This is a fun instructable, something I would take on holiday...
Step 1: Design and CNC Settings
I used the free application easel, to create the file for my CNC router. There are several different fonts available, I settled on Bangers. Once I typed in the message to go on the flip flop, I flipped/mirrored the text, so the message will appear correctly on sand. I scaled the letters to about 4 cm/1.6 inch high.
I choose a depth of cut of 5 mm and made it a fill cut so the inside of the letters will be cut out. Easel adjusts the cutting speeds based on the material, but it doesn't have flip flops as a material... :) I choose expanded PVC and changed the depth per pass to 2 mm, to reduce the overall cutting time.
Step 2: Tools and Equipment
I used my DIY built CNC with a Makita trimmer router. I used a 1/8" single flute solid carbide router bit which I had used cutting plastic before, so I decided to try it out. The flip flop material is a soft foam, so I wasn't sure if it would leave a clean finish.
Step 3: Setup
Securing the flip flops to the work space is difficult due to the non uniform shape. I decided the best method was to use two strips of wood and screwed them to the work table. Due to the straps on the flip flop, I inserted a piece of foam to support to underside of the flip flop.
Step 4: Routing Letters With CNC
The first cut turned out surprisingly well,the carbide single bit worked well. It did leave some rough edges which I cleaned up with a sharp knife.
Step 5: Different Flip Flops and Texts
I decided to try a couple of different flip flops, to see if the cut was better or worse and I changed up the text style also.