If I understand the method you're questioning, it would be described as,
putting the square tubing in the inside corner of a square.
That's not the proper position of the square, for doing the layout work
we've been discussing.

The method I had described is done by placing the square straightedge (or
rule) flat on the tubing side, and perpendicular to the length of the
tubing. The straightedge is laying flat on the side of the tubing.
The thicker part of the square is then touched/held on the adjacent side of
the tubing, with with the length of the thicker part alongside the tubing.
The straightedge will then represent a line across the tubing that is square
to the length.

Most formed square tubing has a significant radius at each corner, but the
face of the thicker part of the square can be extended if needed. The
contact face of the thicker part of the square could be extended (sideways)
with a parallel or similar flat piece of stock, by attaching it with thin
double-sided tape (not the foam type).
The wider surface would bridge the radiused area, and allow surface contact
with the adjacent side.

Generally, I just tilt the thicker part of the square slightly, to make flat
contact with the adjacent side of the tubing. This will cause the
straightedge to stand up slightly on the tubing, but the inner/inside edge
leaves close contact for scribing a square line.

There are squares that are both styles you illustrated, but the more common
type has the straightedge located in the center of the thicker section (A).

When scribing the lines on square tubing with radiused corners, one needs to
"sight" the lines of adjacent sides, since the line scribed on one flat side
of the square tubing won't continue over to the next side.
Solid square stock (like some lumber) has sharp corners, so a scribed line
is easier to extend around the four sides.

If a precisely squared section of angle iron were used for the square tubing
layout job, the two adjacent sides of the angle would show a continuation
from a scribed line on one side of the tubing to the next side.

--
WB
..........


"Jon Danniken" wrote in message
...

Hi Bill, and thank you for bringing up an issue which has been plaguing my
attempts to layout square work pieces for quite some time.

I have a small combination square, and it constructed along the lines of
the
type "A" in the following figure (looking down the long axis of the base):

http://img717.imageshack.us/img717/5493/squaree.jpg

Does a proper machinist's square use a similar method of attaching the
blade
along the centerline of the base? For instance, this one I am looking at
from MSC (SPI 13-626-7):

http://tinyurl.com/2cyrbvc

The issue I am getting at, is that I have trouble scribing a continuous
line
around the circumference of a piece of square tubing, because a scribe
line
along the edge of the blade will not line up with a scribe line along the
edge of the base (it will, of course, be parallel to it).

It would seem to me that square "B" from the first figure would facilitate
scribing a continuous line around a square piece of stock.

Are machinist's squares, like the SPI one I linked to, constructed along
the
lines of drawing "B", or if not, what is the technique to use a square of
type "A" to do so?

Thanks for your help on this issue; this has been a thorn in my side for
some time now.

Jon