David Anderson wrote:
How to get the best machined finish on aluminum casting? With my face mill,
I get a near mirror finish, but with internal boring bar, I get a terrible
finish, despite variations in tool geometry. A cutting fluid?
TIA, Dave


Maybe you need to tell us about the machine, how the boring bar is
mounted, etc.

I got a 3" boring head and set of carbide boring bars a long time ago.
I got horrendous chatter and finish that looked like the scales on a
Carp! I had the boring head mounted on a 1/2" straight shank arbor
into a 1/2" collet on a Bridgeport M head. The speed was probably too
high, and the spindle/arbor wasn't stiff enough. I eventually upgraded
my machine to a Bridgeport J head, and got a boring head arbor that was
a straight mount into the R-8 taper. Suddenly, I could do no wrong!
I've never had chatter with the boring head since. I sometimes bore
aluminum dry, but usually brush on a film of dark cutting oil with
a toothbrush.

Jon