Apparatus and method for determining runout in a machine tool

Abstract

A method and apparatus are described for determining runout of a tool (12) supported by a rotatable spindle of a machine tool, such as a non-contact tool setter (2), using a tool edge sensor mounted on the machine tool. The spindle is arranged to rotate the tool, which may be a flute cutting tool, about a spindle centerline (20). The technique includes rotating the spindle in three or more orientations about the axis of rotation and using the tool edge sensor to measure the position of the edge of the tool (12) for each of the three or more orientations about the axis of rotation. The tool centerpoint is found by fitting the tool edge positions to a function, and the difference in position between the tool centerpoint and the spindle centerline (20) is used to determine tool runout.

Description

[Technical field]

[0001] The present invention relates to an improved technique for determining runout of a tool, such as a fluted cutting tool, using a tool edge sensor mounted on a machine tool. [Background technology]

[0002] It is known to use a Dial Test Indicator (DTI) to measure the runout of a cylindrical shaft. Such a measurement involves placing the tip of the DTI in contact with the shaft and using the DTI to measure the back and forth movement of the tip as the shaft rotates. The difference between the maximum and minimum tip deflection values ​​provides a measure of the axial eccentricity (TIR) ​​of the shaft.

[0003] It is also known to use DTI to measure runout on fluted cutting tools. Such fluted tools have cutting blades that typically spiral or wrap along the length of the tool. All cutting blades are assumed to extend the same radial distance from the central axis of the tool, and runout is determined by using DTI to measure the difference in tip deflection that occurs when various cutting blades are brought into contact with the tip of the DTI. In particular, DTI determines TIR from the difference in radial position for the two blades that deflect the tip of the DTI by the largest and smallest amounts.

[0004] Non-contact breakbeam tool setting devices are also known for use in machine tools (e.g. machining centres, lathes, milling machines etc.). For example, the so-called NC4 non-contact tool setting system is sold by Renishaw Public Limited, Wotton-under-Edge, UK. The NC4 device comprises a transmitter including a laser source for generating a light beam. The light beam is directed towards the receiver through a section of free space in which a rotating tool can be placed. During a tool setting operation, the machine tool on which the tool setter is mounted is programmed to move the tool inside and / or outside the light beam. An output signal from the receiver indicates the absorption of the light beam and the device compares this received intensity signal with a threshold value. A so-called trigger signal is generated by the device to indicate that the tool has reached a certain position relative to the beam. The trigger signal allows the position of the tool to be determined by the machine tool, which is itself capable of measuring the length or diameter of the tool. For the tool run-out measurement, the tool rotates whilst moving relative to the light beam. The tool position is measured twice. A first tool position is found where the longest likely blade is found and a second tool position is found where the shortest likely blade is found. These measurements are taken a fixed distance from the tip of the tool to the top of the tool. The difference in the measured tool positions (tool radius) with respect to these blades is used as a measurement of the TIR in a similar manner to the max / min deflection measurements made by DTI mentioned above.

[0005] However, the inventors have found that such runout measurements (made using DTI, or non-contact tool setting devices) for certain cutting tools are unexpectedly susceptible to error. This is particularly the case for flute tools, where multiple teeth spiral along the length of the tool. In particular, the inventors have found that the TIR measured using the techniques described above varies along the length of the cutting tool. As explained in more detail below, this is because the measured radius for a cutting edge will vary in a manner that depends on both the runout and the circumferential position of the tool for that tooth. The inventors have found that the combination of these two effects, which vary differently along the length of the tool, means that the positional difference between the longest and shortest teeth does not necessarily give an accurate TIR. Summary of the Invention

[0006] The present inventors have reduced at least some of the shortcomings in the prior art and provide more reliable TIR measurements.

[0007] According to a first aspect of the present invention there is provided a method for determining runout of a tool held by a rotatable spindle in a machine tool using a tool edge sensor mounted on the machine tool, the spindle being arranged to rotate the tool about a spindle centreline, the method comprising the steps of: (1) rotating the spindle in three or more directions about the axis of rotation; (2) measuring a position of an edge of the tool for each of three or more orientations about the axis of rotation using a tool edge sensor; (3) determining a tool center point by fitting the tool edge positions measured in step (2) to a function; (4) determining tool runout using the difference between the tool center point determined in step (3) and the spindle centerline;

[0008] In this first aspect, the invention provides a method for measuring runout of a tool (e.g., a flute cutting tool) held on a rotatable spindle of a machine tool using a tool edge sensor (e.g., a breakbeam laser tool setter) mounted on the machine tool. The tool may be, for example, an elongated cutting tool (e.g., a drill, mill, etc.) having a central tool axis with one or more cutting edges (e.g., cutting blades) protruding radially. In use, the spindle rotates the tool about a so-called spindle axis or spindle centerline. The position of the spindle centerline is known in the machine coordinate system (e.g., it will be determined during calibration of the machine tool). Ideally, the long axis of the tool will be perfectly aligned along the spindle centerline (i.e., the spindle rotation axis), but in practice runout (also called eccentricity) occurs when the spindle centerline does not coincide with the tool axis. For a cutting tool, this runout has the effect of increasing the effective cutting diameter of the tool, which can result in the tool removing too much material from the workpiece, or can reduce the tool's life due to increased wear. Thus, any runout measurement is important for several reasons.

[0009] The method according to the present invention comprises step (1) rotating the spindle to rotate the tool to three or more orientations (i.e., azimuthal positions) about the axis of rotation, and step (2) measuring the position of the edge of the tool at each of the three or more orientations about the axis of rotation using a tool edge sensor. The tool edge sensor will be fully calibrated in a known manner, so that the tool edge measurements are performed in a machine coordinate system. As described in more detail below, the tool may undergo rotational movement (intermittent rotation) in each rotational direction and rotation stop to enable the tool edge measurement of step (2) at each such position, or the tool edge measurement of step (2) may be performed while the tool is rotating. It should be noted that the tool edge is the outermost surface of the tool. Thus, the tool edge may be a cutting edge, a non-cutting edge, or any other surface of the tool. In a preferred embodiment described below, the spindle may be moved (e.g., translated) by the machine tool relative to the tool edge sensor to separately measure the position of the tool edge when the tool is located at each of the three or more orientations about the axis of rotation.

[0010] Step (3) of the method involves using the tool edge position measurements made in step (2) to determine the tool center point. This is done by fitting the measured edge positions for each of the orientations about the rotation axis to a function, taking into account the angular orientation of the tool. For example, the measured tool edge positions may be fitted to a circle and the center of the circle may be determined. It should be noted that the edge positions are conveniently measured in the coordinate system of the machine tool that includes the spindle that rotates the tool (i.e., the measurements may be made in the machine coordinate system). However, any suitable coordinate system or systems may be used. The fitting process used to find the tool center point may involve solving equations as described below, or may use iterative or numerical techniques. As described above, fitting the edge position measurements to a circle is preferred because this allows the center of the circle (i.e., the tool center point) to be readily determined.

[0011] The tool center point established in step (3) provides the location of the tool axis. The location of the spindle centerline is also known, and the deviation of the tool center point from the spindle axis can be used to provide a reliable measurement of tool runout. Thus, step (4) of the method uses the difference in location between the tool center point determined in step (3) and the spindle centerline to determine the tool runout (i.e., axis eccentricity or TIR).

[0012] Importantly, knowing the position of the tool centre point relative to the spindle centre line means that the runout can be calculated in step (4) without the errors found to be associated with the prior art methods discussed above, which measure runout from the difference between maximum and minimum radial deviations on the tool (e.g. the difference in radial position of the longest and shortest blades on a cutting tool). In this way, the above-mentioned shortcomings of prior art runout measurement techniques (i.e. the inability to separate the effect of tool runout from any variation due to the circumferential position of the blades which varies along the length of the tool) are overcome using the method of the present invention. In this way, a more reliable runout measurement is provided which can be used for all types of tools. It should be noted that the step of determining the tool runout may include providing a value of the runout (e.g. for use in a subsequent machining process) or checking whether the amount of runout is within an acceptable range.

[0013] As mentioned above, the measurements in step (2) may be taken when the tool is not rotating. Thus, step (1) may include stepping the cutting tool in three or more orientations about the axis of rotation. Alternatively, tool edge measurements may be collected at each desired orientation about the axis of rotation while the tool is still rotating. Thus, step (1) may include continuously rotating the cutting tool, and the measurements in step (2) may be taken in three or more orientations about the axis of rotation while the tool is rotating. It should be noted that even if the tool is not rotating, there may be other motions of the tool between measurements. For example, the tool may be translated into the light beam of a non-contact tool setter to measure the position of the tool edge.

[0014] The tool center point may be found in step (3) by fitting the measured positions relative to the tool edge to an arbitrary function. The function may describe the shape of the tool. Conveniently, the function may include a circle. Thus, step (3) may include fitting three or more measured positions relative to the tool edge to a circle to find the center of the circle, which determines the tool center point. Although not essential, a circle is generally preferred for analytical simplicity. Moreover, most cutting tools are typically manufactured using a grinding process, so it is a good assumption that they are highly symmetrical. For example, each cutting edge of a cutting tool tends to protrude the same distance from the tool axis.

[0015] Step (4) may include calculating the runout using vector differences in the positions of the tool center point and the spindle center line. Step (4) may include determining the runout of the tool at one position along the tool length (i.e., in one plane). Alternatively, step (4) may include determining the runout of the tool at multiple positions along the tool length. In a preferred embodiment, the tool center point may be detected at multiple different positions (planes) along the tool length. Such multiple tool center points may be used to determine the tool axis (i.e., the tool center line). Knowing the tool center line and the spindle center line allows the runout to be determined at any position along the tool length. In other words, the angular runout may be measured. This may also allow the runout at the tool tip to be determined, for example, using measurements made further up the tool.

[0016] The method according to the invention may be carried out on any tool. The tool is preferably a cutting tool. The cutting tool may have a plurality of blades. The cutting blades may have a major axis coinciding with the tool centre point, each of the plurality of cutting blades extending slightly radially from the major axis of the tool by the same distance. The cutting tool may have three or more cutting blades. The tool may be a fluted cutting tool. The fluted cutting tool may include one or more cutting blades (sometimes also referred to as cutting edges). The fluted cutting tool may include two or more cutting blades. The cutting blades may be straight (e.g. one or more straight cutting edges may be provided along the longitudinal direction of the tool). Conveniently, the one or more cutting blades may spiral or wrap around the cutting tool. A helical blade profile may be provided. The nominal profile of the cutting blade may be known.

[0017] As mentioned above, a cutting tool (e.g., a flute cutting tool) may include a plurality of cutting blades. A cutting tool may include three or more cutting blades. A cutting tool may include four or more cutting blades. Conveniently, the plurality of cutting blades are equally spaced apart from one another along the tool circumference. Alternatively, an asymmetric arrangement of the cutting blades may be provided. A cutting tool has a major axis (i.e., the tool axis or tool centerline), which major axis passes through the tool center point. Each of the plurality of cutting blades may extend slightly radially from the major axis (tool axis) of the tool by the same distance. In other words, all the blades of a cutting tool may have the same height.

[0018] If multiple cutting blades are provided on the tool, it is preferred that such cutting blades are each spaced apart by a first angular distance. In other words, there may be a regular and known circumferential spacing between the blades (e.g., three blades may be 120° apart from each other, four blades may be 90° apart from each other). The orientations / positions about the rotation axis in step (2) may then be spaced apart by a second angular distance. At least three positions about the rotation axis are used for step (2). The number of positions about the rotation axis may be set based on the number of cutting blades. The first and second angular distances are set so that the measurement in step (2) can be performed from any initial angular orientation with respect to the cutting tool. It is advantageous that the second angular distance is a submultiple of the first angular distance. So, for example, if a tool is provided with four blades spaced from one another by 90°, the rotational positions at which the tool is measured may be 90°, 45°, 30°, 18°, 15°, 10°, 9°, 6°, 5°, 3°, 2°, or 1° apart. This ensures that measurements are made at the same point for each tool, regardless of the initial angle of the cutting tool. Where increased measurement accuracy is desired, multiple measurements are used, but only a small number of measurements (e.g., four for a four-blade cutter) can be used to reliably find the center position of the measured circle.

[0019] It should be noted that this method may also be applied to tools with irregularly spaced blades, although a large number of measurements may be required to obtain the desired measurement accuracy. The number of orientations about the rotation axis in step (2) may be selected based on the number of cutting blades. For example, the tool may include a plurality of cutting blades that are substantially equally spaced from one another along the tool circumference. The three or more orientations about the rotation axis in step (2) may be substantially equally spaced from one another along the tool circumference. In such a case, the number of orientations about the rotation axis conveniently shares a common factor with the number of cutting blades, the common factor being an integer greater than one. In this manner, the same position on each blade will be measured regardless of the initial angular orientation of the tool.

[0020] The measurement in step (2) may be performed in a plane substantially perpendicular to the axis of rotation of the tool (i.e. the spindle centre line). Conveniently, the tool edge sensor comprises a non-contact tool measuring device. For example, it may comprise a break-beam non-contact tool measuring device (such as the NC4 tool measuring system described above). The non-contact tool measuring device may measure the obscuration of the light beam by the tool. Step (2) may use the machine tool to move the spindle relative to the tool edge sensor (e.g. the non-contact tool measuring device). The non-contact tool measuring device may output a trigger signal when a light intensity threshold is exceeded. The non-contact tool measuring device may output an intensity signal that varies in relation to the received light intensity. The tool edge sensor may comprise a one-dimensional sensor. This sensor may be a contact or non-contact sensor. A contact tool setter may be used to radially engage the tool and trace a path along the tool circumference as the tool rotates to provide the tool edge position measurement.

[0021] The method is performed on a tool held in a spindle of a machine tool (e.g., lathe, milling machine, drilling machine, etc.). The machine tool may be a computer numerically controlled (CNC) machine tool. The machine tool may include motorized axes (e.g., for translating the spindle along one or more axes). The spindle of the machine tool may be motorized to drive the rotation of the tool. The spindle may also be coded so that the rotation angle of the spindle (and tool) can be measured. The tool edge sensor may be mounted on a moving or stationary part of the machine tool. For example, the tool edge sensor may be mounted on a bed or table on which a workpiece to be machined can be placed.

[0022] According to a second aspect, there is provided a tool detection apparatus for measuring runout of a tool held by a spindle of a machine tool, the spindle being arranged to rotate the tool about a spindle centre line, the apparatus comprising a tool edge sensor for measuring the position of an edge of the tool in each of three or more orientations about the axis of rotation, and a processor for fitting the measured positions of the edge of the tool to a function to determine a tool centre point, the runout of the tool being determined from the difference in position between the tool centre point and the spindle centre line.

[0023] The invention also extends to a method for measuring runout of a cutting tool supported by a rotatable spindle of a machine tool, the spindle being arranged to rotate the cutting tool about a spindle centreline, the method comprising the steps of: (a) measuring the position of an edge of the tool at three or more positions circumferentially around the tool; (b) calculating a tool centre point from the positions of the tool edges measured in step (a); and (c) determining the runout of the tool using the deviation of the tool centre point calculated in step (b) from the spindle centreline. Step (a) may include rotating the spindle in three or more orientations about the axis of rotation and measuring the position of the edge of the tool at each of the three or more orientations about the axis of rotation. Alternatively, step (a) may include using a contact tool setter having a sensing tip and contacting the tool with the sensing tip from three or more different orientations. This allows the runout of a tool that is not rotating to be measured, thus providing an advantage over machine tools that do not include means for measuring the angular orientation of the spindle. Providing multiple non-contact tool setter devices at different orientations, or being able to turn a single non-contact tool setter device, can also be used to avoid the need to rotate the tool.

[0024] Also described herein is a tool sensing apparatus for measuring runout of a flute cutting tool held on a spindle, the apparatus comprising a tool edge sensor for measuring a position of an edge of the cutting tool at each of three or more positions about an axis of rotation, and a processor for fitting the measured positions of the edge of the tool to a function to thereby determine the runout of the tool. Any of the features described with the associated method above may be implemented by the apparatus.

[0025] Also described herein is a method of measuring tool runout on a flute cutting tool supported by a rotatable spindle. The method includes measuring at two or more positions along the length of the cutting tool, i.e., at a first tool position when the longest likely edge is detected and at a second tool position when the shortest likely edge is detected. In other words, maximum and minimum tool diameters are measured according to the prior art. However, such measurements are made at multiple positions along the length of the tool. The variation in maximum and minimum tool diameters for various tool lengths allows the effect of runout to be separated from the effect of the helical edge profile on the cutting tool.

[0026] Also described herein is a method for determining runout of a flute cutting tool supported by a rotatable spindle using a tool edge sensor, the method comprising the steps of: (a) rotationally moving the spindle in three or more orientations about an axis of rotation, (b) measuring a position of an edge of the tool at each of the three or more positions about the axis of rotation using the tool edge sensor, and (c) fitting the measured positions of the edge of the tool to a function, thereby determining the runout of the tool. [Brief description of the drawings]

[0027] This method will now be described, by way of example only, with reference to the accompanying drawings in which: [Figure 1] FIG. 1 shows a break beam type non-contact tool measurement device and a dual edge cutting tool. [Figure 2a] FIG. 2 illustrates the effect of runout on the rotation of the tool in FIG. 1. [Figure 2b] FIG. 2 illustrates the effect of runout on the rotation of the tool in FIG. 1. [Figure 2c] FIG. 2 illustrates the effect of runout on the rotation of the tool in FIG. 1. [Figure 3a] FIG. 13 further illustrates the effect of shaking. [Figure 3b]FIG. 13 further illustrates the effect of shaking. [Figure 4a] FIG. 1 shows a helical flute extending along the tool. [Figure 4b] FIG. 1 shows a helical flute extending along the tool. [Figure 5a] FIG. 1 shows the inner and outer radii measured at two locations along the length of the tool. [Figure 5b] FIG. 1 shows the inner and outer radii measured at two locations along the length of the tool. [Figure 6] FIG. 13 illustrates values ​​of inner and outer tool radii. [Figure 7] FIG. 13 shows slices representing inner and outer tool radius values ​​for various positions along the length of the tool. [Figure 8a] FIG. 13 shows measured properties of a tool as a function of position along the length of the tool. [Figure 8b] FIG. 13 shows measured properties of a tool as a function of position along the length of the tool. [Figure 8c] FIG. 13 shows measured properties of a tool as a function of position along the length of the tool. [Figure 9a] FIG. 9 shows measurements made on a tool, confirming the theoretical analysis relating to FIGS. 1 to 8. [Figure 9b] FIG. 9 shows measurements made on a tool, confirming the theoretical analysis relating to FIGS. 1 to 8. [Figure 10a] FIG. 8b is similar to FIG. 8a, but shows the analysis for a three-blade cutter. [Figure 10b] FIG. 8b is similar to FIG. 8b, but shows the analysis for a three-blade cutter. [Figure 10c] FIG. 8c is similar to FIG. 8c, but showing a three-blade cutter. [Figure 11a] FIG. 10B is similar to FIG. 10a, but shows the analysis for a four-blade cutter. [Figure 11b] FIG. 10B is similar to FIG. 10B, but shows the analysis for a four-blade cutter. [Figure 11c]FIG. 10C is similar to FIG. 10C, but showing a four-blade cutter. [Figure 12a] FIG. 12c shows data from an experiment carried out on a triple-blade cutter as shown in FIG. [Figure 12b] FIG. 12c shows data from experiments carried out on a four-blade cutter as shown in FIG. 12d. [Figure 12c] FIG. 12b shows a triple blade cutter associated with FIG. 12a. [Figure 12d] FIG. 12b shows a four-blade cutter. [Figure 13] FIG. 13 illustrates the run-out error for a two-flute cutter. [Figure 14] FIG. 13 illustrates runout error for a three-flute cutter. [Figure 15] FIG. 13 illustrates runout error for a four-flute cutter. [Figure 16] FIG. 13 illustrates the radii measured for the four blades of a four-blade cutter at a given runout. [Figure 17] FIG. 1 illustrates the organization of an input matrix. [Figure 18] FIG. 13 is a diagram showing the sum of elements. [Figure 19] FIG. 2 illustrates Laplace's method of calculating a determinant and creating an inverse matrix. [Figure 20] A diagram showing the solution for x0 and y0. [Figure 21] FIG. 1 shows a comparison of TIR measured using the improved and prior methods. [Figure 22c] FIG. 1 shows an experiment based on the theory outlined above. [Fig. 22d] FIG. 1 shows an experiment based on the theory outlined above. [Figure 23a] FIG. 13 shows the measured TIR against a reference value for three runouts at various positions along the length of a 0.07 mm diameter drill. [Figure 23b] FIG. 13 shows the measured TIR against a reference value for three runouts at various positions along the length of a 0.07 mm diameter drill. [Figure 24a] FIG. 1 shows the measured TIR against a reference value for three runouts at various positions along the length of a 0.05 mm diameter drill. [Figure 24b] FIG. 1 shows the measured TIR against a reference value for three runouts at various positions along the length of a 0.05 mm diameter drill. [Diagram 25] FIG. 13 illustrates the measured difference in runout for multiple measurements taken at various distances from the tool tip and at various angular increments between tool diameter measurements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] With reference to Figure 1, a non-contact tool setting device 2 is illustrated. The device comprises a transmitting module 4 including a laser for generating a light beam 6. The light beam 6 is passed to a receiving module 8 including a photodetector for detecting the intensity of the received light. Both the transmitting and receiving modules are mounted to a common base 10 which may be mounted to the bed, or any certainly suitable part, of the machine tool. Although not shown, the non-contact tool setting device 2 is connected to the machine tool through an interface in a known manner during installation.

[0029] In use, the amount of light received by the receiving module 8 is measured. If a tool is placed inside the light beam 6, the amount of light reaching the receiving module is reduced (i.e. because the beam becomes obscured). The received light intensity is continuously compared to a threshold value and a so-called trigger signal is emitted when the threshold value is exceeded due to the tool moving inside or outside the light beam. This trigger signal is passed to a SKIP input of the associated machine tool controller and the position of the tool, as measured by the machine tool, is captured on receipt of the trigger signal. This allows a tool size measurement (e.g. tool length or tool diameter) to be made.

[0030] With reference to Figures 1 to 15 it will now be explained how prior techniques for measuring cutting tool runout using non-contact tool setting devices are prone to error.

[0031] Referring again to FIG. 1, a flute cutting tool 12 is shown (note that the spindle on which the tool 12 is held is not shown). The cutting tool 12 (which may also be called a cutter) is shown in a perfectly aligned position on the zc axis, which is perpendicular to the yc axis to which the light beam 6 is parallel. The tip of the cutting tool 12 has two cutting edges or blades, and the cutting tool 12 is oriented in the direction of the axis of rotation such that the cutting blade 14 at the tip of the cutting tool 12 is perpendicular to the light beam 6 (i.e., it projects in the xc direction). It is also assumed that the cutting tool 12 is perfectly straight, and that both cutting edges have the same radial height or length (i.e., both blades extend radially the same distance from the tool centerline).

[0032] Figure 2a shows the effect of introducing runout. In particular, consider the case when the tool tip is pivoted (rotated) relative to the spindle centerline by 0.5° along vector xc (noting that the drawing shows a much larger inclination for illustration purposes). If the axis of the pivoting motion is assumed to be 57.295 mm long above the laser beam, this results in a TIR of 1.0 mm at the tool tip. Figures 2b and 2c show in more detail how this causes the spindle axis 20 (i.e., the axis about which the spindle rotates, often referred to as the spindle centerline) to deviate from the tool axis 22 (i.e., the central axis of the elongated tool).

[0033] Figure 3a shows the movement of the tool tip (again exaggerated for illustrative purposes) as the spindle holding the tool rotates. This runout at the tool tip will mean that in use the tool will drill a hole with a larger diameter than its physical size. Figure 3b shows a top-down view of the same effect.

[0034] With reference to Figures 4a and 4b, the effect of a helical profile on the cutting edge will now be considered. As explained above, consider the case where the tool tip 12 is positioned such that the blade 14 projects in the xc direction. Moving further to the top of the tool, the helical profile on the cutting edge means that the blade 14 no longer projects outwards in the xc direction. In this example, the cutting tool has a helix that completes one full revolution every 32 mm. If we consider slices moving 2 mm apart up from the tip of the tool, then each slice shifts by 22.5°. Figure 4a shows such a slice from the side of the tool, and Figure 4b shows a first slice 30 at the tip and a second slice 32 at a point 2 mm above the tip.

[0035] Figures 5a and 5b illustrate the effect of the change in the angle of the tool cutting edge between the first and second slices relative to the position of the tool, as measured with a tool measurement device.

[0036] FIG. 5a shows a first slice in which the spindle axis 20 and tool axis 22 are 0.5 mm apart as the first blade 14 of the cutting tool expands along the xc direction. The effect of the runout is thus to make the first blade 14 appear 0.5 mm longer than its physical length (i.e., the first blade expands further in the xc direction). So, for a tool with a 3 mm radius, the first blade would be measured to be 3.5 mm long. Similarly, the second blade of the cutting tool would be measured to be 0.5 mm shorter than its physical length. The second blade would thus be measured to be 2.5 mm long. The difference in the measured blade lengths (note that blade length is used herein as shorthand to refer to the radial extent of the blade) would thus be 1.0 mm, which is equal to the amount of runout.

[0037] However, FIG. 5b shows the situation when tool measurement is performed on the second slice (i.e. 0.2 mm above the tip). The effect of the helical rotation on the cutting blade means that the cutting blade no longer protrudes along the xc direction when the spindle axis 20 and the tool axis 22 are separated by 0.5 mm. Instead, the effect of runout is reduced due to the additional rotation 22.5° that is required for the cutting blade to extend along the xc direction. In particular, the spindle axis 20 and the tool axis 22 are separated by only 0.483 mm in the xc direction for this second slice. This results in the maximum and minimum blade lengths being measured as 3.451 mm and 2.561 mm, respectively. In this way, it can be seen that the difference in tool length is now less than the actual runout of 1.0 mm.

[0038] FIG. 6 summarizes the inner / outer radii and TIR for the first and second slices.

[0039] Figure 7 shows diagrams similar to those of Figures 5a and 5b for each of the slices of the tool taken at 22.5° intervals along 32 mm. The diagram at the top left shows the first slice (i.e., the tip) and the subsequent slices are displayed in order from left to right from the top row to the bottom row. It is noteworthy that no runout is measured for the fifth and thirteenth slices (i.e., 90° and 270°) because both the spindle axis and the tool axis are located at the same position along the xc axis (i.e., eccentricity is only due to differences in the yc direction) and therefore its effects are not seen by the tool measurement device which is insensitive to positional variations along the laser beam (i.e., in the yc direction).

[0040] Figure 8a shows the radii measured for the two tool tips at each of the slices described above. Point cloud 80 shows the radii measured for the first cutting edge 14, and point cloud 82 shows the radii measured for the second cutting edge. Figure 8b shows the outer radius of the tool (point cloud 84), and the measured TIR (point cloud 86). Thus, it can be seen that variations in the TIR occur based on the location of the radius measurement along the length of the cutting tool.

[0041] Figures 9a and 9b show how such errors arise when measuring a tool. Figure 9b shows a 6mm diameter 2-flute end mill mounted in a Nikon Zero Fit tool holder. The pivot point of the tool holder is located 130mm from the tool tip. The runout of the shank above the cutting flutes (located 27mm from the tool tip) is measured using an LVDT (Linear Variable Differential Transformer) with a resolution of 0.1μm. The tool holder is set to give a runout of 107μm (as measured by the LVDT) and the tool diameter and runout are measured using the non-contact tool measurement device 2 as described above. The change in the measured tool diameter and runout is plotted in Figure 9a. As in the theoretical model, it can be seen that the measured runout varies with position from the tool tip.

[0042] Figures 10a and 10b show that a similar effect occurs in a three-edged cutting tool, with Figure 10a showing the measured radius of each cutting edge and Figure 10b showing the measured outer radius and TIR.

[0043] Figures 11a and 11b show that a similar effect occurs in a four-blade cutting tool, with Figure 11a showing the measured radius of each cutting blade and Figure 11b showing the measured outer radius and TIR.

[0044] Figures 12a and 12b support the models of Figures 10 and 11 with experimental measurements. It is noteworthy that according to Figure 12a the runout change does not track the measured diameter change for a three-edged cutting tool, but it does track the measured diameter change for a four-edged tool as shown in Figure 12b.

[0045] Referring now to Figure 13, it is seen that the maximum possible error that can occur when measuring a two-blade cutter is 1.0 mm. Figure 14 shows that the maximum possible error that can occur when measuring a three-blade cutter is 0.110 mm, and Figure 15 shows that the maximum error for a four-blade cutter is 0.340 mm. This is all based on the fact that a TIR of 1.0 mm actually exists.

[0046] The prior art measures tool runout and diameter at a fixed height above the tool, which means that accurate results cannot be guaranteed. In the worst case, the smallest diameter will be measured and the TIR will show zero, however the tool will move further up the flute to cut further at a larger diameter. If a single full 8mm deep ledge cut is made, the edge of the ledge will not be straight (i.e. the edge will be wavy and / or have form errors).

[0047] 16-25, a technique for runout measurement according to the present invention will now be described that overcomes at least some of the shortcomings of the techniques discussed above.

[0048] It will now be explained how tool runout can be derived from measurements of each of the individual cutting edges and then calculating the average of the tool center point for a four edge cutting tool.

[0049] Figure 16 illustrates the radial location of each cutting tip from the spindle centerline for multiple slices along the length of the cutting tool, assuming that each tooth extends the same distance from the tool center and that the angular spacing of the teeth is equal.

[0050] We will now look at the mathematics of determining the centre point of a two-dimensional circle. In particular, we take any number of XY tool measurement "skips" that describe the measured locations of the tool edge in the machine coordinate system. These are converted to polar coordinates that describe the angle and radius at each point relative to the spindle centreline, using the known angular orientation of the spindle at the time each measurement is taken. These polar coordinates are then converted back to a Cartesian coordinate system and fitted to a circle. The method applies a best fitting circle to the point cloud data, which allows the centre point of the fitted circle to be derived, given in an XY coordinate system relative to the spindle centreline. The radius or runout can then be calculated for a particular measurement location along the tool (programmed from the tip).

[0051] The purpose of this function is to find x0, y0 and r such that:

[0052]

number

[0053] This is x i and y i This is done by solving the equations for x0 and y0, given that x0 is a given point.

[0054] To solve this equation, an input matrix (2x2) and a known vector (1x2) are needed. For purposes of demonstration, a 4-flute tool with 0.5mm runout will be used, as shown in Figure 16.

[0055] The input matrix is ​​defined with reference to Figure 17. In particular, the following steps are performed: Point cloud data is an array of two XY coordinates, i.e. x i and y i are calculated as follows. x i and y i The average of each is x i and y iis subtracted from the new array x c and y c Creates. Each of the six arrays is cross-multiplied and the elements are summed for each array.

[0056] Referring to FIG. 18, the sum of the squares is calculated, then cross-multiplied with other array combinations, and then summed.

[0057] The Laplace method is then used to calculate the determinant and invert the matrix, as shown in Figure 19, before multiplying the matrix by the vector and solving for x0 and y0 according to Figure 20. It should be noted that alternative mathematical techniques can be used to fit the data points to a circle. Of course, different coordinate systems can be used.

[0058] 21 shows TIR values ​​210 calculated using the method according to the present invention, which are seen to be more predictable and reliable than prior art TIR measurements 214 and outer radius measurements 212.

[0059] Figures 22c and 22d show experimental data collected to demonstrate the method of the present invention. In particular, experiments were performed using two micro drills, 0.07 mm and 0.05 mm in diameter. Each drill bit has two flutes that spiral up the tool in a helical fashion. Each drill is mounted in a Nikon Zero Fit tool holder to allow for adjustable runout. Figure 22d shows the linear trigger position for the tool of Figure 22c converted to polar coordinates, noting the machine resolution of 0.001 mm.

[0060] The run-out benchmark values ​​of the two drilling tools were measured by a camera based non-contact tool setter and compared with a laser based non-contact tool setter implementing the method according to the invention.

[0061] Figure 23a shows a comparison (for a drill with a diameter of 0.07 mm) of run-out measurements made using a reference non-contact tool setter (camera based) and a laser based non-contact tool setter implementing the method according to the invention for three levels of run-out (0.001 mm, 0.003 mm, and 0.008 mm) at various distances from the tool tip. A good correlation is observed between the measurements. Figure 23b plots the measurement difference for three levels of run-out (0.001 mm, 0.003 mm, and 0.008 mm) at various distances from the tool tip.

[0062] Figures 24a and 24b plot similar results for a 0.05 mm diameter drill. Again, good correlation is observed.

[0063] The TIR measurements according to the invention presented in Figures 23 and 24 were made with 360 data points collected for each slice along the tool (i.e., 1° increments). Figure 25 shows the effect of the size of the measurement increment on the runout error when measuring a 0.05 mm diameter drill. If the angular distance of such points is set taking into account the angular distance of the cutting blade, it is also possible to use much fewer points. For example, to measure a tool with three equally spaced cutting blades, a minimum of three equally spaced points may be appropriate.

[0064] While the method described above uses tool rotation to measure points along the tool circumference, an alternative method is to use a contact tool setter (or a rotatable non-contact tool setter) and approach the centre of the contact tool setter stylus from three or more different angles, but on a tool that does not rotate. For example, a first measurement can be taken along the X-axis, a second measurement at 60° to the X-axis, a third measurement at 120° to the X-axis, and so on, until three or more (e.g. six) equally spaced measurements of the tool edge have been taken. The mathematics described above for tool rotation then remain the same, and the measured tool edge positions can be fitted to a circle to determine the tool centre point.

[0065] It should again be noted that the foregoing is merely illustrative of the present invention and should not be construed as limiting the scope of the present invention.

Claims

1. 1. A method for determining runout of a tool supported on a rotatable spindle of a machine tool using a tool edge sensor mounted to the machine tool, the spindle being positioned to rotate the tool about a spindle centerline, the method comprising: a first step of rotationally moving the spindle in three or more orientations about an axis of rotation; a second step of measuring a position of a tool edge for each of the three or more orientations about an axis of rotation using the tool edge sensor; a third step of determining a tool center point by fitting the tool edge positions measured in the second step to a function; a fourth step of determining runout of the tool using the position difference between the tool center point and a spindle centerline determined in the third step; A method comprising:

2. 2. The method of claim 1, wherein the first step includes intermittently rotating the tool in the three or more orientations about an axis of rotation, and the second step measurements are taken while the tool remains stationary.

3. The method of claim 1 , wherein the first step includes the step of continuously rotating the tool, and the second step is performed in the three or more orientations about the axis of rotation while the tool is rotating.

4. 4. The method of claim 1, wherein the third step includes fitting the three or more positions measured at the tool edge to a circle to find a center of the circle, the center of the circle defining the tool center point.

5. 5. The method of claim 1, wherein the fourth step includes determining runout of the tool at a plurality of positions along the length of the tool.

6. The method of any one of claims 1 to 5, wherein the tool comprises a cutting tool having a plurality of cutting edges.

7. 7. The method of claim 1, wherein the tool comprises a cutting tool having three or more cutting edges.

8. 8. The method according to claim 6 or 7, characterized in that the cutting tool is a fluted cutting tool.

9. 9. The method of claim 6, wherein the cutting tool has a major axis coincident with the tool center point, and each of a plurality of cutting edges extends slightly the same radial distance from the tool major axis.

10. 10. The method of claim 1, wherein the tool is a cutting tool including a plurality of cutting blades spaced substantially equally apart from one another around a circumference of the tool, the cutting blades being equally spaced a first angular distance apart and the rotational direction in the second step being spaced a second angular distance apart, the second angular distance being a submultiple of the first angular distance.

11. 11. The method of claim 1, wherein the tool comprises a plurality of cutting blades spaced substantially equally apart from one another along a tool circumference, and the three or more orientations about the axis of rotation are substantially equally spaced from one another along the tool circumference, and a number of orientations about the axis of rotation share a common divisor with respect to the number of cutting blades, the common divisor being an integer greater than one.

12. 12. A method according to any one of the preceding claims, characterized in that the second step comprises the step of taking measurements of the tool edge in at least one plane substantially perpendicular to a tool rotation axis.

13. 13. The method of claim 1, wherein the tool edge sensor comprises a break beam non-contact tool measuring device, and the second step comprises using the machine tool to move the spindle relative to the tool edge sensor.

14. 1. A tool detection device for measuring runout of a tool held on a spindle of a machine tool, the tool detection device comprising: a tool edge sensor for measuring a position of an edge of the tool in each of three or more orientations about an axis of rotation, the spindle being arranged to rotate the tool about a spindle centerline; and a processor for fitting measured positions of the edge of the tool to a function to determine a tool center point, the tool detection device being characterized in that the runout of the tool is determined from the difference in position between the tool center point and the spindle centerline.

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