Re: [CAD_CAM_EDM_DRO] cast iron

Hi Carlos,

> 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.

The ageing of cast-iron is a necessity on dimensionally critical parts. The
larger the part the more critical. I have seen lathe beds twisted out of
true because this process was rushed.

The basis of the process is to stress relieve under conditions that won't
induce other stresses, like with the use of heat.

If the parts are for use in a home shop and are not something that will need
final finishing such are precision grinding of long ways then there is a way
to speed up the process a little.

Firstly the object must be rough machined to remove the surface skin and
expose any blow holes. The skin is very abrasive because there will be some
of the mould material mixed in. The first cut should get under this layer in
one go and I tend to use old TCT tooling for this purpose. The skin should
only be removed in places where it is necessary for the finished article.

The ageing process relies on the constant slight heating and cooling of the
weather. Which is why it is called weathering. It is possible to speed the
process up by placing the casting under glass such as in a south facing
green house without heating. A cold frame will do just as well. After three
months the casting can be finished machined. If the object is not required
for some time I often do another rough machine and put the casting back
outside until it is needed. Then finish machine and grind.

Terry