A method for measuring an object

[0001] The present invention relates to the use of a measuring device for determining whether or not the dimensions of an object are tolerant. In particular, the invention relates to a method for determining whether object dimensions are compatible with a tolerance using a measuring probe mounted on a coordinate determining device.

[0002] The conventional method of checking whether objects meet tolerances is to use type-checking elements, it passes - does not pass', as shown in Fig. 1. Such type elements, which pass, do not pass - usually referring to the plug element, ring, tapered or threaded, they are reliable with deviations up to about 0.05 mm. The transitive gauge 26 must either slide unobstructedly into the machined hole 30, as in the case shown in Fig. 1, or, on the contrary, enters the machined projection without obstacles, and if this is not the case, the object exceeds its maximum material condition. The intransigent gauge 24 cannot be inserted into the machined hole (see Fig. 1) or onto the projection, and if it can, the object is below its minimum material condition.

[0003] The disadvantage of this method is due to the fact that the gauges themselves must have tolerances set as they are also produced. All test tolerances must be within performance tolerances to ensure that no items outside the tolerance range (bad) are accepted, but some good items will be rejected. Another disadvantage of this method is the time it takes to check items because everyone

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- The EP2029967 test must be manually guided into place. To prevent bad items from being passed through incorrectly, the transitive gauge should only be used to check one dimension or shape of the product, and the intransitive gauge should be used to check only one aspect of some property (i.e. the length and width of the rectangle should be controlled separately).

[0004] The 'passing - not passing' type test elements only give an answer whether the detail is good or bad. If the detail is wrong, the test element does not indicate why it is defective. For example, if a tool becomes dull during cutting and causes a machining error, then it is worth knowing exactly where it happened. However, this information cannot be obtained from test items.

[0005] Another known way of checking that an object is within the tolerance is by checking the object with a coordinating device such as a machine tool. Items inspected by machine tools are often items that have been machined by such a machine tool. The machine tool has a spindle on which the probe is mounted (contact or no contact). The spindle can be driven in three mutually perpendicular directions X, Y, Z inside the machine's working space.

[0006] Touch probes comprising a touch trigger and analog probes typically have a housing with a contact needle contacting the pivotable needle relative to the housing. In the touch probe that triggers the needle to deflect from its resting position, a signal is generated indicating that the needle has touched the surface of the workpiece (see, for example, patent GB1445977). In an analog probe, the probe deflection is continuously measured while the needle is

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- 3 EP2029967 transmitted by computer).

moved along the surface of the workpiece (see, for example, Patent No. GB1551218). Of course, an analog probe can be attached to a digital processor to give a digital output signal.

[0007] Non-contact probes are placed close to the surface of the workpiece without touching it. Such a probe detects the proximity of surfaces using, for example, capacitive, inductive or optical elements.

[0008] For each property, touch and non-contact probes are based on a significant amount of data from dimensional measurement, to the controller (which may include a program. For example, analog probes transfer thousands of dimensional measurements. Together with information about the location of the machine, the measurement data from the probe allow the controller to build an accurate image workpiece dimensions. To assess whether the workpiece is within tolerance or not, the dimensional measurements obtained by the probe must be compared with the required workpiece measurements.

[0009] The properties must be measured at a sufficiently low data transfer rate received from each point where the probe is deflected. In some types of coordinate positioning devices, such as machine tools, the probe is battery powered and the measurement data is sent from the probe to the controller via a wireless link, for example an optical or radio link. These telecommunication signals consume a lot of battery power. Because of the large number of points to be measured, and therefore the large amount of data to be transmitted, this whole process consumes a lot of battery power and requires a lot of time.

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[0010] US Patent US-A-4831741 discloses a known test system for quality control. Patent WO-A1-2005 / 031254 discloses a known measuring method for use in coordinate measuring machines. [0011] The above-mentioned object can be achieved by the method of independent claim 1.

Preferred embodiments are the subject of the dependent claims 2 to 15.

[0012] This method can be carried out using a touch probe. In a touch probe, such as a flip-flop touch probe, the change in the state of the probe measurement data received by the probe can take place between the probe positions deflected and not deflected.

[0013] The touch probe may also be an analog probe. A change in the state of the analog probe measurement data can occur, for example, when the probe measurement data changes outside a predetermined measurement range, or when the probe measurement data exceeds a predetermined measurement threshold.

[0014] A change in state between the deviation and no deviation of the analog probe occurs when the probe deviation measurement exceeds a predetermined threshold value of the deviation measurement. For example, a bias condition occurs when the probe bias is greater than a predetermined bias measurement threshold value, and a non bias condition occurs when the bias probe is less than a pre-defined bias measurement threshold value.

[0015] Such a method can also be carried out using a non-contact probe. In non-contact probes data change

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The probe measurement occurs, for example, when the probe measurement data changes outside a predetermined measurement range, or when the probe measurement data exceeds a predetermined measurement threshold.

[0016] Preferably, the predetermined measuring range is dependent on at least one object tolerance. Preferably, the predetermined threshold measurement value depends on at least one object tolerance.

[0017] Item tolerance defines the maximum and minimum acceptable size of the item. For example, for a cylinder, the minimum tolerance will be a concentric cylinder with a smaller diameter, and the maximum tolerance will be a concentric cylinder with a larger diameter. The path along which the probe is displaced is based on the tolerance of the object, i.e. the probe is displaced by coordinates defining the object tolerance, or the probe is displaced by coordinates shifted relative to the coordinates determining the object tolerance.

[0018] The method according to the present invention is faster and more versatile than the known "pass, do not pass" test elements described above. The probe can measure any number of features immediately in 'pass or not pass' mode, while 'pass' items can check only one aspect of the property at a time to avoid mistakenly passing defective items through control. The invention also solves the problem of rejecting good parts due to deviations of the test elements themselves. When using the present invention, it is also possible to determine the exact point at which the object

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EP2029967 no longer meets the requirements of the tolerance test, which is not possible when using simple gauge elements. [0019] Preferably, the step of monitoring the obtained measurement data using any probe when the probe is moved along said path is carried out in the probe itself. In other words, the measurement probe may include a control assembly for performing the monitoring step of the data obtained by measuring the probe when the probe is moved along said path.

[0020] This has the advantage that when an analogue or non-contact radio probe is used, it draws much less battery power and is much faster than traditional probing methods. This is due to the need to send signals only if the attempt fails. Consequently, only very few signals (if any) are sent when testing object tolerance. This significantly reduces the amount of data transferred compared to the thousands of signals needed to reproduce an accurate three-dimensional image, which means a decrease in battery power consumption.

[0021] Alternatively, the measurement probe reports all or most of the measurement data taken by the probe to the associated control assembly that performs the step of monitoring all probe measurement data. The interface or driver may contain an associated inspection assembly. This method has the advantage of being faster than previously known methods, because the three-dimensional imaging of the workpiece does not need to be calculated by associating the machine tool position and probe measurement data.

Preferably, where the machine tool measuring device has a main processor for controlling the operation of the machine tool, step

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- indicating that the dimensions of the object do not comply with the tolerance includes providing this indication to said main processor. This allows the main processor to take action.

If, for example, if the method according to the coordinate decision to take the next one is too large, the main processor of the machine may move the machine so as to carry out the reworking of the object.

[0023] The sensor output is preferably provided somewhere on the probe or on the associated control unit to signal to the operator if the dimensions of the object do not comply with the tolerance. The sensor output may for example be a light emitting diode or buzzer.

[0024] The measurement probe may be a touch probe, for example a touch trigger probe, or an analog probe, or a non-contact probe, such as a capacitive, induction or optical probe.

[0025] The measuring device used in operating the present invention may include a positioning device such as, for example, dedicated coordinate measuring machines (CMM), or machine tools such as lathes, machining centers, etc.

[0026] Preferably, the step of moving the probe along a certain path relative to the object is carried out with the probe in "fast" mode, i.e. as soon as the probe is moving without giving false measurement data.

[0027] Preferably, the travel path relative to the object can only include the first path. In this example, because the probe is moved along said first path, probe measurement data must remain in one state for the object to be within tolerance. To

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The probe measurement data remained in one state, they must remain within a predetermined range of measurement data, the range being dependent on at least one object tolerance. If the probe measurement data goes out of the specified range, changing the state of the probe measurement data, then a signal is sent indicating that the object does not meet the tolerance. Preferably such a method is used with contact probes such as analogue probes and non-contact probes such as capacitive, induction or optical probes.

[0028] Preferably, the path relative to the object may comprise a first path and a second path. The first and second roads listed can be based on the permissible workpiece dimensions with maximum and minimum tolerances. The permitted dimension corresponding to the maximum tolerance is, for example, the maximum acceptable cylinder size for a cylinder. The permitted dimension corresponding to the minimum tolerance is, for example, the minimum acceptable cylinder size for a cylinder. When the probe is moved after the first path, the probe data must remain in the first specified condition, and when the probe is moved the second path, the probe measurement data must remain in the second specified condition for the object to be within tolerance. This method can be used with contact probes such as a touch trigger and with analog probes and non-contact probes such as capacitive, induction or optical probes; this is best suited for touch trigger probes.

[0029] The method may include the additional step of rescanning the area of ​​the object to accurately determine the position

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Defects if the initial scan signals a defect in said area. The rescanning step is preferably performed at a slower speed than the initial scan to more accurately find the point where the defect occurred in the object.

[0030] Preferred embodiments of the invention will now only be described by way of example on the basis of the attached drawing, in which:

Figure 1 is a side view of the known gage element "passes, does not pass" and the object whose feature is to be checked by said gage element;

Figure 2 schematically shows a probe mounted in a machine tool above the workpiece;

Figure 3 is a plan view of the workpiece having the hole;

Fig. 4a is a perspective view of the cylinder and Fig. 4b is a top view of this cylinder; Fig. 5a is a top view of the workpiece having an opening; and Fig. 5b shows the variation of the needle deflection as a function of time when scanning the hole shown in Fig. 5a; Fig. 6a is a block diagram of outputting probe tolerance output; and fig. 6b shows a block diagram of outputting data regarding breach of tolerance by an interface or a controller.

[0031] In Figure 2, the workpiece positioned on the machine tool is shown. The measuring probe 6 is mounted on the machine spindle 2 and is fixed by means of the tool holder 4. This is the same position in which the cutting tool would be clamped when machining feature 12 in the workpiece

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- The object 14. The probe comprises a probe body 5, a needle 8 and a needle tip 10. The workpiece 14 is clamped on the machine table 16.

[0032] In this case, the spindle 2 and the measuring probe 6 can move in the X, Y and Z directions under the action of the X, Y and Z drive units controlled by the computer, interface or machine controller, while the table remains stationary. The X, Y and Z scales (which contain counters for the scale output signals) represent in three dimensions the instantaneous coordinates of the position of the spindle 2 on which the measuring probe 6 is mounted. The measurement readings sent from the measuring probe 6 are combined with the readings from the X, Y and Z scales, which allows you to calculate the position of the needle tip and thus the surface of the workpiece.

[0033] Although a stationary table is shown, the machine tool may include a spindle and a probe that only move in the Z direction and the table moves relative to it in the X and Y directions. Any combination of three degrees of freedom of movement of the probe relative to the workpiece is possible.

[0034] Although machine tools have been described above, other measuring devices can be used, for example dedicated CMMs, robots and machines with non-Cartesian measurement.

[0035] Fig. 3 is a top view of the workpiece 14 containing the machined hole 20. The device described in Fig. 2 can be used to check the tolerance of this hole.

[0036] When checking the bore 20 in "pass" mode, the probe needle is initially not deflected and is moved by the controller along a "pass" path 22 based on

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- EP2029967 smallest acceptable diameter d1 of the hole. If the probe needle stays open at all points of its path, no signal is sent to the data capture device (e.g. machine tool controller) and the workpiece passes the pass check.

If the needle is deflected at any point along the 'passing' path, the machine tool reacts here by writing the XYZ coordinates of the position at which the probe deflects. The hole has a diameter smaller than the smallest acceptable diameter d1 and the workpiece does not pass the 'pass' tolerance test.

[0037] In this case, passing through the 'pass' control means that the hole diameter is larger than the smallest diameter determined by the tolerance. If the workpiece fails the test, it passes with a positive result, then it must be further processed and checked again.

[0038] When checking the bore 20 in "not passing" mode, the probe needle is initially deflected from the surface of the bore and driven by the controller along the path 18 "does not pass" according to the largest acceptable diameter d2 of the bore. This path 'does not pass' is chosen such that the position of the needle re-seat is outside the tolerance limit. If the needle remains deflected at all points on the road, no signals are sent and the workpiece passes with a positive result by the 'does not pass' check. If the needle re-seats, i.e. the needle is not deflected, then during its movement the machine tool reacts at this point, recording the position of the XYZ re-seat the needle. The tolerance test result 'does not pass' gives a negative result.

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In this case, the passage through the control does not pass' means that the diameter of the hole is smaller than the largest diameter determined by the tolerance. If the workpiece does not pass the 'does not pass' check, the workpiece should be rejected.

[0040] The maximum and minimum dimensions of the workpiece can be calculated by adding and subtracting tolerances from the known desired surface profile of the workpiece.

[0041] Such an embodiment of this method is best suited for use with the tactile trigger probe described above. The analog probe can be used in the same way as a tactile trigger probe when following this embodiment of the method, i.e. by driving the probe in two ways, one in which the probe must remain deflected (i.e. the probe deflection is greater than the limit upper), so that the workpiece is within the tolerance and the second, in which the probe must remain without a deviation (i.e. the probe deviation must be less than the lower limit), that the workpiece is within tolerance.

[0042] This embodiment is also suitable for use with non-contact probes such as optical, capacitive and induction probes. In this case, the distance of the workpiece from the probe is measured at points along the road. Following the path 'passes', if the distance reading is greater than the specified value (i.e. the workpiece is further away from the probe than the tolerance limit), the workpiece will pass positively through the test passes positively, however, if the distance reading reduces 'passes', it goes below specified level, then this part will not pass

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- EP2029967 in the 'passes' test. Following the path 'does not pass' if the distance reading remains less than the specified level (i.e. the workpiece is closer to the probe than the tolerance limit), the workpiece will pass the 'does not pass' test, however if the distance reading increases above the specified level, then the workpiece will not pass the test 'fail'. In each case, a certain level separates the two probe states from each other (e.g., triggered and not triggered).

[0043] The 'passes' and 'does not pass' paths are opposite to the holes and the mandrels. Fig. 4a shows a workpiece 40 which is a cylinder with a nominal diameter of 42. Fig. 4b shows a top view of the cylinder with a marking of its diameter tolerance. In this case, path 44 describes the smallest acceptable diameter d1 'of the road cylinder' does not pass ', while path 46 describes the largest acceptable diameter d2' of the road cylinder 'passes', [0044] A further method according to the invention will be described from Fig. 5. Fig. 5a shows the workpiece 50 having an opening 52, while in Fig. 5b the changes in the needle deflection as a function of time during the workpiece inspection are shown. In this example, the probe needle has a maximum of 56 and a minimum of 54 deviations, which correspond to a minimum of d3 and a maximum of d4 of the tolerated bore diameters 52. If the probe needle deflection remains within the range of 54-56 when the probe moves along a certain path, then the workpiece is within tolerance and passes the control with a positive result. If the probe needle is swung above the upper limit 56 (as in point 36), the machine tool will react and send a signal to the machine controller indicating that the workpiece

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EP2029967 requires further processing. If the probe needle is deflected below the lower limit 54, the machine will react and send a signal indicating that the workpiece should be discarded.

[0045] For the cylinder, the minimum acceptable diameter of the cylinder can be determined as the minimum acceptable deviation, and the maximum acceptable diameter of the cylinder is consequently determined as the maximum allowable deviation. In turn, if the probe needle is swung above the upper limit 56, the machine tool will react and send a signal to the machine controller indicating that the workpiece needs further processing. If the probe needle is deflected below the lower limit 54, the machine tool will react and send a signal indicating that the workpiece should be discarded.

[0046] This method can be carried out using an analog probe as described above. This method is also suitable for contactless probes, e.g. optical, inductive and capacitive. Such probes measure the distance of the workpiece from the probe. A maximum and minimum distance that corresponds to the tolerance limits is set to determine the tolerance range of the probe output signals. The machine tool will react and a signal will be sent only if the probe output signal goes out of the allowed range.

[0047] The method of determining if the dimensions of the workpiece are in accordance with the tolerance may include the additional step of re-scanning a certain area of ​​the workpiece. If the initial scan signals a defect in a certain area, then the scan can be repeated in that area with a smaller one

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- speed, which will allow a more accurate determination of the point at which the defect occurred in the workpiece.

[0048] Other types of machining features that the invention can be checked for compliance with the tolerance include, for example, corners, bevelled edges and straight edges.

[0049] Fig. 6a is a flowchart illustrating the method of the invention in which the probe 60 only outputs the exceeding of tolerance data transmitted to the interface or controller 62. Fig. 6b shows a block diagram with the probe 60 outputting all measurement data to be transmitted to the interface or to the controller 62.

[0050] The example described above with reference to Figures 2 - Fig. 5 relates to signals sent from the probe to the controller / interface only when the workpiece does not meet the required tolerances, as shown in the block flow diagram of Fig. 6a.

[0051] Alternatively, the probe may send signals to the controller / interface at all points along its path, and the controller / interface may send a signal to the controller or machine interface if the workpiece dimensions do not comply with the tolerances. This is shown in the flow chart of Fig. 6b. In this case, the interface or controller 62 outputs only breach data on its output.

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- EP2029967