Author Topic: Tubal Cain "Polly" without a lathe  (Read 10508 times)

Offline Andersgj

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Tubal Cain "Polly" without a lathe
« on: January 07, 2016, 11:27:21 AM »
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
« Last Edit: January 07, 2016, 11:59:58 AM by Andersgj »

Offline b.lindsey

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Re: Tubal Cain "Polly" without a lathe
« Reply #1 on: January 07, 2016, 12:33:28 PM »
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


Offline tangler

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Re: Tubal Cain "Polly" without a lathe
« Reply #2 on: January 07, 2016, 01:36:14 PM »
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

Offline Andersgj

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Re: Tubal Cain "Polly" without a lathe
« Reply #3 on: January 07, 2016, 02:07:00 PM »
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!

Offline Dan Rowe

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Re: Tubal Cain "Polly" without a lathe
« Reply #4 on: January 07, 2016, 02:31:20 PM »

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
ShaylocoDan

Offline Andersgj

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Re: Tubal Cain "Polly" without a lathe
« Reply #5 on: January 07, 2016, 05:30:58 PM »
Does tinning of the inside of the boiler work ok to avoid this? Saw it mentioned in the article

Offline Andersgj

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Re: Tubal Cain "Polly" without a lathe
« Reply #6 on: January 08, 2016, 11:50:41 AM »
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

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Re: Tubal Cain "Polly" without a lathe
« Reply #7 on: January 08, 2016, 01:47:04 PM »
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

Offline Andersgj

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Re: Tubal Cain "Polly" without a lathe
« Reply #8 on: January 11, 2016, 02:04:30 PM »
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.

Offline Andersgj

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Re: Tubal Cain "Polly" without a lathe
« Reply #9 on: January 11, 2016, 02:33:44 PM »
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?
« Last Edit: January 11, 2016, 09:39:24 PM by Andersgj »

Offline Andersgj

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Re: Tubal Cain "Polly" without a lathe
« Reply #10 on: January 12, 2016, 12:49:02 PM »
The tube I found is a table x tube, rated to 290 psi, so sounds pretty safe to me
Any takers?

Offline b.lindsey

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Re: Tubal Cain "Polly" without a lathe
« Reply #11 on: January 12, 2016, 01:28:38 PM »
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

Offline kev

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Re: Tubal Cain "Polly" without a lathe
« Reply #12 on: January 12, 2016, 02:55:50 PM »
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
« Last Edit: January 12, 2016, 03:33:04 PM by kev »

Offline tangler

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Re: Tubal Cain "Polly" without a lathe
« Reply #13 on: January 12, 2016, 03:56:36 PM »
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

Offline Andersgj

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Re: Tubal Cain "Polly" without a lathe
« Reply #14 on: January 12, 2016, 06:34:54 PM »
I will stick with it, making a steel firebox shouldn't be too hard.

But first the actual engine, then off to the boiler

 

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