The rear axle PB bearings for the 1/20th scale Burrell traction engine actually turned out quite a delight to turn. Phosphor bronze (PB) can be quite difficult to machine. Hence I thought I would try the tungsten carbide tips that are designed to work with aluminium.
We reviewed the Glanze lathe tools before with the original tungsten carbide bits. In this case though I swapped the tips from the gold coloured ones suitable for cast iron to bright silver ones designed for aluminium.
Even in plan view it is possible to see the sharp point and steep angle.
The stock PB material was approximately 13.5mm in diameter. All of the turning was done with the Warco WM240B at 1000rpm. I started the machining without any cutting fluid and found it to be fine. Hence all of the machining on the PB was done without cutting fluid.
Bearings Fitted to Tender
The phosphor bronze bearings are designed to be a tight fit into the tender. Thus locating the bearings accurately. Between the right and left hand tender sides is a brass tube of 8mm ID. The bearings protrude through the tender side and locate the inside this tube. Thus holding the tube in place.
This means the 1/4 inch axle will be supported on the PB bearings each side. Between the bearings the axle is covered by the brass tube.
Finally the rear axle PB bearings have 3 off 12BA bolts fixing each side. Quite a challenge to locate and tap as 12BA is rather fine. The tapping drill is just 1.05mm in diameter. It would be best to have drilled this hole at 0.8mm and then 1.05mm to get a better shaped and sized hole. This would have made tapping the hole easier.
The normal clearance for 12BA is 1.3mm. Increasing the clearance hole to 1.5mm allowed more freedom for the bolt alignment. Made possible by the bearing being located with the inner shoulder aligning to the frame.
Cheesehead bolts as that’s what I had available.
A temporary aluminium axle in the rear bearings. Aluminium is useful at this stage as it is softer and so will not damage the bearings.
The centre section through the bearings is 1/4 inch diameter. The outer section that locates inside the wheels is 6mm diameter. Mixing imperial and metric is ok in the model making world. Originally I had challenged myself to stick to metric. However, this small step will allow me to create a shoulder for locating parts.
Engine on 4 Wheels
With the rear axle now in place I just had to assemble the traction engine. Standing on my A4 notebook, demonstrating my original design for it to fit onto a sheet of A4 paper.