[How do I?] Best Way To Machine This Part

Mutt

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Hey y'all, I'd like to get some advice here. This is a cylinder head off a 2 stroke motor. It HAD a hemi combustion chamber with no squish band (this is from a antique motorcycle) So I desire to up the compression ratio, like on one of my other 2 stroke racing bikes of the same vintage. So I welded the combustion chamber up. let it cool, put in in a fixture (seen in pics, thick rectangular chunk of billet aluminum.) I put this fixture on my Bridgeport on a rotary table, cut the center with a large counter bore, followed on the bottom edge with a ball mill, then did a light face cut with the rotary table. I made sure the face cut protuded out just shy of teh o.d. I really want.

Now I need to machine a 12º cut on the top edge of this weld, from the beginning of 2.425" o.d. inward. This creates a squish band, then I can do the final i.d. of the band for a width of .4375", then polish every thing out.

My question is , what would be the easiest way to cut this 12º angle? in the mill with a rotary table or in my lathe? I have never done wither one of these, but I have to now !!! The block is pretty good size. About 8x10 I guess. My lathe is a 12x36 with a 6" 3 jaw and a 8" 4 jaw

The machined circle here is slightly less than the 2.425".
I need to start my 12º cut basically where the machining starts on this part and go inwards towards the center of the chamber. Whatcha think?
Thanks,
Mutt
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I have mounted them to a stub arbor with a bolt through the spark plug hole to do exactly the job you are doing.
 
Yes, in a lathe. Sorry, I should have mentioned that.
 
In a lathe indeed, it may take far more time to build the fixture to hold the part then the actual machining time to turn it, you are not in a hurry I suspect.

With a dimension desired at four decimal places it will require a machine with excellent control and rigidity (or a good deal of fussing about if not) , if one were to to aim at .437" +- .005 it would be much less irritating and far easier to measure, just my opinion however.

I spent all of today turning bearing bores with retaining ring grooves, the bores were .8755" +.0005 -.0000, all of the parts were rectangular, (squared in mills with a reamed hole for location) requiring 4 jaw chuck work, we have 5 CNC mills including a 4020 Fadal and a Haas 12 X 12, none will interplolate a circle within 5 tenths that is round.
 
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If your lathe can swing it, on a fixture or however you have to hold it, it will be more straightforward and conventional than doing it on the mill. Your surface finishes will likely be better also, leading to less polishing time. I would put a lot of thought into working it out on the lathe before resorting to the mill.
 
It is still on the fixture that I milled it on. I'm not sure that the block will fit into my 4 jaw chuck. My fixture measures 6" x 7" x 1½" thick.

What about something along these lines, so I can chuck it up in my 3 jaw chuck? I can clamp the fixture down to the mill table and use a close fitting rod thru the spark plug hole to keep it centered. Remove the rod , drill a pilot hole, remove head from fixture and drill all the way thru the fixture, Continue to drill fixture until I can use a 1" reamer for the final hole, drill and tap a 5" long section of 1" aluminum round bar for a 1/2-13 bolt to hold the head thru the spark plug hole, then press the bar into the hole in the fixture til it;s flush, then drill and tap the block for a 1/4-20 cup
point set screw to insure the round bar won't ever turn in the fixture?

So I get it where I can chuck it up in the 3 jaw, how do I go about cutting this 12º angle?
 
Thanks Tony. I see you are in my neck of the woods.
 
Yep, not too far. My mother was born and raised in Myrtle Springs. I get up to visit friends in Edgewood once in a while. I still have some distant family down in Ben Wheeler.
 
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