geebee509 writes:

What is the proper procedure for checking the flatness of a part using
a surface plate?

Step one is, "buy or borrow a copy of Connelly's _Machine Tool
Reconditioning_".

Do you coat the part with Prussian Blue?

That works, and you can get it at an art supply place. But it is a
horrible, horrible mess to use, and not really any better than what I
improvised. It is toxic. It dries quickly (you don't want that for
marking/scraping). It stains your skin. It is impossible to completely
clean off of you or your tools.

I made my own compound by simply mixing red iron oxide pigment powder
with a little light machine oil. A tablespoon in an old candy tin would
be a lifetime supply for most people. Works great, cleans up with soap
and water, doesn't stain you chemically, doesn't dry out, not toxic.

How, a finger tip, what thickness?

Yep, rub it on the granite very thin. You'll quickly figure out how
much with practice.

Clean the stone first. Running your hand over the stone will be quite
sensitive to detect any debris.

How do you test it on the surface plate - press it down gently, slide
it, rotate it, what?

Set the part down without sliding, nudge it around a few millimeters,
and lift off, again without sliding. Again, practice and you'll see.

You need a way to vise the work without distorting it.

How would you prepare the surface plate - clean with alcohol, use a
damp kem wipe, what?

Not alcohol, you need a fast-drying, non-polar solvent. Paper towels,
and WalMart store brand camp stove fuel (cheapest source of naphtha) in
a Sure Shot sprayer. Don't get cheap on the paper towels. Wear old
clothes, they'll be ruined.

Do you test flatness on different sections of the surface plate?

The plate should be flat to better tolerance in all directions than you
will be able to scrape to, so it shouldn't matter what direction you're
marking from.

To clean up your hands, scrub with lots of liquid soap and a Scotch
Brite pad. Try not to scratch your nose while you're working!

It your work item is not flat, it is a bit hard to judge how much off it
is and where by simply marking, unless it is way off enough to, say,
slip feeler gages underneath.

Made my own scrapers just as good as the hideously priced European
imports: handles by flattening and drilling the end of 1/2 EMT, and
epoxying a sphere (found as: old trackball, craft shop "doll heads", old
billiard balls) to the other end. For the bit I used 1/8 x 1 inch mild
steel bar, with a carbide strip brazed on the tip (refill from Warner
scraper from paint dept at Lowes). Cadmium braze and flux from
Grainger.com brazes carbide to steel with just a propane torch. Grind
the carbide square, on a round radius of about 6" inches or less (that
is, the plan view is shaped like the letter "D", with a sharp-cornered
blunt edge, not a knife edge); you'll need at least a green wheel on a
tool grinder, or a diamond wheel, for this. A sharp edge is critical,
so shortcuts won't do; you have to resharpen frequently, like every hour
or so. Before carbide they used just tool steel scrapers, but that
requires much more frequent sharpening. Scraping is simply pushing down
and away at a low angle, producing crispy crumbs on cast iron, or thin
shavings on steel, removing a fraction of a thou per scrape.

If this sounds good to you, I have lots of photos I took while scraping
my Bridgeport back to new, but never put onto my Web pages. Maybe if
there's any interest here I will do that.