I made these at TechShop in Menlo Park. Check out TechShop at www.techshop.com. Find a location near you and take a free tour.
Material - Delrin (in this case), or HTPE, or 316L stainless if your feeling crazy, or titanium if you're feeling fancy!
Desired earring - to measure diameter of gauge
Steel wool (or scotch brite if you're working with metal)
Step 1: Size It Up
Chuck your material up in either a 3-jaw chuck or in this case, I used a collet closer. Get a good cutting tool and a parting tool as well. I just used the small tool to part, 'cuz this isn't rocket surgery.
The material I used was already cut into a bit, didn't matter since turning the taper was going to remove all of it anyways.
Step 2: Set Your Taper Angle
Its a good idea to lock the carriage out (also pictured). Once you are in the vicinity of where you are going to be working, all the work you do on the taper will be done with the cross-slide and the compound.
Step 3: Turn On
Feed into the y-axis with the cross-slide ( I moved in increments of 0.030, taking .060 off the diameter in each pass; Delrin rules for this very reason ) and move the compound down the taper in the -x direction, taking it from the tip and working your way down to the thicker base (hold jokes please). For a finishing pass I took off .005 and moved very slow. You really want the surface to be smooth so there aren't little nooks and crannies for bacteria to kick it.
Now that you have a nice taper going here, you want to reference the desired gauge diameter. In my case it is .313".
I simply measured with the calipers where this size was on the taper and started cutting the flat landing a little above (or bigger diameter) where this was. After you take a pass, see what the diameter of the flat is. Subtract your desired diameter from this and you know how much smaller you need to make it. REMEMBER if your working in diameter movements or in radians: when you take .005 off the face (or the radius), you are taking .010 off the diameter!
Step 4: Parting
First: measure where your are parting so you can mimic it on the second taper.
Next: Cut that fool!
I didn't fully sever because the finished piece started flappin' around all crazy-like, and I didn't want to ruin the nice finish. So parted most of it off, then broke off the work and cleaned up the uncut pip with a knife ( again, I love Delrin ). If you were using 316L or titanium, you could file the pip down, or turn it around and face the end off with the lathe and make it look nice; being that is what everyone will see when they look at your face.
Clean it up with some steel wool or scotch brite (depending on your material), rub it down with an alcohol pad and jam that sucker in your ear!
HELPFUL HINT: each one of these steps should be made on both "gauges" before moving onto the next step. This will help you out so you don't have to reset the tools twice and run the risk of having the two come out totally different. This is why I prefer the collet closer, because there is substantially less run-out than a 3-jaw chuck when taking the work in, and out, and in again.
Hit me up with any questions or comments, and don't forget to checkout TechShop - 'cause I made these there!