Flat bottom holes.

Illinoyance said:
The hole will have to be bored or reamed to get the finish needed for a cylinder wall. ,,,

As a practical matter a little extra clearance space at the end of the cylinder will not matter.
Click to expand...
In case of a compressor, clearance space limits the compression in a single stage, and would be
important to minimize. I'd consider machining a plug with one flat face, and fitting that into the
cylinder (secure with solder or braze, or pin...).

Finishing a cylinder wall can be done with grinding, or a hone, but reaming is more convenient.
I'm unsure that boring generates a good flat bottom, the center might not cut smoothly,
and of course the chip removal is problematic (could score the sidewalls).

Does anyone ever mill/bore with an orbital cut? Such as setting a boring bar for 0.5 inch diameter,
offcenter by 0.25 inch, and rotate the workpiece to make a 1.00 inch flat bottom?
 
My point was that additional clearance space in the cylinder doesn't really matter in model (toy) engines. Clearance is detrimental to the performance of real world steam engines and air compressors.
I like the idea of a separate head. Being able to pass a boring bar, reamer, hone, or lap through the whole length of the bore is the ideal way to finish the cylinder bore.

A D-bit is essentially the same as a half-round bit but with a shorter flute.
The end of a half-round drill can be modified to cut a flat bottomed hole.
 
My 2 cents. Jump in and point out what I have missed. It is a number 24 beam engine and once you look at all the cylinder parts, it is not real critical. The cylinder bore is 1/4" x 1-132" (1.0313) long. You have a number 70 hole at 27/32". That means the piston can not come past .8438 (27/32") - .0140 (1/2 # 70 drill) or .8298 from open end of cylinder or .2015 from closed end. Open end has a #70 hole at 11/64" (.1719) + .0140 means that the piston can't go closer then .1859 from open end. If it goes past this, it will cover the steam inlet holes. Do the math and at closed end the piston is .2015 clearance or cylinder volume. At open end the piston is .1859 clearance or cylinder volume. About .016 difference. The flat bottom is so the volume differences don't get too great.
Next, two piston rings are equally spaced on a 3/16" piston or 1/16" spacing. Subtract .0625 from .8438 at closed end and the wall only need to be smooth to .7813 from open end. Or .25 from bottom hole. As long as the reamer taper stops before .25, the bore is good.

I would drill 15/64" so tip touches final depth. Grind the point off and flat bottom drill. Run a 1/4" chucking reamer until it exactly touches and that should do the job. Store brand reamers are around $9, nothing stops you from grinding a little of the taper if you want. I would try this on scrap first.

People, I have a whole page of numbers, arrows and directions to get this to make sense, and add up to what I would do. Go to the web site and look at the whole page and see if I got it right, or totally blew it. Something fun to do on a cold windy day.
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Follow up. I had to know. The taper on my .250. .251 and .312 reamer is about .040. That means you don't have to bottom out. just go past the steam hole and let it fly.
 
I have been making and testing D bits. I think that I am pretty close. I have 2 new bits I need to try when my wife lets me. Another tactic that I may try is to make it a through hole and press fit a plug to give me a nice "square bottom". At this point I will most likely end up using an appropriate D bit to get the bottom square and ream to size. I am waiting for material.
 
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