Cylinder Boring Techniques for Steam Engines

You won't get a cross hatch by feeding out unless you also alter the direction of rotation but then the tool will be the wrong way up, on a worn lathe like you are using you will likely just get another helix that is offset from the first by the amount of play in the lathe.

Best to just stop the lathe and wind the carrige back by hand on all but the last spring cut as the tool will leave a line on the bore where it is dragged back across the finished surface, this will be machined away with subsequent cut sbut obviously not the last.

rather than rely on a sloppy hand wheel dial if you have a DTI set that to measure the cross slide travel and you should be able to return to the same point.

J

rusted in position and penetrating oil for several days followed by persistent use of a small hammer couldn't move it.

You deserve an award for model wengineering in the face of adversity! next you will tell us the difficulties of being distracted by a polar bear in teh shop...

Neil

D'oh - sorry - yes - you're right - I wouldn't have got a cross-hatch pattern that way. I'd have had to take the workpiece out and turn it around.

Thanks for your advice about using the DTI - I hadn't thought about using it in these circumstances thought I should have - a significant help since parts of the readout on the handles are too rusted to be easily readable.

Thanks very much for all your advice and taking the time to teach me these things.

I often cut in both directions when boring, this can give a very good finish, especially on a spring cut. But be aware- a worn lathe or badly adjusted one can cut deeper on the back cut, or not at all, depending on the wear. I've just ruined a small bore in aluminium with an extra 10 thou' or so cut.

I've got more machining done for the restoration of the S50 - I'll try to post some photos. Every step of the way I've been guided by advice from people on this forum - I'm very grateful.

For my own engine I'm thinking more about the problem of heating the stock for the cylinder and the mounting flange for the valve assembly to 600 C to solder them. The high temperature heating wire I got turned out to be much springier than I was expecting so I looked again at the idea of a paraffin or diesel fuelled furnace.

I built a diesel burner with a simple atomiser and fan fed air supply but the atomiser produced droplets of diesel which were too large and had far too high a flow rate, resulting in a large, yellow low-temperature flame - useless. One hint of hope did come as the diesel in the pressure tank ran out and a mixture of diesel and air passed through the atomiser - that gave a very fine colloidal diesel mist which looked almost like smoke and burned beautifully with an intense hot, blue flame. If I could only create that reliably... That gave the idea of using a compressed air to make something similar to those perfume bottles with a rubber bulb and air tube attached that you see in 1930s films but I don't know if this would give the very fine droplets of diesel that I'm looking for.

Now I'm thinking about vaporising the fuel instead. Does anyone have any advice about how to make the orifice that the vaporised diesel passes through in the Venturi effect mixer of the burner? I made this sort of burner often when I was a child but the fuel was ways LPG - I made the orifice by gently hammering a piece of copper brake pipe until the right diameter of hole was made and I got the right sort of flame. If I burn vaporise diesel, I need to be more aware of the need to prick the jet to clean it and keep it free of the deposits that tend to build up in this sort of burner.

I don't know what facilities you have, or really what you are trying to do, but - I once saw a home made furnace ,made of bricks, the burner was rescued from an old central heater boiler, the owner used it to melt aluminium scrap by feeding old engines etc. in at the top and molten alloy out the bottom, might be worth a thought.

Hi Gordon,

This sounds very similar to what I'm planning - I've got heat resistant bricks and my furnace will be much smaller than your friend's (only for soldering at 600C - not for foundray work). I'd love to get my paws on the burner from an old central heating boiler but so-far I haven't managed.

Will

Back to the cylinder bore for a moment - I think I've got a supplier for Veritas lapping grit (silicon carbide). After boring should I hone the cylinder bore with the 600 grit or should I use a coarser grit (e.g. 400 grit) first then the 600 grit? I don't mind if using the 600 grin means that the honing will take a little longer - washing everything to change grit would take a lot of time as well.

I'd like to use 1000 grit or paste but the 1000 grit always seems to be diamond, not carbide.

Will

Lap with the coarsest grit then the next finer and work through them all to the finest you can get.

If you start straight away will the 600 it will take you forever particularly as you said the finish off the tool was not that fine.

This is a good supply of polishing/grinding media - Its used in lapidary (polishing stones) http://www.ukge.co.uk/uk/grits.asp

Hi Jason,

Thanks - I'll take different grade of sandpaper and make scratches on some scrap steel sheet with it then I'll compare the visual appearance and feel of each 'grade' of scratches with the visual appearance and feel of the bore and that should give me an idea which grade to start at (hopefully...). Veritas go all the way to 90 but I suspect that that might be too coarse. I'm guessing from the feel and appearance of different grades of sandpaper compared to the bore that I should maybe start at somewhere round 300 - does that seem reasonable? I'll try it out as described above and let you know how I get on.

Hi HomeUse,

Thank you very much - this is the only place I've seen 1000 grit carbide for sale. I might also order some more stuff from them and get around to polishing some of the stones I find on the mountains some day (or some of the stones that find me in the case of rock falls...)

Will

Hi Will R - I think they still do the 1600 grit - I use this mixed to a thickish paste with veg oil (Usually Linseed) and find that it is a good polishing compound for all metals

Thanks - I think I need to be a bit cautious though - there seems to be agreement that the cylinder should be ground to a finish that still has microscopic scratches because these microscopic scratches help to hold some oil to lubricate it and that a polished surface with none of these scratches wouldn't hold the lubricating oil so well - I might be wrong in this though.

I've found someone who carries stock of "Steel 50" and "Steel 51" now - unfortunately I'm having difficulty working out what Steel 50 or Steel 51 means - can anyone suggest what it's machining properties are like or what the equivalent BS number would be?

Sorry - one more question - if I get a diamond grinding wheel for honing my HSS (or carbide) lathe tools should I get the "hard bonded" or "soft bonded" type?

I found some advice on hard bonded and soft bonded diamond blades here

**LINK**

- I'm not sure how well this applies to diamond grinding wheels but I think it seems to imply that for finishing something hard like a lathe tool I should use a soft bonded diamond grinding wheel - I could be wrong though - fingers crossed...

I haven't got anywhere with finding out what "Steel 50" and "Steel 51" mean - think I'll just need to give them a try and find out what happens - any suggestions welcome!

What's the maximum temperature of superheated steam that would normally be used in a model stationary engine?

I've been trying to make a final selection of metals for my engine. I'd like to quickly check coefficients of thermal expansion to make sure that the main cylinder and piston valves don't cease when it heats up (I think I'm OK but I'd like to do some quick calculations to check). What is the maximum temperature of superheat that model engineers normally use? (I know that initially I'll be using 120 C warm fog but I'd like to design the engine so that it is hopefully capable of running on superheated steam in the future.)

I reckon that the thermal expansion might be smaller then my machining tolerances but I'd like to check and make sure.

I think I found an answer from a lubricating oil manufacturer - they state "Light Super Heat
550 Farenheit" 550 Farenheit = 288 Cenigrade so that should give me an idea of the kind of temperatures involved. 288 Cenigrade seems like a high temperature to use in a model so please let me know if I'm wrong.

Sounds about right as Model Engineers Handbook says 500F could typically be found on a 5" loco.and goes on to give tables for upto 700F so base your calcs on say 300C and you should be fine.

If you use a cast iron cylinder, piston set.. no problem, if the mix is bronze cylinder, and cast iron piston, no problem, except that as they get hotter, the gap will increase a fraction, greater coefficient of expansion for bronze. Ian S C

Thanks very much. I have to admit that when I started out I didn't realise that model engineers used so much superheat - all adds too my respect to folk like Greenly and their small and amazingly powerful engines. I'll see how the thermal expansion works out - was thinking about using bronze and steel for the piston valves - I'll play around with the numbers and give it some thought.