Ringbom 1: Tim's Ringbom Stirling Engine Build

[Admiral_dk]

Nice work Tim 

I can't see the pictures at work on a Win 10 with Chrome - but here @ Home on a Win 7 with Chrome -> no problem 

I'm sure that the reason, is that here Chrome is in http format and af work it's in https .....

Per

[tvoght]

The photos should be viewable now in all browsers. A lot of back-and-forth effort between myself and my web host provider. I'll be posting some more build material after I get some other duties knocked out.
--Tim

[crueby]

YES!  Pictures now show for me in both Firefox on pc and chrome on tablet!      The fins look great!   

[fumopuc]

The photos should be viewable now in all browsers. A lot of back-and-forth effort between myself and my web host provider. I'll be posting some more build material after I get some other duties knocked out.
--Tim

Hi Tim, my Opera does show it now too.

[Admiral_dk]

Total succes - I can see them on Win 10 now too 

[tvoght]

All of the Stirling machines I have built use displacers of very thin-walled stainless steel. I machine these parts from solid bar stock. I have made a number of them, each time refining my technique. This is my latest refinement.

First, a mandrel was made starting with a length of 1" cold-rolled steel in a collet. The extended part was turned to the inside diameter of the displacer shell. The end was faced.



The mandrel was then reversed in the collet and drilled for a center.




To make the displacer shell, a solid rod of 1.125" 303 stainless was chucked in the 4-jaw. The first operation would be to drill it to depth at .938" diameter, and then bore it to slip onto the mandrel.
I know many like to "step drill" with a number of progressively larger drills when drilling a large diameter hole. Here, just two steps were taken.

First, the width of the .938 drill's "web" was measured.



Then a small drill was selected just a hair larger than the large drill's web, and a starter hole was drilled to depth.



The large .938" drill was then used in a second drilling. I got this technique from a book and it works for me.



An endmill just slightly smaller in diameter than the drilled hole was then used to flatten the bottom of the hole.



The hole was then bored for an easy (but not loose) fit on the mandrel. No photo is shown of the boring operation.
The mandrel was then loctited into the still-chucked stock. A dead center was brought up to the center drilled in the mandrel.



With that setup, the outside could be turned to achieve the desired .01 inch (.25mm) wall thickness. Look carefully to see the transition from displacer shell to mandrel.



The displacer shell was parted off just beyond the depth of the hole, leaving plenty of end material, then the mandrel went back into the collet, and the end of the displacer shell was faced to desired thickness. .01 inch end wall thickness was the target here too, but it's a little scary since any slight miscalculation could result in an open ended tube.



 Later, propane heat was used to break the loctite and remove the part from the mandrel.

The surface finish is not pretty, but on an engine of this type, the displacer's outside surface serves a regenerative function in the engine. The rough surface should actually enhance that.

[Kim]

Thanks for taking the time and effort to figure out the issue with the pictures.  I've just read all your posts and am finding your build very interesting.  I really like the step by step on how you did the thin wall displacer!  Fascinating!   

Kim

[tvoght]

Thanks Kim! And thanks to everyone who provided moral support during that pictures snafu.

The displacer cap was to be made of 6061 aluminum. 1 inch rod stock was placed in a collet and a boss turned.



A short section was turned on the outer diameter that will fit the inside diameter of the shell. A small carbide-tipped boring tool was used to remove material between the boss and the outside flange. Weight reduction in the displacer of a Ringbom engine is critical.



A check to verify the fit of the shell onto the short shoulder section.



More work was done with the boring bar to thin the outer wall of the cap, and the outer diameter beyond the shoulder was turned to match the outer diameter of the stainless stell shell.



The cap was parted off and then the boss was placed in a collet. The part was faced to length.



A blind hole to accept the displacer rod was drilled, bored, and reamed.



A shot of the displacer shell with cap test fitted.



And a trial fit of the rod. The shell, cap, and rod will be epoxied together with JB Weld at a later stage



The displacer rod itself is made from purchased precision stainless tubing. Lacking the availibility of this, it might have been machined from solid stock.

[tvoght]

The bearing block attaches to the engine frame and holds 2 ball bearings for the crankshaft to run in. As with all the parts I'm building, there's a drawing attached to this log entry.
Since in many cases there aren't a lot of photos, I'm hoping the drawings will help to fill the gaps.

1" 303 stainless stock is put in a collet and drilled past the length of the part with a drill size that will allow finishing with a boring bar.



Then the hole is bored to fit the outside diameter of the flanged ball bearing.



The shoulder portion will fit within a bore in the frame. It's turned to diameter, and the design plan also intended for the shoulder to fit within a collet on hand, so that the other end of the part can be machined. A sharp tool is used to remove any inside radius at the should transition.



A recess for the bearing flange is turned using a small boring tool.



A bearing is test fit.



The part is cut off with length to spare, and then the shoulder end is put in
a 7/16 collet. A recess is also bored here for a bearing flange.



Another test fit.



At the mill, I use my "3d taster" to locate the center of the bored hole



And then center and drill for 4-40 tapped holes equidistant on a .688" circle.



The holes are tapped.  The DRO's bolt hole circle function makes it so easy to return exactly to each hole
location for center drilling, drilling, and tapping.



The part is complete.

[tvoght]

The frame is a pretty simple affair. There is a drawing attached.

The material is hot-rolled steel angle 3/16 inch thick with 2.5" legs.

Set up in the Bridgeport vise backed from below with a 1-2-3 block, a first
leg was cleaned up with a facemill. Then in a similar setup with that face against the fixed jaw, the other leg was cleaned up.



While still at the Bridgeport, all important holes were drilled, including the
large hole which will hold the bearing block. That hole was bored to size.
What is meant by important holes? All the holes which must have a precise locations.

Then to the CNC mill, where the part was clamped in the vise in a similar way.
The spindle was zeroed at the center of the bearing block hole for reference in the CNC program to be run. The program cut a profile in a series of progressively deeper cuts.



A separate program was used to cut some decorative slots.

[tvoght]

Next up is an overhung crank consisting of a web, a shaft, and a pin.
The web comes first. A drawing is attached.

1.25" 303 stainless stock is put in the 4-jaw and drilled .015" or so less than
the .25 inch intended shaft bore, and deeper than the final length of the part. The hole is then reamed to .25" for the shaft. A shoulder is turned to space the web off of the ball bearing it will butt against.



The part was cut off with extra length and taken to the mill. The center hole was accurately located and a hole drilled at the crankpin offset.



The web was milled to thickness, and then the crankpin hole was drilled and reamed.



At the CNC mill, a tooling plate was clamped down and drilled and tapped for
shoulder screws to clamp the part. A program cut the web outline a little
oversize in progressively deeper cuts.



Then another program cut to final outline at full depth. Notice that there was no margin of error for that smaller shoulder screw. I don't recall how I managed to prevent it from being unscrewed by the cutter. It's possible I used loctite on the threads.



The shaft is purchased 1/4" stainless ground rod. The only machining necessary was to cut it to length. A drawing is attached.

The crankpin is made of stainless and unfortunately there are no photos of the
build. A fairly straightforward collet job. See the drawing.

I don't seem to have any photos of the parts first asesmbled, but they were put together with loctite 609.

[tvoght]

A brass piston rod was made.

1/8" brass stock was setup in the Bridgeport vise. A hole for the big-end bearing (a ball bearing) is drilled, bored, and reamed.


]hr]


Then the hole for the gudgeon pin was drilled and reamed.



At the CNC mill. A tooling plate was drilled and tapped for two hold-down screws. Shoulder screws are usually used here, but with no correct sizes in stock, close-fitting bushings were made and used with ordinary cap screws.



A CNC program was unleashed.

[crueby]

 

[tvoght]

Thanks for looking, Chris!
The piston is in two parts: the piston, and a gudgeon insert.

To make the insert, a rod of aluminum was placed in a collet and the two diameters shown in the drawing were turned. A hole for a 2-56 thread through the depth of the part was drilled.



The collet was put in a square collet block and taken to the mill to make a hole for a gudgeon pin. A spot-face was made, a center-hole was drilled, and the part was drill through and reamed.






With the collet block upright in the vice, a slot for the piston rod was milled.



Back in the lathe, the insert was parted off.



The part was reversed in the collet. The clamping in the collet of that split boss seems a little tenuous, but light cuts were taken to arrive at the correct thickness of the base of the part which mates with the piston. Photos don't show that the edge of the part was deeply chamfered to clear any fillet left in the bore of the piston. Also in this setup, the previously drilled center hole was tapped.



This photo reminds me that on this day in the shop, two other parts were made: 1) the displacer stop, which fits on the displacer rod and limits the travel of the displacer. 2) the piston rod holder, which is essentially a washer that bears on the inner race of the big end bearing and accepts a screw into the crank pin.



There are no photos of the making of those small parts (I must have been "in the zone"), but drawings are attached.

All the complexity of the piston is in the insert, with the piston itself being quite simple. The one pictured is made of graphite, and that is a tried and proven material to use here, but in the process of troubleshooting a problem, another was later made of cast iron with the same dimensions. The cast iron version runs in the engine now.

A quick picture of the graphite piston in process...



The piston, piston insert, and rod with big-end bearing and gudgeon pin installed.



Here is the piston attached to the insert with a screw. Looking back, you'll see that the cylinder head has a hole to clear this screw head.



Thanks for looking.

[tvoght]

The hot cap of the engine is the last part I have any real build photos of. After this, I'll post drawings of the few remaining parts with a brief description for each. Then I'll be essentially finished with the log.
The hot cap construction is similar to the displacer shell done earlier. The only real difference is the inclusion of a flange to clamp to the cooler.

It started as a solid hunk of 303 stainless. The original stock was in better focus.



Same approach to drilling as with the displacer. Selecting a drill for the pilot hole about the size of the large drill's web.




Drilling in two steps and flattening the hole bottom with and endmill.





Then boring to the final inside diameter. The endmill used to flatten the bottom was a little small, so some work with the boring bar was necessary to complete the square bottom.



Making the mandrel. A lesson was learned when making the displacer: it was tough to remove the mandrel because of suction. Here a pressure relief channel is cut to fix that issue.



After turning the outside diameter, leaving the flange. Cutoff just beyond the hole bottom would be the next step.