Yet more hot air

Today I had a rummage around to see what materials I could use to make another Stirling engine. I've got some very thin 1/2" steel tube I can use for the displacer cylinder and some 12mm steel tube with a thicker wall which hopefully I can turn down to leave a suitable radial gap while reducing its weight for the displacer. That'll mean I can solder on the end caps to reduce the likelihood of leaks (air and water leaks were the bane of the last one).

Faces the ends of the disp cyl to length and made a cap for the hot end with about 1mm inside and out. Cut a piece of 1"x1/8" steel for the fixing flange and turned it to the right size (1" square) and centre punched the holes for the disp cyl and fixing screws.

I don't have any aluminium bar of a suitable size for cooling fins but I do have a roughly sawn block of aluminium (sawn by someone with one eye and no motor control I think) so I sawed it up into three roughly equal pieces and faced them. I'll fasten them together to turn the OD, cut the fins and bore the hole so they can be fixed to the displacer cylinder with Loctite (should be ok at the cold end). I need to practice on some scrap with a parting tool as I've never done that on aluminium before. Second photo shows roughly the space they'll occupy on the disp. cylinder.

I decided that with the plan as it stands, the displacer would be too long: difficult to turn down so thin over such a length and heavier than ideal. So I rejigged things, shortening the displacer and cylinder by about 10mm while keeping the same SV ratio.

I made a plug for the tailstock end so I could use a live centre and gripped the uncut length of tube in the chuck. It was too long to use the topslide so I had to move the carriage bodily along. I turned it down to about 0.5mm wall thickness which was rather exciting (aka scary). Here it is inside the disp cyl, the radial clearance is only about 0.5mm so I hope I can get the end cap/rod accurate enough to keep it concentric.

Also glued the three pieces of ali together and started filing some of the edges so I can grip them in something to drill a hole.

Realised I didn't really need to glue those lumps of ali together, just put a hole in each one to fit on a mandrel to turn the outsides al to the same dia.  No big deal. Although they have nice parallel faces now, they're all different thicknesses so I need to work out how many find will fit on each piece.

Made the end caps for the displacer, the bottom end has 1mm inside and 1mm out, the top has 2mm inside and 1mm out and is threaded M3 for the rod. I soldered the bottom cap and the hot cap on the cylinder. Drilled some small holes in the fixing flange that I can transfer to the frame before opening them out further and tapping.

And he's off ..............such fast design and now into the build, impressive, you must be encouraged by the last success! 

Quote from: Sanjay FAnd he's off ..............such fast design and now into the build, impressive, you must be encouraged by the last success! 

LOL there aren't many parts and they're all simple shapes so drawing up a plan isn't hard - I spent more time working out what will work based on the tubes etc I have available.

I glued the disp cyl flange and the frame together for drilling the holes. Drilling went fine but when I came to take it out of the vice I noticed that the two pieces had come apart  The four outer screw holes were fine but the central hole (the important one!) was completely off centre. All was not lost however: since the power cylinder has fixing holes in the same place I could flip it over and re-do the holes on the other end. It just means the hole where air enters/leaves the power cylinder won't be central but that doesn't matter at all and it won't be visible. Also bored out the flange to accept the cylinder, just need to solder them together.

Here's the state of play, you can see the horribly off centre hole in the middle of the right hand five (the part looks like a double 5 domino)

Soldered the disp cyl and flange and made the displacer rod bush. Decided to try the HSS parting tool on some scrap ali. Using that method to create fins I'd need to go about 6mm deep using a ~1.5mm thick blade on each cut. I made two such cuts without any issues but had to turn the scrap around to try again - I have next to no aluminium so I want to make sure I've got the hang of things before trying it for real. Unfortunately, the third attempt went wrong - at a depth of about 2mm the tool decided to dig in and yanked the piece partly out of the chuck  I'd read quite a bit about the subject online and decent speed, constant cut and plenty of lubrication seemed to be called for. I think the last area is where I fell short - I'd tried cutting oil and WD40 but when I picked up the scrap it was really hot.

After a bit of pondering another plan has emerged which I think will do the job and eliminate the need for using the parting tool. Step 1, get all the rounds to the same diameter while mounted on the 6mm mandrel. Step 2, bore out each one to slide over the disp cyl. Step 3 turn down a length of 2mm to the collar diamter. Step 3, flip the part over in the chuck and repeat on the other end. Step 4, saw down the middle of the part. Step 5, grip each half by the turned down section and face the large diameter of the fin to thickness (also 2mm I guess). I can get two of these out of each round of ali and can then slide them along to the cold end and Loctite them in place.

So far I've got them all to the same OD and have bored one out to fit the disp cyl (1/2").

I had parting off problems with my small Sieg lathe until I made a carriage lock, that improved things dramatically - have you got one? They may come with them now, mine was an oldie! 

What speed is recommended for Ali, it's not something I use a lot?

Quote from: Sanjay FI had parting off problems with my small Sieg lathe until I made a carriage lock, that improved things dramatically - have you got one? They may come with them now, mine was an oldie! 

What speed is recommended for Ali, it's not something I use a lot?

Cheers Sanjay. I don't usually have issues with brass or steel on this machine (oddly, I sometimes did on my bigger old lathes a Clarke CL300 and a Myford ML7) but it doesn't have a lock. Ali isn't something I've  a lot of experience with either, as to speeds I don't think the scale on the pot is particularly accurate, I just go by experience (or experiment in this case). I'm pretty sure it was a lubrication issue although I don't know what grade of aluminium it was either so that probably doesn't help!

Hi Paul, plenty of WD40 sprayed on to help prevent a built up edge on the tool and retract the tool regularly to prevent a build up of swarf. If you have a rear tool post that might also help.
I would also recommend a tungsten carbide insert tool with a GT insert, you can get 1.5 and 2mm wide tips, they are designed for cutting aluminum. I'd send a picture if I knew how.
Andy.

It would appear that I do know how 

Quote from: Spinner21It would appear that I do know how 

A picture is worth a thousand words - thanks. I was even  having an issue with turning the ali this morning, using the tool insert for the job - one of the silver coated jobbies with the upturned cutting point. I swapped it for one of the dark grey uncoated inserts and that worked like a dream, which is good because I bought a few at the show last week

Well that plan went remarkably well  the photos show the process. Photo 1, collar diameters turned, photo 2 one piece sawn and faced (slid onto the cylinder), photo three second piece completed and last piece sitting at the end. I can't decide what to do with this last one as it isn't thick enough to saw or even part off so either I leave it 'as is' with a very thick fin or I face away most of it to leave a fin that matches the others. Suggestions welcome! 

Maybe make an arbor to hold it for turning?

Quote from: cruebyMaybe make an arbor to hold it for turning?

There's no problem turning it down (so there would be five fins instead of the planned six) but I can't decide whether that would be better than leaving it as it is as far as shedding heat goes. Sorry if I didn't make that clear 

Looking very tidy