GB2117289A - Grinding tools - Google Patents

Abstract

A grinding tool, especially a rim grinding wheel, in which grinding grits 1 and intervening spaces D are disposed on the surface of the tool in regular patterns so as to facilitate access of coolant to the grits during use. The grits may be arranged in bands in which they are closely- packed in the direction that lies transverse to the motion of the tool in use. In the circumferential direction the grits in each band should not be more than three-deep, and a space separates each band from the next. A grit band may comprise only a row of single grits, closely spaced in the transverse direction. The surface of the wheel may be recessed in the spaces, to improve coolant capacity, and the invention applies particularly to wheels having metal hubs on which grits of "super abrasive" materials are attached by electrodeposition techniques. <IMAGE>

Description

SPECIFICATION Grinding tools This invention relates to grinding tools such as wheels, belts and blocks, which carry grinding medium upon their surface and which in use are held in contact with a workpiece and in motion relative to it. While the invention applies particularly to tools such as wheels and belts that are typically moved against a stationary workpiece, it applies also to tools such as blocks that may be held stationary while the workpiece itself is moved.

The invention applies especially to the tools used in grinding processes in which there is at any moment a long arc or other continuous line of cut between the contacting surfaces of tool and workpiece, such as in the process known as creep-feed grinding. In any grinding process heat is generated at the workpiece surface by friction and but the cutting action of the grinding medium. If it becomes too great, this heat can damage the workpiece in the manner known as "grinding burn". In the conventional processes known as reciprocating surface grinding and surface cylindrical grinding burn can be avoided by transversing the workpiece at high speed and by applying coolant to the workpiece closely adjacent where it is in contact with the grinding tool.By this means, almost all the cooling action to which the workpiece is subject at any instant of time takes place outside the instantaneous area of contact between workpiece and tool, and very little within that area, but because of the traverse speeds just mentioned any generated heat is removed quickly enough. With the greater depth of cut and slower traverse of creep-feed grinding, the heat must be removed more directly from within the area of cut itself: heat flux densities of 3 to 4 kw/cm2 are possible, and grinding burn is likely to arise unless coolant reaches the workpiece surface before it parts contact with the tool.A known way of apppying coolant when effecting creep-feed grinding with a conventional wheel composed of particles of aluminium oxide are silicon carbide is to direct a jet of liquid coolant into the general area of the arc of cut so that the pores of the wheel carry the coolant into the arc itself Many known grinding wheels utilise an enhanced porosity in their construction which helps them to retain and transport coolant in this way. If the wheel should be of non-porous structure, however, the transport of coolant into the arc of cut may be less than desirable when the coolant is supplied as a jet in this way. Figure 1 of the accompanying drawings is a diagrammatic plan view of the surface of a conventional wheel on which individual particles or "grits" 1 of grinding medium are mounted, closely-spaced, in a random pattern.Such close spacing leaves little interstitial volume for the transport of coolant.

Moreover a new generation of wheels is emerging in which the structure is non-porous: grits of socalled "super abrasives", for example natural or synthetic diamond, cubic boron nitride (CBN) or Sialon, are attached to the surface of a metal hub by the electro-deposition of nickel or other metals around the grits.

Figures 2 and 3 of the accompanying drawings, which are both also diagrammatic plan views of the surfaces of grinding tools, illustrate two other known ways of increasing the volume of coolant that the tool itself can transport into the area of cut. However it is both predictable and well known that if the grits are spaced both randomly and relatively wide apart as in Figure 2 then due to the lower number of active grits there is more rapid wear than with the wheel of Figure 1, and the surface finish imparted to the workpiece is poorer because of the wider spacing.

Figure 3 shows the grits mounted in bands 2, with gaps 3 between the bands providing open spaces in which quantities of coolant might be transported. However the depth D of each band 2 is typically many times-say ten times-the width W of each individual grit 1, so it will readily be seen that there will still be an appreciable separation between the coolant in gaps 3 and at least those grits that lie in the middle of the depth of each band 2. Two further disadvantages of such an arrangement are firstly that efficient cooling requires not only that there should be sufficient volume of coolant but also that it should be highly agitated, preferably adjacent the source of heat (i.e. an individual grit) that it is to cool: there is evidence that in the construction of Figure 3 the masses of liquid carried within the shallow trough-like cavities of gaps 3 are insufficiently agitated.Secondly, that in the middle of the depth of each band 2 the close packing of the grits leads to the heat input from one grit following closely upon that of another before any appreciable quantity of coolant has managed to have effect upon the latter.

Test suggest that grinding tools according to the present invention may achieve improved access and agitation of coolant without suffering, like the construction of Figure 2 did, the disadvantage of inferior finish. According to the invention the grits are disposed on a grinding tool in regular patterns, closely spaced in the direction perpendicular to that of the relative motion of the tool and the workpiece in use with wider spacings in the direction parallel to that motion, with no more than about three individual grits arranged in close sequence in the latter direction between adjacent spacings.

The grits may be arranged in parallel bands in the perpendicular direction, with the depth of the spacings (mesaured parallel to the motion) preferably being of the same order as that of the band itself. Preferably the band comprises a single row of closely-packed individual grits and is therefore equal in depth to the width (W, Figure 3) of a single grit.

The bands may be straight, and may lie either exactly perpendicular to the direction of motion or other patterned shape.

The bands may be straight, and may lie either exactly perpendicular to the direction of motion or at an angle to it. Alternativley the bands may have a wavy or other patterned shape.

The quantity of coolant adjacent to each band of grits may be improved by etching away or otherwise depressing the surface of the tool in the spacings between the bands of grits, so increasing the volume of coolant that each spacing can contain.

The tool may be in the form of a block or the like, held stationary in use while the workpiece moves in contact with it. More typically, however, the workpiece will be held still and the tool, typically in the form of a wheel or belt, will move.

The invention applies particularly to rimgrinding wheels comprising metal hubs and where grits of "super abrasive" materials are attached to the rims by electro-deposition techniques similar to those used in the manufacture of electronic printed circuits.

The invention is also defined by the claims at the end of this specification and will now be described, by way of example, with reference to the following further figures of diagrammatic drawings, in which: Figures 4 to 7 show four different grinding wheels, each viewed in a radiai direction; Figure 8 is a section through a grinding belt, and Figure 9 shows another wheel, viewed in anaxial direction.

In Figure 4 each band of grits 1 comprises, as is preferred, a single row and the dimension D' of the intervening gap 3 is of the same order as the width W of the grits 1 themselves which are packed tightly in the transverse direction T, that is to say the direction perpendicular to that in which the grits move when in use relative to a workpiece. The wheel of Figure 5 is similar except that each band 2 now comprises three rows of grits arranged closely alongside each other so that the effective depth D' of each band is about 3W: the dimension D" of the adjacent gap 3 is of the same order.Analysis suggests that limiting the bandwidth to not more than about three times an individual grit width allows coolant sufficiently close access to all individual grits even to grits in the middle "row" of each band-and ensures that all the coolant in each gap 3 is close enought to the profiles of the grits in the adjacent bands to be adequately agitated by the action of them. Of course with the preferred "single-row" arrangement of Figure 4 the proximity and the agitation are even greater.

Figures 6 and 7 show alternative wheels in which the lines of the bands 2 lie generally, but not this time exactly, in the transverse direction T.

In Figure 6 the bands and intervening gaps 3 lie slightly obliquely across the face of the wheel, and in Figure 7 the bands and gaps both follow a wavy path across the rim of the wheel. Other patterns would of course be possible.

Figure 8 shows a grinding belt comprising a flexible backing layer 5 onto one face of which grits 6 of a conventional abrasive-for instance tungsten carbide-have been stuck by a layer 7 of conventional abrasive. In this instance the grits are distributed in rows only onegrit deep. Such a pattern could have been achieved by applying adhesive on to the backing 5 in the appropriate, lined pattern by spraying or by applying it with a pen or other applicator, then scattering the grits on to the backing and allowing the adhesive to set; by shaking the belt, the grits that have not stuck to the adhesive would then fall off.

Alternatively the grits could have been electrostaticaliy attracted in the desired pattern on to a backing surface evenly coated with adhesive.

After that, additional adhesive could have been applied to the backing surface to retain such grits more firmly.

For wheels, as shown for instance in Figures 4 to 7, the grits are preferably of relativley new "super abrasive" kind and are applied to the rim of a metal hub by electro-deposition. For instance the surface of the rim of the wheel could first be coated with resin. A phhotographic negative or other transparent device, carrying a pattern of fine lines, could then be placed over the resin-coated surface and be subjected to ultra-violet radiation which degrades the resiri where it is transmitted through the transparent part of the negative. The resin remains undegraded however where it has been protected by the opaque part of the negative. A pattern of fine lines of resin is thus formed on the rim, and super-abrasive grits are then electro-deposited on the rim in the normal way and will attach in lines where the resin has been degraded but not where it remaIns.

Subsequently the resin can be rmeoved, by many known methods including ultra-violet or other thermal degradation, and further electrodeposition can be applied to the rows of grits to improve their adhesion to the metal rim surface.

To achieve the form shown in Figure 9, in which the bases 9 of the gaps 3 between the grit rows are depressed below the level of the normal rim surface 8, thus increasing the volume of coolant that they can transport, the metal rim surface once coated with a pattern of fine lines of undegraded resin could be subjected to a chemical etch which would create the depressions where the resin was not present. The whole surface could then be re-coated with resin such that these depressions were first filled with resin and then physically smoothed so that the original resin protection was removed, so leaving exposed metal strips separated by resin-filled depressions. Grits could be electro-deposited on the exposed strips, and the resin in the depressions could then be removed.

Claims (11)

Claims

1. A grinding tool of the kind carrying grinding grits upon its working surface, in which the grits are disposed on that surface in regular patterns so that they are closely and continuously packed in the direction perpendicular to that of the relative motion of the tool and a workpiece in use but in which spacings occur between grits parallel to that relative motion, no more than about three individual grits being arranged in close sequence in the latter direction between adjacent spacings.

2. A grinding tool according to Claim 1, in which the grits are arranged in parallel bands in the perpendicular direction.

3. A grinding tool according to Claim 2 in which the dimension of a spacing, measured in the parallel direction, is of the same order as the dimension of the one of the bands measured in the same direction.

4. A grinding tool according to Claim 3 in which each band comprises a single row of closely-packed individual grits.

5. A grinding tool according to Claim 2 in which the bands are straight and lie exactly in the perpendicular direction.

6. A grinding tool to Claim 2 in which the bands are straight and lie at an angle to the perpendicular direction.

7. A grinding tool according to Claim 2 in which the bands have a wavy or other patterned shape.

8. A grinding tool according to Claim 2 in which the surface of the tool in the spacings between the bands of grits is depressed below the level of the surface on which the grits themselves are carried, so increasing the volume of coolant that each spacing can contain.

9. A grinding tool according to Claim 1 in the form of a block or the like, adapted to be held stationary in use while a workpiece moves in contact with it.

10. A grinding tool according to Claim 1 in the form of a wheel or belt, adapted to move in use and to be applied to a stationary workpiece.

11. A grinding wheel according to Claim 10 in which the grits are disposed on the radiallyoutward face of the wheel rim.

1 2. A grinding wheel according to Claim 11 comprising a metal hub, and in which the grits have been applied to the rim surface by electrodeposition techniques.

1 3. A grinding tool according to Claim 1, substantially as described with reference to the accompanying drawings.