US3537216A - Method of smoothing - Google Patents

Description

Nov. 3, 1970 H. P. s. BORGH METHOD OF SMOOTHING Filed March 13, 1967 FIG. 70

United States Patent 3,537,216 METHOD OF SMOOTHING Hans P. S. Borgh, Kungalv, Sweden, assignor to Norton Abrasives Limited, Welwyn Garden, England Filed Mar. 13, 1967, Ser. No. 622,638 Claims priority, application Great Britain, Mar. 11, 1966, 10,908/ 66 Int. Cl. B241) ]/00 US. Cl. 51-327 6 Claims ABSTRACT OF THE DISCLOSURE Smoothing of a workpiece is effected by contacting the workpiece with a rotating grinding wheel having areas of differing friability while effecting relative movement between the grinding wheel and the workpiece, this movement including a component of movement parallel to the axis of the wheel. The surface of the workpiece is contacted with an area of higher friability after being contacted with an area of lower friability while the area of lower friability is still in contact with the workpiece, the areas of lower and higher friability being located at different radial distances from the wheel axis.

The present invention relates to a method of grinding a surface of a workpiece to remove blemishes and to produce a relatively smooth surface.

The invention is especially concerned with the grinding of relatively large workpieces. It is applicable, for example, to the grinding of steel slabs and billets prior to rolling of such workpieces into sheet form. Such a grinding operation is commonly known as snagging or fettling and involves removal of a comparatively large amount of material in order to obtain a surface free fiom blemishes or defects which otherwise may remain pres ent throughout the rolling process and spoil the final sheet or strip.

In grinding processes of this nature with a grinding wheel of a given type of abrasive and given grade hardness, it has been shown that grinding efficiency (expressed as a ratio of total metal removed to wheel wear) increases as the average particle size is increased and this is more particularly observed when the type of abrasive has a high resistance to fracture. However, it is also observed that the surface finish increases in roughness as the particle size is increased and as, in some cases a relatively fine surface finish is required, it has not always been possible to use a composition giving the highest grinding efficiency. Such coarse surfaces could naturally be reground with a grinding wheel of finer particle size but this would be an even more expensive procedure than that if initially using a finer particle size grinding wheel of somewhat lower grinding efiiciency.

Snagging operations are often performed using swing frame grinders on which the wheel may be moved in any plane or by off-hand methods on a floor-stand machine in which the article to be ground may be moved in any plane. We have observed that, in both cases, the major grinding work appears to be performed by the outer portions or edges of the grinding wheel and the wheel develops a convex grinding face.

Another very important type of machine is the automatic or mechanical slab and billet snagging machine in which the wheel is maintained substantially perpendicular to the relatively uneven slab or billet surface, is maintained in high pressure contact with the workpiece by a suitable source of fluid pressure, is traversed in an axial direction along the workpiece (or the workpiece similarly traversed relative to the stationary wheel head) and, at the end of the traverse in one direction the wheel ice head or workpiece is suitably cross indexed and the traverse repeated in the reverse direction. Again we have observed that the major work is performed with the outer portions or edges of the grinding wheel, the traverse in one direction first causing the wheel to wear more rapidly from the leading edge and from the op posite edge on the reverse traverse, again leading to the formation of a substantially convex wheel face.

The present invention provides a method of smoothing a workpiece comprising contacting said workpiece with the peripheral surface of a rotating grinding wheel, said peripheral surface including a first surface area of relatively high grinding efficiency and a second surface area of relatively low grinding efficiency, said first surface area being located at a different radial distance from the wheel axis than said second surface area, while effecting relative movement between said workpiece and said wheel such that said workpiece is contacted by said second surface area after being contacted by said first surface area, said relative movement including a component of movement parallel to the axis of rotation of said wheel, whereby to impart to said workpiece a surface finish superior to that which would be achieved by contacting the workpiece solely with said first area while achieving a grinding efficiency greater than that which would be achieved by contacting the workpiece solely with said second area.

The invention also provides a grinding wheel having a peripheral surface including a first area of relatively high grinding efficiency and a second area of relatively low grinding efficiency, said first area being located at a different radial distance from the wheel axis than said second area.

The invention is especially applicable to snagging or fettling operations in which a grinding wheel is brought into contact with the workpiece while effecting relative movement between the grinding wheel and the workpiece with a component of movement parallel to the grinding wheel axis, the grinding wheel usually being moved over the workpiece in a path substantially parallel to the grinding wheel axis. By contrast with known operations of this type wherein the grinding wheel has a peripheral surface of uniform grinding efliciency the present invention involves the use of a grinding wheel having a peripheral surface having at least one area of higher grinding efficiency than another area thereof.

Thus, for example, when the invention is applied in association with a known snagging or fettling operation where the grinding wheel is moved forwardly and back- Wardly in reciprocating fashion over the surface of the workpiece, the grinding wheel employed may be so formed as to have a higher grinding efficiency at each side of its peripheral surface than at the centre of the peripheral surface. The simplest constructional form of such a wheel is represented by a wheel formed of two outer layers of high grinding efliciency and an inner layer of lower grinding efliciency sandwiched between the two outer layers.

When a cylindrical wheel of this type is used in a snagging or fettling operation wherein there is reciprocating movement between the wheel and the workpiece it is found that both outer edges of the wheel periphery acquire a camber so that the peripheral surface of the wheel becomes smoothly convex. Continued grinding of the workpiece with the cambered wheel then results in the production of a finer surface finish than that which would be attained by'using a wheel made solely of the material of the outer layers while achieving a grinding efficiency higher than that which would be achieved by using a wheel made solely of the material of the inner layer.

It should be noted that the wheel may have difierent structures from that just described. It may for example be so formed that there is a continuous gradation in composition and grinding efliciency from one peripheral edge of the wheel to the opposite edge of the wheel with this efficiency being at a minimum at the centre line of the peripheral face. However, wheels of this type are relatively difiicult to manufacture and it is generally more practical to provide the desired gradation of grinding efliciency across the peripheral face of the wheel by assembling a number of layers of differing grinding efiiciencies, as in the case of the three layer wheel described above. Clearly the number of layers can be increased beyond three. For example, a five layer wheel may be formed by sandwiching an inner layer of low grinding efficiency between two intermediate layers of higher grinding efiiciency and then applying two outermost layers of still higher grinding efiiciency.

These various types of wheels all become cambered at both peripheral edges when used in a reciprocating type of fettlin-g or snagging operation. As a result the workpiece is always first contacted by an area of higher grinding elficiency and is last in contact with an area of lower grinding efficiency irrespective of the direction of relative movement between the wheel and the workpiece. The presence of the cambered edges on the wheel is an important feature of the method of the invention and in some cases itmay. be desired to form the wheels with such cambers during their manufacture rather than form the cambers at the start of the grinding operation.

Snagging and fettling operations where the grinding wheel is moved across the surface of the workpiece in one direction only, rather than being reciprocated over it, are uncommon. However, in such a case the method of the invention would involve the use of a wheel one peripheral edge of which Was of higher grinding efiiciency than the other peripheral edge. The wheel could simply consist of two layers, one of higher grinding efliciency than the other or anumber of layers could be used arranged in order of increasing grinding efliciency. Alternatively the composition of a one piece wheel might be so varied from one peripheral edge to the other as to produce a continuously increasing grinding efiiciency across the peripheral face. Whatever the particular structure chosen the grinding operation is so conducted that the peripheral edge of higher grinding efiiciency first contacts the workpiece. This edge acquires a camber (or may be so preformed) and the results obtained are then similar to those described in respect of the reciprocating type of snagging or fettling operation, i.e. a good surface finish is obtained while attaining a grinding efficiency not normally feasible in conjunction with such a finish.

The grinding wheels described above for use in the method of the invention are characterized by having the first area of relatively high grinding efl-iciency located at a lesser radical distance from the wheel axis than is the second area of relatively low grinding efliciency. When using such wheels it is possible to operate by movmg the wheel over the workpiece with its median plane substantially at right angles to the surface of the workpiece. Alternatively, the wheel can be moved over the workpiece with its median plane at an acute angle to the surface of theworkpiece in the direction of the relative "movement between the wheel and the workpiece. It is only when the wheel is moved over the workpiece with its .median plane at an obtuse angle to the direction of movement. Wheels of this second type are characterized by having the first area of relatively high grinding efficiency located at a greater radial distance from the wheels axis than in said second area. It will be appreciated that this second type of grinding wheel is completely opposite in construction to the first type of grinding wheel.

Thus, a grinding wheel of the second type may be so formed as to have a lower grinding efliciency at each side of its peripheral surface than at the centre of the peripheral surface. The simplest constructional form of such a wheel is represented by a wheel formed of two outer layers of low grinding elficiency and an inner layer of higher grinding efiiciency sandwiched between the two outer layers.

When a cylindrical wheel of this type is used in a snagging or fettling operation wherein there is reciprocating movement between the wheel and the workpiece it is found, as in the case of the wheels of the first type, that the peripheral surface of the wheel becomes smoothly convex. Provided that the obtuse angle between the wheel and the workpiece mentioned above is maintained it is found that the workpiece acquires a finer surface finish than that which would be attained by using a wheel made solely of the material of the inner layer while achieving a grinding efiiciency higher than that which would be achieved by using a wheel made solely of the material of the outer layers.

As in the case of the Wheels of the first type it is possible to form the wheels of the second type with various structures. Thus, the wheel may have a continuous gradation in composition and grindingefiiciency from one peripheral edge of the wheel to the opposite peripheral edge with this efficiency being at a maximum at the centre line of the peripheral face. Alternatively, the wheels of the second type may include more layers than three with the middle layer having a high grinding efficiency and outer layers of grinding efliciencies which decrease towards the edges of the Wheel. All of these wheels acquire a convex peripheral face when used in the method of the invention if they have not been preformed with such a face.

The invention will be described by Way of illustration and without limitation with reference to the accompanying drawings in which:

FIGS. 1, 2 and 3 show grinding wheels suitable for use in the method of the invention, and

FIGS. 1a, 2a and 3a illustrate more clearly the contours of the peripheral surfaces of the wheels of FIGS. 1, 2 and 3.

FIGS. 4, 5 and 6 illustrate different modes of movement of a wheel over a workpiece.

As shown in FIG. 1a wheel 1 includes two outer annular layers 2 of high grinding efiiciency composed of a relatively coarse abrasive material and an inner annular layer 3 of lower grinding efliciency composed of finer or more friable abrasive material.

Such a combination in a grinding wheel gave the surprising result that, without showing excessive wear of the inner portion, the wheel gave, when used in a reciprocating type of fettling or snagging operation with the Wheel moving substantially parallel to its axis of rotation, a surface finish better than that to be expected from the coarse outer layer composition and, coincidentally, a grinding efiiciency higher than that to be expected from the liner or more friable abrasive inner layer or layers.

Such a combination of layers may be moulded as a single wheel by known methods, layers may be made separately and cemented together to form a single wheel,

'or layers may be made as separate wheels and mounted appropriately on a single spindle on the grinding machine to provide a unitary laminated structure of the desired layer distribution.

FIG. 1a illustrates in exaggerated form contour of the peripheral surface of the wheel of FIG. 1. According to the invention the wheel 1 can be formed with such a contour during its manufacture. Alternatively, the wheel can be formed cylindrically; the peripheral edges of the wheel then become cambered at the start of the fettling or snagging operation so that the peripheral surface of the wheel becomes slightly convex as in FIG. 1a. It will be appreciated that because of this convexity the workpiece surface is past contacted by the peripheral surface of layer 3 in both backward and forward passes of the wheel over the workpiece.

FIG. 2 illustrates another type of wheel suitable for use according to the invention in a reciprocating type of fettliug r snagging operation. The wheel 4 0f FIG. 2 includes outermost annular layers 5 of high grinding efficiency, intermediate annular layers 6 of lower grinding efiiciency and a middle annular layer 7 of still lower grinding efiiciency. FIG. 3a shows in exaggerated fashion how the peripheral surface of the wheel is formed in manufacture or how it becomes convex at the start of the grinding operation so that the peripheral surface of layer 7 is always last in contact with the workpiece and determines the nature of the finish.

FIG. 3 illustrates a grinding wheel suitable for use according to the invention in a grinding or fettl'ing operation in which the wheel passes over the surface of the workpiece in only one direction. The wheel 8 is composed of an annular layer 9 of high grinding efficiency and an annular layer 10 of lower grinding efficiency and is moved across the workpiece with the layer 9 providing the leading edge. At the start of the grinding operation the peripheral surface of the wheel acquires a camber as indicated in exaggerated fashion in FIG. 3a. Alternatively the wheel may be so preformed. The grinding efficiency is comparable with that corresponding to use of a wheel made solely of the material of layer 9 while the surface finish achieved is comparable with that corresponding to use of a wheel made solely of the material of layer 10.

FIGS. 4 and 5 illustrate alternative ways in which a wheel of the type shown in FIGS. 1, 2 or 3 is moved across the surface of a workpiece 11. In each of the figures the wheel moves in the direction indicated by the arrow. In FIG. 4 the wheel 1 is moved with its median plane at right angles to the surface of the workpiece 11. In FIG. 5 the wheel 1 moves with its median plane at an acute angle to the surface of the workpiece 11 in the direction of movement of the wheel. It will be appreciated that whereas in the arrangement of FIG. 4 the wheel 1 can simply be reciprocated without changing its attitude at the end of each pass, in the arrangement of FIG. 5 the tilt of the wheel 1 must be reversed at the end of each pass so as to maintain the desired acute angle between the median plane of the wheel and the surface of the workpiece 11. Moreover, in the case of the wheel of FIG. 3 the wheel would have to be rotated through 180 at the end of each pass in order that the surface of lower grinding efiiciency is last in contact with the surface of the workpiece.

The wheels of FIGS. 1, 2 and 3 are all of the type in which the first area of relatively high grinding efficiency is located at a lesser radial distance from the wheel axis than the second area. It will be readily understood from the foregoing disclosure that wheels having the first area of relatively high grinding efiiciency located at a greater radial distance from the wheel axis than a second area of relatively low grinding efficiency can be made by simply reversing the construction of the wheels of FIGS. 1, 2 and 3. Thus, the wheel of FIG. 1 can be modified by having the layers 2 of low grinding efiiciency and the layer 3 of high grinding efficiency. Similarly, the wheel of FIG. 2 can be modified by having the layers 5 of lower grinding efficiency, the layers 6 of higher grinding efliciency and the layer 7 of still higher grinding efiiciency. Also, the wheel of FIG. 3 can be modified by having the layer 9 of lower grinding efficiency and the layer 10 of higher grinding efliciency. It will be appreciated that the wheels may be preformed with the peripheral edge cambered in the manner described or alternatively may be caused to acquire such a camber at the beginning of the snagging or fettling operation.

FIG. 6 illustrates the way in which these modified wheels of the second type are moved over the surface of the workpiece. The wheel 12 moves with its median plane at an obtuse angle to the surface of the workpiece in the direction of movement of the wheel. The object is to ensure that the surface of lowest grinding efiiciency is last in contact with the workpiece. As will be understood, the angle of tilt of the wheel 12 is reversed at the end of each pass when the wheel is used in a reciprocating type of operation.

In a particular example a wheel of the type illustrated in FIG; 1 was constructed with two outer layers 2 each 25 mm. thick composed of grains of sintered bauxite, particle size 14 mesh, bonded together by a synthetic resin binder and an inner layer 3 25 mm. thick composed of grains of sintered bauxite, particle size 20 mesh, bonded together by the same resin binder. The layers 2 and 3 were also bonded together by means of the same resin binder to form a unitary assembly.

This wheel (referred to as Wheel A below) was used in an automatic slab snagging machine for smoothing the surfaces of stainless steel slabs, the wheel being moved over the slab in a direction substantially parallel to the wheel axis and with its median plane substantially at right angles to the surfaces of the workpiece. Comparative tests were also carried out using in the same machine two wheels of similar dimensions, one 75 mm. thick composed entirely of sintered bauxite of particle size 14 mesh (Wheel B) and one 75 mm. thick composed entirely of sintered bauxite of particle size 20 mesh (Wheel C).

As was to be expected it was found that the grinding efiiciency of Wheel B (expressed as ratio of total metal removed from the slab to the loss of material from the wheel) was higher than that of Wheel C but that the surface finish imparted to the slabwas smoother with Wheel C. With Wheel A on the other hand the grinding efficiency was only slightly less than in the case of Wheel B while the finish was only slightly less smooth than that in the case of Wheel C.

In a further series of experiments wheels were made having layers of different abrasive materials but of similar particle size and were tested in the same automatic snagging machine on austenitic steel workpieces and on ferritic steel workpieces. The wheels were all bonded using the same synthetic resin binder and all of the abrasive particles had a particle size of 14 mesh. The structures of the wheels were as follows:

The grinding efficiencies of these wheels on austenitic steel and ferritic stainless steel were as follows, the efiiciency being expressed in each case as the ratio between kilograms of metal removed to the volume of wheel wear in cubic decimetres.

Wheel D Wheel E Wheel F Wheel G Austenitic steel- 14. 5 35. 3 38. 9 44. 7 Ferritie steel 14. 4 28. 9 30. 8 35. 9

Wheel E produced in both cases a surface finish almost as good as that produced by Wheel D. Wheel F produced in both cases a surface finish inferior to that produced 7 y by Wheel E but considerably superior to that produced by Wheel G.

The invention has been described with reference to particular embodiments thereof and it will be appreciated that it is not limited thereto. Many variations and modifications are clearly feasible within the scope of the invention. As has been described, the wheels used in the invention may comprise layers of the same abrasive material but of different particle size or layers of different abrasive materials of the same particle size It will be understood that'the layers could also be formed of different materials of different particle size. 7

Any of the materials and techniques known in the prior art may be utilised. Sintered bauxite and fused alumina have been specifically mentioned but other abrasive materials can be used. However, sintered bauxite is a preferred material since .it has such a high impact resistance. The manufacture of the wheels can be undertaken with the aid of such known techniques as hot pressing using resin binders of known types.

In conclusion it may be noted that grinding wheels comprising planar layers of differing composition of abrasive materials are known but in previous instances each composition is designed to perform a complete grinding operation upon a section of a given article. Also, in some cases, very thin outer layers of a composition of harder grade than the main body of the wheel may be used to avoidbreakdown or rapid wear of corners when .cylindrically grinding -into fillets and similar operations. 1 l

Additionally, grinding wheels for snaggingor fettling are known having annular layers increasing in grade hardness from periphery to centre to compensate for deduction in surface speed as the wheel wears downy.

None of these knowncases are comparable with the present method wherein all layers traverse the whole of the workpiece area almost simultaneously. Knowledge of these known procedures could not lead to a prediction of the outstanding combination of grinding efliciency and finish-provided by our invention.

Moreover, the wheels of the prior art have been of a cylindrical nature. by contrast with the wheels of the present invention which have a preipheral surface including a surface. area of relatively high grinding efliciency radially displaced from the wheel axis a different distance than a surface area'of relatively low grinding efficiency. 7

What I claim is:

1. A method of smoothing a workpiece comprising contracting said workpiece with the peripheral surface of a rotating grinding wheel,'said peripheral surface including a first surface area of relatively low friability and a second surface area of relatively high friability, said first surface area being located at a different radial distance from the wheel axis than said second surface area, and effecting relative movement between said workpiece and said wheel such that the surface of said workpiece is contacted :by said second surface area after being contacted by said first surface area and while said first surface area is still in contactwith the surface of said workpiece, said relative movement including a component of movement parallel to ;the axis of rotation of said wheel, whereby to impart to said workpiece a surface finish superior to that which would be achieved by contacting the workpiece solely with said first area while achieving a grinding efliciency greater than that which would be achieved by contacting the workpiece solely with said second area.

2. The method of claim 1 wherein said relative movement is. substantially parallel to said axis of rotation.

3. The method of claim 1 wherein said wheel is moved reciprocatingly over said workpiece.

4. The method of claim 1 wherein said wheel comprises two outer annular layers of low friability and an inner annular layer of higher friability sandwiched between said outer layers, and said wheel is moved over said workpiece with its median plane substantially at right angles to the surface of the workpiece-in the direction of said movement.

5. The method of claim 1 wherein said wheel comprises two outer annular layers of low friability and an inner annular layer of higher friability sandwiched between said outer layers, and said wheel is moved over said workpiece with its median plane at an acute angle to the surface of the workpiece in the direction of said movement.

6. The method of claim 1 wherein said wheel comprises two outer annular layers of high friability and an inner annular layer of lower friability sandwiched between said outer layers, and said wheel is moved over said workpiece with its median plane at an obtuse angle to the surface of the workpiece in the direction of said movement.

References Cited UNITED STATES PATENTS 1,009,877 11/1911 Winn.

1,616,531 2/1927 King 51-207 2,198,377 4/1940 Dunbar 51-48 3,019,562 2/1962 Price 51-3 HAROLD D. WHITEHEAD, Primary Examiner

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