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Seastar
03-17-2005, 12:17 PM
I need to machine a 1.68" diameter spherical cavity in the two halves of an aluminum mold to cast lead balls for a small cannon.
I have a small lathe (9X21) and a small bench mill.
How should I go about this.
I have very little experience with this sort of thing.

SGW
03-17-2005, 01:10 PM
Well, there's a couple of possibiities. One is to either make or buy a radius-turning attachment for your lathe. (If you look at what they cost to buy, the idea of making one will probably look attractive.) But if that doesn't seem feasible for one reason or another, here's another idea, if you have a rotary table for your your milling machine.

Compute a table of X-Z coordinates for the hemisphere you want to cut. That is, for position X on the diameter, how deep Z do you need to cut to touch the "bottom" of the cavity at that point. Start at one side and compute Z for each .05" increment of X, or whatever seems reasonable, until you get to the center. Then, put an end mill in the spindle, move to the outer edge of where the hemispherical cavity is going to be cut, and zero the Z axis when the end mill just touches the top of the blank. Move over the first delta-X, start the mill, plunge the caclulated Z, and rotate the rotary table to cut a circle of that depth. Then move over another delta-X, plunge to the calculated Z for that X position, rotate the blank, etc.

You'll end up with a more or less rippled surface, depending on how small you made the delta-X increments. And of course you need to take into account the diameter of the end mill you're using, when you set your initial position, etc. But you can get a pretty good hemispherical cavity this way, albeit slowly, and with suitable smoothing/polishing it ought to work.

And of course you'll need to locate some dowel pins to match the two halves of the mold, put in a sprue slot, etc.


[This message has been edited by SGW (edited 03-17-2005).]

[This message has been edited by SGW (edited 03-17-2005).]

Lynn Standish
03-17-2005, 01:10 PM
I think that's about the diameter of a golf ball. Maybe you could use those instead of casting.

lynnl
03-17-2005, 01:50 PM
Since you're into casting anyway, how about making or obtaining a core of proper size, and then casting the mold around it?

[This message has been edited by lynnl (edited 03-17-2005).]

Carl
03-17-2005, 03:54 PM
Hemispheres can be generated on a vertical mill on which the head can be tilted. A flycutter will cut a circular pattern, and if the head is tilted and the work is revolved on a rotary table, it will generate a hemisphere. I've seen it done on a video tape I bought in which the machinist was generating diopters for optical work.

[This message has been edited by Carl (edited 03-17-2005).]

alwynoak
03-18-2005, 05:26 AM
Carls got the right idea,any spherical curve can be produced by fly cutting.better still use a boring head as a fly cutter this then means you can adjust the cutter radius (most important). the bridgeport is tool to make optical mirrors ect but any machine with a swivling head is ok,you need a rotary table and a boring head.to cut a convex hemisphere the head angle needs to be 46deg the machine spindle needs to be on the centre line of the rotary table.and the blank you are cut the radius on needs to be fixed on the centre of the rotary table.turning the rotary table by hand is laborius so think about electrifying it windscreen wiper motors are ideal attach a pulley to the handle of the rotary table and one to the wiper motor and watch it go round or go and go and make a cup of tea.use a battery charger to drive the wiper motor,this will give you 4 speeds.if any one needs more depth info,like head angles for varius radi ect feel free to ask

psomero
03-18-2005, 05:40 AM
<font face="Verdana, Arial" size="2">Originally posted by alwynoak:
Carls got the right idea,any spherical curve can be produced by fly cutting.better still use a boring head as a fly cutter this then means you can adjust the cutter radius (most important). the bridgeport is tool to make optical mirrors ect but any machine with a swivling head is ok,you need a rotary table and a boring head.to cut a convex hemisphere the head angle needs to be 46deg the machine spindle needs to be on the centre line of the rotary table.and the blank you are cut the radius on needs to be fixed on the centre of the rotary table.turning the rotary table by hand is laborius so think about electrifying it windscreen wiper motors are ideal attach a pulley to the handle of the rotary table and one to the wiper motor and watch it go round or go and go and make a cup of tea.use a battery charger to drive the wiper motor,this will give you 4 speeds.if any one needs more depth info,like head angles for varius radi ect feel free to ask</font>


why 46 degrees?

alwynoak
03-18-2005, 06:24 AM
why 46deg you ask,it can be any angle more than 45 deg,if say you want make a 25mm hemi the blank needs to be 51dia 26 high leaves 1 mm all round to clean up.46 deg is just past hemi this means that you use a micrometer to measure the diameter.anything les than 45deg cant be measured because the mike is point contact.

kap pullen
03-18-2005, 07:28 AM
I have made cannon ball molds and other radius work in this way.
I always set the machine at 45 degrees.
You need to preset the boring bar to 1.68 diameter.

I used a 3/4" x 45 degree angle boring bar.
This is more rigid than a boring head.

I preset the diameter by picking up an aluminum block with edge finder, moving the machine center away from the block the required radius and setting the tool with a .001 shim from the block.

I center drilled the molds at the pin locations, and pocket center and indicated the pocket center in on the rotary table.

It is hard to get the "flycutter" centered over the block but doable.
You can use an edgefinder one way, but have to make a cut, rotate 180 degrees make another cut and compensate the 45 degree offset way.

Most purchased radius attachments will not do this because of the pivot pin interferance with the block.

Most will not bore a full spherical radius.

I also built a unit to use in the lathe with an old milling attachment as an actuator. This was for 12# full sized molds (4.687 dia. I think?).

The attachment for this tool fit inside the 4..... dia hole.

I can post a picture later if you want.

If your bench mill won't set over, you may be able to set the rotary table on 45 degrees.

kap

[This message has been edited by kap pullen (edited 03-18-2005).]

aboard_epsilon
03-18-2005, 10:48 AM
Alwyn who posted above is a friend of mine. here is an example of his work.

http://img.photobucket.com/albums/v190/aboard_epsilon/hemi2.jpg

http://img.photobucket.com/albums/v190/aboard_epsilon/hemi1.jpg

all the best.mark