Hello all;
It's been three months to the day since I finished my last engine, and during that interval, much
time was spent trying to decide what I wanted to build next. Well, actually, I built a number of
things during that time, but they were not engines.
Not to long ago, I finished up my version of Jerry Howell's small propane burner, and I made it
as a prelude to building an engine to go with it. I knew I wanted to make some kind of heat
engine, but until a few weeks ago, hadn't settled on the design. I'd seen a Duplex Vacuum engine
some years ago, and it struck me as an attractive design then, and it's where I finally landed for this build.
I got the prints for it from the late Mr. Howell's business, along with a piece of graphite for the
piston, and two small ball bearings needed for the crank shaft. The name of the engine is
deceptive, until you know, as Mr. Howell writes, that it was made by the Duplex Vacuum Company,
so, it's really a proper name, rather than a descriptive one. It's a Heinrici type stirling cycle engine.
The displacer and power piston run on the same axis, or centerline, with the displacer rod running
through the center of the power piston, and the two of them go up and down in the same cylinder.
There's a picture of the engine at this link, if you'd like to see it. (It's copyrighted, so can't be
put up here.)
http://www.model-engine-plans.com/engineplans/images/Duplex.jpg
I started with the displacer arm. Similar to a con-rod, but has a clamping eye on one end.
It may be an odd place to start, but my latest materials order has been lost in shipment, so
until something shows up, I'll make parts with the metal I have on hand.
Starting with a piece of rectangular aluminum stock, milled to length, the holes for the shafts
it will need are bored.
The big end has to be radiused, so a shoulder stud is made for holding the arm securely.
The end of a close fitting shaft for the bore in the arm is turned down enough to thread for
a 4-40 nut.
Really, just putting this here so someone won't be able to accuse me of doing nothing
between this new build, and my last engine. See, Zee? Made a collet closer for the Atlas.
There. It's ready.
Now the shoulder stud is mounted in the vise nice and square, and the arm slipped over it.
The shoulder on the stud is left just slightly proud of the arm, so the nut can be tightened down
without pinching the arm. Then the radii are cut on both sides. One half of the radius, toward
the end of the arm, is quite shallow to leave room for the clamping end of the arm.
I made this radius slightly larger than called for in the print, so there will be a little for clean up
with a file, later.
Originally, I did this step using a ball end mill, but the smallest of that type I have is 3/16", and
the radius it produced didn't look right to me, so I went over it with a regular end mill.
The end here will be the clamping part. It's narrower than the boss, so after getting my dials
set to zero, I made a number of passes taking bites that run up almost all the way to the boss,
but leaving just a bit for filing later.
Now the piece is rotated 90 deg and the hole for the clamping screw is drilled and tapped.
I've also blued the piece and marked out a radius, then nibbled away the waste up to the
mark out line.
Then the clamping part is slit so it can pinch on the shaft that holds this arm.
The other end is slotted too, for a link that will eventually hold it to the displacer shaft.
About the same thing as the shot above, so no pics of that.
Back on its side again, and the waste is removed between the eyes to form the middle part
of the arm.
Turned over one more time, to get rid of the waste on the small end, so it can be rounded over with a file.
Up to this point, I've left the flange of the arm, (the main part in the middle), full width, to
provide a little extra work holding support. 'Bout time to get rid of the excess.
Using 1/16" rods through the two eyes of the arm just happen to give the proper angle to give
the arm a tapered look.
Now I need a couple of filing buttons so I can get rid of the lumpy bits without messing up my
work with the file. A 3/8" piece of drill rod is just the right o.d., so a piece is chucked up and
drilled/tapped for 10-32. A #10 screw is the same size as the bore in the big end of the rod,
so will hold the buttons nicely centered. The nut for a #10 is too large, though, and that's the
reason for taping one of the buttons. It can act as a nut, too.
After the first button is parted off, the threads remaining in the second one are drilled out for
clearance of the #10 screw.
My parting tool left these ridges on the buttons that need to be turned off. Using a piece of
the same 3/8" drill rod behind each button in the collet lets the collet squeeze down on the
little buttons firmly, and helps with positioning them out far enough to clean up.
The buttons are then heated up and then quenched to harden them. If you harden your buttons,
they will not be deformed when you do your filing. The file will just skip over them.
I took a number pics while doing this, trying to get the camera set right to show something close
to the proper color for a hardening temperature. This is as close as I could get, and pretty much
what it looks like indoors. If you have trouble telling colors when doing this, use a magnet to test
the heat. When the steel gets to the right temp, it will no longer be attracted to the magnet.
For small things, like these, you can tell pretty easily, but for larger pieces, it takes longer, and you
find yourself wondering, "Is it there, yet?". Use a magnet, and you'll know for sure.
I just leave these dead hard, and they will be fine that way as long as you don't do something like
squeeze them in a vise. They'll break if you do something like that, or hit them with a hammer.
If they were going to be used in a vise, tempering would be a good idea.
Here are the buttons in place, and I can get to filing on the piece to clean it up some.
After the 'file a while' game, it started looking half way decent.
Well, that's one piece down. Probably won't be anything new for a few days. I'm a slow builder!
Thanks for having a look.
Dean
