Although glass is a brittle material, it can be cut using an abrasive waterjet and the results can be quite stunning. Most glass cutting is done for artistic purposes.
There are a few main items to consider when cutting glass:
- Quality of glass
- Abrasive and water delivery
- Support of the glass
- Design of the part
Quality of glass
Some types of glass cut much easier than other types of glass. Generally speaking, higher quality glass that is thicker tends to be easier to cut than low quality glass or thin glass. The manner in which the glass was cooled at the factory affects the temper and how brittle and how difficult it is to cut later on, and some of the cheaper glass is not as good.
Tempered glass is impossible to cut with a waterjet, as the glass is under stress. When you begin cutting it, tempered glass will shatter into many small fragments.
Abrasive and water delivery
Timing of the abrasive delivery is absolutely critical during piercing of the glass. Use the lowest pressure you can while still being able to flow abrasive (something in the range of 10,000 to 20,000 PSI [700 to 1400 bar]). You want to have the abrasive hit the material as soon as the water begins flowing. Water without abrasive cracks glass.
Some techniques for doing the above include:
- Turn on the abrasive before you turn on the water, or at the same time, or only slightly after, but before the water has a chance to reach full pressure. Your particular setup will dictate the best approach. Typically these timing things are adjusted in your software.
- Use vacuum assist to pull the abrasive through the nozzle, so that as soon as it starts, the abrasive is already flowing.
- Use a sacrificial material on top of the glass to take the force of the water until the abrasive has a chance to flow properly. For example, a sheet of plywood on top of the glass can provide enough time for the abrasive and water to reach pressure before it reaches the glass.
- Cut from from the edge of the sheet, or a previously drilled hole, so that the water and abrasive flow is completely stabilized before beginning reaching the glass.
- Slowly ramp the pressure from the pump beginning at zero, so that only the lowest pressure water hits initially while the abrasive is beginning to flow.
Typically, waterjet machines support at least one or two of the options mentioned above. Experiment with them to get the right settings. Once you have the right combination, then you should be able to use this combo forever. You may, however, trade reliable piercing for an increase in nozzle plugging, so you’ll probably want to use these settings only for brittle materials.
timing setup screen for abrasive waterjet control software
A typical screen for controlling timing for an abrasive waterjet machine
Support of the glass
It’s important to support the glass on the bottom side with something that provides even support, yet is soft enough that the jet does not ricochet back into your glass and frost the bottom surface. Waterjet brick is a great option for this, though it does wear out faster than metal slats. Some people use dense Styrofoam™ sheeting or a piece of plywood, as long as the entire piece of glass is evenly supported.
Design of the part
When designing a part that you’ll be cutting from glass, you can make some adjustments that will minimize the effect that any cracking will have on your part. If you have the room, use long lead-ins so that if a crack does appear at the pierce point, it is less likely to make its way all the way to the edge of your part.
There are many kinds of piercing methods that can be used with glass. With the proper setup, any of these can be used without trouble, but you may find through experimentation that some are better than others. If you are having problems getting a good pierce with glass, you may want to try a different piercing method.
If you are cutting something with a lot of pierce points, the odds become much higher that it will crack on at least one of the pierces. Consider pre-piercing all the start holes, and then cutting the final features. That way, if something does crack, you can throw it away and start over before you waste a lot of time (and money) cutting the path.
Of course, reducing the amount of piercing needed is always helpful as well.
Watch out for dramatic temperature changes between a hot water tank and cold air which can cause your glass to shatter. For example, if you pull your freshly cut glass out of a hot water catch tank (and most waterjet catch tanks heat up from dissipating the energy of the high-pressure water), avoid hosing it down with ice cold tap water.
A good quality abrasive always provides better waterjet parts, but it’s particularly useful when working with difficult materials such as glass. You may find that higher mesh sizes (100, 120, or 150) give smoother results with less microchipping on the edges.
Sample glass part
In the series of pictures below, a bicycle is first cut from metal, and then the exact same shape is cut in a sheet of glass. Note that the bicycle is carefully designed as one continuous tool path, which means that there’s only one pierce in the sheet of glass. The waterjet achieves such high precision that the metal bicycle shape fits precisely into the glass for a perfect inlay.