Workpiece production and measurement

Abstract

To provide a method that does not reduce overall efficiency in a continuous machine tool.SOLUTION: A method of manufacturing a workpiece is disclosed, in which the workpiece is sequentially loaded into two or more machine tools (10, 12, 14, 16), and one or more machining operations are performed on each machine tool, so that one or more features of the workpiece is manufactured. After machining operations on both or all machine tools, the workpiece is passed to a common dimensional inspection station (20, 22, 24). In the common dimensional inspection station, dimensions of the feature of the workpiece manufactured by the machining operation on two or more machine tools are measured. Two or more output signals are generated, each associated with performance of the two or more machine tools that performed the machining operation based on a result of measuring the dimensions of the feature. Each of the output signals is fed back to the machine tool that performed the respective operation so as to adjust the manufacturing process of each corresponding machine tool.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to the manufacture and measurement of workpieces or parts, and to methods and manufacturing systems for such manufacture and measurement. The terms "workpiece" and "part" are used interchangeably herein. [Background technology]

[0002] Description of the Prior Art An automated factory manufacturing system may include one or more production machines (such as machine tools) for producing workpieces. Typically, these may be manufactured as a series of nominally identical workpieces. The manufacturing system may also include one or more inspection stations for inspecting the workpieces being produced. The inspection station may include conventional gauging, such as fixture gauges, or even manual gauges, such as height gauges or calipers. Alternatively, it may include a coordinate measuring machine (CMM) for measuring the workpieces, or a comparative gauging machine for comparing them to a master reference. Each of these production and inspection machines may have numerical or computer control and are linked by a network to one or more server computers. An example can be found in U.S. Pat. No. 5,189,624 (Barlow et al.).

[0003] A percentage of the workpieces manufactured by a manufacturing machine (or even all workpieces produced) may be inspected at an inspection station. The server may schedule the workpieces to be transferred to the inspection station and may control a transfer robot or conveyor for this purpose.

[0004] In some prior art examples, the inspection result may simply be a pass or fail decision. In the case of a rejection, this can be fed back to adjust the manufacturing machine, so that the production process can be controlled and improved thereafter. Such control of the manufacturing process is performed manually in the example of U.S. Pat. No. 5,189,624. Alternatively, even in the case of a pass, automatic feedback can be provided to adjust the production machine if a single workpiece dimension exceeds a control limit, for example, updating the cutting tool offset by only the appropriate percentage of the error in the dimension. In this case, the control limit can be set at a level lower than the acceptable limit at which the workpiece would be rejected. Alternatively, the control limit can be set at a level at which the workpiece is rejected.

[0005] It is known to perform more sophisticated analyses of the inspection results of multiple workpieces within a series of nominally the same workpieces. For example, a series of measurements of a particular dimension of successive workpieces may be filtered to remove outliers. Alternatively, a series of measurements may be analyzed to detect trends. For example, if the production machine is a machine tool with cutting tools that wear out during use or are subject to temperature drift, there may be a gradual trend of the dimension of a feature of the manufactured workpieces increasing or decreasing over time. Such analyses may be performed after inspecting the workpieces in a quality control or laboratory, separate from the production machine. Manual corrections to the production process may be applied later by a skilled machine operator, but will have no beneficial effect on the workpieces produced in the meantime.

[0006] US Patent Application Publication No. 2003 / 0040830 (Parikh et al. / Applied Materials) shows a multi-stage semiconductor processing system. Individual features of a workpiece are processed sequentially by independently operating process tools, each performing a different type of manufacturing operation on the feature involved. Between each manufacturing process, or after all manufacturing operations, the workpiece is removed and placed in a metrology station where it is measured to identify whether the workpiece features are within certain parameters. Metrology data analysis equipment can use the collected data to feed forward or feed back control signals to adjust the processing tools.

[0007] US Patent Application Publication No. 2003 / 0040830 (Cameron / Rolls-Royce) shows a machining cell with a plurality of processing tools capable of performing operations on a workpiece. The cell also includes a measuring device for measuring the workpiece after the operations have been performed.

[0008] US Patent No. 6,571,145 (Matsumiya et al. / Mitutoyo) shows a manufacturing system with multiple machine tools. The machine tools process identical parts of a workpiece. A measuring device measures the workpieces produced by each machine tool and generates a compensation data file for the associated machine tool, which includes compensation values ​​for multiple dimensions of the workpiece. The compensation data file is fed back to adjust the machine tools.

[0009] The applicant's unpublished UK patent application no. GB1708730.5 describes a manufacturing system with multiple machine tools and multiple inspection stations, where a workpiece produced by one of the machine tools can be measured by any of the inspection stations and the results fed back to adjust the machine tools involved.

[0010] It can be efficient to arrange a manufacturing system so that multiple machine tools perform machining operations on individual workpieces in succession, balancing the workload between the machine tools and machining different workpiece features that cannot be easily machined on a single machine tool with the same setup, but measuring the different features separately after each machining operation on successive machine tools reduces overall efficiency. Summary of the Invention

[0011] The present invention provides a method for manufacturing a workpiece, the method comprising: loading a workpiece into two or more machine tools in succession and performing one or more machining operations to manufacture one or more features of the workpiece on each machine tool; passing the workpiece to a common dimensional inspection station after machining operations on both or all of the machine tools; measuring, at a common dimensional inspection station, dimensions of features of a workpiece produced by machining operations on two or more machine tools; generating two or more output signals respectively related to the performance of two or more machine tools that performed the machining operations based on the measurements of the dimensions of the features; and feeding back each output signal to the machine tool that performed each respective operation to adjust the manufacturing process of each corresponding machine tool; Includes.

[0012] In at least a preferred embodiment of the present invention, measuring features at a common dimensional inspection station after machining operations on both or all machine tools can improve the overall cost-effectiveness and efficiency of the manufacturing system. In addition to proving that the final part is manufactured within its design specifications (quality assurance), the common inspection station can improve quality control by feeding back associated output signals to the involved machine tools. This allows appropriate corrections to be made to the machining process without or with fewer intermediate inspection stations between the machine tools.

[0013] Preferably, the workpiece is one of a plurality of nominally similar workpieces, on each of which machining operations are performed successively using two or more respective machine tools, and each of which is measured at a common inspection station. The method may include analysing results of the measurements of corresponding features of the workpiece, and generating, based on the analysis, said output signals respectively related to, and fed back to, the performance of the two or more machine tools. Analysis of the results may include detecting trends in the measurements of successive workpieces.

[0014] The output signals fed back to the machine tool may update the tool offset values ​​and / or the location where the feature is machined and / or the workpiece coordinate system, which may modify the production of future workpieces.

[0015] In one preferred embodiment, the method includes calculating one of the second output signals dependent on one of the first output signals prior to feeding back the second output signal to a corresponding machine tool. The first output signal may result from measuring a datum feature on a workpiece, and the machine tool to which the second output signal is fed back may perform an operation related to the datum feature.

[0016] A machine tool in the context of this application should be understood to be a type of manufacturing machine, and thus references to multiple machine tools herein are references to multiple production machines (each of which is a machine tool), rather than to multiple tools (e.g., different types of cutting tools) that may be used within a single production machine.

[0017] The present invention also provides a manufacturing system comprising two or more machine tools, at least one dimensional inspection station, and a server and / or one or more controllers configured to control the machine tools and the inspection stations and to perform any of the methods described above.

[0018] The invention further provides a software program or programs which, when executed on a server and / or one or more controllers of such a manufacturing system, cause the manufacturing system to perform any of the above methods. The software program with computer code may be recorded on a non-transitory machine-readable medium such as a disk or memory device, or may be stored on a remote server for download. [Brief description of the drawings]

[0019] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1] FIG. 1 is a schematic diagram of a manufacturing system layout in a factory, including manufacturing and inspection machines. [Diagram 2] FIG. 2 is a workflow diagram of a method according to a preferred embodiment of the present invention. [Diagram 3] FIG. 3 shows an example of a workpiece that can be manufactured using the present method. [Figure 4] FIG. 4 is a schematic diagram of updating tool offsets in manufacturing the workpiece of FIG. [Diagram 5] FIG. 5 shows an example of another workpiece that can be manufactured using the present method. [Figure 6] FIG. 6 is a graph illustrating a possible process control analysis that may be performed in a preferred embodiment of the present invention. [Figure 7] FIG. 7 is a graph illustrating a possible process control analysis that may be performed in a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Description of the Preferred Embodiments The manufacturing arrangement of FIG. 1 includes several manufacturing stations, each equipped with computer numerically controlled (CNC) manufacturing machines 10, 12, 14, 16 for manufacturing parts (workpieces). The manufacturing machines may use any manufacturing technology. They may be machine tools such as milling machines, lathes, mill-turning centers, grinding, drilling, laser cutting, lapping, honing, polishing, or they may be coating machines, forging machines, presses, or additive manufacturing machines (3D printing). The exact number of these machines is not important, and they may be two or more. There may be any combination of different types of production machines, or they may all be identical.

[0021] Each production machine is controlled by a respective controller 11, 13, 15, 17, which may comprise a conventional CNC control. Optionally, any or all of the controllers may include a separate computer in communication with the CNC control.

[0022] The manufacturing arrangement also includes one or more inspection stations equipped with inspection machines, preferably CNC gauging machines 20, 22, 24, each for inspecting parts (workpieces) manufactured by the manufacturing machines. A suitable flexible comparative gauging machine is sold by the applicant Renishaw plc under the trademark EQUATOR. As explained in our previous international patent application No. WO 2013 / 021157, which is incorporated herein by reference, the gauging machine has a motorized structure with a non-Cartesian geometry. It moves a probe in three dimensions relative to the manufactured workpiece in order to compare the manufactured workpiece with a master reference workpiece. Each gauging machine is controlled by a respective computer or controller 21, 23, 25, which also obtains measurements from the machine. Optionally, the computer or controller may also take care of processing the measurement results, for example determining whether the dimensions of the workpiece are within tolerances.

[0023] Instead of these gauging machines, the inspection stations can be equipped with computer-controlled coordinate measuring machines (CMMs), or other dimensional measuring equipment such as inspection robots. Alternatively, they can be equipped with gauging fixtures or jigs, in which gauges with LVDTs or other transducers are custom designed to measure specific dimensions of the workpiece. The measurement results of these gauges can be automatically or manually fed to the respective computers or controllers 21, 23, 25. It is also possible to have inspection stations that manually measure the workpiece using traditional hand-held gauges, such as height gauges or calipers. If computer motion control is not required, the computers or controllers 21, 23, 25 can be replaced by one or more terminals of a common server 28 (discussed below). The measurement results are then fed manually to the server via one or more terminals.

[0024] The manufacturing arrangement further includes a transport system 26 for transporting parts (workpieces) from any of the machine tools 10, 12, 14, 16 to any of the gauging machines 20, 22, 24, where they can be inspected for conformity to specified dimensional tolerances. The transport system can comprise computer-controlled robots, vehicles, or conveyors, or can simply involve manually transporting workpieces or pallets of workpieces. It can be part of a larger transport system that also supplies the machine tools with raw billets or castings for machining, and / or removes the workpieces after manufacturing or inspection. If necessary, the workpieces can be returned to the machine tools for remachining after inspection.

[0025] Also provided is a server 28, whose programs or software modules are responsible for scheduling the production of workpieces and are connected by one or more data buses 30 to the CNC controllers of the machine tools and gauging machines. The server 28 also controls the transport system 26, for example, to transport workpieces between the machine tools and the measuring machine when required. For example, the server 28 may take the form of a programmable logic controller, such as is conventionally used to control a manufacturing cell with multiple machine tools, but with different programming as described below.

[0026] Server 28 may provide the machine tools and gauging machines with the CNC part programs necessary to machine and inspect each particular design of the part (workpiece) being manufactured, as needed. Alternatively, these part programs may be stored in the machine tools and gauging machines' controllers and selected for use based on instructions received from server 28.

[0027] Typically, the server 28 may schedule a series of nominally identical parts (workpieces) of a particular design to be produced on the machine tools 10, 12, 14, 16. It may then command each of these workpieces to be transferred to one of the gauging machines (e.g., gauging machine 20) and schedule inspection at that machine. Alternatively, a periodic sample of the workpieces may be scheduled to be inspected (e.g., inspect every 10th workpiece, or inspect workpieces after a certain amount of time has passed, such as one workpiece per hour).

[0028] Inspection of each workpiece at the inspection stations 20, 22, 24 generates multiple dimensional measurements that are returned and recorded to the server 28 on the bus 30. When the measurements are processed by the inspection station computers or controllers 21, 23, 25, e.g., to determine whether the workpiece dimensions are within tolerance, the results are then also returned to the server 28. If a workpiece is determined to be out of tolerance, either by the server 28 or by the inspection station computers or controllers 21, 23, 25, the server may schedule it for rejection or rework, as appropriate. Alternatively, the computers / controllers 21, 23, 25 may directly command the transport system 26 to send it to a "pass" or "reject" bin or pallet.

[0029] As explained in unpublished co-pending UK patent application No. GB ​​1708730.5, the various functions of server 28 may be shared between two or more servers. One or more of the servers may also perform other process control and quality control functions as explained in that application. In simpler systems, the production schedule, the use of the various machines, and the transportation of workpieces between the machines may instead all be determined and performed by human operators.

[0030] FIG. 2 illustrates the workflow in a preferred method of the present invention to produce a series of nominally identical workpieces 90, each of which may have characteristics such as those shown in FIG.

[0031] In this example, the first machine tool 10 machines a first feature of the workpiece in the form of a shoulder 92. The machine tool 10 may be programmed to position the shoulder 92 at a distance d1 relative to a first datum plane 96 on the back surface of the workpiece according to a design document for the workpiece. The design document may be a drawing or a design file from a computer-aided design (CAD) system. The datum plane 96 may be defined by the original surface of the workpiece before machining or by an abutment surface to which a fixture is attached.

[0032] The transfer system 26 then passes the workpiece to the second machine tool 12, which is programmed to machine additional features of the workpiece, such as two holes 98. The design documents may specify that the centerlines of these holes should be spaced a distance d2 from a second datum plane 100 defined by shoulder 92.

[0033] 3 shows examples of additional features that may be machined by successive machine tools 14, 16, respectively, programmed according to the workpiece design documents. Machine tool 14 may further machine holes 102, which may be a different size than hole 98. The design documents may specify that they are spaced apart from hole 98 by a distance d3. Machine tool 16 may machine additional features, such as spaced apart flanges 106 on the underside of the workpiece.

[0034] While some of these features can be machined on the same machine as one another, it may be advantageous to machine them on separate machines to balance the workload among all the machines available in the shop. For example, the different sizes of holes 98 and 102 require different cutters, so it may be more efficient to machine them on separate machines. Also, it may not be possible to manufacture flange 106 on the underside without changing the workpiece setup, which may be most efficiently accomplished by transporting the workpiece to another machine.

[0035] After all machining processes are completed, the transport system 26 then passes some or all of the completed workpieces to a common gauging machine 20 for inspection as instructed by the server 28. The server may schedule to inspect all workpieces or just a sample of them (e.g., one in ten workpieces). The sampling frequency may be pre-programmed or may be determined by the server according to a programmed algorithm. For example, the sampling frequency may be increased if significant differences are found in the inspection results, or conversely decreased if there are no significant differences.

[0036] The gauging machine 20 is programmed to measure each dimension d1, d2, d3 along with other dimensions specified by the design documents, such as the diameter of some or all of the holes 98, 102, and the width and spacing of the flanges 106. Depending on the need for efficient scheduling of machine usage in the factory, one of the other measuring machines 22, 24 may instead be a common measuring machine to which the workpieces are passed after all machining processes. That is, one gauging machine 20 commonly measures all desired features of a given workpiece 90, but a series of other workpieces 90 may be measured on other gauging machines 22 or 24.

[0037] All measured dimensions are passed to the server 28. As previously described, if not already done by the computer or controller 21, the server compares the measured dimensions to the design values ​​in the design document. As an alternative to a pass / fail / rework decision by the computer / controller 21, the server 28 may determine if one or more measured dimensions or coordinate locations (such as the diameter of one of the holes 98, 102 or the location where a feature was machined) differ from the design value by more than a predetermined tolerance specified in the design document. If so, it schedules the workpiece for rework or rejection (scrap).

[0038] If there is a difference from the nominal value, the server 28 also calculates an offset value and feeds it back to the controller of the machine tool that produced the relevant measured feature in order to adjust the machining process of the subsequent workpiece. This may update the tool offset table of the machine tool controller or may modify the workpiece coordinate system. This ensures that a series of subsequent workpieces are produced within the tolerance. The server may also determine if the measured dimension is merely approaching the nominal value without exceeding it, for example, by comparing the measured dimension with the control limits set in the design document. In this case, no rejection or rework is required, but the server similarly calculates an offset value and feeds it back to the controller of the machine tool that produced the relevant feature. Again, this adjusts the machining process to ensure that subsequent workpieces continue to be produced within the tolerance.

[0039] That is, for example, in the case of measuring the position of shoulder 92 relative to datum plane 96 (dimension d1), as shown in Figure 4, the server feeds back the offset value to update the position of the associated cutting tool T1 in machine tool 10 for subsequent workpieces, as necessary. If the correct design value for dimension d1 is 10 mm, as shown in Figure 4(a), but the measured value on gauging machine 20 is 10.1 mm, as shown in (b), then the cutter offset for the diameter of cutting tool T1 may be increased by 0.1 mm. Alternatively, if datum plane 96 defines the origin of a workpiece coordinate system, the position offset of tool T1 in that coordinate system may be decreased by 0.1 mm, or the origin of the workpiece coordinate system itself may be modified.

[0040] In the case of measuring the position of the centerline of hole 98 relative to datum plane 100 (dimension d2), the server feeds back the offset value, if necessary, to update the position of the associated cutting tool T2 in machine tool 12. In this case, however, the server takes into account any offsets that have already been fed back to machine 10 to correct the position of shoulder 92. The same offsets will also affect the position of hole 98 when subsequent workpieces are machined.

[0041] Figure 4 shows that the design value of dimension d2 is 15 mm. In Figure 4(b), the measured value of d2 shows that it is too small, e.g., 14.9 mm instead of 15 mm. However, after the offset of tool T1 on machine tool 10 is corrected by 0.1 mm, no further adjustment of the offset of tool T2 on machine tool 12 is required relative to datum plane 96 or the workpiece coordinate system. The update of the offset of tool T1 automatically corrects the measurement of dimension d2 of the workpiece thereafter.

[0042] If the measurement of d2 shows that it is clearly correct at 15 mm, as shown in (c), then updating the offset of tool T1 will make it inaccurate in subsequent workpieces. Therefore, the position of the centerline of hole 98 should also be adjusted. This can be done by updating the position offset of tool T2 relative to datum plane 96 or the workpiece coordinate system by 0.1 mm increments, in the opposite sense to updating tool T1.

[0043] As shown in (d), if d2 measures too large, e.g., 15.1 mm, then additional adjustments are required to the datum plane 96 or workpiece coordinate system to account for the offset that is fed back to tool T1 to correct for the oversize. The offset of tool T2 on the machine tool 12 is updated accordingly.

[0044] Similarly, dimension d3 (the position of the centerline of hole 102 relative to the centerline of hole 98) takes into account both previous dimensions d1 and d2. The offsets associated with machine tool 14 are updated accordingly.

[0045] The server 28 may also feed back tool offset values ​​to the controllers of the associated machine tools 12 , 14 , 16 to modify the diameter of the holes 98 , 102 and the width and spacing of the flanges 106 .

[0046] The workpiece shown in Figure 3 is suitable for manufacture using a machining centre or milling machine as the machine tools 10 to 16. Figure 5 shows an alternative workpiece, some or all of the features of which may be machined using a turning machine or lathe as the machine tools 10 to 16.

[0047] 5 has an end face 110 perpendicular to the vertical lathe or turning centerline 108, an inner face 112, and outer and inner turning faces 118, 120. An annular groove 116 (e.g., for receiving an O-ring) is provided in the inner turning face 120. A flat-bottom slot 114 is machined into a portion of the periphery of the outer turning face 118.

[0048] 2, these various features may be machined on different machine tools 10-16 to balance the amount of work between the machine tools. In particular, the flat-bottom slot 114 is most conveniently machined on a milling machine rather than a vertical lathe or turning machine.

[0049] After every machining process, the dimensions d4-d10 are measured by the gauging machine 20 and the offsets are fed back to update the appropriate tool offsets of the corresponding machine tools, which corrects the subsequent machining of the workpiece in the same way as described with respect to Figures 3 and 4.

[0050] End face 110 may be considered, for example, as a datum surface, and the offset of a tool for machining inner face 112 may be updated based on the measured dimension d8.

[0051] The offsets of the tool that machined the sides of the annular groove 116 are then updated based on the measured dimensions d9 and d10, taking into account any updates based on dimension d8, in the same manner as described in Figure 4. Similarly, the offsets of the tool that machined one side of the slot 114 are updated, taking into account any updates based on the position of the other side of the slot (dimension d11).

[0052] Dimensions d4 and d5 relate to the position of the inner and outer revolution surfaces 120, 118 relative to the centerline 108. The offsets of the tool manufacturing the bottom surfaces of the groove 116 and slot 114 are updated based on dimensions d6, d7 while taking into account the updates based on dimensions d4 and d5.

[0053] In any of the above examples, the server 28 is pre-programmed to configure which machine tools 10-16 each offset value is fed back to depending on which machine tool machined the relevant feature. Also, the measured dimensions and feature measurements used when calculating the offset values, as well as which offsets need to be updated on the relevant machine tool are pre-programmed depending on the measured dimensions and the cutting tool used on that machine. This is possible because measurements are performed simultaneously after machining operations on different machine tools. This is true not only for the inner surface 120 but also for the slot 114, which is manufactured on a different type of machine tool.

[0054] When manufacturing the workpiece of FIG. 3, for example, a positioning offset may be fed back to the machine 12 related to the measured dimension d2, and a cutting tool offset related to the measured diameter of one of the holes 98. Unless there is a significant error, for example due to chips or tool breakage, it would not be necessary to feed back the cutting tool offset for the diameter of the other hole 98. If both holes are machined with the same cutting tool, it can usually be assumed that any necessary corrections will all be the same. In fact, it may not even be necessary to measure the other hole 98.

[0055] Alternatively, rather than preprogramming server 28, the program can accept input from a machine tool operator to specify which offsets to update. For example, if the operator specifies a cutting tool to be used for a particular machining operation on the machine tool, he / she may also specify a corresponding offset value to apply to that cutting tool.

[0056] In addition to feedback based on measurements of a single workpiece, the server 28 also has software modules programmed to perform process control. As it inspects a workpiece, it checks for trends in the inspection measurement results of successive workpieces. It may, for example, determine a trend toward a gradual increase in size of a particular dimension as successive workpieces are produced. This may be caused by wear of the associated cutting tool of the associated machine tool 10-16, or by gradual thermal growth of the machine tool, or the raw material stock, billet, or casting from which the workpiece is machined. The server 28 may then feed back updated offset values ​​of the corresponding cutting tool via the bus 30 to the CNC controller of the machine tool. This may result in corrections to the machining process to ensure that the series of future workpieces remains within tolerance.

[0057] Of course, if more convenient, this process control can be performed on a separate server in communication with server 28.

[0058] To determine measurement trends, all parts (workpieces) may be presented to the same gauging machine 20 in the same order that they are produced by the machine tools 10-16. However, to increase the overall production flexibility in the factory, it is possible to present them to different gauging machines 20-24. In this case, the parts may be labelled in the order of production to help determine any trends, for example separating the measurement results by the order of production on the machine tools 10-16. This may be explained in co-pending UK patent application No. GB ​​1708730.5. The process control module may keep a historical record of the inspection results of all machined parts over the required period.

[0059] One suitable form of process control involves sequentially analysing the results from a series of successive parts to generate an order history for the performance of the production machines 10-16, or the associated cutting tools or tool turrets of the production machines that machined the features of interest. This is done according to pre-set rules that are dependent on the manufacturing process and the tolerance requirements of the parts of interest. Suitable rules will be known to those skilled in the art. Some possible rules are illustrated in FIG. 6.

[0060] In FIG. 6, dashed line T indicates the maximum tolerance limit for the relevant measured dimension or measured coordinate point. Successive measurements show an increasing trend towards the tolerance limit T. This may be a trend in the overall performance of the machine tool 10 or other production machine, or in the performance of the machine tool's tool-holding turret, for example caused by temperature drift. Or it may be a trend caused by gradual wear affecting the performance of an individual cutting tool used to machine the part. Dashed line L indicates a predetermined lower control limit selected to allow the manufacturing process to be corrected before the tolerance limit is exceeded, thereby allowing production of parts within the tolerance range to continue uninterrupted.

[0061] One possible pre-set rule might simply evaluate whether a measured dimension or point coordinate exceeds the control limit L. In FIG. 6, this applies to the fifth measurement bar. A more sophisticated rule might statistically analyze successive measurement results, for example using least squares analysis. This might look for trend results, as shown by line 78. The rule might be triggered when such a trend is detected, according to appropriately selected criteria, such as detecting when the slope of line 78 exceeds a predetermined value. Or, as another possibility, the rule might evaluate whether and when a trend exceeds the control limit L. In FIG. 6, this applies to the sixth measurement bar, but may be predicted from previous measurements. Other possible rules might detect whether and when a decreasing trend in the measurements, or a decreasing trend (or individual measurements) exceeds a predetermined control limit in a negative direction before a minimum acceptable level is reached. Other possible rules filter the series of measurements to smooth the results or to remove outlier results that do not contribute to the general trend. This might be completed before determining whether the filtered series of measurements exceeds the control limit or indicates a trend.

[0062] If a rule has been triggered, corrective action is requested. For example, the server 28 may generate a control signal or value, such as to calculate a new tool offset. The new tool offset can be, for example, a percentage of the error in the measured dimension, placed to counter the detected trend. This is fed back to a controller corresponding to one of the machine tools 10-16. In this case, the new tool offset adjusts the cutting tool of the machine tool 10-16 responsible for cutting the part feature whose dimensions were analyzed. In this way, the server 28 generates a control signal or value that is used to adjust the production process of the machine tool to ensure that it continues to produce good parts within the tolerance limits T.

[0063] Other feedback actions are possible. For example, if the analysis indicates that the tolerance limit T has been suddenly and unexpectedly exceeded, indicating that a cutting tool has broken, the corresponding machine tool 10-16 may be instructed to swap out a replacement cutting tool for future production. The server 28 then also schedules the out-of-tolerance workpiece to be rejected or remachined. It is also possible for the process control module to generate only an alarm or send a message to request action to investigate the machine tool problem by a human operator.

[0064] It is also possible to perform statistical analysis to automatically perform statistical process control in real time on the manufacturing floor, rather than as a result of subsequent analysis in a quality control or laboratory. Such statistical process control may determine the process capability of a tool or tool turret of a production machine or machine tool, which may, for example, be a C pk , C p or P pk The process capability index (RPI) is the ability to produce a part to a given desired tolerance with respect to a known process capability index such as the tolerance of the part. This may simply be output as a control report, or may be used as feedback to adjust the production process as described above. Alternatively, a production machine may be perfectly capable of producing a part to the required tolerance, but offset from the nominal required dimensional value. In this case, a correction is fed back to adjust the manufacturing machine to remove the offset.

[0065] FIG. 6 shows the measuring bars 70, 72, 74 simply arranged in the production order on the machine tools 10-16, so that they are equally spaced. This may be appropriate, for example, if tool wear is anticipated and monitored. Alternatively, it is possible to monitor trends etc., taking into account the actual recorded production times and the time intervals between the production times of each part on the machine tools 10-16. In this case, the spacing of the measuring bars would be non-uniform. This may be appropriate, for example, if changes caused by stepwise temperature drifts are monitored.

[0066] FIG. 7 shows a further example of uneven spacing of the measurement bars, along with gaps 75 between measurements. One reason for uneven spacing is that the process control module may include an initial step to detect outliers in the measurement results, such as outlier 76, which is not accounted for. Other reasons for gaps 75 are that the machine tool may suffer some downtime, or parts may not be selected for inspection at a regular frequency; and / or some inspection results may be unavailable because one of the inspection stations is busy and on hold. Nevertheless, the analysis is performed continuously, detecting trends and executing other analysis rules based on the available measurement results. Of course, the spacing may also simply be uneven, since it takes into account the time intervals between the actual recorded production times of the workpiece.

[0067] It will be appreciated that Figures 6-7 relate to only one set of measurement results of a dimension or point of a nominally identical workpiece. In practice, dimensions or coordinate points of multiple features of the workpiece may be measured, giving multiple sets of such measurement results. Each set of measurement results may be evaluated in the same way, with appropriate feedback to the controller of the corresponding machine tool. However, it may not be necessary to provide feedback from all sets of measurement results. For example, if a variation in the manufacturing process of the machine tool is due to tool wear, this may be evaluated and corrective feedback may be provided from only one of the sets of measurement results affected by the corresponding worn tool. If the variation is due to thermal growth, it is possible to evaluate this and provide corrective feedback from only one, or only a few, sets of measurement results (corresponding to only one or a few of the nominally identical features measured).

Claims

1. A method for manufacturing a workpiece, comprising the steps of: loading the workpiece into two or more machine tools in succession, the two or more machine tools being controlled to perform two or more different respective machining operations to sequentially process the workpiece according to two or more different respective manufacturing processes to generate two or more different respective features of the workpiece; performing said machining operations on said two or more machine tools to produce two or more different respective features of said workpiece and then delivering said workpiece to a common dimensional inspection station; measuring, at the common dimensional inspection station, dimensions of the two or more features of the workpiece produced by the machining operations on the two or more machine tools; analyzing the measurements of the dimensions of the two or more features and generating, based on the analysis, two or more output signals respectively related to performance of the two or more machine tools that performed the machining operations; feeding back each of said output signals to the machine tools that performed said respective operations to adjust the manufacturing process of each corresponding machine tool; A method comprising:

2. A method as described in claim 1, comprising calculating an output signal for a second one of the machine tools in dependence on an output signal for a first one of the machine tools.

3. 3. The method of claim 2, wherein the output signal for the first machine tool is a result of measuring a datum feature of the workpiece manufactured by the first machine tool, and the second machine tool performs an operation relative to that datum feature.

4. the workpiece being one of a plurality of nominally similar workpieces, on each of which the machining operations are performed successively using said two or more respective machine tools, and each of which is measured at a common inspection station, the method comprising: analyzing the measurements of corresponding features of the workpiece; and generating said output signals respectively related to and fed back to the performance of said two or more machine tools based on said analysis; The method according to any one of claims 1 to 3, comprising:

5. The method of claim 4 , wherein there is more than one dimensional inspection station, and each workpiece is measured at one of the inspection stations.

6. The method of claim 4 or claim 5, wherein the analysis of the results includes detecting trends of increase or decrease in successive workpiece dimensional measurements.

7. A method according to any one of claims 1 to 6, wherein the output signal fed back to the machine tool updates a tool offset value of a tool of the machine tool.

8. 8. The method of claim 7, wherein the tool offset values ​​that are updated are pre-programmed to correspond to a cutting tool of the machine tool that performed the respective operation.

9. A method according to any one of claims 1 to 8, wherein the output signal fed back to the machine tool updates the position at which the feature is machined and / or the workpiece coordinate system.

10. 10. A manufacturing system comprising two or more machine tools, at least one dimensional inspection station, and a server and / or one or more controllers, the server and / or the one or more controllers configured to control the machine tools and inspection stations to perform a method according to any one of claims 1 to 9.

11. A software program which, when executed on a server and / or one or more controllers of a manufacturing system according to claim 10, causes said manufacturing system to carry out a method according to any one of claims 1 to 9.

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