Fine Scale Brass Gearbox - 4.5mm to 3/16" Axle

[Guest Isambarduk]

I was recently asked for help to open out the final drive gear and the axle bearings of several gearboxes from 4.50mm to 3/16” (4.76mm) diameter so that they would be suitable for use with Slater’s axles.

 

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The motor/gearbox

Backing right off the grub screw in the boss of the final drive gear allows the axle to be withdrawn and the gear to be removed.  In principle, the top hat bushes that form the axle bearings could then be opened out in position using a 3/16” reamer with a reasonable lead (taper) but there were good reasons to dismantle each gearbox to work on the bearings:

• The gearboxes were not screwed tightly to the motors and the lower of the two screws could not be reached with the gearbox assembled and in place.  The only way to reach the lower screws to tighten them is to dismantle the gearboxes
• The bearings are made of a tough phosphor-bronze and, even with a new reamer, the torque was sufficient to rotate the bearings in the side frames so the bearings needed to be held independently of the side frames.
• Even taking the greatest of care, there is a danger that swarf may enter the gearbox

A complete motor/gearbox (left) and with the axle, final drive gear and one side frame removed (right); a replacement Slater’s S7 axle lies in front

 

 

A final drive gear is held by its boss in a collet in the lathe whilst the hole is opened out with a 3/16” reamer (left) and the chamfered (right)

 

 

With the frames removed from the gearbox, the brim of the top hat bush may be held in a three-jaw chuck for reaming and chamfering

Ideally, I would have used a collet but the brim of the bush was too shallow for the collet to reach beyond its holder to grip it.  There was no such difficulty with the jaws of the three-jaw chuck, which was sufficiently self-centring for this purpose.

 

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James May: The Reassembler? A gearbox stripped down with one of the 4.5mm stainless steel axles (note the milled ‘flat’ for the retaining grub screw, more of a notch than a flat)

 

 

I milled similar notches in the mild steel Slater’s axles.

 

I would like to make it clear that these motor/gearboxes were old stock from FineScaleBrass and that the current motor/gearboxes, which are advertised on their website (www.finescalebrass.co.uk/components.htm) and which are available at shows, will have 3/16” axles and were made specifically for the 0 gauge market, so you would not need to worry about this exercise.  However, should you come across on older motor/gearbox or wish to replace the wheels in one of their models, you know what it is about.

David

[The Nth Degree]

Thanks David, sterling job.

Steve