Using the 3mm Society SQ Wheels for Finescale

Last updated: 6 May 2011

Overview

The new SQ range of wheels by the 3mm Society have a square ended axle which is a big advantage over the current range of finescale wheels (and any other wheels which use round ended axles). The square automatically quarters the wheels and prevents them moving round their axles.

When the 15mm wheel was introduced I experimented to see if it could be modified to meet the finescale standard. I have now acquired some 18mm and 19.5mm diameter wheels to further my experiments. The 19.5mm wheel is of particular interest because it fills a gap in the finescale range.

The Society has ordered a small run of 14.2mm gauge square ended axles to further these experiments. If they prove successful then we could also have tyres turned specifically for finescale use.

Tyres

As supplied, the tyres are machined to the RP25-79 profile and look like this:

To meet the finescale standard the width of the tyre needs to be reduced from 2.1mm to within the range 1.65mm to 1.75mm. I made a jig to hold the tyre in my Unimat while I shaved 0.4mm from the front of the tyre. It is machined from 19mm diameter brass bar. This is just big enough to hold the 19.5mm tyre.

I cut and faced off a short length of the bar to act as a base. It needs to be just long enough to be securely held in the chuck. Mine came out at 12.35mm This is drilled and tapped 6BA.

Another short length of bar is cut and faced off for each tyre size. I used another piece of the 19mm bar for the 18mm and 19.5mm tyres but it is more convenient to start with a smaller diameter for smaller tyres. This is drilled 6BA clearance. One of these is needed for each wheel size.

The length of these pieces will be determined by the minimum size that can be held in the chuck for machining them. Mine are just under 12mm long. The diameter of one end is reduced so that it is a sliding fit inside the rim of the tyre. The length of this reduction should be sufficient to accommodate the tyre and the tool used for trimming the tyre. I used a parting off tool and the length of my reductions is about 3.4mm.

Here is the base and the blocks for 15mm, 18mm and 19.5mm tyres.

To assemble the jig, place the back of a tyre on the base, insert the appropriate centre into the tyre and screw the assembly up tight. The tyre is held by the lip on the back so it doesn't matter if the tyre is not quite centred in the chuck. This makes is much easy for amateurs like me! Mount the base in the chuck like this.

I use a parting off tool as shown to remove 0.4mm off the face of the tyre. The first time a centre is used let the tool continue beyond the tyre to further reduce the centre in front of the tyre making it easier to use thereafter.

Assembly

Clean up the tyre removing any swarf before assembly.

Place the spoked centre in the tyre and press it home. For the 19.5mm wheels this can be done between fingers. For the 18mm and 15mm wheels I prefer to lay the tyre on a flat surface and then push the centre into it with my fingers.

This produces a wheel with a plastic rim protruding from the inside of the metal tyre like the one on the left in the next picture.

I tried removing the plastic rim with a file but this is extremely tedious. You have to be very careful not to catch the front face of the spokes.

I now mount the wheel on an axle, mount the axle in the chuck and remove the plastic with a parting off tool. This is much quicker and produces a neater result. Do make sure that the rim of the wheel is supported by the jaws of the chuck before you start.

Here are some assembled wheels before and after removal of the plastic rim.

To me the combination of metal and plastic rims makes these wheels look a bit heavier round the edge than the Stapleton finescale wheels but I think they look much better when the rims are backened. I drop the whole wheel in Birchwood Casey Brass Black for a few seconds and then buff up with a cloth and stiff brush to produce this, a 19.5mm and an 18mm wheel.

To be continued when we have some SQ axles for 14.2mm gauge.

Mechanical Fixing

Having developed a method for screwing the Stapleton wheels to their axles as described in A Mechanical Fixing for Stapleton Wheels. I am keen to use a similar method for the SQ wheels.

The ends of the SQ axles are 2mm square and it is feasible to drill and tap them 10BA as shown below.

I have yet to countersink the SQ wheels and reduce the axle ends for the 10BA screw fitting but I see no reason why it should not work.