[AR] Re: Thrust Chamber Manufacture
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 15 Aug 2020 16:50:45 -0400
On Sat, Aug 15, 2020 at 05:28:40AM +0100, Peter Fairbrother wrote:
As to rotational symmetry, yes a rotating drill and a rotating workpiece can
be rotationally symmetrical - but only up to the point where the drill
wanders.
Consider a stationary two-flute drill and a rotating workpiece. If the drill
is centered the edges of the drill will both be cutting at the same speed s.
Now offset the drill half a diameter to the side, so that the rotational axis
is at one edge. That edge will be cutting at zero speed, the other edge will
be cutting at 2s (relative to the material to be cut).
That is where the breaking of symmetry (and drills!!) comes in.
Except you can have the same thing with a rotating drill: if the tip
of the drill is off the axis of rotation, one flute cuts faster than
the other.
As I have said I don't quite know the geometry and action of the single edged
drill, though all the two or three gun drills I have used have just worked;
but gun makers have used the technique for centuries to drill probably the
most accurate holes ever - the hole in a sub-MOA match rifle barrel has to be
extremely straight, straight to a couple of tenths of a thou in 28 inches -
and that is how they are made.
I've actually heard the opposite: "a barrel doesn't have to be
straight to shoot straight". That is, though the quality of the hole
(smooth, uniform diameter, rifling consistent in every way) is of huge
importance, perfect straightness isn't. Though I'm not entirely sure
of that, it stands to reason: even if the barrel is straight to start,
during firing it bends a lot as the gun recoils (unless you have a
very unusually symmetrical gun). And I think even just the force of
gravity will make a 28-inch barrel sag a lot more than 0.0002 inches.
For the rifle to be accurate, the barrel's bending just has to be
repeatable; it doesn't have to be zero.
Getting a hole that goes straight down the middle is still necessary
for ordinary manufacturing purposes, even for barrels of ordinary
quality.
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