A Different Material for Gears – A Second Attempt

For several years I’ve been producing replacement gears in Shapeways’ Smooth Fine Detail, formally know as Frosted Ultra Detail (FUD).  But what about other materials?

Back in June of 2019 I tested a different material, Multi Jet Fusion Plastic or PA12.  You can find the post here.  This test was not successful, as you can see below, because the definition simply wasn’t there.

To be fair these gears were small, measuring only 4.38mm (0.172″) in diameter.

But this material is a lot stronger than the Smooth Fine Detail.  The Smooth Fine Detail is an acrylic and although it’s very hard, it’s brittle and impacts can cause cracking.  That’s why my replacement Bachmann HO 4-8-4 gears needed to be much thicker than the original.  The PA12 is a nylon plastic and some of its listed key properties are high strength and durability, as well as good impact resistance.

To give it a good test I needed to try a larger part and the ideal opportunity came along when a  customer wanted some replacement gears for an O Scale 3 Rail brass AC6 Cab Forward.  These 4-8-8-2 are huge and a lot of power is transferred to the driving wheels to be able to pull prototypical trains.  This particular model has an idler gear that runs between a metal worm gear and a metal axle gear. An idler gear is a gear that spins freely on an axle without turning that axle between two other gears, the purpose is either to change the direction of the second gear or to simply space the gears out. (Image take from https://www.notesandsketches.co.uk)

The original idler gear was plastic, and due to the constant wear from the two metal gears, had become damaged.

The reason for this gear being plastic rather than brass was it had a left-hand twist, as you can see below.  This is not impossible in brass but it’s very expensive compared to regular gears with straight-cut teeth.

But this sort of complex 3D geometry is ideal for 3D printing.  This gear has a diameter of 9mm (0.3543″) so is twice the size of the previous gear and has much larger teeth.  In order to allow for spares, several were test printed at once on my typical sprue.

The PA12 material has a coarser finish than the Smooth Fine Detail but given the scale of the components, this is not an issue.  As you can see below the new idler gear fits well into the AC6’s transmission.

After assembly, the locomotive had several hours of use with no problems at all, so in this case the PA12 material is a success. At the time of writing the cost of the set of five PA12 gears is the same as a set of Smooth Fine Detail gears.  When the PA12 was first introduced it was more expensive.  The base color for the PA12 is gray but the black, as used here, can be chosen for a little extra cost.

With this successful test, I now have another material available for larger gears that don’t rely on tiny smooth parts.  The point where Smooth Fine Detail will need to be used over PA12 will be different from model to model and will depend on things like teeth size, but I’ll endeavor to use this where possible.

I still have several new gears to try for projects I’m working on, but if you have a need for a gear that’s proving difficult to source please get in touch and hopefully I can produce it for you.