On Thu, 15 Mar 2012 18:14:51 -0500, Tim Wescott
wrote:

On Thu, 15 Mar 2012 14:11:59 -0700, Bill Martin wrote:

On 03/15/2012 10:14 AM, Tim Wescott wrote:
On Thu, 15 Mar 2012 09:36:39 -0700, Stanley Schaefer wrote:

On Mar 15, 9:54 am, Tim 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).



How about you turn it inside out so it's an external slotting/milling
operation? :-)
That's not as nutty as it sounds...mill your transfer ports into the
outside of the cylinder proper, than press a hollow tube (sleeve) over
the outside. Presto, you have transfer ports! One thing that is going to
be a problem though: steel/iron doesn't get the heat out very well, and
two strokers have a lot to get rid of. Good luck,

The standard construction for model airplane engines of this sort is to
have a cylinder lining of steel or (if you have friends in the right
sorts of shop) hard chromed brass. It's a thin tube, usually with a lip
on top, that slides into the crankcase. The crankcase has passages cast
or milled into it for the transfer and for the exhaust.

So the gas travels up from the crank case to the port along the outside
of the sleeve, then turns 90 degrees to go into the cylinder. Meanwhile,
the exhaust is exiting through its port without (we hope) leaking into
the crankcase because the liner is sealed to the case by a close
mechanical fit.

I'm not sure of the detailed heat-flow -- I just know that it must be
adequate, or all my engines would have burnt up long ago.
Greetings Tim,
I don't know how many of these you need. If I had to make just a few I
would mill out the square holes with a .062 dia endmill and then
finish the square corners with a square needle file. I would grind the
teeth off one side first to make a safe side on the file. But I have a
surface grinder that would make the job easy. A fellow could stone off
the teeth too since the file is so fine and narrow. If I had to do a
lot I would grind a broach to square up the corners but I would still
mill out most of the metal. First a .094 drill, then mill, then
broach. The tubing I would mount on a mandrel to support it, that
would keep distortion within your tolerance range. Having a safe side
on the file makes it easy to just remove material from the corner
while not removing material from the side. Furthermore the safe side
will burnish the side.
Eric