This invention is concerned with improvements relating to cutting devices, particularly cutting devices which are operative at high (ultrasonic) frequency.

There is disclosed in DE-A-2504019 a cutting member which is secured at an axial plane to a source of ultrasonic vibrations, the cutting member comprising a cutting element which extends radially from the cutting member.

According to this invention there is provided a cutting device comprising:

• [a] a generally annular cutting member having on one side of a diametral plane a driving face to be attached to a source of ultrasonic vibrations, said vibrations acting in a direction radially of the member; and
• [b] a cutting edge extending circumferentially of the cutting member on the other side of said diametral plane.

Preferably the cutting member, whilst being generally annular, is provided in its outer surface with "flats", or surfaces curved on different centres.

By attachment of a circumferential portion of the cutting member to (e.g.) an ultrasonic horn at an anti- node, the centre of the annulus constitutes a theoretical anti-node, and vibration will occur alternately radially outwardly and inwardly from the centre of the annulus, so that the cutting member effects a "pulsation".

Thus, the cutting edge may subtend at an angle of between 45° and 270°, although in practise a cutting edge above 180° is unlikely to be of wide value, and preferably the range of the cutting edge is between 60° and 160°, with a cutting edge vibrating radially of the annular cutting member.

The cutting edge may be V-shape in cross-section, or two parallel cutting edges may be provided by machining a V-shape or U-shape groove into circumference of the cutting member at least over the circumferential distance thereof required to perform a cutting operation.

Such a cutting device may be used for cutting brittle and hard materials, such a ceramics or concrete, or soft materials, such as fabric, or foodstuffs.

A theoretical analysis of radial vibrations in thin annular discs can be found in Young, Winsper and Sansomes papers in Applied Acoustics vol 3 1990 P212-216. The wave equation for pure radial simple harmonic motion can be written as: Ur is the particle displacement in a radial direction at radius r and k is the wave number.$$\text{K =}\frac{\text{W}}{\text{Cr}}\text{and Cr =}\sqrt{\frac{\text{E}}{{\text{ρ(1 - V}}^{\text{2}}\text{)}}}$$

Where

• W is angular velocity
• Cr is wave velocity in a radial direction

Equation (1) is a well known Bessel equation and a discrete solution is obtained for radial motion (after Watson 1962 Theory of Bessel Functions; London, Cambridge University Press) for an annular ring with inner and outer radii R1 and R2 or at R1 and R2 with zero stress.

In the region 1>KR1> 0.3 a simple approximate solution can be written$$\text{K(R1 + R2) = 2n}$$ Where n is an integer, which means that radial motion occurs when the mean circumference of the annular ring (R1 + R2) is equal to an integral number of wavelengths. The analysis shows that radial motion is independent of disc thickness. Finite Element modelling (eg PAFEC'S FEA Computer Programmes, Nottingham, UK) now enables the disc dimensions to be calculated to show that the radial mode vibration of a disc 150 mm outer and 50 mm inner diameter, and of aluminium with 4% copper alloy giving vibrational frequency of 18.2 kHz.

Thus, preferably the ratio of the inner radius R1 to the outer radius R2 of the annular disc is determined by the equation K (R1 + R2) = 2N, to within an accuracy of plus/minum 5%.

The device may comprise a plurality of cutting members secured (e.g. by welding or bolting) in parallel array to a single drive member, to enable a plurality of parallel cuts to be produced simultaneously.

The invention will become more clear from the following description, to be read with reference to the accompanying drawings, of cutting devices which have been selected for the purposes of illustrating the invention by way of example.

In the accompanying drawings:

• FIGURES 1a and 1b are respectively side and front elevations of a cutting device which is the first embodiment of the invention;
• FIGURES 2a and 2b are respectively side and front elevations of the cutting device which is the second embodiment of the invention; and
• FIGURE 3 is a schematic view showing a multi-bladed cutting device.

The device which is the first embodiment of the invention comprises an annular cutting member 6 having an inner radius of 50mm and an outer radius of 150mm, the cutting member being of aluminium with 4% copper alloy. The member 6 is provided with a flat drive face 8 on one side of diametral plane, by which the cutting member is attached to an ultrasonic horn 10 providing a source of ultrasonic vibrations at a frequency of 18.2 kHz. The face 8 will constitute an anti-node (portion of maximum displacement) and the centre of the aperture 12 of the member 6 providing a theoretical node. With such a construction, the member 6 vibrates radially inwardly and outwardly, as shown by the arrows, in a pulsating manner. The member 6 is provided with a circumferential cutting edge 14 subtending an angle of about 180° at the centre of the member which in the operation of the drive vibrates at a substantially uniform amplitude, allowing when the device is moved in the direction of the arrow A through a substrate 18, a cutting action to be effected.

In the second embodiment, parallel cutting edges 20a, 20b are afforded by the provision of a generally U-shape groove or channel into the circumferential edge of the cutting member, to enable twin cutting tracks to be provided.

In the embodiment illustrated in Figure 3, a plurality, specifically three cutting member 6a, 6b, 6c, each similar to the cutting member illustrated in Figure 1, are connected in parallel to a multi-stage ultrasonic horn 28, whereby the cutting edges 14 are all caused to vibrate together, enabling a multiple cut to be obtained in the substrate.

It will of course be appreciated that by the utilisation of a cutting member of the kind illustrated in Figure 2, a multiple increase of the number of cuts may be obtained.

By the use of the invention a significantly deeper cut than may conventionally be obtained may be made, and the device may be used to cut a wide variety of materials, including those with which difficulty may otherwise be experienced.

Whilst in the preferred embodiment the cutting member is annular, if desired "flats" or alternatively curved faces may be provided on the other circumference.