GB2271072A

GB2271072A - Thickness measurement

Info

Publication number: GB2271072A
Application number: GB9212052A
Authority: GB
Inventors: Andrew Ioannides; Roger Phillips
Original assignee: CMB Foodcan PLC; CarnaudMetalbox PLC; Metal Box PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1992-06-06
Filing date: 1992-06-06
Publication date: 1994-04-06
Also published as: GB9212052D0

Abstract

An apparatus and method for measuring the thickness of a drawn article such as a can body in which the distance between the surface of the can and each of typically four proximity sensors (140 - 143) disposed around the can circumference is measured. The measurement is carried out while the drawn article is still carried by a forming punch (128) and an indication of mean thickness of the can body together with its variation in thickness is provided. Can bodies with an excessive thickness variation can thus be rejected immediately after manufacture. <IMAGE>

Description

"THICKNESS MEASUREMENT" This invention relates to measuring the thickness of a drawn article such as a can body.

Articles such as can bodies are normally manufactured by a drawing process in which a blank is drawn to form a cup having an end wall and an integral sidewall. In one common process known as "draw-redraw" (or DRD), the drawn cup is redrawn to form a can body. In an alternative process, known as "draw and wall-iron" (or DWI), the drawn cup is redrawn to a reduced diameter and thereafter the drawn sidewall is passed through one or more ironing rings to further thin and elongate the sidewall. There is also an intermediate process known as "partial wall-ironing", in which the drawn cup is mounted on a punch and put through a redrawing die which makes with the punch a clearance smaller than the thickness of the cup so that the thickness of the sidewall is decreased.It should be understood for the purposes of this specification, the term "drawing' is meant to include all of these processes.

Drawn articles such as can bodies will inevitably have some variation in the thickness of theIr sidewall around the circumference thereof. Most frequently the internal and external surfaces will be circular, but the two circles may not be concentric, producing areas of minimum and maximum thickness. This may be caused by mis-alignment of the die with respect to the punch, to wear of the die, or other reasons. At present a random selection of drawn articles is removed from the production line, and tested with contact measurement equipment such as an micrometer in order to assess the degree of variation of the thickness of the article. The relatively slow speed of the testing means that it is not possible to perform an on-line test on 100% of the production output.

It is an object of the present invention to provide a wall thickness measuring technique for drawn articles, which can form the basis for an on-line monitoring system.

Accordingly there is provided apparatus for making a drawn metallic article, the apparatus including a punch carried by a reciprocable piston; one or more dies through which the punch is adapted to force the metallic material to form the drawn article; one or more proximity sensors located adjacent the drawn article whilst it is still carried by the punch, the one or more sensors each being adapted to give an electrical output proportional to the proximity of the metallic surface of the drawn article; and electronic processing means adapted to receive the electrical output of the one or more sensors and give an indication of the thickness of the drawn article adjacent the one or more sensors.

Since the position of the internal surface is known due to the fact that the drawn article is still carried by the punch, any detection of the location of the external surface will automatically give an indication of the thickness of the drawn article at that point. Where the proximity sensors employ the detection of induced currents for their operation, they will measure the location of a conductor such as the metallic surface of the drawn article. Metal/plastics laminates are increasingly employed in the manufacture of drawn articles such as can bodies, so it should be understood that the metallic surface is not necessarily the exposed external surface of the drawn article. Nevertheless, even in such laminates, detection of the location of the metallic surface will still give a useful indication of the wall thickness of the drawn article.

The invention relates more specifically to a press machine for making can bodies, the machine further including one or more sensors located adjacent to the can body as it emerges from the die section of the machine, the one or more sensors each being adapted to give an electrical output proportional to the proximity of the metallic surface of the can body; and electronic processing means adapted to receive the electrical output at the one or more sensors and give an indication of the thickness of the can body adjacent the one or more sensors. Whether employed in connection with can bodies or other drawn articles, there is conveniently provided a plurality of proximity sensors disposed around the circumference of the drawn article.According to one preferred arrangement there is provided four proximity sensors, conveniently disposed around the circumference of the drawn article equidistant one from another.

The invention further resides in a method of making a drawn metallic article comprising the steps of: i) using a punch to force material through one or more dies to form the drawn article; ii) measuring the distance between the metallic surface of the drawn article and one or more predetermined fixed locations whilst the drawn article is still carried by the punch; and iii) interpreting the distance between the metallic surface of the drawn article and the one or more predetermined fixed locations in terms of the thickness of the drawn article.

The invention further resides in a method of measuring the thickness of a drawn metallic article comprising the steps of: i) measuring the distance between the metallic surface of the drawn article and one or more predetermined fixed locations whilst the article is still carried by the punch on which it was drawn; and ii) interpreting the distance between the metallic surface of the drawn article and the one or more predetermined fixed locations in terms of the thickness of the drawn article.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which; Fig.l is a sectional side elevation of apparatus according to the present invention; Fig.2 is a schematic plan view of the apparatus of Fig.l; Fig.3 is a sectional plan view, highly exaggerated, of a can body produced by the apparatus of Figs. 1 and 2; Fig.4 is an example of the output of one of the sensors of Figs. 1 and 2; and Fig.5 is a schematic diagram of the electronic processing unit of Figs. 1 and 2.

Referring to Figs. 1 and 2, there is shown a machine 130 for making can bodies which is of well known construction and will be described only briefly. The machine 130 makes metal can bodies of the type in which each can body comprises a cylindrical sidewall closed by an integral dome-shaped end. The sidewall is provided with a flange in another machine. In a well known manner, after the can body has been filled, it is closed by an end which is connected to the can body by a double seaming operation.

In the machine 130, each can body is formed from a workpiece in the form of a shallow cup by a redrawing operation, a series of wall-ironing operations and a doming operation. The forming components of the machine 130 include a toolpack 131, a doming station 132 and a piston 133. The toolpack 131 contains a redrawing ring and three wall-ironing rings. The workpieces are pushed through the toolpack 131 by a punch 128 which is mounted on a ram 129. The ram 129 is mounted for sliding movement on a pair of bearings 134, 135. The workpieces are supplied to the machine 130 by a infeed conveyor 136 and they are removed by an outfeed conveyor 137. Each can body is removed from punch 128 by stripper fingers 138.

Each cup is initially held in position by the piston 133 which moves to the left, as shown in Fig.l, for this purpose. The piston 133 carries part of the redraw tooling. Adjacent the toolpack 131 are four proximity sensors 140 - 143, connected to an electronic processing unit 144 by means of leads 145.

Figure 3 shows, in exaggerated fashion, the cross-section of a can body produced by the machine 130.

The proximity sensors 140 - 143, spaced around the can body, give an electrical output proportional to the proximity of the external metallic surface, shown generally at 146. The proximity sensors 140 - 143 are inductive linear sensors, available from Kaman Instrumentation Corporation under their reference 2Sl.The sensors give a substantially linear output within their operating range, which is between 1 and 2 mm. The output of one sensor is shown in Figure 4, and clearly shows how the thickness of a can body is greater in the region 147 towards the top of the sidewall, as compared with region 148 which is the remainder of the sidewall. Signals from the proximity sensors 140 - 143 are passed to the electronic processing unit 144, which calculates the maximum and minimum thickness of the can body.

The operation of the electronic processing unit 144 will now be further described with reference to Figure 5.

The output from each of proximity sensors 140 - 143 is fed to differential amplifier 10 via sample and hold circuits 20 and 30, typically National Semiconductor type LF398.

Sample and hold circuit 20 transmits signals from the sensor 140 to one input of the differential amplifier 10 on a pulse A provided at a time in the cycle of the machine 130 when there is a metal can body on the punch 128. Similarly, the sample and hold circuit 30 transmits signals from the sensor 140 to the other input of the differential amplifier 10 on a pulse B provided when there is no can body present on the punch 128, the can body having been removed by the stripper fingers- 138. Thus the differential amplifier 10 is able to compare the signals from the sensor 140 when a can body is present on the punch and when the punch is bare. The output of differential amplifier 10 is an analogue voltage proportional to the can wall thickness adjacent sensor 140.Similarly a differential amplifier 11 and sample and hold circuits 21 and 31 are associated with the output from proximity sensor 141; amplifier 12 and sample and hold circuits 22 and 32 with sensor 142; and amplifier 13 and sample and hold circuits 23 and 33 with sensor 144.

A differential amplifier 40 measures the difference between the outputs of amplifiers 10 and 12, whilst differential amplifier 41 measures the difference between the outputs of amplifers 11 and 13. The output of amplifier 40 is squared using a precision analogue multiplier 45, such as a Burr Brown MPY634. Similarly, the output of amplifier 41 is squared by a multiplier 46.

The outputs of multipliers 45 and 46 are then summed using an operational amplifier 47, and then the square root of the output of amplifier 47 is obtained by a further multiplier 48 figured in square root mode. The output of the multiplier 48 can be used to give a reading of the range of wall thickness from the can body under investigation.

A further summing amplifier 49 set with a gain of 0.25, receives the outputs of the differential amplifiers 10, 20, 30 and 40. The amplifier 49 sums the outputs of the differential amplifiers and divides by 4, in order to give a mean value for the thickness of the can body.

As described herein the present invention can give an indication of the mean thickness of the can body together with its variation in thickness. Can bodies with an excessive thickness variation can be rejected as unacceptable, immediately after manufacture, without the need for extensive sampling and batch testing commonly employed at present.

Claims

CLAIMS:

1. Apparatus for making a drawn metallic article, the apparatus including a punch carried by a reciprocable piston; one or more dies through which the punch is adapted to force the metallic material to form the drawn article; one or more proximity sensors located adjacent the drawn article whilst it is still carried by the punch, the one or more sensors each being adapted to give an electrical output proportional to the proximity of the metallic surface of the drawn article; and electronic processing means adapted to receive the electrical output of the one or more sensors and give an indication of the thickness of the drawn article adjacent the one or more sensors.

2. Apparatus according to claim 1, wherein there is a plurality of proximity sensors disposed around the circumference of the drawn article.

3. Apparatus according to claim 2, wherein there is provided four proximity sensors disposed around the circumference of the drawn article.

4. Apparatus according to claim 2 or claim 3, wherein the proximity sensors are disposed around the circumference of the drawn article equidistant one from another.

5. A press machine for making can bodies, the machine further including one or more sensors located adjacent the can body as it emerges from the die section of the machine, the one or more sensors each being adapted to give an electrical output proportional to the proximity of the metallic surface of the can body; and electronic processing means adapted to receive the electrical output of the one or more sensors and give an indication of the thickness of the can body adjacent the one or more sensors.

6. Apparatus according to claim 5, wherein there is a plurality of proximity sensors disposed around the circumference of the can body.

7. Apparatus according to claim 6, wherein there is provided four proximity sensors disposed around the circumference of the can body.

8. Apparatus according to claim 6 or claim 7, wherein the proximity sensors are disposed around the circumference of the can body equidistant one from another.

9. A method of making a drawn metallic article comprising the steps of: i) using a punch to force material through one or more dies to form the drawn article; ii) measuring the distance between the metallic surface of the drawn article and one or more predetermined fixed locations whilst the drawn article is still carried by the punch; and iii) interpreting the distance between the metallic surface of the drawn article and the one or more predetermined fixed locations in terms of the thickness of the drawn article.

10. A method of measuring the thickness of a drawn metallic article comprising the steps of: i) measuring the distance between the metallic surface of the drawn article and one or more predetermined fixed locations whilst the article is still carried by the punch on which it was drawn; ii) interpreting the distance between the metallic surface of the drawn article and the one or more predetermined fixed locations in terms of the thickness of the drawn article.

11. A method according to claim 9 or claim 10, wherein the drawn metallic article is a can body.

12. Apparatus according to any of claims 1 to 8, and substantially as hereinbefore described.

13. Apparatus substantially as hereinbefore described with reference to Figures 1 to 5 of the accompanying drawings.