Small Sterling Fan

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shred

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After some discussions around the hot-air engine posted in the Plans area, I decided to get off my duff and make the mini-fan I've been thinking about for a while. Since I asked for this forum, I'm going to post parts as I make them, with plenty of commentary and gory details, and encourage others to do so too, no matter what their experience level.

I'm never one to follow directions exactly, so this one is mostly based on Jerry Howell's "Mini Sterling Fan" plans, but with modifications and additions from Dr Senft's Moriya (and the half-size version) and other things I think up. Hopefully it'll all work in the end, since this will be the first Stirling for me. Anybody with more experience is welcome to chime in with hints and other ideas as I track off into the deep end.

Much of the reason for building in this scale is I'm working with scraps from a friend's CNC machine shop. You'll note that "one inch" is a very common dimension for major parts, with very few larger than 1" in two dimensions. That works well for the scraps.

My usual pace is one part a night, though that varies a little. Somehow easy parts mostly end up taking all night too.. I'm definitely no Master of the Machine Shop, mostly self-taught and barely getting by on the experience from previous screwups.

So, going with the rake60 patented 'make the hard part first', I started off with the displacer cylinder. Jerry calls for this all to be one piece of metal, but not having a slitting-saw the right size and not being keen on long overhangs of the parting tool needed to clear the base, I elected to make this in two parts similar to Moriya. In addition, the parting tool I had handy wasn't the thickness specified for the fin spaces, so those also had to be resized. Two design changes and I've not even made the first part yet. Poking in the scrap box, I went with brass for the finned piece and aluminum for the base. Brass conducts heat pretty well and it looks nice, so there it is.. Three changes, now..

Senft's instructions for fin cutting by putting the compound parallel to the lathe axis and cranking it to set the fin-space distance works. I started out with a table of spacings, but ended up just zeroing the dial and cranking the same distance each time.

Then, thinking about how to attach the the base, I thought ahead to the hot cap and how I had some nice 1" round stainless. The plans call for 1" square, which I don't have, but by moving the mounting screws in a little, not only could I add room to hold the fins on, but I could fit the mount screws into a 1" circle. That turned out great. Even though there will be some insulators holding them apart in the end, 1" round fits perfectly inside the 2-56 SHCS heads holding the fins on. I'll probably counterbore them flush anyway, but I was impressed with my own serendipity ;)

(a note on the photos. Right now they're 'as-cast'. I'll tidy them up later, hopefully. (for scale, the fin block is 1"x1"x1.2")



Well, one weekend over and I have a new shiny paperweight, for small values of 'paper'.. Onward to the hot cap and power cylinder.


 

shred

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The Hot Cap is specified as "CRS or stainless, stainless much preferred" (for thermal reasons), so, not having worked with stainless since getting my 10" lathe, and thus having forgotten the irritation thereof, I went with stainless. I think the 1" round was 17-4, or at least my friend thinks it was. Anyway, my opinion of stainless hasn't improved. This stuff generated thousands of tiny porcupine needles that lept off the workpiece into my fingers, plus left an ugly finish (the source bar of course has a flawless smooth shiny finish) :mad:

The hot cap is specified at 1.00" long, with a .950" deep bore and a .050 sidewall at the 'waist', to limit heat conduction. My cleverness here at not-wanting to dink with precisely boring a flat bottomed .950 hole was to bore it over deep (~1"), then measure and face it off so the hole was .950 deep. Probably old hat to the pros, but I was happy to have thought of it. After snapping one drill drilling out the stock, I tried a tip I read somewhere and clamped a 1/4" end mill into the boring bar holder with one flute on center as a mini boring bar-- that worked surprisingly well to enlarge the initial drill hole enough to get regular boring bars in.

The next smart thing (Jerry suggests this on his web site) was to turn up a close-fitting piece of Al with a center hole in one end. Putting that in the finished bore, it was easy to judge the wall thickness and prevent it from collapsing as I turned the outside profile, plus let me get the tailstock into the fray.



The plans call for 2-56 stainless screws to lower heat conduction. Asking around for some of those led me to an excellent bolt and fastener supply house just a few miles away-- no minimums and $0.03 apiece "how many do you want?". "how many you got?" "35" "ok, I'll take them" "that'll be $1.05, cash or charge?". :) With a place like that I may have to abandon my usual practice of ordering small screws in the longest available size and grinding them down as needed.
 

Powder keg

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Great job so far! You could make one of those arbors with the flat spot on them that uses a small dowel to cam the piece tight. Using this, Use some Emery and sand the hot cap to a shiny finish.

Wes
 

shred

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Jerry's plans have the power cylinder with fins. I'm not sure why except maybe to look cool, since few others I've seen have them. Stirling gurus, is there a thermal reason I'd want a cool power cylinder?

Anyway, I had some 7/8" brass rod ends and sorta-finned mine. Turns out my parting tool had got loose and the fins were only about half as deep as the plans show, so they'll be even less useful, if they ever were. After drilling and boring the cylinder, I turned up another close-fitting piece of Al and with a little JB Bore compound, lapped out the bore. JB is used to clean heavy deposits of copper and lead out of rifle barrels and works great for light lapping (plus I have a ton of sample jars)

The 7/8"-ness of the stock rod almost got me in trouble with the mounting screws-- I didn't check beforehand and they only barely fit (Aside: I drilled the displacer base already on my little CNC-- I've taken to drilling not quite all the way through to avoid drilling into my parallels and finishing the last little bit on the drill press. So far it's worked well)

I milled the flats using another little trick I picked up a while back for no particular reason-- my screwless vice has a little v-notch in the center. Makes it easy to hold and locate round stock with an edgefinder off the sides of the vice. Anyway, In order to mill the flats on the cylinder base full depth without setting the top of the cylinder up on parallels, I drilled a 3/4" hole in some smooth scrap .125" Lexan, put the power cylinder top-down through the hole, clamped it so the lexan acts as a sacrificial spacer between the part and vice and milled the flats. I should go back and stage a photo of that.







 

shred

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Powder keg said:
Great job so far! You could make one of those arbors with the flat spot on them that uses a small dowel to cam the piece tight. Using this, Use some Emery and sand the hot cap to a shiny finish.

Wes
That's a good idea.. I'll remember that come 'bling' time-- I tend to bang the parts around a little while I'm building, so I don't do much finishing until late in the game.
 

shred

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Made the bearing standards a couple days ago. Pretty straightforward cutouts, leaving just a little stock to ream for the bearings. The plans were a little vague on some of the contours, so I made up some that looked right. Later on once I determine the need for the holes I may go back and redo them with some fancier contours.

I spent a while contemplating how to do the workholding to cut them out before I came to my senses and realized the finished parts were only an inch across-- slap some 1.5" flat in the vice and say 'go'...

Bearing standards and the rest of the assembly so far:


I also made my first try at the cold end cap today. The milling went great.. until I realized I had only left myself .085" to grip for turning the next .115" into a boss. I think I'll try that again in reverse order. Lathe first, then mill.
 

Powder keg

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Good Idea on leaving some to ream out on the bearing holes. I'll do that on my LTD that I'm planing:O)

Great job so far! Sometimes it's easer to "finish" the part buy emerying it down, before you part it off. Also I've noticed that shiny parts don't scratch up as easily as ones that are just lathe turned. Just something to think about?

Wes
 

shred

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Yesterday I built version 2 of the cold cap (the first scrapped part so far, not including a few fin-practice bits). Here it is with the bearing standards attached using overly-long SHCS (the loose screws are what hold the cold cap to the displacer cold end):



After the learning experience from the first part, it was pretty straightforward, though with a bunch of operations for a little part-- put a short bar end of 1.5" Aluminum round in the lathe, turn the .624 boss, ream the gland hole, skim the bar then move the bar over to the mill vice, get centered on it (using the above "edge-find the vice & use the centered v-notch" setup-- the previous skim cut keeps things aligned if the lathe chuck was off center or the original stock was non-round), and mill around four sides of the round to make the end into 1" square. Back to the lathe (or bandsaw) to part that off, then back to the mill the other way up to cut one of the central notches, rotate 90' and cut the other and drill the first four 2-56 holes. Flip it onto one side to drill, tap and cut the notch for the bearing standards, then flip and do the other side.

No doubt the entire process would be easier done with 1" square stock, but I had none in the bin, and plenty of 1.5" round.

(Aside: 1.5" round can make 1" square stock with a tad to spare. Remember your nemesis Pythagoras from middle school? The diagonal of a rectangle with sides of length A and B? A squared + B squared = C squared. Grab the shop calculator, add 1" squared to 1" squared, which equals 2, and take the square root. That's 1.414... So the diagonal of a 1" square fits in a 1.5" circle.. perfect. The easiest way I found to actually cut this is was to get centered on the round, then move half the length of the sides of the square plus the mill diameter (in this case 0.5 in both X and Y, plus the diameter, then drive the mill around in a square with sides of length 0.5+mill diameter. The same concept works for arbitrary rectangles as well.)

Looking back on it now, I'm thinking the Moriya and even more so, the posted Hot Air Engine plans, are simpler in regards to the cold cap and the bearing standards. Were I to do it again, I'd probably rescale one of those designs instead of using this one. I'm getting a little tired of tapping 2-56, and I'm sure tolerance stack errors are creeping in.

Here's the parts so far, more or less how they're supposed to be assembled:


One important milestone: completion of the cold cap completes the parts from the first page of the plans. Only two more pages to go.

[note; my server which hosts the pictures is acting up. If you don't see anything, try again in a ten minutes or so]
 

shred

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Today's part was the displacer piston body. Pretty straightforward conversion of solid rod into 1/32" wall tubing. I elected to make the bottom integral to the body instead of a separate part, so I did the bore-too-deep-and-face-back-to-depth trick again. A DRO would render such shenanigans moot, but I've yet resisted the idea of putting a DRO onto a circa 1938 lathe.



After the displacer is completed, I have to sort out what to do about the stand. I'll probably go with something closer to Moriya than Jerry's plans.
 

Powder keg

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That is starting to look Awesome!!! You are doing a great job! Keep the pictures coming:O)

Wes
 

shred

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Remember what Uncle Marv said in another post "if you scrap a part, take a break and don't try to make it again the same day" ???

Last night a couple different ways to salvage the first cold cap came to me. Making a second one didn't take that much time, but it's a good lesson to pay heed to (and one I've ignored previously)-- if you make a scrap part, break a tap, whatever, stop right then, put it down and just think about it for a while. Sometimes the first mistake can be fixed, but the next hasty attempts to repair it can't.


 

shred

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Quick update: Not much happening in the past few days; real life is attempting to intrude.. oh well, that's why it's a hobby and not a job.

I did get around to making the displacer top, over-drilling the precision 0.0625 press-fit hole, and patching it by using some .068 brass wire the hobby shop had. Brass isn't great when the aim is to be as light as possible, but it's not that different from steel and the next steel size up they had was 3/32". If all else fails I'll make a new displacer top.

I also started in on the crankshaft parts, making the crank pins and disks-- they're going to need some finishing work since contouring the outsides with the mill didn't turn out as nice as I'd like. I won't be able to loctite up the crank until I get some parts from page 3 made anyway.

Amazingly enough, the only parts left on page 2 are a couple cut-to-fit gaskets, the power piston, fan blades and spider (and I have a different plan for the latter two). That means I've got close to half the parts complete.
 

Powder keg

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Here's something to give you a little push:O)

[ame]http://www.youtube.com/watch?v=QvGbrM8RgYY&feature=related[/ame]

Wes
 

shred

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Other activities have been taking me away from the shop, but since the last installment, I designed and built the fan blades, spider and spinner, plus finished the power piston.

The power piston I made out of the end of my cylinder polishing mandrel. It was just the right size on the OD already and fits nicely.

I'm pretty happy how the fan turned out especially since I didn't really like either plan. I got the design for the fan blades from an 1898 patent that the Emerson fan company used a version of until the 30's. A little late for the heyday of the stirling fan, but not too bad and it looks much more interesting than the usual fan. At this point I'm not planning to guard the blades, so the shape may also help with that.

Somehow in what must be ascribed to leaving the cap off the acetone, my design needed twelve more 2-56 tapped holes ::)

The spinner was made by roughing at increasing angles in the lathe, followed by blending with a file. The four holes in the nose are extensions of the spider mounting screw holes, drilled through for the "nose-cannon" effect.

The blades themselves were cut from a brass door-plate-- a local hardware store is going out of business and at 75% off, kick plates are a bargain for brass sheet.



"Hokey Smoke Bullwinkle! They turned you into a fan!"

"Yeah Rock, I'm feeling pretty cool" :D
 

Powder keg

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That is beautifull!!! I like how it's coming together.

Wes
 

Brass_Machine

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Wow Shred. I am dully impressed with those fan blades. Very stylin'


And yes I am old enough to know who Rocky and Bullwinkle are ;D

Eric
 

shred

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While the weather kept me out of the shop most of the weekend, I did get some small parts made:



Mostly rod-ends of various sorts. The two con rod ends on the right were interesting to make. I cross-drilled and profiled them in the mill on the end of some 1/4" brass rod using a collet block, then moved the rod to the lathe to drill/ream the 1/16" hole in the long end. I then parted them off, and super-glued the newly drilled hole onto a little mandrel-- a length of 3/16" drill rod with .100 of the end turned down to 1/16". Rechucking the drill rod, I drilled the oil cups in the other end of the part. A quick heat with a torch afterwards and the superglue comes right off.

Why 3/16" drill rod? I had just used that earlier to super-glue the crankshaft parts on to clean them up as well.

Here's a fuzzy picture of the setup. The ersatz tailstock DRO is just a caliper and a couple very strong magnets standing in until I chop it down and bolt it on for real (note how it barely clears the handwheel)


SStriFan1.JPG
 

Powder keg

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Lookin good!!! I like Updates:O) Those parts turned out nice!!!

Wes
 

shred

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Major milestone. All the machined parts are done. "some assembly required". ;D



All I need to do now is make some gaskets, pins and a raft of cut-down screws to hold it all together.

I did try the pin-vise itty bitty hole drilling method on one of the .050" holes just to try it and it worked amazingly well.

 
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