There is always the possibility of some small variation in the size of castings. To get the exact thickness of the tyres and sidewalls you can take the actual internal diameter of the casting and add to it twice the scale tyre thickness for your model outside diameter.

MY PREFERRED METHOD
Measure the casting and make a sketch of its cross section to find the safe areas outside the tyre where turning shoulders can be cut. Also draw the tyre to show clearances in front and behind the tyre. I usually remember to send copies of my 6x4" workshop card with the castings.

Mount the casting BACK OUT on its cast shoulder and cut a small shoulder on the outer edge in the safe area outside the flanges, I set from the back of the tyre plus 1.0 mm for 1.5 mm wide flanges and 0.5 for 1.0 m wide flanges (See diagram 1).

TURNING THE FRONT OF THE CASTING
Mount the casting FRONT outwards on your new shoulder in a 4 jaw chuck centring with a dial indicator.
Take a cut over the tread at about 0.5 more than the final diameter, and just short of the root of the flange – usually about 1.2 mm out from the chuck face. (See diagram 2)
Skim the hub just enough to make a level surface. There may be a low hump in the centre (for contraction allowance) to be removed.

Face the front of the tyre (the turning card will show how far it is below the hub). It is at this stage that you will start to see the benefit of using vee pointed tool end on to the work, the depth of cut reduces to nil as you reach the inner edge – which cannot then crumble as it might with a shaped insert. For best appearance use a slow rate of feed. Be very careful not to cut into the weights of some wheels which should project beyond the tyre as in the LNER A3. Turn the chuck by hand to deburr the inner edge with a hand scraper.
Centre with a Slocombe drill and drill (eg with a Titanium coated stub drill) - this will need to be undersize for press fitting. Eg for 1/4" drill 6.3 and ream 6.33 or 6.34

FACING THE BACK
Mount the wheel BACK OUT, centre with the dial indicator in the bore, cut to the tyre thickness of your chosen standard.

TURNING THE TYRE PROFILE
Mount an aluminium backing disc in the jaws of the lathe (See diagram). This is a double sided disc with a step turned on the inner face to sit on the corners of the chuck make the back step at least 50 mm dia to help reaching in with spanner to a nut on the back of the mandrel), its outer face is to the nominal diameter of the wheel, drilled as a loose fit for your mandrel.

Take a small cut off the outer face of the disc each time you mount it. Pass the mandrel through from the back, then on the front the casting, front disc and nut. Make sure that the front disc is sufficiently recessed to clear the hub so that all the grip is on the tyre sides. As the casting is slightly loose on the mandrel, finger tighten the nut, tap it central with a dial indicator and tighten up.

Make several cuts at the appropriate angles, or make one more accurately repeatable but slower cut with a form tool (if your lathe will turn slowly enough). If using a form tool, line it up carefully, lock the saddle and turn slowly to size. I use about 0.03 mm/rev feed at about 40 to 70 rpm, depending on diameter. Hand turning the chuck with the lathe in neutral for the last little bit gives a smoother burnished finish. Wheels gripped at the centre only may slip or break the spokes during turning so ALWAYS GRIP BETWEEN DISCS WHEN TURNING THE TYRES, otherwise breakages are at your risk.

Slip off the front disc, replace it with a smaller spacer, recentre, retighten, and turn off a small bevel on the front edge of the tyre.

MOUNTING WHEELS ON AXLES
See under 'AXLES' for a note on adhesives and press fitting sizes. For press fitting I make discs recessed to fit the wheel exactly with just the right extra allowance at the hub to prevent either wobble, if too little or axles pushing through if too much. In the back I leave a 10 mm dia stem to hold the discs central. This could be forced on between the lathe chuck and back post, though I prefer to use a machine vice converted with new jaws having 10 mm holes dead opposite to each other. Slots are milled in at 90º the width of coupling rod pins for quartering. The press discs are very handy to use as sizing gauges during turning to make sure the wheels are all the same size.

FORM TOOLS
I can supply drawings for you to have a form tool made by your local tool maker. State whether for 2.0, 1.5 mm or Scaleten flanges. Toolmakers may charge about £110 for a tool (£80 for the work plus £12 for the steel, plus VAT.)

CARBIDE TIPPED TOOLS,
the sharp pointed shape known as 'roughing' is good for cast iron. There is little chance of edge crumbling as the cut depth reduces to nil as you finish a cut across a tyre.

SUPPLIERS include GLR Distributors at Unit 1, Geddings Road, HODDESDON, Herts. EN11 0NT. Tel 01992 470098