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 gauge elements of the type, passes - does not pass', as shown in Fig. 1. Such test, type, pass, do not pass type components usually referring to plug, ring, tapered or threaded components are reliable with deviations of up to about 0.05 mm. The transitive gauge 26 must either slide smoothly into the machined hole 30, as in the case shown in FIG. 1, or on the contrary - he enters the processed projection without obstacles, and if it is not, the object exceeds its maximum material condition. The intransigent gauge 24 cannot be inserted into the machined hole (see fig. 1), or on the projection, and if it can, then the item 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 objects, because each test must be manually guided to the site. 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.

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[0005] Another known way of checking that an object is within tolerance is by checking the object with a coordinate fixing 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 the analog probe, the probe deflection is continuously measured while the needle is 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 dimensional measurement data sent to a controller (which may include a computer program). For example, analogue probes transmit thousands of dimensional measurements. Along with information about the location of the machine, the measurement data from the probe allow the controller to build an accurate picture of the dimensions of the workpiece. 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 by a wireless link, such as an optical or radio link. These telecommunication signals consume a lot of battery power. Because of the significant number of points to be measured, therefore

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- a significant amount of data to be transferred, the whole process consumes a lot of battery power and requires a lot of time.

[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 subject to the dependent claims.

[0012]. 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.

[0013] 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 bias condition occurs when the bias probe is less than a bias predefined bias measurement threshold value.

[0014] The predetermined measuring range depends on at least one object tolerance. The predefined threshold measurement value depends on at least one workpiece tolerance.

[0015] 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 tolerance of the object, or the probe is displaced by coordinates shifted relative to the coordinates determining the tolerance of the object.

[0016] 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 fail' mode while

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- elements 'does not pass' can only check one aspect of the property at a time to avoid mistakenly passing defective items through the check. 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 no longer meets the requirements of the tolerance test, which is not possible when using simple gauge elements.

[0017] 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.

[0018] This has the advantage that when using an analog radio probe, it draws much less power from the battery 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.

[0019] Alternatively, the measurement probe provides 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.

[0020] Preferably, where the machine tool measuring device has a main processor for controlling the machine tool operation, the step of indicating that the dimensions of the object do not comply with the tolerance includes providing this indication to said main processor. This enables the main processor to make decisions about the next action. For example, if the workpiece is too large, the main machine tool processor may move the machine so that the workpiece can be reworked.

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[0021] 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.

[0022] The measuring device used in operating the method according to the present invention comprises a coordinate positioning device i.e. machine tools such as lathes, machining centers etc.

[0023] 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.

[0024] 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. For probe measurement data to remain in one state, they must remain within a predetermined range of measurement data, which range depends 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. [0025] 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. [0026] The method may include the additional step of re-scanning the area of ​​the object to accurately determine the location of the defect 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.

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[0027] 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 is a block diagram of outputting tolerance data by an interface or a controller.

[0028] In Fig. 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 14. The probe contains the probe body 5, the needle 8 and the tip 10 needles. The workpiece 14 is clamped on the machine table 16.

[0029] 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 tool 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.

[0030] 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 that provides three degrees of freedom of movement of the probe relative to the workpiece is possible.

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[0031] Machine tools have been described above. [0032] 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.

[0033] When checking the bore 20 in "pass" mode, the probe needle is initially not deflected and is displaced by the controller along the "pass" path 22 based on the 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.

[0034] In this case, a positive pass 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.

[0035] 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.

[0036] In this case, passing through the check 'does not pass' means that the hole diameter 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.

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[0037] 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.

[0038] An analog probe is used, when this example of this method is followed, that is, by driving the probe in two ways, one, in which the probe must remain deflected (i.e. the probe deflection is greater than the upper limit), that the workpiece is within the tolerance and the second, in which the probe must remain without a bias (i.e. the probe bias must be less than the lower limit), that the workpiece is within tolerance.

[0039] The 'passes' and 'does not pass' paths are opposite to the holes and the mandrels. In fig. 4a shows a workpiece 40 that is a cylinder with a nominal diameter of 42. In fig. 4b shows the top view of the cylinder with tolerances of its diameter. 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', [0040] Based on Fig. 5 a further method according to the invention will be described. FIG. 5a shows a workpiece 50 having an opening 52, while in FIG. 5b shows the change in needle deflection as a function of time during workpiece testing. 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 in the range 54-56 while the probe is moving along a certain path, then the workpiece is within tolerance and passes the check with a positive result. If the probe needle is deflected 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 needs 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.

[0041] 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 tilted above the upper limit 56, the machine tool will react and send a signal

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- EP2029967 to the machine tool 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.

[0042] This method is carried out using an analog probe as described above. [0043] The method of determining if the dimensions of the workpiece are in compliance 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 this area at a lower speed, which will allow you to more accurately determine the point where the defect occurred in the workpiece.

[0044] 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.

[0045] 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.

[0046] The example described above with reference to Figures 2 - Fig. 5 relates to signals sent from the probe to the controller / interface only if the workpiece does not meet the required tolerances, as shown in the block flow diagram of Fig. 6a.

[0047] 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 in Fig. 6b. In this case, the interface or controller 62 outputs only breach data on its output.

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- EP2029967