Most people with hobbyist CNC’s produce 2D and 3D carvings made of wood, and these machines can do an exceptional job with plywood or hardwood. Except for prototyping, very few of my projects have been made of wood. My first project was to engrave glass with a diamond bit! If I only knew then what I know now…

The project was to personalize Mason Jar mugs with names of the married couple. Since the wedding was at a lodge and everyone would be using them throughout the weekend, we also added the name of each guest for a personal touch.

I was going to laser etch the names with my CNC, but since the laser I wanted was not yet FDA-approved I decided to engrave the glasses instead using a diamond drag bit. A drag bit, as the name implies, drags across the surface of the glass with an industrial diamond to impart the design in the surface. The bit doesn’t spin at all.

Glass is one of the hardest surfaces to engrave; bits can break or wear out relatively quickly. I replaced the bit after every dozen or so cups. A drag engraving setup consists of a holder and a spring loaded bit. The spring load provides uniform pressure and allows for engraving on surfaces that are not perfectly flat (like a Mason Jar).

Assembled Drag Bit, with components shown below. Diamond is in the holder on the left hand side.

Holder Jigs:

Once I determined it was possible, I had to devise work holding jigs, one for the married couples’ name and date, and one for the name of the attendee. Since the surfaces were not flat I used foam to carve out a holder for the mug. The holder also solved the problem of setting a consistent starting point (known in CNC as “XYZ0” or the zero point of the X, Y, and Z axes). I had to cups from two different suppliers so I needed to make two jigs for each operation. While the cups looked similar they had different curvatures, handle sizes and “flat” areas.

With the design aspects squared away, it was time to engrave. Here’s a video of the process with a photo of the the other jig:

CNC engraving with a drag bit