Turning in a New Design

Next, I had the machine switch tools and put a small undercut at the end of the threaded shank, so the threading tool had a place to run off.

After that was the threading operation, a custom thread that I will discuss in more depth in another column. The print called for a 2.875"-20 unified thread custom fit to a mating lock nut. I simply touched off the threading tool in my initial setup, told the machine the diameter of the thread and the distance between the threads and the machine spun away, with two additional spring passes to ensure it cleaned up properly.

Our machine is a conversational-type CNC. You simply input your start and end dimensions, and the control calculates tool nose offset and triangulations for you. Great for small lot work, or for those who aren’t savvy with G- and M-Code programming, yet want some CNC capabilities. Being able to put in the diameter, distance between threads, desired spindle speeds and thread lengths with mere numerical decimal values and getting a finished part at the end of the cycle is a big selling point for this type of machine. Since I was using these parts as the thread gauge for the lock nuts, I checked the threads with a pitch gauge and called it good.

The last operation in this setup for the rear wheels is the pilot drill for the half-shaft pocket and grease fitting tap drill. Using the tailstock, I stepped up drill sizes until I reached the diameter of the round-bottom pocket. Then, using a drill that I ground to have a rounded cutting surface instead of the standard drill point tip, I sank that drill to the desired depth. Lastly, I sank the tap drill for a 3/8"-24 UNF thread through the center deep enough that it would open out in the opposing counterbore in the second side. For the front wheel hubs, since they served no power transmission purpose, they were simply drilled and bored out to leave a 0.250" thick wall to the smallest outer diameter feature to give the hubs a high strength-to-weight ratio.

For finishing these parts on the second side, I did not want to use the factory hard jaws on the chuck because I would be gripping the parts on a finished diameter, the bearing surface. I needed a set of custom soft jaws bored out to the diameter of the bearing surface. (To see how we tackled the making of soft jaws for our lathes, see my earlier article “Eccentricity Rings True” in the January 2025 issue.)

On the advice of a coworker who spent many years doing finish and repair work by making soft jaws, I bored the jaws to about 0.002" smaller than the desired diameter, ensuring a strong grip on these hubs as they were being finished.

After taking a skim cut on both the face and the OD of the unfinished side, and setting the tools to it, I programmed the remaining features and let the machine turn all that excess stock into chips. This included the turning of the shoulder for the wheel’s center hub to rest on, the OD for the stud flange, and the undercut between that and the brake rotor flange. For the front wheel hubs, the same bore was cut and match turned as it was on the first side. For the rears, a weight reducing counterbore was sunk in, leaving a wall for the half-shaft to press against and capture the grease for lubricating the half-shaft ends.

With all the turning completed at this point, all six parts were sent over to our CNC mill to get the holes for the studs and the cutouts for the floating brake rotor pins (see Figure 3). The rears received the three-lobe pockets for the tripod rollers to ride in, as well as the three threaded holes used to hold down the collar that captures it all together (see Figure 4).

One thing I was able to do while programming at the control — and something I reinforce when training other employees in the use of the machine — was the practice of burying the insert’s tool nose radius until I was doing the light finishing passes. This allows the chip breakers to do their job, preventing those ghastly bird nests that can show up when stringy chips and a spinning lathe chuck meet. Carbide inserts have come a long way since I started in my career; most of them are able to outperform whatever speeds and feeds our machines are even capable of achieving! For most of you reading this, the mantra of “money is only made when chips are flying” is doctrine. Make that happen!

A special thank you goes out to Mechanical Engineering Professor Scott Hummel, Ph.D., for providing the wheel hub sample for photographs.

About the author: Robert Layng is an engineering technician/machinist at Lafayette College in Easton, Pennsylvania. He specializes in prototyping and practical design. He can be reached at layngr@gmail.com.