Home Log In / Sign Up Contact Us

Nozzles

All abrasivejet systems use the same basic two-stage nozzle design shown in Figure 2. First, water passes through a small-diameter jewel orifice to form a narrow jet. The waterjet then passes through a small chamber where the Venturi effect creates a slight vacuum that pulls abrasive material and air into this area through a feed tube. The abrasive particles are accelerated by the moving stream of water and together they pass into a long, hollow cylindrical ceramic mixing tube. The resulting mix of abrasive and water exits the mixing tube as a coherent stream that cuts the material. It is critical that the jewel orifice and the mixing tube be precisely aligned to ensure that the water jet passes directly down the center of the mixing tube. Otherwise the quality of the abrasivejet will be diffused, the quality of the cuts it produces will be poor, and the life of the mixing tube will be short. In the past, most nozzle designs required the operator to adjust the alignment of the jewel and mixing tube during operation. Modern nozzle designs rely on precisely machined components to align the jewel and mixing tube during assembly, thereby eliminating the need for operator adjustments.


Figure 2: Typical abrasivejet nozzle

The typical orifice diameter for an abrasivejet nozzle is 0.010" to 0.020" (0.25 mm to 0.50 mm). The orifice jewel may be ruby, sapphire or diamond, with sapphire being the most common. Ruby and sapphire are very similar in their life expectancy and cost. Diamond is recognized to last as much as ten times longer than the other two, and so more and more operators are finding the extra cost of a diamond to worth it.br>
In theory, a jewel orifice should operate reliably until dissolved solids and minerals in the water build up next to the water passage. The jewel does not really fail, but it no longer produces a straight, smooth stream of water because of scale build-up.

Many ruby and saphire jewels fail when struck by dirt or abrasive particles that have managed to get upstream of the jet during nozzle changes or overhauls. This causes the jewel to crack or pit, substantially altering water flow through the jewel. Once water flow through the jewel is disturbed, the cut quality will be poor and the mixing tube life will be shortened dramatically. A cracked jewel assembly can quickly ruin a ceramic mixing tube. Many operators change the jewel orifice as a matter of course whenever they overhaul a nozzle. Diamond on the other hand is much less prone to cracks or pits and may last hundreds and hundreds of hours.

The majority of mixing tubes used with abrasivejet cutting systems are manufactured by Boride Corporation. A division of Kennametal, these nozzles are made using a proprietary process originally developed by Dow Corning Corporation. The standard OMAX mixing tube has a 0.030" (0.75 mm) inside diameter and is 4" (10 cm) long. It features a smaller bore and longer length than the more-common 0.040" (0.35 mm) diameter by 3" (7.5 cm) long tube used by most abrasivejet system manufacturers. Tests indicate that the OMAX tube delivers a more precise cut with less taper than the other tubes.

Mixing tubes are expensive, typically around $100 regardless of brand. Many ceramics and tooling companies have tried to enter the market with mixing tubes at prices as low as $50, but none of these low-cost tubes have demonstrated an acceptable life span.

The venturi chamber between the jewel orifice and the top of the mixing tube is an area that is commonly subject to wear. This wear is caused by the erosive action of the abrasive stream as it enters the side of the chamber and is entrained by the waterjet. Some nozzles provide a carbide liner to minimize this wear. Other designs simply recommend periodic replacement of the entire nozzle body.

Precise alignment of the jewel orifice and the mixing tube is critical to maximizing mixing tube life. This is particularly true for the relatively small diameter 0.030" (0.75 mm) mixing tube used by the standard OMAX precision nozzle. The jewel orifice is mounted to a precisely-machined stainless steel holder which fits into a precisely-machined bore in the nozzle body. The mixing tube fits into a precisely-machined tapered section in the lower part of the jewel holder to ensure proper alignment of the mixing tube and jewel holder. If dirt, particles of abrasive, or any other contaminants prevent the jewel holder from mating precisely with the nozzle body or mixing tube, then the mixing tube life will be shortened substantially.

Regardless of the brand of nozzle, nozzles must be kept scrupulously clean when being overhauled. For maximum mixing tube life, a small ultrasonic cleaner should be used to insure that the nozzle body is free of all contaminants. The nozzle should be overhauled in the cleanest space available, not just anywhere in the shop.

In addition to cleanliness, it is critical that good water quality be maintained to insure the longest possible mixing tube life. If water entering the mixing tube has high mineral content, scale deposits will build up on the orifice jewel and affect the jet quality and mixing tube life. OMAX offers standard particle filtration and optional water conditioning as needed to insure best water quality. OMAX will also test your water before installing your equipment to help determine the need for a water purification system.