On Sun, 14 Apr 2013 00:43:43 -0400, Tom Gardner Mars@Tacks wrote:

On 4/12/2013 10:17 PM, Ed Huntress wrote:
On Fri, 12 Apr 2013 20:02:37 -0400, Tom Gardner Mars@Tacks wrote:

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?

The flexing -- that is, the spring rate without taking a set -- is the
same for all grades on steel, within a couple of percent. More
correctly, they all have about the same Young's Modulus, and it
doesn't matter how they're heat treated or work-hardened. The modulus
will be the same.

That is, except for stainless, which is a little springier (lower
Young's Modulus).



My cousin told me that, I was hoping he was wrong. I could make it out
of Ti and/or go from 5/8" to 3/4" or a scooch more.

Not titanium! It's MUCH springier than steel. The Young's modulus for
steel runs around 30 (don't worry about the units). For titanium, it's
around 16 -- about the same as gray cast iron (14) . Aluminum is
around 10.

Tungsten is a little over 60. Tungsten carbide is close to 70.

On the other hand, stiffness is a function of the *cube* of thickness
(called "plate stiffness"). So making your part a little thicker will
improve its stiffness by a lot more than you might think.

In your example of going from 5/8 to 3/4, the part would be 1.2 times
thicker but 1.73 times stiffer.

--
Ed Huntress