Stirling Engine : Laura

Oh, so it IS supposed to be tilted like that ? Thank you.....I thought I was doing something wrong.

John as these are the standard SCLCR type tools which are designed for boring and facing how do you manage to take a facing cut if the tip is set above ctr height? Running above ctr is a sure fire way to chip the tip as it gets pushed up by the cone left in teh middle of the work.

You would not be able to make cuts like this with it above ctr

I see them as more for facing shoulders in bores Jason not facing the bottom of blind holes. However on a shallow one such as you show there would be little overhang and not much chance of the tool deflecting.

I can think of a number of occasions when I have had to set a tool significantly over centre just due to the size of cut being taken and the size of the bar that has to be used so even the term SLIGHTLY needs qualifying. I would generally see it as meaning thou's when boring or during straight turning. When boring deflection can introduce chatter but often the best solution is to turn the speed right down.

All messed up when deliberately turning well above centre using tools with excessive amounts of clearance but that is a rather specialised subject and needs a pretty rigid set up. That is the other aspect of tool setting. Below centre reduces the clearance angle when boring. Above centre reduces it when turning. This can be pretty critical on low diameter parts

John

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Well, that went very well : my first use of the boring bar did not throw up any real problems. I used a piece of 6mm brass bar that came with the stirling engine kit to jack up the height and I put one shim underneath it to raise it slightly above centre and one shim on top of the boring bar to protect it as per the above advice.

I was not getting a smooth finish with manual feed but the auto feed gives a perfect finish. I made the recess on the top of the cylinder too wide at 17mm (supposed to be 16mm). I am not sure what went wrong there ! Anyway I can just adjust the collar to suit when I make it.

This is also my first use of an indexable tip. They do cut very nicely.

NOTE : where can I buy 6mm square brass bar in Australia or the UK ? My usual suppliers only have 1/4 inch X 1/4 inch. I will need more for the Stirling engine kit as not enough has been supplied.

Edited By Brian John on 20/12/2015 06:27:15

Edited By Brian John on 20/12/2015 06:52:35

Edited By Brian John on 20/12/2015 06:52:52

Brian, you should have more than twice the amount of 6mm square that you need in the kit. It's only used for parts 12 (12mm long) and 18 (6mm long) and my kit came with a piece about 60mm long.

I've been following this thread with interest as I am also in the middle of building one of these - my first attempt at any form of model engineering in about 40 years. I was also confused by the boring bar tip angle, but I have a friend who is a 'proper' engineer who put me right.

You may have to have a think about the recess that you've made 17mm instead of 16mm - it retains the O-ring that forms the seal for the glass cylinder. Even at 16mm it's not a tight fit!

Edited By Andrew Holdaway on 20/12/2015 09:50:11

Thank you for that Andrew. I was not aware that the recess was for the O ring. I will have to do it again. I always expect to make at least two attempts before I get it right which is why I need the extra brass

Edited By Brian John on 20/12/2015 10:07:24

I don't know what the 6mm sq bar is for Brian but the fact that 1/4" sq is 0.014" / 0.35mm bigger might not matter at all. I have a very good none ferrous supplier not far from me and the bulk of their stock is imperial. Over the past 5 years or so they have added some metric stock but only where some one wants significant amounts of it or on certain materials where it is only supplied that way now. The only one I can think of off hand is oddly cast iron. They keep a limited amount of that.

This area is a problem you might often come across so the easiest answer is to consider what are the important aspects of a design. As far as engines go that is the fits of certain parts and timing associated parts.

John

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I would not be too worried about the diameter where the o ring fits. If it is snug on the cylinder and trapped between two other parts then it will do its job. If however it does need a accurately sized diameter then consider soldering in a ring to machine back to size. It may require a little more machining off the recess before the ring is soldered in depending on where the join line falls.

Martin

Martin : yes, I have been thinking about how I could salvage the piece. It is always worth trying these things just to see if it can be done. Something is always learnt in the attempt.

Andrew : I have just looked and there are two O rings in my kit but they are both 20mm in diameter. They do not fit in that recess as they are too big for either 16mm or 17mm. How big are your O rings ?

My instructions say '' part 42  1 X O ring glass tube 16mm 2mm filament''. I definitely do not have that. They are easily obtained on ebay though.

Edited By Brian John on 20/12/2015 13:38:33

Brian, I also had two in the kit and they are indeed 20mm x 2mm. I confess I didn't measure the O ring, I just shoved it in the recess - it fits, and gives the seal a bit more bulk.

I've just checked the drawing, and the recess in the cylinder cap is 20mm, so the O ring sits nicely in the cap and I presume then distorts down into the cylinder recess to effect the seal. So I apologise for sending you off on the wrong track - the recess in the cylinder cap is to house the O ring, which makes the recess in the cylinder a lot less critical.

Andy

I bored out another displacemnt cylinder today and I also made the collar for the glass tube (part 27). These boring bars are useful things : how did I manage without one !

1. I must admit I am a bit puzzled about the 16mm recess in the displacement cylinder and the 16mm recess in the collar. Is this just to take the excess from the O ring ?

2. If you had to make a cylinder with a 10mm bore, what would you use : drill to 9.8mm and ream out to 10mm or a boring bar ?

Edited By Brian John on 21/12/2015 06:11:25

I would bore it unless it was very long which would mean the boring bar would have to hang too far out the toolpost.

J

I am also a bit puzzled about Stirling Engines in general : the displacement cylinder has a 14mm bore, the glass cylinder is 13mm ID and the piston is 12.5mm. It would seem that the bore of the displacement cylinder is not crucial as nothing is touching it. And I would have thought the piston would have to be a tighter fit inside the glass tube ?

Actually the parts list show the piston (part #22) as 13mm X 45mm but the drawing shows 12.5mm X 45mm. I do not think 12.5mm can be correct as this would surely be too loose in the 13mm ID glass tube ?

Edited By Brian John on 21/12/2015 08:41:28

There needs to be a certain amount of resistance to air flow around the displacement cylinder so loosely speaking the clearance could be more important than the actual sizes - within reason.

To ream or not to ream. I would say that might relate to the measuring gear that is available. 3 point bore micrometer can measure bores very precisely but are rather expensive. We are usually stuck with the transfer measurements such as

Hole gauges

Or telescopic gauges for larger sizes

Personally I would only buy full ball and telescopic gauges the spring out on one side - good ones too. The fit of the side that telescopes out needs to be rather precise. There is a bit of knack to using them as when a measurement is taken they need to be precisely across the diameter.

Another type are these, so called bore bore gauges but often listed as bore micrometers on ebay

These should be set in a ring gauge of the required size but they can be set with a micrometer after a fashion but some sort of jig to hold these and the mic in alignment is likely to do that more precisely. They come in various sizes.

John

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Hi Brian,

I would go with the smaller diameter for the displacer piston (12.5mm) There has to be a clearance between the piston and the cylinder on the displacer cylinder. When the piston is at BDC, the air is all at the hot end of the glass tube. It heats up and exands, which is what pushes the power piston down the other cylinder.

Then as the crank rotates under power from the power cylinder, the displacer moves up the displacement cylinder. pushing the air out of the hot end of the glass tube and down to the cool end with the brass fins on it, where it cools and contracts and the cycle starts again.

So the gap around the displacer piston is to allow that air to flow back and forth from hot end to cool end of the displacer cylinder. The reason they use glass tube is because the glass is a poor conductor of heat so not too much heat can travel along the the displacer cylinder and make the cool end warm.

I am still not sure how I will get the round nose bullet shape on the displacement piston. If I am going the use the glass tube then I am sure that the piston will have to have corresponding shape.

.

Have a look at some designs for 'Ball Turning" or "Radius" attachments.

No need to make anything too fancy; just understand the principles.

MichaelG.

You could do a ball end via some calculations - turn a series if steps and then smooth them out with a file. it's possible to make the steps a few thou short of the sphere and use the bottom corners as witness marks - file down to them evenly and then carefully remove them via more careful filing.

Lock the saddle and use the compound slide for that axis.

Accurate balls via a ball turning attachment need careful setting up.

The other alternative is a file and radius gauges but smoothed out steps are easier.

Rather than a file a graver can also be used. Sometimes a piece of bar held in the tool post can be used as a rest.

John

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Edited By John W1 on 21/12/2015 11:23:27

Edited By John W1 on 21/12/2015 11:25:46

I was thinking about doing it by eye but I have dismissed that idea.

How about this tool ; it has a 12mm shank and my lathe will take a 13mm shank.

**LINK**

But I am not sure why it is unsuitable for a Sieg C2 ''insufficient clearance behind the spindle''. That reason is not clear to me ? Anyway, it is only for larger lathes. Does anybody make anything similar for smaller lathes ?

Edited By Brian John on 21/12/2015 11:29:45