Phants wrote:
"Kelly Jones" wrote in message
...
Hi all. I just got a new lathe for Christmas and I have been trying
it out. I have been trying to turn a piece of unknown CRES from
about 0.75 OD to about 0.38 OD. I have tried various speeds and
feeds but still keep getting a very rough finish. The finish is so
bad that it looks as is the metal is being torn rather than cut.
The surface is full of small slivers that would cut your hand i a
heartbeat. I am having similar problems with an unkown carbon
steel, although not as bad. I have checked the Machinery's
Handbook, the Army metal working PDF, and some engineering reference
books I have, but no help. I thought that I might even be using the
tools wrong (cutting on the wrong face) but I can't seem to find the
source of the problem. The lathe is a Grizzly 12 X 36. Any ideas?

Hi Kelly;
The speeds, feeds and the amount being cut will all have an effect on
the surface but those are probably not the problem.

Do you have a bench grinder for your tool bits?

Two points:
1) line up the tool bit (where it is cutting) with the exact center of
the piece being turned. You can see this by taking a "face cut" - if
you are low you will see a little round "stub" when your tool bit
passes the center and under it. Shim the tool bit up repeatedly until
the cutting point passes thru (or almost) dead center. (better to be
a few thousands low than high - if high the "relief" or under side
will be "rubbing" rather than cutting)

2) Find a reference on the angles you should grind on the tool bit. In
general the softer the material, the sharper the angles. Also check
the rasdius at the cutting point - anywhere from a "hand stoned" very
slight radius to large "form tool" for a radiused shoulder on the
work.

What you ask is not really "simple" and you will still be learning
these operations next year. You can learn about your "speeds and
feeds" (for each material) from the colors (steel) on the continuous
"chip curl" - or the small separate chips that fall to the bed...
Aluminum will curl and spiral or be in ling straight "strings."

I suggest you find a machinist, toolmaker or modelmaker in your area
that will let you watch and ask questions as he demonstrates - You
will learn more it two hours that way than with all the instructions -
then read the tables and instructions and play... You're going to love
this...
JHbs
Sounds to me like your 'tool' is too sharp (read no radius).
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