US2272405A - Cutter mounting for well reamers and the like - Google Patents

Description

Feb. l0, 1942. J, GRANT E'rAL 2,272,405

CUTTER MOUNTING FOR WELL REAMERS AND THE LIKE Filed Feb. 20, 1939 2 Sheets-Sheet l 5 35' v 55 f n, A f

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Il.. Il Il' -I 2 Sheets-Sheet 2 J. GRANT ETAL CUTTER MOUNTING FOR WELL REAMERS AND THE LIKE Filed Feb. 20, 1939 Il Il Il il Feb. 10, 1942.

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Patented Feb. 10, 1942 CUTTER MOUNTING FOR WELL RAMERS AND THE LIKE John Grant and James J. Santiago, Los Angeles, Calif.; said Santiago assignor to said Grant Application February 20, 1939, Serial No. 257,427

(Cl. Z55- 73) 2 Claims.

This invention has generally to do with the mounting of cuttersy in such tools as drills and reamers used` in deep well work. The invention, as willvbe readily understood from the following, is applicable equally well to various types of such tools, including for instance reamers which have expansion of the cutters as well as reamers orv bits which involve nosuch expansion. For sirnplicitys sake `the invention will be hereinafter described typically as applied to a non-expansive or plain reamer, but it will be understood that no limitation is implied thereby.

As is well known, all drilling and reaming tools lose their full cutting diameter, or gage, by the wear to which they are subjected. In some cases and when operating in some formations the wear is comparatively rapid, so that a tool may very quickly lose its gage to such an extent that it must frequently be pulled out of the hole and replaced. Loss of time involved in provide a method and means of keeping the cutters out to the gage diameter; to so move the cutters, as drilling or reaming operations progress, as to compensate either wholly or substantially for wear that takes place.

The invention will be described in typical form as applied to illustrative and typical roller cutters such as are used on tools of the character here under consideration. Further, this illustrative type will be of roller cutters applied to a typical plain reamer, but it will be readily understood how the same provisions may be applied to the cutters, 0r roller cutters, of any well drilling reaming tool. For the purpose of the following illustrative descriptions it will also be assumed that the roller cutters are mounted upon central spindles which form either their rolling axes or bearings; but also as will be understood from what follows, such specific form of axis or bearing is also typical and not limitative.

Thus, assuming for purposes of discussion a roller cutter which is mounted on an axial spindle or bearing, it will be very readily understood that the wear causing the cutter to lose gage is made up of ktwo factors, rst the wear on the periphery of the cutter which reduces it in size, and second, the wear on the bearings supporting the cutter, which wear allows the cutter to move bodily in toward the axis of the tool. As a more specic illustration, the case of the roller cutter mounted upon an axis substantially parallel to the tool axis may be considered. Here, the wear on the periphery of the cutter reduces the cutter diameter and 'causes the cutter to lose gage in proportion to that reduction. Additionally, wear takes place on the axial bearings of the cutter and allows lthe cutterto move inwardly bodily. In this case then the total loss of vgage is made up of the two specied factors'.

As applied typically to such a cutter as just described, our invention constantly provides a new bearing for the cutter and by doing so keeps the cutter centered on its proper and original axis. In its present form our invention does this by feeding the cutter along its axis or spindle onto a new bearing surface. And by doing that the loss of gage resulting from bearing wear is substantially or wholly compensated.

Our invention not only compensates for the bearing wear` but also is capable of compensating for the loss of gage due to peripheral wear on the cutters.A To accomplish the latter, the cutter is placed upon anaxis or spindle which is inclined with reference to the central axis of the tool. Typically such a cutter axis will incline away from the tool axis in an upward di` rection, so that as kthe cutter moves upwardly along that inclined axis it also moves outwardly away from the tool axis. In such a situation, our invention provides` the means whereby the cutter is fed upwardly along the divergent axis in such a manner as to keep the cutter centered on that axis, and provides means for so regulating the amount of upward divergent movement of the cutter that the peripheral wear is compensated for as well as bearing wear.

With these preliminary observations in mind the invention will now be better understood from a consideration of the following detailed and specific description of the illustrative embodiments of the invention, reference for this purpose being made to the accompanying drawings, in which:

Fig.' 1 is a side elevation .of a typical plain reamer equipped with our invention;

Fig. 2 is a fragmentary longitudinal section on line 2--2 of Fig. l;

Fig. 2a is a fragmentary section showing a variation in the form of cutter bearing;

Fig. 3 is a cross section on line 3-3 of Fig. l;

Fig. 4 is a fragmentary longitudinal section similar in aspect to'Fig. 2 but showing a modified cutter mounting;

Figs. 5 and 6 are cross sections, respectively, on lines 5--5 and 6-6 of Figs. 2 and 4, and Fig. 4;

Figs. 7 and 8 are further fragmentary longitudinal sections showing other variant forms of cutter mountings; and

Figs. 9 to 13, inclusive, are diagrams illustrative of the invention and its mode of operation.

For explanatory purposes we refer rst to the diagram of Fig. 9. In this diagram the center line of the tool and the center line of the cutter, labeled as such, are shown as being parallel. A cylindric cutter IU has an initial peripheral size indicated by the full lines II. The dotted lines I2 indicate the periphery of the cutter after it has worn down in diameter, but still centered on the original center line of the cutter. The position of the outer dotted line I2 thus indicates the loss of gage due solely to peripheral wear of the cutter. If in addition the cutter wears on its axial bearings so that the axis of the cutter is displaced toward the left in Fig. 9, then the worn periphery of the cutter may be displaced to the position indicated by the dash lines I2a. The total loss of gauge is thus seen to be made up of the two described factors, and the amount of loss is indicated by the distance between the lines II and I2a at the outer side of the cutter.

The diagram of Fig. 10 again shows the center line of the cutter and the center line of the tool in parallelism. Indicated in this diagram are upper and lower conical bearing surfaces I3, which are assumed to be alined on the center line of the cutter and fixed in position relative to the center line of the tool. The cylindric cutter I4 has an upper conical bearing surface I5 and a lower conical bearing surface I6 which bear both radially and endwise (upwardly) against the conical bearings I 3. subjected to two thrusts, a lateral inward thrust which may be taken as substantially radial to the cutter axis (although not usually radial to the tool axis) and an upward thrust due to downward progress of the tool into the formation. The cutter is thus thrust inwardly against the conical bearings I3, and also upwardly onto them. As either the fixed conical bearings, or the cutter bearings, wear, or both, the cutter is fed upwardly onto the conical bearings by the upward thrust. Thus, as the conical cutter bearings I5 and I6 wear out larger, the upward thrust moves the cutter upwardly on the downwardly facing conical bearings I3, to such a position as shown illustratively in dotted lines at IfIa. In this position the cutter, although the bearings have worn, is still centered upon its original center line. And thus any loss of gage due to bearing wear is compensated. In the typical arrangement shown in this diagram the loss of gage due to peripheral wear center line of the tool but tilted so as to be upwardly divergent from the center line-of the tool. Fig. 11 contains two diagrams of cutters; the one at the left will be rst considered. Upper and lower fixed conical bearings I3 are again shown `in this figure as in Fig. l0; and a cylindric cutter I4 is shown as before with upper and lower conical bearings I 5 and I 5. As will readily be understood from what has been said in connection with Fig. 10, it will be understood how the cylindric cutter I4 of Fig. 11 will be moved upwardly along Such a cutter, in operation, is

the bearings I3 and upwardly along the tilted cutter center line, as wear takes place. But in this arrangement upward movement of the cutter along the outwardly divergent center line, moves the cutter bodily outwardly as well as upwardly. In the original position of cutter I4, shown in full lines in Fig. 11, the original outer periphery of the cutter is shown by the full line I'I. The gage diameter to which such a cutter cuts is determined by the radius of the corner point I8 from the center line of the tool. When the cutter has reached such an upper position as is indicated at Ilia in dotted lines, then, if it has had no peripheral wear, its effective cutting gage will have been increased. If, on the other hand, it has been worn down peripherally to such an external surface as is indicated by the dotted line I'Irzy then, as will be seen from the diagram, its loss of cutting gage due to peripheral wear has been exactly compensated by the outward bodily movement of the cutter which has accompanied its upward movement on the bearing faces I3.

With reference to any particular type of formation through which a well is being drilled or reamed, and with reference to the lateral and vertical pressures under which the cutting is being done, the rate at which a given roller cutter looses gage by reason of peripheral wear may be at least approximately ascertained. Assuming a suitable angle of tilt between the center line of the tool and the center line of the cutter, then the ideal conditions for fully compensating peripheral cutter wear are attained when the Wear of the bearing sufaces I3, I5 and I6, is at such a rate as to allow the cutter to move upwardly on those bearings, along the cutter center line, fast enough to make the bodily outward movement of the cutter equal to the radial loss of gage due to peripheral wear. Within reasonable limits. bearing materials, and also the conical angles of the conical bearing surfaces I3, may be so selected as to allow the cutter to move upwardly and outwardly substantially at the specified compensatory rate. For instance, if materials are selected for the bearing surfaces I5 and I5 which by wear have radial enlargement at a rate substantially equal to the radial decrease in size of the cutter due to peripheral wear, then the half angle of the conical bearing faces I3 should be substantially equal to the angle of tilt between the center line of the cutter and the center line of the tool. Such an arrangement is approximately shown in Fig. ll. If the materials of the bearings I5, I6 have a relatively faster rate of wear, then the corresponding conical angles of the xed bearings I3 will be larger; if the former have a relatively slower wearing rate, then the conical angles will be less. Or to prevent the conical angle of the bearing from becoming too small, the angle of tilt of the cutter axis may be increased.

The foregoing expresses the ideal compensatory relations of the various factors involved, and that ideal relation can be expressed more denitely as follows. Fig. lla is a diagram indicating the relations. In that figure a represents the divergent angle between the tool axis and cutter axis. The radial wear of the cutter periphery is represented by w, and the radial wear of the bearing surfaces, say of the surface in the cutter, is represented by wl. The angle Nalo c lil-tan w tan a Thus, if angle ais say 20 and angle is then, for full compensation, the radial wear on the bearings is approximately half that on the cutter periphery. This expression states the ideal conditions for full compensation of peripheral cutter wear. It is to be noted however that we do not consider it necessary to attainment of desirable results with our invention that the ideal conditions be attained and peripheral wear on the cutter be completely compensated. Any substantial partial compensation results correspondingly in increased efficiency of operation.

Typical bearing materials suitablefor the purposes of either full or partial compensation are the following. For instance if one of the bearing surfaces is made of any good alloy steel hardened by heat treatment, and the other bearing surface of common low carbon steel, the bearing wear is relatively very rapid especially in the presence of a lm of very fine mud. If on the other hand, an alloy steel no1; heat treated be substituted in the above combination for the low carbon steel, the wear is controlled to be less rapid.

Although, as we have indicated, the bearing surfaces will probably at all times have a lm of line mud between them, it is one of the advantages of our invention that coarse matter, such as sand, etc., that causes destructive and irregular bearing wear, is kept out of these bearings because of their being continuously kept in close fitting engagement.

There is one other factor that more or less controls the conical angle of the bearing surfaces I3, I5, etc. The inward thrust and the upward thrust on the cutter result in an inwardly and upwardly directed resultant, such as indicated at R in Fig. 11. If, for instance, that resultant makes an angle of say 45 with the horizontal, then the outer conic element (line) of the conical bearing surface that is co-planar with the thrust line should make an angle with the vertical of something less than 45, say 30, so that the resultant force, or its vertical component, can force the cutter upwardly on the conical face. If the angle of cutter axis tilt is then say 10 from the vertical, the half angle of the cone is then approximately And, under the conditions named, any conical angle less than that is operative.

The right hand part of the diagram of Fig..11 shows an arrangement in all respects the same as that shown in the left hand part just previously described, except for the following particulars:

The same numerals are applied to the righthand part of this diagram as to the left, insofar as applicable. The cutter here is shown as being more or less barrel-shaped, the upper part of the cutter being shown as frusto-conical, with a conical half angle equal to the angle of tilt between the center line of the cutter and the center line of the tool. Thus, the original periphery of the upper part of the cutter, indicated by the full line at Hbdetermines the original cutting gage of the cutter. If this cutter be fed upwardly on the bearing faces I3 to the position indicated in dotted lines at la, then the gage determining peripheral line of the cutter would be fed out to line Hc if in the meantime the cutter had not been worn down in diameter. Under the ideal conditions provided for by the selections previously described, the cutter will have been Worn down an amount represented by the distance between the lines I 'lo and llc while it is moving up from the position indicated at I4 to the position indicated at Illa, and will thus remain at gage.

The arrangements which have so far been considered are those embodying placements of the cutter on an axis located in a common vertical plane with the center line of the tool. Figs. 12 and 13 are diagrams by the aid of which the application of the invention to a cutter otherwise disposed will now be explained. Fig. 12 is a diagrammatic elevation in which the center line of the tool is shown in vertical position and the center line of the cutter is indicated as being in front of, directly toward the observer from, the tool center line, but extending across the tool center line at an acute angle. Fig. 13 is a plan in which the point position of the center line of the tool is indicated by label and the relation of the center line of the cutter to the center line of the tool is indicated in plan. As will be seen from these two diagrams, the center line of the cutter may lie in a vertical plane which is parallel to the center line of the tool; and, as here chosen to be illustrated, the center point C of the delimited center line of the cutter (and also of the cutter itself in approximately its normal position) lies at a point where it is intercepted by the radial line ri which is radial to the center line of the tool and normal to the Vertical plane of the center line ofthe cutter. In such an arrangement, as will readily be seen from consideration of Fig. 13, the radius r from the center line of the tool to the upper end of the center line of the cutter, and likewise the corresponding and equal radius from the tool center line to the lower end of the cutter center line, is larger than the radius rl. Consequently, in an arrangement of this kind, although the center line of the cutter is straight, its upper part in eifect tilts away from the tool center line in an upward direction, as does likewise its lower part in a downward direction. As a consequence of this, if the cutter, which is indicated at i4 in Fig. 13, is made with a barrelled periphery of such shape that its outermost peripheral line, indicated at l'ld in Fig. 13, is in plan projection concentric with the center line of the tool, then the outermost or cutting parts cf the cutter will have an equal cutting gauge throughout the whole cutter length. The specific and illustrative application of our invention which we have shown in Figs. 1 to 8, is an. application of the invention to the particular cutter type and cutter arrangement which has been explainedin connection with Figs. 12 and 13, and we will directly explain preferred mechanical designs for this adaptation of the invention. However, before going to those details we point outthat if the cutter Hi of Figs. l2 and 13 is moved upwardly along the indicated tilted center line of the cutter, then the results, insofar as the upper half of the cutter is concerned, are the same as have been explained in connection with the diagrams of Fig. 1l. As we have stated, the upper half of the center line of the cutter in Figs. 12 and 13 has an effective outward tilt from the center line of the tool. And thus, the diagrams of Fig. 11, and particularly the right hand diagram in that gure, may be taken as in effect illustrative of the conditions obtaining in the upper half of the cutter of Figs. 12 and 13; and the upward feeding of the cutter of the last mentioned gures, in accordance with the conditions specified in connection with Fig. 11, will cause the upper half of the cutter of Figs. 12 and 13 to maintain gauge just as the upper half of the cutter in the right hand diagram of Fig. 11 is caused to maintain gauge. Of course, in the arrangement shown in Figs. l2 and 13, upward displacement of the cutter will decrease the efiective gage of the lower half of the cutter; but that is not of so great importance as long as that part of the cutter which last performs cutting action on the formation (the upper part of the cutter) is maintained to or approximately to gauge. The lower half of the cutter, although somewhat decreased in cutting gauge, still performs its share of the cutting action because it rst comes in contact with the relatively small diameter of the well bore which is being cut to enlarged diameter. This is especially true where a cut of any substantial radial depth is being taken.

Fig. 1 of the drawings shows in elevation a plain non-expansive reamer having three cutters each arranged in the relative positions shown in the diagrams Figs. 12 and 13. In Figs. 1, 2 and 3 the numeral 30 designates the body of the reamer, its axis being central and vertical. The three cutters, each designated generally by the numeral 3| are equally spaced about the body axis and are identic in structure and mounting, so an explanation of one will suice.

Fig. 2 shows a central section longitudinal of the axis of one of the cutters and shows the details of the illustrative form of mounting. A central spindle 32 is mounted in projecting body parts 33 and 34 and in bushings 35, and held in place both longitudinally and rotatively by keys 36, 31 and a stud 38. The details of the spindle mounting need not here be described specically as they form no particular part of this invention, and as this spindle mounting is in the subject-matter of prior Patent No. 2,029,770 to John Grant dated February 4, 1936.

Immediately surrounding spindle 32 and fitting snugly thereon is a bearing sleeve 40. This bearing sleeve has at its upper end a head 4|, and immediately below that a conical bearing face 42. The plan configuration of head 4| is shown in Fig. 5. As there shown, and shown also in Fig. 2, the head 4| has a flat rear face 43 which lies against a flat face 44 of the body, thus holding head 4| and the bearing sleeve 40 at its upper end against rotation.

The lower end of bearing sleeve 40 is reduced as at 45 and extends through a head washer 46 to which it is welded, as indicated at 41, after the cutter 3i is in place on the sleeve. Washer 46, as shown in Fig. 3, has a flat rear face 43 which bears against a flat face 49 of the body to prevent rotation of the washer and of the bearing sleeve at its lower end. The bearing sleeve assembly, including the head washer 46, ts endwise snugly between the body projections 33 and 34, which projections lie at the upper and lower ends of the cutter receiving recess 50.

As shown in Figs. 1 and 2, the cutter 3| has an internal longitudinal bore which fits the bearing sleeve 40 fairly snugly, but preferably with a little clearance. In the ordinary and prior type of cutter mountings, the cutter 3| would have merely such a cylindric bearing either upon the bearing sleeve 40 or directly upon the pin 32. In the present construction the roller cutter 3| has a cylindric bearing upon the sleeve 40 which is eiective to give the lower parts or lower end of the cutter a supporting bearing. The effective bearing for the upper end of the cutter is formed by the conical bearing face 55, in the upper end of the cutter, bearing laterally and upwardly against the xed conical bearing surface 42.

The outer or cutting periphery of the cutter 3| is shown as being barralled substantially in accordance with the shape described in connection with Figs. 12 and 13, so that initially the cutter may be of substantially equal cutting gauge from top to bottom. The conical angle of the bearing faces 42 and 55, and the wearing qualities of the materials of those b-earing faces, are selected as we have previously described; for the purpose, and with the result, of wholly compensating for loss of gauge by wear of the bearings, and either partially or substantially wholly compensating for loss of gauge by peripheral wear of the cutter. This action of compensation, in the particular design shown in Figs. 1 and 2, takes place only at the upper end or upper parts of the illustrated cutter. As we have explained that is the part of the cutter where the preservation of full cutting gauge is most important. In this particular design the lower end of the cutter, having a simple cylindric bearing, may gradually decrease its cutting gauge as a result of both bearing wear and peripheral wear on the cutter.

Fig. 2a shows how the conical bearing members may be reversed; this figure showing a reversed arrangement of conical surfaces which may be substituted for the conical arrangement 42055 of the form of Fig. 2 or of any similar form. In Fig. 2a the general arrangement is the same as described for Fig. 2, except that the fixed conical bearing surface is here shown as an internal conical surface 42d on the underside of head 4|, while the cutter has a corresponding external conical surface 55d. The action of these reversed conical suriaoes is the same as before described.

In Figs. 4, 5 and 6 we have shown a cutter mounting similar to that of Fig. 2, but here the cutter is provided with compensating bearings in both its vupper and lower portions, so that the lower portion of the cutter as well as the upper is prevented from moving bodily inwardly as a result of bearing wear. The construction in these gures is for the most part the same as that shown in Fig. 2, and like numerals are applied to all of the corresponding parts. The upper end of the bearing sleeve 40a is provided with the head 4| and with the conical bearing surface 42, as before. On a lower part 40h of this bearing sleeve, another conical bearing surface 42a is provided, and the internal bore of the cutter is provided with a corresponding conical bearing surface 55a. The upper and lower parts 40a and 4Gb of the bearing sleeve may either be integral or may be separated into two parts as shown in Fig. ll. The lower end of the lower part 40h of the bearing sleeve is cut to semicylindric form, as shown at 45a, and enters a correspondingly shaped recess in the lower collar 46a, so that the lower part of the bearing sleeve is anchored against rotation by thus being rotatively locked to the anchored washer 46a. The interior bore of the cutter Bla, is of such size as to have cylindric clearance at 60 and 6| from the cylindric parts of the bearing sleeve, so that the cutter preferably obtains its only bearings upon the upper and lower conical bearing surfaces which have been described.

'I'he conical bearing angles and materials being selected as'before, and so that the bearing wear at both the upper and lower conical bearing surfaces are equal, it will be seen that in this form of cutter, the lower part as well as the upper part of the cutter will be maintained centered at all times on that axis, by moving upwardly along the cutter axis. Consequently, in this form the upper part of the cutter will be compensated for both bearing wear and peripheral wear as before described, while the lower part of the cutter will be kept at all times in position on the proper cutter axis and its position therefore compensated for bearing wear.

It may be remarked that both Figs. 2 and 4 do not show the conical bearing surfaces 42 extending any substantial distance above the upper ends of the cutters or above the upper ends of the cutter conical surfaces 55, The xed bearing surfaces 42 may extend on upwardly, as it is indicated that they do in the diagrammatic iigures hereinbefore discussed; but in practice we find that for some purposes it is preferable not to extend the conical bearing surfaces 42 on upwardly but to construct those bearings originally with the over-hanging shoulders such as are indicated at G2. We have found in practice that, with such an initial construction, the upper ends of the cutters, at the upper ends of their conical bearing faces, tend to wear their way into the overhanging parts of head 4|, and thus to cut new conical bearingsurfaces for themselves as they move upwardly.

Fig. 7 shows a form which in essentials is the same as that shown in Fig. 2, except to provide roller bearings in the cutter. The upper bearing sleeve head 4l and its conical bearing face 42 are substantially the same as before. The bearing sleeve l0 extends down from this head and is set in the lower washer 46 the same as in Fig. 2. The upper conical bearing face of the cutter 3l is, however, provided with frusto-conical bearing rollers 55h. The general action, the Wear of the effective bearing surfaces, and the upward movement of the upper end of the cutter along the cutter axis is the same as before described. While it might appear that the substitution of rolling bearings at 55h might quite largely eliminate the bearing wear upon which the upward feed of the cutter depends, we have found in practice that bearing rollers of any ordinarily used material wear quite rapidly and reduce in diameter quite rapidly when operating in such a medium as the mud and sand laden circulation iiuid which surrounds and permeates all vparts of tools of the class under description.

Fig. 7 also shows how roller bearings 10, with intervening spacers 'H and end rings 'I2 may be inserted in the enlarged bore of the cutter 3l. As here illustrated these spacers 1| and end washers 12 have annular grooves 'I3 in their faces engaging the rollers '10; and the rollers have correspondingly shaped conical points 14 entering the grooves. The construction facilitates the assembly of the roller bearings within the cutter before the bearing sleeve 40 is put in place. A retaining washer 'l5 is either force fitted or Welded into the open end of the cutter bore to retain the bearing assembly in place endwise.

Fig. 8 illustrates another typical construction which in various respects is similar to the several constructional forms heretofore described. Here the upper bearing sleeve head 4l is the same as before described and has the described conical bearing surface 42 immediately below it, the cutter having at its upper end the same conical bearing surface 55 as it has in Figs. 2 and 4. The bearing sleeve 40e has one or more annular recesses in its exterior, as indicated at 40d, to take roller bearings 10d which operate between the reduced portions of the bearing sleeve and the internal bore of the cutter 3l. Near its lower end the bearing sleeve 40o has another conical bearing face 42e, engaged by a corresponding conical bearing face 55e of the cutter. The whole construction is very much similar to that of Fig. 4, the upper and lower sets of conical bearing faces operating in the same manner as in Fig. 4, but the cutter being provided with one or more sets of roller bearings intermediate its ends.

We claim:

1. A well cutting tool having a body rotatable about a central vertical axis and having rotatable cutting means mounted thereon to operate upon the wall of a hole, said means being subjected to a relatively constant rate of wear by reason of such operative engagement with the wall which Wear tends to reduce the effective cutting diameter of the tool, including xed bearing means for said cutting means, the center line of said bearing means which forms the axis about which said cutting means rotates being disposed at an upwardly and outwardly diverging angle to the vertical axis of said tool, said bearing means and said cutting means having cooperating conical seats, said seats being symmetrically disposed about said center line with the smaller ends of said seats being directed downwardly, the material forming the contacting surface between said seats having a known rate of wear, the conic element line through said seats furthest from the body axis making an angle with said axis such that as said cutting means is forced downwardly against said wall a sliding force is established between said seats to cause said cutting means to diverge from the body axis along said center line, said surface rate of wear being proportional to said peripheral rate of wear of said tool whereby said cutting means slides along said bearing means parallel to the center line to tend to increase the effective diameter of said tool as rapidly as the periphery of the cutting tool is worn off to maintain a substantially constant cutting diameter.

2. A well cutting tool havinga body rotatable about a central vertical axis and having rotatable cutting means mounted thereon to operate upon the wall of a hole, said means being subjected to a relatively constant rate of wear by reason of such operative engagement with the wall which wear tends to reduce the effective cutting diameter of the tool, including xed bearing means for said cutting means, the center line of said bearing means which forms the axis about which said cutting means rotates being disposed at an upwardly and outwardly diverging angle to the vertical axis of said tool, said bearing means and said cutting means having cooperating conical seats, said seats being symmetrically disposed about said center line, the material forming the contacting surface between said seats, including the seat of the cutting means, having a known rate of wear, the conic element line through said seats which makes the largest angle with the body axis making an angle with said axis such that as said cutting means is forced downwardly against said wall a sliding force is established between said seats to causesaid cutting means to diverge from the body axis along said center line, said surface rate of Wear being proportional to said peripheral rate of wear of said tool whereby said cutting means slides along said bearing means parallel to the center line to tend to increase the effective diameter of said tool as rapidly as the periphery of the cutting tool is worn off to maintain a substantially constant cutting diameter.

JOHN GRANT. JAMES J. SANTIAGO.

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