New Axles for a Second Generation Bachmann HO 4-8-4 Northern – Part 2

As promised in my last post I’ve now received the first test print for my new 3D printed axles for this locomotive, and in this post I’ll share with you how well they worked out.

This was the first test print for this design and although the design has been well thought out there’s always a chance that something’s not right, so I printed three sets.

Each set contains one drive gear and three axles; below you can see an original axle and gear for comparison.

With the new parts removed from the sprue, a closer comparison can be made. And for me, the first thing that stands out is the size of the square hole. It appears much smaller on the new part than the original, which in fact it is, but by design.

The reason for the smaller size is based on the actual axle size on the wheels. I need the replacement parts to be a tight fit but not so tight that the new parts split. Sadly they do. The reason for this is due to variances in the wheel axles. The wheels are not machined, they are cast. Then they’re plated which gives them their shiny finish and conductive coating. However, this leads to a large variance in the thickness of the axles. It’s not enough to be really visible, but it’s enough to make a difference in splitting an axle or not. So I’ill need to adjust my 3D model to allow for this.

In order to complete this test, I still wanted to see if all the other aspects of the gear and axles worked, so I filed the square hole with a square needle file to enlarge them slightly.

Now the main drive gear is fitted. Quartering the wheels, or making sure the wheels are at 90° to each other, was very easy with the square holes, and the two wheels pushed into the axle without splitting it. Although the axles have been designed to correctly space the wheels it’s good to check the distance between the back of the wheels, which should be 14.4mm for HO. I use a laser-cut ‘back-to-back’ checker as you can see below. This should be a snug fit between the wheels.

All the axles have now been fitted and spaced. At this point, I check to make sure all the wheel assemblies can move freely and none of the valve gear is misaligned.

The last part is refitting the base plate to hold everything in. For this locomotive, the base plate also holds the pilot and trailing truck and clips onto the chassis. There are two screws in the base to hold it on.

And then comes the test, does it work under power? At this stage, I’m looking for three things; does it run freely, does it run at slow speeds without binding, and most importantly, how does it perform under load? If you followed my previous design post about the first generation of this locomotive, the original gear was very thin and the first design for the replacement gear cracked or stripped its teeth under load. Luckily the second generation already has a much thicker gear. In the short video below you can see I tested all of these.

And I’m pleased to say it passed all three tests. Given that I had to enlarge the square hole in all the parts I’ve now updated my 3D model to reflect this and I’ve ordered a second test print. I also made a slight change to the angle of the teeth on the main gear. I don’t see a problem but it brings the angle more in line with the teeth on the contacting gear.

Hopefully, the new test sets will be here soon and I’ll replace the gears in this chassis to see how well they fit. If everything is okay, with no splitting, I’ll release the Bachmann HO 4-8-4 Gen 2 replacement axles and gears for sale.