Bellcrank Beta Stirling Engine Project

[tvoght]

Next on my list of things to make is the gudgeon insert for the piston.
I'll attach a drawing and rendering to show what I'm up to.


--Tim

[tvoght]

I clamped a piece of 1" brass stock in the Bridgeport vise and side-milled the end.


In a series of steps I cut away to leave a raised portion that will become the
gudgeon pin pivot flanges. Between cuts I aligned with previous cuts using
an adjustable parallel and square.









I attempted to round over the top of the flange section using the DRO's arc function. The results were not the best, but can be cleaned well with a file.



The slots for the two arms of the connecting rod were sawed with a slitting saw.It is a 1/16" slot and the saw is .032 thick, so each slot took 2 passes.





I simply cut the part off of the stock with my bandsaw leaving just enough for cleanup on the flat face. Virtually no waste of brass here. The stock goes back in the brass box.
Here I've clamped it on the width of the flange block and milled the bottom plate to thickness and also drilled for the relief of the displacer rod gland.
I drilled the holes to pass the 4-40 screws that will attach the gudgeon piece to the piston proper.
 



I need to mill away the center portion now for which I have made witness marks with the saw.



With no good place left to clamp, I whipped up a fixture so that the piece can be attached via its mounting screws, then clamped and milled away.





I test fit the piece in the piston. The steel countersunk screws are temporary. These are planned to be brass.







Thanks for looking in,
--Tim

[Admiral_dk]

Great description on how you approach each Part 
+ a fine result     

Per       

[tvoght]

Great description on how you approach each Part 
+ a fine result     

Per       

Thank you Per.  I probably spent more time laying awake thinking about possible operation sequences than I did in the machining.  --Tim

[tvoght]

Next up is the displacer shell. Drawing and rendering attached.
--Tim

[tvoght]

To start the stainless steel displacer shell, I first made a mandrel of 12L14 leaded steel. It was turned to the intended internal diameter of the thin displacer shell.



Then it was reversed in the chuck, faced and center drilled. Sorry for the focus.



A rod of 416 stainless steel was chucked. It was pilot drilled and then
drilled with this large drill to just over the intended depth of the shell.



I bought this big center-cutting endmill and made a holder specifically to flatten the bottom of the drilled hole.



Then I used a boring bar to open up the internal diameter until the mandrel I made would nicely slip inside it.




Now with a live center supporting the mandrel I could begin turning the wall
of the displacer shell to the desired thickness. I usually go for around .01" (.25mm) here, but I had an idea, so I took the wall to .03" and then cut a 60 tooth per inch thread down the entire length of the shell to a depth of .014".


The idea is that this will serve as a more effective regenerator. On these types of engines we usually just rely on the displacer wall to provide a regenerative function. My aim is just to provide more surface area on the wall. Threading was a convenient way to create this raised ridge. It is not intended to serve as a thread in any way. In fact the thread form is at 100% depth so the peaks are sharp. It would be a very poor thread. Such a fine thread came out pretty ragged looking. Functionally it should be fine. Appearance-wise not so much, but it's not a visible part.






Then... Uh. Oh. I couldn't remove the mandrel from shell! It seemed as if the threading operation had somehow shrunk the shell onto the mandrel. I wrenched and mumbled and cursed. Finally, I remembered something I had read in one of Guy Lautard's "Machinist's Bedside Reader" books.
I drilled a hole through the length of the mandrel and threaded the hole for a grease fitting. The way oiler for my Bridgeport uses such a fitting, so I began pumping way oil at pressure into the mandrel. Slowly but surely the mandrel was backed out with hydraulic pressure.

 
With the mandrel removed, I measured the depth of the inside cavity of the displacer, and then cut the wall down until the correct depth was reached. Then
I cutoff the part with plenty of margin past the hole bottom.




I super-glued the shell back onto the mandrel and took light cuts to face the end until the desired end wall thickness was reached.



--Tim

[tvoght]

The aluminum displacer cap closes off the displacer shell. A drawing and rendering are attaced at the bottom of this post.


With a chunk of aluminum in the chuck, I turned the various diameters that make up the profile of the cap.



The spigot extension that houses the displacer rod was made a diameter that will fit a collet. The part is reversed and clamped on that diameter, the large flange was faced to length, then a hole drilled and reamed to accept the displacer rod.




The finished cap was test fit to the shell.



--Tim

[tvoght]

I was ready to start on the bellcrank linkage, and besides the bellcrank itself there are three links to be made. Drawings of these parts are attached to this post.   --Tim

[tvoght]

For the "bellcrank actuator link", I stared with a piece of 1/4" thick aluminum stock I dredged from the scrap box and sawed just over a quarter inch wide.
I milled down the sawn side to arrive at a length of 1/4" stock.




I cut the slot in the end with a slitting saw.



I drilled through for a 2-56 thread, and then drilled and reamed the top part
of the fork for 3/32". The thread in the bottom part of the fork could then be tapped with spindle alignment with the tap dropping through the reamed upper hole.



Then I drilled and reamed a 3/16" hole on the other end.



I used the arc contour feature of the DRO to round off the forked end.



In this setup I milled down half of the width on one side and then flipped the part to mill down the other side.



The displacer link and displacer rod end were made using similar methods. The finished parts:



Thanks for looking,
--Tim