The present invention relates to honing and is concerned with the preparation of superabrasive materials for use in honing.

Hones containing industrial diamonds, though hard and therefore highly abrasive, have the disadvantage that the diamonds cannot withstand high temperatures and also react with the steel or iron being honed. It is also difficult to obtain a strong bond between the diamonds and the metal bonded matrix in which they are supported, with the result that the diamonds are prematurely dislodged from the matrix.

There are also now available hones which comprise vitrified bonded cubic boron nitride (CBN). The cubic boron nitride is extremely hard and the bond achieves a strong adhesion with the surface of the cubic boron nitride particles so that much of a particle is worn down before the particle is dislodged from the bond. A further important advantage is that the strong bond secures the particles such that during, wear the CBN crystals fracture on cleavage planes thus regenerating sharp cutting edges.

The present invention is concerned with the preparation of a hone of vitrified bonded cubic boron nitride or industrial diamonds and the term superabrasive is intended to include not only these two materials but other equivalent materials that are, or may in future, be used in hones.

When honi-ng an engine cylinder bore, for example, the surface finish achieved is dependent to a great extent on the method of preparation of the hones. These hones in this case are typically strips mounted on an expandable honing head which if a drum formed f several sectors which can be moved radially in synchronism with one another to urge the hones into contact with the inner surface of the cylinder bore. The drum is rotated and simultaneously moved axially relative to the cylinder so that the hones follow a helical path often

with a predetermined helical angle. The cylinder bore is intended to be formed with helical grooves so that oil may be retained for the purpose of lubrication. It is furthermore important that the surface of the cylinder bore should not folded, that is to say distorted and stressed by dragging. The surface finish is thus critical and special importance must be attached to the preparation of the hones.

At present, hones containing diamond or CBN are prepared with the use of a vitrified grinding wheel while mounted on the honing head or drum. The grinding wheel is rotated and brought into tangential contact with the drum which is rotated at low speed. The grinding wheel and the drum are then moved axially relative to one another so that the hones are ground down to a cylindrical surface. When metal bonded diamond or CBN hones are used, relatively slowly rotating grinding wheels of silicon carbide or similar material are used to prepare the hones.

The result of this operation is to leave the hones with a relatively smooth cylindrical surface with the abrasive particles almost flush with the surface of the bond or matrix, in which state the surface is not conditioned for use. While the hones can be used in this form, their metal removal capacity is not sufficiently high to cut efficiently until the bond has been worn down from contact abrasion with the wor piece material, to expose the abrasive particles. The hones are therefore rubbed down with a grinding stone to roughen the surface and expose the diamond or CBN particles before use, but it is difficult by this.method to achieve an accurate and repeatable surface finish from the hones.

Improperly conditioned hones initially exhibit unpredictable properties which may verge on burnishing. In such-a case, they require higher specific contact pressure to cut and this produces surface metal deformation in the honed surfaces for which reason, amongst others, these materials have not been commercially used for cylinder bores.

The surface of the hone wears down during use and, after a few workpieces have been honed, a consistent surface finish is obtained but the first results achieved with the hone after it has been conditioned are unpredictable.

The present invention seeks to mitigate the foregoing disdvantages and to provide a method of conditioning hones . which allows the hones to be used immediately and which enables the surface of any honed workpiece to be accurately predetermined by the conditioning operation.

According to the present invention, there is provided a method of conditioning vitrified bond superabrasive hone, which method comprises rotating the hone, rotating a diamond abrasive wheel, urging the diamond abrasive wheel and the hone into contact with one another and axially traversing the diamond abrasive wheel and the hone relative to one another, characterised in that the surface speed of the hone is substantially greater than the surface speed of the diamond abrasive wheel such that the contact between the hone and the diamond abrasive wheel crushes the vitrified bond and the -superabrasive and in that the depth of cut and the speed of traverse are accurately controlled to determine the surface finish to be achieved by the hone in a workpiece.

When a surface is being ground, it is possible to vary the surface finish by altering the conditions used in truing the grinding wheel. A grinding wheel is trued by turning it at its high working speed and moving axially relative to it a diamond or a diamond wheel which wears down the surface of the grinding wheel. The speed of the grinding wheel during truing relative to a stationary diamond or a rotating diamond wheel varies the effective grinding hardness of the grinding wheel. Furthermore, the speed of axial movement of the grinding wheel relative to the diamond or the diamond wheel varies the surface finish obtained in the workpiece ground by the grinding wheel. Such grinding wheels, unlike hones trued in a conventional manner, are ready for immediate use and the

surface finish in the workpiece can be controlled by the truing process of the grinding wheel.

The present invention is predicated upon the realisation that if one were to substitute a drum carrying hones for the grinding wheel in the process described above, the same result as can be achieved with a grinding wheel can be obtained with hones.

This represents a major departure from the technique previously used in conditioning hones wherein the drum is rotated at low speed and the grinding wheel is rotated at a higher speed not directly related to that of the drum. It may at first glance seem equivalent to rotate the drum Instead of the diamond wheel at high speed since one can reason that it is the relative speed at contact that is of importance. In practice, however, this has been found not to be so and the reason for this is believed to be that the

- surface density , size and strength of the abrasive particles in the truing wheel and the hones are sot the same. ^• By conditioning the hones in the manner proposed in the present invention it has been found that they may be used without any further preparation and the control of the surface speeds, the depth of cut and the traverse speed can provide control of the surface finish which can be obtained by use of the hones.

Preferably, the drum surface during conditioning is moved at a speed at least equal to its speed under operating conditions. This ensures that the hones will operate correctly at their normal speed conditions to cut properly and with the correct clearance. Lower speeds can achieve similar results if the speed of the abrasive wheel is correspondingly reduced but the hone preparation then takes longer to achieve.

It has been found experimentally that the relative size of the particles in the conditioning wheel is important if the desired crushing effect is to be achieved. The particles in the conditioning wheel should be significantly larger then the particles in the hones and are preferably between 4 and 5 times as large.

The method of the invention can be applied not only to expanding hone heads of the type used in honing cylinder bores, but also to tapering one pass hones which remain of fixed diameter during a honing cycle. Currently, such one pass hones are all formed as diamond hones and no procedure is adopted for conditioning them prior to use. Because the invention allows control over surface finish and enables a hone to be re-conditioned if worn by prolonged use, the invention enables one pass hones to be formed of vitrified bonded superabrasive materials. As in the case of hones mounted on an expandable head, as used for cylinder bores, the conditioning of the one .pass hones under controlled speeds of rotation and traverse relative to a conditioning diamond wheel provides control over the surface finish that may be achieved by the use of the hone in a workpiece.

While the greatest benefits arise with vitrified cubic boron nitride, hones of other vitrified bonded superabrasive materials, including diamond, aluminium oxide and silicon carbide, will also benefit from conditioning in the manner proposed by the invention.

The apparatus which may be used in implementing the method of the invention is itself conventional and does not differ from apparatus in current use. It is not therefore deemed necessary to describe the apparatus in detail within the context of the present invention.