Stirling Engine : Laura

Sorry Brian, I have only just joined your party. I looked at the pictures of your first attempt at parting off and came to the conclusion that the reason for the non flat surface was because the saddle was not locked down to the bed. As the tool you used has an angle on its cutting edge as it cuts it puts a side pressure on the saddle which then moves until it has taken out all the slack in the lead screw/rack. If the tool was put in at this angle it would have squealed so loudly, your neighbours would have complained.

frank

No, it is not squealing. I used the angled edge tool for parting off and this works well without problems. The flat edge tool I use to make recess cuts and this is giving me problems.

I have cut three small and three large cylinders ; I needed practise making recess cuts. It is much easier once the carriage is locked down. All cylinders are drilled out to 8mm waiting for the boring bar to arrive.

Which will give the most accurate results for the smaller cylinder : drilling out to 9.8mm and finishing with a 10H7 reamer or drilling out to 8mm and finishing with the boring bar ? Or should I always finish with the reamer even when using a boring bar ?

Edited By Brian John on 24/11/2015 06:40:31

The best way, for accuracy and finish, is to drill first to "rough" it out, then bore the hole to ensure concentric location, then ream for fine finish and accurate size. Also, you should take a finish facing cut over the end of the cylinder that is going to mount to the crankcase/frame while the job is still set up from the boring and reaming. This ensures the finished hole is dead square to the mounting surface.

You can however get away with drilling and reaming if accuracy is not super important. I would drill, bore and ream in this instance. Stirling engines are critical on friction, and low friction needs good alignment and smooth finishes all over.

Edited By Hopper on 24/11/2015 07:06:03

Okay, I think I will need a 14H7 reamer for the larger cylinder then.

Friction : it would seem that the fit of the piston in the cylinder is more critical in the Stirling engine then a steam engine. The Stirling engine must turn VERY freely but still have a good seal.

As far as I can tell, the larger cylinder that holds the glass test tube should be a fairly loose fit, as it relies on this to transfer the hot air to the 'working' cylinder. Therefore no reamer required.

It might be worth you having a small bottle of cutting oil around Brian. You could use all sorts of things some specific to the materials you are working but that's a bit OTT really. On a reamer for instance just smear a little on with a small brush. It will help get an H7 tolerance and improve the finish. You should find it on ebay Oz. It oxidises and rapidly so keep the screw cap on the bottle except when you get a little on a small brush. Something like a 1/4" wide artists brush will take up more than enough of it.

I've not built a Stirling engine but my understanding is that there is zero friction from the piston. They have clearance according to the design and the motions prevent the pistons from actually touching the cylinders. That aspect needs a high degree of precision.

I've resisted building any sort of engine for a long time but of late am weakening.

John

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I do have ''Tap Magic'' which is use on some things It can be useful for recess cuts but I have not used it with reamers.

What is OTT ?

Edited By Brian John on 24/11/2015 10:47:54

OTT, either over twenty one tiswas or over the top.

John

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The test for the cylinder/piston fit, first the cylinder is bored, and lapped to a mirror finish, round and parallel, then the piston is turned to a size were it just won't fit the cylinder, now the way I do it is take a bit of "Wet and Dry" paper, start ing with 600/800 grit(depends on what I have), backed with a steel rule polish the piston down until it just starts to enter the cylinder, now change to 1200 grit paper. It is the right size when the piston will drop through the cylinder uner it's own weight (dry), but if you put finger over the end, it should almost stop, slowest is best. No actual measurements, except when checking that the bore is parallel ie., the same diameter at each end, and in the middle.

If the piston is made of graphite, the method is similar, except that the "Wet and Dry" paper is replaced by ordinary paper, copier paper is ok. This rubs off the graphite, and leaves the surface glassy smooth, and as long as there is NO OIL EVER this is about the ultimate in low friction pistons.

Ian S C

Ian : that is about how I do it too. But I use 2000 grit for the final polish and fit.

Yes, you may not have to ream the displacer cylinder as the piston does not touch it. But it will need to be accurately concentric and square etc to hold the glass tube displacer cylinder in place.

I had some problems today when attempting to make parts 10 and 11 which involved cutting a 2mm thread on the end. The threaded piece broke off inside the die. Is there any way to remove it from the die without damaging the die ?

If you are lucky you can get a junior hacksaw blade into the slot and cut most of teh rod away then bend it into itself and remove. Don't overcut and run the saw against the teeth of the die.

Whats the metal, just wonder if the heat from your dremel cutting discs has had an effect.

Use your tapping fluid, open the die right out and wind the die back regularly to break off the burr eg every half turn.

The material is silver steel. It is the smallest thread I have ever tried to cut so I will have to be more careful.

I've not had ocassion to do this (yet) but if that's a split die then can't you just take it out of the holder, tap a screwdriver blade gently into the split to open it up and lever/drift the dross out?

Depending on the die pgk pgk that method is quite possible to crack the die.

Brian, don't worry about the finish in the displacer bore, some actually say that a rough surface is helpful in that it increases the regenerative properties of the displacer chamber(don't worry about that rubbish either). What is the clearance for the displacer in the displacer cylinder?

Ian S C

I think I will just buy a new die.

Ian : it says to make the front clearance as small as possible......not very helpful !

With the displacer, a diametrical difference of about 2 mm between the displacer and it's cylinder, and a gap at each end of the stroke of 1 to 1.5 mm is quite a good figure for a little motor like this. The exact measurement would need to be found by experiment, but a gap of 1 mm to 1.5 mm would make very little difference. There is probably a mathematical formula for this, but I use the method of , that looks about right, let's try that. Some times it's just a case of what tube I make the displacer, and the hot cap from.

Ian S C

A couple of things to make using a die easier:

Turn the diameter of the bar down to 3 or 5 hundredths of a mm smaller than the nominal 2mm.

Turn a good chamfer on the end of the bar to help the die get centred and get started.

Make sure you use the end of the die that has the gently tapered teeth as the leading end so the first teeth cut only a shallow thread, the next teeth a bit more, and so on.

Turn the die about half a turn forward then about quarter of a turn backwards. You will feel the chips break off and the die move more freely. This helps break up the swarf and lets it drop out instead of jamming up the die.

Use some kind of lube, tapping fluid, oil, WD40 etc.

Keep checking the side view to make sure you keep the die and die stock level in all directions as you progress. Or hold the job in the lathe and use the tailstock to hold the diestock square. But still use the half a turn forward, quarter of a turn back routine.