Gamma1-VPA Stirling

[arnoldb]

Nice engine Tim 

Thanks for the write-up as well; some very interesting operations in there.

Kind regards, Arnold

[Ian S C]

Tim, I gave up on aluminium displacers after I had three melt downs on one of my motors, and I like to use stainless tube, .014" wall thickness, unknown spec.  Apart from higher heat resistance, there is a lower rate of conduction.  Apart from three or four small motors, mine run on LPG, and spend most of their running time at red heat.   
       A source of suitable containers to make hot caps is old NiCad batteries,  I think your size might be C size cells, an Alkaline one is OK if you don't mind the + terminal bump on the end.  If the size doesn't suit, it may be ok for the displacer.      Ian S C

[tvoght]

Mman, thanks for the tip on fin-cutting. Arnold, thanks for your comments.

Ian, the displacer shell is made of 303 stainless just like the hot cap. Only the end-cap (the "cool" end) of the displacer is aluminum.. I gave up on aluminum displacers before I even  started...

--Tim

[Hugh Currin]

Tim:

Very nice engine which runs great. Thanks for showing us.

Are you using CAM software to create CNC paths? I've found on similar parts that each depth leaves marks on the part as well as the rough finish you found. I now typically leave some 5 thousands material initially, then take a full depth finish pass to size. With a little cutting fluid, or coolant, that final surface usually comes out nicely finished. This is fairly easy with the CAM packages I've used. Though, it would be a little more involved with manual coding it's still possible.

Following along with interest. Thanks again.

Hugh

[vcutajar]

Hi Tim

First of all congrats on a great runner and also following along your tale of the build.

Vince

[tvoght]

Vince, I'm glad you're watching!

Are you using CAM software to create CNC paths? I've found on similar parts that each depth leaves marks on the part as well as the rough finish you found. I now typically leave some 5 thousands material initially, then take a full depth finish pass to size. With a little cutting fluid, or coolant, that final surface usually comes out nicely finished. This is fairly easy with the CAM packages I've used. Though, it would be a little more involved with manual coding it's still possible.

Hugh,
After this part, I've begun doing roughing passe(s) followed by finishing passe(s). I'm getting much better results. My finishing passes have been with many depth passes, and I haven't
been too unhappy with that, but your suggestion of doing the finishing pass at full depth has a lot of merit. It would have been easy to change my code to do that, it just didn't occur to
me. Something to try later.

I haven't seen any affordable CAM software that I liked, and writing g-code can get laborious, so I have struck a happy medium.
My favorite programming language is Python, so I wrote a Python "wrapper" around g-code which essentially allows me to write g-code in the context of a high-level rapid prototyping
language. It suits me just fine for the time being. When I get home tonight, I'll try to post a short program to give some flavor of what that's like.

Thanks again to those watching.

--Tim
 

[b.lindsey]

Still following along Tim, and thoroughly enjoying your account of the build!!

Bill

[tvoght]

Bill, thanks for the encouragement.

This post is specific to CNC programming and anyone not interested in the subject may safely skip it without missing any of the flow of
the build.

As I replied to Hugh above, I've been generating g-code using a Python language module that I wrote. To write a CNC program, I write it
in Python and in Python I write to a "gcode wrapper" object. I can create functions, loop, define data structures, make calculations,
read data from files, etc.

Once the Python program is written, it is executed, and it spits out pure g-code.

If you wanna see what it looks like, the link here shows the program that generated the gcode that was used to cut the contour of the Bearing
Stand shown before. Anyone who is comfortable with programming should be able to make some sense of it.

This method is not for everybody, but as I've said, it suits me fine.
http://www.voght.com/gamma1-vpa/bearing_stand/1-contour.py

That's enough of that for now,
--Tim

[Ian S C]

Sorry Tim, got the wrong end of the motor.  I ran one little motor for many years on meths, with the aluminium body of a white board marker, but I up dated that to a size AA NiCad battery case, the performance improved on long runs, but due to extra weight the short run was not quite so good.  With aluminium the water in the little tank boiled quite quickly.  2.5 CC and enough power to drive a generator to run a 3V radio.
                                                 Ian S C

[tvoght]

Ian,
Yeah, reduction in reciprocating mass is one of the few reasons for using an aluminum displacer, though given the higher strength of stainless, the walls could be made thinner, and
the weight competitive. I managed to get the walls of the stainless steel shell to about .01 inch thick, and  could have gone thinner without too much teeth-clenching..

The displacer in the photo a few posts back is 2 inches long and .94 inches in outside diameter. It weighs 16 grams, 11g for the SS shell, and 5 for the aluminum end-cap.

--Tim

[tvoght]

The Bearing Block fits in the large bore of the Bearing Stand made previously.
It holds the ball bearings in which the driveshaft runs.

I started with a chunk of 1144 steel. The material choice is uh.. immaterial
I wanted steel, and this was the closest thing in size that I had.
The piece is first drilled then bored to a sliding fit of a trial bearing.




A recess was cut to accept the flange of the bearing on the near end,
and the near end was also reduced in diameter. The smaller diameter surface
is where the Cylinder Frame will rotate to effect phase-angle changes.

The part was centered up on the Bridgeport and three holes drilled and tapped
for the "Cylinder Frame Clutch". I often find myself grasping to find the
correct names for these little pieces.  I have a very similar experience
finding names for symbols when writing software,



--Tim

[tvoght]

Now to the Cooler Stand. I hoped to improve the results I had gotten with the
Bearing Stand on this part, which is similar. A piece of 3/4" aluminum was
drilled at the Bridgeport with the holes which will be used to mount the cooler
to the stand. These holes could then be used to clamp to the CNC tooling plate.
A flat sacrificial plate was clamped to the CNC mill table and drilled and
tapped for the mounting holes in the workpiece. A program was run to cut a
rough contour leaving 50 thousandths or so to cut on a finishing pass.




The next program cut a series of steps onto the front.




Then a finishing pass profile was cut with a .1875 diameter endmill to take
off that last .050 or so.




The results on the contour were much better when the endmill was allowed to cut
only conventionally. I still have some issues with chips welding to the work.
This is most evident on the steps. I am sure it's now speed and feed issues.
It'll take me some more cutting to figure it out...



Thanks for watching!

--Tim

[Hugh Currin]

Tim:

You're better than I. I'd continually make small stupid errors in programing using your method. It does look better than writing G-code directly though. I've been using CamBam lately and find it a very nice CAM package. I see it's around $150 now. It  took awhile to find a package that runs under Linux but CamBam does. Not sure what magic they use to run it, but it's a very workable port. I'm using Kubuntu.

The finish on this last part looks good. I've been leaving 0.005" or so for the finish pass, I may try leaving more. Are you using climb or conventional cutting? I find climb cutting gives a better finish and it should work on a CNC with ball screws. Backlash can cause problems with climb cutting on a manual machine.

Thanks for documenting the build. Very nice.

Hugh

[tvoght]

Hi Hugh,
I don't make so many small stupid errors as I do great big stupid errors. Fortunately, those are usually caught by examining the preview paths before hitting the 'go' button. My approach moves are usually not rapids, so I have plenty of time to hit the "stop stop stop" button before the tool reaches the part. that first time..

I am cutting conventionally. I've not had much luck with climb on this mill (even with ball-screws). The fact is, compared to a Bridgeport, this small import mill feels kinda like a flimsy toy.
It doesn't take much of a cut for things to start shaking badly.

--Tim

[tvoght]

The cylinder mounts upon a Cylinder Frame which rotates concentric to the
crankshaft axis. The previously-made Bearing Block -in addition to containing
the crankshaft ball bearings- provides a plain bearing surface on its outside
for the Cylinder Frame to rotate upon. Rotation of the cylinder frame allows
adjustment of the engine's phase angle.

I started by laying a paper cutout of the Cylinder Frame upon a tooling plate
on the CNC mill. That allowed me to quickly determine a good touch off point
for the center of the large bore. Having chosen a spot, the coordinates were
zeroed, and 6 mounting holes were drilled and tapped in the tooling plate. The
frame blank will be pre-drilled at the Bridgeport with this same hole pattern.
Finally, these holes are the mounting holes for the cylinder to the frame.




The raw workpiece has an odd shape owing to the stock it was chopped from.
At the Bridgeport, it was drilled with cylinder mounting holes (with
counterbores) and bored accurately for the rotation bearing. 6 #2-56 screws
held the piece to the tooling plate.



Rough profiling was done to bring the contour to within .05 of finished
dimension.





The finishing pass followed.



Kinda pretty huh? Except WRONG. The mounting screw counterbores are on the
wrong side of the part! I had to do the whole thing over again except this
time cutting the contour with a program that placed the little feature at
the lower right instead of the lower left. No pictures of that, because there's
nothing new to see (and I was grumbling too much to take any).



In the Bridgeport vise, and aligned at the correct angle A radiused internal
corner was squared up for looks. This feature is for a handle for changing
phase while the engine runs. A hole was drilled and tapped to receive the
phase-change handle.





Oh, I guess I just admitted that the original intent was to be able to
change phase angle while running. Initial attempts at this were complicated by
kinking of the tubing between the cooler and power cylinder.
It looks like I may have a solution to that problem. Video when I get
it dialed in,

Do stop by soon (and feel free to comment good or bad),

--Tim