EP1618993B1 - Process for grinding and/or polishing surfaces - Google Patents

Abstract

The assembly (1) to remove material from a workpiece surface (S), by grinding and/or polishing, uses abrasive particles (5) carried in a liquid (3) mostly of water. A gas (4) and especially air is fed into the liquid or its holding tank (12) through a volume control (8) by a pump (7). At the workstation, a jet (9) accelerates the flow of liquid and particles against the workpiece surface. A gas pressure is used for grinding, and no pressure is used for polishing.

Description

The invention relates to a method for removing material during the grinding and / or polishing of surfaces of workpieces according to claim 1.

It is known to grind or polish surfaces with abrasive particles conveyed in a liquid. This has the advantage that it is possible to dispense with a grinding tool.

On the one hand, the removal rate of the surface depends on the size of the abrasive particles in the jet. On the other hand, it depends on the energy and thereby the speed of the particles at the time when they hit the surface. The material removal rate per time is considered as the removal rate.

Out US 5,700,181 For example, a method of grinding and a nozzle for accelerating the abrasive liquid are known. The abrasive liquid is conveyed under high pressure into the nozzle and accelerated from there to the surface. The acceleration takes place on the one hand in the nozzle, on the other hand in the gap between the nozzle and the workpiece to be machined.

A similar procedure is off EP 1 409 199 known. Instead of a nozzle, a tool with a wide outlet is used. The acceleration of the abrasive particles takes place in the gap between the outlet and the surface.

These known methods have the disadvantage that coarser abrasive particles must be used in the highly abrasive machining, especially grinding, than for fine grinding and polishing. The coarser particles also result in coarser sanding marks, which then have to be smoothed with the finer grain. The transition from grinding to polishing can not be continuous. Between grinding and fine grinding / polishing, the abrasive must be changed.

The use of differently coarse particles requires extremely thorough cleaning of the tool and the workpiece if it is to be finely ground or polished. If coarser particles are present during fine grinding or polishing, they lead to deep scratch marks, which can render the entire workpiece unusable.

Often different devices are used for grinding and polishing. These devices can work with different pressures, for example. This increases the procurement or manufacturing costs. In addition, the workpiece must be transported during the machining process. This in turn increases working time and manufacturing costs. This can also cause contamination or the workpiece can be damaged,

It is the object of the present invention to avoid the disadvantages of the known, ie in particular to provide an improved method for grinding and / or polishing surfaces in which a change between grinding and polishing can be done in a simple manner. In particular, the removal rate should be set individually as simply as possible.

This object is achieved by a method for removing material during grinding and / or polishing surfaces according to claim 1. It has surprisingly been found that with increasing proportion of gas with the same size of the abrasive grain and the same process parameters (for example delivery pressure), the removal rate is increased. Therefore, when the device is provided with means for adjusting the gas content in the liquid, the rate of removal can be easily controlled or changed. Thanks to the present invention, it is possible to achieve high removal rates with relatively small grain sizes. By setting a high proportion of gas, the removal rate is increased without the need to use larger abrasive grain.

Advantageously, the removal rate and also the transition from coarse grinding to fine grinding and polishing can be varied continuously. Also, the grain size of the abrasive grains and the delivery pressure of the abrasive liquid for coarse and fine grinding and for polishing can be the same. By adjusting the gas content in the liquid, the removal rate is easily controlled and changed as needed. It can be worked with the same delivery pressure and with the same abrasive grain. The change from one grain size to another and the associated cleaning of the machine is eliminated.

The device for removing material from a surface of a workpiece during grinding or polishing of the surface by means of abrasive particles conveyed by a liquid for this purpose contains a device for adding gas to the liquid.

This gas is preferably air because air is inexpensive and does not pollute the environment. Other gases or gas mixtures, for example argon, helium, nitrogen, CO ₂ or oxygen, but can also be used and are not excluded from the invention. The gas can be contained either in the form of gas bubbles or in dissolved or bound form in the liquid.

The present invention has the advantage that with a device as needed a workpiece can be both ground and polished. The more gas the abrasive fluid contains, the greater the removal rate. Expensive and procedurally delicate steps such as cleaning the workpiece or the device or the transport of the workpiece from one device to another thus eliminated.

The advantages of the present invention are particularly advantageous in the grinding and polishing of high precision parts, e.g. Parts of optics or medical technology. Due to the broad applicability of the device and the method, both spherical and aspherical shapes can be precisely ground and polished.

It is advantageous if the device for adding the gas to the liquid comprises an arrangement for adjusting the amount of added gas. It is advantageous if the amount of gas supplied is continuously variable. This can be used e.g. increase the removal rate by increasing the amount of gas. Reducing the amount of gas leads to smaller removal.

The device for adding the gas to the liquid can be arranged, for example, in a tank for the liquid be. It is also conceivable to install the device in a pump for conveying the liquid or in a conveying line for the liquid. Combinations thereof for more efficient and controlled addition of the gas are advantageous.

The gas can be added with a propeller in the tank, for example. Furthermore, a pressure connection to the line, the tank or the pump can be provided. Moreover, it is also possible to chemically oxidize the gas, e.g. to be enclosed with tablets. For example, in the form of effervescent tablets, a chemical substance pressed in tablet form can be added to the water, which reacts on contact with water and gives off gaseous reaction products. It is also conceivable to add gas to the water electrolytically or to generate gas electrolytically in the water. By applying a voltage in the abrasive liquid, e.g. Water molecules in the grinding fluid to be separated into hydrogen and oxygen. Combinations of said possibilities are also included in the invention.

The gas content is controlled and adjusted according to the invention. Typically, the amount of gas (in volume percent) is adjusted to desired levels. In this case, the gas can either be dissolved or bound in the liquid or present in the form of bubbles therein.

It is also possible to use a regulating arrangement for regulating the gas content. The gas content in the liquid is continuously measured and adjusted depending on this measurement.

When the gas is dissolved or bound, the mixing ratio can be determined by the viscosity of the abrasive fluid. ever the greater the proportion of gas in the liquid, the lower the viscosity.

If the gas is in bubbles, the bubble size may be e.g. be determined optically. In order to have a favorable influence on the removal rate, it is advantageous if the resulting gas bubbles are larger than the abrasive particles.

In both cases, the measurement preferably takes place between a pump for the abrasive fluid and an outlet port for the fluid.

The gas content in the liquid controls the removal rate at the surface of the workpiece. The greater the gas content in the abrasive fluid, the greater the removal rate.

The removal rate depends on the impact energy of the abrasive particles. The transition from grinding to polishing surfaces can be directly controlled by the gas content in the abrasive fluid. The smaller the gas content in the abrasive fluids, the smaller the removal rate, and the sooner the surface is polished. For brittle surfaces, eg workpieces made of ceramic or glass, the transition from grinding to polishing can at the same time correspond to a transition from brittle to ductile removal of material from the surface. (See, eg Appl. Opt. 37, pp. 6771-6773 (1998) )

It is believed that the removal rate is increased by the gas content in the liquid, because the gas inclusions are more compressible on impact than the liquid itself. The abrasive particles are thus braked faster than the impact of the liquid just before the impact. The bigger the gas content, the greater the impact energy of the abrasive particles.

Further, particles that are in a gas bubble are less strongly cooled when hitting the surface than particles that are surrounded by liquid. It is believed that the increased temperature increases the likelihood of cracks forming. As a result, the removal rate is increased, because in the brittle removal of material the erosion is preceded by cracking on the surface.

If the gas is not present in bubbles but is dissolved in the liquid, the viscosity of the abrasive liquid decreases. At the same delivery pressure, the abrasive fluid is thereby accelerated more. This acceleration increases the impact energy of the abrasive particles.

It is also possible that other factors contribute to the increased ablation rate.

A method according to the preamble of claim 1 is disclosed in document US 5,283,991 described.

The inventive method is designed to generate a delivery pressure of less than 100 bar, more preferably less than 50 or even less than 20 bar. Since a gas is added to the liquid, the impact energy of the abrasive particles can be increased. This allows a reduction of the delivery pressure with the same removal rate. Compared to high-pressure arrangements energy is thus saved during conveying, which has an economic and ecological advantage. A further reduction of the delivery pressure can also be achieved by appropriate, suitable arrangements for accelerating the abrasive fluid. So, for example an arrangement for accelerating the abrasive liquid are provided, in which the abrasive liquid is advantageously accelerated to a speed of about 20 m / s. As a result, the abrasive particles receive the necessary energy, so that material is removed. The energy of the particles grows quadratically with the speed of the beam.

The acceleration arrangement can have, for example, one or more nozzles.

The arrangement for accelerating the abrasive liquid may also include an arrangement for adjusting the distance between at least one exit opening of the liquid and the surface. The acceleration is then adjustable between the at least one outlet opening and the surface as a function of the cross section of a liquid feed opening and the cross section of the gap between the surface and the at least one outlet opening. The smaller the gap compared to the cross section of the liquid feed opening, the stronger the acceleration of the abrasive liquid. For example, the arrangement according to EP 1 409 199 be designed.

The combination of mechanical measures to increase the exit velocity and the addition of gas to increase the impact velocity of the particles on the surface to be machined leads to a particularly effective manner to a reduction of the necessary delivery pressure. At the same time an individual adjustment of the removal rate is possible due to the adjustability of the gas content. It is particularly advantageous that with the same process parameters and the same abrasive liquid By supplying gas a better removal rate can be achieved.

The abrasive particles have an average grain diameter less than 50 microns, preferably 1 to 10 microns .. The grain size varies but depending on the application. The finer the surface to be polished, the finer abrasive particles are used.

If gas bubbles are generated, their diameters should be greater than the diameter of the abrasive particles

The delivery liquid is preferably predominantly water. However, other known liquids are also within the scope of the invention. Other liquids, e.g. oil or alcohol based may also be suitable.

The use of gas inclusions of a conveying fluid for abrasive particles for influencing the removal rate when removing material of a surface of a workpiece during grinding or polishing of the surface is also within the scope of the invention. The gas is preferably air.

In the method for removing material from a surface of a workpiece for grinding or polishing the surface with abrasive particles that are conveyed by a liquid, the removal rate is influenced by the adjustment of the gas content in the pumped liquid, by adding a gas to the abrasive. Liquid. Preferably, the gas is air, but other gases are not excluded from the invention.

The abrasive liquid is added to gas from 0 to 70% of the total volume. The greater the amount of added gas, the greater the removal rate. To polish the surface little or no gas is added, since during polishing a low removal rate is desired.

It is quite conceivable during the removal process to vary the amount and / or nature of the gas bubbles produced or the mixing ratio of the dissolved or bound gas to the liquid. Thus, it is particularly conceivable to achieve high removal rates in a first step by supplying a large amount of gas and to reduce the amount of gas supply in a second step during the same processing method and thus to reduce the removal rate. In this case, polishing takes place in a second step with the addition of less gas or without addition of gas.

In a preferred embodiment, the reduction of the gas supply is continuous. The transition from grinding to polishing is not discrete.

The delivery pressure of the abrasive liquid is kept constant for grinding and polishing at below 100 bar, advantageously below 50 bar, particularly advantageously below 20 bar. This simplifies the process because the delivery pressure of the liquid does not have to be changed if the removal rate is to be varied.

Figur 1

ein Ausführungsbeispiel einer Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens in einer schematischen Darstellung,

Figur 2

eine alternative Ausführungsform der Vorrichtung in einer schematischen Darstellung und,

Figur 3

eine weitere alternative Ausführungsform der Vorrichtung in einer schematischen Darstellung.

In the following the invention will be explained with reference to exemplary embodiments and drawings. Show it:

FIG. 1

An embodiment of an apparatus for carrying out the method according to the invention in a schematic representation,

FIG. 2

an alternative embodiment of the device in a schematic representation and,

FIG. 3

a further alternative embodiment of the device in a schematic representation.

In FIG. 1 a first embodiment of the invention is shown schematically. A workpiece 2 is located on a holder 10 in a closed container 11. The surface S, which is to be ground and polished, is located on the upper side of the workpiece 2.

The holder 10 is displaceable along a longitudinal axis and rotatable about a vertical axis. The movement of the holder 10 is indicated by the arrows B.

In a liquid tank 12 is a liquid 3, in this example water. To the water 3 are added abrasive particles 5 in a concentration of 10% by weight. There are used abrasive particles 5 of silicon carbide with a particle size of 7 microns in diameter. The abrasive liquid 3 is conveyed by a pump 7 in the direction of the arrows 6 through the delivery line 14. The delivery pressure is 12 bar.

The removal of material takes place at room temperature, ie at about 21 ° C instead. Due to the mechanical stress, the abrasive fluid can heat up to approx. 25 ° C, which does not affect the removal rate negatively.

The delivery line 14 leads into the interior of the container 11. There, the water 3 is accelerated with a nozzle 9 to about 40 m / s. The nozzle 9 is directed to the surface S, which is moved with the holding device 10 below.

In a pressure tank 13 is air 4, which can be introduced via a connecting line 16 in the water 3. The air supply is varied via a valve 8. The greater the desired removal rate, the more air 4 is added to the water 3. The maximum air content in the water 3 is 70% of the total volume. The pressure in the tank 13 is 250 bar. The air 4 is supplied at a pressure of 14 to 50 bar. It is essential here that the supply pressure of the air is greater than the delivery pressure of the water. For polishing, the air supply is prevented or at least reduced.

The used water 3 is fed via a drain 15 back into the tank 12. The abrasive particles 5 are thus in a closed circuit, which is indicated by the arrows 6. As a result, the material consumption is kept as low as possible.

The air supply is calibrated on a reference workpiece (not shown). The removal rate can then be adjusted as needed on the workpiece 2 due to this calibration.

An alternative embodiment of the device 1 is shown in FIG FIG. 2 shown. As in the embodiment according to FIG. 1 is the abrasive liquid 3 in this embodiment in a closed circuit, which is indicated by the arrows 6. The liquid 3 used is again water.

To accelerate the water 3 to the surface S of the workpiece 2 is in this example, in contrast to the example according to FIG. 1 no nozzle used. Instead, the abrasive liquid 3 exits through a delivery head 17. The acceleration of the water 3 takes place in the gap 18, which is formed between the head 17 and the surface S. To regulate the width of the gap 18, the holding device 8 as indicated by the arrows B also displaceable in a vertical direction. The acceleration is greater the smaller the area of the gap compared to the cross-sectional area of the delivery line 14. The ratio of the cross-sectional area of the delivery line to the area of the gap is 10: 1 in this example.

The removal rate is controlled by the supply of air 4 in the water 3 in the tank itself. Air 4 is located in the tank 12 above the water 3 and is stirred with a propeller 19 under the water 3. The propeller 19 is driven by an electric motor M. The speed of the motor M determines the amount of air 4 supplied. Since the water 3 is used in a closed circuit, the level of the water 3 is relatively constant. The propeller 19 is large enough in comparison to the water tank 12 that fluctuations in the water level do not adversely affect the amount of air supplied.

FIG. 3 shows a device analog FIG. 1 , In addition, however, a device for reducing the air content in the water 3 in the tank 12 is provided for carrying out a method not according to the invention. The gas content of water may vary due to environmental parameters.

According to FIG. 3 3 ultrasonic transducers 20 are mounted in the tank 12 to reduce the air content in the water. These send out ultrasonic waves 21, which displace the air inclusions in the water 3.

Claims (5)

1. Process for removing material from, in particular grinding and/or polishing, a surface (S) of a workpiece using abrasive particles (5) which are delivered by a liquid (3), wherein the gas content in the liquid is set, the abrasive liquid (3) is supplied with gas (4) amounting to 0 to 70% of the volume, characterized in that the abrasive particles (5) have a mean grain diameter of less than 50 µm, preferably from 1 to 10 µm, and in that a delivery pressure of the abrasive liquid (3) less than 100 bar, in particular less than 50 bar is generated.
2. Process according to Claim 1, characterized in that gas (4) is admixed with the abrasive liquid (3) during a first step for grinding the surface (S), and in that no gas (4) is admixed with the abrasive liquid in a second step for polishing the surface.
3. Process according to one of the preceding Claims 1 to 2, characterized in that the delivery pressure of the abrasive liquid (3) is kept constant at less than 100 bar, in particular less than 50 bar, for both grinding and polishing.
4. Process according to one of Claims 1 to 3, characterized in that the liquid (3) used is predominantly water.
5. Process according to one of Claims 1 to 4, characterized in that gas bubbles which are larger than the abrasive particles (5) are generated in the liquid (3).

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