Kevin Faris and I recently worked out a barter for some RV parts.  I traded him some Van's standards, non-locking fuel caps for a Tru-Trak auto pilot wing servo mount.  When I got the cnc mill, Kevin gave me the caps back so I could try my hand at engraving them for him.

Not pictured, but the first thing I needed to do is to figure out  how to hold the fuel cap during the engraving process.   I decided to use a block of 6061 extrusion to hold the fuel cap.  I milled a pocket the size of the fuel cap so that I could stick the fuel cap in the milled hole and secure it with the rubber o-ring (just like when it is installed on the plane). 

Next thing I had to do is make sure that the fuel cap was straight with respect to the milling table so that the text would be straight on the fuel cap.  I used a dial indicator and run the "lever" of the dial indicator along the straight edge just behind the lift tab.  When I got a deviation of less than .001" across the straight edge distance, I called the alignment good. 

Here is what the setup looks like....  Lets engrave...

I ran the engraving file once sitting above the fuel cap about .005" to make sure that the engraving would fit on the fuel cap.

More engraving as the engraving continues...  I am using a .020" 80 degree taper engraving cutter that is cutting .005" into the fuel cap.

First cap is done.  It is hard to see in the pictures, but the polishing of these fuel caps turns out a pretty nice cap. Significantly better than how they come in the box.  Here is how I did my polishing:

• Disassembled the fuel cap since the cap and the lift tab are not at the same heights.
• Started sanding with a 220 grit sandaper.  I put the sandpaper in an electric hand sander and mounted the sander upside down in the bench vise.
• After I got the highs and lows sanded out of the cap, I changed the sandpaper to a 400 grit wet/dry sandpaper.  I kept the sandpaper wet with just a little water.
• Changed the sandpaper to 600 grit wet/dry and repeated the sanding process.  As the cap started to shine (show a reflection), I broke out the Mothers aluminum polish and sanded, buffed, and repeated.
• I once again changed the sandpaper to 1000 grit, sanded, polished with more Mothers aluminum polish until I got a really nice finish.
• All the sanding was done PRIOR to engraving.
• After engraving, I dropped the fuel caps (still disassembled), into a vibratory polisher and let them polish for about 12 hours.

Here is a shot at both fuel caps after engraving.  They both turned out really well.

Tami and I had to run to Omaha to run some errands and stopped by Kevins hangar to drop off the fuel caps and some foot plates he wanted me to cut with his shear.  The peanut gallery took up residence in the swing.  There sure was a lot of noise contamination coming from that swing area...

Hard to see here, but the polishing of the fuel caps looked great.

The engraving process doesn't take to long, however, the polishing (which makes the caps look great), takes a long time.  I have several hours in polishing these caps.  I am now setup to engrave standard fuel caps.

However, I still haven't figured out how to engrave my own fuel caps since I have the deluxe, locking fuel caps.  Those caps are a different size (slightly) and secure to the filler ring of the fuel tank differently than Van's standard caps.