Richard J Kinch wrote:
I'm trying to assemble the tools for scraping the saddle ways of a 1970s
Bridgeport series I vertical milling machine back from the typical
swayback wear pattern into flatness. One of the requisite tools is a
straightedge for testing flatness. I've seen the cast iron type made by
Challenge Machinery, which cost $100s even used. And I've followed
Michael Morgan's efforts to supply similar cast iron designs at various
stages of finish.

But I wonder, could not a granite surface plate be sliced up into
straightedges? I note that Starrett sells straightedges made of granite
instead of cast iron:

The problem with this is that a granite straightedge just slabbed off a
large surface plate will be VERY heavy. You probably want a 36" long
straightedge. A 24 x 36" granite surface plate will usually be 4"
thick, and weigh about 15.625 Lbs per inch of width, calculating off my
surface plate. So, a 3" wide straightedge would be about 50 Lbs. I find
Michael Morgan's cast iron 25" straightedge to be heavier than I'd like.
Comparing it to photos of Starrett iron straightedges and their descriptions,
Michael's is at least 50% heavier than a Starrett the same size.
The granite job would be 3 times heavier than mine! Imagine curling
a 50 Lb weight bar for 8 hours straight! You'd be in the hospital!
I DO have a 2" wide granite straightegde salvaged from an air-bearing
slideway, and it is WAY too heavy for any spotting/scraping use. It even
has a series of holes bored in it to lighten it.

http://www.starrett.com/pages/437_straight_edges.cfm
(e.g., 2" x 4" x 24" black granite for $426)

Given that a new 12" x 18" B-grade import surface plate sells for about
$30, and could be sliced up into about 3 18-inch straightedges, ya gotta
wonder, eh? Machine some handles and epoxy them on?

What use would an 18" straightedge be on a Bridgeport? The worst wear
is usually on the bottom of the table, and that is going to be at least
32" long. As all hand scraping books advise, you need a straightedge
as long as what you are scraping, and then some. it is INCREDIBLY
hard to get something bigger than your straightedge flat across its
full length. I know! I have done it, and it was a total bear. I needed
some very exotic tools that few people have access to, such as a
"Talyvel" ultra-precision level.

Jon