On Thu, 15 Mar 2012 20:27:03 -0700, etpm wrote:

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.

This is a hobbyist thing, so one or a few is what I'm looking at (or
maybe it's just mental masturbation -- time will tell if I actually
_build_ something). If I were doing it in production I'd certainly be
thinking EDM or broach.

--
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