Wheels, axles and flycranks

Wheels

Experience from the SNBR, plus advice from someone who'd already built this project, suggested that the best way to make the wheels is to turn them from solid. Cast Iron ones are fine, but get grooved quite quickly by the railhead. Therefore, 4x8" diameter, 28mm thick steel blanks were obtained from Parkside Electronics. Work then commenced on hacking 6.75" diameter wheels out of these. The process was:

  1. Drill to suit the available wheel turning jig
  2. Turn outside diameter down to just over 7.25"
  3. Rough out the tyre (not yet coned)
  4. Profile the tyres, including coning the tread, radiusing the flange etc.
  5. Finish all four wheels to have exactly the same diameter
  6. Dish the centre section (to make them look like cast iron centres with steel tyres)
  7. Bore out and ream the axle hole (1" diameter)

About 12 hours effort was expended to turn them from the solid.

Axles

The axles deviate from the drawings a little. I was very concerned about the axle end design – as drawn, the axle end (over 3.5” of it), which holds the wheel, axlebox and flycrank, needs to be a push fit for the wheels and they have to be pushed right on. This strikes me as difficult to achieve. I chose to have roller bearing axleboxes, and these are 25mm inside diameter. Indeed, I used take up units as combined axlebox/bearing units. They may not look pretty, but they’re functional and save a lot of machinig work. Thus, there is a small step between the 1” (25.4mm) axle seat for the wheel and the part for the bearing. I managed to find a 15/16 reamer, which is slightly less than 25mm, so the fly crank seat is slightly smaller again. In this way, each part has its own specific seat, and they can all be pushed on easily.

The keyways for the flycranks were machined using a technique I found on the internet. It is so simple, yet ensures correct quartering. Take a short (say 2”) length of square steel (in this case 1.5” square) and bore it EXACTLY in the centre to suit the axle diameter (1.125”). Cut in half, and tap each half for a grub screw (to attach it to the axle). Put each half at one end of the middle part of the axle, stand on a surface plate (to ensure that the two parts are exactly flat) and tighten the grub screws. Now, put it in the milling machine vice and press the square steel right down oin the bottom of the vice. Tighten, and mill out the slot at one end. Now, loosen the vice, and rotate the axle 90 degrees (remember she’s right hand lead). Tighten and mill out the other keyway. Now, fit the squares to the other axle, ensuring the same surfaces are pointing up. By using this method, even if the square you use is not perfectly square (althought there’s no reason it shouldn’t be), the angle between the two keyways will be the same.

Next, I loctited the wheels on to the axles, not forgetting to put the 4 eccentrics on the rear one. Then, I fixed the bearings on with loctite bearing retainer.

Flycranks

The supplied castings are way oversize, and you end up with a lot of iron in the bin. This also means that they take a long time to machine. I reamed out the center hole, then cut a keyway. I set the chuck with the end of the crank pointing towards me, engaged the backgear (to hold it all still), with a weight to take up the slack. Then I put a length of ¼” tool steel (which I’d checked for size – it was .247”) at center height + 1/8”. Now, using the saddle, I moved it in and out of the crank, advancing the tool 2 or 3 thou at a time, to cut the keyway. You can feel when the tool needs advancing again – it may well take several goes to remove the metal. Advance the tool slowly, and take your time and you’ll get an excellent keyway.

Crankpins

I decided to use washers to retain the rods on the crankpins – the drawings are not specific as to how it should be done. Therefore, the crankpins have an M6 tapped hole at the end, with some countersinking, so that a suitable washer and cap screw can be put on. The rear left hand one will also drive the lubricator, so I made it longer than the design and milled a square at the end to hold the return crank for the drive. The crankpins were then loctited into their respective fly cranks.

Final Assembly

Finally, the fly crank/crankpin assemblies were loctited (with suitable keys) on to the axle ends. Now (at last) I had the means to put her on her wheels !!!

Postscript

Having made up these assemblies, they were fitted to the frames. Then, I completed the connecting rods and tried them on, but they were binding badly. Measurement against the frames (by eye) showed that the rods were identical and the correct size. This left just one possibility – that the quartering of the fly cranks was not the same. By resting the axles on the lathe bed, resting one crankpin on the cross slide, then locking the chuck against the other, I was able to compare them. Unfortunately, I found that they were, indeed, different. So, I had two options:

·        Make a new axle up, or

·        Remove one flycrank and reset it

The first option would take a long time, be expensive, and could lead to more inaccuracy, so I decided to go for the second option. First, I drilled out the key. Then I heated up the flycrank and removed it. Then, I put the rear (assembled) axle onto the lathe bed, with one crankpin resting on the toolpost. Then, I moved the chuck so that one of the jaws was touching the other crankpin. Then, I engaged the backgear to keep it in place. Carefully, I removed the axle, and put the other one in place. A bit of loctite on the end of the axle, slide on the flycrank, and leave it there to set. Phew! If only I'd checked it the first time....