Abrasive Flow Machine, Method for Ascertaining Material Removal on a Workpiece, and Method for Determining the Cutting Power of a Grinding Medium

Abstract

An abrasive flow machine is indicated, having a media drive device which is adapted to move the abrasive medium over a surface of a workpiece and/or through the opening of the workpiece in a flow direction, having a workpiece holder for mounting the workpiece with two parts adapted to be positioned on opposite sides of the workpiece, having a structure-borne sound sensor for measuring structure-borne sound generated in a workpiece when the latter is machined with an abrasive medium, and having an evaluation unit which is adapted to infer a cutting power of the abrasive medium and/or a rate of material removal on the workpiece based on an integrated measurand of the root mean square of the structure-borne sound measured by the structure-borne sound sensor over time. Further indicated are a method of ascertaining a material removal on a workpiece and a method of determining the cutting power of an abrasive medium.

Claims

1. An abrasive flow machine for moving abrasive media over a surface of a workpiece and/or through the opening of a workpiece, comprising a media drive device which is adapted to move the abrasive medium over a surface of a workpiece and/or through the opening of the workpiece in a flow direction; a workpiece holder for mounting the workpiece, having two parts adapted to be positioned on opposite sides of the workpiece; a structure-borne sound sensor for measuring structure-borne sound generated in a workpiece when the latter is machined with an abrasive medium; and an evaluation unit which is adapted to infer a cutting power of the abrasive medium and/or a rate of material removal on the workpiece based on an integrated measurand of the root mean square of the structure-borne sound measured by the structure-borne sound sensor over time.

2. The abrasive flow machine according to claim 1, wherein the evaluation unit has a look-up table stored therein, from which a material removal on the workpiece and/or a cutting power of the abrasive medium can be read based on the integrated measurand of the root mean square of the measured structure-borne sound signal over time.

3. The abrasive flow machine according to claim 1, wherein the evaluation unit includes an amplifier for amplifying the signal measured by the structure-borne sound sensor.

4. The abrasive flow machine according to claim 1, wherein the evaluation unit includes at least one filter for filtering out machine frequencies from the signal measured by the structure-borne sound sensor.

5. The abrasive flow machine according to claim 1, wherein the structure-borne sound sensor is in direct or indirect contact with the workpiece during operation of the abrasive flow machine.

6. The abrasive flow machine according to claim 1, wherein the abrasive flow machine comprises a bypass duct that extends parallel to a fluid main channel, and in that the workpiece is a dummy workpiece arranged in the bypass duct, the fluid main channel extending over a surface and/or through an opening of an additional workpiece to be machined.

7. The abrasive flow machine according to claim 6, wherein the abrasive flow machine comprises two structure-borne sound sensors, wherein, during operation of the abrasive flow machine, a respective structure-borne sound sensor is positioned on the dummy workpiece in the bypass duct and also on the additional workpiece to be machined.

8. The abrasive flow machine according to claim 1, wherein the abrasive flow machine comprises a checking unit which is adapted to adjust at least one process parameter based on the cutting power and/or the material removal rate ascertained by the evaluation unit during the machining of a workpiece.

9. The abrasive flow machine according to claim 8, wherein the process parameters that can be adjusted by the checking unit are a flow velocity of the abrasive medium, a fluid pressure of the abrasive medium, a back pressure on the abrasive medium, and/or a temperature of the abrasive medium.

10. The abrasive flow machine according to claim 1, wherein the abrasive flow machine comprises a fluid conveying device that is adapted to discharge worn abrasive medium from the abrasive flow machine and to supply unused abrasive medium.

11. A method of ascertaining a material removal and/or a rate of material removal on a workpiece when the workpiece is machined in an abrasive flow machine, comprising the steps of: passing an abrasive medium over a surface and/or through an opening of a workpiece to be machined; measuring the structure-borne sound generated in the workpiece during machining; forming a root mean square of the measured structure-borne sound signal; integrating the root mean square over the machining time; and determining the material removal and/or the rate of material removal on the workpiece on the basis of the integral formed.

12. The method according to claim 11, wherein a cutting power of the abrasive medium is determined based on the integrated root mean square of the structure-borne sound signal divided by a flow rate of the abrasive medium.

13. The method according to claim 11, wherein based on a material removal rate, at least one of the following process parameters is set: a flow velocity of the abrasive medium, a fluid pressure of the abrasive medium, a back pressure on the abrasive medium and/or a temperature of the abrasive medium.

14. The method according to claim 11, wherein at least one process parameter is adjusted if the material removal rate measured during machining of the workpiece deviates from a desired material removal rate by more than a defined tolerance value.

15. The method according to claim 11, wherein a reference curve describing a desired profile of the material removal rate is established by storing a profile of the material removal rate of a machined workpiece in an evaluation unit, and wherein a tolerance range about the reference curve is defined, within which the material removal rate is to be.

16. The method according to claim 15, wherein at least one process parameter is adjusted during the machining of the workpiece if the actual profile of the material removal rate is outside the tolerance range.

17. The method according to claim 11, wherein based on a material removal rate, a request to replace at least part of the abrasive medium is made.

18. The method according to claim 11, wherein a further structure-borne sound signal is additionally measured on a dummy workpiece and a difference of the two structure-borne sound signals is formed in order to monitor an improvement in the surface finish of the workpiece.

19. A method of determining the cutting power of an abrasive medium, comprising the steps of: passing an abrasive medium over a surface and/or through an opening of a reference workpiece; measuring the structure-borne sound generated in the reference workpiece; forming a root mean square of the measured structure-borne sound signal; integrating the root mean square over the machining time; dividing the integrated measurand by the media flow rate; and determining the cutting power of the abrasive medium using the divided integrated measurand.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Further advantages and features of the invention will be apparent from the description below and from the accompanying drawings, to which reference is made and in which:

[0068] FIGS. 1a and 1b each show a part of an abrasive flow machine according to the invention;

[0069] FIG. 2 shows the profile of the root mean square of a structure-borne sound signal;

[0070] FIG. 3 shows a partial area of a further abrasive flow machine according to the invention in the region of a workpiece holder;

[0071] FIG. 4 shows a further abrasive flow machine according to the invention;

[0072] FIG. 5 shows a flow chart for the processing of a structure-borne sound signal;

[0073] FIG. 6 shows a reference curve describing a desired profile of the material removal rate;

[0074] FIG. 7 shows a profile of a material removal rate during machining of a workpiece; and

[0075] FIGS. 8a and 8b show profiles of a directly measured material removal rate and an indirectly ascertained material removal rate, respectively.

DESCRIPTION OF THE INVENTION

[0076] FIGS. 1a and 1b each show a sectional representation of an abrasive flow machine 10 for machining a workpiece 12 with a viscous abrasive medium 14. To machine the workpiece 12, the abrasive medium 14 can be moved through an opening 16 in the workpiece 12 to be machined.

[0077] In particular, the abrasive medium 14 moves in a fluid main channel 15 that extends through the opening 16 of the workpiece 12.

[0078] Alternatively or additionally, the abrasive medium 14 may be moved across a surface of the workpiece 12 that is outside of the opening 16, with the fluid main channel 15 being at least partly defined by the surface of the workpiece 12.

[0079] FIGS. 1a and 1b depict the abrasive flow machine 10 in respective different states in which the abrasive medium 14 is moved in opposite directions, as illustrated by arrows, i.e. is pumped back and forth.

[0080] For mounting the workpiece 12, the abrasive flow machine 10 comprises a workpiece holder 18, which includes two parts 20, 22 adapted to be positioned on opposite sides of the workpiece 12. During machining, the workpiece 12 is, e.g., axially clamped between the two parts 20, 22 of the workpiece holder 18.

[0081] In order to obtain a particularly good sealing, the parts 20, 22 may have seals fitted to them, which rest against the workpiece 12 during machining. The seals are preferably metal seals or ceramic seals; rubber seals would in fact dampen the structure-borne sound signal.

[0082] The abrasive flow machine 10 further comprises a media drive device 24 that is adapted to move the abrasive medium 14 over a surface of a workpiece and/or through the opening 16 of the workpiece 12. This process removes material from the workpiece 12, thereby finishing and polishing the surface of the workpiece 12.

[0083] In the exemplary embodiment shown, the media drive device 24 comprises two displacement pumps 26, 28; depending on the direction of flow of the abrasive medium 14, one of the two displacement pumps 26, 28 forces the abrasive medium 14 through the opening 16 and the respective other displacement pump 26, 28 constitutes a device counteracting the abrasive medium 14 and which counteracts the flow of the abrasive medium 14. A displacement pump 26, 28 each has a piston 30 guided in a cylinder 32.

[0084] The media drive device 24 furthermore comprises a drive element 34 for each displacement pump 26, 28, the drive element being, for example, a hydraulic actuating element or a linear motor actuating element.

[0085] When the workpiece 12 is machined using the abrasive medium 14, structure-borne sound waves are produced in it, in particular acoustic and/or elastic waves. Based on the intensity of the structure-borne sound waves, conclusions can be drawn about the effectiveness and the extent of the machining of the workpiece 12. In particular, an intensity of the structure-borne sound waves correlates with a rate of material removal on the workpiece 12 and/or with a cutting power of the abrasive medium 14.

[0086] The cutting power of the abrasive medium 14 is given in units of millivolt-seconds per liter [mV.Math.s/l].

[0087] The cutting power indicates how much material is removed per liter of abrasive medium 14 moved over the workpiece 12.

[0088] To measure the structure-borne sound waves, the abrasive flow machine 10 comprises a structure-borne sound sensor 36 that is in contact with the workpiece 12 to be machined.

[0089] Furthermore, the abrasive flow machine 10 comprises an evaluation unit 38, in particular an electronic evaluation unit 38. The evaluation unit 38 can receive the structure-borne sound signal measured by the structure-borne sound sensor 36 and, in particular by means of software, form the root mean square of the structure-borne sound signal and integrate it over time. Alternatively, the evaluation unit 38 may, for example, already receive the root mean square of the structure-borne sound signal from an amplifier. The structure-borne sound signal is measured in millivolts or milliamperes, for example.

[0090] For evaluation of the structure-borne sound signal, it is preferably rectified before the root mean square is formed.

[0091] The profile of the root mean square of the structure-borne sound signal in millivolts over time in seconds is shown for a machining operation as an example in FIG. 2.

[0092] In addition, the evaluation unit 38 is suitable for inferring a cutting power of the abrasive medium 14 and/or a rate of material removal on the workpiece 12 based on an integrated measurand of the root mean square of the structure-borne sound measured by the structure-borne sound sensor 36 over time.

[0093] To draw conclusions about the material removal rate or the cutting power, in particular the material removal is first determined in the evaluation unit 38 on the basis of the integral formed. In fact, the integrated measurand is specific to a particular material removal.

[0094] To determine the material removal rate, the evaluation unit 38 is configured to relate the material removal to the machining time.

[0095] To determine the cutting power, the evaluation unit 38 is configured to divide the integrated measurand by the media flow rate.

[0096] For evaluation of the structure-borne sound signal, the evaluation unit 38 may have a look-up table 39 stored therein, from which a material removal on the workpiece 12 and/or a cutting power of the abrasive medium 14 can be read on the basis of the integrated measurand of the root mean square of the measured structure-borne sound signal over time. Such a look-up table 39 is illustrated in FIG. 5 below.

[0097] The abrasive flow machine 10 further comprises a checking unit 40, which is adapted, based on the cutting power and/or the material removal rate ascertained by the evaluation unit 38, to adjust at least one process parameter, in particular during the machining of a workpiece 12. In this way, it is possible to react to changes in the cutting power of the abrasive medium 14 and/or to other fluctuations in the machining process, for example to temperature fluctuations.

[0098] The process parameters that can be adapted by the checking unit 40 include, for example, a flow velocity of the abrasive medium 14, a fluid pressure of the abrasive medium 14, a back pressure on the abrasive medium 14, and/or a temperature of the abrasive medium 14.

[0099] With the exception of the temperature of the abrasive medium 14, the aforementioned process parameters can be set by means of the media drive device 24. For this purpose, a position of the two displacement pumps 26, 28 relative to each other and/or a speed of movement of the individual displacement pumps 26, 28 can be adjusted.

[0100] For example, when the distance of the displacement pumps 26, 28 or of the pistons 30 from each other is reduced, the fluid pressure of the abrasive medium 14 is increased. As a result, the abrasive particles of the abrasive medium 14 are pressed against the surface to be machined of the workpiece 12 with a higher contact force and a material removal rate can be increased.

[0101] By increasing the flow velocity, the material removal rate can also be increased since more abrasive particles are moved over the surface of the workpiece 12 in the same period of time.

[0102] If the pistons 30 of the two displacement pumps 26, 28 are moved at different speeds, in particular if a piston 30 mounted upstream in the direction of flow is moved more slowly than a piston 30 mounted downstream or its movement is met with greater resistance, a counterpressure on the abrasive medium 14 can be increased. Whether a piston 30 is mounted upstream or downstream depends on the respective current direction of flow of the abrasive medium 14, which changes after each machining cycle.

[0103] To adjust the temperature of the abrasive medium 14, a heating and/or cooling sleeve or the like may additionally be provided. The heating and/or cooling sleeve is arranged around the cylinder 32, for example.

[0104] In the exemplary embodiment illustrated in FIGS. 1a and 1b, the structure-borne sound sensor 36 is in direct contact with the workpiece 12.

[0105] It is, however, also conceivable that the structure-borne sound sensor 36 is in indirect contact with the workpiece 12, in particular via an additional component 42 such as an aluminum disk. This is illustrated schematically in FIG. 3, where for the sake of simplicity only the area around the workpiece 12 is shown. The additional component 42 may be part of the workpiece holder 18 here.

[0106] FIG. 4 shows a further abrasive flow machine 10 according to the invention. The same reference numerals are used in the following for identical structures with identical functions that are known from the embodiment above, and reference is made to the preceding discussions in this respect; in the following, the differences of the respective embodiments will be discussed in order to avoid repetition.

[0107] In the embodiment shown in FIG. 4, the abrasive flow machine 10 comprises a bypass duct 46 that extends parallel to the fluid main channel 15, which is not visible in FIG. 4.

[0108] In this case, the workpiece 12 is a dummy workpiece 12a disposed in the bypass duct 46.

[0109] In the exemplary embodiment illustrated, the fluid main channel 15 extends through the opening 16 of an additional workpiece 12b to be machined.

[0110] Alternatively, as already mentioned previously, the fluid main channel 15 may extend over a surface of the workpiece 12b to be machined.

[0111] In this case, the abrasive flow machine 10 may comprise two structure-borne sound sensors 36; during operation of the abrasive flow machine 10, a respective structure-borne sound sensor 36 is positioned on the dummy workpiece 12a in the bypass duct 46 and also on the additional workpiece 12b to be machined. This allows the machining process to be monitored even more closely.

[0112] Such a structure of the abrasive flow machine is advantageous in particular if the workpiece 12b to be machined is shaped such that the structure-borne sound sensor 36 cannot be properly mounted to the workpiece 12b, in particular if the workpiece 12b is relatively small.

[0113] A further advantage of such a structure of the abrasive flow machine is that the cutting power of the abrasive medium 14 can be determined on the basis of the structure-borne sound signal measured at the dummy workpiece 12a, independently of the material removal rate. In addition, such a setup can be used to monitor the surface improvement particularly precisely by forming a difference of the structure-borne sound signal measured at the dummy workpiece 12a and that measured at the workpiece 12b to be machined. This difference correlates with the surface improvement.

[0114] The dummy workpiece 12a is preferably made of a harder material than the workpiece 12b. As a result, no or only little material is removed from the dummy workpiece 12a, and it can remain in the abrasive flow machine 10 for a large number of machining processes.

[0115] For holding the dummy workpiece 12a, preferably a workpiece holder is provided which, for the sake of simplicity, is not shown and which is configured like the workpiece holder 18.

[0116] In a further embodiment that is not illustrated, a structure-borne sound sensor 36 may be arranged only at the dummy workpiece 12a. This is the case in particular if the workpiece 12b additionally to be machined is either too small or is shaped in such a way that the structure-borne sound sensor 36 cannot be properly positioned at the workpiece 12b.

[0117] FIG. 5 illustrates the processing of a structure-borne sound signal that was measured by the structure-borne sound sensor 36.

[0118] The structure-borne sound signal is first output as a raw signal 37 by the structure-borne sound sensor 36.

[0119] Subsequently, the raw signal 37 is rectified in a rectifier 41, which may be part of the evaluation unit 38.

[0120] FIG. 5 further illustrates that the evaluation unit 38 may include an amplifier 48 for amplifying the structure-borne sound signal measured by the structure-borne sound sensor 36.

[0121] Furthermore, the evaluation unit 38 optionally includes a filter, in particular an HP filter 50 and/or a band-pass filter 52 for filtering out machine frequencies from the signal measured by the structure-borne sound sensor 36.

[0122] The rectifier 41, the HP filter 50, the amplifier 48, and the band-pass filter 52 are contained, for example, in a so-called acoustic emission coupler, which are distributed, for example, under the trade name of Piezotron® coupler. Such acoustic emission couplers already have an integrated RMS converter for evaluation of the structure-borne sound signal. That is, such an acoustic emission coupler can already determine the root mean square of the structure-borne sound signal and make it available for further evaluation in the evaluation unit 38. In addition, the raw signal of the structure-borne sound signal can be made available.

[0123] In the following, a method according to the invention of ascertaining a material removal and/or a rate of material removal on a workpiece 12, 12a, 12b when the workpiece 12, 12a, 12b is machined in an abrasive flow machine 10 and/or of determining a cutting power of the abrasive medium 14 is discussed, in particular when machining the workpiece 12, 12a, 12b in an abrasive flow machine 10 as described in connection with FIGS. 1 to 4.

[0124] When machining a workpiece 12, an abrasive medium 14 is directed over a surface and/or through an opening 16 of the workpiece 12, 12a, 12b to be machined. This is performed in particular by means of the media drive device 24 described above.

[0125] In the process, the structure-borne sound produced in the workpiece 12, 12a, 12b during machining is measured, in particular using the structure-borne sound sensor 36.

[0126] Subsequently, the root mean square of the measured structure-borne sound signal τ.sub.RMS is ascertained in the evaluation unit 38, in particular in an acoustic emission coupler.

[0127] Following this, the root mean square is integrated over the machining time.

[0128] The material removal and/or the material removal rate on the workpiece 12, 12a, 12b may then be determined using the integral formed.

[0129] Alternatively or additionally to the material removal and/or the material removal rate, a cutting power of the abrasive medium 14 may be determined on the basis of the integral formed.

[0130] In addition to the integrated measurand, the raw signal 37 of the structure-borne sound signal may also be output.

[0131] If the material removal rate measured during machining of the workpiece 12, 12a, 12b deviates from a desired material removal rate by more than a defined tolerance value, preferably at least one process parameter will be adapted.

[0132] In particular, at least one of the following process parameters is adjusted based on a material removal rate: a flow velocity of the abrasive medium 14, a fluid pressure of the abrasive medium 14, a back pressure on the abrasive medium 14, and/or a temperature of the abrasive medium 14.

[0133] To be able to determine in a particularly simple way whether the material removal rate is within a desired range, a reference curve is preferably established which describes a desired profile of the material removal rate.

[0134] The reference curve is established, for example, by plotting, during machining, a profile of the material removal rate of a machined workpiece 12, 12a, 12b. The machined workpiece 12, 12a, 12b is subsequently subjected to a quality inspection and a measurement of the material removal. If the workpiece 12, 12a, 12b has been found to be in order, the material removal rate as plotted is stored as a reference curve in the evaluation unit 38.

[0135] Such a reference curve is illustrated in FIG. 6. Here, the profile of the material removal rate has been plotted over time.

[0136] In addition, a tolerance range about the reference curve is defined; the material removal rate is to be within this tolerance range. The tolerance range is illustrated in FIG. 6 by dashed lines about the reference curve.

[0137] If it is found during the machining of a workpiece 12, 12a, 12b that the actual profile of the material removal rate is outside the tolerance range, at least one process parameter, for example, is adjusted during the machining of the workpiece 12, 12a, 12b. This is to achieve that the material removal rate again follows the profile of the reference curve. More precisely, it is to be achieved that the machined workpiece 12, 12a, 12b is of good quality after the machining has been completed.

[0138] However, when the abrasive medium 14 is worn beyond a certain extent, that is, when a cutting performance of the abrasive medium 14 has significantly decreased, the profile of the material removal rate can only be slightly influenced by a variation of process parameters. Effective machining of a workpiece 12, 12a, 12b that leads to an acceptable result in terms of quality is then no longer possible.

[0139] Therefore, preferably based on a material removal rate, there will be a request to replace at least part of the abrasive medium 14, in particular when the actual material removal rate is below the tolerance range, as shown in FIG. 7, for example.

[0140] It is furthermore apparent from FIGS. 6 and 7 that the material removal rate approaches a constant value towards the end of the machining time. The difference between a maximum value occurring at the beginning of the machining time and the final value at the end of the machining time describes the surface improvement obtained.

[0141] In order to ascertain an optimum machining duration for a workpiece 12, 12b, at the beginning of a machining process the final value of the material removal rate of an already machined workpiece 12, 12b may be compared with the maximum value of a subsequently produced workpiece, in particular the difference may be formed. In this case, the difference correlates with the desired surface improvement.

[0142] According to a further method according to the invention, a cutting power of an abrasive medium 14 can be determined by passing abrasive medium 14 over a surface and/or through an opening of a reference workpiece 12, measuring the structure-borne sound generated within the reference workpiece 12, and measuring the root mean square of the measured structure-borne sound signal. Subsequently, the integral is formed over the root mean square and the integrated measurand is divided by the media flow rate of the cutting medium 14. The divided integrated measurand can be used to determine the cutting power of the abrasive medium 14. This makes the method according to the invention suitable for examining or for developing novel abrasive media.

[0143] If it is only intended to examine the cutting power of an abrasive medium 14 without intending to produce a workpiece, usually only a dummy workpiece 12a or a reference workpiece is arranged in the abrasive flow machine 10.

[0144] FIGS. 8a and 8b graphically illustrate a profile of a directly measured material removal rate over a number of machining cycles (FIG. 8a) and a profile of an indirectly ascertained material removal rate, that is, a profile of the integrated measurand of the root mean square of the structure-borne sound signal over the number of machining cycles (FIG. 8b).

[0145] The directly measured material removal rate is given in mg/L here. The indirectly measured material removal rate is given in mV.Math.s/L.

[0146] Here, the material removal rate or the integrated measurand is ascertained individually for each machining cycle. In particular, a machining cycle corresponds to a cycle in which the abrasive medium 14 is moved in a flow direction by the media drive device 24.

[0147] It is apparent from FIGS. 8a and 8b that the profile of the directly measured material removal rate strongly correlates with the profile of the integrated measurand over the machining cycles. This clearly shows that the integrated measurand of the root mean square of the structure-borne sound measured by the structure-borne sound sensor 36 over time can be used to draw conclusions about a cutting power of the abrasive medium 14 and/or a material removal rate on the workpiece 12.