Turning in a New Design

Here’s how a new part design became the “hub” of future victory for students working on the Formula SAE car project at Lafayette College in Easton, Pennsylvania.

The year was 2013 and our Formula SAE car team had just returned from a successful run at their competition, albeit with some scorching critiques from the design judges. They really hammered us on the wheel assemblies, and the laundry list of potential failure points on them. Just about every single point where the wheel packages (wheel hubs, brakes, upright bearing housings, steering and suspension pickup points) interacted with something else, they had scathing feedback. We pushed the car back into the garage and licked our proverbial wounds, and then discussed what we’d do differently.

As is our history, the team was all college seniors, and they went on to graduate and start their careers, leaving the car to the next batch of incoming seniors. They did, however, leave behind a wish list of suggestions to my incoming team. (That list found its way to the recycle bin.)

Fast forward to the Fall semester of 2014. I met my new team assembled in the garage to pick up where my last team left off. This was a dream team of designers, drivers, gearheads and marketing gurus! Their elected leader for this new team was a man of so few words that, if he did speak, the words were either of wisdom or frustration, and everyone in the room listened. This team redesigned everything from the brake rotors and motor mounts to the cockpit and motor box, as well as everything in between. They managed to do all that in record time, something that has rarely happened in our history.

This brings us to today’s focus, turning out new wheel hubs. By new, I mean they are an original design and were completely machined from bar stock instead of being hobbled together from butchered Volkswagen Rabbit drivetrains.

Figures 1 and 2 show the outside features of the parts as turned. This view is identical for all six pieces. I made the two for the front, the two for the rear, and a spare of each to pack in the toolbox in case the worst happened at the track.

This new design embodied the principle of “monolithic integration” or making a single part that performs many jobs at the same time. Instead of bolting flimsy rings of steel to a flange for brake rotors, which in turn was bolted to another part that became the hat/hub/brake rotor, this hub integrated the flanges with the bearing diameter, pre-loading threads, wheel lug flange and the half-shaft input. This single part performed five separate operations all at the same time!

While it may sound wasteful to take a large chunk of stock, in this case a 5" diameter bar saw cut to roughly 6" in length, this method saves the added hardware and weight of redundant parts. Despite the pile of chips and shavings from cutting a half dozen of these things, you must remember two important points:

First, I work in a prototyping shop, and these are acceptable losses in the quest for a successful design.

Second, these parts still retained a substantial amount of the material while weighing in at a fraction of what their predecessors did.

For this part, I started the machining with use of the factory hard jaws on the three-jaw chuck, gripping on the part with a little extra bite while sticking enough of the material out to turn most of the features in one setup before needing to reverse the part to finish it.

As a result of the increased precision and repeatability, I chose to machine these parts in our CNC lathe. I programmed the machine to face the part, turn the OD of the threaded area, the bearing diameter, the shoulders, and the OD for the brake rotor mounting flange. I also programmed the machine to turn the brake rotor flange back a bit past the desired width so I could clean up the back surface in the finishing setup.