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Building another Stirling

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cidrontmg: Some more. I rounded the base board corners. I don´t have a "proper" tool for this, so I took a wood router bit, it has carbide tips, so it´s all OK with aluminium. Took several passes, small chips everywhere... Ran the mill probably faster than at any time before. I also polished the board, tool marks and scratches incl. And the rest of the stand. They can now be put aside for a while. Then it was about cutting the air passages in the cylinder supports. In the platter that goes next to the cylinders, there are two holes for passing the air to the cylinders, and a channel uniting the holes. In the other platter, there´s just the channel. I used a bit of scrap ali to hold the platters in the mill vise. Drilled and tapped some M4 holes in the sacrificial bit, and attached the platter with some screws. And drilled the M4 (so far) holes. And milled the channel Here´s the two plates, and the ali "jig" to hold them. The displacer cyl. will have a hole that aligns with this hole. It will go straight "up" to the bottom of the displacer cylinder. The hole for the power cylinder in the previous engine needed a bit more complications. There is a brass pipe that goes right through the cooling fins towards the cylinder top. There is a horizontal hole, bored into the cylinder top, that unites this pipe and the cylinder. And then the hole in the cylinder side was plugged with a small piece of ali. You can see the said pipe and the ali plug in the other engine, in here. It´s sort of "hidden", not too obviously visible. But it´s there.  :D In this engine, there´s a separate cylinder head, so the brass pipe will pass right through along the whole cylinder, and also go some way into the head. And then I´ll mill a channel in the cyl. head to pass the air above the piston. More when there´s more  :wave:

NickG: Thanks Olli, now I know how it works!  :thumbup: Nick

cidrontmg: Some more photos. I drilled the hole for the tubing alongside the power cylinder, and milled the hole in the cylinder head. The tubing end protrudes slightly into the head. Then there´s need to open up the holes in the cylinder support plates for the power cylinder, to create space for the connecting rod. Not much space above the lathe ways, but enough. I drilled it to 20 mm. The piston will be 19 mm (cylinder bore = 19.05mm = 3/4"), so it can enter freely. And then bored the cylinder to 18,5 mm, and opened it with a 3/4" reamer. I don´t have a drill chuck nearly big enough to hold 19 mm, but the reamer has a centre, so no big problem. You can see there´s a homemade handle to facilitate turning the mandrel by hand, and slowly feeding with the tailstock, a lot of cutting oil, a spanner holding the reamer from turning, it worked quite well. The bore is straight, uniform in size, and with a rather good surface finish. I had a slight change of plans. I was intending to use graphite for the power piston, but turning graphite is a messy business. The graphite bar is 20 mm dia, so about 1 mm needs to be turned to dust. It´s not terribly much, but... So I was thinking about a Teflon piston. Or rather a Teflon wrapper around an aluminium core. Teflon is also a very low friction material... I turned an ali bar to 12 mm dia, 23 mm long, and made 10 mm deep hole with M4 thread into one end. And turned a 23 mm stump of Teflon to 19 mm dia, with a 12 mm hole in one end. And pressed the two together. Rummaging in my scrap bin, I found a 70 mm long 6x6 brass bar, and bingo! - a con rod. Unfortunately it was some 4.5 mm short... I also found a bit of 13 mm bronze bar. I turned the bar to a nicer(?) shape, and milled a 6 mm slot in it. And soldered the two together. Now the con rod is long enough. I made a small brass piece, 6 mm dia, with a M4 thread to go into the piston, and a flexible joint with the con rod. So there´s the power piston as it now stands. There´s a reddish tinge on the Teflon, don´t know why. But it´s really snow white. The bronze "big end" really is rather reddish, clearly different from the brass. I turned two thin grooves in the Teflon, and wound them full of plumber´s Teflon tape. Not that it´s really needed, the piston has a good compression even without it. But it seems to wipe away "something" from the cylinder bore, and it can be easily replaced. And the compression "fine tuned". The stroke is 20 mm, it will be the same also in the displacer cylinder. The power piston now breathes freely through the tubing, and with a hefty flick of a finger on the flywheel, it turns something like 22-23 revolutions (a bit difficult to count). If you block the tubing, it just bounces back and forth, not even one full rev. So there´s still some hope - it hasn´t jammed solid yet...   :wave:

NickG: Olli, Nice work. I was wondering about the use of teflon for a piston but I bet it's more difficult to achieve a good fit than with graphite? I suppose the heat won't be (hopefully) much at the cold end so it should cope with that ok. It sounds like you've got a good seal but a high compression ratio - it shiould be a fast runner provided there's enough volume of air heated to overcome the compression, or rather you heat the air enough. Nick

Stilldrillin: Olli. Some time back, I used PTFE for the 5/16" diameter pistons, in a special Mamod steam engine. When the steam hit, they expanded by around .0025", jammed solid........ Modified, to a thinner PTFE sleeve, still gave .0015 expansion.....  :doh: I think you will have to experiment, even at Stirling temperatures......  :wave: The experiment was a success....... The engine is still performing as hoped for, with it's new owner, in Australia.  :thumbup: David D

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