RP25-110

Next up in the sage of making wheels for Ivor is the bit I've been dreading the most; profiling the wheels to add the flange. In theory this should be easy as in essence all it involves is reducing the diameter of the casting using a profiling tool, but that glosses over a whole bunch of issues.

Let's start by defining exactly what we mean by "to profile" a wheel. Put simply it's to shape the tread and flange of the wheel to a specific shape. There are lots of different profiles all defined by different guidelines and associations. For Ivor I've settled on using wheels profiled to match RP25-110. This means I'm using the RP25 profile, as defined by the NMRA, with code 110 wheels, i.e. wheels that are 0.110" in depth (or for those who prefer metric, 2.794mm). Now a lot of the work of making the wheels fit this profile was done during the design of the 3D model as it set the depth etc. but it is impossible to 3D print thin enough and accurately enough to produce the flange.

To add the flange (and to ensure a nice smooth turned tread) I'm going to finish the wheel profile on the lathe, using a specific profiling tool. You can get profiling tools from numerous places but I picked up this one from Carbide Solutions as it had a square shank which made mounting it in the lathe easy.

Even with a sharp profiling tool (I'd not used mine before) this is actually quite hard work for the lathe and involved taking very very tiny cuts and quite a few breaks even during each wheel to ensure I didn't over heat the lathe or the wheel (it got exceptionally hot).

As you can see I've held the wheel in the lathe using a tool designed specifically for the job (another purchase from fohrmann-WERKSEUGE). The tool ensures the wheel is perpendicular to the lathe bed and clamped tight, although I did manage to over tighten the clamp on two out of the four wheels which unfortunately opened out the axle hole in the wheels slightly -- not a problem as I'll fit insulating bushes to these two wheels so will need to open the hole further anyway.

You'll also notice that I've left as little of the profiling tool hanging out of the toolpost as possible to try and reduce chatter, and I clamped the crossslide tight so as to remove as many sources of movement as possible. On the first wheel I started with the lathe on a slow speed but that was a disaster as the tool tended to dig in and bounce around, even with the lathe locked as solid as possible. I reconfigured the belts to run the lathe on it's fastest setting (4000 rpm) and with very very tiny cuts this worked a lot better, and fortunately I figured this out before destroying the first wheel. Once I'd reduced the first wheel to the correct diameter (21mm for 5' 3" wheels at 4mm to the foot) I took a note of the position on the cross slide, and then repeated the process for the other three wheels.

It took me all morning (in blocks of a couple of minutes here and there) but I've managed to profile all four wheels. While I'm not going to claim any of the four as perfect, I'm happy enough with them all to try and assemble the loco. All four have a difference of no more than 0.02mm when measured and all are slightly above 21mm rather than below, so they are close enough that I think they should sit level. The main issue with them is that when I removed them from the sprue, and filed the excess away, I didn't take into account that the edge of the flange was the edge of the casting (something I'll change if I produce any more) and so in a couple of places there is a slight straight edge on the flange, but hopefully not enough to cause a problem, if it is I can always order another set and replace them but fingers crossed...