Techniques for milling internal bowl shape

Hello, I'm not a machinist but now we have a mill at work so I've got to get up to speed pretty quickly.

Fortunately I get to use the mill all I want to make my own parts in the name of learning.

One of the parts I want to make is part of a diaphragm pump. It's a thick plate (probably aluminum) that get s a 6" diameter dish about 1-1/2" deep.

Question is, what's the best way to machine this? I would imagine roughing out with an endmill then gently going at it with a large ballmill, but that takes a good amount of time. Not that I care for this particular application, but are there better ways to machine smooth internal radii? It would be a constant radius, if that helps.

This is obviously a better lathe job, but our lathe can't handle 9" round stock.

I think I have a process down for removing tool marks and anodizing, but it does little for averaging grooves left by taking too coarse of steps. Elbow grease is always an option to smooth it out, but is an extra step that would ideally be avoided.

Link to an old thread with lots of information: https://www.practicalmachinist.com/...9400/?highlight=milling+radius+telescope+lens

LexD said:

Link to an old thread with lots of information: https://www.practicalmachinist.com/...9400/?highlight=milling+radius+telescope+lens

Click to expand...

Unfortunately the pictures are gone but I got the idea. It's very clever.

I probably should have mentioned the mill at work is a 3 axis Fadal (with a 4th axis table) so that particular trick is out for this, but would be very useful with the rotary axis to make arbitrary radius grooves along a cylinder.

Strostkovy said:

One of the parts I want to make is part of a diaphragm pump. It's a thick plate (probably aluminum) that get s a 6" diameter dish about 1-1/2" deep.
.
.
It would be a constant radius, if that helps.
.
.

This is obviously a better lathe job, but our lathe can't handle 9" round stock.

Click to expand...

Hold on. My bet is that - in THIS case - your lathe CAN DO.

Let us assume a SB 9", a Hardinge, or even a H-F LSO / abomination.

- your "round" is only a thick disk either cut off a bar, but more likely a plate cutout, perhaps store-bought, already Blanchard ground.

- your shape is CONCAVE, not convex, and it is shallow relative to its diameter.
First, that means it need not clear the cross. Second that a(ny) radius-turning attachment can easily be DIY'ed for the tasking. Because it needeth not the "Cee" frame to place the pivot "under" the shape - as a CONVEX hemisphere would need.

- If you have a faceplate you are in business.

- If you have a chuck, no faceplate, pull the jaws, use their slots for tee-bolts.

- If you have NEITHER, order-up a typically under $100 "backplate" for your lathe's spindle. It "becomes" a faceplate. For this task.

If you cannot back-bolt directly into the stock with through holes or blind holes? TiG the outer rim of the stock to a sacrifical plate and back-bolt into the plate. Skim-off the TiG when done. One uses a boring-bar to "reach around" the outer edge of diameters larger than the usual a conventionally-mounted tool can be made to reach. Or mills, belt-sands, etc. to remove the weld as an off-lathe op.

Turning a shallow concavity on a lathe is dead-simple, just a mite TEDIOUS.

But you can polish the bugger to as smooth a surface finish as you care to do. "Optical grade", even. Or not even NEED to polish.

DONE, and soon, too.

Learn something mill-ish on the mill. It's what THEY are good at!

Lathe work? Not so much!

As with trying to teach a pig how to whistle. It just wastes your time and annoys the pig.