Sorry for the long delay in replying...

After some experimenting, the answer is...

Raising the boring bar ~1/16" above the centerline reduced the rippling
but did not eliminate it. i then switched over to a brazed on tool as
opposed to a insert tool and tried again. MUCH better finish. not
polished bright, but that nice sort of "honed"finish. and i couldn't
feel or measure any effective rippling.

The difference? the relief on the carbide. the insert had very little,
so when the brass was cutting in a tight bore, it would "pack" slightly
between the wall of the brass and the insert, causing it to push out
from the wall till it "unpacked" and so on and so forth causing the
rippling. raising the bar 1/16" gave it a bit more clearance and reduced
but did not eliminate it. the brazed on bar easily has 3-4deg more
relief, and didn't pack nearly as much.

Thanks for all the help everyone!

--joel
In article ,
spaco wrote:

I ain't the expert, but here goes: If you are using an "insert" I
assume it is a carbide insert. If so, there probably isn't enough
relief on the nose. You know, carbide is brittle so the inserts need a
lot of support under the cutting edge. I have caused the same thing
with boring bearing bronze. I had to switch to HSS cutters, sharpened
with lots of front relief so the heel doesn't drag in the cut. Also,
you need to make a pretty sharp point so it doesn't take much force to
get the tool into the work. If you don't, then springiness will always
be a problem. You probably already know that tools want to dig in to
brass/bronze, so sharpen accordingly.

You would not have this problem on the od because the relief is working
in your favor, whereas in a bore, it's got to be handled carefully.
Maybe you can get down there with a magnifier to see the problem to
confirm it.
I hope others jump in here so I can learn more about boring smooth
holes, too. I have just about totally abandoned inserts for boring for
the reasons mentioned above. As it is, I have a bunch of 3/16" square
cobalt lathe bits that I cut off to fit in my home made boring bars.
One nice guy on this list talked about the "spring constant" of steels
for boring bars and that helped me with stiffness problems. You are
probably in pretty good shape since you are using a Kennametal boring bar.
You may be creating a big part of the problem with the small boring
bar, since the heel of its insert is probably scraping even worse than
the bigger one because of the smaller diameter. And then, because of
the springiness created by the larger insert's geometry, those ripples
are amplified or at least not easily removed by the bigger bar. I would
at least try taking several dead cuts to try to get rid of the ripples
before proceeding.

Pete Stanaitis
----------------------

joel wrote:
trying to figure out where I'm going wrong...


1 3/8" brass hex stock chucked up nicely in the lathe, need to bore a
7/8" x 3" deep hole. i center drill then progressively drill up to 1/2"
and slightly shallow on the depth so i can finish later. everything good
so far. i switch over to a small boring bar to open it up another 1/8"
so i can switch to the 1/2" bar for the finish. The small bar leaves
some ripples which i attribute to tool flex, but i figure the bigger bar
will be stiffer and there shouldn't be a problem. However, with the
bigger bar, the problem is remaining the same. i have lots of ripples,
some as big as 3-4thou down the length of the bore. all combinations of
fast/slow feed rate, fast/slow RPM, large/small cut seem to make no
difference. the larger boring bar is a 1/2 kennametal with clean, sharp
insert. Turning the outside diameter down results in clean, smooth
finish.

I'm ripping my hair out on this. aluminum, steel, etc. seems to have the
same effect.

help?

--Joel