Firstly, we need to understand how glass laser engraving works. If you are carving wood with a laser, the laser beam hits the wood and vaporises it - that’s why the smoke is generated.
However, glass behaves differently. Instead of vaporising the glass, the laser beam hits the glass and causes it to rapidly expand at the point where the laser beam (which is only 0.1mm wide) hits. The surrounding glass does not expand as quickly and so the area under the beam fractures. This micro fracturing is what creates the engraving on the surface of the glass.
Depending upon the additives in the glass, this process behaves slightly differently and so having a couple of spares available for testing before committing is essential.
For example, adding lead to the glass to create lead crystal creates a glass that is softer than standard glass and is significantly harder to engrave successfully as the glass absorbs heat faster and so needs a higher power laser but that can then lead to the glass overheating and shattering.
The Roller Rotary Attachment comprises two long rollers which rotate the workpiece and hold it in place throughout the engraving process. As the stepper motor turns the rollers, the object rotates in the opposite direction. These laser rotary attachments work with cylindrical objects such as water bottles, but anything with a handle can be very difficult to position correctly. For simple cylinders, they are quick and easy to set up. They do, however, rely on friction between the rollers and the workpiece and if they slip, the entire engraving design can be misaligned. This is also why performing a second pass of the laser is not recommended with roller rotary attachments.