Re: [CAD_CAM_EDM_DRO] cast iron

Well, I've seen lots of answers posted about this one. Wonder how many of
those answers are based on REAL experience?

At any rate, having worked in the auto industry for more than 20 years, I
can tell you how they do it and the reasons for it.

Castings develop stress due to various sections cooling at different rates.
This is caused by varying thicknesses throughout the castings. This process
will take place on both investment cast and sand cast parts. This WILL
happen even if the casting is left to cool completely in the sand bed.
Since the entire sand bed is of a much lower temperature than the molten
metal, it is impossible for the casting to cool uniformly as the metal
solidifies..hence the stresses. Even cast flat plates will develop some
stresses.

The auto industry stress relieves the castings by bring the temperature of
the castings up to about 1800 degrees (this temperature will vary based on
the alloy being cast) and HELD there for an hour or more (depends on casting
size and complexity). The furnace is turned off and the castings are
allowed to slow cool IN THE FURNACE (doors are all left shut).

If your getting small casting made, this same procedure can be done at your
local ceramics shop (if you can talk them into using the kiln for this
purpose.

Or, if you do your own castings at home via investment castings, you could
use your wax burnout oven to do this.

With this process, you won't have to wait months for the casting to age
before machining. This same techniques is also used for weldments.

Smoke

>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
>