On Thu, 15 Mar 2012 09:36:39 -0700, Stanley Schaefer wrote:

On Mar 15, 9:54Â*am, Tim Wescott wrote:
How would one make nice sharp-cornered, clean-sided (I'm not sure of
the surface finish, but mirror-bright would be nice) square holes,
about 0.1" on the sides, off center from the axis of the bar (so not
square to the bar surface), in a round steel bar about 0.75" in
diameter?

Alternately, how might one make those same holes in a cylinder machined
out of that same bar, with a wall thickness of about 0.06", without
distorting the cylinder by more than a couple of thousandths (I am
assuming that one would have to do some post-operations to clean up the
cylinder after making the holes, unless one hand-filed them).

This is a thought experiment for making cylinder liners for 2-stroke
engines of about 0.2 in^3 displacement; the holes would be the transfer
ports, and the cylinder liners need to have their diameter controlled
to about 0.001" on the outside and less than that on the inside for
proper sealing (or if not controlled, then at least matched to the
crankcase that they slide into, and the piston that slides in them).

EDM and you really, really don't want sharp corners. Nice stress risers
if you do. Have a look at some weedwhacker motors. You also don't want
a configuration that'll grab your piston rings.

Stan


Well, square-ish then. One wants square holes for more gas flow, but at
the same time one doesn't want one's cylinder liner to break.

Weedwhacker motors aren't exactly on the same point in the production
volume/performance potential/cost curve that I'm trying to address -- if
I go looking at bigger engines I'll be looking at race bikes.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com