Carl Mulhausen wrote:

~  "Patent Norris High Pitch Setting Device"

~  The illustration shows a bent metal gizmo screwed to
~  the back of a plane iron. It looks like it was meant
~  to take the place of the cap iron and allow you to
~  install the iron bevel up in the plane giving a
~  "cutting" angle >90 deg. I guess the device also 
~  helped close up the mouth with the bevel up.
~  Functionally like the scraper attachment that Lee
~  Valley is now selling.

Out of curiosity, To enable a Stanley blade honed at 30 degrees to be
fitted bevel-up, I have just sawn the last 5/16" from the head of a
spare Stanley cap-iron. No weak puns about de-cap-itation please. 8-).

The shaving aperture is now cavernous, of course, but a trial on a 7"
x 1-1/4" (or so) piece of some fairly hard flowering cherry, even with
a pretty fine set, caused it to violently skitter for the whole
length. I doubt that the shaving aperture size has much to do with
this.

It worked after a fashion on some pine, but required much extra
effort.

Conclusion of a very brief trial: under these circumstances, a 75
degree cutting angle with a 45 degree clearance angle is impracticable
and not worth perservering with, but someone must have made his gadget
work well enough to satisfy Norris and go to the trouble and expense
of getting a patent.

~  Just say: Doesn't say anything about how it will work on
~            Wenge.

Now you know why!

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Rather academic speculation follows. Ignore according to your
inclination. Tell me if I've got it all wrong, but it /is/ useful
to have somewhere to bounce ideas against! 
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Trusting some very rusty trigonometry and school physics, I think that
the turning effect from the reaction on the end a beam inclined at
differing angles to the horizontal, when biffed by a
horizontally-acting force against a vertical face, might be
proportional to the cosines of the angles. 

The beam is the plane blade. The angles are the cutting angles. The
cutting angles are the inclinations of the upper face of the cutter to
the sole. The horizontally-acting force is you, pushing the plane. The
vertical face is the end of the workpiece. Gerrit now?

Sorry about that, but what can one do without a decent diagram
facility? 

Cos 75 divided by cos 45 = 2.74, hence the greater likelihood of the
plane involuntarily lifting from the shock of the edge when it first
engages with the leading edge of the workpiece. This quantifies (I
hope) what one perceives intuitively.

It is appreciated that this applies to the special case of the iron at
the start of the cut, but intuition also suggests that it will be
somewhat applicable to cuts started within the body of the workpiece.

One conclusion? Lower cutting angle = less chance of skitter. A Lower
cutting angle is easier to obtain with a low-angle bevel-up plane.

A question remains, however. I have made rather casual backfacing work
on wood. The casualness might have meant that the cutting angle was
maybe about 60 degrees. Does the greater clearance angle have any 
ill-effect? Off-hand, I can't think why it should, but am open to rational
explanations. I suppose that the flexing effect (golly, I nearly said
"chatter") on the un-damped (since no effective cap-iron) 75deg.
cutting edge could be 2.74 times as great, hence a release of stored
energy would increase the lifting effect, or would it?
 
Engineers/physicists to the rescue please!

Enough of this self-indulgence. Work to be done!

Jeff.

-- 
Jeff Gorman - West Yorkshire
jeff@m...