I was turning down some 1045 taking off .250" dia per pass (.125 doc) and
was getting a beautiful mirror finish. After the last pass I was still
0.006" big so I took that. Surface finish went to chit.

What happened?

Wes

Wes wrote:

After the last pass I was still
0.006" big so I took that. Â*Surface finish went to chit.

What happened?

You can't split atoms. :-)
The lighter the cut, the sharper the tool has to be. With carbide (was it
one?) and their dull edges, you can't go below a certain depth of cut (in
that range). Same for HSS, but lighter cuts are possible.
The more flex and bend and rattle the lathe has, the heavier the cuts have
to be to get a nice finish.


Nick
--
The lowcost-DRO:
http://www.yadro.de

Wes fired this volley in news:13ddl8inee0pm33
@corp.supernews.com:

I was turning down some 1045 taking off .250" dia per pass (.125 doc)
and
was getting a beautiful mirror finish. After the last pass I was still
0.006" big so I took that. Surface finish went to chit.

What happened?

Wes


Lost motion somewhere in the machine. The DOC wasn't deep enough to
spring all the lost motion to its limit.

LLoyd

Could be tool sharpness as Nick said or could be that the DOC went
below the nose radius of your tool or could be too much flex in the
setup. This has happened to me so many time that I have a personal
rule. Once you get a cut that produces a good finish don't change
it. The old timers put it this way "Don't sneak up on the final
diameter."

wrote:

Once you get a cut that produces a good finish don't change it.

Right!
Don't change depth of cut, don't change rpm, don't change lubrication.
Except you *know* it works (or you desperately hope so).


Nick
--
The lowcost-DRO:
http://www.yadro.de

Nick Mueller wrote:

What happened?

You can't split atoms. :-)
The lighter the cut, the sharper the tool has to be. With carbide (was it
one?) and their dull edges, you can't go below a certain depth of cut (in
that range). Same for HSS, but lighter cuts are possible.
The more flex and bend and rattle the lathe has, the heavier the cuts have
to be to get a nice finish.


It is a very tight machine that swings 24" or so. I may be ruined by almost
exclusively turning 12L14 The insert was carbide.

After posting, I remembered a few things from my days in college like a
finish cut should be some significant fraction of the nose radius.

Tonight during lunch, I'm going to try a .030 cut which is too small for the
next part I need to make to see how the finish looks. I guess learning just
what DOC makes a nice finish is part of learning the craft.

Another thought is using a diamond stone the next time I miss my desired
diameter and want a nice finish. Touching up a corner on a used portion of
insert might turn it into a finisher.


Wes

Wes wrote:

I was turning down some 1045 taking off .250" dia per pass (.125 doc) and
was getting a beautiful mirror finish. After the last pass I was still
0.006" big so I took that. Surface finish went to chit.

What happened?

Wes



`Your final cut should be at least .010 on most steels unless you are
doing hard turning with ceramic or cbn. If I am holding tenths I take
two final cuts of .010 each. See what the first one does and then
correct the second cut accordingly. If you don't have a dro, put a
dial indicator on the cross slide. A tenths indicator is easier to see
if you are working close. Remember to multiply by two fot diameter
reading. Another trick is to set your compound at 5.7 degrees and that
will give you a 10 to 1 ratio on the compound. It makes it a lot more
accurate because you reduce the error in the acmce screw by a factor of
10. Its also easier to see the numbers rather than the little division
lines.

John