There is a wide range of cast iron wheels available from the National 2 ½" Gauge Association that can be used for building James and Sam and also some of the other prototypes. The casting No. 6916 for a 5' 2" diameter wheel with 16 spokes and crank pin on the spoke is correct for Sam. There is sufficient metal to enable this to be turned to close to 5' 4" diameter. A correct casting for James with 18 spokes and the crank pin between the spokes does not exist, however there is one that has the crank pin on the spoke - No.7518, so yours truly has been obliged to "do-a-Nelson". Full size wheel standards Before describing the manufacture of the wheels for the model, it is perhaps timely to explore the topic of wheel standards. The drawing below shows the cross section of a full size driving wheel, manufactured in this case by Dubs in 1892. Driving wheels manufactured in the UK were generally 5 ¼ inch thick. The wheel centre was originally made from wrought iron until the use of cast iron was developed by Mr Webb of the LNWR. The tyre was made of steel and shrunk onto the cast iron centre using special heating device that enabled the steel rim to be enlarged just enough so that it could be slipped onto the cast iron centre. The width of the tyre in contact with the rails is generally 3 9/16. The drawing below is of a wheel that is 5 ½ inches thick with a tyre width of 3 13/16 inches.
The drawing below is fromThe EngineerJuly 1889. The narrative extract describes the American practice for manufacturing driving wheels which had hollow spokes and makes some interesting and acerbic comments illustrating that American practice was not well thought of, this side of the pond.
"These cast iron wheels look rather heavy, but are not otherwise unsightly, the outlines and sections of the spokes and rim are more rounded and smoother than the type of cast iron wheel introduced into England by Mr Webb; the whole appearance conforms more to that of the wrought iron wheels, and although the sections are not such as cast iron structures would be expected to take, and in theory are not so good as those used by Mr Webb yet the whole wheel looks better than his design, has ample strength, and does not offer receptacles for the accumulation of dirt, and is therefore much easier to keep clean and inspect. These driving wheels and tyres strike one as being very far from first-class practice. The tyres are also held on in a very crude and apparently haphazard way, which an English engineer would not think of pursuing." Model wheel standards There has been much debate in the National 2 ½ " Association during 2010 and also in Gauge 3 Association about wheel standards. The standards adopted by the Gauge 3 Society were set by Alan Headech some 20 years ago and specify a wheel width of 8 mm or 312 thou, a wheel tyre of 5.5 mm, flange depth of 2 mm and a tyre cone of 2 degrees. The back to back measurement is set at 58 mm. The 2 ½ Association have recommended that these standards are adhered to by their members as the vast majority of models built to LBSC, Martin Evans and Don Young designs all used these same standards. Fine scale wheels are all very well if you feel that the wheels have a steamroller visual impact. However there will be some difficulty is running fine scale rolling stock on other tracks which are not to fine scale standards. The main problem is that wheels will tend to ride up over the frog when traversing through points, unless the check rails are set to finer dimensions. Increasing the back to back to 59 mm will make this even more important. The picture below shows three driving wheels turned to different standards, can you really spot the difference? The "finer" scale wheel has a tyre, that part which is in contact with the rail, that is very close to scale at 4.4 mm whereas the true scale is 4.1 mm. Extreme Right - Standard profile 0.312 (8 mm) thick, an Alan Headech Reference wheel Middle - James driving wheel at 0.290 (7.5 mm) thick Extreme Left - "Finer" scale wheel at 0.270 (7.1 mm) thick
Wheel turning Wheel turning has been described many times in the model engineering press so this is not going to be an exhaustive description, however certin important points learnt over the years are stressed. Experience shows that the average time taken to make a set of wheels for a live steam locomotive consisting of driving wheels and bogie or pony wheels, is on average one hour per wheel. Unfortunately turning cast iron wheels is a very messy business and generates a lot of unpleasant grit and dust which seems to get sprayed everywhere. Wearing thin disposable rubber gloves (Lakeland ) will save your hands a lot of agro and safety glasses are advisable. The castings available from the National 2 ½ "Association, which now incorporates the Alan Headech designs, seem to be quite rough these days with a lot of casting tags that have to be filed off from the rims, prior to mounting in the lathe for turning. The quality of the cast iron also varies, sometime the iron is a lovely fine light grey colour and turns beautifully, and on other occasions it is much darker in colour and gritty and hard. After filing off the rough bits, turn the backs of the wheels in the three jaw chuck. Use a very slow speed using the back gear and a tungsten tipped tool to cut through the outer skin of the casting. Change up to a faster slow speed and cut the wheels until they are circa 1 mm from the desired thickness. Turn the casting around in the chuck and centre the wheels until they spin nicely such that the inner rim appears to spin as evenly as possible. Some wheels castings are quite perfect and concentric, whilst others can almost never be properly centred, no matter how many time you try - oh well! The next step is to drill and ream the axle hole to 5/16 inch using a selection of sharp drills finally using a 15/64. Tip - when reaming at 5/16 introduce the reamer very slowly, both inwards and outwards, as this will prevent any scarring of the axle hole which may cause difficulties when the wheel is mounted on the turning mandrel. Wheels can get stuck fast on the mandrel if the hole is scarred or if the mandrel is slightly too large.
The mandrel can be made from a length of easy cutting mild steel rod about 1 ½ inch long and say 1 inch diameter. Turn 8 mmvery carefullyas close as possible to diameter 0.312 and dead parallel to take the hole reamed in the wheel and to be a tight but sliding fit. If it is too tight you may have difficulty getting the wheel off. The end of the mandrel is turned ¼" diameter for a screw thread, BSF 26 or similar for a retaining nut, washer and spring. Countersink the back of the wheel, clean the inside of the hole with kitchen paper or loo paper to remove any stray cast iron dust before mounting on the mandrel, tighten up with the washer, locking nut and spring and off we go.
Turn the face of the wheel by just taking a small amount of the face, bearing in mind that you have already cut the thickness down to circa 1 mm of the final width. Finally turn the wheel tyre to the final profile using a cutting tool shaped with a 1 mm radius end at the tip using a slow speed but without the back gear. Rub the tool on a carborundum stone to sharpen and turn finally at higher speed to finish. Before removing the wheel from the mandrel, use a fine file to round the tyre edge, and if you wish polish the wheel tyre with very fine emery cloth.
To turn the bogie wheels, reduce the diameter of the mandrel to ¼ inch and if you are careful you might get away without having to reduce the diameter of the spigot for the retaining nut.
To finish the driving wheels, make a jig for drilling the crank pin holes. The crank pin throw for James is 11 inches, so a jig was made using a piece of gauge plate drilled to take a bit of axle turned to 5/16 at one end with a shoulder such that it can be inserted into a hole drilled in the plate. The drill hole of diameter 0.100 or 2.5 mm for the crank pin is 12.5 mm from the axle centre. The plate had been tempered to dark straw or blue, just in case I might use the tool some time again for other wheels. Hold the jig firmly in place and drill the crank pin hole right through.
Here are the eight wheels, ready for the next stage, making the axle and axle box for each pair of wheels.