Thanks for the insights.

be sticky instead of allowing something to float. If your surface plate is
flat it doesn't much matter how much or in what direction you move it as
long as you move it through a the blue enough so the reading is effected by
any differences in how thick the blue is at various places on the plate.

If there was a high spot and I moved the piece too much, I just
assumed that after a certain point, too much blue would cling such
that it would spread to a neighboring low spot and affect the reading.
I was trying to control the degrees of freedom in my procedure.

3x6 is plenty big enough to scrape. I don't know why you'd expect dots
rather than smudges.

Both the scraping video demo (machinerepair.com??) and the tony's
precision scraping page show dots that are clearly circularish. These
are the only markings that I have seen, other than my own. As an
aside, I did try cutting the blue with oil, and now the paint is very
smooth and the marking is much more than what I expected. Without the
oil, I noticed that small clumps would form on the plate and roller
that had to be periodically removed. With the oil in the mix, I can
spread it with no clumping, and arbitrarily thin.


imo you're making this too complicated, the principal simple, you are using

I agree 100%.

paint to indicate where one piece isn't flat by comparing it to a flat
piece. spread some blue on the plate, lightly grip the work by the edges
and move it around. The spread of blue will be heavier when you start which
is more aggressive and as you to get it closer to the finished state, the
lesser amount of blue left on the plate will show more detail. If you are
getting a circle of blue, either your plate has a high spot, or the blue
isn't even close to evenly covering the plate. don't weight it down, you're
just bending it to the shape of the plate. think about it - how is bending
the work to the plate going to show you what's high and low?

That is a good point. I wasn't trying to bend, just kind of eliminate
any hydroplaning or air-pocket riding. I tried doing a thought
experiment with the 2 extremes - light and heavy. I thought that it
should be possible to scrape a coin, like a quarter, but that a piece
this light might ride the paint a bit. Also, I see that a lot of
scraping goes on with 100+ pound surface plates. Therefore I figured
that weighting it a bit should not change the reading, but might
eliminate any float. I am pretty new to all of this, so my logic
might be a bit warped....


two things to consider 1) if the piece is so flimsy that you're concerned
about your grip distorting it (i.e., why are you using tweezers), what's the
point of scraping it? its going to bend in use or conform to what ever
shape its bolted to, and 2) this is a flat thin piece, there may be enough
internal stresses that its constantly changing shape, it least enough to
mess you up trying to scrape it down to a tenth of a thou (which may be the
case if you're putting the blue on so lightly you can barely see it). You
don't say what material it is, but I'd use cast iron if you have a choice.
What is the intent anyway for this piece??

The piece started as hot rolled steel (I think). It's intent is
twofold, one as a practice scraping piece, and two as the part of the
carriage on the Gingery lathe that rides the ways (that is if I don't
practice scrape it down to nothing!). I don't know anything about the
metallurgy in general, or about the suitability of this piece for the
purpose, I just know that it is the only metal I can reasonable get
right now (I am overseas). Any comments on practicality would be
appreciated.