If you hang around this forum long enough you will see mention of Tubal Cain's "Building Simple Model Steam Engines". It's a fantastic book and even if you have no intention of building things for yourself it will greatly help you to understand how toy steam engines are constructed and work. But I do build things for myself. There is a very well equipped model engineering workshop here at Henry Towers that sees activity most days. Most of the projects I have been working on over the last couple of years have been fairly complex so I thought it would make a nice change of pace to build something simple. Polly is about as simple as they come. The nice thing about Tubal Cain's engines is that they are very easy to modify to suit the tastes and available materials and tools of the builder. Building an engine like Polly also means that I get to play with all the toys in the workshop. So here is my version of Polly - erm… "Polly-esque"... "Polly-ish"... "Polly 2.0"?
Just need to clean up a few parts and stick them together...
Time to give some attention to the portface and port block. Because I have very definite ideas about how I want the finished engine to be I deviated slightly from the plans in the book. Brass bar was sawn roughly to length then the ends were squared up and machined to their correct length on the milling machine. Then they were coated in layout fluid ready for marking out.
These are not very big parts - only 22mm long. There are many different ways of marking out such pieces but I like to use a height gauge. A drill bit or bar of known diameter is used to set the pointer which is locked in place and then a line is scribed on the workpiece. I thought it would be a good time to make the pivot for the cylinder - mainly so I could check that parts were lining up correctly. I decided to make it from some 3mm mild steel bar. I've got some 3mm silver steel (somewhere) but EN1A-Pb is easier to work with especially when you want to cut threads. Trying to grip small diameter steel bar in a 3-jaw chuck is not going to work very well in conjunction with a tailstock die holder. The bar will spin in the chuck and threads won't get cut. Fortunately I have ER collets which grip the bar really firmly.
The stages of making the pivot -
Going from right to left: Find some 3mm bar and cut it to length. Face the ends in the lathe. Chamfer the ends on the lathe. This makes it easier for the die to start cutting the threads. Cut the threads. Why are there so many parts? I only need one of each. Well, I like to try different techniques and sometimes* I make mistakes. It can be a little bothersome** to find you've poked a hole in the wrong place after several operations have been performed on a part and you have to start all over again.
First attempt at making the parts. Good job I've got those spares. Right? * More often than I care to admit. ** Understatement of epic proportions.
I have been giving some thought to how I want to heat the boiler. Since it's a "simple" steam engine the simplest solution might be something like this -
The aluminium base of a tealight candle. Add a good dollop of gel fuel and away you go.
But I thought it would be nice to have a meths burner. There are all sorts of designs of meths burners that would work perfectly well here though in this example I decided to go for something similar to Tubal Cain's design from his book... sort of.
Meths (a.k.a. Methylated Spirits, Denatured Alcohol, etc) is not what it used to be since it is now primarily used as a cleaning product and degreasant rather than a fuel. You may get better results using bioethanol. YMMV.
As he says in the book, "First find your wick..." I have some fibreglass wick that is (nominally) 5mm diameter. Fibreglass wick is not the cheapest option but it does last much longer than cotton wicks. A wick needs some kind of support to hold it in place. Tubal Cain's burner uses three wicks (or two, it depends on what size wick you can find) and the parts that hold them need to be identical. Making singular items in the home workshop is real easy. Making multiple identical parts requires a little planning and forethought. The C3 was already set up with the collet chuck and I decided to use the same brass bar I used to make the bearings from.
First the bar was faced and then drilled. Experimentation had shown that a 4mm drill would make a suitable hole for the wick. The hole was drilled deeper than the length of the wick holder. This will be important in a little while...
With the carriage stop set, the bar is turned down to about 6.0mm diameter for a length of 12mm. Because I did not want to change the settings on the C3 the bar is removed and set up in the C0 for parting off. (Oh the joys of having more than one lathe!)
Ooh! Look! An "action" shot!
A wooden cocktail stick held in the tailstock chuck prevents the part from falling into the mound of swarf beneath the lathe. I could have just sawn the part off but this way the bar is faced again before it goes back to the C3 for more drilling and turning another 12mm. Repeat the above steps to make more wick holders. I could have stopped there. After cleaning up the tops of the wick holders they would have functioned just fine. However, they looked a little plain so back to the C0...
The tops were countersunk to make it easier to stuff in a wick. Then the compound slide was set up so a 45 degree chamfer could be turned. A set of angle gauges make this task very simple and accurate. The brass bar got shorter and the pile of wick holders grew.
They will go in the burner something like this. The C3 is great for general lathe work and for "shifting metal". However, for fine, detailed work the C0 excels. It's also a lot quicker to set up too!
Time to make the bearings for the flywheel/crank shaft. Because the next few jobs on the C3 lathe will involve round bar less than 12mm diameter I changed the 3-jaw for a collet chuck. (This isn't the only project on the go at the moment and I tend to do jobs in batches depending on how my machines are configured.) If you have a lathe collet chucks are very handy for working on small(er) diameter round bar and tube. I use ER collets on the lathe because they are also used in the milling machine and for general work-holding e.g. Stevenson's collet blocks.
A length of brass bar is faced in the lathe before turning a step to match the holes in the engine standard. Doesn't have to be too precise - a slip fit will do. Any minor errors will be corrected when the engine is soldered. Next a hole is drilled slightly undersize for the 4mm shaft. Then the hole is reamed 4H7. This does need to be precise. Finally the bearings are parted off to length. About 5 or 6mm long will do for the one that goes next to the crank. The length of the one that goes next to the flywheel must be long enough to prevent the flywheel fouling the firebox.
Tubal Cain goes to some detail about what flywheels you can use on his engine designs and the different methods of making them depending on what facilities are available to you. Since Tubal Cain wrote his books many more model engineering suppliers have come along and it is now very easy to get ready made and part-finished flywheels for a reasonable sum. This can save a lot of time and effort. It is also a considerable boon for those model engineers who do not have access to larger lathes. In this instance I will be using a 50mm flywheel I got from Winfried Niggel.
Making more bits. The base and firebox are made from 0.5mm tinplate. I spent a happy couple of hours marking everything out before using a centre punch to mark the locations of the many holes that will be required. Time to play with the 3-in-1 machine. For those with limited space in their workshop a 3-in-1 machine is a very useful thing to have it you're doing sheet metal work. It can cut, fold, and roll sheet metal. Working with thin sheet metal presents its own challenges. Ideally you want to avoid distorting the pieces you make while you are working on them. The major cuts were done on the 3-in-1's guillotine. Then it was time to make the holes. Drilling holes in thin sheet metal can be tricky so I decided to punch them instead. Because I do quite a bit of sheet metal work I have a Roper-Whitney No. 5 hand punch. This is an incredibly useful tool and I do not know why they are not more commonly available in UK. The opening for the burner was cut using a fret saw and bench hook. Finally the firebox was rolled in the 3-in-1 machine.
The engine standard was marked out on 1.0mm brass sheet. The holes were punched before cutting out the standard with a hacksaw. (One day I'll have a bandsaw to make jobs like this easier...) The curved section was formed by hammering the standard over a steel bar. Once the curvature matched the firebox the ends were folded in the 3-in-1 machine. It occurs to me that the engine standard from Polly would also work well on a horizontal boiler... Well, that's the next project sorted. What I love about engineering, and model engineering in particular, is that there is always more than one way of doing things. If you want to build an engine like this using different methods and tools no-one will mind.
Making the boiler is really straightforward. There are no surprises or "gotcha"s along the way. I decided to silver solder it rather than use soft solder. There's nothing wrong with soft soldering a boiler - plenty of commercial toy steam boilers use it and we have all enjoyed playing with them for many years with no problems. The advantage with silver solder is that the boiler will suffer no harm if it runs dry. There's no sight glass on this boiler (though I did add a water level plug) so there is always the risk that some future owner might be tempted to run the engine after refilling the lamp without checking the water level - regardless of firm warnings! The thing with silver soldering is you need heat - lots and lots of HEAT. The parts to be joined have to get up to around 650C for the solder to flow. This is one instance where having the right tools for the job is important. The type of blowtorch used is dictated by the mass of metal you are heating. As you heat the metal it is radiating heat away to its surroundings. When making boilers, even small ones, a small DIY butane torch is no good. For this boiler I used a propane torch with a 25mm head. For smaller jobs, e.g. adding union cones to pipes, a smaller burner can be used. For larger boilers I use honest-to-god liquid fuel blowlamps because they give a nice bushy flame that heats the metal well. While not as convenient as gas torches they are very effective. A word of caution - novices who have only ever used butane torches may find they suddenly need to change their underwear the first time they use a blowlamp!
The boiler was assembled in stages. First the top cap was soldered in place. Once I was satisfied with that the bottom cap and centre flue were added. Finally the bushes for the safety valve, steam outlet and water level plug completed the job. To prevent the top cap moving during the soldering process I was going to use a couple of rivets to hold it in place and file them smooth afterwards. Then I thought "Why not make a feature of them?" It wasn't that much more effort to mark out and drill eight holes spaced equidistantly around the boiler shell. The rivets are simply pushed into the holes. Structural integrity comes from the silver solder. For the bushes on the top of the boiler I used silver solder paste (55% Ag). It is not a cheap product but really worth it when neat joints are called for. The steam outlet will have a standard union cone and nut rather than just soldering the pipe to the boiler. This will make future cleaning and maintenance much easier.
The boiler was tested to 60psi. Its operating pressure will be around 20psi. The boiler is designed to use a modern Wilesco safety valve. I like these safety valves. They work well and are readily available. They also have a 6x0,75mm thread which discourages the hard-of-thinking from replacing them with an M6 screw in the hope of getting "more power" from a boiler.
Although I am building this for me I am mindful of the fact that the finished article should be good for 60 years or so. I would like the future owner(s) to enjoy using it in safety for a l-o-n-g time.
The first job is making the boiler. Well, boiler ends actually. In this case the boiler is made from 1.0mm thick brass. Brass is a perfectly good material for toy steam boilers. It polishes up nicely and is waaaay easier to machine than copper. (You have no idea how much I loathe machining copper!) A quick rummage in the workshop produced a former I made some time ago for 2" vertical boiler ends. It's made from aluminium (mostly). Much bashing with a Thor #1, plus several rounds of annealing, produced the end caps.
I find it easier to poke holes in the end caps while the metal is still a flat sheet before the ends are cut out and flanged. The former also acts as a mandrel to hold the end caps when it's time to tidy them up on the lathe.