Stirling Engine : Laura

[Brian JohnParticipant @brianjohn93961]

I have made a start on the Stirling engine kit ''Laura'' by Bengs. I thought I would tackle what I think might be the most difficult part first : the cranks. I intend to face off the brass work piece, reduce diameter to size, drill out centre hole, part off and polish the other face . Now I have to remove metal from two sides to get the correct shape. I was thinking of drilling a (say) 8mm hole to get the curve then cutting the rest away with a dremel tool.

1. Without a milling machine, how would you tackle this ?

I do not intend to build this engine with a glass cylinder as per the original design. This will be replaced with an aluminium cylinder so I do not have to build a bullet shape piston to fit the glass tube. But I am curious how you would go about making a bullet shape in the lathe.

2. Do you just round off the end of the piston with a hand held file and do it by eye ?

**LINK**

Edited By Brian John on 11/10/2015 07:41:11

Edited By JasonB on 11/10/2015 07:57:47

Edited By Brian John on 11/10/2015 08:06:32

[Brian JohnParticipant @brianjohn93961]

A premilled kit by Bengs

[JasonBModerator @jasonb]

The 6H7 means it needs an accurate hole so dril smaller than 6.0mm and ream.

Yes drill out the corners but saw and then file to shape, forget the Dremel.

You may have problems with an aluminium cylinder expanding and loosing compression, there is a reason they suggest glass.

Turn end roughly to profile and then file in the lathe, ideally a ball turning tool would be used

[Brian JohnParticipant @brianjohn93961]

I use the dremel cutting wheel as a saw ; I find it quicker and tidier than a hack saw. I round off everything with a hand file.

Expanding aluminium cylinder : won't the aluminium piston expand an equal amount to compensate ?

[HowiParticipant @howi]

I do not think the bullet shaped item is a piston, but a displacer. The cylinder needs to be glass ( pyrex) as this is where the heat is, the glass has very low heat conductivity where as ALU is the opposite and would conduct heat to the cold end – not what you want in a Stirling engine.

As for the bullet shape, not important at all other than asthetic appearance as you can see it through the glass tube.

Start changing things from the original plans ( without knowing what you are doing) and I can guarantee you, you will not get it running.

Beng kits are very good – build as plans show if you do not want to waste good money.

[nigel jones 5Participant @nigeljones5]

they dont seem to sell plans alone? anywhere else you can buy them? (the free ones online look pretty crappy)

[HowiParticipant @howi]

Why would they sell plans desperately? They are trying to sell kits ( beings kits have a lot of parts pre machined 

CNC I would assume) not commercial sense to just sell plans.

Edited By Howard Winwood on 11/10/2015 10:13:21

Edited By Howard Winwood on 11/10/2015 10:14:29

[Brian JohnParticipant @brianjohn93961]

Many Stirling engines do not have a glass cylinder ; the cylinder is aluminium. This includes other Stirling engines by Bengs. I do understand that the engine may not run if I change the design but I think it is worth trying. No harm will be done if it does not work. The only thing that cannot be cheaply replaced in this kit is the cast iron flywheel.

[Ian S CParticipant @iansc]

The glass bit can be changed for a metal hot cap, but I would not use aluminium, stainless steel is the best. Depending on the diameter, a good place to get a ready made hot cap is a NiCad battery, I have a little motor that uses the metal case of an AA size battery. I find it unusual that the displacer is made of solid metal, these are normally hollow, and as light as possible. Some of the engines with test tube hot caps also have a test tube displacer.

I bought a little Bohm Stirling Engine from Jaycar, I find it is not really well designed, but it does go.

Ian S C Here's the little one of my design.  The flywheel is 2" diameter  Stainless steel, main and big end bearings are ball races salvaged from VHS tape machines.

Edited By Ian S C on 11/10/2015 12:28:52

[Neil WyattModerator @neilwyatt]

Use a NiMH batter instead, then you won't have to play with toxic cadmium salts.

Neil

[HowiParticipant @howi]

The glass test tube can be replaced with metal (stainless steel) but why would you? It is supplied in the kit. The glass test tube will be more efficient in stopping heat getting to the cold end than a metal cylinder.

Plus, with a glass cylinder for the displacer you can see the action of the displacer, it is surprising how many people think the displacer is the actual piston………

I have made a couple of marble Stirling's, everyone I demonstrate them to think the movement of the marbles directly moves the pistons, until I explain the principle and demonstrate the timing.

Brian it is your kit, you are free to do what you want with it, but I would have thought it would have been a lot cheaper just to buy some plans and modify to your hearts content  ( whether it worked or not). There are plenty of plans out there, have a look at Jan Ridders designs/plans – although I would still not change anything, Jans designs will work if made exactly to the plans as SOME parts are quite critical, things like flywheels can be changed without compromising the engines performance.

On the subject of flywheels , they can be easily made from steel/ALU tube and will usually work out a lot cheaper.

Cast iron flywheels can be a pig to machine, others can be a pleasure, you will know which you have when you start to machine it!   

 

Edited By Howard Winwood on 11/10/2015 13:32:03

[michael mParticipant @michaelm]

Why would they sell plans desperately? They are trying to sell kits ( beings kits have a lot of parts pre machinedÂ

CNC I would assume) not commercial sense to just sell plans.

Maybe they don't know that, because they do sell plans on ebay.

Search " laura construction plans"

Michael

[HowiParticipant @howi]

Posted by michael m on 11/10/2015 14:31:34:

Why would they sell plans desperately? They are trying to sell kits ( beings kits have a lot of parts pre machinedÂ

CNC I would assume) not commercial sense to just sell plans.

 

Maybe they don't know that, because they do sell plans on ebay.

Search " laura construction plans"

Michael

 

So they do, there you are fizzy

 

[nigel jones 5Participant @nigeljones5]

so they do…its the plans for John that im after though, any offers?

[Brian JohnParticipant @brianjohn93961]

I made a start on the small crank today (part 8) and I have encountered an unexpected problem. I drilled out the work piece to 6mm and this time I used : 3mm spotting drill, 3mm jobbing drill, 5mm jobbing drill, 5.8mm jobbing drill then finished with the 6mm reamer. All went well this time and I could actually see the reamer doing something unlike when I was using the 6mm jobbing drill.

I then reduced the outside diameter down to 19mm ; I left enough material to make two crank …ever the pessimist !

The problem was in parting off. I ended up with discs that were 4mm at the edge as required but 0.7mm thicker in the centre. I remember practising parting off when I first received my lathe and I do not recall having this problem ; perhaps I just did not notice it.

1. Why did this happen and how can it be avoided ?

2. What is the best way to salvage the two discs I have already cut ie. how to grind/polish one side down so that it is flat ? They are too thin to hold in the lathe chuck….or is there a trick to doing that ? (I could use a mandrel to remove most of the metal except for the part under the nut and later grind this bit off with a dremel tool.) 

NOTE : If nobody ever deviated from an existing plan then nothing would ever be improved !

Edited By Brian John on 12/10/2015 07:16:57

Edited By Brian John on 12/10/2015 07:27:04

[JasonBModerator @jasonb]

Those thin parting tools don't need all the grinding that has been done to yours, just front rake ground square across. Any angle on the end will tend to push the thin blade sideways.

Pack out behind the disk when setting in the chuck so you are only holding by say 3mm, packing will keep the work parallel to the chuck face. Remove packing before turning on.

When you come to do the big one face off the bar, just touch with the spotting drill to mark ctr and then do all your marking out before putting back in lathe to part off, its easier to hold than a thin disc

Note, if it ain't broke don't alter

Similar parting tool as yours note the top and sides have not been ground and end is ground square across, cut all those grooves in steel no problem

Edited By JasonB on 12/10/2015 07:54:01

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 12/10/2015 07:10:16:

2. What is the best way to salvage the two discs I have already cut ie. how to grind/polish one side down so that it is flat ? They are too thin to hold in the lathe chuck….or is there a trick to doing that ? (I could use a mandrel to remove most of the metal except for the part under the nut and later grind this bit off with a dremel tool.)

.

Spend a few minutes learning about 'Wax Chuck' technique, as used by watchmakers.

… Originally [and best] done with Shellac; but 'SuperGlue' is a reasonable alternative for jobs like this.

MichaelG.

[HopperParticipant @hopper]

It may be that the parting tool was not set dead square to the lathe axis. If I want precision parting i will sometimes check the alignment of the parting tool with a dial indicator to make sure it is dead on.

Ian SC is right about using stainless rather than aluminium for the displacer cylinder. The cylinder needs to conduct as little heat as possible, so that there is a considerable difference in temperature between the hot end with the candle on it and the cool end with the fins on it. This temperature differential is what causes the pressure differential that makes the engine work. Ally will quickly transfer the candle's heat to the cool end of the cylinder and warm it up, resulting in no-go. Stainless is a poor conductor of heat so is a pretty standard material for the displacer cylinder on Stirlings. Glass is even better. As there is a small gap between the displacer piston and the glass cylinder, you can dome the end of the displacer piston with a file in the lathe, using a shop-made radius gauge cut from a piece of cardboard or plastic as a guide.

To flatten out those crank blanks, use the reverse jaws in your lathe to hold them, with a stack of washers or similar in behind them to space them out beyond the jaws but running true on the back face of the job. Then just face the crank blank off with a facing tool. (LH turning tool)

Yes, drilling holes to get the radius on the profile of the crank is a good idea. I would then cut with mini hacksaw and finish with a file. So needs to be marked out first. Either by blueing and scribing in the traditional manner, or by making a full size photocopy of your drawing and cutting out the shape of the crank and gluing it to the piece of brass as a guide. That is the easy way. To do it the traditional way, use a Texta pen to blue the brass. Then set the round piece in the lathe chuck and use a pointed tool bit to scribe a centre line across the diameter and then another at 90 degrees to that line. Centre pop the cross int he middle and this is the reference point for layout.

You could also scribe the two lines for the throw of the crank by raising or lowering the height of the tool used to scribe the first centre line and scribing two lines to define the straight edge of the crank. To mark up the two lines at 120 degrees for the angled faces of the counterweights on the crank, you can again use the lathe to markout. Your three jaw chuck can easily be used to mark out as each of the 3 jaws is 120 degrees from the next. So setting one jaw dead vertical, then rotating the next jaw to the same position give you your 120 degrees and you can use the lathe tool tip to scribe the line again. Kind of hard to explain in words but hopefully you get my drift.

[HowiParticipant @howi]

Changing plans is OK if the person making the changes is more experienced than the original designer.

I would imagine you will want the engine to work after you have finished it, so would suggest make as per plan, get it running, THEN make your changes and see what differences your changes make.

I doubt whether you will better the original, please feel free to prove me wrong. 

Howard

[JasonBModerator @jasonb]

There is also the question of whether the machine is upto boring a stainless steel cylinder that long. If you are only changing the glass to avoid rounding the end of the displacer that would be easier than boring the hole.

While I'm on what sort of drill do you have, can't remember if its handheld or a bench drill?

Edited By JasonB on 12/10/2015 09:16:37

[Ian S CParticipant @iansc]

I would suggest that instead of making the hot cap from solid, try and get some tube, I'v been getting a stainless disc TIG welded in the end of mine, but my tame TIGer was having eye sight problems last time, so the next one will be brazed in, I use either a narrow strip, or piece of brass wire, It's cheap, and stands the heat on the displacer. The hot cap on my motors needs to withstand red heat. The hot cap for my first tthree engines were made from solid 316 stainless steel, and it took quite some time.

Ian S C

[Brian JohnParticipant @brianjohn93961]

A. I realise now that I should have marked out that smaller hole BEFORE I drilled the larger centre hole ! But I have stuffed up like this before and as Hopper suggested it can still be marked correctly. Yes, I can see how the three jaws of the chuck will help to mark it out.

B. Regards the glass cylinder : I just did not want to make any part of the engine out of glass. I can see now why aluminium will probably not work and that stainless might be really stretching my small lathe. I do have a set of engineers drills (1 to 6mm in 0.1mm increments ) so maybe if I took it slowly, I might be able to make a stainless steel cylinder (or find one from somewhere). I can drill mild steel in this lathe. How much tougher is stainless steel …twice as hard ?

C. All the drilling I am doing here is done on the lathe. I do not have a bench drill, only two hand drills (corded and cordless). I was thinking of mounting the corded drill in a stand and making a bench press out of it but I am not sure how successful that idea would be. You can buy the stands from ebay.

D. The parting tool was definitely set dead square. The edge of the parting tool lines up exactly with the edge of my top slide which is very convenient. I was very careful to get it square. I need to get this problem sorted out before I do anymore parting off. The top edge of the tool is thicker than the bottom edge but that should not be the cause as only the top edge is making the cut.

E. I will try some sort of packing with washers in the lathe and attempt to grip the discs for a facing cut using the external jaws. This should be interesting…working very close to the chuck !

[Jeff DaymanParticipant @jeffdayman43397]

Especially when working close to the chuck, after setting up, it is a good idea to turn the chuck and spindle one or two full turns by hand (with power cord unplugged). Do this to be sure nothing is fouling the chuck jaws or carriage, before switching on.

With most drill stands I have seen for holding corded hand drills, it is difficult to get the drill set up truly 90 degrees to the table and keeping it there. The frustration is not worth the effort. Small drill presses have gotten very affordable in recent years and a good one will be a pleasure to use for dozens of jobs in any shop. I'd look for one if I were you. If shopping for one, take a 6" square and verify the table is at 90 degrees to the column and a drill in the chuck is 90 degrees to the table. Also extend the quill as far out as it will go and wiggle it – if there is perceptible play, look for a different one. For this reason it is wise to buy a far-eastern-made drill press in person.

Good luck, JD

[Brian JohnParticipant @brianjohn93961]

I have machined the ''domed'' surface of those two disc. I used the same internal jaws and simply set the discs in the chuck to run true by eye (with a small amount sticking out) and took facing cuts with a LH tool. It actually worked ! I did try to use the external jaws and washers for packing as per the above suggestions but I just could not see how to do that. Anyway, it is done now and I have two discs to work with. I placed them on my graph pad to get them centred, marked off the M2 hole and then punched it….looks good. I also punched the hole in the edge for the M3 grubscrew. I then realised that I could not drill this by hand ; I am going to need a drill press before I go any further.

There is an axle supplied and it is the correct diameter of 6mm (actually 5.97mm). I just had to face off the ends to get it to the correct length. I then decided to try turning my own axle from some 6.35 steel rod I have. What a mess : I just could not get a good finish on it. I did not think I would need a fixed steady for this as it is only 60mm from the chuck to the dead centre ? I did try to polish it up with some 400 grit paper but it seemed to make things worse. I am really not sure what went wrong here today but it was not the fault of the lathe.

 

 

 

 

Edited By Brian John on 13/10/2015 08:05:27

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[Ian S CParticipant @iansc]

You may be able to devise a way of drilling the holes, if you look at your lathe you'll see that it is a drill press laid on it's side, so you might be able to hold the parts in the tool post, and the drill in the lathe chuck, or if the tail stock drill chuck will fit in the head stock maybe use that.

Ian S C

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