A Mechanical Fixing for Stapleton Wheels

Last updated: 27 March 2008

Warning! I suggest you read to the end of this article before trying it as my techniques are still under development.

Overview

I like to be able to dismantle the mechanical components of the kits that I build. That is partly due to a lack of confidence in my ability to build a working model first time around and partly to take advantage of improvements in products and my skills as they arise.

With this in mind I embarked on a project to develop a mechanical fixing for Stapleton 14.2mm gauge finescale driving wheels, available from the 3mm Society. Unfortunately the techniques described here depend on a variety of tools which may not be accessible to the average modeller. However if the method proves to be successful maybe some of the ideas can be incorporated into future products making the advantages available to us all.

The wheels are fixed to the axle using a 10BA countersunk screw. The fixing screw includes a 0.5mm hole which is used to align the wheel in the quartering jig. It is also proposed to use this hole to align an etched cover plate which will include a ridge on the back to locate in the slot of the screw and a 0.5mm hole in the centre to align with the hole in the screw and represent the hole in the centre of the prototype axle.

The crankpin in the Stapleton wheel is replaced with a 1mm diameter tube tapped 16BA through the centre to accept a 16BA screw for testing purposes. In the final assembly the actual fitting will depend on the prototype. It could be a 16BA stud and nut or a 16BA bolt shaped to represent the prototype fitting.

Quartering is achieved using a jig which is a modified version of a jig produced for 3mm Society members by Bruce Hoyle.

Axle

The axle is based on a standard Stapleton driving axle. The length of the shoulder on each end of the axle is reduced to suit the wheel being used. For the SR/BR Mogul kits the boss has been removed from the face of the wheel to bring it in line with the tyre. In this case a shoulder of 1mm was found to be acceptable. One end of the axle is centre dilled in the lathe and then drilled half way through using a 1.4mm drill. The axle is then turned round, centre drilled again and drilled 1.4mm right through. Each end of the axle is tapped 10BA. The holes in the ends are slightly countersunk using a 2.9mm drill.

Screw

The wheel is screwed to the axle using a standard 1/8" long 10BA countersunk brass screw. A 0.5mm hole is drilled through the centre by screwing the screw into an axle which is held in the chuck of the lathe. The screw is centre drilled and then drilled right through 0.5mm.

Wheel

A 2mm drill is used to align the centre of the wheel in a vertical drill. The wheel is clamped to the drilling table and the axle hole countersunk using a 2.9mm drill. At this stage it should be possible to fit the wheel to an axle using a 10BA countersunk screw. The face of the screw needs to be flush, or slightly inset to take the etched cover. This can be achieved by filing the screw head flush with the boss or increasing the countersink slightly as appropriate.

Crankpin

The original crankpin is removed from the Stapleton wheel. If the whole of the back of the crankpin is visible then it can be pushed out of the back of the wheel. However the back of the crankpin is often partly or completely covered by plastic. In this case hold the pin in the chuck of the lathe, centre drill the back of the wheel and then drill the pin out using a 1mm drill. When the wheel starts to spin remove it from the chuck and push it out of the front of the wheel with the end of an old drill from the back.

When the crankpin has been removed a 1mm drill is used to align the crankpin hole in the wheel in a vertical drill. The wheel is clamped to the drilling table and drilled 1.5mm. A 1.5mm OD 1mm ID tube is fitted through the crankpin hole and secured using Loctite 601. The tube is filed flush to the back and front of the wheel. This provides a fitting for the crankpin itself and also a metal bearing surface for the coupling rod. A 1mm OD 0.5mm ID tube is cut to fit though the wheel, coupling rod, connecting rod etc as required. It should be slightly longer than required so that it can be filed back to length during final fitting. This tube is held in a pin vice and tapped 16BA (carefully!). Then it is fitted into the 1.5mm tube using Loctite 601. If the crankpin is left protruding slightly at the back of the wheel the 601 can be applied without risk of running into the screw thread and gumming up the securing screw later. The tube should be filed flush with the back of the wheel. A 16BA bolt and washer can be used to hold the coupling rods on to the crankpin during testing.

Quartering

Bruce Hoyle produced batches of at least two types of quartering jig. The later one of these was used as the starting point for a quartering jig.

The retracting pin assemblies were removed to leave two plates, one fixed to guide rods and the other sliding on the rods. Each plate had a 1/8" hole in the centre with a slot to take the crankpin. The sliding plate can be removed and turned round to suit a left or right hand lead.

Two "screwdriver blades" were produced from 1/8" silver steel rod. The centres were drilled to take a 0.5mm pin. Broken 0.5mm drills were used for the pins. The "blades" were mounted in a chuck in the milling machine and flats milled either side of the 0.5mm holes. The ends of the resulting blades were filed to match the 10BA countersunk screw heads. The screwdriver blades need to be held firmly in some sort of handle. I used an old drill chuck and a tap holder. This is easier to see in the picture above than to describe. Check that the pin and blade fit the screw head when the wheel is screwed on to the axle.

Two sleeves were made to fit over the crankpins and locate them in the slots in the jig. I used 1.5mm OD 1mm ID tube but while writing this I noticed that a 1.6mm OD 1mm ID tube might have been a better fit.

The jig is used by loosely assembling the wheels on the axles with the sleeves over the crankpins. This assembly is inserted in the jig making sure that the sliding plate is the correct way round for left and right hand lead, as required. The blades are inserted through the holes in the plates and the pins located in the centres of the fixing screws. The wheels will have to be turned so that the crankpin sleeves line up with the quartering slots in the jig. Then the jig is gently closed and the blades used to tighten the screws.

If the wheels are not tight on the axles when the assembly is removed from the jig then put the assembly back in the jig and tighten some more. Practice makes perfect!

Refinements

I use a brass block with 3 holes in it, 1/8" for the axle, 1.5mm for the crankpin sleeve and 1mm for the crankpin itself. These are used to check that the axle and crankpin parts are perpendicular to the wheel. The crankpin holes are used to hold the sleeve and crankpin in alignment while the Loctite sets. The hole for the sleeve has been relieved to avoid the sleeve being glued to the block! The crankpin hole will be similarly treated. The crankpin sleeve is shown projecting slightly from the face of the wheel so that the Loctite can be applied to the outside face without risking seepage inside. This will be filed flush when set. The extension at the back was held in the block while the Loctite was applied. It too will be cut and filed flush to the back of the wheel.

22 October 2007

I just messed up a wheel in the quartering jig by screwing too far and chewing up the hole in the wheel. The "blades" need stops so that the screw stops when it is flush with the boss, or very slightly countersunk to leave space for an axle end cover plate.

An end stop for countersinking the hole in the wheel would also be useful.

27 March 2008

I have bought a 1/8" diameter 90 degree countersink bit for countersinking the centre of the wheel. This should match the angle on the 10BA screws precisely giving a firmer hold on the wheel and stopping the chewing up of the plastic as the screw is tightened.

I have made some plugs for the centre hole in the wheel so that the countersink bit can be set at a known height. Knowing the starting height of the bit and the thickness of the wheel the depth of countersink can be measured precisely.

I calculated the depth of countersink required as follows:

thickness of etched cover plate 0.125mm

depth of countersink + blank thread on screw

blank thread on scre