GB1584299A

GB1584299A - Electromagnetic clamping devices

Info

Publication number: GB1584299A
Application number: GB5207377A
Authority: GB
Original assignee: Czech Technical University In Prague
Current assignee: Czech Technical University In Prague
Priority date: 1976-12-22
Filing date: 1977-12-14
Publication date: 1981-02-11
Also published as: CS190819B1; FR2374999B3; JPS53111584A; NL7713746A; IT1089788B; DE2757418A1; FR2374999A1

Description

(54) IMPROVEMENTS RELATING TO ELECTROMAGNETIC CLAMPING DEVICES (71) We, CESKE VYSOKE UCENI TECHNICKE V PRAZE, of Praha, Czechoslovakia, a corporation organised and existing under the laws of the Czechoslovak Socialist Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electromagnetic clamping device for the clamping of workpieces during their working, for instance grinding, milling, turning and the like.

Electromagnetic clamping devices previously proposed are provided with a damp ing plate with various arrangements of magnetic poles and with a body made of magnetic material, housing one or more exciting coils, fed from a DC source.

The drawbacks of these electromagnetic clamping devices are known. They include a high magnetic reluctance in the air gaps, the pole plate proper and the body of the electromagnetic clamping device, which reduces the holding forces; furthermore relatively lower mechanical stability of the pole plate: air gets sucked into the coil space in the course of cooling of the electromagnetic clamping device with consequential impairment or even loss of insulating properties of the coil due to moisture; and also relatively poor magnetic properties of the material of the body and a limited possibility of clamping of small and thin workpieces.

Other clamping devices previously proposed are electromagnetic clamping devices without a clamping plate. A coil with many windings or tums, usually protected by synthetic resin, is situated in a main body, generating in the whole clamping plate a single open magnetic circuit. This solution has relatively poor magnetic and mechanical stability and is not suitable for heavy working conditions, particularly if cooling liquids are used. Such a magnetic clamping device is suitable for single purpose and special application, particularly for the manufacture of working fixtures.

The aim of this invention is to provide an electromagnetic clamping device which would allow clamping of workpieces of different size and thickness and which would be at the same time efficient and of reasonable weight.

The invention provides an electromagnetic clamping device comprising a body made of one piece of ferromagnetic material having a clamping surface provided with a plurality of straight, parallel and equally spaced slots, at least one insulated current conductor being situated in the slots, the slots being closed by non-magnetic wedges which retain the conductor or conductors in the slots, and means for the connection of the or each conductor to a DC source.

In a preferred embodiment the device includes said DC source, the source being provided with an arrangement for the adjustment of its output current. The current source may be a controlled current rectifier. The rectifier may be connected to the output winding of a transformer, at least one winding of said transformer being provided with change-over taps.

The electromagnetic clamping device according to this invention provides substantially uniform distribution of the density of the attractive force along the whole clamping surface. The intensity of the attractive force is comparable to, or higher, than that of leading electromagnetic clamping devices presently available. Intensities of the attractive force of 6.3 kp/cm2 (0.62 MPa) have been measured on an operational embodiment at 100% occupation of the clamping surface at a coarseness of the surface corresponding to an air gap of lv = 20 sum.

The simplicity, rigidity and comDactness of the construction together with the low operating voltage of the electric current ensure substantially trouble-free operation and thus a high reliability.

One or more current conductors of large cross-section are situated at the bottom of the groove, which is from the top hermetically closed by a non-magnetic pressed-in metal wedge of sufficient height, providing both the possibility of an occasional additional grinding of the plate and also the mechanical strength of the plate and a substantially perfect metallic contact of the current conductors with the body of the clamping device with the possibility of an increase of the current density up to 10 A/mm2.

Due to the low drop of the DC voltage on the current conductors (of the order of tenths to units of volt), virtually no insulation of the coil is required.

The manufacture of the coil, of a meander from profiled insulated conductors--bandss- is simple and facilitates replacement of copper by aluminium.

Only about one-fifth of the copper (aluminium) of comparable conventional clamping devices is required.

By elimination of the body of cast steel from the magnetic circuit, a higher degree of energy utilisation is obtained (a rather more advantageous magnetization characteristic of soft steel against cast steel).

The body.of the electromagnetic clamping device according to this invention can be made of magnetic material of better magnetic properties than steel, i.e. of a material with high values of Br and low values of Hk, enabling reduction of the excitation current while maintaining the holding forces. Thus the consumption of electric power is substantially reduced.

The constructional height of the damping device is also reduced from between 10 and 12 cm to between 3 and 4 cm and thus also a reduction of weight of the clamping device up to 60% is achieved.

The distance between poles can be selected according to requirements and the manufacture of the grooves is simple.

The extent to which the construction of the multipole clamping surface (density of poles) can be varied and the variable current source enable a wide field of application, from grinding of rather thin and small workpieces up to rough working, requiring a large attractive force.

Another advantage is the substantial reduction of labour required for manufacture compared with conventional constructions with pole plates and conventional coils together with saving of material of the current conductors of the magnetic circuit and the electric insulation of the coils, enabling a substantial reduction of costs of manufacture and material; according to rough estimates the costs of manufacture and material are reduced by a factor of between 5 and 10.

The invention is illustrated in an exemplary embcdiment in the attached drawing, wherein Fig. 1 is a top view on a multipole clamping surface, Fig. 2 is a cross-sectional view of a detail of the body of the electromagnetic clamping device, and Fig. 3 is a longitudinal sectional view of a detail of the body of the clamping device.

The electromagnetic clamping device com prises a single solid body 1 (see Fig. 1) of ferromagnetic material, for instance of mag netic soft steel, the upper part of which is formed as a multipole clamping surface, ob tained by a uniform distribution of one or more current conductors 2 of a relatively larger cross-sectional area (about 5 mm2) fed by an adjustable DC current source 6 (of the order of several tens of amps). The current conductors 2 are situated in n grooves 3 with a suitable pole pitch t so that n + 1 magnetic poles are created on the multipole clamping surface. One or more current conductors 2 (Cu of Al insulated strips) are situated on the bottom of the grooves, which are from the top hermetically closed by suitable pressed-in non-magnetic wedges 4 (for in stance of brass or aluminium of a height of about 10 mm). Thus mechanical strength of the clamping plate and equally a substantially perfect heat contact of the current conductors 2 with the body 1 of the clamping device are ensured so that the current density in the current conductors 2, for instance Cu strips, may be increased up to 10 A/mm2 without any danger of heat degradation of the insulation of the current conductors 2. At the ends of the grooves 3 spaces 5 are provided in the body 1 for heads of the meanders of the current conductors 2, which spaces 5 are sealed by casting of some suitable material.

The current conductors 2 are substantially embedded in the grooves 3 in the surface of the body 1 in such a way that, in the simplest embodiment shown in Figure 1, they form a closed circuit. The current conductors 2 form a meander.

In addition outlets of current conductors 2 from the body 1 of the electromagnetic clamping device are provided for connection to a current source 6.

WHAT WE CLAIM IS: 1. An electromagnetic clamping device comprising a body made of one piece ferromagnetic material having a clamping surface provided with a plurality of straight, parallel and equally spaced slots, at least one insulated current conductor being situated in the slots, the slots being closed by non-magnetic wedges which retain the conductor or conductors in the slots, and means for the connection of the or each conductor to a DC source.

2. A device according to Claim 1 including said DC source, the source being provided

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. cross-section are situated at the bottom of the groove, which is from the top hermetically closed by a non-magnetic pressed-in metal wedge of sufficient height, providing both the possibility of an occasional additional grinding of the plate and also the mechanical strength of the plate and a substantially perfect metallic contact of the current conductors with the body of the clamping device with the possibility of an increase of the current density up to 10 A/mm2. Due to the low drop of the DC voltage on the current conductors (of the order of tenths to units of volt), virtually no insulation of the coil is required. The manufacture of the coil, of a meander from profiled insulated conductors--bandss- is simple and facilitates replacement of copper by aluminium. Only about one-fifth of the copper (aluminium) of comparable conventional clamping devices is required. By elimination of the body of cast steel from the magnetic circuit, a higher degree of energy utilisation is obtained (a rather more advantageous magnetization characteristic of soft steel against cast steel). The body.of the electromagnetic clamping device according to this invention can be made of magnetic material of better magnetic properties than steel, i.e. of a material with high values of Br and low values of Hk, enabling reduction of the excitation current while maintaining the holding forces. Thus the consumption of electric power is substantially reduced. The constructional height of the damping device is also reduced from between 10 and 12 cm to between 3 and 4 cm and thus also a reduction of weight of the clamping device up to 60% is achieved. The distance between poles can be selected according to requirements and the manufacture of the grooves is simple. The extent to which the construction of the multipole clamping surface (density of poles) can be varied and the variable current source enable a wide field of application, from grinding of rather thin and small workpieces up to rough working, requiring a large attractive force. Another advantage is the substantial reduction of labour required for manufacture compared with conventional constructions with pole plates and conventional coils together with saving of material of the current conductors of the magnetic circuit and the electric insulation of the coils, enabling a substantial reduction of costs of manufacture and material; according to rough estimates the costs of manufacture and material are reduced by a factor of between 5 and 10. The invention is illustrated in an exemplary embcdiment in the attached drawing, wherein Fig. 1 is a top view on a multipole clamping surface, Fig. 2 is a cross-sectional view of a detail of the body of the electromagnetic clamping device, and Fig. 3 is a longitudinal sectional view of a detail of the body of the clamping device. The electromagnetic clamping device com prises a single solid body 1 (see Fig. 1) of ferromagnetic material, for instance of mag netic soft steel, the upper part of which is formed as a multipole clamping surface, ob tained by a uniform distribution of one or more current conductors 2 of a relatively larger cross-sectional area (about 5 mm2) fed by an adjustable DC current source 6 (of the order of several tens of amps). The current conductors 2 are situated in n grooves 3 with a suitable pole pitch t so that n + 1 magnetic poles are created on the multipole clamping surface. One or more current conductors 2 (Cu of Al insulated strips) are situated on the bottom of the grooves, which are from the top hermetically closed by suitable pressed-in non-magnetic wedges 4 (for in stance of brass or aluminium of a height of about 10 mm). Thus mechanical strength of the clamping plate and equally a substantially perfect heat contact of the current conductors 2 with the body 1 of the clamping device are ensured so that the current density in the current conductors 2, for instance Cu strips, may be increased up to 10 A/mm2 without any danger of heat degradation of the insulation of the current conductors 2. At the ends of the grooves 3 spaces 5 are provided in the body 1 for heads of the meanders of the current conductors 2, which spaces 5 are sealed by casting of some suitable material. The current conductors 2 are substantially embedded in the grooves 3 in the surface of the body 1 in such a way that, in the simplest embodiment shown in Figure 1, they form a closed circuit. The current conductors 2 form a meander. In addition outlets of current conductors 2 from the body 1 of the electromagnetic clamping device are provided for connection to a current source 6. WHAT WE CLAIM IS:

1. An electromagnetic clamping device comprising a body made of one piece ferromagnetic material having a clamping surface provided with a plurality of straight, parallel and equally spaced slots, at least one insulated current conductor being situated in the slots, the slots being closed by non-magnetic wedges which retain the conductor or conductors in the slots, and means for the connection of the or each conductor to a DC source.

with an arrangement for the adjustment of its output current.

3. A device according to Claim 2 wherein the current source is a controlled current rectifier.

4. A device according to Claim 2 or 3 wherein the current source is a rectifier connected to the output winding of a transformer, at least one winding of said transformer being provided with change-over taps.

5. An electromagnetic clamping device substantially as herein described with reference to, and as illustrated in, the accompanying drawing.

6. A machine tool comprising a clamping device according to any one of the preceding claims.