GB2028860A - Grinding wheels - Google Patents

Abstract

Resin-bonded, reinforced grinding wheels are provided comprising a matrix of resin-bonded abrasive particles in which matrix is embedded a fibrous reinforcement composed of fibres greater in length than 10 mm and having an E-modulus greater than 1x10<4> kp/mm<2>. The wheels are of high stability, low reinforcement content and usable at high peripheral grinding speeds.

Description

SPECIFICATION Grinding wheels The invention relates to a grinding wheel comprising grains, a synthetic resin-based, preferably duroplastic binding agent and preferably filler material with a high-strength fibre reinforcement which is embedded in the grinding body.

In grinding, in particular in severing grinding, endeavours are made to attain the highest possible peripheral speeds of the grinding body.

In this respect, the term severing grinding is used to mean grinding through materials, preferably in bar form, with grinding wheels of relatively small thickness; that is to say, the thickness T is generally less than 0.01 of the diameter D:AT 0.01 . D.

In technical use, maximum peripheral speeds of 100 m/sec are encountered at present, but peripheral speeds of 1 30 m/sec and more have already been achieved in testing and laboratory work.

In this respect it will be appreciated that the stricter safety requirements which apply to highspeed grinding operations must be especially considered. Thus, for a grinding wheel operating speed of 100 m/s, a break-up speed (burst speed) of at least 1 50 m/s is required.

In accordance with the laws of mechanics, increasing the wheel speed to 1.5 times results in an increase to 2.5 times of the maximum stresses which occur in the wheel. This follows from the fact that the tangential stress at the bore of a rotating wheel does not rise linearly with the peripheral speed v, but quadratically, that is to say: tans. max = prop. v2 However, the advantages of high-speed grinding are so great that this method is being used to an ever increasing extent in practice. The most important advantages are that the cutting forces at the grain material decrease with increasing speed and the efficiency factor and the specific workpiece removal efficiency (volume of material removed) and the level of edge or profile stability increase in an over-proportional manner.

The efficiency factor here is the ratio between the cross-sectional surface area severed, and the disc surface area consumed. The specific removal efficiency denotes the volume of workpiece which is removed in a unit of time per millimetre of wheel width.

Also, endeavours are made to produce severing grinding wheels which are of the smallest possible wheel thickness T. This provides the advantage that on the one hand a lower drive power is required on the part of the severing machine, and that on the other hand the degree of material waste (cutting loss volume cut away per cut) falls.

This is important particularly in respect of expensive materials such as high-alloy steels, titanium, tungsten and the like. In addition, the degree of heating of the workpieces is lower, and this is advantageous on the one hand in regard to materials which are sensitive to heat or cracking and on the other hand in regard to protection of the environment (reducing the required content of grinding-active filler materials in the wheels).

A further requirement made on severing grinding wheels is that the wheel diameter D and thus the useful wheel surface area should be as large as possible. Such a wheel affords the possibility of cutting through larger cross-sections or longer service periods (reduction in wheel replacement costs). At the present time, severing grinding wheels of up to 1 200 mm diameter can be produced by a mass production operation, and severing grinding wheels up to 1800 mm can be produced individually.

It is known however that the lateral rigidity of a wheel falls in proportion as the thickness of the wheel is decreased and the diameter of the wheel is increased, that is to say, the greater the ratio D to T of a severing grinding wheel, the greater is the tendency of the wheel to flutter and the greater is the tendency for the cut to wander off course. This can result in rough cut surfaces, cuts which are not correctly angled, and damage to the grinding wheel.

For this reason therefore, a high degree of static rigidity and a high level of dynamic stability are required particularly for high operating speeds.

It follows from the laws of mechanics that a high degree of rigidity is linked to a high resonance frequency fres. For planar circular plates, the following formula applies:

in which E denotes the modulus of elasticity, a denotes the density and p denotes the Poisson number of the wheel.

In practice, T, D, a and v are virtually fixed predetermined parameters, so that an increase in stability is possible only by means of an increase in the modulus of elasticity (abbreviated to E-modulus).

However, like mechanical strength, the E-modulus is virtually predetermined with the conventional grain-resin combinations, as the selection (kind of grain, grain size, proportion of resin, etc) must be effected primarily from the points of view of the grinding operation. Now, in order to withstand the centrifugal and tangential forces which occur at the high peripheral speeds mentioned above and to increase the modulus of elasticity and thus the rigidity and stability of the wheels, with the grinding bodies which are in technical operation at the present time, it is usual to provide a fibre reinforcement.

In particular, glass fibres are used in the present state of the art, as conventional organic fibres generally have insufficient heat resistance.

This latter consideration applies both to the grinding operation and also to the curing operation, which, in the case of organic grinding body binding agents (phenol or epoxy resins and the like) is generally effected at temperatures of from 1 50 to 1 900 C.

In this respect, glass fibres are used: a) in the form of short fibres statistically distributed in the binding agent, b) in the form of tangled fibre fleeces, c) in the form of woven cloth.

In these cases, the fleeces and the woven cloth are provided with resin impregnation which generally comprises phenol resins so as to provide satisfactory transmission of force from the grinding material to the reinforcement, or so that all individual fibres are uniformly loaded ('take their share of the load'). In the course of technical development, many attempts were made to increase the effectiveness of the glass fibre cloth, and some of this work is already described in the patent literature, for example woven cloth with roving threads (little or no rotation of the individual fibres in the cloth hank), special types of cloth, such as three-directional cloth, round cloth discs with preferably radial thread direction and improvements in adhesion by special surface treatment of the glass fibres (for example by silane).

In practice however it has been found that the above-mentioned advances, which can be achieved by the fibre reinforcement, are nullified to a considerably extent by damage to the fibres in the production of the grinding bodies. The sharp edges of the grinding grains cut through the fibres or produce nicks in the surface of the fibres, simply in the pressing operation. The more the grinding grains are splintery and sharp-edged (for example silicon carbide, high-quality corundum), and the higher the density of the wheel structure, the more serious do such phenomena become, as a high compressing pressure is required to produce such wheels. For example, glass cloths which are removed again from uncured grinding wheels present reductions in strength of between 5 and 90%.

This damage to the fibres in the production of the grinding wheels also represents one of the main difficulties when using high-strength carbon fibres as the reinforcement.

Apart from this, it should also be mentioned that, considered from the purely technical point of view, glass fibre threads, in particular the high-strength types, age relatively easily and quickly. Thus, particularly under conditions of severe dampness, the fibres may suffer from a reduction in strength of up to 30% in one hundred days. Glass fibres suffer from a further irreversible fall in strength simply in the curing operation, the fall in strength at a curing temperature of 2000C being in fact from 10 to 20%.

In order to avoid the fall in strength in the reinforcing fibres due to damage and ageing, it has been necessary to incorporate a safety reserve in the form of higher proportions of reinforcement.

As the mechanical strength and the E-modulus of the conventional organic grinding wheel binding agents (resins) could not be increased, or could be increased only slightly, an ever increasing volume of reinforcement is used at the present time in high-speed grinding wheels, in the production thereof.

However, such an increase in the volume of reinforcement in the grinding wheel gives rise to serious disadvantages. In particular, it necessarily results in a fall in the content of wheel material which is active in the grinding operation (grain and active filler materials) and thus a drop in the efficiency factor. When using a glass fibre reinforcement, it is also found when grinding that there is a tendency for the wheel surface to be smeared by molten glass, so that the cutting capacity falls severely and the wheel grinds hotter. Moreover, similar phenomena also occur generally when using thermoplastic fibre materials.

This invention seeks to provide a grinding wheel, in particular a severing grinding wheel, which, while having the same or a smaller content of reinforcement, permits higher peripheral speeds with a sufficient degree of stability and cool grinding.

According to the invention, this is achieved in that the reinforcement has fibres of more than 10 mm in length, whose E-modulus is higher than 1 x 104 kp/mm2.

The invention preferably provides that the reinforcement is provided with a coating which has good adhesion to the fibre and which with the wheel binding agent forms a mechanically sufficient chemical and/or physical connection and which is formed by a resin or a mixture of resin and the binding agent used in the grinding body, wherein the reinforcement is laid into a layer of fine grain material or fine-grain filler material which is active in the grinding operation.

According to the invention, the reinforcement fibre material are high-strength glass fibres, singlecrystal whiskers, polycrystalline metal whiskers, carbon fibres, boron fibres and drawn metal wires. In addition, boron nitride, silicon carbide and high-strength heat-resistant organic fibres with a high Emodulus (for example polyaramide).

Metal wires which may be considered in accordance with the invention include in particular wires comprising a high-alloy C-steel.

The following values may be given as basic values, in regard to order of magnitude, of the modulus of elasticity and the tensile strength of these fibres: Modulus of elasticity E > 100 GN/m2 (kN/mm2) Tensile breaking strength 5 Z, B > 2 GN/m2 (kN/mm2) In order to improve the adhesion of the fibres in the matrix of the binding agent, the invention provides that the fibres are provided with a surface layer which has a high chemical and/or physical affinity with respect to the binding agent and the fibres.

For this purpose for example the metal wires may be provided with a primer. Carbon fibres are preferably subjected to surface oxidation or C-whiskers or SiC-whiskers are refined at their surface.

High-strength fibres often give rise to difficulties in the production of cloths of standard structure (approximately equal strength and rigidity in the weft and warp directions). Therefore, the invention further provides that the wheel has as reinforcement at least one cloth whose weft (or warp) comprises fibres with the above-mentioned high strength and E-modulus properties and whose warp (or weft respectively) is formed by more elastic and more flexible fibres, for example conventional glass or organic fibres (so-called unidirectional cloth).

A preferred embodiment provides at least two cloths of this kind, in a crossed arrangement.

The fine grain used includes the known grinding agents such as for example corundums, silicon carbide, boron carbide, while the grinding-active filler materials include for example cryolite, pyrite, etc.

In this respect, the reinforcement may be laid into the fine grain material, either when impregnating the reinforcement material, as a preparatory operation, or only when finishing the grinding body.

By virtue of this fine-grain enclosure, which is obviously saturated by the above-mentioned coating material in the manufacturing process, which is not the subject of this invention, on the one hand the reinforcement fibres are protected in the process of producing the grinding bodies (pressing), and on the other hand the above-mentioned effect of smearing of the surface of the grinding wheel when grinding is reduced. The high mechanical strength and E-modulus values are maintained, even when using reinforcing materials which are very sensitive to nicking, for example carbon fibres.

The construction according to the invention of the grinding wheel or severing grinding wheel provides for a high permissible peripheral speed with a low tendency to flutter, high dynamic stability and thus the lowest possible tendency to wandering of the cut. In addition, the danger of the wheel breaking up is substantially reduced.

The construction of the invention also provides an improvement in the surface quality of the materials which are ground or cut through with such grinding wheels.

It has been found essential that the reinforcing fibres or wires which are used according to the invention, while having the same or even higher tensile strength, have a substantially higher E-modulus, than conventional reinforcing fibres, which higher E-modulus is fully maintained or suffers only a slight fall, until the wheels are used.

According to the invention, it was possible to produce grinding wheels whose E-modulus was five times that of conventional wheels provided with a conventional glass fibre reinforcement. Likewise, the mechanical strength values (tensile strength, rupture limit peripheral speed of the wheels) were double the usual values.

This is of particular importance because an increase in the E-modulus is possible only to a limited extent, with the organic grinding body binders as such (phenol, epoxy and polyester resins and the like), which are at present in technical use. Therefore, in practice only the course which involves fibrereinforced composite materials is suitable.

Hereinafter a table of the E-modulus and strength values achieved in fibre-reinforced test bodies.

The sample grinding bodies were produced in the usual manner from abrasive grain and a mixture of phenol resins with inorganic filler materials, as the binding agent. The ratio (by weight) of resin to filler material was 1:1.

With all types of fibres, the proportion forming the reinforcement was 20% by volume of the grinding mass (grain + binding agent). The fibres were used in the form of unidirectional cloths in order to reduce the influences of the weaving operation or in order to be able to produce the cloths as easily as possible.

The surface of the fibres were provided with surface coatings to improve adhesion; thus, the Cfibres were oxidised and the steel wires and glass fibres were provided with a primer.

Data on the reinforcing fibres used:

Breaking E-modulus Strength Name More precise identification GNhn2 GN/m2 Glass E-glass, low alkali content 73 2.2 S-glass 85 1 4.4 Steel Carbon steel ZOO 4.0 C-fibre Carbon fibre of medium strength and E-modulus 300 2.5 B-fibre Boron filament with tungsten core 420 3.0 The impregnation of the cloths and density of the fibres was of the same composition as the binding of the wheels of phenol resin and fine-grain inorganic filler material. the cloth was saturated with phenol resin solution, laid into the filler material, and then pre-dried in conventional manner to such an extent that it had the necessary working consistency.

Basic recipe: Normal corundum 30 mesh 70 parts Phenolresol (fluid) 10 parts Phenol resin/Novolak (powder) 4 parts Cryolite (powder) 14 parts Pressing density (without reinforcement) = 2.70 Two crossed cloths were then incorporated into the grinding body, as internal reinforcement, in such a way that the two outer layers of grinding material are each half the thickness of the middle layer.

The wheels were pressed and cured in the usual manner.

The E-modulus and strengths were then measured and the resonance frequency was calculated from the E-modulus. As already stated above, the resonance frequency is a measurement of the lateral rigidity and dynamic stability of the wheel, and thus a criterion for the possible peripheral (working) speed when grinding.

The results are diagrammatically summarised in the following table. The measurement values of the grinding bodies according to the invention, with glass fibres as the reinforcement, were in each case taken as the base (100%).

Possible E-modulus of Resonance frequency peripheral Material of the the test of the test bodies speed when reinforcement bodies in % in 9' (X) grinding in % Glass 100 100 100 Fine steel wire 250 150 150 C-fibre aOo 200 200 fibre 500 220 220 (X) Values for resonance frequency calculated from the measured values of the E-modulus.

It was found that there is a close agreement between the possible peripheral speeds which were determined by way of the calculated resonance frequencies, and the dynamic performance which was found in a grinding test. A further criterion is obviously the permissible speed which is fixed on the basis of the burst peripheral speed and the safety regulations. For example, when using S-glass, this permissible speed would be about 30% higher than when using Glass, but the dynamic stability with both types of glass is approximately the same. Therefore, it is possible to make use of high strength values only in regard to fibres which also have a high E-modulus. A further prerequisite is that the Emodulus is maintained during the process of producing the grinding bodies.

When performing a grinding operation with the test grinding bodies, substantially less coating of the wheel surface was found, than in the case of conventional wheels.

Therefore, the present invention makes it possible on the one hand to achieve increases in operating speed, with high-strength fibres such as carbon fibres, steel fibres, boron fibres, etc, without an increase in the relative volume of reinforcement in the grinding wheel, and on the other hand, the loss of strength which occurs with conventional production methods and which, as already mentioned above, is from 5 to 90%, can be greatly reduced by the embedding effect according to the invention.

This latter therefore also provides considerable advantages when using the usual glass fibres.

In a further aspect, the present invention also provides a method of manufacturing a resin-bonded fibre-reinforced grinding wheel which reduces the problems associated with the physical damage which occurs to the fibres during moulding of the grinding wheel caused by the grains of abrasive material cutting or scratching or otherwise damaging the surface of the fibrous reinforcement. This method comprises precoating the fibrous reinforcement with a layer of resin which may be the same as or different from, but compatible with, the resin used in the grinding wheel. Preferably the same resin is used, and preferably the protective coating formed on the fibres comprises fine grain particles of a filler and/or the abrasive material used in the construction of the grinding wheel. After precoating the fibrous reinforcement is laid into the resin/abrasive mix and moulded in a conventional manner.

In this aspect, the invention is not restricted to fibrous reinforcement of any particular length or modulus but may be used in the formation of any fibrous reinforced resin bonded grinding wheels, where physical damage to the fibres by the abrasive during moulding is a problem.

Claims (11)

1. A resin-bonded, reinforced grinding wheel comprising abrasive particles bonded together with an organic resin binder to form a matrix, and embedded in said matrix, a fibrous reinforcement, wherein the fibrous reinforcement comprises fibres having a length greater than 10 mm and having an Emodulus greater than 1 x 104 kp/mm2.

2. A grinding wheel according to claim 1, wherein the fibrous reinforcement comprises single crystal fibres, polycrystalline metal fibres or boron fibres.

3. A grinding wheel according to claim 2, wherein the fibres are synthetic fibres.

4. A grinding wheel wherein the fibres have or have been provided with a surface layer having a high chemical and/or physical affinity for the resin binder.

5. A grinding wheel according to claim 4, wherein the fibres are carbon fibres having an oxidised surface.

6. A grinding wheel according to claim 4, wherein the fibres are metal fibres coated with a primer.

7. A grinding wheel according to claim 6, wherein the fibres are of a high alloy C-steel.

8. A grinding wheel according to any one of the preceding claims, wherein said fibrous reinforcement is in the form of one or more unidirectional cloths whose warp or weft, but not both, comprises said fibres of length greater than 10 mm and E-modulus greater than 1 x 104 kp/mm2, the weft or the warp, as the case may be, comprising fibres of greater elasticity and flexibility.

9. A grinding wheel according to claim 8, wherein the reinforcement comprises two or more crosslapped unidirectional cloths.

10. A method for the manufacture of resin-bonded, fibre-reinforced grinding wheels which comprises a fibrous reinforcement into a mix of abrasive grains and a resin binder, and moulding t'ne reinforced mix to a desired shape, wherein before laying into the abrasive/resin mix the fibrous reinforcement is precoated with a layer of the same or different resin.

11. A method according to claim 10, wherein the fibrous reinforcement is precoated with a layer of the same resin as used in the abrasive/resin mix and containing a proportion of a fine grain filler and/or fine grain particles of the abrasive used in the abrasive/resin mix.