On 4/12/2013 2:54 PM, Terry Coombs wrote:
"Tom Gardner" Mars@Tacks wrote in message
...
On 4/12/2013 8:13 AM, Terry Coombs wrote:
"Harold & Susan Vordos" wrote in message
...

"Terry Coombs" wrote in message
...

I just got to wondering about internal stresses in 4140 round stock .
I'm making a part for an oilless compressor , and have it roughed a bit
oversize . I know that some steels and other metals develop internal
stress from forming operations , and need a few hours to let the
stresses
equalize before finishing machining so the finished part doesn't
distort
. Got to wondering if 4140 is one of them .
--
Snag


It is generally accepted that all parts have stress, the result of the
manufacturing procedure, as you alluded. Hot rolled materials tend to
have much less.

Good shop practice dictates that one roughs parts, leaving sufficient
material for finish machining, with total cleanup. The less material
remaining, the better, as the piece will more closely represent the
finished object, and the small amount of material removed in the process
should make very little difference in movement of the part. I expect
that unless you have a very critical dimension, less than .0002"
tolerance, you should be fine.

Nice job. I spend a lot of my time trying to have folks understand
that
roughing is a very important part of machining. Some don't pay
attention
(until they have the pleasure of reviewing the resulting scrap, anyway).

Time can be a factor, but the roughing should release much of the
stress.
If you have doubts, allow the part to age a day.

Harold

I backed off the chuck jaws a tad before I came in , rechucked it
after a
couple of hours . After recentering it I found about .002 runout on the
last
area I'd machined same setup . Looks like it might have had a bit of
stress . I'll finish it up today .


Speaking of which, would you have a piece of 4140 that is 6" x 2" x 3/4"
or thereabouts that you could sell?

Nope , all I have is rounds in 1.250 and 1.5" or maybe 1.75 , didn't
measure it because the 1.25 was big enough here in . Now I have some
unidentified bigger 2.375 and 3.125 IIRC rounds in Memphis that machine
and harden like 4140 , but I really don't know for sure what they are .

Thanks anyway. I need to make an arm that holds a carbide square like
an arm on a pair of scissors. The current one is cold-roll and is
flexing too muck under pressure. Ya' think 4140 would be a better choice?