Re: [CAD_CAM_EDM_DRO] Re: cast iron

Hi,

I have had some experience with machining cast iron that has been aged
properly. On the whole heat treating has produced a casting that machine
very easily but I have found that they warp to a greater degree than one
that has been aged. As to the violent methods of ageing, I have seen it
done, but the effort involved and the possibility of scrapping the casting
because it didn't work or ended in a pile of metal fragments would put me
off trying.

The removal of the skin is very necessary before ageing because it is here
that the most warpage happens. An added bonus is that you can see the hard
pots and the blow holes, if any, and be able to do some thing about them. I
have used a furnace to heat treat a casting to remove hard spots but have
then aged the casting in the normal way.

I'm sure you realise that you would only remove the skin in the areas that
are required for ways and such to save machine time. It would take a great
deal of time and effort to machine a casting all over.

Any machining induces stress in any metal which is why it is good practice
to normalise after rough machining.

I have had over an 1/8" of twist appear in a 5' long lathe casting but it
could also have been bowed as well. Even small thin walled castings warp. I
was asked to finish a model vertical steam engine a few years ago and when
the column casting was placed on a surface plate for marking out it rocked.
Because it had already been finished machined it was necessary to reduce the
models height once the warp was machined out. At the end of the day you pay
your money and take your choice. The old ways work but that is not to say
that the new ones don't.


Terry

> What about "artificial" heating and cooling in small amounts, but
> cycled more often than every 24 hours (well, 12 or so ,really)
> I.e., a 'weathering' environment chamber made from, say, an icebox or
> refrigerator, with some heating coils...? And a 'lawn sprinkler'
> timer?
>
> Obviously this implies more than 'occasional' need.
>
> Thank you in advance.
>
> Ballendo
>
> P.S. Is the removal of 'skin' NECESSARY to proper aging, or is it
> more a convenience. I ask because you say it's ok to remove
> skin "only where you need to". This would leave a lot of parts
> with "mostly" skin. So why not just age and then do what cutting is
> necessary? Is it that the removal of skin (like wrought aluminum)
> will cause the piece to warp?