Gamma1-VPA Stirling

[tvoght]

This is the retrospective build log of an engine I've just nearly completed.
Retrospective as it is, I start with some engine photos and a video.


The "Gamma1-VPA" is a gamma-type Stirling engine of my own design. Mostly for
novelty, it features a variable-phase mechanism which allows the lead angle
between the displacer and the power piston to be changed.


I have always planned to do a build log, but it was to come afterward, after I
had proved that a Stirling engine of my own design would actually run.


With a log here in mind, I took photos during the build and made an effort to
rough out some notes of what had transpired. Sometimes I "fell down on the job"
and didn't keep notes. Some parts lack photo documentation.


In the next few days or weeks, I'll be working on the write-ups a part at a time
in roughly the order the parts were built. Some of the later parts lack any
notes at all, so I'll have to take cues from the photos to remember how they
were made.


There is still work to be done, such as making a new flywheel. The one seen
here was just for testing.


"Beauty shots" and a video.





<a href="https://www.youtube.com/watch?v=lpzM9CvXTVI" target="_blank">http://www.youtube.com/watch?v=lpzM9CvXTVI</a>

Questions welcome,


--Tim

[b.lindsey]

What a nice engine Tim !  I love the geometric touches to the supports and cylinder exterior, etc. The variable phase angle is also another very interesting feature. Will look forward to seeing more on this one, even if retrospectively

Bill

[tvoght]

Thanks Bill.
The geometric touches are to some degree CNC learning exercises, and only successful to some degree, as will be seen as the log develops.

--Tim

[mklotz]

Very attractive engine.

I'm sure we would all like to hear what you learned with the variable phase angle feature.  How about a shaft power versus phase angle plot?

[tvoght]

Hi Marv,

I have not learned much thus far, as the engine has not been running that long.

I actually did some first-order isothermal analysis resulting in some highly optimistic plots of shaft power vs phase angle, and
especially shaft power versus swept volume ratio. That is how I chose my initial design parameters for these things.

I had grandiose plans of fitting a pressure sensor and shaft position sensor for data acquisition so that I could directly compare experimental values to indicator plots from my first-order analysis.
There's noting to keep me from doing that later. I'd also want to rig a little prony-brake type dyno.

If there is any interest, I could do a separate thread with some information on how my analysis was done.

--Tim

P.S. Initial impressions are as the analysis suggested: phase angle does not make as much difference as you might think.

[Roger B]

Interesting engine and an instructive video.  The power cylinder looks like the body of a high wattage resistor to me?

I would certainly be interested in your analysis and any power figures   

[tvoght]

Roger,
The resemblance of the cylinder to a large power resistor was not lost on me, but it is in fact purely coincidental. The cylinder is made of cast iron and CNC milled to look like that.

Thanks for your comments!

--Tim

[MMan]

Hi Tim,

I would like to see your analysis too. I want to understand how mine works and would be glad to learn from you.

All the best,

Mman.

[tvoght]

Thanks for your comments MMan. I am trying to get together some material for a thread with notes on the analysis (hoping that noone
mistakes me for an expert in any way).

Construction will start at the hot cap.

A length of 1.25 inch diameter 303 stainless steel was centered in the 3-jaw
chuck, faced, center-drilled and then drilled with a 15/16 drill to a bit more
than the final internal depth of the hot cap.

I had reservations about starting with such a large drill, but I researched the
proper speeds and feeds and tried to stay in the correct neighborhood. It
worked just fine, with only a couple of belt-slips when I got too aggressive.
The photo suggests I might have gotten just a little too hot.



I used a 7/8 endmill in the tailstock and faced to the depth of the drill tip.



From here, a boring bar was used to flatten the bottom further, and to open
up to the desired inside diameter give or take a couple of thousandths.

Once bored, I measured the depth of the bore and then faced the end to
acheive the desired internal depth. Then I thinned down the walls to about
.050" thickness, to be thinned in a later operation on a mandrel.



Finally I cut off the cap at a length from the faced end calculated to give me
about a .050 end wall thickness. I used a cutoff tool until the noises became
unpleasant, at which point I shut off the lathe and finished the job with a
hacksaw.  Again, the end wall will be thinned down further with the part
inverted on a mandrel.
Hmm, I don't have a picture here of the part in that intermediate state.

Next, the workpiece will be placed on a special mandrel and the walls thinned
down further. The thinner the better, since I want heat to transfer well
through the end wall, but conduct badly lengthwise toward the cold end of the
engine. Thin walls will work in my favor on both counts.

Heat conduction is also one rationale for using stainless steel, as it is a
pretty awful heat conductor. Thats an advantage for preventing the heat from
conducting down the length of the hot cap, but a detriment for conduction
through the end.

More follows,

--Tim

[tvoght]

Here's the mandrel I made to thin down the walls of the hot cap.
The mandrel fits in a 1 inch collet. It's drilled and threaded 1/8-27
(tapered pipe thread). The drill hole extends throughout the length of the
mandrel, but the threaded depth is just a little more than half the length of
the sawn slots.



The expander is a pipe cap with a socket hex head. The other end is drilled
and tapped for a 8-32 cap screw. The cap screw is shortened and then silver-
soldered into the cap. Now I can drive the expander from either end of the mandrel.
This allows me to tighten the mandrel from the rear when the hot cap is
covering one end. This expander design got me through this build, but it's
not really satisfactory, as I'll discuss later.



Here's how the mandrel is tightened from the rear (collet) end. by a left
hand turn to pull it further into the tapered threads.



In use. Ah, there's the missing photo of the roughed out part.


The end and side walls were thinned to about .015 inches. With this expander,
it was impossible to get the mandrel tight enough. The part would slip on all
but the lightest cuts, It took many very shallow cuts to reduce the wall
thicknesses.

The finished part.




Thanks for stopping by.

--Tim

[Ian S C]

Tim, looks like a well thought out motor.  I'v made hot caps that way from 316 stainless(all I could get), only difference mine have a thread cut in the open end, and around that area six radial holes for a C spanner. I'v gone to fabricated hot caps lately, thin walled tube, end TIGed on.       Ian S C

[tvoght]

Thanks Ian. I've never machined 316 stainless, but from what I hear, I wouldn't have wanted to do this with 316. Seems it would have been quite a different story. Kudos to you!

The displacer shell was made in the same way the hot cap was. The final
step (the thinning down of the walls on the mandrel) was done after cutting
the same mandrel down to the smaller diameter of the displacer I.D. I won't
 belabor the point with more photos, except for this one of the finished shell.



The displacer cap is turned from aluminum. Here is the stock in a collet
with a shoulder turned back to make a sliding fit in the stainless shell.
It has been drilled and threaded 5-40 in this setup for concentricity (note
that the rod will screw in from the other end of this thread). A tool with
a sharp point was used to square up the shoulder and relieve the inside corner.



The aluminum cap was sawed off and placed in the 3-jaw chuck with sawed end
facing out, and then faced to length, leaving a top flange. Here's a shot of
a trial fit.




Thanks,

--Tim

[tvoght]

The Cooler came into the shop as a length of 2.25 inch 6061 Aluminum bar. 
I sawed off a length with generous chucking stub, and centered it up in the
4-jaw.  The outside was turned just to clean up, and the end was faced and
a boss formed where the displacer rod packing gland would go.
I then tried to cut a fin with a .063 inch HSS cutoff tool.


This photo shows how the cutter skewed off to the side. I realize I could have
fiddled around and maybe gotten things straightened up, and then I started
thinking about cutting 10 or 11 spaces using this method. I quickly went to
plan 'B'.





Plan B. CNC. This is something I had tried before and gotten decent results.
The cooler stock was clamped upright between v-blocks in the mill vise.


The program was to lower a 3/32" slitting saw to each fin space position and
to circle the saw around the piece in decreasing diameters until the fin depth
was reached. The process took a while, but was fairly stress-free and the
results are pretty good.







I'd gotten this nice lathe faceplate a couple of years ago but not used it til
now. I started by facing off the new plate to get it true to the lathe. Then I
threw together this tooling plate with clamps which would clamp on the topmost
thick flange of the cooler (clamping on the first fin).






After centering, I cut off the most of the chucking stub and faced the part to
length, followed by drilling all the way through the part with a drill of about
.23 inches diameter.
Then I drilled with a 7/8" diameter Silver and Deming bit, taking the tip of
the bit to the depth of what will be the blind bore of the Cooler.
I bored to the correct depth and diameter, trying my best to get a flat bottom.
This left a .23" hole at the far end, which was then reamed to .250". The
displacer rod packing gland will fit in that reamed hole.
Not shown is the shallow recess I cut around the open end of the chamber. The
flange of the hot cap will sit in the recess.





I had an off-the-shelf expanding mandrel of 1", which happens to be the
internal bore of the cooler. I put the part on the mandrel and was able to
access the socket head screw that tightens the mandrel by reaching the key
through the reamed gland hole. Thus mounted, I cleaned up the outside to be
concentric with the inside. That left a nice finish on the fins, just needing
some deburring later to crispen the edges.





The Cooler is not finished, but that's as far as I'm going for now until some
other parts are finished.


Thanks for looking in,


--Tim

[tvoght]

To make the Bearing Stand I started by rough-sawing a bar of 3/4" 6061 Al.
The first milling operations were at the Bridgeport. The work was clamped in
the vise and two reference holes drilled. The hole for the Bearing Block was
first drilled with a large bit:



And then bored using the boring head. I've finally gotten the knack of
using these telescoping gauges to a really decent accuracy.



The bored part was clamped at the CNC mill. The paper pattern was used to
help me get the clamps in a non-interfering position. After clamping, it was
torn away. This photo was taken before I realized that it must be clamped atop
a sacrificial plate (that's seen in the following photo).



After centering up on the bore, a program was cut loose on the work. It cut
the outline contour at many steps of depth. One thing to note is that I just
cut to the finished dimension with the cutter cutting on both its sides (slot
cutting).  I have since concluded this is a mistake, as will be shown.



Here is the finished contour (before any deburring of course).



And here's the ugly truth. The cut appears to have been chewed by shop-rats.
I blame this on cutting the contour with the end mill cutting on both its sides.
Future work will attempt to rough the cut first, with a finish pass cutting
only conventionally.



I used files and sand-paper to clean up as best I could. A general roughness
here will have to be a part of this engine's personality. Chalk it up to
experience (or lack thereof).

Til next time,

--Tim

[MMan]

Hi Tim,

I ran into the same problem of recutting in a slot making the plate for my Bas, looked much like yours.

I had to remake for other reasons and what I did to avoid this was cut first with a 9.5mm cutter and then, once cut to full depth, go round with a 10mm cutter. The 10mm cutter is 0.25mm per side (or 10 thou) bigger and so a reasonable finishing cut and cleans up the marks left from the first cut.

Mman.

[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

[smfr]

Interesting process there, Tim! Too bad about the part being wrong-side-up the first time. This seems to be one of the easiest mistakes we make; I've certainly done it myself at least once!

Simon

[fumopuc]

Hi Tim, also following along quietly. A nice runner.

[Ian S C]

Tim, re the kinked hose; I seem to remember something about a pair of curved brass tubes telescoped together.  Might have been in a Model Engineer article about a Dutch hot air engine.
                                               Ian S C

[tvoght]

Thanks for your comments Simon and Achim. Ian, it looks like I have a solution for the tube kinking problem, I'm saving it for a future
post since a photo and/or video will be so much simpler than my inarticulate word choices.

The cylinder was machined from a chunk of "Durabar" cast iron 1.25" square.
First in the Bridgeport vise, the end of a 1 foot bar was machined on top,
sides, and end. The width was cut to the finished width dimension of the
cylinder.



6 #2-56 mounting holes were drilled and tapped referencing from the known
locations of the machined end and sides.



The workpiece was then separated from the bar on the bandsaw.



The part was placed sawn end up on parallels back at the Bridgeport vise.
Care was taken to clamp  so that the side with the drilled mounting holes
was parallel to the mill spindle.



The end of the cylinder was milled down to finished length dimension, and
the center of the bore located.




The cylinder was drilled then bored to about .001 less than final diameter.



At the CNC mill, the part was placed on parallels in the vise with the mounting
base on the parallels. A contouring program was run to get a desirable shape.




A separate program written as an after-thought, cut the top of the head flange
to size and rounded the top corners.



I liked the results.




The cylinder still needs to be lapped, and I show here the commercial
lap I used. I'm not offering details but will mention that there is at least
one excellent thread on lapping on this forum to be referred to.
I think here of the thorough coverage by Ramon, whose thread I read at least
twice before undertaking this job.



Thanks again,

--Tim

[Jo]

Coming along nicely 

I am intrigued how does that commercial lap work? I could understand Ramon's with the little screw in the side providing the adjustment  But I can't see how that one adjusts 

Jo

[tvoght]

Thanks Jo. The replaceable barrel on the lap has an internal taper and the screw in the end is tapered to match. I thought Ramon had shown similar shop mad ones.

Note that the one in the photo cost less than $15 US, so I don't mind buying one.

--Tim

[tvoght]

The cylinder head was machined from a nice piece of 1/4" thick brass.
A blank was sawed out a bit oversize and the mounting holes drilled and
counterbored.



Once again, the mounting holes were used for mounting to the aluminum tooling
plate on the CNC mill. A profiling program was run to bring the outline to
size.




I had intended to leave the top face as seen in the previous photo, but it
looked too plain, so I whipped up another program to add some fins.



At the Bridgeport, a first hole was made for a barbed tube fitting.



I set up for, and milled an angled channel to go from the cylinder to the tube
fitting.




The threads were turned off of a purchased barbed fitting to fit the
drilled hole in the head. Here shown in the chuck of my trusty Taig lathe.



The head, the modified fitting, and an unmodified fitting for comparison.



And the assembly ready for soft-solder (process not shown).



Check in soon,

--Tim

[tvoght]

We take a break from the regularly-scheduled build to a time in the future -when the engine is running-.
I said before I'd originally intended to make the engine's phase angle variable even as the engine was running.
I had some problems with tubing kinking, but I have found a suitable solution to the problem.

Heres a video explaining:

<a href="https://www.youtube.com/watch?v=yjcjhlVbio4" target="_blank">http://www.youtube.com/watch?v=yjcjhlVbio4</a>

--Tim

[tvoght]

The piston has a graphite outer piece and an aluminum inner gudgeon piece.

The aluminum gudgeon was made from a length of 6061 aluminum held in a collet,
first cutting down a smaller portion.



And in a collet block at the mill, cutting in a slot for the rod.



And then drilling and reaming 3/32 for the gudgeon pin.



Still in the collet block, the piece was sawed off.



With the smaller diameter in a collet, the large end could them be finished.
I wanted this portion to be pretty close to .063 thickness, but I found I
couldn't get a measurement instrument in place to measure it. I "eyeballed" the
thickness by holding a 1/16" drill bit alongside. I got really very close using
that technique.




The piece was then drilled and tapped for a #2-56 screw.



The corner was generously chamfered to allow for a non-square corner at
the bottom of the bore where it will seat.



I have no process shots of the machining of the graphite portion. The shots
here are in fact taken this morning at a disassembly of the broken-in engine.

This shot shows the graphite piece at the bored end. The small screw hole can
be seen at the end.



The components ready to go together. A counterbore for the screw head has been
made, and was drilled by twisting the counterbore bit by hand.



This photo shows how the rod fits and how the gudgeon pin is trapped by the
piston walls.



Thanks for stopping by!

--Tim

[tvoght]

The Displacer Rod Gland is made of 932 bronze.

In the 4-jaw chuck, I turned the 1/4" plug-end which will fit in the reamed
hole in the end of the Cooler. The larger diameter was turned, then the rod
hole drilled to perhaps .11 inches and then reamed to .125 (careful that the
reamer did not reach the bottom of the hole).



I see that I am missing a photo which shows that, after cutoff, the plug-end
was held in a 1/4" collet and a boss turned on the other end, leaving a
flange with a thickness of 3/32".

I clamped an aluminum tooling plate on the mill and drilled and reamed a 1/4"
hole to accept the plug-end. By zeroing the DRO at the center of the hole,
the center of the part was thus well-established.

A hole had been drilled and tapped just to the side to allow a 10-32 screw
to act as a clamp on the flange.



I drilled three mounting holes.



The finished part.



Using another purchased lap, I lapped the reamed hole. Here's the cute little
1/8" lap.



--Tim

[tvoght]

The crank was built up of 303 stainless steel and held together with loctite
retaining compound.

The first photo shows the beginning of a fixture for building the crank. An
aluminum block was clamped in the CNC mill and faced off to be square with the
machine.



A paper cutout helped me to locate the position where I would drill for the
crank shaft.




A crank disk had already been turned with a hub and a reamed shaft hole (sorry
about the missing lathe pictures). The fixture was drilled and reamed for the
shaft, and the hole was enlarged near the top to clear the hub.
A large chamfer was made to accommodate the radius between the hub and the
crank disk.



Here's the disk in place. You can see how the back of the disk sat nicely
on the fixture block.



A second disk drilled and reamed slightly off-center was placed on top of the
first and was aligned with a .25 mill shank. The fixture had been drilled
and tapped for clamping screws, and a strap clamp held the disks in place.



With the parts thus aligned and clamped, two crank pin holes were drilled and
reamed through both disks. The holes extended well into the fixture block to
be used for later alignment references.



With the bottom disk removed, the previously reamed holes provided for
alignment of the upper disk with drill and mill shanks. The disk was clamped.



The disk was milled to define a small crank web between the crank pins. Note
the thin bridging piece left to keep the larger part with the center hole
intact. Keeping the larger piece attached facilitated fixture clamping and alignment.



The parts were re-clamped, this time with the short piston crank pin loctited
in place. The drive shaft was temporarily used in the center hole for alignment,
along with a drill bit in the yet unused crank pin spot. Also, a 1/8 inch lathe
bit was used to establish the proper spacing between the disks.



These photos shows the progress after the loctite cured on the first crank pin.




Shown in this photo is a fixture to help hold the displacer crank pin in
alignment. The crank pin is seen protruding out of the aluminum alignment fixture.



Here's the assembly with the displacer crank pin being loctited in. The aluminum
alignment fixture obscures the pin itself. Note the crank shaft is still inserted
to ensure alignment of the two disks during loctite cure.



After the loctite had cured, the main shaft was loctited in its proper position
in the lower disk.




Finally, the extraneous fixturing part was sawed away with a jeweler's saw
to leave just the small web between the crank pins.



A note about the shaft and crank pins. They are made from 303 stainless
steel "Miniature Drive Shaft" material purchased in 3 inch lengths from
McMaster-Carr. The diameter and straightness tolerances are excellent.

--Tim

[tvoght]

The Cylinder Frame Clutch (for want of a better name) is the part which traps
the Cylinder Frame against the Bearing Stand and allows the Cylinder Frame to
rotate in order to effect phase angle change.
A pad made of thin Teflon sheet is trapped between the clutch and the Cylinder
Frame to provide a smooth adjustment with a tendency to hold place once set.
Is there a term for the kind of action I'm trying to describe?

I started by turning and parting a .125" thick washer of 1144 steel (chosen
because I had stock of appropriate diameter). At the Bridgeport, I drilled and
countersunk for 3 #4 flat-head screws.



I mounted the part on a store-bought expanding mandrel and tapered back the
face for appearance.



I mounted the part on a tooling plate on the CNC mill and ran a program to shape an outer contour..




The finished part. The 3 holes match the holes tapped on the face of the
Bearing Block seen before. The three "arms" which extend past the screw holes
lie over the Cylinder Frame, with the Teflon pad between.



Thanks for looking,

--Tim

[Roger B]

Coming along nicely 

I am intrigued how does that commercial lap work? I could understand Ramon's with the little screw in the side providing the adjustment  But I can't see how that one adjusts 

Jo

Jo, it looks like one of these:

http://www.acrolaps.com/

The price list is attached.

I ordered the 15mm and 25mm sizes along with two spare laps in each size. Postage was 28 USD

[tvoght]

Roger, I'm quite certain that is the source of the laps I use. Thanks for posting the link.


Photographic evidence of the connecting rods is woefully inadequate, but I'll
show what I've got.

First the piston connecting rod. Starting with 360 brass, I made an end cap
by drilling for 2 0-80 screws in the end, close fit through the cap and threaded
in the larger rod portion. The cap was then sawd off with a slitting saw and
then screwed back on.

In this first photo, you can see the rod in the Bridgeport vise with 5 holes
drilled. The hole the the left is the big-end crankpin hole and you can see the
split where it was sawn. That hole is reamed 3/16. The 3 middle are lightening
holes which double as clamping holes in a further operation. To the right is
the gudgeon pin hole which is also reamed 3/32. The milled portion reduces the
thickness of the rod everywhere except at the big end. There is a matching
milled slot on the other side.



At the CNC mill, the part is clamped to the tooling plate with cap screws.
There's a reference hole in the tooling plate to align the part to a known
mill reference (so the programmed cuts are made with reference to the crankpin
hole).



Here's the part after the contour is cut.



The displacer connecting rod was made in the same way, except that it does
not have a detachable end cap. I have no process photos.

Thanks for watching,

--Tim

[Ian S C]

Tim, I,m a bit uncertain about the reason for the separate crank pin for the displacer, was tis required to attain the correct ratio between the two sides of the motor?
      My first Stirling Engine was a V type, with a single crank pin, the displacer driven by a Scotch Yoke.  The ratio was obtained by increasing the diameter of the displacer.  I did, at one stage think of making an adjustable phase angle on it, but it ended up getting pull to bits, and converted into a Ringbom motor.
                                                          Ian S C

[tvoght]

Hi Ian,
  I'm glad you asked, that's something I wanted to point out.

 Whereas most Gamma Stirlings would have crank pins 90 degrees apart, mine are just 41 degrees apart. The angle between the displacer and piston is added to the 41  to make the entire displacer lead angle. In other words, to get the tradiitional 90 degree displacer phase lead, the angle between piston and displacer would be just: (90 - 41) = 49 degrees. I did that so that the tube between would not have to be
so long which would have added to the dead volume in the engine.

Oh yes, as you suggested, there is a slightly longer stroke on the displacer pin to help make the difference in swept volume ratios.

Make sense?

--Tim

[Ian S C]

Tim, that explains the angle of the power cylinder in the video, thank you.  My thing is that I do minimum maths on the build of my motors, so I haven't tried any thing fancy yet, I'v built one GAMMA motor with parallel cylinders, a bell crank, and single crank pin.
                                                           Ian S C

[tvoght]

It seems I've run out of build photographs and yet the flow of the narrative seems quite incomplete. I'll attempt to provide closure by offering this rather
grungy video showing the engine first partially disassembled, followed by a re-assembly. At the end, there's a short and slightly more graphical description of what I was trying to explain to Ian about how the Variable Phase Angle feature is accomplished.

Sorry in advance for the quality of the video.

<a href="https://www.youtube.com/watch?v=bm25oCfxbEA" target="_blank">http://www.youtube.com/watch?v=bm25oCfxbEA</a>

Thanks,

--Tim