A post on my experiences and observations while rastering glass, and the methods I followed.
Refer to my first laser post (How to… laser cutting) about the general use and basics of laser cutting.
Here, I’m looking specifically at rastering images onto glass. (The lasers that I’m using aren’t strong enough to vector cut through glass.)
To get a clear, well rastered image, settings have to be fine tuned, because the power speed, and laser movement vary per laser cutter. These settings also differ between the glass that you are restoring onto, since different glasses have a different composition and impurities.
The final rastered pieces!
Preparing an image for rastering (using Inkscape as an image editor)
- Determine the size of the glass face that you will be putting the image on, and create a document of that exact size.
- import your image in, and convert it to a vector file (black and white)
- convert the image colour to 50% grey. This helps preserve the integrity of the glass and does not cut as deep. It also creates a “whiter” finish
- (optional: tidy up the image)
- place the image in the location wanted
- export file as a PDF and save to a USB drive
Using the laser
- turn on all fans and ventilation equipment
- check laser is calibrated correctly and focus the laser, if not using the autofocus
- open file on computer linked to laser cutter, and hit file, print
- depending on laser cutter, settings are adjusted differently. in advance settings, select rastor, setting power to 100, and speed to a value between 20 and 40%
- hit print on the computer, and on the laser machine.
Experimentation and learning:
Particular selections that I had to make were selecting rastor instead of vector cutting, setting the highest power possible (100), and testing various speeds and restoring styles to get the best finish.
I tried a series of test combinations:
black image, speed 50
black image, speed 40
50% grey, speed 40
50% grey, speed 30
For rastering my image, I found that a grey image at 25% speed worked best, and for rastoring text, black at 30% worked best on this glass. Also tried using grey text at 30 and 25%, but they both leave areas not quite fully rastered.
Another experimental point is the way the motion of the rastor cut: this was the difference between “standard” and “jarvis”.
standard vs jarvis
Rastering circumferential surfaces:
The previous experimentation was all done on a flat surface so that the laser draws a flat image. To be able to raster onto a circumferential surface, a special jig must be used to rotate the surface as the image is engraved.
Depending in jig setup, a number of constraints must be considered for optimal engraving.
Non-varying diameter with no external protrusions is the best shape to use
for a circular glass.
examples of suitable glasses
Here is an example of a very inappropriate glass, which would not be possible to rastor onto easily:
Features that make this inappropriate are:
- handle will get in the way of the laser when the glass rotates, so a workaround would need to be considered in terms of the engraving space.
- the base rim means that the glass does not sit level, which it needs to do for uniform rotation.
- engraving area has to be planned very carefully. if it were a blank glass then it wouldn’t matter, but if I want what I’m engraving to line up with what is already on the glass, then careful measurements have to be made.