Tubal Cain "Polly" without a lathe

[Andersgj]

I am planning to build a Polly engine for/with a kid I know.
I don't have a lot of tools, only drills and hand tools.
I am reasonably good at brazing and work as an aircraft mechanic, so I am used to precise work.

I have a couple (lot) of things I wonder about:

Is there any reason to make the boiler and fire chamber as separate parts or could I just use a longer 50mm tube to make a combined unit?

Will it be alright to use a 1,5 mm walled 50mm tube or will it be to much of a heatsink?

Will a spring from a pen be alright or do I need a special spring?

Anders

[b.lindsey]

Anders, given that it includes a boiler as well as the engine itself, unless you are an experienced boiler maker I would stick to the proved design in the plans. No need asking for problems. I am not sure what the wall thickness on the plans is or how that compares to the 1.5mm you mentioned, but I certainly wouldn't go any thinner than called for.

Let me also welcome you to the forum. When you have a chance please post and introduction in the "Introduce Yourself" section so that everyone can welcome you to the forum. It's a very helpful bunch of folks here so you will likely get some good answers to your questions.

Do you have access to any machine tools  (perhaps at work)? I am not personally familiar with the "Polly" plans, but wonder if it is doable with hand tools alone.

Bill

[tangler]

Anders,

Tubal Cain actually wrote a series on making a Stuart 10H by hand and without machine tools.  Model Engineer vol 148 June 1982 and continued in alternate issues through most of vol 149.

Polly was one of Tubal Cain's designs in "Building Simple Model Steam Engines" which includes a boiler.  The firebox is made from steel but I would guess that if the whole thing was made from copper, rather than brass, then that wouldn't be a problem.  I think you could make it without machine tools.  You will keep us up to date on progress, won't you 

Cheers,

Rod

[Andersgj]

We have lots of machine tools where I work, but it is only cnc super precision stuff, only skilled machine workers are allowed to touch them.

The boiler is supposed to be from 0.6mm or 24swg brass, so 1.5 mm is well above that!

Is there a reason that copper is better than brass? I have access to the brass tube, not so sure about copper.
There is a smaller engine in his book that uses a unified boiler and firebox design from a one inch brass tube. That is why I thought a unified design would be convenient on the Polly too, since the other engine looks like a scaled down Polly.

I think it is feasible without machine tools, the most difficult part will be the flywheel and crank, but I might have a plan. I am going to make a treaded hole in the middle, rough cut it and then use an electric drill and  a belt sander to turn it nice and circular.

The cylinder will be a tube and for the piston I will use a matching steel rod, ground down in the middle to make a skinnier conrod.

The wobble plates I hope to be able to grind and lap by hand.

It will definitely be a challenge!

[Dan Rowe]

Is there a reason that copper is better than brass?

Yes, water can attack the zinc in brass and cause failure.
https://en.wikipedia.org/wiki/Selective_leaching

Dan

[Andersgj]

Does tinning of the inside of the boiler work ok to avoid this? Saw it mentioned in the article

[Andersgj]

After some thought I decided to build the actual engine first, after all what is the point of a boiler if I never get to the actual engine.
When it is up and running, I am going to think about the boiler.

[fcheslop]

I dont see why you could not build the boiler and firebox as one piece but definitely go for copper as you simply may not know the grade of brass you have  and as already mentioned it suffers from de zinc.
The crank could be simply a bent up one as per Doll from the turn of the last century
The cylinder is easily made from K&S tubing as its reasonably true.
I seem to remember in the second book him mentioning building techniques for hand tools only but I would need to check
Good luck
cheers
frazer

[Andersgj]

It seems I can only get hold on copper 50mm pipe in 2meter of longer lengths, which I can't afford, the biggest diameter I can easily get a hold of is 22mm, so I guess that is it for this project.

[Andersgj]

Found some on eBay.
Is all copper equal or are there some things to shy away from here too?

I was thinking about using the end caps that are supposed to be soldered on. Will that be a good way to go about it?

[Andersgj]

The tube I found is a table x tube, rated to 290 psi, so sounds pretty safe to me
Any takers?

[b.lindsey]

You are planning on silver soldering right?  As for the design and end caps, it would certainly be safest to stick with the original design, or at least consult with someone with more experience making even model boilers before changing the design.

Bill

[kev]

Brass is fine for a toy boiler such as the poly, ALL of the popular toy makers use brass, wilesco, mamod etc True it does degrade and if not dried out after use in 50 to 70 years time it MAY be a problem. The heavier wall is not a problem but you may end up using all the fuel getting it up to temperature so simply start with hot water Have fun and stop worrying your not making some 200 PSI bomb a working pressure of 20 to 25psi will be more than enough

[tangler]

I agree with Kev.  However, Tubal Cain specified steel for the firebox underneath the boiler.  He seemed somewhat concerned about the stability of the model so added a steel disc to give some low down weight.  I think thin gauge brass might get a bit soft and possibly buckle if used for the firebox.  Personally, I'd stick with the original design.

Rod

[Andersgj]

I will stick with it, making a steel firebox shouldn't be too hard.

But first the actual engine, then off to the boiler

[kev]

I cant see heat being a problem, I have a poly with a rivited copper boiler and brass firebox/base and its fine, I do have it mounted on a heavy wood base to make it more stable

sorry cant figure out how to rotate them back .... Thanks for the tidy up bill

[tangler]

 

Practical  experience is worth a thousand theories 

Rod

[10KPete]

 

Practical  experience is worth a thousand theories 

Rod

Word!

I like that little unit, Kev!!

Pete

[kev]

Thanks Pete, they are a nice little introduction engine and dont need to be rivited I just did it for fun. It does have the sheet of lead under it for weight but the wood base provides enough stability, been meaning to make a burner for it and this thread has raised my interest again.

[tangler]

At the Alexandra Palace show last Saturday I had a chat with the chaps on the SMEE (Society of Model & Experimental Engineers).  They run a course on making Polly engines and had some examples on show on their stand.  They reckoned that several had been made with a brass firebox, as a continuation of the boiler, without problems.  Just be sure that you have the model properly fastened to a solid/wide base so that it wouldn't fall over.  They also recommended using Chafing Gel http://www.amazon.co.uk/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=chafing+gel for heating - it can be put into a simple open dish burner.

HTH

Rod

[Andersgj]

Here is the end result.
I got a bunch of brass at work, so wound up using a thick walled tube as the mail part of the boiler, silver soldered the top and bottom on to it. I tested the boiler to 2.5x safety valve pressure.

I borrowed a lathe to make the piston , flywheel and crank. The rest is made with hand tools on our living room table.

[MJM460]

Hi Anders,

That looks like a pretty good effort without a lathe, even if you borrowed one for the piston.  Well done, and congratulations on getting it running.

You last posted before I joined the forum, so I did not know you were working on this one.  I have also built a small single acting oscillator using tube for the cylinder, though it was around 10 mm or 3/8 brass tube.  I have a lathe, so made the piston to suit, but without a lathe, some oil grooves wound with cotton or other suitable packing material on a piston made from the closest size rod you can source will work quite adequately, especially if you can size it a little with file rests and your drill.  A bit larger size is less critical in my experience, some 3/8 or 1/2 inch hard brass tube is a good size basis to start with.

Eventually the lathe will come, in the mean time you will have plenty you can do with your other skills.  Have a look at Steam Guy Willy's freelance engine for what can be done with files, skill and patience.  Though I believe you would need a lathe for a double acting engine to get everything on the cylinder concentric.  Others might also comment on this.

I can understand deferring the lathe until the family is grown, I had to wait until all had left home, there are always other pressing purchases with children.  If you browse old threads on the forum, you will see people are making amazing engines on very small lathes if you have no space for a larger one.

So what is your next project now?

MJM460

[Ian S C]

I'v even heard of a hand drill clamped in a vice and used to turn a piston for a steam engine.
Ian S C

[Andersgj]

Thanks!
My next project will be Elmer's wood beam engine with a boiler. I am thinking of making the boiler fully enclosed since the polly boiler has so little heated area and that was something I wanted to improve on my next build.

My neighbour just got a lathe and he will lett me use when ever i need, so now I practically own a lathe

This engine will probably take me a long time as my wife is pregnant and I have to do a lot more around the house for the forseeable future.

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