LTD Stirling

[Cogsy]

Hah! I put it in an electric skillet set to 350F (probably way hotter than necessary, and only as I write this do the words "temper" and "draw" zip through my brain), and it popped right out. Next time I won't get it so warm, if there even needs to be a next time.

Although it's unlikely to make much of a difference on a tiny bearing like that, most bearings are only dimensionally stable to around 110 Celsius (230 ish Fahrenheit) and shouldn't be heated above that or they may permanently deform. SKF 'Explorer' bearings can handle a bit more (about 150 C from memory) but in general you don't want your precision bearings getting too warm.

 

[Tim Wescott]

After I did the above, I decided that I wanted the groove to fit entirely inside my 1/2" square bar. This entailed not just making the diameter smaller, but reworking my tool because it was not clearing the 0.44" OD of the trepan. But the test cut came out pretty good; I feel confident in doing it for real.

One lesson I think I should learn here is "don't take pictures of your work under a microscope". Clearly, it isn't sticking.

 

[Tim Wescott]

Gaah! I just mucked up my bearing block! I should have used blocks to keep the chuck from digging into the ends. Next one I'll do the mount ring first (when the ends are solid!) then bore it out and do the bearing holes. Which I have to get right again, after sweating bullets the first time over them.

 

[Tim Wescott]

Because if you need half a dozen 1/4" hex spacers, 1" long, then of course, the sensible thing to do is get some 1-1/8" spacers surplus (saves money!) and cut them down. Not, like, find some 1" long ones (it's an off-the-shelf size), and just buy them.

And if you need a dozen matching 6-32 screws 1/4" long, then of course it makes sense to cut down the 1" long ones you have, rather than making a run to the hardware store...

The spacers came out within 1 mil of each other, except for the one that was 0.002" short. I'm going to keep it -- I don't think that much length mismatch will be a problem. I plan on assembling the heads to the displacer cylinder with bathtub caulk, and you don't want the displacer to be a tight fit anyway, so I think I'm golden here.

The middle picture shows how I mass produced my six spacers -- I have a 3" long shaft from somewhere; I just used the Jacob's chuck to set the distance. It's not as good as it could be -- the face of the chuck isn't quite flat -- but it's OK.

 

[Tim Wescott]

End caps (heads?) assembled onto the displacer cylinder. Everything fits -- probably because after drilling 3/32" pilot holes in one end cap, I taped them together with double-sticky tape and drilled them through together.

A 5/64" drill is a really snug fit on a 6-32 screw, so things are nicely located. The cylinder is a snug slip fit (if that's a thing) between the spacers. And blowing on it shows obvious but very small leakage -- so the challenge with RTV will be applying a small enough bead so that it Just Doesn't Show. Part of me is tempted to try to machine a groove for an O-ring in the top and bottom of the cylinder. I'm not going there -- nope, nuh-uh.

That clear acrylic cylinder is really, really good at showing fingerprints. Final assembly is going to be a bear, I can tell. Gloves will probably be involved.

 

[a41capt]

What material are your spacers made of? The only reason I ask is that if they’re of a conductive material, they’re going to conduct the heat/cold to the opposite plate. While it probably won’t negate the temperature differential completely, in an LTD Stirling you need all the differential you can muster to make it efficient. That’s the real reason for the acrylic cylinder, it doesn’t transfer as much heat/cold from one end plate to the other.

Your work looks great however!

John W

 

[Tim Wescott]

• I hadn't thought about that
• Aluminum
• You're absolutely correct
• There's some 1/4" Nylon, Delrin or Bakelite rod in my future, but I think I'll forge ahead with this as it is for now. (I may just buy some hex spacers in Nylon, if I need to make an electronics order)

 

[Tim Wescott]

When I was going to school, getting an electronics engineering degree, I was overjoyed when the rules changed and I could get my degree without taking thermodynamics. Now I have just enough scratched-together knowledge to size heat sinks correctly (which is essential for power electronics), and I really wish that I could do things like sit down and analyze a Stirling engine for the degree to which aluminum spacers vs. nylon really makes a difference -- but I can't.

 

[a41capt]

When I was going to school, getting an electronics engineering degree, I was overjoyed when the rules changed and I could get my degree without taking thermodynamics. Now I have just enough scratched-together knowledge to size heat sinks correctly (which is essential for power electronics), and I really wish that I could do things like sit down and analyze a Stirling engine for the degree to which aluminum spacers vs. nylon really makes a difference -- but I can't.

Go with the aluminum and see if it works! You can always change them out later if you want to test the difference, but if it works, why not leave it be! Frankly, I’m looking forward to seeing your LTD take its first spin!!!

John W

 

[Atlantis]

Hallo Tim,

Ich habe auch LTD-Stirling gekauft. Zwei von ihnen laufen mit Handwärme. Ich habe das andere aus Fotos aus dem Internet verkauft. Es läuft mit einem Delta T von ca. 5 ° C. Der kleine Handwärme Stirling läuft mit Delta T ab ca. 6-8 ° C und ist auch Ringbombe bauen. Er hat eine warme Hand. Eine weitere kleinerer LTD besteht über eine Magnetsteuerung.

Grüße Günter

 

[Atlantis]

Es ist eine Schande, dass Googles Übersetzungen so schrecklich schlecht sind .......

 

[Tim Wescott]

Günter, I'm glad there's no contest for "good looking", because I've already lost. I'm going to have to fall back on "if it works, it looks good". At least -- if it works.

I may use the magnet idea -- I'll have to think about it. It certainly saves a few cranks and seals.

 

[Tim Wescott]

I'm kinda torqued at myself for making a nice pretty trepanning tool that only works in reverse. So I decided that I'd try again. This time, rather than spending tedious hours with a Dremel Dangerous Disk and a 1/4" square tool blank, I'm going to spend tedious hours with some drill rod and my lathe, cutting a blank that I'll finish off with saw and file, and then braze to the end of a mild steel shank.

The mild steel shank is because (A) it was there, and (B) I'm a tightwad, and drill rod is expensive. In fact, the bar I used for the shank has been lying around on my bench for just about ever, and it was warped. A thump or three with a sledge hammer on an anvil straightened it out enough to fit in my boring bar adaptor.

My Evil Plan is to finish off the blank, braze it onto the shank, then quench everything in oil, then draw the thing a bit. That's probably a horrible misuse of O2 steel, but I'm only making a few cuts, and into aluminum at that, so I think it'll be OK. And, if I screw it up, I can get out the Dremel Dangerous Disk.

Of course, I could be wrong about the O2 steel -- the package says it's Rulon. I guess if it melts when I try to braze it I'll know that I was just horribly wrong.

 

[Tim Wescott]

Handy tech tip: don't put an itty bitty part, with an hour's work in it, on a piece of wood on your welding bench, then decide to toss that piece of wood off your welding bench for fire safety. At least, not without retrieving your itty bitty time-consumed thing.

I never did mention -- this is going into a 3/8" diameter boring bar holder. Hence, the nominal diameter of the thing is 3/8". I'm cutting a grove of 7/16" diameter, so having a tool with an outer face of 3/8" diameter will, I hope, do a good job of providing clearance.

So on my first attempt I made the thing 7/16" in diameter. Looking at it, I decided that 3/8" would be better. So I turned a new bit out of tool steel, turned down the end of the shank, and got as far as actually making the cutter blank.

On my second try, I got a blank all cut out -- annulus cut on the lathe, bit end cut out with a hacksaw in the vice, then filed by hand to the rough shape, then back to the lathe for parting. This is where the handy tech-tip comes in -- that one is somewhere in my welding shop.

On my third try I did all the above, and managed not to lose the thing before it was brazed on. My original plan was to braze it on and immediately quench it. This may have worked, but I was so paranoid about not dropping it into the oil I pulled it out before it was really cool -- a file definitely touched it. This afternoon I got it all nice and "won't attract a magnet" hot, then quenched it properly -- it's plenty hard. I didn't draw it at all -- I'm going to make a few cuts with it and retire it, so I'll trade ease of heat treatment and no worries about softening it too much for (slight) worries about it being too brittle.

And if I screw it up, well, I have experience making these things...

The thing is all over oxidization and brass and a bit of brazing flux -- but going on the principle that beauty is what beauty does, based on the test cut it is, de-facto, beautiful. This thing works pretty darned good. There's a bit of vibration as it goes in (very reminiscent of parting off -- gosh, I wonder why that is?). But it leaves a very nice finish, and cuts a groove that's almost the right size -- it's just a hair over 1/16". Since I'll be making the mating part to fit, I'm just going to claim that I meant it to be that way all along, and proceed.

 

[Tim Wescott]

Since I'm feeling comfortable with the cutter, onward and upward to the second try at the crank bearing block. I've cut out a piece a bit over 1" long, marked it out for center and used my scribe to make a shallow punch mark in that center.

According to Gimp, the center is about 0.002" off from the true side-side center of the square bar. I'm calling that good enough. The photomicrograph is because in my other life as an electronics engineer I've obtained a nice stereo microscope, which I used to more precisely make my center-pop.

I'll deepen the punch mark, then use a wobbler to get it centered on the lathe.

 

[Tim Wescott]

I'm definitely committed to using only the most up-to-date precision-made tools.

One day I'm going to get myself an indicator with an actual dial. And maybe buy tools instead of making them out of bits of wire lying around the shop.

 

[a41capt]

Is that a Startett 64A? Those look like a reasonably good substitute, but as you said, no comparison to a good DTI!

Enjoying your work around, thanks for sharing with us!

John W

 

[Tim Wescott]

Plain old 64, according to it. It's not going to get me down to tenths, but I feel pretty confident that if it says I'm centered to half a thou or so then I'm really there.

(I'm not that close on this one -- it's a hair less than a thou, and I'm calling it good).

 

[a41capt]

Old technology like this served machinists well for decades. I looked at picking one up locally that they wanted $50 for, but I couldn’t justify the expenditure.

 

[Tim Wescott]

The mounting features of the block are finished. If I got my math right, then that screw in that hole, tightened on the post that holds everything together, should clear the 3/16" shaft by 0.01".

If not -- there's cutting tools.

The hole was cut by first using last month's setup to drill a clearance hole for a 2-56 screw with a #43 drill. Then I swapped ends, centered on the hole using the drill as a wobbler (way high tech!), cut 0.430" deep with a plain old 3/16" drill, then cut 0.430" deep with a 3/16" mill. I wanted to do most of the cut with a plain old drill because I wasn't sure about the wisdom of using the mill in a Jacobs chuck in my tail stock.

Now I just need to get my courage up and do the bearing holes.

And now, here where I've put my own back against the wall, I realize that I could have made the scary fiddly bits (the bearing holes) first, and then made plugs to put in 'em so that I could make the mounting features without risk of marring my piece -- and it was being scared of marring things that drove me to doing the scary bits last.

Oh well -- live and learn. We measure progress by the discards pile...