On Thu, 15 Mar 2012 12:25:19 -0400, Ed Huntress wrote:

On Thu, 15 Mar 2012 10:54:15 -0500, 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).

I think that the only ways are non-traditional machining. The most
obvious is EDM, as Pete says. I also suspect it's the only *practical*
way to do it. Laser would be a problem on the corners and perhaps on the
finish. Other energy-beam methods, likewise. ECM (electrochemical
machining) would do the job very nicely but that's a production method
that requires a lot of custom setup.

You can get mirror finishes with conventional, or sinker-type EDM. If
corner radii can be 0.002", no problem. Sharper corners are possible but
require really good technique and equipment.

I wouldn't try to do it with home-made EDMs. You'll have a problem with
surface finish and it would be really slow. This is a job for a jobbing
EDM shop -- usually mold shops -- that is sympathetic to your interest
and that would give you a good rate.

Burn rates should be very quick. That's not where your money would go.
Electrodes would be simple -- you could make them at home. It's setup
time that could cost you.

If you find a shop and visit, you should be able to figure out how to
fixture the parts and get specs for making electrodes (graphite or
copper). If you deliver parts already loaded in a simple-to-mount
fixture, that could make the whole job really cheap.

"I wouldn't try to do it with home-made EDMs." Do you wave red towels in
front of bulls, too?

My first alternative is to just use an end mill, and accept that a round
hole isn't going to pass as much gas as a square one. Or use an end mill
to establish the hole position and angle, then use a plain old file,
elbow grease, and patience to establish the squareness.

If I actually find myself wanting to build more than one motor then yes,
I'll consider finding a sympathetic shop; that's a good idea.

--
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