Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch

Wheels, Axles and Cranks.

1. Wheels.

Cut four slices approximately 8 mm thick from a piece of 45 mm (1 3/4") mild steel bar. Chuck each of these in turn, face the cut surface, turn round and face the reverse. Drill a 4.2 mm hole and ream 4.5 mm for the axle. Repeat for all four driving wheels.

It is quite common that your chuck is slightly out of true. Therefore to ensure the wheels are concentric, make a wheel turning mandrel to fit in the chuck as follows. Firstly, cut a 30 mm length of 6.5 (1/4") mild (or silver) steel rod. Thread one end 1/4" by 40 tpi for 10 mm. Place on one side. Take a piece of mild steel bar, around 30 mm diameter and 20 mm long. (These dimensions are not critical.) Mount this in the chuck, face the end, centre drill and drill 7/32" and tap 1/4" by 40 tpi. (If you wish, mark the mandrel alongside No 1 Jaw of the chuck for future use, although you may find it is not accurate on a second use. However, retain the bar as you can easily make a new mandrel for another set of wheels.) Hold the piece of 6.5 mm rod in the tailstock chuck and screw it into the piece of bar as tightly as possible. Loosen from the tailstock. Face the end of the rod then carefully turn down until one of the wheel


blanks will just slide along the rod without any slackness. From this point on ALL the wheel turning must be done without disturbing the mandrel. Thread the end of the rod 4 BA leaving approximately 6 mm plain to mount the wheels. Place a wheel blank on the mandrel and tighten a lock washer and 4 BA nut to hold the wheel firmly.


Using a freshly sharpened tool, turn the wheel blank to 43 mm diameter. Note the setting on the cross slide. Remove the blank, and repeat with all the other wheels. Now turn the tread of the wheel to 40.5 mm diameter for a length of 5 mm. Remove the wheel and repeat for all of them. Now, after re-sharpening the tool, turn the wheel to 40 mm diameter. Note the cross-slide setting. Remove the wheel and do the same for each one in turn TO THE SAME SETTING. You can leave the treads parallel if you wish or turn a 2 degree taper on them by now setting the cross-slide over to the desired angle and working from the flange root, carefully turn the taper. Again repeat for each wheel. . Using a file, break the edge where the face meets the tread, to leave a slight bevel. Also, using the file, shape a slight angle on the flange, and round off the edge. You should now have a set of wheels that are all the same diameter.


To turn the recess in the wheel face, use a round nose tool with sufficient overhang or a boring tool. Treat each wheel in turn noting the cross-slide setting for the first one and working to the same measurements. You have now finished with the mandrel for this set of wheels. Remove it from the chuck. You may use it again with a fresh stub axle for the pony wheels.

Take a wheel, mount it in the chuck gripped by the tread, with the flange pressed hard against the jaws. Face off the back of the wheel until the flange is 1.5 mm thick. Note the setting on the top slide. I usually lock the saddle traverse and use the top slide to ensure that all the wheels finish to the same flange thickness. Before removing the wheel from the chuck, use a file to create a slight bevel at the flange. As before repeat with each wheel in turn.


I usually secure my wheels to the axles with 8 BA grub screws. A 1.8 mm hole is drilled through the wheel boss into the axle hole and tapped 8 BA. If I am certain that I will not want to move the wheels at some later date, I also drill a 1 mm hole and secure a pin through the wheel and axle with Loctite. Alternatively, drill two 8 BA holes and use two grub screws for each wheel. This model is capable of changing gauge, and therefore you may not wish to secure the wheels on the axles permanently. However, as the frame axle bushes are removable it is possible to remove the complete wheelset and replace with a different gauge.


2. Axles

The axles are cut from silver steel rod of appropriate size. A small centre pop in the end of each axle has a nice effect and may be useful to check the coupling rod dimensions. You may leave the axles overlong until the cranks are made and then by fitting the crank to the axle and adjusting to allow free rotation in the axle bushes, the finished length can be marked and cut to size.


3. Cranks



The cranks are cut from 6.4 mm (1/4") mild steel bar. To ensure that all the cranks have an equal throw, we need to make a jig for drilling the crankpin holes. This is shown on the drawing, made from a piece of 10 mm by 4.5 mm mild steel. A short piece of axle steel is fixed into this (make the hole a push fit) and a 3.2 mm hole (see note below re: 3.1 mm) drilled at the crank throw dimension. The jig should preferably be case-hardened by heating to red heat, dipping in 'Kasenit' and quenching in cold water. However, if you are only going to drill 4 cranks with it you can probably get away without hardening it.

Cut four pieces of bar long enough for each crank. Drill the axle holes first in each piece, taking care to ensure that these holes are perpendicular to the crank. Place the blank on the jig, clamp in position and drill through the hole in the jig. (NB. Although the hole size is shown on the drawing as 3.2 mm, if you drill the hole 3.1 mm, then just enter the taper of a 3.2 mm reamer into the hole, you will probably find that the crank pins will then be a firm push fit into the cranks. If they are slack they can be secured with Loctite 601 and should also be pinned.) Do each crank in turn. File the crank to the outline shape. Drill and tap for the 8BA fixing screws. Note that the cranks are relieved at the rear. Chuck a short stub of axle steel in the lathe. Fasten the crank to this with the rear face towards the tailstock. Face the crank for 1 mm to form a boss. (One advantage of this boss, is that should the clearances between the crank and the crossheads be a little tight, you can file away some of the boss to ease the restriction.)


4. Crankpins

 These are made from 3.2 (1/8") silver steel. Cut to length shown on the drawings. The front pins are centred, drilled 1.8 mm and tapped 8 BA. A hexagon headed steel screw is fitted to retain the coupling rod. The rear pins are turned down to 2.2 mm diameter for 2.5 mm length. This is then threaded 8 BA for the return crank. Push the crankpins into the cranks, retaining with Loctite 601 if not made to be a push fit. Alternatively drill a 1 mm cross-hole through crank and pin. Insert a piece of 1 mm silver steel to retain the crankpin. Use adhesive to retain the locking pin. The coupling rods are retained on the front crankpins by 8 BA nuts and steel washers.

If you have any questions, or comments or find any errors in these notes please contact me by email. Email Keith Bucklitch.

Copyright © 1998, Keith Bucklitch
Last Edited - October 1998