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An abrasive water jet is created by pressurizing water up to 90,000 psi (6200 bar) with a water jet intensifier pump (see Figure 1). The water is forced through a tiny jewel orifice as small as .007 in. (smaller for water-only cutting). The supersonic water then draws an abrasive media such as garnet into the water stream, accelerating the abrasive media and focusing the slurry through an abrasive nozzle as a high-velocity coherent abrasive jet stream capable of cutting virtually any material.
Water jet operators typically use orifice/nozzle combinations of .010/.030 in. (10/30), .013/.040 in. (13/40), .015/.045 in. (15/45), and .020/.060 in. (20/60). The larger combinations are used for thicker cutting while the smaller combinations are used for thinner cutting when less energy is required to cut the material. Precision water jet cutting machines are controlled with a PC-based motion control system.
GlassA wide range of untempered glasses, including float, decorative, Pyrex® and more, can be machined through water jet cutting. A clean, flat surface is essential to a quality cut, and glass projects should begin with a spray-down of the water jet grating; even a tiny amount of grit on the work surface can be problematic.
"To avoid fogging or chipping the glass, place it on a clean, flat sacrificial backer material," says Brian Wallace, a machinist with Jet Edge, Inc. "I would recommend 3/16-in. corrugated PVC. This eliminates fogging, because when the water shoots back up, it gets displaced through the corrugation rather than bouncing back at the part." Other acceptable backers include expanded PVC, Plexiglas® or sheet rubber. Soft materials such as Styrofoam® do not provide adequate support.
Before cutting, the part should be weighted or clamped to hold it in place. It is also beneficial to place a rubber cushion or cardboard under the weight or clamp to avoid scratching the glass. Glass can be cut above or below water; cutting below water is much quieter and does not affect the quality of the cut.
In addition, it is essential to use the right abrasive. "I use 120-grit garnet abrasive almost exclusively," says Wallace. "But if you don't care about the edge chipping, use 80-grit so you can cut faster."
It's important to remember that bigger equals faster when considering orifice/nozzle combinations. "I prefer to use a 10/30 with the appropriate amount of abrasive (from .3 lbs/min for thin material to .75 lbs/min for thick material) because the stream is smaller and doesn't exert as much force on the material, giving you a better chance to reduce breakage," Wallace explains. "That being said, if you want to increase cut speed, a 15/45 with up to 1.5 lbs/min of abrasive will allow you to cut faster at the risk of more edge chipping.
The initial pierce must be made at a very low pressure to avoid shattering the glass. "You have to use low pressure, typically 10,000 psi," Wallace says. "At that pressure, you need to lower your abrasive rate almost as low as it will go. Otherwise, you're going to be wasting abrasive and you run the risk of plugging the nozzle, or the abrasive will pile up in the delivery tube. You can pierce faster at 12-15K, but you run the risk of chipping the glass."
Wallace recommends using a dynamic pierce (piercing by cutting a tiny circle) rather than a static pierce. He typically cuts a 0.040 in. diameter circle as the pierce rather than concentrating the water jet in one spot. "You can reduce your piercing time from two minutes to 10 to 15 seconds, depending on the thickness of the glass, by having it cut a circle rather than having it stay in one place," he says. "The circle eats through the material rather than bouncing back, and it disperses the energy better."
Frequently raising and lowering water jet pressure at the water jet pump to make low-pressure pierces can cause pump components to wear out faster, raising maintenance costs and lowering productivity. Using a dual-pressure valve allows water jet operators to raise and lower water jet pressure independent of the intensifier pump. This allows parts to be cut faster because it is not necessary to wait for the pump to raise and lower to the desired pressure. It also reduces wear and tear on the pump components, lowering maintenance costs and reducing downtime.
For best quality, cut slowly to avoid chipping the edges of the glass and minimize the taper on thicker pieces. Water jet manufacturers can provide recommended cutting speeds for desired cut qualities. "The faster you try to cut, the worse you are going to chip the edges, especially on the back side," says Wallace. "You get more chipping on the back side because it is the exit point for the jet."
By increasing cutting pressure to 75,000 psi, water jet operators can greatly increase their cutting speed and minimize chipping. "You can increase your cutting speed from 55K by 50-75% at 75K," Wallace says. "It also helps with the edge chipping. You get a much nicer edge." When cutting a sharp corner, cut beyond the corner, make an external loop and return to cut around the corner. This achieves a sharp corner rather than one that is slightly rounded.
Laminated glasses, such as bullet-proof glass, can delaminate during the low-pressure pierce. To avoid delamination, make the low-pressure pierce well away from the edge of the part and increase the lead-in cut to the part.
When cutting glass with an uneven surface, such as stained glass, be sure to watch the nozzle height. A 1/8-in. stand-off is recommended. It's also best to avoid getting too far away or the edges of the cut will round over.
Sidebar: Tips for Water Jet Glass Cutting
- Reduce the risk of chipping by using a low-pressure pierce of approximately 10,000 psi.
- Reduce the low-pressure piercing time from two minutes to 10-15 seconds (depending on thickness) by cutting a tiny circle rather than leaving the jet fixed on one spot.
- 120-grit garnet produces the best edge quality. If speed is the priority and chipping is not a concern, 80-grit garnet can be used.
- Cut at 75,000 psi to increase cut speed by 50-75% and improve edge quality.
- Support glass on a sheet of 3/16-in. corrugated PVC rather than directly on the metal slats to reduce the risk of chipping and fogging.
- If frequent raising and lowering of pressure is required, install a dual-pressure valve to save time and prolong pump component life.
- Tempered glass cannot be cut using the water jet process.
CeramicsThe water jet process can easily cut most ceramics (with the exception of 100% alumina ceramics, which are impractical to process using water jet), but, like glass, ceramic can chip and break easily if not properly cut. Cutting ceramics is very similar to cutting glass, according to Mike Jensen, a water jet operator with Chukar Waterjet, Inc. For example, ceramic materials also require a solid backer material and must be pierced at a very low pressure.
It is also sometimes a good idea to place a thin sheet of material on top of the ceramic to avoid chipping it. "I usually cut on plywood or thin plastic," says Jensen. "When it comes to the low-pressure pierce, I go as low as you can go and still have the water jet cutting head come on. I shoot a split second of abrasive before the water to minimize chipping."
Typically, 80-grit garnet is used for cutting ceramics, and a 10/30 or 15/45 orifice/nozzle combination is recommended.
It can sometimes be tricky to stop tiny pieces from dropping into the water jet tank. "I try to make sure there is a slat under the part so there's something there to grab it, or sometimes I will stop it before it is done and leave a small tab that I grind down later," says Jensen. "Sometimes you can catch it with a dentist's tool, or you can always put tooth picks around it so it has some resistance to prevent it from falling in. Of course, sometimes you have to cut several before you can catch one before it falls in."
For more information about water jet cutting, call (800) 538-3343, e-mail email@example.com or visit www.jetedge.com.