CD LTD Stirling

[Arthur Dent]

Here are the support posts and the cd flywheel hub that I made several months ago, along with a few graphite slugs sent to me by a very kind forum member.



I have never worked with graphite before, this should be interesting.  I haven't yet started looking for the glass for the power cylinder.  If anyone has a suggestion for a US source I'll be interested.  I have thought about making the power cylinder out of brass, but that would loose the visual appeal of the glass.  I do have an Airpot dashpot I could use, but that would be cheating.
The flywheel hub was made from a chunk of an aluminum grounding rod of some sort found by a dumpster diving friend.  The material is very odd, much lighter than you would expect for the size, and the surface has a kind of powdery grainy look.  It almost looks like it has been anodized or sandblasted or something.  I'm nor sure it's actually aluminum, but I don't know what else it would be.  The rods are a little over 1" in diameter and about 22" long.  There is a 1/8" dia steel core down the center, which I presume was to provide a hard tip for pounding into the ground.  There was also a copper connector on the top of each, which my friend cut off to keep for his copper scrap pile.  You can sort of see the grainy surface finish in the photo.  Compare that to the support posts to the right, which are 6061 aluminum.

[wlb]

Not sure if this will work I have attached two photos of my hot air engine which looks like the one on topic. My working piston was plastic. It runs on the heat of a 12V car bulb

[zeeprogrammer]

I meant to ask earlier...how did you go about cutting the carafe?

[Arthur Dent]

The carafe wasn't very difficult to cut actually.  I cut a length of paper the exact width I wanted, taped it around the carafe and drew a line around.  After removing the paper I took it over to the paper cutting bandsaw at work (part of my lunchtime project) and cutting very lightly sawed it off.  Not sure what kind of plastic the carafe is made of, but it wanted to melt into the blade so I had to take it very slowly.  After that, a light touch-up on the belt sander and the cylinder looks pretty good.     

[Arthur Dent]

I did get a little time in the shop today... very little.  Anyway, I decided to play around with turning one of the discs down to diameter.  I put a dimple at the center of one of the discs, then used a live center to butt it up against the lathe chuck (with jaws removed).  A thin piece of cork sheet between the disc and the chuck provided a little traction.



I saw someone do something similar for a flywheel in a build thread somewhere.


 
I had to move the lathe bit out to the outside position in the holder, wind it out all the way, then angle it in a bit, but it worked just fine, as you can see.



I started out just taking off a few thou, but got braver as it went, and eventually found that I could take off .010" at a time without chatter.  This will be fine for the top plate, all it gets is a few holes and a groove for the top of the displacer cylinder to fit in.  The bottom disc however, gets hollowed out a bit to thin down the bottom surface for faster heat transfer.  For that one I'll probably try the double sided tape method.

[Bezalel]

The flywheel hub was made from a chunk of an aluminum grounding rod of some sort found by a dumpster diving friend.  The material is very odd, much lighter than you would expect for the size, and the surface has a kind of powdery grainy look.  It almost looks like it has been anodized or sandblasted or something.  I'm nor sure it's actually aluminum, but I don't know what else it would be.  The rods are a little over 1" in diameter and about 22" long.  There is a 1/8" dia steel core down the center, which I presume was to provide a hard tip for pounding into the ground.  There was also a copper connector on the top of each, which my friend cut off to keep for his copper scrap pile.  You can sort of see the grainy surface finish in the photo.  Compare that to the support posts to the right, which are 6061 aluminum.

Hi Arthur


Your description is identical to what one would expect to see for a magnesium sacrificial anode out of a solar hot water storage chamber.


there are two ways to confirm it ( well it is only one if you actually intend to use the material for some thing else )


1. scratch a piece down to clean bare metal add a few drops of white table vinegar, if the vinegar bubbles up it is indeed magnesium.
2. this one is not recommended but fun for confirmation, if you have a few spares.
Ignite the end of the bar in a very hot timber fire or furnace or blowtorch. once alight you have a hand held torch good for about 10 to 15 minutes with the brightness about the same as a 3000 Watt light bulb.   


This is an out doors activity in the clear absence of kids, pets and spouse.

[Arthur Dent]

   You might well be right about the magnesium.   Every time I pick up one of those rods that little voice in the back of my head says "this isn't aluminum".  I just never followed it through.
I suppose that is the danger of using "mystery metals". 
If I get the chance I'll test a sample of it after work tonight.

[Arthur Dent]

I have been thinking about the glass for the power cylinder.  Several months ago I read a long thread somewhere discussing the suitability of various types of test tubes and syringes for this kind of use, though of course I can't find it now.  There are lots of online suppliers for such things, but being a creature of habit I of course first turn to:
Amazon.com 
Wow, what a variety of choices.  16 X 150mm rimed Pyrex test tubes look good.   That would give me a lot of material to play with, and hopefully out of a dozen or so I'll end up with a piece round enough to make a good cylinder.   So now while waiting for those to arrive I have plenty of other work to do...

[rleete]

Are you also known as "Rocketcaver" on that other forum?

[rleete]

Also, for test tubes that are the right size for this (as well as most of Jan Ridders engines), try Edmund's Scientific.  Buy several, as you will find some aren't round, some are tapered, etc.  Of course the ones that are perfect you will break while cutting, so you need a selection.  I bought about 6 of each size, and got enough usable to make several engines.

[Arthur Dent]

Yes sir, that's me.  (I thought you might recognize some of my photos)
I haven't ordered the test tubes yet, but plan to in the next day or so.  Have been looking at the Corning Pyrex Vista series. 
I hadn't thought of Edmund's, will take a look.  Thanks!

[Arthur Dent]

Bezalel, you were right.  It's magnesium.

[Dan Rowe]

Arthur,
One of the demonstrations at the fire fight training I had was to take a double handful of oily magnesium swarf and set it on fire in an open field. We used a  three way nozzle and hit it with fog first and it really flared up. Then we switched to high pressure and the swarf became a 20 foot ball of flame. Wow that was impressive.

The machine shop was only too happy to donate the swarf to the school as it is a real hazard.

Dan

[Arthur Dent]

Thanks for the warning.   
I am well aware of what magnesium can do, I used to make rocket engines with magnesium powder and... other stuff.  I just never had a big solid chunk of it before.
Rest assured that I don't plan to machine any more parts out of this stuff.  I'm not sure what I will do with it, but it has already been removed from my metal working area.

[Arthur Dent]

My diamond coated glass cutting discs and test tubes arrived a few days ago, so I logged some more shop time.
Here you see me set up to cut a test tube.  Most folks pad the test tube with paper or tape or something to keep the chuck jaws from crushing them, but I found that un-necessary.  A light touch on the chuck key and the tube was securely anchored.
 
Mr "out of focus" decided to visit for a while, sorry about that.   

The lathe is set to run in reverse, so the the tube and cutting discs rotate in opposite directions.  I'm not certain that this is really necessary, but it made me feel better.  Also, lathe and Dremel running at full speed.  A few drops of 3 in One oil every now and then, plus a very light touch on the hand wheel, and...

This particular tube was a culture tube that I happened to have a whole bunch of.  The culture media had expired and the lab was going to pitch them, so I got permission to "liberate" them from the waste bin.  I was just using this for practice before cutting up a Pyrex tube, but the culture tube turned out to be quite a bit more "in round" than the Pyrex tubes I got, so I'll try it in the engine first.  In this low temp engine the heat tolerance won't matter, so the lime glass should be fine.  I got a few different sizes of Pyrex tubes, I'll save them for use in my Ridders high temp "Stirling Bas" when I get around to building it.  Some of them just happen to be the right size.   
I turned down a slug of delrin to see how good a seal I could get with this tube, and the results are encouraging.  With my thumb closing one end of the tube the slug barely moves down the tube... when I release my thumb it falls through freely.  I'm actually tempted to use the delrin for the power piston.  But I have the graphite, so I HAVE to use it, right?
Back to the top plate, I thought I would be clever (or lazy) and use a parting tool to make the groove for the displacer cylinder to fit into, but because of the way the parting tool holder is made, there is no way to mount it so that I could crank it into the proper position for the groove.  So, I ground up a quick tool to do the groove. 

You can see that it worked ok, but after I got in a few thou in it started squalling like a scalded cat (no actual cats were harmed in the process).  Yes, I was wearing hearing protection, and it's a relativity shallow groove, so I just kept going until I reached spec depth.  But I would be interested to know how to properly grind a tool for this purpose.  I did search first and didn't find anything, so just did what I thought would work.  It worked fine, other than the squalling.  I'm sure it was caused at least partly by the outside wall of the groove rubbing on the side of the tool, I can see the side of the tool is rubbed dull by this action.  I thought I had allowed for that while grinding it, obviously I was incorrect.