In article , Joe Gwinn
wrote:

In article , Jim Wilkins
wrote:

"Joe Gwinn" wrote in message
...

Hmm. I will remeasure the Vee angle. These measurements are a bit
clumsy to make, and errors are a danger. I think I'll try some
variation of the two rods (different diameter penetrates to
different
depth in the V) approach.

Joe Gwinn

You could trim a small piece of sheet metal to nearly the correct
angle, press the straighter edge against one side of the internal vee
and scribe a line parallel to the other side.

Measure it, construct a more precise sheet metal template at the
suspected angle with a vernier protractor, and see if it fits
light-tight.

I did think of that, and I do have a vernier protractor, but the
two-rods method seems simpler and more accurate. Certainly less
fiddly.

But I do have the precisely-made rods (drill blanks) and a indicator.
That plus some math should yield a precise answer.

The two rods approach is a standard way to measure inside Vee angles,
according to some old toolmakers books I've read. (Don't recall which
one.)

Following up to my own post, I performed the rods measurement using
five different test rods.

Details: Clamped the rest upside down in a vice, so the base is on
top. Cleaned machined surfaces with acetone and a razor blade.
Clamped a magnetic base to the flat, with arm and digital indicator
over the Vee groove, with indicator probe moving vertically and more or
less perpendicular to the plane of the flat. Installed the 0.500" rod
and zeroed the indicator. Took indicator measurements on five rods,
being 5/8, 1/2, 13/32/ 23/64, and 5/16 inch diameter. Fitted a line to
the data, and also used a formula on measuring tapers using discs of
various diameters from Machinery's Handbook (27th edition, page 715, in
Angles and Tapers).

The consensus answer is 79.7 degrees included angle; one assumes that
the target was 80 degrees. One would assume that people don't sweat
getting Vee-groove angles exact on steady rests.

This measurement is likely accurate enough to exclude the 75 degrees of
Harrison lathes.

By the way, after I cleaned and inspected everything, I think I misread
the hand-stamped number. I had read it as 68, but I now think it's
actually 89, as in 1989.