Member postings for Martin Johnson 1

I always advise prospective builders of larger scale traction engines to consider how you will transport it, where you will store it, and how you will manhandle it. There are huge (expensive) implications for what vehicle you drive or are likely to drive. A part built 4" scale engine is about the same footprint as a lathe - so could you easily cram another lathe in your workshop - it's a rare workshop if you can! Finally, how fit (and old) are you? - Don't answer in public on this forum! The fact is you will be shifting best part of 1/2 ton of machinery around on a regular basis so is that going to be feasible 10 years from now?

As to the models you mention, a few comments:

The Plastow drawings have a "reputation" of the wrong sort, being difficult to read and having their fair share of inaccuracies.

The Plastow 4 1/2" scale Burrell is of the 7 HP engine (I think), so makes up into quite a large model, roughly 6 feet long. These days, a copper boiler for this engine would not really be considered, purely on cost. So a steel boiler is probably the way to go - and that has implications for storage and long term care.

There are plenty of examples of this model around, so you will be one of the crowd. However, plenty of build advice and tips on where the drawing errors are will be available.

I assume you are considering the EKP 4" scale Burrell Gold Medal Tractor. I have not heard about EKP's drawings, but generally the GMT is a very detailed and good model. I have not seen the 4" scale model around, so they are not that common. It is to a slightly smaller scale and is a smaller prototype than the Plastow, so will be significantly smaller and lighter. I believe it to be considerably more detailed and closer to prototype than the Plastow.

Plenty to think about!

Martin

Since you are only generating a straight line for the edge of the tool, why do you need a cup wheel? Grinding on the periphery of a 5 or 6" disk wheel will give a perfectly adequate tool for cutting acme (or most other thread forms), albeit with a slightly hollow clearance face.

I have both types of wheel on my homespun cutter grinder, but the periphery is the "go to" option when possible.

Martin

Another vote for buy it finished. By the time you have bought the brass, stainless steel, silver solder, balls, fasteners, the tank etc. etc. you will save money by buying it finished. I have been using a bought one for a couple of years now and it is so much more convenient and works very well. You can also use it to top up pressurised central heating systems.

Martin

Several points worth flagging up on this saga:

1) "1/4 to 1/2 thou undersize" for what is probably about a 4" register, that is pretty fine tolerances anyway.

2) Another piece of received wisdom is that lathe tools have to be at centre height. If you are trying to take wee shaving cuts and the tool is actually a shade OVER center height, then the tool won't cut. So you put on a bit more and try again. Then, the tool finally gets "in" and takes off a load more than you bargained for. Moral - keep tools a shade BELOW centre height - especially on fine finishing work.

3) Funny stuff is cast iron. The finish straight off the tool (HSS at any rate) will be "whiskery", which gives a false over size reading on a mic or calipers. Before measureing take the whiskers off with a bit of wet & dry paper.

4) Another way to rescue such a situation is to let in and loctite 4 plugs, say about 20 mm diameter for your example, equispaced at 90 degrees and straddling the register. Then machine the plugs back to (hopefully) finish at size. Assemble it and hope you are long gone by the time someone else finds out your rescue bodge. Seen it done in my working days to rescue a very urgent part.

5) I agree with the others - no need to resort to bodges in this case. Crack on and well done.

Hope that helps,

Martin

I don't think anybody has mentioned that a hole bored on the lathe by rotatin the work depends on the accuracy of the lathe to maintain it's parallelism - if the lathe is "out" then the hole will be tapered. I know this because I have a lathe that is slightly out.

However, if you rotate the cutter the hole will be parallel, but not necessarily at a perfect right angle to the table. I know this because I have a mill that is "out" as well.

Tool deflection is another matter that will also cause inaccuracy.

To answer the OP's question. I have recently bored the pump cylinder for a Southworth steam pump. (65 mm end to end and 23 mm bore). I did it on the lathe cross slide with a between centre boring bar. Actually, it is a boring bar I grip in the four jaw chuck at one end and on centre at the other. By gently adjusting the chuck jaws and using a DTI against the tool tip, you can put on a fine cut in a very controlled manner to finish at a precise diameter.

Hope that helps,

Martin

I have one that I bought at a club sale for half a crown. That gives you some idea of when that was.

It has done sterling service over the years, setting up lathes and truing up work. With the arrival of a pucker dial gauge (for a lot more than 2/6), it is now still used on a crude stand for setting lathe tool height - on centre is centre of the scale, so easy to see if you are high or low. I ground the rounded "stylus" flat for this job.

Highly recommended for those on a tight budget.

Martin

I am glad others have noticed that some of the "Colonial" centre drills are dire. If you put a colonial against a good brand, the differences are striking. Buy cheap and buy twice!

Martin

I agree with all the above construction ideas. But keep a double width door somewhere - you never know what machines you might want to get in or out. Bargains pop up unexpectedly, house moves can be forced upon you and how are you going to get that milling machine out? Or you might decide to build a 4" scale traction engine, or take up motorbike restoration.

Plan for the unexpected.

Martin

If it makes Turbine Guy feel better, my old company that I trained with (W.H. Allen & Co, Bedford - see **LINK**) used to have an air turbine testing rig that was used for developing blading on steam turbines. So there is every good reason for using air as a convenient medium for development purposes.

Martin

As part of my continuing analysis of model locomotive boilers, I am trying to use some test results on a Roger Marsh Romulus to check my prediction software.

Can anybody help me with some leading dimensions of the boiler, please? I am trying to get the following (rather formidable), but any help would be greatly appreciated.

BOILER INTERNAL DIMENSIONS

Grate width & length

Firebox wall area

Length between tubeplates

Number & diameter & thickness of firetubes

Number & diameter & thickness of superheater flues

Outside, Inside diameter of superheater element, & how many elements to a flue.

Length of superheater element & length inside firebox (if applicable)

Boiler Outside Dimensions

Overall length of boiler shell

Diameter of boiler shell

Thickness of lagging (if any)

Many thanks in advance,

Martin

A couple of points:

Burning a candle in a boiler for storage - so that hydrocarbon fuel will produce CO2 and H2O, which will condense and combine with the CO2 to make carbonic acid, and then lay in the nooks and crannies - Doh!

Filling to the brim. Does nothing to address crevice corrosion - in fact encourages it. You will find plenty of crevices around all fittings, around the foundation ring weld and around stays - none of which can be cover welded from the inside of the vessel. Maybe if you are filling to the brim with an inhibitor laden water.

For my boiler, anything over a couple of weeks it is always dry - blow down hot and let the residual heat do the rest. For over winter, take the plugs out as well. My boiler is stored inside in a heated room. For less than a couple of weeks (between rallies) then store well filled. I use boiler treatment anyway which will scavenge most of the oxygen out.

Martin

Clause 5.4.3 makes perfect sense and the friction losses of another offtake can make the water reading false - no problem with that.

However, if that clause is satisfied, I am with Duncan in that what possible difference could GWR style columns make? So we are actually no nearer to answering the "why?"

Martin

Turbine Guy,

I have just caught up with your post of 14/3/19 doing outline design calculations for a turbine and a turbine / ejector combination. Carefully thought through again, and interesting that it shows things to be roughly the same - i.e. ejector losses balance turbine efficiency gains. Thanks for that.

How well do you think the Ds Vs. Ns chart reflects things in your tiny turbines?

I will be very interested to see what you get from the two stage open pocket design. - I dare say you will be as well.

Thanks for a great thread,

Martin

" You might be OK with a commercial test and a commercial insurer as long as you can do the numbers and get a design approved. Could be complicated and expensive though! "

I have just completed that route for a conventional steel vertical firetube boiler. 18 pages of calculations, many pages of risk assessments, installation and operation instructions etc. etc. later, plus over £1500 lighter in the wallet department just for design approval and independant testing. I cover some of this in an article coming up in the next issue of EIM.

In short, that sort of money will buy you a hell of a lot of copper and silver solder.

Martin

Thanks for making me think, chaps.

You are right for constant momentum, but greater mass flow, the TORQUE would remain constant for a given design of blades, but at the same time the optimum running speed would fall in the ratio of the mass flow increase (same as velocity decrease). Hence power falls as the ratio of mass flow increase or velocity decrease. All assuming motive and entrained fluids of identical density.

BUT earlier in this thread TG is reporting a blade speed to fluid speed ratio of 0.022 - way down on where it should be for a two velocity staged machine. AND even if he can get the running speed up to where it should be, how long will the bearings stand up to it? Or, indeed, how long will the blades stay attached to the hub? AND what losses would there be in a wee gearbox to reduce the speed to something manageable?

So, the question remains for tiny turbines, would an ejector lose more than it gains?

For all that, I am in awe of the tiny rotor and blade cutting that TG is undertaking. Also, the methodical way it is being thought through. Keep the reports on progress coming, please.

Martin

The last time I got something stress relieved (about 30 years ago) I was able to go to a local heat treatment company (in Loughborough) and get it put through with a batch of something else. If you can find a company, it costs nothing to ask.

Martin

Since I have remembered the idea of the ejector driven turbine for so long, you can tell it fascinates me.

Duncan, I agree that the momentum flux stays the same but as you say the velocity drops and the mass flow increases. and from Newtons law the force would stay just the same.

HOWEVER, designing model turbines seems to me a battle against bearing problems, disk friction and how to make stupidly small passages / blades. It seems to me that converting the momentum to more mass flow and lower velocity answers all three of those problems. I think we can accept some inefficiency in the ejector because turbine efficiency at very low flows just plummets, so to offset the ejector loss we have a gain in turbine efficiency.

Turbine Guy - Your argument is the one I swallowed for years. But as Duncan says, the momentum flux stays just the same and since turbines actually need momentum flux, then are the losses not small?

One of us is going to have to make one!

Martin

I think there could be a book on de-burring methods. I have a selection of hook type deburring tools (Noga and similar), a very lashed up version of the OP's picture which is a broken stump of a largish drill shoved into a file handle.

However, my weapon of choice for lathe work is a triangular file, about 1/2" face width, shortened, ground to a point but with the file teeth left on the back half. This was inherited from my Dad who used to in industry to make Clearview Screens for the Navy. To use:

• Internal holes - use the pointy end as a hand turning tool (no rest needed) to take off any burr.
• External shoulders - use the file teeth end to knock off the aris.
• Make sure Elfin Safety is not around.

Another good way of deburring multiple holes is a largish drill in the battery drill - you can do dozens of holes in the time it takes to type this screed.

Martin

Hi T.G.,

" I may have confused you by using the velocity diagram of an 3 velocity stage axial impulse turbine. "

Well, I have been constructing and reading velocity diagrams for long enough to realise that what you showed was not what you were talking about. Thanks for the extra explanation on the return passages in the Terry turbine casing. So it seems in practical terms the axis of the cutter for the return passage has to be tilted over from radial (viewed in the axial plane) to achieve the return of steam to the "next" blade in the rotor.

Sorry I can't find you a reference to the ejector - turbine combination, but it was a long time ago. The builder was looking to replace a Stuart Double 10 with a turbine, and was trying to get equivalent power from the same boiler - IIRC!

Martin

Turbine Guy,

Thank you for these posts they are a useful input to a topic that has defied model engineers for a long time.

I was not aware of the Terry turbine (having been brought up in a company that made big steam turbines with proper fir tree root blades), but the concept looks very good for miniature builds. I cannot quite see how the Terry velocity staging works (despite studying your drawings and the Terry illustration), so more clarity on the casing return passages would help.

It seems to me the fundamental problem in model turbines is that we are dealing with tiny steam flows. Hence, disk friction losses in the turbine are going to be relatively huge. Therefore, whatever means we can use to reduce running speed will all be to the good. I am sure I had an efficiency estimation chart at one time, which showed that as the turbine type number reduces then the efficiency also heads south - due to the disk friction losses.

One method of reducing running speed (and increasing type number) that I saw in an ME, probably from the '50s comprised a steam ejector working into an axial flow turbine. I rather discounted the idea at the time as ejectors are notoriously inefficient. HOWEVER, with further thought it is the momentum input to a turbine that matters and ejectors are very good at changing momentum from a very fast small jet to a slower big jet with THE SAME MOMENTUM. Given that turbocharger rotors can be readily found in scrapyards, I wonder if an ejector / radial inflow turbine combination would give good results.

My own interest is in developing a turbo generator for charging a 12 volt car battery - so quite big.

Martin