Member postings for Will Robertson

As folk have described, fire-bricks can be used to form a mini-furnace to reflect heat back to the onject being soldered but another option is to use ceramic wool as the lining of your mini-furnace - it's got a lower specific heat capacity so heats up faster.

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

Apologies for my silence - work's been heavier than usual and there's been a delay getting carbide tips through customs.

Hi Clive, Thank you for the SAE 303 and 316 steel grades - now that I've got these I've looked up the chemical compositions of the steels and their properties and have a number that I can hopefully use in dealing with suppliers Thank you also for your advice about clearance and higher cutting speeds - what sort of cutting speed and feed rate should I aim for for these steels? (At the moment I don't have any cooling.)

Re. Dressing off with a stone, should I use the fine diamond stone I use for honing HSS or should I use a different sort of stone?

Hi John, Unfortunately, at the moment my steel supply comes as random lumps of unidentified scrap - so when I find a piece that works well there's unlikely to be another piece the same in the box I'll try to build up experience to tell more quickly whether a given lump is likely to work or not. Let me know if you can find any sort of identification number or classification for the steel that gave good results for you.

Will

Hi John,

Thank you very much - again you've saved me a lot of confusion and heartache.

Thanks for your advice about graphite string and viton o-rings. (I used to have an enormous tub of graphite flakes when I was a child which were a source of constant fascination because of their appearance, texture and electrical and mechanical properties.)

Thanks for your advice about avoiding nitrile and silicone - sounds like it's handy stuff in the chemistry lab but no good for steam (I wish they'd mention this in the catalogues!).

Is there any particular type of stainless steel, brass or bronze that I should use to give a good balance between corrosion resistance and ease of machining? I know there are BS specifications for different stainless steels, brasses and bronzes but I've no idea which one to use. (Here people often talk about 'chrome steel' and say it's got a reputation for being indestructible but difficult to machine.)

Thanks for the photos of the engines - I don't think I've ever seen such beautiful oscillating engines!

Will

Hi Dick,

Thanks - I thought about looking at the hydraulics community for machining cylinders but I didn't think to look there for valves - I'll see what I find out.

Hi Clive, Thanks very much ffor metnionging Reves - i didn't know about them - I noticed that they've got nitrile o-rings - wonder if they'd be suitable for glands?

Will

Aluminium has a low density compared to other metals - typically a more dense metal would be used for flywheels.

Hi John,

Thank you very much for the advice to avoid silver steel - that's probably saved me a lot of heartache.

Is mild steel also too rust-prone for use with steam?

I've found the stainless steel road on RS.

Do you have any general advice about selecting metals for valve and engine building? (I don't have a large enough junk collection so I'll need to buy some of the metal.)

Hi Clive,

Thank you very much for the advice about packing. Any more info about the woven square section PTFE would be great! Is the PTFE tape for the glands just the ordinary Boss Blue or Boss white type PTFE tape used for plumbing?

What did you use to lap the valves? Course and fine valve grinding paste or basso - or something else?

Will

Sorry - I forgot to ask - what type of steel is best for the pistons of the piston valves and what is best for the valve blocks? Would Silver Steel (BS-1407) be suitable for the pistons or would this be too difficult to machine?

I'm guessing that the piston and valve block should be made from dissimilar metals to reduce wear - because of the precision of the valve do we need to think about differences in the coefficients of thermal expansion of the two metals or is that insignificant?

Sorry for another question and sorry if my guesswork is way wide of the mark.

If it's any help, ready-made magnetic bases are available now from the far east via ebay, etc at a much lower price than they cost 10 or 15 years ago so a ready-made solution might be the answer. Modern Neodymium magnets give you a much stronger field than their predecessors.

If nobody is nearby then maybe posting a photo of the motor and a close-up of its terminal block might help jog people's memory. It would also let an owner of the same lathe as you confirm whether the motor is original or a replacement.

Hi John,

Thank you very much - I didn't know you could get ground rod stock. Very excited now about the opportunities it opens out.

Is it suitable for making piston rods as well? (Pistons even?)

Am I right in thinking that ordinary ground stock is OK and that there's no need for the 'mirror finish' sort?

Could you suggest some suppliers for ground rod stock?.

A little steam leakage is fine (I sometimes suffer a little steam leakage myself). My plan is for the valve block to be easily detachable from the cylinder and replaceable so that once the engine is made I can experiment with different sort of packing for the piston steam gland and try making a second version of the valve block with o-rings if necessary.

I thought about an oscillating engine but decided to go for one with separate valves - I was fascinated by mill engines when I was a child and wanted for years to build something like a miniature version of one of them. I also like the flexibility of being able to trim the valve timing. (If I'm crazy enough, in future I might even think about one of those complicated regulators that can adjust valve timing - that's unlikely though.)

Will

And whatever you do, don't wire it up by guess, throw the switch and hope for the best - my late father tried that one and he got a loud bang, a blue flash, a workshop full of smoke and no motor...

Many thanks - good to know!

Hi Shaun,

Motor wiring like this is often beyond the scope of domestic electricians. I reckon that the safest thing to do would be to speak to the manufacturer of the lathe (Myford) or the manufacturer of the motor by phone or email them (perhaps with a photo of the terminal block) and ask their advice. They should be able to send you a data sheet clarifying things.

Bear in mind that it's possible that someone could have replaced the original motor in your lathe with a new motor and that this could be the cause of the confusion.

If you're really confident with motors then some investigating with a multimeter may help clarify things but this is only helpful if you're really sure of what sort of motor you've got and the characteristics of the different coils so in this case it's probably not helpful.

Will

There are digital micrometer callipers available now very economically - I bought mine on Ebay, a few months later a large supermarket chain started selling them as well (albeit at higher cost), I noticed that Model Engineer are now giving them away as a free gift for new subscribers. They all look similar although the manufacturer isn't named.

Has anyone tested how accurate these are? The literatures with mine says "Accuracy: +- 0.02mm/0.001in (<100mm) +- 0.03mm/0.001in (>100mm)" and "Repeatability: 0.01mm/0.0005in".

To independently verify their accuracy I think it would be necessary to compare them to certified precision standards or an instrument with certified calibration but I don't have access to either of these.

The last time I used callipers they were read via a Vernier scale an the electronic version was considered an expensive luxury...

Thank you very much for such carefully thougth out work and for sharing it with us.

Thanks also for the link to the Sandvik and Sumi info.

>I think there's a rule when using inserts that the depth of cut should be greater than the tip radius. I'll have to look into this, I also feel an experiment coming on!

Let us know the results!

Will

Thank you very much - I've started reading Bog's blog and it's wonderful

What level of accuracy do I need to be able to achieve for the valves to work correctly? (I'm using an old borrowed lathe with a 3 jaw chuck - no 4 jaw - so I'm not sure whether it will be capable of the required accuracy.) Paxman's literature about the Paxman-Lentz engines said "Our tools and men are quite accustomed to work to one two-thousandth part of an inch." so I'm guessing that that's the level of accuracy needed for piston valves as well - I could be completely wrong though.

http://www.paxmanhistory.org.uk/paxlentz.htm#brochure

My plan was to do a trial-run of one valve, check it with micrometer callipers then check its function with compressed air. Is this appropriate or can anyone suggest a better way of determining whether or not the lathe I'm using is up to the job?

Thanks again for all your help!

Will

I've got a tool for carbide tips from the garage next door (who're kindly letting me use their small lathe). It looks like it takes 10mm wide triangular carbide tips (ISO Shape code A). Would anyone be willing to give me a recommendation for the best tips to buy to get a high quality finish on steel and brass with light cuts?

Based on what Graham, KWIL and Chris said, I think I've some idea what tip to buy but a recommendation from someone with more expertise would probably be much better!

First thing I'm hoping to do when I get the carbide tips is cut the same piece of steel with carbide tipped and HSS tools and see what gives the best finish. Any suggestions on how to measure the finish without an expensive digital meter would be valued!

Just had a look through a machine tool suppliers catalogue and the new milling tools (some of 'em with 11 carbide tips on one tool) make me standing at the gridnding wheel with my chunk of HSS look like something out of the Flintstones! - Maybe the old technology is best though - only one way to find out...

Will

Thanks - I'll try to give Peter a call.

Thank you for the advice about the chip breaker and DOC. Is this the reason why a lot of people report having big problems getting an acceptable finish with carbide tips?

I've read that the sharpness of tips affects the surface finish - how significant is this?

Thanks for the advice re. ISO - I'll definitely stick to ISO.

Will

I'm trying to design and build my first model steam engine. Having thought over the possible drawbacks of slide valves I thought it might be worth trying to build a piston valve but I can't find much information about designing and building piston valves (all the books concentrate on slide valves there doesn't seem to be much information online about general design of piston valves.).

I'm not sure how to make the cylinder in which the piston valve would operate - it would probably be too small to bore and I'm not sure if reaming would give a good enough surface. At the moment drilling and reaming seems the best option.

I'm also not sure how to seal the piston valve - whether to use an o-ring or cut oil groves in the piston.

Maybe there better valve designs that I've overlooked.

Any advice gratefully received.

Will