Improvements To Machining Process Control

Abstract

A method of machining a workpiece using a machine tool, the machine tool comprising a tool mount carrying a tool, a workpiece mount carrying a workpiece, a drive mechanism for moving at least one of the tool mount and the workpiece mount relative to the other, and a control arrangement for controlling the drive mechanism. The method comprises moving at least one of the tool mount and the workpiece mount with the drive mechanism under the control of the control arrangement so that the tool contacts a portion of the workpiece to commence a machining operation, and the tool then removes material from the portion of the workpiece until completion of the machining operation, the movement being such that the relative velocity between the tool and the workpiece decreases continuously during the majority of the time that the tool and the workpiece are in contact with each other during the machining operation.

Claims

1. A method of machining a workpiece using a machine tool, the machine tool comprising a tool mount carrying a tool, a workpiece mount carrying a workpiece, a drive mechanism for moving at least one of the tool mount and the workpiece mount relative to the other, and a control arrangement for controlling the drive mechanism, wherein the method comprises moving at least one of the tool mount and the workpiece mount with the drive mechanism under the control of the control arrangement so that: the tool contacts a portion of the workpiece to commence a machining operation; and the tool then removes material from the portion of the workpiece until completion of the machining operation, the movement being such that the relative velocity between the tool and the workpiece decreases continuously during the majority of the time that the tool and the workpiece are in contact with each other during the machining operation.

2. The method of claim 1, wherein the relative velocity between the tool and the workpiece decreases continuously during at least 80% of the time that the tool and the workpiece are in contact with each other during the machining operation.

3. The method of claim 1, wherein the machining operation is completed when the portion of the workpiece has been machined to its finished size by the tool.

4. The method of claim 1, wherein the control arrangement calculates a profile of relative velocity against time to be achieved between the tool and the workpiece over the course of the machining operation using a velocity profile generation algorithm.

5. The method of claim 4, wherein the relative velocity profile is calculated such that the relative velocity between the tool and the workpiece at the start of the machining operation corresponds to a material removal rate that is less than a predetermined specific material removal rate threshold.

6. The method of claim 4, wherein the velocity profile generation algorithm comprises a second or higher degree polynomial.

7. The method of claim 4, wherein the velocity profile generation algorithm comprises a term which decreases exponentially with respect to time.

8. The method of claim 4 or claim 5, wherein the relative velocity profile is calculated with reference to a plurality of predefined combinations of time and velocity.

9. The method of claim 4, wherein the relative velocity profile is calculated with reference to a plurality of predefined combinations of the depth of material remaining to be removed from the workpiece with fractions of the relative velocity achieved when the machining operation is commenced.

10. The method of claim 4, wherein the relative velocity profile is calculated using a spline fitting algorithm.

11. The method of claim 1, wherein at a predetermined point partway through the machining operation, at least one dimension of the portion of the workpiece being machined is detected, and a new relative velocity profile to be achieved between the tool and the workpiece over at least a portion of the remainder of the machining operation is determined by the control arrangement with reference to the detected dimension.

12. The method of claim 11, wherein removal of material from the workpiece is suspended during the step of detecting at least one dimension of the portion of the workpiece.

13. The method of claim 11, wherein removal of material from the workpiece by the tool continues during the step of detecting at least one dimension of the portion of the workpiece.

14. The method of claim 11, wherein the step of determining the new relative velocity profile comprises: calculating a value of a velocity modifier parameter which corresponds to a displacement forwards or backwards in time along a current relative velocity profile, by reference to the difference between the measured size of the detected dimension and the expected size of the detected dimension at that point in the machining operation, the displacement being calculated to correct for this difference in order to achieve a desired final value of the dimension at the end of the machining operation; and adjusting a current position along the current relative velocity profile with reference to the calculated value of the velocity modifier parameter.

15. The method of claim 23, including a further step of: determining an intervening velocity profile which provides a smooth transition between the current relative velocity profile and the new relative velocity profile.

16. The method of claim 11, wherein, after the dimension has been detected, the measured size of the detected dimension is compared to upper and/or lower threshold values for the expected size of the detected dimension at that point in the machining operation and a new relative velocity profile is only determined if the detected dimension is beyond one of the thresholds.

17. The method of claim 1, wherein the tool is a grinding wheel.

18. A machine tool configured to carry out the method of claim 1 comprising a tool mount for carrying a tool, a workpiece mount for carrying a workpiece, a drive mechanism for moving at least one of the tool and the workpiece relative to the other and a control arrangement for controlling the drive mechanism.

19. A computer-readable medium storing computer-executable instructions adapted to cause a machine tool to carry out the method of claim 1.

20. An electrical carrier signal carrying computer-executable instructions adapted to cause a machine tool to carry out [[a]]the method of claim 1.

21. (canceled)

22. (canceled)

23. The method of claim 13, wherein the step of determining the new relative velocity profile comprises: calculating a value of a velocity modifier parameter which corresponds to a displacement forwards or backwards in time along a current relative velocity profile, by reference to the difference between the measured size of the detected dimension and the expected size of the detected dimension at that point in the machining operation, the displacement being calculated to correct for this difference in order to achieve a desired final value of the dimension at the end of the machining operation; and adjusting a current position along the current relative velocity profile with reference to the calculated value of the velocity modifier parameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Prior art processes and embodiments of the present invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein:

[0039] FIG. 1 shows plots of tool velocity and position against time for a known machining operation comprising “fast”, “medium” and “slow” tool feed rates;

[0040] FIG. 2 shows a portion of the tool velocity plot of FIG. 1 with a larger scale on the vertical axis;

[0041] FIG. 3 shows a plot of distance to finished size of the workpiece against time for a known machining operation and embodiments of the invention;

[0042] FIG. 4 is a table of combinations of distance to the finished size of the workpiece against the tool velocity as a percentage of its velocity at the start of the machining operation;

[0043] FIG. 5 is a velocity profile generated from the parameters indicated in the table of FIG. 4;

[0044] FIG. 6 is a plot of tool position against time corresponding to the machining operation represented by FIG. 5;

[0045] FIGS. 7 and 8 are plots of tool velocity and position against time for a machining operation embodying the invention which includes a pause for a measuring procedure, with a portion of the tool velocity plot of FIG. 7 shown in FIG. 8 with a larger scale on the vertical axis;

[0046] FIG. 9 is three plots of tool position against time showing different adjustments following a measuring operation;

[0047] FIG. 10 shows plots of tool position and target end position against time for a machining operation embodying the invention;

[0048] FIGS. 11 to 13 show plots of tool position, target end position and size gauge reading against time for three machining operations embodying the invention;

[0049] FIG. 14 shows plots of tool position, upper and lower tool position thresholds and the target end position against time according to a further machining operation embodying the invention; and

[0050] FIG. 15 is a sketched plot of tool velocity against position for a machining operation similar to that illustrated in FIG. 11.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0051] FIGS. 1 and 2 illustrate a known machining operation using phase-by-phase piecewise feed control. FIG. 1 shows a plot 2 of tool velocity relative to the is workpiece against time. It includes three distinct phases 4, 6 and 8 corresponding to “fast”, “medium” and “slow” feed rates, respectively.

[0052] FIG. 2 shows the same machining operation in which a portion 2′ of the tool velocity plot 2 is shown with a larger scale on the vertical axis. In FIG. 2 (and other similar figures), negative values of tool velocity on the y axis represent movement towards the workpiece, with positive values representing movement away from the workpiece. Feature 10 is a spike in the plot corresponding to a short controlled retraction of the tool from the workpiece surface to avoid microscopic surface finish defects in advance of a rapid retraction and movement of the tool to the next workpiece portion to be machined. The magnitude of the tool velocity towards the workpiece during the slow feed phase is greater than zero, but too small to be discernible in this plot.

[0053] Graph 12 in FIG. 1 is a plot of the position of the tool along the physical machine axis carrying the tool against time.

[0054] For clarity, the plots of FIGS. 1 and 2 show the demanded tool position and therefore do not include effects occurring in practice caused by the cutting process.

[0055] In FIGS. 1 and 2 it can be seen that in the transition from one phase to the next, there is a brief, rapid deceleration of the tool relative to the workpiece until the velocity of the next constant velocity phase is reached.

[0056] FIG. 3 is a plot of the distance to be travelled by the tool to the finished size of the workpiece against time according to three different machining operations.

[0057] A plot similar to plot 12 of FIG. 1 is also shown in FIG. 3 as plot 20. Plot 20 corresponds to a known type of machining operation including fast, medium and slow phases (an “FMS” procedure). In this example, the fast feed velocity is −0.55 mm/s, the medium feed velocity is −0.12 mm/s and the slow feed velocity is −0.018 mm/s. The workpiece is machined to its finished size in approximately 9.5 seconds. The operation is completed during a further 0.5 seconds in which the workpiece is rotated without further infeed of the tool. These feed rates are consistent with typical feed rates used in crankshaft grinding as an example, where the amount of material to be removed (the “stock level”) is of the order of 1 mm.

[0058] Plot 22 represents a velocity profile calculated in accordance with the present invention. It was determined by fitting a second order curve (in other words a second degree polynomial expression) to the start and finish points of the FMS procedure. It can be seen that the relative velocity between the tool and the workpiece decreases continuously from the start to the finish of the machining operation.

[0059] Plot 24 represents a further embodiment of the present invention. This velocity profile was determined by fitting an exponentially decreasing curve to the start and finish points of the FMS procedure. In another embodiment, a spline fitting technique is used to generate the velocity profile (an example is not shown in FIG. 3).

[0060] It has been found that a continuously decreasing velocity profile may be fitted to the start and finish points of a conventional FMS operation, with the initial velocity of the tool infeed lower than the FMS initial infeed velocity, whilst still achieving the finished workpiece size over a similar or even shorter timescale.

[0061] FIGS. 4 to 6 illustrate how a velocity profile embodying the present invention may be determined with reference to a plurality of predetermined intermediate points in a machining operation.

[0062] By way of example, FIG. 4 is a table identifying a number of intermediate points representing target points identified by a user to guide a grinding operation. At each point, for a given distance remaining to be travelled by the tool to the finished position at which the workpiece has been ground to size, a relative velocity between the tool and the workpiece is specified as a percentage of its velocity at the start of the machining operation. These points are then inputted into a curve fitting process to derive a velocity profile guided by these points (but not necessarily passing exactly through each point between the start and end points) in which the relative velocity between the tool and the workpiece decreases smoothly and continuously during the machining operation. A curve 30 based on the points shown in FIG. 4 is illustrated is in FIG. 5.

[0063] The curve of FIG. 5 may then be used to calculate a velocity profile having a continuously reducing velocity as the target workpiece size is approached. The timescale of the machining operation is determined such that the initial velocity does not result in an excessive SMRR at the start of the machining operation. In a grinding procedure, SMRR is generally measured in terms of the volume of material removed per unit width of the grinding wheel per second.

[0064] An example of a resulting velocity profile is illustrated in FIG. 6. In this embodiment, the tool engages the workpiece at the time 12.25 seconds, at a position along the physical machine axis carrying the tool measured at 512.575 mm. The machining process then is completed after around 10 seconds, with the tool position at 511.95 mm along its axis.

[0065] In a preferred embodiment, a method of machining a workpiece may involve the following sequence of steps: [0066] The target position for the tool, at the point where the portion of the workpiece to be machined has been ground to its finished size, is determined; [0067] The starting position for the tool is determined with reference to the target position plus the amount of stock material to be removed plus any additional “runout”; [0068] The initial relative velocity between the tool and the workpiece is determined so as to be less than the maximum that the process can tolerate before burn occurs; [0069] A relative velocity profile is calculated with reference to the initial feed velocity and achievement of a velocity at or very close to zero at the finished workpiece size, using a curve fitting routine or algorithms which may be stored in the controller of the machine tool or a separate processor; [0070] Rapid movement is added at the start of the machining operation to arrive at the start position where the tool is in contact with the workpiece, to minimise the time taken for the machining operation. This includes a deceleration to the initial feed control velocity calculated above; [0071] Where the component has been finished using target positions only (designated as a “timer grind”) then the feed velocity is controlled to follow the velocity profile until it reaches zero at the target position. The final size accuracy of the workpiece relies on the calculations for the target position based on the original calibration of the machine's feed control axis and its absolute position with respect to the location of the workpiece and, additionally, on small allowances which are included for process related variations in temperature of the machine's axes and deflections caused by process forces. With this approach, no measurements of residual stock levels are taken during the machining process which would allow accurate compensations to be included which may be related to changes in ambient conditions such as temperature and humidity.

[0072] FIGS. 7 and 8 illustrate a machining operation according to an embodiment of the invention which includes a brief pause in the material removal whilst the size of the workpiece is measured using a size gauge partway through the machining operation. In FIG. 7, plot 40 is a plot of relative velocity between the tool and the workpiece against time. Plot 42 is a plot of tool position along an axis perpendicular to the workpiece against time. FIG. 8 shows plot 40′ which is a portion of plot 40 shown with a larger scale on the vertical axis.

[0073] In this embodiment, a size gauge is applied to the workpiece when the tool feed position passes a predetermined point in the machining operation. This point may be defined with reference to the position of the tool along its machine axis such that sufficient stock is left on the workpiece to allow a satisfactory completion of the feed control from the point at which the gauge acquires information of its near complete size (designated as a “gauge grind”). The gauge reading provides compensation for the process and environmental size variations which may otherwise cause a loss of size accuracy in the previously described “timer grind”.

[0074] When the predetermined point is reached, the infeed of the tool relative to the workpiece is suspended and the gauge is brought into contact with the workpiece for sufficient time to acquire the required size measurement. The distance to be travelled by the tool from the position reached during the preceding material removal process to its target, finish position is calculated with reference to the gauge measurement. The velocity profile for the remainder of the machining operation is then re-calculated on this basis. The revised velocity profile is modified relative to the previous version to take into account any difference between the current distance to size as indicated by the tool machine axis and the distance to size indicated by the gauge measurement.

[0075] FIG. 9 illustrates modification of the machining operation illustrated in FIGS. 7 and 8 according to two different scenarios. It can be seen that the gauge measurement was taken in this embodiment when the workpiece was around 150 microns above its finished size. The tool position profile corresponding to the original velocity profile for the part of the machining operation after the gauge measurement is shown as plot 50 in FIG. 9. Plot 52 shows the tool position profile corresponding to a recalculated velocity profile where the tool finishes the machining operation at a tool machine axis position which is further from the workpiece than originally calculated, as a result of the grinding wheel size being larger than expected. Alternatively, plot 54 shows the tool position profile corresponding to a revised velocity profile where the finish position of the tool along its machine axis is closer to the workpiece than originally calculated as the wheel size is smaller than expected.

[0076] Embodiments of the invention involving a different way of adjusting the velocity profile having regard to workpiece measurements carried out during the machining operation will now be described with reference to FIGS. 10 to 14.

[0077] For the purposes of comparison with subsequent plots, FIG. 10 shows a plot 60 of a tool position against time according to an embodiment of the invention, with the relative velocity between the tool and the workpiece decreasing continuously during the machining operation. Also shown on the graph is a plot 62 representing a target end position. The two plots 60 and 62 are generated using different vertical axes which have been overlaid for the purposes of this illustration. Similar considerations apply to the vertical axes of FIGS. 11 to 14.

[0078] FIG. 11 illustrates the introduction of a size gauge reading partway through a machining operation. Plot 70 shows the tool position during the machining operation, plot 72 is the target end position, and plot 74 corresponds to the size gauge reading output.

[0079] In this example, the size gauge reading is found to indicate that the grinding wheel size is smaller than expected, and so the workpiece is larger than expected at this stage. Accordingly, the tool must travel further for the workpiece to reach the target size end-point. The control arrangement of the machine tool therefore carries out a calculation to determine where the tool needs to move to so that the workpiece will be ground to the correct size taking into account the size adjustment for the grinding wheel. This calculation results in a modification to the target end position, indicated by the vertical drop in the plot 72 shown in FIG. 11. This may be implemented by reducing the rate of decrease of the tool infeed velocity for a period to accommodate the extension to the target end position. Alternatively, it may result in the machining operation moving back along the relative velocity profile to a point where the feed velocity is higher. In this example, this movement is relatively small and so the change in the gradient of the tool position profile 70 shown in FIG. 11 at time t.sub.1 is barely perceptible.

[0080] FIG. 15 illustrates how a profile of velocity against position may be modified in the scenario to which FIG. 11 relates, that is, where the size gauge reading indicates that the grinding wheel size is smaller than expected. Initially the machining operation is following a velocity profile 110. This is calculated such that the operation is completed as the tool velocity reaches zero and the tool reaches the position p.sub.2. At tool position p.sub.1, a size gauge reading is taken which indicates that the grinding wheel is smaller than expected at this point. In response to this measurement, a velocity modifier parameter is calculated, which represents an amount by which the velocity profile is shifted along the position axis so that the end of the shifted profile (that is, when the tool velocity reaches zero) coincides with the tool reaching the position (p.sub.3) where the workpiece has been ground to the correct size. The shifted profile 112 is shown in FIG. 15. The control arrangement calculates an intervening velocity profile 114 (shown by a dashed line in FIG. 15) which provides a smooth transition between the old velocity profile 110 and the shifted velocity profile 112. As a result, the velocity falls at a slower rate until it blends into the shifted velocity profile 112.

[0081] In contrast to the example of FIG. 11, in FIG. 12, the grinding wheel size is determined by the size gauge reading of the workpiece to be larger than expected. Accordingly, the tool must travel a shorter distance to machine the tool to the required size end-point. In FIG. 12, plot 80 is the tool position, plot 82 the target end position, and 84 the size gauge reading output. Following analysis of the size gauge reading, the target end position is modified at time t.sub.2 causing the feed velocity to decrease more quickly than would otherwise be the case at this point. In this example, a second adjustment is made at time t.sub.3. In each adjustment in this example, the current velocity is adjusted by moving further along the relative velocity profile, resulting in a greater reduction in the feed velocity than would otherwise be the case, with a view to the workpiece reaching the target size at the end of the machining operation.

[0082] A further example is shown in FIG. 13, with plot 90 being the tool position, plot 92 the target end position, and plot 94 the size gauge reading output. As in the example of FIG. 12, in this case the grinding wheel size is found to be larger than expected. Again, the tool must travel a shorter distance to reach the target size end-point. In this example, the grinding wheel is not in contact with the workpiece until after the gauge has engaged with the workpiece. The first calculation by the control arrangement in response to the signal from the gauge causes a step change in the target end position at time t.sub.4. A pronounced reduction in the relative velocity of the tool can be seen in the profile 90 at time t.sub.4.

[0083] The control arrangement then determines in response to the signal from the size gauge that the initial estimate of the deviation from the expected workpiece size was too large. There follows six smaller corrections to the target end position between time t.sub.4 and t.sub.5 to enable the machine tool to move to a tool position where it is able to resume following the relative velocity profile such that the end-point of the profile coincides with the workpiece reaching the required size.

[0084] FIG. 14 illustrates a further method embodying the invention. Plot 100 corresponds to the actual tool position. Plots 102 and 104 correspond to upper and lower thresholds respectively for the measured workpiece dimension through the course of the machining operation. Plot 106 shows the target end position. In this example, the feed compensation incorporated in the relative velocity profile is dependent on the degree to which the predicted end-point, as determined with reference to the size gauge output, is found to deviate from the desired position. The upper and lower thresholds define a control band around the workpiece size as measured by the size gauge, such that the relative velocity profile is only adjusted when the gauge reading falls outside the area limits defined by the control band. The width of the control band may reduce to improve the precision with which the relative velocity profile is adjusted, as the workpiece approaches its target size. If the workpiece size deviation falls outside the control band, a value for a velocity modifier parameter is determined which changes the feed rate, such that its position along the relative velocity profile is changed and the desired size is reached at the end of the machining operation. It can be seen that the target end position is adjusted four times in the example shown in FIG. 14.

[0085] As illustrated in FIG. 14, the effect of the modification of the infeed velocity and the recalculation of the target end position is to keep the tool position centralised in the error window. This results in smooth and accurate control of position and velocity during the machining operation, through to achievement of the desired workpiece size.

[0086] Where embodiments refer to tool movement relative to a workpiece, it will be appreciated that the teaching is also applicable to procedures involving movement of the workpiece relative to the tool instead (or in addition) in order to achieve the desired relative velocity profile.