[TheForge] TheForge Digest, Vol 106, Issue 24

[Andrew Vida]

On 11/14/2012 2:28 PM, Peter Fels & Phoebe Palmer wrote:
> OK...point made ( again). Metal content.

> I'm a terrible hand
> at my old lathe..pig-ignorant. I was ineptly cutting a tapered hole
> in the length of a chunk of what i presume to be naval bronze
> shafting i'd drilled out, with an improvised boring bar... (it's
> about done now) , but it was slow and  there were horrible squealing
> and crunching-gravel like sounds . The shavings were masses of tiny
> sharp pins..ugly! I tried changing all the parameters i thought might
> help ( bit level, cutting face angle, feed rate), to little avail.
> Perhaps the cutting bit had too wide a face? The boring bar wasn't as
> rigid as i would have liked. Any pointers would be appreciated.

Boring bar must be maximally rigid.  You want the thickest and shortest 
bar that will get the job done.  In some cases you will want to start 
with a smaller diameter bar and change to a larger diameter when a lot 
of material is to be removed.  If you are still getting chatter, then 
reduce your depth of cut but try to use the fastest feed possible until 
your final pass.   Try not to cut too lightly on your last pass.  On 
many lathes a very light cut will not produce the expected result.  Your 
tool must be very precisely on center, though better a thousandth or two 
BELOW than above.  If your lathe is sufficiently trustworthy to put the 
tool on path, I would never go less than .010" on a final cut as a good 
chunk will deliver better results than a light cut.  Depends on the 
lathe to some extent, the tool geometry, and how precisely the tool is 
set to center.  A centering scope is probably a good investment, though 
I confess to not having one myself.

The geometry of your cutting tool must be right for the hole to be bored 
without chatter or to have rubbing, which is even worse.

The machine talks to you.  One can feel when it is cutting right and can 
hear and see it as well.

I will point out that some bronzes and brasses do not machine readily. 
Beryllium copper is notoriously uncooperative.  Hard oxygen-free copper 
has a very bad habit of tearing in the direction of tool action, leaving 
a lousy surface.  I have found that this can be corrected with the right 
combination of tool geometry, depth of cut, speed, and feed.