Abrading – Abrading process – Utilizing fluent abradant
Reexamination Certificate
1999-09-02
2001-08-21
Scherbel, David A. (Department: 3723)
Abrading
Abrading process
Utilizing fluent abradant
C451S036000, C451S038000, C451S089000, C451S090000, C451S091000, C451S097000, C451S101000
Reexamination Certificate
active
06276993
ABSTRACT:
This invention relates to apparatus to produce a flow of abrasive particles in a pressurised carrier fluid for the machining of materials. The carrier fluid is normally water and the abrasive a sand such as garnet. The apparatus described is particularly suitable for producing abrasive water jets less than 100 &mgr;m (microns) in diameter to meet a growing need for micro machining of high technology, difficult to machine metals, ceramics, polymers and composite materials.
The use of abrasive particles in a fluid jet is a well known method of machining. Commercially the most important form of apparatus for abrasive fluid machining uses highly pressurised water discharged through a 0.2 to 0.4 millimeter diameter nozzle to form a jet travelling at up to 800 meters per second. The water jet traverses an enclosed space before entering a ceramic focusing tube. Air laden with abrasive particles is inducted by the jet into the enclosed space and entrained into the focusing tube by the jet. In the focusing tube energy is transferred from the water to the abrasive particles to accelerate the particles to a velocity greater than 300 meters per second. The jet of water, abrasive and air leaving the focusing tube is directed onto the workpiece to be machined. In order for the entrainment process to work the focusing tube cross sectional area needs to be about 10 times greater than that of the initial cross sectional area of the high velocity water jet. This increase in area, combined with energy losses in the focusing tube results in the mean energy density of the water and abrasive impacting on the workpiece being about one twentieth of that of the water jet. However, by using ultra high pressures of 250 to 400 MPa (2500 to 4000 bar) to drive the water jet abrasive particle energy densities at the workpiece are sufficient to economically machine a wide range of engineering materials.
Typically abrasive water jets form by entraining abrasive into a high velocity water jet have diameters between 0.7 to 1.2 millimeters and these jets produce cut widths of 0.75 to 1.3 millimeters. Machining efficiency drops off rapidly as jet diameters are decreased below 0.7 millimeters and additional features are required at diameters less than 0.5 mm to induce sufficient air flow to convey the abrasive particles prior to their entry into the focusing tube. Patent No. EP 0 391 500 A2 describes apparatus that extends the operation of entrainment abrasive water jets down to 0.25 millimeters diameter, which is probably the lower economic limit for entrainment jets.
To produce abrasive water jets that do not suffer from the jet size restrictions inherent in the entrainment process, it is necessary to mix the abrasive with the water before the water is accelerated to a high velocity in a nozzle. The abrasive particle acceleration process in nozzles designed for abrasive in carrier fluids are efficient, so the jet energy densities at workpieces are close to those in the nozzle and the cut widths produced are similar to the nozzle diameter. Because of the efficient acceleration of the abrasive particles, water pressures can be reduced to 25 percent of those needed by entrainment abrasive water jet equipment. Lower water pressures are desirable because the abrasive storage, metering and flow control systems have to operate at the carrier fluid pressure, resulting in more complex apparatuses than those needed to generate entrainment abrasive water jets. The apparatus also has to accommodate complex fluid/particle and flow phenomena, and these phenomena grow in importance as abrasive particle size and fluid flow rates decrease.
One form of apparatus for generating abrasive in a carrier fluid meters abrasive particles from a storage vessel by directing about 10 percent of the water flow from a pressurised water source through the abrasive storage vessel to fluidise and carry abrasive particles out of the vessel into the approximately 90 percent of the water that bypasses the vessel. The division of pressurised water between the vessel flow circuit and the bypass circuit is achieved by restrictors in the vessel flow circuit and in the bypass flow circuit. The relative sizes of the restrictors in the vessel and bypass flow circuits determines the abrasive to water concentration at the nozzle. In the nozzle the pressure energy of the water is converted to velocity energy and the water and abrasive particles are accelerated to velocities of about 300 m/s for a water pressure of 500 bar and 500 m/s for a water pressure of 1500 bar.
Fluid compressibility is important at the pressures required to drive abrasive in carrier fluid jets. In the case of water, the volume decrease is about 1 percent per 250 bar of pressure. Water in the voids between abrasive particles occupies about 50 percent of the volume of a bed of abrasive in abrasive storage vessels and there is usually a water filled space above the abrasive bed. The volume of compressed water in abrasive storage vessels is typically equivalent to 2 to 10 seconds of the steady state pressurised water flow to the apparatus. When the supply of pressurised water is reduced or stopped the compressed water in the abrasive storage vessel expands to relieve the pressure in the vessel via the nozzle. As the compressed water expands it can cause high concentrations of abrasive in the water flowing out of the vessel and carry abrasive particles into parts of flow circuits where they can damage valves and other components.
The amount of abrasive that is expelled from an abrasive storage vessel when it is depressurised via the cutting nozzle is at a maximum when the restrictor in the abrasive vessel flow circuit is located on the inlet side of the vessel. The easiest flow path for expanding carrier fluid is out of the bottom of the vessel, generating flows that can be 80 percent of abrasive by weight. This is well above the 40 to 50 percent abrasive concentration level at which nozzle blockage is highly likely. The restrictor is usually located on the inlet side of the abrasive storage vessels because restrictors on the outlet side are more prone to blockage and wear.
It is normal to provide a means of depressurising the abrasive storage vessel via a valve to a low pressure region but it is not practical to operate the valve to deal with all upset conditions. Patent Application No. PCTIGB95/00979 describes flow circuits that use a jet pump located in the abrasive storage vessel flow circuit on the inlet side of the vessel. One of the functions of the jet pump is to provide an easier route for carrier fluid from the top of abrasive storage vessels to the nozzle during depressurisation. Another function is to reverse the flow in part of the abrasive storage vessel flow circuit to clear abrasive from the circuit. With cutting nozzles less than 0.2 millimeters in diameter the Reynolds Numbers (ratio of inertia to viscous forces) of the 10 percent of the pressurised water supply passing through the abrasive storage vessel flow circuit are so low that laminar rather than turbulent flow occurs. Fluid entrainment processes, on which the operation of jet pumps rely, are very poor in laminar flows, so the apparatus described in PCT/GB95/00979 will not function effectively at the jet sizes required for micro machining.
One method of preventing the fall off in jet pump performance is to increase the jet pump driving pressure by decreasing the diameter of the restrictor that produces the driving jet. In practice, this is not an option for jet pumps operating on the 10 percent or so of the flow that passes through the abrasive storage vessel flow circuit because they have restrictor diameters close to those of the abrasive particles and are therefore at risk of blockage by particles that reach the restrictor. The flow circuits disclosed in this invention utilise a jet pump that operates on the total supply of pressurised water to the apparatus thereby overcoming Reynolds Number and pressure drop limitations.
It is desirable that the machining action of abrasive water jets can be started and stoppe
Heslin & Rothenberg, P.C.
McDonald Shantese
Scherbel David A.
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