cutting internal radius?

[Tony]

i'm hooked on this pulsejet thing. at least
for this week. have a new design that calls for a tricky venturi.

basically i need to cut a 1.5" radius inside of a 3.5" aluminum round.

here's the tricky bit: its slightly more than a quarter circle AND i have no access from the other side. so the internal radius will form a bit of an undercut.

if it were simply a quarter circle (like a big internal fillet) i'd cut in steps and file/sand smooth. the undercut seems tricky.

-tony

[Dave Opincarne]

A profiler atachment for your lathe? If it's just a litle undercut a radius tool might get in there.

Dave