GB2321524A

GB2321524A - Coating thickness measurement

Info

Publication number: GB2321524A
Application number: GB9801102A
Authority: GB
Inventors: Helmut Fischer
Original assignee: Helmut Fischer GmbH and Co
Current assignee: Helmut Fischer GmbH and Co
Priority date: 1997-01-28
Filing date: 1998-01-20
Publication date: 1998-07-29
Also published as: GB9801102D0; DE19702950A1; DE19702950C2

Abstract

The coating thickness on coated, substantially cylindrical workpieces 14 such as screws, pins, etc., is measured by magnetic induction or eddy current methods using a measuring probe 4 whose measuring pole 5 is brought into tactile contact with a predetermined measuring point on the workpiece, the measuring probe being positioned in a receptacle 2 and the workpiece being inserted in an adaptor 11, 12, 13 exposing at least the measuring point on its circumference or end face and positioning same with respect to the measuring probe. The measuring probe and adaptor are moved relative to one another in forcibly guided manner up to contact between measuring pole and measuring point.

Description

2321524 METHOD AND DEVICE FOR MEASURING THE COATI5G THICKNESS ON

CYLINDRICAL WORKPIECES The Invention relates to a method for measuring the coating thickness on coated, substantially cylindrical workpleces or small components, in that a measuring probe operating according to a magnetic induction or eddy current measuring method Is brought into tactile contact with a predetermined measurement point on the workpiece.

The invention also relates to a device for measuring the coating thickness on the circumference or on the end face of coated, cylindrical workpieces with a measuring probe with measuring head operating according to the magnetic induction or eddy current method and which comprises a measuring table with a holder positioning the measuring probe and a receptacle for guiding the workpiece and orienting a predetermined measurement point on the same with the measuring head.

Cylindrical workpieces such as screws. rivets, dowel pins, etc. are generally made from a ferromagnetic material. For certain applications said workpleces must be coated e.g. in order to protect them against corrosion. Frequently said coatings consist of nonmagnetic or organic materials. For the effectiveness and quality of the coating mainly its thickness in decisive. To the extent that standardized structural elements are involved, eg. screws, a coating thickness with a corresponding tolerance is prescribed.

For quality control purposes an optical examination of the coating is not adequate and Instead the actual coating thickness must be checked. For this purpose It is e.g. necessary to check from a batch coated In a single process a large number of workpieces to ensure that the coating thickness or thickness tolerance has been respected using standard quality control cards.

The known magnetic Induction or eddy current measuring methods are suitable for coating thickness measurement purposes. In both cases they are contacting measuring methods using a measuring probe to be placed on the part to be measured. In the first case the induction current produces a low frequency magnetic field and in the second case a high frequency magnetic field. The spacing between the measuring pole of the measuring probe and the substrate formed by the coating on the said substrate In the first case influences the strength of the magnetic field between the same and in the second case the strength of the eddy currents induced in the substrate. The divergence compared with the field in an uncoated subsúrate constitutes a measure for the coating thickness.

The measuring pole of the measuring probe has a spherical surface, generally in the form of a ball, which is placed on the measured product, so that an almost punctiform contact exists. However, the induced field has a spatial extension, so that In the case of a small surface on the measured product the extension of the measurement field in certain circumstances can be a multiple. The resulting errors can admittedly be eliminated by complicated and costly calibration, but this always presupposes a very exact placing of the measuring pole on the measured product. For the measurement of workpieces, e.g. &crews up to M1.6. only probes with an adequately small structural form can be used. The requirement still exists that the probe or measuring pole have to he very exactly positioned. This is problematical in the case of workpleces, because their handling is made much more difficult due to the limited size. In addition, the coating thickness must be measured on the one hand on predetermined partial areas of the measured surface and on the other on very small partial surfaces, e.g. on the termInal edge of a hexagonal socket-head screw. With a purely manual measurement errors necessarily occur making necessary repeat measurements, which involves a corresponding loss of time. The reproducibility of the measured values also suffers. because a manual measurement cannot ensure that measurement always takes place at the same contact point.

The problem of the invention is to propose a method and a device for measuring the coating thickness on cylindrical workpieces or small components, which allow within the framework of an operational quality control a rapid checking of large numbers under constant measurement conditions.

On the basis of the aforementioned method this problem Is solved In that the measuring probe is fixedly positioned in a holder and the workpiece is inserted in an adaptor, which at least exposes the measuring point and positions the latter with respect to the measuring probe, and that the measuring probe and workpiece In the adaptor are moved relative to one another in forcibly guided manner up to contact of measuring probe and measuring point.

Through a fixed positioning of one of the two parts, preferably the measuring probe, it is ensured that the probe head or measuring pole assumes a clearly defined, local position. Through the use of an adaptor, in which is inserted the workpiece to be measured and which can have a larger size than the workpiece, the latter can be more easily handled with the adaptor and supplied in a clearly defined manner. The workpiece Is so inserted In the adaptor, that at least the measuring point is exposed and can be positioned with respect to the measuring probe. An a result of a forcibly guided movement of the adaptor with inserted workplece, the exposed aeasurIng point of the workpiece can be brought into a clearly defined contact with the measuring probe. This ensures that the coating thickness is measured at the predetermined measuring point, e.g. on the circumference or on the end face or only in partial are& thereof. Through the loose insertion of the workpieces in the adaptor It is possible to measure large numbers in a short time. which is unavoidable for guiding a quality control card.

The Indication or display of a correct performance of the measurement, as well as the evaluation of the measured results, their statistical fluctuations andlor the tolerance& of the coating thickness can take place with operating programs, which are known for such measuring methods.

ln a preferred embodiment, the measuring probe Is resiliently positioned against a stop in the receptacle. Through the spring and stop It is ensured that the measuring probe or measuring pole always assumes a clearly defined starting Position. The spring also ensures a constant, adequately great pressing of the measuring pole against the measured product. The spring also prevents excessive impacting of the measured product on moving up the adaptor and consequently to an excessive compressive load at the measuring point, which on the one hand deforms the coating to be measured and therefore leads to errors of measurement, and on the other could damage the measuring probe.

For the measurement of the coating thicknesa on the circumferential surface of the workpiece, the latter is Inserted in the adaptor with its axis perpendicular to the probe axis and Is engaged by the adaptor on its circumference with a centre angle smaller than 36C, so that in the remaining sector the measuring point is exposed. In this case the adaptor is moved in forcibly guided manner axially parallel to the measuring probe until contact takes place between the measuring pole and the measuring point.

If, however, the coating thickness Is to be measured at an end face, the workpiece is inserted in the adaptor with its axis in the direction of the probe axis and the adaptor Is fixedly positioned with respect to the measuring probe before or after insertion, whilst the workpiece is axially displaced against the probe In the adaptor. The workpiece is circuaferentially guided during its displacement in the adaptor.

If the measuring point on the end face of the workpiece is outside its centre, i.e. is eccentrically positioned, the workpiece in the adaptor is guided with a corresponding axial displacement with respect to the measuring probe. This method is e.g. used In the case of hexagonal sockethead screws.

If the workpieces are provided on the end face with a through slot (clotted head screws, grub screws) or crossing slots (cross-slotted screws), then the measuring point is outside the slot or slots on the end face. In this case the workpiece must be so positioned In the adaptor that it contacts the measuring probe with the eccentric measuring point and not with the slot. In this case the slot is used for positioning the workpiece in the adaptor, said positioning ensuring that the measuring probe only has contact with the measured Product if the contact point Is located outside the Blot on the end face.

A device aultable for solving the problem of the Invention in known manner comprises a measuring table with a holder positioning the measuring probe with the measuring pole and a receptacle for guiding the workpiece and orienting a predetermined measuring point thereon with the measuring pole. For the case of measuring the coating thickness on the circumference of the workplece, such a device is characterized in that the receptacle is constructed as an adaptor, which has at least one depression, with an axis perpendicular to the measuring probe guiding the workplece on most of its circumference and exposing the measuring point on the remainder of the circumference. and that the adaptor is guidable on the measuring table parallel to the measuring probe axis, so as to bring about contact between the measuring point and the measuring pole.

The construction Is preferably such that the adaptor has several cylindrical depressions with parallel axes and different cross-sections for receiving workpieces with different cross-sections.

This embodiment makes it possible using the same adaptor to measure cylindrical workpieces with different diameter&. As a function of the adaptor size and the cross-acction or diameter of the workpieces, it Is possible on the same adaptor to Implement three or four depressions, so that by simply turning the adaptor by In each case 90 It is possible to position workpieces with different cross-sections and transfer the measurement point.

In order to perfectly guide the adaptor with respect to the measuring probe on its side remote from the depression the adaptor can have a central guide pin with which it is displaceable In a slot guide of the measuring table.

Preferably the guide pin is constructed as a multi-surface component and in each case two facing surfaces as guide surfaces are associated with one of the depressions in the adaptor, so that even when measuring workpieces having different diameter and which are inserted in correspondingly different depressions of the adaptor, it is always ensured that the measuring point exposed by the adaptor is in a clearly defined contact with the measuring probe.

For measuring the coating thickness on the end face the adaptor has a workpiece guide oriented with the measuring probe axis and exposing the measuring point. The adaptor is Interchangeably guided on vertical guides of the measuring table. Advantageously the procedure is such that firstly the adaptor with the cylindrical guide adapted to the workpiece is inserted in the vertical guides of the measuring table and positioned with respect to the measuring probe, in that the adaptor e.g. runs agarnat a stop or against the measuring table. The workpiece is then inserted in and displaced In the cylindrical guide on the adaptor until its end face contacts the measuring pole. If the end face is planar over Its entire extension, the guide for the workpiece only has to be positioned in such a way that a random point on the end face comes Into contact with the measuring pole.

If, however, it is necessary to inspect the coating thickness at a point on the end face eccentric with respect to the workplece axis, then the cylindrical guide axis Is displaced with respect to the probe axle by the eccentricity of the measuring point. This e.g. applies in the case of workpieces with a central depression, but as hexagonal socket-head screws and the like.

However, if the cylindrical workpiece is provided on Its end face with a through slot, such as is e.g. the case with a slotted-heed screw, then the measuring point is located outside the slot on one of the two segments of the end face. In this case, on its side exposing the measuring point the adaptor has a positioning aid, Which on displacing the workplece In the guide of the adaptor engages in the slot In such a way that the measuring point is oriented towards the measuring probe.

Preferably, the adaptor is rotatable in the vertical guide receiving it, in order to be able to orient the positioning aid prior to the start of a series of measurements.

In this case it is also advantageous if for receiving the cylindrical workpiece, there is a cylindrical mounting mandrel exposing Its end face and which can be inserted into the guide on the adaptor until the end face contacts the measuring head. The mounting mandrel increases on the one hand the application surface for the hand of the user, so that the workpiece can easily be manually displaced in the guide and on the other the workpiece can be rotated with the mounting mandrel until the slot of the slotted head screw engages over the positioning aid and subsequently the measuring pole rests at the appropriate measuring point.

.Ol The Invention is described in greater detail hereinafter relative to nonlimitative embodiments and the attached drawings, wherein show:

Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8 A perspective view of an embodiment of the invention, whilst diagrammatically reproducing the functional sequence.

A perspective view of the underside of the adaptor shown in fig. 1 for measurement on the circumference.

A perspective view of a hexagonal socket-head screw in the measuring position.

An axial partial section through the holder and adaptor of the device for measurements on hexagonal socket-head screws according to fig. 3.

A section displaced by 90 compared with fig. 4.

A perspective view of a slotted head screw with measuring probe in the measuring position.

A perspective view of the adaptor of the device for measurements on slotted head screws according to fig. 6.

An axial section through the adaptor of fig. 7.

The device shown in fig. 1 has a measuring table 1, on whose surface is fixedly mounted a holder 2. The holder 2 has a cylindrical bore 3, in which is inserted a measuring probe 4 operating according to the magnetic induction or eddy current measuring method and whose measuring pole 5 projects by a few tenths of a millimetre over the front end 6 of the blocklike holder. The measuring probe 4 is located in a guide sleeve 7, which Is Inserted from the rear in the guide bore 3 on the holder 2 and is secured by means of a setscrew 8. The measuring pole 5 is precisely mounted and axially displaceable against a spring tension In the guide sleeve 7, so that on moving the measured product up to the measuring pole a resilient giving way is possible.

Directly In front of the holder 2, two vertical guide pins 9 are fixed to the measuring table 1 symmetrically with respect to the guide bore 3. In its area in front of the holder 2 the measuring table 1 also has a linear guide 10, whose longitudinal axis is parallel to the axis of the guide bore 3 or measuring probe 4 and which is eccentric with respect to the guide bore.

The device also has several adaptors 11, 12 and 13 for receiving the cylindrical workpieces 14, e.g. screws, which are generally made from ferromagnetic material and are externally coated, e.g. with a metallic or nonmetallic coating.

The adaptor 11 shown in fig. 1 is used for measuring the coating thickness on the circumference or jacket of the cylindrical workplece and in the specific case on the circumference of the screw head 15. For this purpose the adaptor 11 has at least one depression 16 In the form of a cylindrical bore, in which is inserted the workpiece 14 or screw head 15. The guide 16 surrounds the screw head 15 over most of Its circumference. on the remaining part the adaptor 11 has a window 17 through which is exposed an area on the circumference of the screw head 15 forming the measuring point.

In the represented embodiment the adaptor 11 has two further cylindrical 9 - depressions 18 and 19, each of which has a narrow window. The depressions 16, 18 and 19 have different diameters for receiving different diameter workpieces and are in each case displaced by 90. The depressions on the adaptor can naturally also have a non-circular cross-section, e.g. for receiving the head of polygonal screws.

On its underside the adaptor 11 has a guide pin 20, which in the represented embodiment is constructed as a square plate and in each case has facing guide surfaces 21 and 22. The edge length of the guide pin is matched to the width of the guide 10 in the table 1 of the device, so that as a function of the orientation of the depressions 16, 18 or 19 one or other pair of guide surfaces 21 or 22 takes over the guidance task. For stabilizing the guidance movement of the adaptor 11 on the measuring table 1, in addition to the guide pin 209 it is possible to have a screw- in head 23, which runs on the bottom 24 of the guide 10.

Adaptor 12 Is used for measuring the coating thickness on the end face 25 of the workpiece 14, e.g. the head 15 of a round head or internal polygonal screw. For this purpose it has a cylindrical bore 26, in which the screw head 15 can be slid. The adaptor 12 also has a circumferential groove 27 by means of which it can be inserted in clamping, but rotary manner between the guide pins 9 on the measuring table 1.

Whereas the adaptor 12 Is intended for larger diameter workpieces, which can be held without difficulty between the fingers, the adaptor 13 in used for receiving smaller diameter workpiecee 14, which are difficult to handle. The adaptor 13 is again provided with a circumferential groove 28 for guiding on the pins 9 and has an extension 29. The entire adaptor 13 Is traversed by a guide bore 30, which has a larger diameter, cylindrical mounting mandrel 31 for smaller diameter workpieces 14.

The operating procedure on measuring the coating thickness on the circumference will now be described. As can be seen in fig. 1, the cylindrical workpiece 14, e.g. a screw, Is inserted from above with the screw head 15 in the matching cylindrical depression 16 of the adaptor 11. Before or after the adaptor is slid into the guide 10. The window 17, where the screw head circumference is exposed, to oriented by the guide surfaces 21 or 22 on the guide pin 20 towards the measuring head 15 of the measuring probe 4. The adaptor 11 is moved in front of the measuring probe and engages between the guide pins 9. Finally, the circumference of the screw head 15 reaches the measuring pole 5, which can resiliently retreat into the guide bore 3, so that there is no hard running up of the measured product. The spring tension ensures a clearly defined pressing action of the measuring pole 5 on the circumference of the screw head-15. The coating thickness is then measured in conventional manner by means of a a&gnetic induction or eddy current field. The measured values of a large number of screws are statistically evaluated. If the characteristic values. are outside the tolerance, it is possible to eliminate the batch from which the random sample was taken or also the single workplece.

Figs. 3 to 5 illustrate the procedure when measuring on the end face of workpieces in the fore of hexagonal socket-head screws. In this case the screw head 32 has a depression 33 with a polygonal cross-section. Thus, in this case the end face 13 is formed by a substantially circular surface, on which the coating thickness, as indicated with the measuring pole 3 in fig. 3, is to be measured. For this purpose the guide bore 26 in the adaptor 12 (fig. 4) is displaced with its axis 33 with respect to the axis 36 of the measuring probe or the measuring pole 5 by the eccentricity of the end face with respect to the axis of the workpiece 14. The hexagonal sockethead screw Inserted in the guide bore 26 of the adaptor 12 is thus so positioned that Its circular end face 34 strikes against the measuring head 5. In order to guarantee a clearly defined spacing of the adaptor 12 from the front end face of the holder 2, a calibrating foil 37 can be provided and is inserted between the two parts.

The adaptor 13 according to fig. 1 is not only used for receiving very small diameter or short length workpieces 14, but also e.g. for receiving slotted screws. Such a screw is shown in fig. 7 and has a head 38, which is provided on its end face 39 with a cross-slot 37. In this case the screw head 38 must be moved up to the measuring pole 5 in such a way that the coating thickness is measured on the segments of the end face 40 outside the slot. For this purpose a positioning aid 39 is provided and is located on the adaptor 13 directly upstream of the front opening of the guide bore 30 (figs. 7 and 8). In this case the positioning aid 40 is formed by a spring wire 41, which is positioned centrally in front of the opening of the guide bore 30 and is inserted in the adaptor 13. As shown in fig. 7, the slotted head screw is inserted from the front in the mounting mandrel 31, in which it acquires a moderate fit. The mounting mandrel 31 Is then inserted from the rear into the guide bore 30 of the adaptor 13. If necessary, the mounting mandrel 31 is rotated until theslot 37 engages in the spring wire 41. In this position the segments of the front end face 39 of the screw head 38 and consequently the measuring point are located in front of the spring wire and the latter can be contacted with the measuring pole 5.

Claims

1. Method for measuring the coating thickness on coated, substantially cylindrical workpieces, in that a measuring probe operating according to the magnetic induction or eddy current measuring method is brought into tactile contact with a predetermined measuring point on the workpiece, characterized in that the measuring probe is positioned in a holder and the workpiece is inserted in an adaptor at least exposing the measuring point and positioning the same with respect to the measuring probe, and that the measuring probe and the workpiece in the adaptor are moved relative to one another in forcibly guided manner up to contact between measuring probe and measuring point.

2. Method according to claim 1, characterized in that the measuring probe is resiliently positioned against a stop in the receptacle.

3. Method according to claim 1, characterized in that for measuring the coating thickness on the circumferential surface of the workplece, the latter is inserted in the adaptor with its axis perpendicular to the measuring probe axis and Is embraced by the adaptor on its circumference with a centre angle smaller than 360 and accompanied by the exposure of the measuring point and the adaptor is aoved In forcibly guided axially parallel manner to the measuring probe until contact occurs between the latter and the workpiece.

4. Method according to claim 1 or 2, characterized in that for measuring the coating thickness on an end face of the workpiece, the latter is inserted in the adaptor with its axis in the direction of the measuring probe axis, the adaptor is fixedly positioned before or after with respect to the measuring probe and the workpiece in the adaptor Is axially displaced against the measuring probe.

5. Method according to claim 4, characterized in that the workpiece is circumferentially guided during the displacement In the adaptor.

6. Method according to claim 4 or 5, characterized in that in the case of an eccentric measuring point on the end face of the workpiece, the latter is guided in the adaptor with a corresponding displacement with respect to the measuring probe axis.

7. Method according to claim 6, characterized in that in the case of workpleces with a through slot on the end face and a measuring point located outside the slot, the workplece is positioned in the adaptor by means of the slot and consequently the measuring point is oriented towards the measuring probe.

8. Device for measuring the coating thickness on the circumference of coated, cylindrical workpieces (14) with a measuring probe (4) operating according to the magnetic induction or eddy current method, comprising a measuring table (1) with a holder (2) positioning the measuring probe (4) with measuring pole (5), and a receptacle for guiding the workplece and orienting a predetermined measuring point on the circumference thereof towards the measuring pole, characterized in that the receptacle is constructed as an adaptor (11) having at least one depression (16. 18, 19), with an axis perpendicular to the measuring probe (4), which guides the workpiece (14) on most of its circumference and exposes the measuring point on the remaining part, and that the adaptor is displaceably guided on the measuring table (1) parallel to the measuring probe axis, in order to contact the measuring point with the measuring pole.

9. Device according to claim 8, characterized in that the adaptor (11) has several cylindrical depressions (16, 18, 19) with parallel axes and different cross-sections for receiving workpieces (14) with different cross-sections.

10. Device according to claim 8 or 9, characterized in that on Its side remote from the depressions (16, 18, 19) the adaptor (11) has central guide pins (20), with which it is displaceable In a slot guide (10) of the measuring table (1).

11. Device according to claim 9, characterized in that the guide pin (10) is constructed as a multi-surface component and in each case two facing surfaces (21, 22) are associated as guide surfaces with one of the depressions (16. 18, 19).

12. Device for measuring the coating thickness on the end face (25, 34, 39) of coated, cylindrical workpieces (14) with a measuring probe (4) with measuring pole (5) operating according to the magnetic induction or eddy current method and comprising a measuring table (1) with a holder (2) positioning the measuring probe (4) and a receptacle for guiding the workpiece and orienting a predetermined measuring point thereon towards the measuring pole, characterized in that the receptacle is constructed as an adaptor (12, 13), which has a cylindrical guide (26, 30) oriented towards the axis (36) of the measuring probe (14) and exposing the measuring point, for the workpiece (14) axially movable therein and is interchangeably guided on vertical guides (9) of the measuring table (1).

13. Device according to claim 12, characterized In that there are several adaptors (12. 13) with guides (26) having different cross-sections for workpieces (14) with different cross-sections.

14. Device according to claim 12 or 13 for cylindrical workpieces with a measuring point on the end face (34) eccentric to the axis thereof, characterized in that the axis (35) of the cylindrical guide (26) is displaced by the eccentricity of the measuring point with respect to the axis (36) of the laeasuring probe (4).

15. Device according to one of the claims 12 to 14 for cylindrical workpieces (14) with a through slot (37) on the end face (38) and a measuring point outside the slot, characterized In that the adaptor (13), on its side exposing the measuring point, has a positioning aid (40), which on displacing and rotating the workpiece in the guide (30) of the adaptor (13) engages in the slot (37) in such a way that the measuring point is oriented towards the measuring pole (5).

16. Device according to one of the claims 12 to 15, characterized in that for receiving the cylindrical workpiece (14) is provided a cylindrical mounting mandrel (31) exposing Its end face (38) and which can be inserted in the guide (30) on the adaptor (13) until the end face contacts the measuring pole (5).

17. Device according to claim 15 or 16, characterized in that the positioning aid (40) is a wire (41) crossing the cylindrical guide (30) directly upstream of its opening facing the measuring probe (4).

18. Device according to one of the claims 12 to 17. charact-erized in that the adaptor (13) is rotatable on the vertical guides (9) r-e'- ceiving it for orienting the measuring point towards the measuring probe (4).

19. Device according to one of the claims 8 to 18, characterized in that the measuring probe (4) is placed in a guide sleeve (7) fixable on the holder (2) and is positioned by means of a spring against a stop and when the measuring point on the workpiece runs up against the measuring pole (5) it gives way counter to the spring tension.