 
Imagine the mold was made of rubber and what it would look like with the same clamping force

I am surprised that the amount of distortion was so great, but not that it was distorted
 
If the parting line was PARALLEL to the vise jaws the clamping force would be PERPENDICULAR to the parting line, wouldn't it?

I said "I don't quite understand this." :-)
 
those lee blocks are pretty thin, then you took more meat out by cutting a 9mm mould to .50 cal, further weakening it. You simply distorted the blocks by clamping to hard with the vise, it doesn't take much. Good news is that you may still have a useable mould. As stated before sizing the bullets will round them back up, even bullets from the most expensive custom moulds can be oval/warped due to inconsistent cooling, hand pressure, releasing, etc.
Also the few that I opened up or modified, I found that I needed to go .001 to .002 over the diameter I wanted the bullets to drop. I personally like to use my lathe for this with a 4 jaw chuck. Ill clamp the blocks together with a toolmakers clamp then indicate the hole in. Its faster to me that using a mill and a boring head and I think the 4 jaw puts less pressure than a mill vise and its on all four axis. Plus I can grind a custom tool to do what I want.

This time if you use the mill,don't squeeze the crap out of it, hold it just enough to make the light cuts you need. Maybe even hold it in a 4 jaw on the table or clamp it in tool makers clamps and hold them down with toe clamps.
 
It's a double cavity mold, so there's support in between the holes. I kinda doubt it got squooshed out enough to be measurable. And its oversize along the parting line, which should be smaller if it was distorted outward while it was being cut.

The vise was mounted sideways on the table. So my theory is now that the knee is sagging and he trammed the quill to that. So the quill is angled back slightly with respect to the ways on the column.

So as he cranked the knee up, the centerline of the cavities moved backwards with respect to the centerline of the cutter. Which elongated the bores at the parting line.

They should however, in theory, be square to the top face.
 








 
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