Abrading – Abrading process – Utilizing fluent abradant
Reexamination Certificate
2002-02-05
2004-02-10
Eley, Timothy V. (Department: 3724)
Abrading
Abrading process
Utilizing fluent abradant
C451S038000, C451S053000, C451S056000, C451S102000, C451S449000
Reexamination Certificate
active
06688947
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to fluent abrading processes and apparatus. More particularly, this invention relates to an improved nozzle for an abrasive fluid jet cutting apparatus.
2. Description of the Related Art
Cutting with water is a well-known technology that has been prevalent since the 1970's. Water jet cutting is one of a number of technologies known as power beams. These include laser cutting, plasma arc cutting and oxy-acetylene gas cutting.
By utilizing a high-pressure pump to pressurize water to ultra high pressures and then forcing the water to flow through a tiny orifice, one can produce water jets that have velocities that are up to three times the velocity of sound. Such a focused water jet has sufficient kinetic energy to cut through most hard-to-cut materials, and when abrasives are mixed with the water flow so as to yield an abrasive water jet, one can efficiently cut almost any type of material.
Because of their greater cutting power, abrasive water jets account for nearly sixty percent of the water jet cutting market. Typical applications include the cutting tasks associated with fabrication of structures using extremely hard materials, such as titanium and the super-alloys, and in various mining and drilling applications where hard rocks must be cut. Meanwhile, plain water jets are used for industrial cleaning, surface preparation and paint stripping applications, and for the cutting of food products, paper and plastic materials, and woven (e.g., carpet) and nonwoven (e.g., filtration materials) products. Saline, water cutting jets have also been used in medical applications.
The primary equipment associated with a typical, abrasive water jet cutting system is shown in FIG.
1
. It consists of an incoming water treatment system, a booster pump for optimal operation of downstream filters, an intensifier pump that raises the water's pressure to ultrahigh levels, high pressure plumbing that delivers the ultrahigh pressure water to the system's cutting head, an abrasive feeder system that supplies the abrasive particles that are mixed with the water either before or in the cutting head, and an outgoing water catcher and treatment system.
Two types of cutting heads for abrasive water jets are in common use today. These are denoted as either an abrasive entrainment jet (AEJ) head or an abrasive suspension jet (ASJ) head.
The abrasive entrainment jet (AEJ) head utilizes an orifice constructed from a very hard material (e.g., sapphire, diamond) to create a high velocity water jet. A dry abrasive, such as garnet, silica or alumina, is then aspirated or entrained into the mixing chamber by the vacuum created by the water jet. It mixes with the water jet and the mixed slurry jet is then collimated by a mixing tube (also called a focusing tube) before exiting the cutting head through the mixing tube's exit orifice. See
FIG. 2
for cross-sectional view of the typical AEJ head that is used in an abrasive water jet cutting system.
The abrasive suspension jet (ASJ) head utilizes a premixed slurry of abrasives and water from which to create a high velocity jet by forcing the premixed slurry through an orifice or nozzle that is typically made of diamond. See
FIG. 3
for cross-sectional view of the typical ASJ head that is used in an abrasive water jet cutting system.
Despite the apparent similarities between these cutting heads, they possess significant operational and performance differences. These include:
(a) the mixing process in an AEJ is extremely inefficient and only a fraction of the energy of the high-speed water is transmitted to the abrasive particles. Hence, the speed of the collimated jet is substantially reduced. In an ASJ however, the energy transfer is quite effective resulting in higher particle velocities. For similar hydraulic and suspension concentration parameters the specific cutting power (particle's kinetic power per unit jet diameter) of an ASJ is four times that of an AEJ;
(b) the process of entrainment by a vacuum in an AEJ limits the amount of abrasives that can be added to the jet and this results in a sparse distribution of particles in the cutting jet. Also, the particle velocities in an AEJ decrease as the abrasive mass flow increases. Higher abrasive mass flow rates can be pumped with an ASJ, which improves their cutting effectiveness;
(c) the entrained abrasives in an AEJ becomes fragmented during the mixing process as a result of the higher-velocity water jet hitting them. Since the cutting effectiveness of the jet has been shown to increase with the size of the abrasive particles, the reduced size particles hitting a workpiece make the AEJ less efficient than it might otherwise be;
(d) for a small diameter AEJ to have sufficient abrasive particle entrainment capabilities, it must be operated at high pressures;
(e) an ASJ is capable of utilizing a smaller orifice diameter, which means that the cutting width of such a jet can be smaller than that of a comparable strength AEJ, and
(f) For the same abrasive particle velocity, an ASJ utilizes lower pressure pumps than an AEJ. An ASJ can better handle operation with dirty water than can an AEJ, as the AEJ needs highly purified water and clean operating conditions to raise the carrier fluid (water) pressure to ultra high pressures.
Both AEJ and ASJ cutting heads are plagued by the wear and erosion problems that are associated with their use of abrasive particles. Even using very hard materials, the high speed of the fluid through such cutting heads can rapidly destroy the ASJ's nozzle or the AEJ's mixing tube. Further, as these cutting head elements erode, the cutting jet's kerf, or width of cut, changes, as does the dispersion of the fluid upon exiting from the cutting heads. Consequently, the nozzle and mixing tube elements of the respective ASJ and AEJ heads must be replaced frequently, resulting in constant maintenance and inspection, loss of accuracy, and machine down time, all of which add to the cost of using such cutting apparatus.
Prior attempts to solve this wear problem have generally concentrated on trying the minimize the damage resulting from the occurrence of abrasive particle-to-adjoining wall-contact. Means to accomplish this have included seeding a pure liquid jet with abrasive particles only downstream of the nozzle, use of nozzles and mixing tubes made of very hard materials (such as diamonds, sapphire and tungsten carbide), using abrasive particles that are softer than the nozzle or mixing tube walls, and attempting to modify the flow structure through these elements in order to keep abrasive particles away from the wall surfaces. All of the presently available techniques have major deficiencies.
Seeding downstream of the nozzle reduces the speed of the abrasive particles, and causes considerable expansion, scattering, and unsteadiness of the fluid flow. Nozzles fabricated from very hard materials are expensive and almost impossible to form into desirable shapes. Use of abrasive particles softer than the adjoining walls reduces cutting efficiency. Modification to the jet flow structure by introducing secondary swirling flows near the adjoining walls is useful only with relatively slow flows and small abrasive particles; such modification also causes jet expansion and secondary flow phenomena that limit the capability to control the process. Attempts have also been made to try to minimize the actual occurrence of abrasive particle-to-adjoining wall-contact. Tan and Davidson (1990) and Tan (1995 and 1998) suggested the use of porous nozzles in cutting jet applications. They studied flows through porous nozzles at low operating pressure (1-2 MPa) and where the fluid flowing through the porous walls, which was water, was of the same approximate viscosity as the carrier fluid for the abrasive particles. Because these studies were performed at low pressures (i.e., at low velocities), it is impossible to extrapolate their results to predict how such porous nozzles might perform under the t
Anand Umang
Katz Joseph
Eley Timothy V.
Guffey Larry J.
The Johns Hopkins University
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