Re: cast iron

--- In CAD_CAM_EDM_DRO@y..., "Carlos Guillermo" <carlos@v...> wrote:
List -

I've got some questions about some castings I'll be having made
soon. There was recently some mention of aging the cast iron before
machining, and I'm wondering, how long one should let it age? Is it a
thermal issue? Also, how common and important is stress-relieving for
parts that will only be lightly machined? Several of the foundries
I've spoken with said that stress-relieving was not a "must", but
mostly for troublesome parts.

Thanks in advance,

Carlos Guillermo
VERVE Engineering & Design

Carlos,
There are a lot of stories about how to stress relieve cast iron.
Most of them work after a fashion. The objective of the stress relief
is to shake loose the stresses locked in the crystal structure. That
means that the metal must move mechanically so that the crystals can
slip across one another until the lowest energy levels are reached
(the spring needs to get unwound). Thermal cycling, mechanical
vibration, etc all will work, if you move enough distance to slip the
stresses out. On the other hand, If you are only taking a small cut,
and it will never be stressed in service enough to release the
stresses it should not matter if it has some internal stresses. One
of the Indy car drivers likes to use old bus engine blocks because
they are so stable. The shop I used to work at had a rig with a
vibrator that shook it and relieved most of the stresses in one night.
The cheapest way I know to relieve stresses is to put the casting on
a resilient support (inner tubes or springs) and pound it all over
with a hammer. IF you drag it behind a truck for a few miles, it will
probably be very relieved when you quit ;)

Dave.