The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:
With reference to
One of the key parameters for selecting good abrasives is density, because very dense particles come out of the working fluid 18, or move to the edge thereof, very quickly and are more aggressive. Air in the working fluid 18 rapidly increases the removal rate, because the huge decrease in buoyancy resulting from the air causes the abrasive particles to hit the surface of the component 13 very hard; however, particles with low density (high buoyancy) do not come out of the working fluid 18 easily and do not have much affect on the component 13. If suspension agents are added to keep the particles in suspension then the erosion process seems to stop all together. Accordingly, selecting abrasive particles with high density or low buoyancy in the carrier fluid, e.g. water, is important in creating a relatively rapid removal rate. For example, cerium oxide has a specific gravity of 7.8, and zirconium oxide has a specific gravity of 5.8; accordingly abrasive particles with a specific gravity greater than 5 is preferred.
Keeping the dense abrasive particles in suspension in the working fluid 18 is normally difficult and requires stirring or the use of a suspension agent to maintain. Unfortunately, as hereinbefore noted, the suspension agent, by itself, may prevent the abrasive particles from moving to the edge of the flow and doing work. However, the dilatant additive seems to solve this problem by stiffening the fluid and holding the particles quite firmly in the working fluid 18 and greatly increasing the pressure on the component 13. Accordingly, adding both a dilatant additive and a suspension agent to the working fluid 18 is a preferable combination, which eliminates the need for stirring, while providing good removal rates for a wide variety of particle densities. The aqueous suspension agent can be selected from the group consisting of: stearic acid, palmitic acid, myristic acid, lauric acid, coconut oil, palm oil, peanut oil, ethylene glycol, propylene glycol, glycerol, polyethylene glycol aliphatic polyethers, alkyl sulfates, and alkoxylated alkyphenols. The suspension agent can also be an aqueous mixture containing fat and/or fatty acid; a mixture of stearic acid and a vegetable oil; or a material sold under the trademark EVERFLO®, which comprises mostly water, about 12½ wt % stearic acid, about 12½ wt % vegetable oil, and small amounts of methyl paraben and propylene glycol.
Multiple axis (3, 4, 5 or 6) motion systems may be used to process a wide variety of component shapes. A mechanical linkage 20 may also be added to maintain the angle of the nozzle 17 over spherical or aspheric components 13, and thereby reduce the need for multi-axis motion control systems
During erosion the end of the nozzle 17 and the component 13 are preferably submerged within the working fluid 18, whereby ambient air is not introduced into the closed loop of working fluid slurry. Any air bubbles that are present in the system simply bubble to an air pocket 15 at the top of the erosion chamber 16 and are not re-circulated, thereby producing surfaces with very smooth surface finishes. The air pocket 15 can be vented continuously or at time intervals. A drain pipe 19 at the bottom of the erosion chamber 16 evacuates the erosion chamber 16 and passes the working fluid 18 with eroded particles from the component 13 to a pump 21, which re-pressurizes the working fluid 18. Plumbing pipes 22 are used to return the working fluid 18 back to the nozzle 17.
A motion system 23, which is usually computer controlled, e.g. by computer 50 in
A property controller 24, including switch 25 and bypass pipes 26 and 27, may be added to control any one or more of the various properties of the working fluid 18, e.g. temperature, fluid density, viscosity, pH and Theological properties. If temperature control is required, a temperature sensor in the switch 25 determines the temperature of the working fluid 18 and reroutes all or a portion of the working fluid 18 through the property controller 24 via the bypass pipe 26, wherein the temperature of the working fluid 18 is adjusted higher or lower using suitable heating or cooling means. The thermally altered working fluid is passed back to the plumbing 22 via the return bypass pipe 27. The temperature of the working fluid 18 can be adjusted in order to optimize the removal rate of the component particles and/or the surface roughness of the component 13. In particle heating or cooling the tip of the nozzle 17 can affect the properties of the working fluid slurry thereby increasing or decreasing the removal rate, i.e. cooling the working fluid 18 will lead to a stiffer slurry and an increased removal rate. The property controller 24 can alternatively or also include means for altering the pH of the working fluid 18 by adding high or low pH materials thereto for optimizing the removal rate of component material and the surface roughness of the finished product.
Preferably, some means for vibrating or stirring the working fluid 18 is provided within the property controller 24 to maintain the abrasive particles in suspension and to optimize the removal rate and surface roughness. The fluid circulation system should be designed with as few horizontal surfaces as possible to minimize settling of the abrasive particles. Mixing by the normal flow of the working fluid 18 through the nozzle 17 and the pump 21 may be sufficient to keep the abrasive in suspension without additional stirring or vibrating means.
The profile of the effect of a stationary fluid jet on the surface of a component creates a tool pattern in the shape of an annular ring, e.g. a donut, for a vertical nozzle or in the shape of a teardrop for an angled nozzle. A computer program controlling the motion system 23 is used to optimize the dwell time of the tool pattern on the surface of the component 13 in order to achieve the desired final surface shape and smoothness. Typically, the pressure of the fluid jet of working fluid 18 remains constant and the velocity (or dwell time) of the nozzle 17 is varied to remove the desired amount of material from different areas of the component 13. Alternatively, the pressure of the working fluid 18 can be altered or the nozzle 17 can remain fixed and the component 13 can be moved, e.g. reciprocated, using the moveable platform 12, as hereinbefore discussed. The pressure of the working fluid 18 can be actively changed during the erosion process to provide different removal rates for different portions of the surface of the component 13.
Dwell time calculated for a grid of points distributed over the surface of the optical component 13 can be converted to velocity profile using v(x,y)=d/T(x,y) where v(x,y) is desired velocity between adjacent points and T(x,y) is the calculated dwell time for the second point. Normally, the tool, e.g. nozzle 17, is moved in a raster pattern so the conversion is only applied in one direction.
Preferably, the nozzle 17 is disposed substantially vertically for launching a slurry of working fluid 18 at a constant velocity at the surface of the component 13, traveling back and forth in a simple grid pattern in the x and y directions substantially perpendicular to the surface of the component 13 with the dwell time over each position on the grid determining the amount of material removed. The coordinates of the component 13 are predetermined or determined by the computer system 50, whereby the computer system 50 can then determine the dwell time at each grid position based on the requirements, i.e. desired characteristics, e.g. dimensions, surface roughness, of the finished product. Sensors in the erosion chamber 16 and/or on the part holder 12 can be used to measuring the properties of the component 13, while the component 13 is being processed in order to create a closed loop system, thereby improving the speed and accuracy thereof.
To provide added control over the erosion process, the orifice of the nozzle 17 can be provided with an adjustable opening or a plurality of nozzles 17, each with different sized openings, can be provided. To increase the removal rate, the size of the orifice is increased or a nozzle 17 with a larger orifice is used. To increase the resolution of the removal, the size of the orifice is reduced or a nozzle 17 with a smaller opening is used. Alternatively, the shape or angle of the nozzle 17 can be changed or altered to create various tool profiles, e.g. disposing the nozzle 17 at an acute angle from vertical creates a tear drop shaped profile. Multiple nozzles 17 can also be provided to increase the speed of particle removal. The distance of the nozzle 17 from the component 13 can be adjusted between runs or actively during each run in order to optimize the resolution, removal rate of particulate material and surface roughness of the component 13. Masks can be provided to prevent the working fluid 18 from contacting certain areas of the component 13 to thereby create deep channels and concave areas. Air, or some other suitable gas for decreasing buoyancy, can be introduced into the working fluid 18 proximate the nozzle 17 or any other suitable location to increase removal rate or affect the surface roughness of the finished product.
With reference to
With reference to
In the detailed embodiment shown in
A typical diaphragm pump would operate well above 1 Hz, say 5, 10, 20, 60 Hz+, the pump 21 is preferably slower than 1 Hz, typically a few seconds to several minutes. In the jet polishing process according to the present invention, the slower the nozzle 17 is moved, the more material gets removed, i.e. the faster the nozzle 17 moves, the less material gets removed. Accordingly, on a component 13 in which the shape is to be significantly changed, it is necessary to move fast while making a pass on some rows and slower while making a pass on other rows. Therefore, it is important to have a wide dynamic range in pump speed, e.g. 5 seconds to 5 minutes. However, if there is not enough hydraulic working fluid in the first and second pumping chambers 32 and 33 or not enough abrasive fluid 18 in the system for a 5 minute pass, a double pass for 2.5 minutes can be done. The key is that the switching of the pump 21 is under complete control of computer 50, i.e. the same computer that controls the motion system 23 of the nozzle 17, whereby the pump 21 alternates between the first and second pumping chambers 32 and 33 at the same time as the nozzle 17 ends one pass on the part 13 and starts another. Typically, the pump 21 is operated to alternate between pumping chambers at an interval of between 5 seconds and 1 minute.
The present invention claims priority from U.S. Patent Application No. 60/824,629 filed Sep. 6, 2006, which is incorporated herein by reference.
Number | Date | Country | |
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60824629 | Sep 2006 | US |