I want to experiment a bit with building my own woodworking machines. The hardest part by far, IMO, is fabricating the tops, frankly because I can't make my own castings and I don't have a giant surface grinder.

Does anyone here know of a source of various sized cast iron tables? If I can't find what I need in cast iron, is thick sheet steel a viable alternative?

As for grinding the tops, is there a way to accurately grind a part that's bigger than the capacity of a surface grinder? The part would need to be repositioned halfway through the grinding process. Will the repositioning throw the whole thing off? Accuracy needs to be about 0.003".

Thanks.

Find somebody who has a blanchard grinder. These are suited for griding bigger items and will do it relatively quickly and cheaply.

When we built machines within our own shop, we used tables from a company called AMT..they were the largest in the business at one time.
They had all kinds of tables and tops, including Cast iron of different thicknesses. i did a quick search and did not find their home page, but here is a reference from a Engineering mag.
http://www.engineeringtalk.com/news/mie/mie144.html
Maybe you can find it from there. I think they were located in Illinois.

For a quick source, maybe a ordinary CI surface plate will work ?
I saw a bunch of small ones for 25 bucks apiece at the Cabin Fever show this past January.
When I went back to buy one the next day, they were gone !