Cylinder Boring Techniques for Steam Engines

I'm planning my first steam engine and looking for advice about machining the cylinder. There seems to be a lot of disagreement about this and the more I've read the more confused I've become.

Some accomplished model makers recommend using reamers for the cylinder bore and others say that reamers are completely unsuitable. I can see why a hand reamer would be unsuitable but what about a machine reamer mounted in the lathe and driven by the lathe? (Would the spiral flutes of a machine reamer eliminate the risk of it causing corrugations in the way a hand reamer can?)

A between centres boring bar with a home made HSS cutting tool seems to be the method of choice for many people for finishing the cylinder bore. Does anyone have any advice about this - particularly about how to accurately mount the cylinder on an ordinary cross slide for boring between centres?

Under what circumstances (cylinder diameter, length, etc) does an ordinary boring bar become unacceptable and a between centres boring bar become necessary?

What about a between centres reamer? Does such a tool exist?

Small, inexpensive cylinder honing tools with 3 interchangeable course and fine stones are available for small engine cylinders and brake cylinders. At first sight they'd seem a good option for finishing the cylinder bore - is this correct?

What surface finish should we aim for? Theoretically, a perfect mirror finish would minimise ware but some sources say that microscopic groves in the surface are desirable because they help retain lubricating oil and thus reduce wear.

With the above in mind, does the choice of cylinder O-ring (oil grove, plastic, etc.) affect how the cylinder should be finished?

A lot of people seem to finish the cylinder and the piston by lapping the two together with Brasso or fine valve grinding paste. If I plan to do this what clearance should I leave when machining the cylinder and the piston?

Sorry to post such a complicated question. I've read a lot of posts (this one seemed the most useful) and bought a lot of books but none seem to give definitive advice and boring the cylinder seems critical to the success of the whole engine.

PS - I'm planning to machine the cylinder from stock metal not castings.
The decision as to which metal to use for the cylinder and piston isn't made yet so advice on that would also be valued.

Many thanks,

Will

I am building the Stuart Turner compound steam engine. The cylinder was turned with lathe and honed with honing tool for brake cylinder. The material in the cylinder is made of cast iron. Then i has maked the other steam engines of bronce, brass or aluminium. The honing tool was a wooden stick with sand paper wrapped around. No problem

Will, for the smaller size cylinders for say a 3.5" or a 5" then mounting the raw cast cylinder onto a lathe face plate is the way.

Clamped to an angle plate then the face can be turned flat and having previously marked out your centers then you can drill and bore the cylinders.

larger cylinders may well be bored out by clamping to a table on the saddle and using a between centers boring bar good work can be done.

This is assuming you have a large enough lathe and equipment.

It would be possible to use a milling machine with a boring head, again having adequate movement of Z axis.

Finishing the bore is by lapping with stones on a mandrel and a spiral motion while doing it. The marks at about 45deg angle as you lap.

In any case the piston would be turned to size and be a sliding fit in the bore as you will then use packing of some sort in the piston ring groove.

There is nothing difficult about this as I did a 3.5" cylinder on an ML10 using the face plate and an angle bracket.

Reaming a cylinder in the smaller sizes is OK as you still have to lap the cylinder anyway and the result is a parallel bore, sometimes you may need a tapered bore on small engines.

As you work, a few thou. over or under a size given on the drawings is OK as long as you achieve the fit you need, being pedantic about exact size of a cylinder is not the way.

It will still work a few thou over or under size!

I have a small honing tool with three stones on it and I finish the bores to a nice matt finish as a shiny bore will not allow bedding in of parts.

making from bar or stock metal aim for a Cast iron or Bronze cylinder, the piston can be made of a contrast metal. Bronze to cast iron or cast iron to bronze as long as they are dissimilar its OK.

I used bronze on a cast iron cyl.and stainless steel on the steam valve piston.

Clive

Hi Will

I think ,maybe, all your reading has given you too much information!

You say that you are planning to build your first steam engine. For folk to advise you I think a bit more information would be useful. Without wishing to dampen your enthusiasm I think that the cylinder boring is likely to be one of the easier bits!

What equipment do you have and how experienced are you in its use? Which engine is it that you propose to build and, to address your specific cylinder query, what size is the cylinder and what material is it to be made from?

Boring is likely to be the prefered method and, with care a very smooth and parallel bore can be achieved. If this is your first attempt, honing is unlikely to be necessary.

My best advice really is to chose a design especially for beginners - one which has been written up and gives detailed instructions on all stages of construction. Any problems that occur as you progress - then ask here.

Regards

Norman

This is how I did some for a 5" loco:

..but any of the methods you mention should work. Just choose one to suit your equipment and go for it!

Russell.

A cast iron piston may be used in a cast iron bore, cast iron being the only metal that is happy to run like to like, friction wise this has the lowest coefficient of friction of any metals used in general engineering. Ian S C

Hi Will,

Tribology (study of friction between surfaces) is not my field but we did touch on it in my engineering classes when an apprentice. To try to answer your questions, firstly I would say don't worry too much on a first engine, as Clive said, make your cylinder by whatever means and make your piston to fit the worst mistake is to make it conical. A little clearance won't matter too much, as, being a first engine it is unlikely to be entered for efficiency tests.

As far as surface finish is concerned, yes, you do need somewhere for lubriction to lodge, a mirror finish will allow the lubricant to be wiped off. In i.c. engines such as vehicle engines it was quite common for the bores to be honed to 'break the glaze' when overhauling.

I would not advise honing bore and piston together although it might seem logical to 'size' the piston that way, you will actually create a situation where the surface finish is the same on both surfaces, a condition which should be avoided at all costs. This is because if you have similar surface finishes they can 'interlock' with each other rather like similar gear teeth and this enhances wear. Imaging trying to slide a piece of 60 grit emery over another of the same grit, then imagine sliding it over a surface of 1000 grit and you get the idea.

It is a good idea to use dissimilar metals for piston and cylinder, except, as Ian says when using Cast Iron. As an aside, where a soft material is in sliding contact with a harder material it is likely that it will be the harder material which wears. Think of a rotating shaft such as a crankshaft or axle in a car. When removed when oil leaks occur, they are often badly worn where the rubber oil seal has rubbed on it. That is because the softer material picks up hard particles which in turn become embedded and then it acts like emery cloth on the hard metal surface, so wearing it away.

To summarise, don't worry too much at first, things can get over complicated when over analysed, you should be enjoying the hobby. Concern yourself with the arcane areas of engineering later.

Above all have fun,

Terry

Does it matter if you use dissimilar metals for piston and bore? I don't know about this engine, but in general the piston never touches the bore - the rings or packing do. All the piston does is act as a carrier for the rings, and bungs up the rest of the hole. Make it from what you like in principle, so long as it remains small enough never to touch the bore, even at steam temperatures.

Bore finish. Depends on the rings in general? If you are running iron rings in iron bore, then honing is one way. Personally I have never worried about that, and just finish with a decent tool finish. Run in on my Metre Maid, - its done about 4 hours running, its gone from quite tight all round , and now, if you pull the engine back with reverser in neutral, suction on one cylinder will pull the engine forwards half an inch or so. So the seal is pretty darned good. Nice parallel bore, decent finish on a fine feed, and to the size specified by the ring gap is what is needed.

The alternative generally would be a bronze or gunmetal bore, using o rings or other PTFE/graphite packing as rings. In general they like a high polished finish, but I have no great experience of them, and would leave advice on that to others. However, a properly reamed finish should provide a huge pressure seal with O rings on a pump, so that should be Ok for a bore. As always when reaming a round hole, go to about .003" undersize, ream slowly, stacks of coolant to prevent sticking with temp changes and with a floating reamer if possible.

How to bore it. Between centres has the advantage of always producing a parallel bore, but often tooling is not so easy to adjust, and it can chatter horribly. So my choice is always a good stout boring bar and the 4 jaw. But that would depend on the size of the chuck/lathe against the job, and the confidence oyu have in your lather turning parallel. For small work you could use the Keats type angleplate - used to be opular, probalby less so now. The mill is a good way too, as has been suggested. Possibly easier to get bore and portface all in the right place in the mill.

You know your machine, how it cuts on the backcut, and how to remove spring from the tool. At the end of the day have confidence, its just another hole. And, if you don't have that confidence, there is usually enough spare metal to allow you to have a few practise finishing cuts in the bore as you go, before having to hit final dia.

All the best.

Edited By mgj on 30/05/2012 23:20:45

Thank you for such carefully thought out advice - much better than what I've found in books.

Hi Clive,
Thank you very much for your advice again. I'd originally thought about i) Machine a cylinder. ii) Machine a piston with grove to fit the cylinder. iii) Machine then split a piston ring from a block of PTFE (or similar) to fit both. - is this a suitable approach?

I can't get my paws on a face plate but I can get use of a 4 jaw chuck. I hadn't considered using the milling machine for boring - I'll give it more thought. Where did you get your lapping stones and mandrel / honing tool? What specification of bronze and cast iron do you use?

Hi NJH,
The sise will depend on the stock I can get (no casting skills or facilities). I'll choose materials based on advice from more experienced people. I haven't used machine tools for 10 years so I'll make and test prototypes of all components using scrap for practice.
Have access to a fairly good lathe and milling machine owned by a friend. An enormous challenge is to design the engine myself based on ideas from John's series on puddleducks and existing engines and bearing in mind thermodynamics and reading about André Chapelon's and Corliss's principles.

Hi Ian,
Thank you for your advice about the low coefficient of friction of cast iron. Is there any particular grade of cast iron you'd recommend (I'm not sure what would give the best balance of machinability, corrosion resistance, etc.)?

Hi Terryd,
Thank you for your advice about about the hard metal wearing more than the soft - I hadn't realised that - it changes my understanding of things a lot. (Makes sense as well when you think about someone cutting class with abrasive paste and a coper wheel.) Thank you also for your advice about not honing/laping the piston and cylinder together - I've seen this recommended in several books but your advice seems better. Thank you for helping me to understand things with such clearly explained advice.

Hi mgj
How many piston rings would you generally use in a model steam engine (guessing 2 or 3)? I don't know if the lathe I'm borrowing turns parallel but I should be able to calibrate it so that it does. Thanks for advice about using coolant and mentioning a floating reamer - I didn't know about them. Thanks also for advice about using a milling machine for boring - I've never done that before - any more advice would be interesting. Please could you explain "and to the size specified by the ring gap is what is needed." a little more. Unfortunately, I don't have a good enough knowledge to understand this.

Three other general questions that have been confusing me:

Am I right in thinking that there are two fundamentally different approaches: i) Make piston precisely fit bore with an oil grove in the piston to help it seal. and ii) Make piston a looser fit and rely on piston rings or packing to achieve the seal.?

Dissimilar metals for piston and cylinder seem to be essential (ex. cast iron) - but - different metals have different coefficients of thermal expansion - what should I do to avoid making an engine which turns smoothly at room temperature but ceases up under steam due to different components expanding by different amounts when heated by the steam?

What is best for piston rings - graphite tape, plumbers' PTFE tape, a ring machined from PTFE stock or something else?

Thank you very much again!

My first engine (I'm currently working on No. 3!) was a steam engine with one inch bore. I can't remember if it was bored in the 4 jaw chuck or mounted on the face plate. Anyway, I drilled the cylinder with the largest drill I had using the tailstock (around 18mm IIRC) then used a boring bar with a carbide tip to get close to finished diameter, but the finish was rough. To finish I used a home made silver steel boring bar ground very sharp and then ran the lathe around 50rpm and slowly got a really nice finish. Point being very sharp steel tool run very slowly very light cuts to finish nicely. The engine runs very well without any further work on the cylinder bore. No lubricant on cast iron is usual practice.

I think that Rolls Royce used to bore their cylinders a few thou over size and then with a fine knurl make the bore a few thou undersize. After that they stuffed a a ball ground to size down the bore. This left a fine set of pockets for oil.

Hope this helps

Dick

Will, the best cast iron is what ever you can get, sash window weights are good, find the largest diameter ones you can, the skinny ones are sometimes chilled, and you won't even turn them with a carbide tool, I got suspicious of one weight that shattered when I dropped it. I'v used old car flywheels, and brake disks/drums. I got some cast bar (65 mm dia) not sure of the spesifications, it has a different feel when cutting. Don't know about corrosion, my pistons are in hot air engines, so they are dry. There is little loss if you use a steel (center tube from a car shock absorber) cylinder with a CI piston for these engines. My only steam engines are little (and smaller) wobblers, with brass cylinders, and stainless steel pistons. Ian S C

Will - dissimilar metals and precise fits. You can achieve that, but you do have to make sure that the piston never gets to the same size as the bore. The consequence is known a seizure! Simples I'm afraid. What do you do tp prevent it - the arithmetic.

Rings. Smaller engines tend to have packing rings/ O rings. Larger (iron) bores will have rings. In general iron bores will rip graphite packing to bits.

How many piston rings. Usually 2 usually in 2 separate grooves. But some Stuart designs run 2 rings in a single groove. Better/worse I don't know - I'd rather run a narrower ring in 2 grooves and keep themeparate.

Gaps. - for iron rings in an iron bore, you can run at say .001 gap when new. .

Bore dia. Engines will run with their valve gear all over the shop, and bores thous over and undersize with bits made to fit. Probably OK for a single cylinder engine. Not a grand idea on a double or loco with three!. so for me, if the drawing calls for a 2" bore then a 2" bore it will get (+/- .0002". ) Saves pratting about filing rings etc. Its easy enough to achieve - you just approach carefully, and always cut on the in cut and the back cut. Normally, as you get close, twice. It will be dead right.

Piston size --.002/3" Something like that. Not critical if you are fitting rings.

Liners etc I would make of continuous cast meehanite. Its not very expensive, and it should be very consistent (if ringed.)

No experience of unringed bores, but gunmetal/gunmetal is used a lot, with grapjhite/PTFE packing. Lasts well I understand.

May I suggest - you are in danger of wading in a pool that is way too big for the answers. Decide now, how big and the basics. Then you can ask more specific questionsand get targeted answers. At the moment every answer has to cover all the bases from tiny oscillator to 2" bore traction engine, and that would appear to confuse rather than help.

Edited By mgj on 02/06/2012 15:28:46

Will, perhaps you can give us some indication of the size of the item you want to build? Bore and stroke perhaps or even which named machine.

There is enough information in all the posts now for you to make a decision on what materiel you want to use and even how to do it.

Smaller details you can raise as and when you come to them, plan ahead and raise the questions before you need to do them.

Clive

Hi Clive,

Sorry about the delay.

My present steam supply is 1.2 bar saturated steam (low pressure but I hope it'll do for now - I'll maybe try higher pressure and/or superheating in future). If I say a maximum force on the piston of 30 Newtons (working expansively the average will be less) that gives a piston radius of 8.92 mm - so I thought a piston radius of around about 10mm would be OK. I'm not sure about how to work out the most appropriate stroke so I guessed based on other models - maybe 20 to 40mm to give a cylinder and crank of manageable sise?

For my first attempt at the cylinder I'll start with an unknown piece of scrap stainless steel tube 27mm bore and 3.7mm wall thickness or another of unknown not-so-stainless steel of 20mm bore and 5.2mm wall thickness. Rough cuts with carbide and finishing cuts with better quality carbide or HSS honed with diamond stone. For the piston a lump of unknown scrap brass. If my attempts to machine the unknown scrap steel fail I'll try to get the SAE 303 and 316 steel you recommended in another thread. For the piston seal a grove packed with plumbers' PTFE tape (to be upgraded to the PTFE yarn or graphite yarn from Reeves you mentioned or the woven square section PTFE you recommended - I think Sealrite Limited might supply this) or a ring machined from solid PTFE.

That's the initial plan...

Will

The way to do it is the other way round Will, the cylinder of Bronze and the piston of S/steel.

Firstly you will get a better finish in the bore and secondly its easier to make the piston from S/steel.

S/steel is not very forgiving and it will be hard to get a good bore finish.

This has all been done before so follow the masters on this!

Clive

Hi Clive,

Thank you for your advice. The idea of the steel bore was because of the scrap metal I've got at the moment - thanks for confirming that it's inside-out. Terryd's advice about how metals wear also suggests bronze cylinder and steel piston.

I've no idea what brass scrap I've got (or even if it is brass) - just that it looks and feels like brass and has a similar weight - I'll hopefully get use of a large enough lathe to do some trial cuts on it in the next few days. Is using scrap in this way a good idea or should I concentrate on getting the correct sort fo bronze from a supplier? (I'm over in Switzerland because of work so finding suppliers is more difficult than in the UK.)

The advice from you and from others has been very valuable - much more valuable than what I've found in books - are there any books you'd recommend? (I've bought several books but they all seem limited and I'm running short of cash and space for them!)

Will

Will, there are so many ways of doing things in model making, one mans method is anothers tribulation due to different size machines or inabilty to see through the job.

recognising different metals and how and where to use them is sometimes complex and here you seek answers and someone will come to your aid.

I agree with you that in Switzerland you will have difficulty obtaining materiel for what you want to do. Having spent time in Switzerland I know there are Firms that can supply metals.

I work on Instruments made in Switzerland (LEICA) They are based in Heerbrugg. Are you able to speak German? or are you in another Canton that speaks Italian or French. Ask someone you work with and see if they can help out. Switzerland is quite big in manufacturing.

As to books, I feel you may be better to buy books on basic metal working and machining to learn how things are done. Having been applying myself to Instrument and Toolmaking for nigh on 60 years I seldom refer to a book now unless its some odd thread tapping size.

by all means keep asking questions on here as it keeps some of us awake!

Regards one question that I did not see an answer to was about how many piston rings, well, normally just one ring. This as a 'Ring' (metal/PTFE) or Packing like Graphited string or woven PTFE in square form.Piston glands with again Graphite or PTFE.

Clive

Hi Clive,

Sorry to be slow to reply. Unfortunately an accident (not machine tool or mountaineering related!) has meant I've been in and out of hospital over the last few days.

I live in the German speaking part and I struggle away in German (my French is better...). A friend has a small but busy factory so I think he'd be willing to put my small orders for steel together with his much larger orders. For other metals I've no idea what to do yet apart from getting help from a friend's garage.

Great to hear about your work in instrument making - my first job was as a junior technician at a university helping making mountings for equipment for laser optics. (Not much precision involved there because all the precision alignment was done using the lasers.)

I might have a go at restoring an existing engine before starting work building my own - I've got a Stuart S50 where someone made a real mess of the machining - I bought it on Ebay and I reckon if I re-bore the cylinder and replace the damaged parts I can get it running (I'll use Tubal Cain's set of articles for guidance).

I haven't tried that trial cut on the big chunk of mystery brass yet - I need to get access to a better lathe (the small lathe could turn it but I don't think it would be safe).

In one interview I remember being thrown a lump of metal and told to identify it. It took me a while to work it out but when I eventually did they seemed impressed - Monel alloy.

Will

Hello Will, hope you are on the mend and feeling better.

I think if you work on that engine and follow the instructions you will learn a lot. Somehow one is able to find the metal thats needed and it all works out OK.

At th moment I am working on a 2 cyl. steam engine made mostly from the scrap bin. I will put a picture in the gallery later when its a bit more presentable.

Keep us upto date with your project and if you can post the pics s.

Clive