On Tue, 20 Sep 2005 09:43:18 -0700, bridgerfafc wrote:

iron moves. so does aluminum....

I see the scale on my bies rail change significantly with temperature.

There is a number known as the "coefficient of expansion". It's in the
tiny decimals for cast iron and aluminum. Comparatively speaking, it's
off the map for any wood.

Besides which, if you want cast Fe any flatter, you can just lap it in. Do
a good job of it and it will be flat enough to use as a reference flat
(generations of michinists did just that and it worked fine for 'em.)