Fabricated Cylinders

[baggo]

Waggy posted a thread recently re fabrication versus castings so here's a few pics of a cylinder 'casting' I've just completed for the middle cylinder of the 2-1/2" gauge 4-8-4 I'm currently building. The cylinder 'barrel' is turned from cored gunmetal bar and the steam chest from solid gunmetal bar. The end plates and exhaust manifold are brass. There's still a bit of machining to do i.e. finish boring of cylinder and piston valve bores and general tidying up. Cylinder bore is 7/8" and piston valve 7/16" dia.







Cost of the materials used is about £10 which is considerably less than having to get a one off casting made. The two outside cylinders will be similar fabrications.

John

[gwrfan]

Excellent workmanship and pictures, John, and your 'Karma' must surely increase!

Geoff

[baggo]

Thanks Geoff,
I think I'm improving with age! I'm certainly a lot more patient than I used to be. I think the next loco will be a nice simple 4-4-0 - there's a lot of work in a 4-8-4!

John

[davidimurray]

Hi John

Cylinders look fantastic - I've designed a set of fabricated cylinders for my next big loco project. I'm a big fan of fabrication over castings - there's a lot of work just cleaning up a casting and the cost nowadays is ridiculous!

How do you plan to check for leaks - hydraulic test?

Cheers

Dave

[dscott]

Fabrication,

Yes I am also in favour of this method and appart from looking into doing the cylinders from steel with cast iron liners. I am also looking at wheels, especially the ones with square spokes.
They do make some in 2mm scale, these are about the size of a small fingernail so in our size there is hope. We also get a steel tyre for better grip.

David Scott.

[baggo]

davidimurray said:

How do you plan to check for leaks - hydraulic test?

To be honest I haven't given that much thought! It would be a bit tricky with it being a piston valve cylinder.

I soldered the valve chest to the cylinder first before adding the end plates, exhaust manifold etc. and was very careful with cleaning and fluxing. After soldering and pickling the solder seemed to have flowed through the joint properly so hopefully all will be ok . The only part of the joint that could cause a problem is between the two inlet ports and these are spaced well apart so there's plenty of area for the solder to fill.If any leaks are found later on a spot of soft solder in the right place should sort it out. I don't think the cylinder will get hot enough to melt soft solder. Hope not anyway as the plastic material I'm going to use for the valves melts at 300 degrees C !

John

[davidimurray]

I've been wondering about that same thing! I was thinking of blanking the steam in and exhaust and then maybe putting some endcovers on the bores with a drawbar through the covers and some packing to seal the whole lot. I'm cautious because I would hate to tap a lot of holes only then to have to get the heat on it again.

If you got to 300C I think you would be doing extremely well.

Plastic valves - tell me more!

Cheers

Dave

[baggo]

Hi Dave,

I'll have a think about testing. At the moment the cylinder is in the lathe and being finish machined. I'll probably wait till the valve etc is finished and then devise something using compressed air.

Re the plastic valves - this is just a bit of an experiment. I wanted to use PTFE originally but due to it's high expansion rate it's difficult to get the right fit at steam temperature. After some searching I discovered a material called Peek HPV (has different names depending on the manufacturer) which is a high strength bearing grade plastic. It contains 10% carbon, graphite, and PTFE and so has a very low coeficient of friction, even with no lubrication at all. It will withstand superheated steam and is very easy to machine. It has a rate of expansion about twice that of bronze or stainless but considerably less than pure PTFE so it should be easier to get the correct fit. It's not cheap but I've bought a few lengths of various sizes to give it a try. My thoughts are that with it being self lubricating the valve can be made quite a tight fit to begin with without danger of it seizing. Then by the time it's loosened up it should be a perfect fit and should last a lifetime. If it doesn't work out I'll just substitute bronze or stainless valves. I'll post any findings here anyway.

With regard to fitting bronze or stainless piston valves - Don Young's method was to make the valves a very tight fit, coat them with molybdenum disulphide paste and basically press them into the bore. The valve is then tapped back and forth until it becomes a nice sliding fit after which the valve and bore will have a mirror finish and last as long as the loco!. As Don said, if you make the valve the correct fit by machining only, as soon as the machining ridges wear off the valve will leak.

Incidently, I've used some of the Peek rod to make the bushes for the coupling rods so it will be interesting to see how it stands up with service.

Just another thought - has anyone ever used PTFE for making slide valves? The high expansion rate wouldn't cause too much of a problem for those.

John

[baggo]

Bit of an update on the cylinder.

The machining is now finished and the complete cylinder is nearly ready to fit in the frames. The cylinder and valve chest covers are turned from ordinary brass bar fitted with bushes made from the Peek material. O rings are fitted for the rod seals and the main piston seal. I managed to get an excellent finish on the cylinder bore by bodging up a very fine feed to the lathe leadscrew using a geared motor, an old toothed belt, and a hornby train speed controller to control the speed of the motor. Crude but it works perfectly



The piston valve bore was honed with an expanding aluminium lap and 600 grade Aloxite powder. A LOT of time was spent machining the Peek piston valve to try and get a perfect fit but only a steam test will prove if I've got it right ???At 100 degrees C the valve is slightly tight but at room temperature there is about a thou clearance. The Peek bobbin is held by a brass sleeve threaded onto the valve rod for final valve setting.



The valve heads are much wider than usual because I've used Henry Greenly's design described in ME many moons ago. The cylinder uses two ports at each end of the valve chest instead of just one. One port controls the inlet and the other the exhaust. This means that the valve heads are much longer with a larger bearing surface which should help sealing and reduce wear.




One down, two to go!

John

[davidimurray]

Hi John

Just catching up with the group - cylinders are looking great. One quick point - brass in contact with steam goes crystalline and breaks up - I suspect it is fine for end covers but definitely not for anything like valves! I try to not make anything that comes into contact with steam from brass - using a gunmetal or bronze instead.

I've experienced steam-brass failure at close quarters. WHile firing a full sized engine a gauge glass blew-I tried to shut the frame off and all that happened was that the handle came off in my hand! Luckily we were still on shed at the time so went sent the diesel out and dragged the next engine out of the shed!

Keep up the good work and let us know how you get on

Cheers

Dave

[baggo]

Hi Dave,

thanks for that comment. I think I might change the valve carrying sleeve to bronze to be on the safe side. I must agree that I wouldn't trust brass for boiler fittings!

John

[Frits]

Hi John,

As far as I can see, you did a fine job on your cylinders.
Maybe you already know him, otherwise this side me be in your interest.
www.sci.fi/~animato/3003/060n.html

This guy also constructed his cylinders.

Frits

[baggo]

Hi Frits,

Thank you. Hopefully the two outside cylinders will be easy now that I know what I am doing!

I found Jan-Eric's web site last year when I was searching for web sites about building steam locos. I find such sites a great source of ideas and inspiration and wish more people would publish similar ones. They are a useful source of information for beginners and help explain what is involved in actually building a loco etc.

Regards,

John

[Shawki Shlemon]

Hi
Your cylinders looks great. I also intend to fabricate cylinders for 7.25" Rocket which I started some time ago and it is now on shelf while I am working on Dukedog . I have already purchased cored bronze for the purpose . Your success gives me confidence.
As for PTFE I used it on three engines ie King class, NSW C38, and C36 ( the last two are Australian engines ) giving clearance of 0.008" per 1" dia and it works great . I arrived at this figure by trial and error . I machined the valves a few thou's clearance then assembled and heated in the oven ( my wife was out ) to 180 C and checked it was too tight , and continued doing this 0.002" at a time until I was happy . The only problem with PTFE is the engine will not run well on air and is sluggish until it is warm .
I am a boiler inspector and brass is a no no on boilers but for other parts is OK .
Regards
Shawki

[baggo]

Hi Shawki,

Thanks for your comments. I've not started the outside cylinders yet as I've decided to do some boiler making to give me a break from machining!

With the ever increasing price of castings I think that fabrication is the way to go. A lot of castings available now are very poor quality and much better results can be achieved by fabrication along with lower cost.

The Peek material I am using for the piston valves has a much lower expansion rate than pure PTFE so I am hoping to get away with smaller clearances. Like you, I am working on a trial and error basis for the correct clearance needed and only time will tell whether it is right or not!

John

[baggo]

Just to finish off, here's a couple of photos of the fabricated outside cylinder 'castings' that I've just finished. Method of construction is virtually identical to the inside cylinder but the job went a lot quicker this time around!





All they require now is some pieces on the outside to fill the gap between the cladding and the cylinder ends. These will be soldered on with Comsol as they are not structural, just cosmetic. All in all the whole jobs turned out quite well. They have been fiddly to make and a lot of time was spent on them but it has worked out much cheaper than using castings

John

[Tel]

Nice work John, a credit to you. ;D

[Myford Matt]

Really interesting pics. I've often wondered why we shell out so much for cylinder castings. Spoked wheels I can understand, but some castings really are just lumps that are easy to fabricate.

Cheers

MM

[Nigel Bennett]

Regarding the use of PEEK - I "persuaded" my company to buy some for - ahem - "research purposes". I used it as a seat for safety valves on my Edward Thomas, with every satisfaction. Lovely stuff to machine, but very expensive! It should be excellent for piston valves, and I intend to use it when I get that far with my "Tom Rolt".