Grindstone

May 7, 1935.

2,000,772

Sheets-Sheet 1

W. H. McGILL

GRINDSTONE

Filed Sept. 24, 1934

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May 7, 1935.

W. H. McGILL

GRINDSTONE

Filed Sept. 24, 1934

2,000,772

Sheets-Sheet 2

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Patented May 7, 1935

2,000,772

UNITED STATES PATENT OFFICE ' 2,000,772 '

GRINDSTONE William H. McGill, Winchester, Mass. Application September 24,1934, Serial No. 745,185 7 Claims. (Cl. 51—206)

This invention relates to grinding wheels or grindstones of the type made of natural stone. It is more especially concerned with the grindy stones used in grinding wood in the process of • making pulp, paper, fibre board and the like, although it will be evident that some features of the invention are also useful in grindstones employed for other purposes.

The stones used in pulp grinding machines are 1® substantially cylindrical in form and of relatively large dimensions, usually running from fiftyfour to sixty-seyen inches in diameter and from twenty-four to fifty-four inches in width.

They are of two general types: one made from I· natural stone and the other from artificial abrasive material of some suitable character, such as silicon carbide, or the like. The former have the advantage of producing a better grade of pulp but the latter give a higher rate of pro²⁰ duction and have greater mechanical strength.

The present invention aims to devise a grindstone in which the advantages of these two types of stones will be combined.

The nature of the invention will be readily ²⁸ understood from the following description when read in connection with the accompanying drawings and the novel features will be particularly pointed out in the appended claims.

In the drawings “ Fig. 1 is an end view of a pulp grindstone constructed in accordance with this invention;

Fig. 2 is a plan view of the stone shown in Fig. 1; and

Figs. 3, 4 and 5 are angular views illustrating ³³ additional embodiments of the invention.

Referring first to Figs. 1 and 2, the grinding wheel or stone 2 there shown consists of natural stone but has a multiplicity of inserts in the form of dowels 3 set into it radially. These in⁴⁰ serts preferably are made of an artificial abrasive, such as silicon carbide or aluminous oxide, in a more or less finely divided form, the particles being united by some suitable binder. Or some of the various grades of emery, garnet or ⁴⁶ quartz may be. used, So far as the composition and methods of manufacture of these elements are concerned, the prior practices may be followed. In other words, these inserts may be made of the same compositions and by the same ⁶⁰ methods heretofore used in the manufacture of grinding wheels, and the like, from carborundum and other artificial abrasives. The inserts may be molded to the desired shape and dimensions, the binder cured, and the completed inserts then °® placed in holes drilled in the stone 2 and cemented securely in their operative positions. Or, the composition in its plastic state may be forced into the holes in the stone and the stone may later-be baked'to cure the binder. The practice to be followed in any particular case 5 necessarily will vary with the requirements of individual situations. Various waterproof·· adhesives may be used to cement the inserts'in their sockets, such as sodium silicate, bakelite, ‘ and some of the synthetic resins having the 10 necessary adhesive properties, Glyptal and Durite being common forms. In general, the binders used in the composition of the inserts also make satisfactory cements for securing the inserts in the stone. . ^; 15

Dowels may readily be made of such compositions as those above described which will have considerably greater tensile strength than; does the sandstone from which these grindstones are cut. Consequently, by making these dowels of 20 suitable dimensions, say for example two or three inches in diameter, properly spacing them, and making them long enough to extend substantially through the wearing volume of the stone, the strength of the stone against breakage⁵ while 25 in operation can be increased very substantially. Usually these dowels need not be over seven Or eight inches in length. They should be staggered, somewhat as shown in Fig. 2, so that this abrasive material will be distributed sub- 30 stantially uniformly over the working- surface of the stone. .

A further increase in mechanical strength may be produced by impregnating the stone with a waterproof binder, preferably of a resin- 35 ous nature, as disclosed in my earlier Patent No. 1,959,059. If the same binder is also used in the composition of the inserts then the method above described, in which the introduced elements are forced into the cavities in the stone 40 while in a plastic condition, is greatly facilitated since this operation can be performed at the conclusion of the impregnating step and the entire stone may then be reacted to convert the impregnating and binding agents into ⁴ⁱ⁵' their solid, insoluble condition.

The ratio between the areas of the natural and artificial abrasives may be varied very considerably by changing the shape, dimensions and spacing of the inserts. At present I prefer the ⁵⁰construction illustrated in Figs. 1 and 2 in which round dowels are used and are so spaced and dimensioned that they form about six per cent of the grinding or peripheral surface of the stone when it is new. This ratio, however, will

2,000,772 necessarily vary with the requirements of different grades of work, but my experiments have indicated that the proportion of artificial abrasive surface should seldom be less than five and δ rarely more than fifteen per cent of the total grinding area.

As the stone wears down the proportion of the total grinding area formed by the exposed surfaces of the inserts will increase, in the con10 struction shown in Figs. 1 and 2, due to the fact that these insert^ have a uniform diameter and their exposed area, therefore, remains constant while the grinding area of the stone grows smaller with the diminishing radius. 15 This is a desirable action for the reason that these stones are run at a fixed speed and, consequently, it is impossible to maintain a constant peripheral speed as the stone wears down. Some decrease in grinding rate or production 20 rate thus necessarily occurs. By using a suitable proportion of an artificial grit oi^- abrasive having a higher cutting rate than that of the natural stone, the relative increase in area of this artificial abrasive resulting from wear in25 creases the efficiency of_; the - stone and thus compensates in some measure for the reduction in speed.

The combination of an artificial abrasive with a natural stone is of further advantage in in30 creasing the output obtainable from the natural stone while still preserving most of . the advantage which this stone affords in quality as compared with the artificial stones. This difference in quality results chiefly from the 35 action know as “regrinding”. This means that as the stone revolves it carries fibres removed from one block of wood along under one or more additional blocks where the fibres are further acted upon. In the case of an artificial stone 40 this regrinding does more harm than good, because the sharp abrasive pulverizes a large portion of the fibre and grinds it to a flour. The natural stone, being milder in its action, does not produce this result but improves the quality 45 of the pulp. In the composite stone shown in the drawings most of this regrinding action will take place on the natural stone. At the same time the higher abrasive properties of the sharper artificial stone are utilized in effecting 50 the initial removal of fibre from the wood blocks on which the stone operates. Thus the advantages of the two types of stones are combined.

Fig, 3 illustrates a modification in which the inserts are made in the form of plates or strips 55 4 and are set into slots which extend at an angle to the elements of the cylinder and also lie tangent to an inner circle in the cylinder.

Fig. 4 shows a.different construction in which the inserts have the form of plates or strips, «0 are positioned radially in the stone, and have enlarged inner edges. These strips are inserted endwise in the slots cut for them in the stone and are cemented in place in the same manner as are the dowels 3 and the inserts 4 of the 85 forms of the invention above described. If desired, said inner margins may be reinforced by rods or sections of wire rope, as indicated at 6.

A somewhat similar construction is illustrated in Fig. 5, the artificial abrasive strips I in this form of the invention, however, being tapered or somewhat dove-tailed in shape and having their greater thicknesses located toward the axis of the stone. Both this construction and that illustrated in Fig; 4 give added mechanical security in the anchoring of the inserts in the stone, while the arrangement shown in Fig. 5 has the further advantage of increasing more rapidly the ratio between the exposed surface gxea of the inserts and the total grinding area of the stone as the stone wears down.

While I have herein shown and described typical embodiments of my invention it will be evident that the invention may be embodied in other forms without departing from the spirit or scope thereof.