GB2334468A - Tracking of Abrasive Sheets or Belts - Google Patents

Abstract

A belt (B) is provided with two series of zones (Z1 and Z2) of magnetisation near its edges and any mis-tracking of the belt on its rollers (R1, R2) is detected by magnetic sensors (S1, S2) whose outputs are compared and a control signal applied to an actuator (T) which tilts a rotation axis of one roller to correct for the mis-tracking.

Description

ABRASIVE BELT The invention relates to the abrasion of objects using coated abrasive sheet material more particularly belts. Coated abrasive belts are widely used in industrial applications for grinding, stock removal and finishing and surface abrasion of workpieces and materials, e.g. wood, particle board, metal, plastic veneers, glass, ceramics; seeds, aggregate and precious stones; and the like.

In use, the abrasive belt is normally run on a machine having two or more rollers which tension the abrasive belt and also drive it, in some cases up to very high surface speeds. The material to be abraded, e.g. ground or finished is then applied to the moving belt or vice versa, either by hand or through an automatic feed device.

The action of the workpiece on the belt, belt wear (stretching), belt heating from grinding, or liquid coolants can cause the belt to run over to one side of the machine and commonly the belt will progressively track off the machine which can cause catastrophic damage to the belt and the machine itself.

Various devices are conventionally used to control this effect, ranging from simple 'cut-out' devices which automatically switch off the machine if the belt tracks too far to one side or the other, to highly sophisticated systems which automatically monitor the position of the moving belt and make continuous small adjustments to track the belt in a tight or predetermined oscillating range of positions. In order to monitor the lateral position of the belt on the machine a variety of devices are employed; these include pneumatic sensors, optical sensors and mechanical devices. All of these devices have limitations caused by the environment in which they have to operate, e.g. in a cloud of wood or metal dust, in a coolant spray of oil, water or oil/water mixture.

It is an object of the invention to provide means for the purpose specified in which lateral deviation of the belt is reduced or eliminated and/or corrected.

In one aspect this invention provides an abrading or polishing machine having a moving abrasive sheet the sheet having magnetised particles therein, characterised by a sensor arranged to generate an output signal in dependence upon the magnetism of the particles.

Preferably the sheet has zones having non-uniform magnetisation along its direction of travel and the sensor is arranged to generate an output signal which varies according to the magnetisation of the zone passing the sensor.

In one embodiment the magnetisable particles are magnetised in bands having spaced apart zones of the same or alternating polarity.

The magnetisable particles may be selected from a wide range of materials. Preferably the magnetised particles comprise any one or more of barium ferrite, chromium dioxide, cobalt doped iron oxide, ferric oxide, magnetite, strontium ferrite; or the like.

Preferably the particles are present in a concentration of from about 50 grams per square metre to about 500 grams per square metre of the sheet.

The support may, for example, be woven cloth; paper, fibre, plastic film; or the like.

In one embodiment the sheet comprises an adhesive layer on a support, an abrasive layer being disposed on the adhesive layer and the magnetised particles being incorporated in the adhesive layer.

In the manufacture of the above abrasive sheet, one or more adhesive layers can be applied in known manner to a flexible support backing and the coated abrasive material incorporating the magnetisable particulate material can be applied.

For example, a first layer of adhesive (base or maker coat) can be applied to the backing, abrasive grains deposited on to the adhesive layer followed by drying/curing to anchor the abrasive grains. A second layer of adhesive (sizing coat) can be applied over the abrasive grains followed by a further drying/curing operation. A third layer of adhesive containing grinding aids, lubricants or other additives can optionally be applied over the sizing coat followed again by drying/curing.

The magnetisable material can be magnetised by passage against single or multiple electromagnet or permanent magnets to provide a track containing alternating North and South poles or spaced apart poles of one type only. Preferably in use the sheet (normally in the form of a belt) generates a pulse signal as it moves past the sensing device.

In another aspect, the invention provides an abrasive sheet having its ends joined together to form a belt, the belt having zones of magnetisation distributed along its direction of travel, e.g. at its margins.

The belt may be used in any sanding or abrading machine such as a wide belt sander; or an automatic narrow belt machine; or the like.

As explained, the magnetised portion of the coated abrasive sheet or belt, for example, can be continuous and of the same polarity or in discrete bands or strips or other configurations of the same or alternating polarity. Preferably a sheet of this invention has, by virtue of its structure, a substantially uniform thickness. This helps the belt run true. The magnetisable particles may, for example, extend across the belt or they may be confined to specific areas, e.g. the longitudinal margins.

In one embodiment the apparatus includes a sensor adapted to derive information, e.g. digital or analogue signals, from the magnetised particles to keep the belt running true. The magnetic portion on the coated abrasive belt can be detected for example by a Hall Effect device, magneto resistor, flux-gate sensor, pick-up coil, or any other magnetic field or proximity sensitive device. The output from these devices can be processed electronically, to trigger the tracking mechanism and track the coated abrasive belt in normal use. However, the sensor may be arranged to act in a smart sense, e.g. to react to the information sensed to adjust speed of the belt or the position according to rate of wear sensed; or the like.

The invention also provides a method of abrading or polishing an object or objects using an abrading or polishing machine as defined and maintaining the belt in a predetermined path comprising running a machine as defined and moving the belt into the predetermined path according to the deviation from the predetermined path sensed by the sensor.

In order that the invention may be well understood, a preferred embodiment will now be described by way of example only with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a cross section of part of a belt for use in an embodiment of the invention; Figure 2 is a partial plan view of two magnetic pole designs for the belt; Figure 3 is a plan view of part of an abrasive belt grinding machine in accordance with the invention; Figure 4 is a side elevation of an arrangement for magnetising the belt of Figure 1 or Figure 2; and Figure 5 is a plot of output voltage : speed of a belt for use in an embodiment of the invention.

As shown in Figure 1, a belt B comprises a length of sheet material joined end-to-end.

The belt comprises a support 1 of backing material, e.g. paper cloth or plastics. A layer 2 of adhesive resin containing magnetisable particles M, e.g. barium ferrite has been applied in known manner, and on top of that a second adhesive layer 3 to anchor the abrasive particles A to the support. On top of the second adhesive layer is a third adhesive layer 4 as a supersize. The formed sheet thus comprises four layers, the two uppermost of which have abrasive grains embedded therein. The sheet is cut to length and two ends are joined together in known way to form a belt.

Referring to Figure 4, the belt B is passed under an electromagnet E, energised by a pulsed signal to magnetise the magnetisable particles M into bands made up of zones Z of magnetic polarity; as shown in Figure 2 these can be spaced apart zones of say North pole (Figure 2a) or alternating North and South poles (Figure 2b). The belt is then mounted on a machine such as a wide belt sander for wood, veneer and particle board or for metal finishing and polishing with water, oil or water/oil coolant; or an automatic narrow belt machine.

It has also been observed that the magnetised signal from such a belt can penetrate layers of thin metal (e.g. steel), wood and plastic to such an extent that a strong signal can be measured even with such materials between the moving abrasive belt and the sensor.

Figure 3 shows an abrasive belt grinding machine in accordance with the invention, comprising a belt B (as described above) mounted on rollers R1 and R2 and driven by a motor (not shown). The axis of rotation of roller R2 can be tilted in a horizontal plane by a tracking actuator T, as indicated at P. Actuator T is controlled by the output of a control circuit C, which compares the outputs of two magnetic sensors S 1 and S2 which are located beneath belt B with the centres of their sensitive regions located slightly outside the centres of two series of zones of magnetisation Z1 and Z2 respectively. When belt B deviates to the right on roller R2 then the output of sensor S1 exceeds that of S2 and the resulting difference signal is derived by circuit C to cause actuator T to tilt the axis of roller R2 in the direction Q 1. When the belt deviates to the left, the relative outputs of the sensors S1 and S2 are reversed and the actuator T is caused to tilt the axis of roller R2 in the opposite direction Q2. In this manner any deviations from true running of the belt are corrected.

By incorporating a magnetisable particulate material into the adhesive layers of the coated abrasive, the lateral position of the abrasive belt on the machine can be sensed without interference despite the presence of particulate dust and coolants.

EXAMPLE An abrasive belt 50 mm x 1525 mm was prepared using conventionally manufactured grit size P80 aluminium oxide resin bonded abrasive cloth where the base coat of adhesive contained 84g/m2 addition of barium ferrite. The belt was then magnetised using an electromagnet to produce magnetised areas approximately 10 mm diameter, spaced at approximately 100 mm intervals along the length of the belt.

The belt was then run at a range of different surface speeds and the pulse signal measured at each speed using a pick up coil. The signal voltage measurements are given in Figure 5. It should be noted that the invention is not limited to the embodiments shown. For example, in other embodiments the magnetisable material M could be incorporated in the backing of the belt B or even applied in a coating (e.g. on ink coating) to the rear surface of the backing.

Claims (18)

1. CLAIMS 1. An abrading or polishing machine having a moving abrasive sheet the sheet having magnetised particles, characterised by a sensor arranged to generate an output signal in dependence upon the magnetism of the particles.
2. 2. An abrading or polishing machine as claimed in Claim 1, wherein the sheet has zones having non-uniform magnetisation along its direction of travel and the sensor is arranged to generate an output signal which varies according to the magnetisation of the zone passing the sensor.
3. 3. An abrading or polishing machine according to Claim 2, wherein the zones are of alternating polarity.
4. 4. An abrading or polishing machine according to Claim 2 or Claim 3, wherein the sensor is an electromagnetic sensor.
5. 5. An abrading or polishing machine according to any preceding Claim, wherein the sensor is arranged to generate an alarm or cut-out signal in response to a predetermined movement or rate of movement of the sheet.
6. 6. An abrading or polishing machine according to any preceding Claim, wherein the sheet is in the form of an endless belt.
7. 7. An abrading or polishing machine according to Claim 6, wherein the sensor is arranged to generate an output signal which is responsive to the lateral position of the belt.
8. 8. An abrading or polishing machine according to any preceding Claim, wherein the sheet comprises an adhesive layer on a support, an abrasive layer being disposed on the adhesive layer and the magnetised particles being incorporated in the adhesive layer.
9. 9. An abrading or polishing machine according to any preceding Claim, having control means responsive to the output signal of the sensor to control the path or speed of the sheet.
10. 10. An abrading or polishing machine according to any preceding Claim, which is a wide belt sander or a narrow belt sander.
11. 11. An abrading or polishing machine according to any preceding Claim, wherein the concentration of the magnetised particles is from 50 g/m2 to 500 g/m2 of the sheet area.
12. 12. An abrasive sheet for use in an abrading or polishing machine as claimed in any preceding Claim, wherein the sheet has zones having non-uniform magnetisation along its direction of travel and the sensor is arranged to generate an output signal which varies according to the magnetisation of the zone passing the sensor.
13. 13. An abrasive sheet according to Claim 12, wherein the zones are of alternating polarity.
14. 14. An abrasive sheet according to Claim 12 or 13, wherein the sheet comprises an adhesive layer on a support, an abrasive layer being disposed on the adhesive layer and the magnetised particles being incorporated in the adhesive layer.
15. 15. An abrasive sheet according to any of Claims 12 to 14, wherein the concentration of the magnetised particles is from 50 g/m2 to 500 g/m2 of the sheet area.
16. 16. An abrading or polishing machine substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.
17. 17. An abrasive sheet substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.
18. 18. An abrasive sheet substantially as described hereinabove with reference to the Example.