GB2508246A

GB2508246A - Cold cutting/grinding disc

Info

Publication number: GB2508246A
Application number: GB201221352A
Authority: GB
Inventors: Gordon Iain Priestly
Original assignee: Safehouse Habitats Scotland Ltd
Current assignee: Safehouse Habitats Scotland Ltd
Priority date: 2012-11-27
Filing date: 2012-11-27
Publication date: 2014-05-28
Also published as: GB201221352D0; WO2014083304A1

Abstract

A cold cutting or grinding disc comprises two opposed faces which are adapted to be rotated around a central axis A that extends between the faces. The disc is provided with a multiplicity of apertures (105, fig 2) which extend between the faces of the disc and which provides an increased surface area of the disc. Each aperture (105, fig 2) may have a longitudinal axis (L, fig 4) extending through the disc which is at an angle to the central axis A. The internal surface area of the apertures (105, fig 2) may make up between 20% and 60% of the surface area of the disc, with the apertures (105, fig 2) being distributed evenly in a rings 108 around the central axis A. The disc is particularly useful in grinding or cutting operations in potentially explosive environments. The increased surface area facilitates heat dissipation from the surface of the disc to the surroundings. The apertures (105, fig 2) also reduce the surface area of the faces of the disc and so reduces the amount of heat build up from the friction between the faces of the disc and the material or object being cut or ground.

Description

GOLD CUTTING/GRINDING DISC

Field of the Invention

The invention relates to discs for cutting or grinding and to apparatus for use in grinding or cutting operations in potentially explosive environments, such as offshore and onshore oil and gas exploration and production facilities, in particular DSEAR and ATEX Zone 1 and 2 areas.

io Background to the Invention

Cutting and grinding of metalwork using power tool apparatus is known to generate heat; due to friction between the cutting/grinding implement and the material being machined. Sparks which are thrown up, or frictional heating of the surface of the is cutting/grinding implement, can result in ignition sources and cause a safety haiard in the presence of flammable concentrations of gasses, vapour, mist or dust.

The risk of fire or explosion can be reduced if ignition sources originating from the power tool itself are removed or isolated (e.g. by use of pneumatically powered tools, or tools with flameproof electrical components). However, the surface of the cutting/grinding implement may still be an ignition source and necessitate additional and costly safety measures.

It is known to use cutting and grinding tools which do not generate sufficient heat for their use to require a "Hot Work Permit". This type of work is classified as "cold work" and tools which can be used in potentially explosive atmospheres (or "EX environments") without a hotwork permit are known as "cold" tools. However, "cold" cutting or grinding apparatus of this type typically require a source of cooling e.g. water or air in order to remain below a required temperature and so can be heavy, and costly, : 30 and their use may not be possible in some circumstances.

: Problems associated with known cold cutting and grinding tools include difficulties with cutting tool balance; insufficient heat dissipation; brittleness causing tool failure; costs and short life cycles.

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Accordingly it is an object of the present invention to avoid or minimize one or more of the foregoing disadvantages of known cold cutting or grinding tools.

Summary of the Invention

According to a first aspect of the invention, there is provided a cold cutting or grinding disc having two opposed faces and adapted to be rotated around a central axis extending between the faces, wherein an increased surface area of the disc is provided by a multiplicity of apertures extending between the faces of the disc.

The apertures of the cutting or grinding disc increase the surface area of the disc, in comparison to a disc lacking apertures, and thus facilitate dissipation of heat from the surface of the disc to the surroundings (typically air). The apertures also reduce the surface area of the faces of the disc and so reduce the amount of heat build up from friction between the faces of the disc and the material or object being cut or ground.

The apertures may be cylindrical and may be circular, though oval or other shapes may be utilised.

Each aperture may have a longitudinal axis extending through the disc which is at an angle to the central axis. Each aperture, and the disc as a whole, is thereby provided with a greater surface area.

Each aperture is preferably circularly symmetric about its longitudinal axis. Accordingly, each aperture may define an elliptical opening in each face of the disc and each aperture may define a skewed cylinder.

: The angle between the central axis and the longitudinal axis of each aperture may be greater than 10°, or greater than 20°. The angle between the central.axis and the * 30 longitudinal axis of each aperture may be greater than 0° and less than 60°, or *. *.

* between 10° and 50°. The angle may be around 30°. Preterably the angle between the :: : central axis and the longitudin& axis of all of the apertures is the same. *

The longitudinal axis of each aperture may be at any orientation. The longitudinal axis of each aperture may be perpendicular to a radius extending from the central axis to the aperture, such that the apertures are oriented at an angle to the central axis around the disc. The longitudinal axis of each aperture and, a radius extending from the central axis to the disc may lie in the same plane, such that the apertures are oriented at an angle away from the central axis.

Preferably, the orientation of each of the apertures in relation to the central axis is the same. All other factors being equal, apertures which are aligned in this way provide the greatest distance and amount of material between adjacent apertures.

A cutting or grinding region extending around the periphery of the disc, between the faces is provided, this being of known shape, type and construction. The cutting or grinding region is typically parallel to the central axis. The cutting or grinding region may be adapted to provide a cutting or grinding action when the disc is rotated only in one direction. For example, the cutting or grinding region may be provided with sharpened edges facing towards one direction of rotation. Accordingly, the cutting or grinding region may define a cutting/grinding direction of rotation.

The disc may be sided, such that one face is adapted to be mounted to the rotatable hub of a power tool and the other face faces away from the hub of a power tool. The disc may comprise one or more mounting holes for securing the disc to a power tool which extend from one face of the disc part way through the disc. In use, on a power tool the face of the disc which is not secured to a power tool may be more exposed than the face which is secured to a power tool by a cover or cowling or the like.

The total surface area of the disc includes the surface areas of the faces, the ". : cutting/grinding region around the periphery of the disc and the internal surface area of * the apertures. The multiplicity of apertures therefore increases the surface area of the disc (i.e. in relation to a disc lacking apertures) by at least 30% or at least 50% or 60%, by between 30% and 100%, or between 50% and 50%, and may increase the surface ****..

* 30 area of the disc by around 60% or 65%. The internal surface area of the apertures may 0**** * * comprise more than 25%, or more than 35% or more than 40% of the surface area of the disc. The internal surface area of the apertures may make up between 20% and 60%, or between 30% and 50%, for example around 40% or 45% of the total surface area of the disc.

The disc is substantially rigid, i.e. not adapted or intended to significantly flex in use.

The cutting or grinding region of the disc may not flex or be capable of flexing along the central axis by an amount greater than the thickness of the disc. The disc may flex by only a fraction of the thickness of the disc, e.g. by less than 10% or less than 5% of the thickness of the disc).

The disc may have a thickness between at least a part of each face, of more than 2% of the disc diameter, or between 4% and 10%, or around 4.5%. The internal surface area of an aperture extending through a comparatively thick, rigid disc of this configuration is proportionally greater than the internal surface area of an aperture extending through a disc having a thickness of less than, for example, 2% of the diameter of the disc. The disc may have a thickness of between 2mm and 20mm, or between 2mm and 10mm. The disc may have a thickness of around 5mm. The diameter of the disc may be between 40mm and 250mm, or between 80mm and 150mm, or around 100mm to 120mm. The disc may have a diameter of between 200mm and 250mm, or between 210mm and 230mm, or around 220mm.

The disc may comprise at least 100 apertures, or more preferably at least 200 apertures. The disc may comprise between 200 and 300 apertures. The size of the apertures is dependent on the material and dimensions of the disc and the intended use. The apertures may, for example, have a diameter of between 1mm and 5mm, for example around 3mm. Each of the multiplicity of apertures may have the same diameter, or the disc may comprise apertures of different diameters.

The disc preferably comprises a regular array of apertures. The apertures are preferably distributed in a regular array which is symmetrical about the central axis.

The apertures may be distributed in a ring around the central axis at a first radius and in a ring around the central axis at least one second radius from the central axis. For * .**..

* 30 example the disc may be provided with an array of apertures evenly distributed in rings * * around the central axis at each of several radii from the central axis. For example, the a... disc may comprise up to 10 rings of apertures (e.g. 8 rings), each ring having a different radius.

S *5 -

The disc may comprise a ring of apertures at a first, radius from the central axis and a ring of a greater number of apertures at a larger second radius from the central axis.

Thus, the disc has a larger number of apertures in a region closer to the cutting or grinding region, where the disc is prone to the greatest amount of frictional heating in S use. The disc may comprise a first region extending around the central axis and a second region extending around the central axis and the first region; the second region having a larger number of apertures per unit surface area of the faces than the first region.

Preferably, a radius extending from the central axis and through an aperture at a first radius passes between (and not through) adjacent apertures at an adjacent second radius (which may be larger or smaller than the first radius). An aperture in a first ring having a first radius may be equidistant from adjacent apertures in an adjacent second ring having a second radius. Accordingly, a radius extending from the central axis and through an aperture at a first radius from the central axis may bisect a region between adjacent apertures at a second radius from the second axis. All other factors being equal, apertures which are staggered in this way are provided with the greatest distance and amount of material between apertures at adjacent first and second radii from the central axis. * 20

The disc may comprise apertures distributed on the faces of the disc along a plurality of spirals radiating from the central axis.

In a second aspect of the invention, there is provided a cold cutting or grinding disc having opposed first and second faces and a cutting or grinding region around the periphery of the disc between the faces, the disc adapted to be rotated around a central axis extending between the faces, wherein; the disc is thickest at its cutting or * grinding region. For example, one or both faces may be concave or tapered with * respect to each other such that the cutting disc does not become jammed when cutting tue..

* 30 into material.

C..... * *

The cutting or grinding portion may be generally cylindrical and maybe parallel to the central axis. The cutting or grinding region may extend generally perpendicular to the * .. first face, or may be angled/tapered with respect to the central axis of rotation.

The disc may comprise a multiplicity of apertures extending between the faces according to a first aspect of the invention.

The disc (of the first or second aspect) may be composed of or at least partially coated with a ceramic material, such as a nitride (e.g. titanium nitride or boron nitride) or a carbide (e.g. tungsten carbide or tungsten oxycarbide). The disc may for example be a metal such as steel, coated at least around the periphery with a ceramic such as tungsten carbide, or with a diamond-impregnated ceramic material. At least a periphery of the disc may be diamond coated, i.e. diamond crystallites may be bonded or embedded in a surface around at least a periphery of the disc.

Further preferred and optional features of the second aspect correspond to preferred and optional features of the first aspect and preferred and optional features of the first aspect correspond to preferred and optional features of the first aspect.

According to a third aspect of the invention, there is provided cold cutting or grinding apparatus, comprising a power tool formed and arranged for driving a cutting/grinding disc of the first or second aspect. The power tool may be a pneumatically powered by apparatus of generally known type and construction.

When secured to the hub of a power tool, one face of the disc may be more exposed than the other. For example the apparatus may comprise a cover or cowling to protect an operator, in use, which may extend over more of one face of the disc (e.g. the face closest to an operator's hands in use). The face of the disc pointing away from the hub may be more exposed than the face of the disc pointing towards the hub.

The disc may comprise a multiplicity of apertures (according to a first aspect of the invention) each having a longitudinal axis extending through the disc at an angle to the central axis. The longitudinal axes preferably extend out of the more exposed face of * 30 the disc towards the direction that the disc rotates in use. * *

"r. The apparatus for powering rotation of the power tool is preferably adapted to provide high torque and comparatively low angular velocity (e.g. in the range of 100-1000rpm), so as to minimise heating or sparking caused by friction during cutting or grinding work.

The hub may be hydraulically or more preferably pneumatically powered. The tool may be adapted to power rotation of the disc at around or below 800rpm. or at between 300-600rpm. A disc having a larger diameter may be required to be rotated at a lower angular velocity.

The apparatus for powering rotation of the hub may be EX rated apparatus.

Optionally, the cutting or grinding apparatus may include additional means for cooling the cutting or grinding disc (e.g. a source of water to cool the disc).

DescriDtion of the Drawings Further preferred features and advantages of the invention will appear from the following detailed description, given by way of example embodiments illustrated with reference to the accompanying drawings in which:-Figure 1 shows a schematic diagram of apparatus for cutting or grinding.

Figure 2 shows a perspective view of a cutting disc according to the present invention; Figure 3 shows a plan view of the front face of a cutting disc, shown in Figure 2; Figure 4 is partial side section of the disc in Figure 3; Figure 5 is a schenatic side view of the disc showing its cutting edge; Figure 6 is a schematic cross sectional view of an alternative embodiment of a disc of the present invention; and * * Figure 7 shows a schematic diagram of apparatus for cutting or grinding, including an * 30 optional control unit.

*t,.;.** * * Figure 1 shows a pneumatically powered cold cutting/grinding apparatus 1 suitable for use in EX environments. The apparatus 1 comprises an air hose 3 extending from a compressed air supply (not shown) to a hand held tool 5. The apparatus can be connected to an integrated compressed air supply of a rig or installation, or to a

portable compressor.

The hand held tool has handles 7, 9 by which the tool may be carried and manipulated by an operator, in use. The hand held tool supports and pneumatically powers rotation of a cutting disc 100, which is described in further detail below. The disc is releasably mounted to a hub (which is located behind the disc 100 and so not visible from the perspective of Figure 1) by a lock ring 11 which is threaded on to a spindle 13 extending from the centre of the hub.

The hand held tool has a cowling 15, which extends around a part of the periphery of, and across a part of the rear face (as viewed from the perspective of Figure 1) of the disc 100, to provide some protection to the hands and face of an operator. Pneumatic power to the hub is regulated by way of a lever 17.

As will be described in further detail below, the periphery of the disc 102 is a cutting region and is provided with cutting teeth. The cutting region is adapted to cut or grind material when rotated in a clockwise direction by the hub (as viewed from the most exposed front face). The disc is provided with multiple cooling apertures 105 extending through the disc.

The cutting disc 100 is shown in more detail in Figure 2. The disc has a front face 101 and a parallel rear face 103 (shown). The front face 101 is facing downward and is not visible in the view of Figure 2. Around its periphery, the disc has a cutting region 102.

S

* * * 25 * ** The disc has a central mounting hole 104 (which is coaxial with a central axis A extending through the disc perpendicular to the faces) sized to receive the power tool spindle 13. Lock holes 106 extend part way through the disc and are positioned to fit over pegs located on the hub (not shown) so as to prevent the disc rotating in relation to the hub. S... * . S * .

The disc has eight rings 108a-lOBh of cooling apertures 105. Each ring has a different radius (labelled a-g, respectively), and the apertures in each ring are evenly spaced around the central axis of the disc: As can be seen from Figure 3, the apertures in any given ring are also staggered in relation to the apertures in the ring with the next largest and/ar next smallest radius, such that the apertures are arranged in 66 spirals (indicated by dotted lines B). This ensures that apertures in adjacent rings do not lie along the same radius from the central axis. Far example, an aperture 110 in ring 1 08b is equidistant from adjacent apertures 112, 114 in the next largest ring 108a and is also equidistant from adjacent apertures 116, 118 in the next smallest ring 1 08c. A radius extending from the central axis A through the centre of aperture 110 thus bisects the regions 120 and 122 between the apertures in adjacent rings a and c, respectively. The distance between adjacent holes (and so the mechanical strength of the disc) is maximized by this staggered arrangement.

Rings 108a to 108d each have 44 apertures. Smaller radius rings lOBe to 108h each have 22 apertures. The disc has a 110mm diameter and a thickness of 5mm. Each of the apertures has an internal diameter of 3mm. Therefore, the surfaóe area of the disc is increased by more than 50%, in comparison to a disc lacking the cooling apertures and the internal surface area of the apertures make up around 45% of the total surface area of the disc.

The region of the disc including rings baa to 108d, from around 68mm to 100mm diameter, has an aperture per every approximately 24mm2. The region of the disc from around 38mm to 68mm diameter has an aperture per every approximately 28mm2.

Accordingly, there are a greater number of apertures and a generally greater number of apertures per unit area, in the region of the disc closer to the cutting edge 102.

The apertures reduce the surface area of the faces 101, 103 and so also reduce the amount of friction between the faces and material being machined, in use. Each cooling aperture is cylindrical and has a diameter of 3mm and thus the apertures reduce the surface area of the face of the disc by more than 20% in comparison to a disc lacking cooling apertures. * * . ** *

In Figure 4, the outline of single aperture 124 is shown schematically (and enlarged).

The aperture is cylindrically skewed and has a longitudinal axis L at an angle of 30° to -the central axis A. The apertures are skewed cylinders and the openings extending through the faces of the disc are elliptical in shape -see enlarged view The surface area around the walls of a skewed cylindrical aperture is greater than the surface area ci the walls of a cylinder having the same diameter. For example, a 3mm diameter aperture extending through a 5mm thickness disc with a longitudinal axis parallel to the central axis would have an internal surface area of around 47mm2, whereas the 3mm diameter apertures 105 extending through the 5mm thickness of disc each have an internal surface area of around 58mm2. Accordingly, the surface area of the disc 100 as a whole is approximately 9% greater than a disc with cooling apertures parallel to the central axis and more than 60% greater than a disc lacking any cooling apertures.

The longitudinal axis of each of the cooling apertures 105 is perpendicular to a radius extending from the central axis to the respective longitudinal axis. The cooling apertures are aligned around the central axis in this way so that there is a maximum amount of material between adjacent apertures.

The longitudinal axes of the apertures are also orientated such that the opening of each aperture in the front face 101 is further forward in the direction of rotation R than the opening of the aperture in the rear face 103. Orientation of the apertures towards the direction of rotation (as viewed from the front face of the disc, which faces away from the hub) is intended to more effectively direct a flow of air through the apertures when the disc is rotating in use.

Figure 5 shows a side view of the disc. A section of the cutting region 102 is shown in **.: 25 detail. The cutting region is provided with a series of parallel ridges 128, each provided with cutting teeth 130, cut out of the leading edge (in respect of the direction of rotation A). The cutting/grinding region may be provided with alternative profiles of cutting teeth (not shown). * *

The disc is composed of tungsten carbide. Suitable materials are available from Dymet **** Alloys (a division of Corewire Limited). ** * * . *

U

In use, there is friction between the disc and the material being cut or ground. The apertures increase the overall surface area of the çlisc, in comparison to a disc lacking cooling apertures, and so increase the rate at which heat radiates from the disc to the surroundings. This "self-cooling" of the disc, by virtue of its increased surface area, means that the apparatus 1 need not be provided with an additional flow of water or cold air in order to keep the temperature below around 200°C. Under typical operating conditions, the surface of the disc does not exceed approximately 16000.

Figure 6 shows a side section ot an alternative embodiment of a disc 200 according to the invention. The disc 200 has all ot the features of the disc 100 and like features are indicated by the same reference numeral, incremented by 100.

As for disc 100, thetront face 201 and rear face 203 tapered outwardly towards the cutting region 202 which is cylindrical and parallel to the central axis A such that the disc is thicker at the cutting region than at the region adjacent to the mounting hole 204.

Arranging the disc in this manner ensures that, in use, for example, during cutting, a cutting disc cuts a channel slightly wider than the width of the disc. This reduces the proportion of the time the disc contacts and slides against the sides of the channel which is cut and the tapering of each of the faces reduces the area of contact that can be made at any time, between the disc and the material being machined.

Consequently, frictional heating and the potential for jamming is reduced.

An alternative embodiment 700 of the apparatus for cutting or grinding is shown in Figure 7. Features of the apparatus 700 in conmon with apparatus 1 are provided with : 25 like reference numerals, incremented by 700.

A control unit 750 is connected to an air supply conduit 753 and a water supply inlet 754. The control unit houses an air compressor and a water pump. The control unit receives air from a safe location (remote from combustible gasses) to which the air supply conduit extends. The air is filtered and dried by filters in the control unit, in line with the air supply conduit (not shown). Compressed air is delivered to the tool 705 via air hose 703. The control unit also houses a water pump (not shown) operable to deliver a flow of cooling water to the tool via water supply line 704.

Disc 100 is mounted to the tool 705. As described above, use of disc does not require a water supply. However the apparatus is also compatible with known discs lacking cooling apertures.

In order to ensure sate cold working when using apparatus 700 with other types of cutting disc, the control unit is provided with a flow rate sensor between the water pump and the water supply line. The control unit is also provided with a pressure sensor positioned in the control unit between the water pump and water supply line. A processor (not shown) receives data from the pressure sensor and the flow rate sensor and is configured to interrupt electrical power to the air compressor and thereby regulate the supply of compressed air to the hand tool.

The pressure sensor is operable to detect fluid pressure in the water supply line and the processor is configured to cut off the supply of air to the hand tool if a fluid pressure in the water supply line below a minimum threshold pressure is detected.

The processor is configured to compare signals received from the flow rate and pressure sensors. A lower than expected flow rate in the water supply line at a given pressure (which may be above the minimum threshold) may indicate that air is present in the water supply line. Accordingly, the flow rate sensor is operable to detect flow rates in the water supply line, and the processor is configured to shut off the supply of air to the hand tool if a flow rate of fluid in the water supply line is indicative of the presence of air in the water supply line. The water supply itself is not cut off and so power to rotate the disc (and potentially cause an ignition source) is reduced, but the **.: 25 supply of cooling water remains active. * * * * * * * * ** * *. *

51148775-1 jbretherIofl

Claims

CLAIMS: I. A cold cutting or grinding disc having two opposed faces and adapted to be rotated around a central axis extending between the faces, wherein an increased surface area of the disc is provided by a multiplicity of apertures extending between the faces of the disc.
2. A disc according to claim 1, wherein each aperture has a longitudinal axis extending through the disc which is at an angle to the central axis.
3. A disc according to claim 2, wherein the angle between the central axis and the longitudinal axis of each aperture is greater than 00 and less than 60°C.
4. A disc according to claim 2 or claim 3, wherein the longitudinal axis of each aperture is perpendicular to a radius extending from the central axis to the aperture.
5. A disc according to any one of claim 2 to 4, wherein the orientation of each of the apertures in relation to the central axis is the same.
6. A disc according to any one preceding claim, wherein the internal surface area of the apertures make up between 20% and 80% of the surface area of the disc. * . .* 25
7. A disc according to any one preceding claim, wherein the aperturesare evenly *..*..

* distributed in a ring around the central axis at a first radius and in a ring around the central axis at at least one second radius from the central axis.
8. A disc according to claim 7, comprising a ring of apertures at a first radius from *:::: 30 the central axis and a ring of a greater number of apertures at a larger second -radius from the central axis. * * * * **
9. A disc according to claim 7 or claim 8, wherein the apertures are staggered such that an aperture in a first ring having a first radius is equidistant from adjacent apertures in an adjacent second ring having a second radius.
10. A disc according to any one preceding claim, comprising a first region extending around the central axis and a second region extending around the central axis and the first region, the second region having a larger number of apertures per unit surface area of the faces than the first region.
11. A disc according to any one preceding claim, wherein the faces of the disc are tapered outwardly towards a wider cutting/grinding edge.
12. Cold cutting or grinding apparatus comprising; a power tool formed and arranged for driving a disc according to any one preceding claim.
13. Cutting or grinding apparatus according to claim 13, wherein one face of the disc is more exposed than the other face; the disc comprising a multiplicity of apertures extending between the faces of the disc, each having a longitudinal axis at an angle to the central axis; and wherein the longitudinal axes extend out of the more exposed lace of the disc towards the direction that the disc rotates in use.
14. A cutting disc substantially as described hereinbefore with reference to figures 2 and 3 of the accompanying drawings. * . S * ** ** .**** * SI * I * *** * . * SII * * * *I S I. I * ** I **AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS: CLAIMS: 1. A cold cutting or grinding disc having two opposed faces and adapted to be rotated around a central axis extending between the faces, wherein an increased surface area of the disc is provided by a multiplicity of apertures extending between the faces of the disc and the faces of the disc are tapered outwardly towards a wider cutting/grinding region.2. A disc according to claim 1, wherein one or both faces are concave.3. A disc according to claim 1 or 2, wherein each aperture has a longitudinal axis extending through the disc which is at an angle to the central axis.4. A disc according to claim 3, wherein the angle between the central axis and the is longitudinal axis of each aperture is greater than 00 and less than 60°.cr4-5. A disc according to claim 3 or 4, wherein the longitudinal axis of each aperture 0 is perpendicular to a radius extending from the central axis to the aperture.LO6. A disc according to any one of claims 3 to 5, wherein the orientation of each of the apertures in relation to the central axis is the same.7. A disc according to any one preceding claim, wherein the internal surface area of the apertures make up between 20% and 60% of the surface area of the disc.8. A disc according to any one preceding claim, wherein the apertures are evenly distributed in a ring around the central axis at a first radius and in a ring around the central axis at at least one second radius from the central axis.9. A disc according to claim 8, comprising a ring of apertures at a first radius from the central axis and a ring of a greater number of apertures at a larger second radius from the central axis.10. A disc according to claim 8 or 9, wherein the apertures are staggered such that an aperture in a first ring having a first radius is equidistant from adjacent apertures in an adjacent second ring having a second radius.11. A disc according to any one preceding claim, comprising a first region extending around the central axis and a second region extending around the central axis and the first region, the second region having a larger number of apertures per unit surface area of the faces than the first region.12. Cold cutting or grinding apparatus comprising; a power tool formed and arranged for driving a cold cutting or grinding disc, and a cold culling or grinding disc according to any one preceding claim mounted to a rotatabe hub of the power tool. 15 13. Cutting or grinding apparatus according to claim 12, wherein the face of the disc pointing away from the hub is more exposed than the other face.14. Cutting or grinding apparatus according to claim 13, wherein the disc comprises ° a multiplicity of apertures extending between the faces of the disc, each having 14) 20 a longitudinal axis at an angle to the central axis; and wherein the longitudinal axes extend out of the more exposed face of the disc towards the direction that the disc rotates in use.
15. A cutting disc substantially as described hereinbefore with reference to figures 2 and 3 of the accompanying drawings.