US2512888A - Apparatus for grinding drills - Google Patents

Description

June 27, 1950 M. DOUGLAS APPARATUS FOR GRINDING DRILLS Original Filed 001,- 18, 1945 3 Sheets-Sheet l INVENTOR.

elvjlz flou glag June 27, 1950 M. DOUGLAS APPARATUS FOR GRINDING DRILLS 3 Sheets-Sheet 2 Original Filed Oct. 18, 1943 AX/s PL AME A 6 ONE AX/5 6 DIP/1A AXIS .n 1 7 F m0 June 27, 1950 M. DOUGLAS 2,512,838

APPARATUS FOR GRINDING DRILLS Original Filed 001;. 18, 1943 5 Sheets-Sheet 3 INVENTOR.

Melt 1'11 flolfglds BY MW% Attys Patented June 27, 1950 2,512,888 APPARATUS FOR GRINDING DRIIZLS Melvin Douglas,'Chicag0,'IIll., assignor' to Henry Hildebrandt Continuation of application .-Serial No. 2506;695, .October 18, .1943. This application May-8,.1948,

.Serial No. 25,885

' 2 Claims.

This application is a continuation of my copending application Ser; No.506;695,"filed*October 18,1943, now abandoned.

The ordinary twist .drill-which is 'nearly always used in drilling holes inrmetaland is commonly1used for drilling holes in'manyother'materials must be ,ground very precisely to function at "its best. Thehand vgrinding'or sharpening of these drills requires a very high degree "of skill. "Not only 'are"'the'manipulations of the drill against the 'grindstone J complex, but. the 3 operator must alsol'be able to exercise his judgment accurately as "to' the desired .angles .and variations thereof and theiuniformity of the sides. 'Verycomplex drill .grinding machinery has been developed and used extensively. Not only are these machines extremely :expensive, but they require skilled operators.

The problem of "drillgrindingis complicatedby the desirability of i-ncreasing--the-- clearance behind the cutting edge from the periphery of the drill towards its axis. The end :or :point surfaces of. the drilhthose tworsurfaces at thezen'd of :the helical lands-arecalled the heels. :The ordinarydrill has two lands separated :.by two flutes and thencetwo heels. Thefront-edge :of each heel forms a cutting edge'or lip'of the'dr ill. It is'obviou'sly necessaryfor the heel -to be ground oif enough toprovide a clearance angle behind the cutting edge-so thatthe heel-will not'ri deon thesurface ofthe m'etalat the bottomof "the' ho'le being drilled and prevent the cutting.- edge from bitingintothe metal. If the clearance-ang le i-s too great,the drill is excessively weakened and the cutting edge will be chipped off'fin ser'vice.

- It follows that although 'itis necessary-taprovide'sufficient clearance to" allow the cutting edge to bite into the metal, theclearance angles-should not-greatly exceed this" requirement '--or the drill Will be weakened without any corresponding'benefit.

The theoretical clearance angle is considerably greater '-near the axis of thedr'ill than nearits periphery. The reason for this will be apparent by contemplating the -path of movement of two points of the cutting edge ofthe drill, one'at'the periphery and one" adjacent the central web *of the drill. For example, let us-assume-a one inch drill "and consider "a "point at the-periphery and a point .1 inch "fromthe "axisof the drill. L If the drill is -fed-one ="thousandth of an inch perrevolution;both points moveinto'themetal 'beingdrilled one=thousandth "of an inch feach" revolution but the peripheral point, "in advancing this tonethousandth of an inchrmovesa 'peripheralfldistance equal to the icircumference 'of"a"cir.cle of one inchtdiameter;'namely"3l4l6 inches. The point v.1 ,inch'from'ithe axis,on"the other hand, has aperipheral'movement .equal"to* the-"circumference of a circle f 1:2 diameter,"namely .6283 while "advancing :its '.00 '1inch. Thus itfis seen that thelinner'po'int advances'xdtimes. as steeply into -the'metal "as the outer point and 'hence" it wouldseem that it must have. 5 times thewlearance angle behind. itin orderthatthe movement of the cuttingedgecan bite'freelyi into the metal. However, the metal adjacentthecutting edge is naturally subject to some elastic distortionmnder thepressures applied to theman'cl thisdistortion may be fairly uniform' along "the entire :cutting edge. Asa result-of allowingfor :t'his'elasticity, the clearance angle-required at the-inner point is in "fact not. nearly 5 times 'asiigreatias' at the periphery and'a; great dealmore clearance must be allowed at 'bothpointsthan 'WOLlldJTbB indicated by the. gradual. rates at whicnthey move into the "work. Nevertheless"'the "fact remains' that a' considerably largerclearanceari'gle is desirednear .the center voiithe drillthannear its' periphery.

This complicated shape has been obtained heretofore by considerable:manipulation of the drill on the grindstone, this manipulation being either manualor by more or" less automatic machinery. The more complex shape described-has been obtained in' a-variety of ways, one of-which has involved moving the drill across an-edge of a' grindstone so that it was difierentially *ground while-rotating the drill-atthesame time. of course," only'took'careof 'one' land of the-" drill and "it-was necessary'to go through"the-whole complicated movement-with the drill shifted I degrees between-times in order to grind the other land to match.

According to the present invention,-this"complicated movement of the "drill which requires either special skill-or special machinery or both iSaVoide'd. The "drillmay-simply be pressed-with little or 'norotation-against -a special *grindstone, withdrawn, rota'tedIS'O degrees about its "axis and again pressed without rotation against the grindstone. The shape of the heel surface is determined accurately, uniformly and without the exercise of any operating skill by the predetermined shape of the grindstone and the predetermined positioning of the drill. In other words, the drill is contour ground. Prior to this invention, the provision of the complex shape heretofore described without manipulation of the drill would have been considered impossible. It is accomplished according to the present invention simply by pressing the heel surface to be ground against an inside conicalsurface of a specially contrived grindstone.

An unskilled operator could be trained quickly to do a fairly good job of drill sharpening by" pressing the drill against this grindstone by hand. 3

However, the extremely simple movement of the drill permits the provision of a very simple device for holding the drill in the proper position and shoving it against the grindstone with greater ac- 4 degrees and pressed to the same axial position while the other heel is being ground. This ensures uniformity between the two heels and the positioning of the drill on the internal conical surface of the stone ensures proper shaping of each heel. Proper positioning of the drill in the two positions is ensured by a holding and manipulating unit 2! which can be much more simple than any prior drill sharpening apparatus because of the simple movements of the drill which are sufficient for accurate grinding, according to the present invention.

The drill is accurately positionedin the unit 2| by placing it against a guide 22 which is swung into position as shown in dotted outline in Fig. 2.

"During theinsertion of the drill, the handle 23 (which moves chuck 24 in cradle ring or carriage would be held by lock 28 in the position shown curacy and uniformity than would be possible in holding the drill by hand. This holder has been worked out to be adapted to a large variety of sizes of drill even though drills of different sizes should be centered at different positions with respect to the grindstone. Adjustment is also provided for varyingfthe clearance angles accordingto the work to beperformed.

Additional objects and advantages of theinventio'n'will be apparent from the following description and from the drawings in which Fig.1 is a fragmentary'perspective view of the principal portions of one embodiment of the invention chosen for illustration Fig. 2 is a perspective view of the drill holding apparatus shown in Fig. 1, Fig.2 showing the position .of the parts in setting the drill in the chuckf Fig. 3 is a view' illustrating the proper drilling action of a correctly ground drill; Fig. 4 is a 'sideview of the en'd'portion' of a drill; Fig. 5 is a Viewv of the end of a drill; U FigIi'G is a view" illustrating (one possible positioning ofIthe drill against'the internal conical surface of the grindstone; b

Fig. 7 7'is a plan view of the embodiment of this invention chosen for illustration; 'Fig. 8 is aside elevational' view of the same;

Fig. 8'A is a. fragmentarydetail viewof the In accordance with this invention, a special grindstone ll isprovidedhaving an internal conical surface 12. This grindstone is carried; and driven by the shaft of. a motor I3. Adrill I4 is sharpened by simply pressing the successive surfaces tobe ground against theinternal conical surfacelZ. Ofcourse proper positioning of the drill is important. In the ordinary drill, there are two surfaces tobe ground,;these surfaces being the ends of the two lands "land l1. These end surfaces are called the heels. One of them is seen in Figr i andis numbered 18.

It is, of course; important that the two heels be ground evenly as well as that each have the proper. shape. According to this inventioir the drill is pressed to aI-given position while one heel i P ge oun then best d r t ted 1 0 in Fig. 1. The chuck jaws 26 are tightened on V the drill while the parts are thus disposed.

The guide 22 is swung out of the way, the handle 23 is moved to the intermediate position shown in Fig. 2, and the entire unit 2| is slid along base 21 (Fig. 8) into operating position, predetermined as described below. The handle 23 is then swung to the rightor left. Approximatel'y'at the end of1fthe;slot'2j 9,'the slot curves forwardly as shown at'fifl' soIthat moving the handle as far as it will gate the left also pushes it and chuck 24 forwardly. Thehandle Z3. is rigidly mounted in chuck ,24 which is rotatable and longitudinally movable in ring or carriage 25. Thus as the handle, is'swung to the. left, and pushed forwardly by 'thecu'rve 30 in slot 29, this presses the drilllll 'again'st the internal conical surface of grindstone l2,,thu's grinding .oneheel of the'drill. The handle 23'. is then swung all the way to the right where the other end of the slot is also shaped as shown at 530. This rotates the drill ldegreessojasto' present the other heel to the internal conical surface I2 of the stone and topress' it against the stone to the same axial position as before so that the other heel of the drill is ground to-match the first heel.

Theory] of drill grinding I The proper grinding ,of a 'drill is perhaps most evident in the larger sizes where a properly ground drill will cut-twouniform'coiled shavings 36 from the'metal 31 into which it is being uniformly fed.-These coiled shavings (or chips, as they arev called in the art) are cut-from the metal bytwo cutting edges 38 forming the front edges of the heels l8; v; In order for the cutting edges to bite into themctal'being drilled,- it is, of course, necessary that the remainder of the ,heel behind the cutting edge'have-sufiicient clearancewith the metal at the ,bottom'ofthe hole so that the heel surface will not rest on the bottom of the hole and keep the cutting edge fromgoing deeper Since the drill is being constantly advanced by the feed thereof, the cuttingedges are constantly moving more deeply-into the metal with a result that the surface .atthe bottom of the hole is of generallyvconvolute shape. If theheelsurfaces of the'drill had the same convolute shape as the bottom'ofthe drill hole, they would rest with equal pressure ;at allpointsr -Of course it is desired to concentrate z-the pressure at the cutting edge and hence;theheel-should have a greater pitch or clearanceangle "than the pitch of the convolute formedby the -bottom; of the hole.

If we referto Fig 4 'enlightening comparisons may bemade between then ovements of the rqi i' smi t e Po ts: and; etween the clean ancez angle required :behindeach. Let 11s assume that the point A is at the periphery of thesdrill andlxthe point B. Lisnear. thenaxis-say. onefi'fth of;the :distance outtoathe periphery. .'=As:the drill fed, both: :poir1ts:move .into the, metal and the sameadistance ofor each revolutioni'of the'drill. 51h aperipheral direction,.howeVer,-1the pointA moves five ::times :the. distancesthat the .point' B moves. It follows that since,the;point='Amovesjfivetimes the distance that the point B,moves for the same advance into themetalfthe point B has a pitch or slope of movement-five times that ,of point A. -Likewise the slope .of the helix described by point-Balong the bottom surface of the. hole is five times as steep as the helix described by point, A. ..It follows that, ignoringdistortion of themetal, the clearance angleof the heel I8 must befive times as; steep behind the point B as behind the point .A. .In fact, the resilient distortion cannot be ignored and hence the clearance. angle ..does .notneed to increase inwardly nearly as rapidly as. would be indicated byv these ratios. Nevertheless, .agreater clearance ,angle is required .near theaxisof the drill than at the periphery.

.It. isv not .practicable merelyto provide along the entire heel surface 18 themaximum angle of. clearance that is required because this would unnecessarily. weaken thecutting edge and cause it to chip off .in service.

It is evident from the foregoing that there are important requirements as to the amount of clearance angle and astothe increasing of the clearance angle inwardly from theperiphery of the drill. This 'does not mean, however, that there can be no "departures from one theoretical shape. '.The shape which with the vmaterial being drilled would just barely provide clearance above the bottom of the hole might theoretically be the best. shape but in fact there are probably no objections to'any variations between this. shape andthe higher clearance anglesat which chip: ping of the cutting edge begins to occur. Furthermore, there is considerably more leeway quite close to the axis where the areas and torque arms involved are small and where there isrelatively little metal that has to be removed in drilling the hole. The clearanceangle desired varies according to the material to be drilled, the rate of feed and perhaps other considerations. The drill shown in Fig. 4 illustrates a very common clearance in'which a linefrom the point A to thepoint where the land joins the heel and the flute at the edge opposite theflip forms a 12 degree angle with a plane perpendicular to the axis of the drill. This. is generally referred to as a 12 degree clearance although in fact, the clearancepimmediately adjacent to the point A is; much less than 12 degrees. For the. purpose of comparison, the clearance angle at the point B hasbeen indicated as being 23 degrees. Again, the clearance'angle immediately behind point B may be "lessthan the 23. degrees there indicated. These clearance angles, as commonly referred to, mean the apparent overall angle. However, a more common-way of judging the clearance at the more-central-portions of the drill can be seen with-reference to Fig. 5. Generally speaking, the angle between the cuttingedge 38 and the connectingedge 39 crossing the dead center of the drill increases as the clearance near the axis increases. It is-common 'to' provide clearance at the central portion of the drill such that this angle between edges 38 and 39 is from 120 to 135 degrees. I

.The...angle ofauthe :cutting zedge with the axis of 6. the -Idril1 is also important, although .-:the..59:Lde gree 'anglecillustratedin 'Eig; 6 is very .much more common. than any other angle. Naturally it' is also important to have. the two cutting. edges identical or one will do more .work than the other, .or the drill .mayxdrill off eenter .and.drill ahole that .is i.-too' :large or. irregularly shaped.

-Grindiugon the conical stone I'Ifheshapeoflthe drill surfaces 'in' the various respects.discussedabove can be predetermined inaccordance withthe present. invention by selectinga stonehaving a suitable angularity of its cone. and byproperly selecting. the position offthe drill as it is pressed against'the internal conical grinding surface. It is not meant to imply that the conical grinding surface will shape the heel to' an accurate convolute. It may, however, come closer to providing-a theoretically perfect surface than any previousxcomparable machine and closer than is achieved by the. most skilled hand-sharpening, except in rare instances of good'luck. At least tests have shown that it comes withinthe permissible limits. 'of departure from a theoretically perfect surface.

Any internal grinding surface may be used to obtain the advantages of this invention to some extent. In'fact,'if point shapes not nowcommon were desired, .any internal grinding surfaces of progressively decreasing radius would bewithin the range of'this invention. and simplicity. of this invention may possibly re sult in the production of drill shapes even more desirable than present common drill shapes. and, of course, the inventioncan be used to produce a wide variety of shapes whether better than common drill'shapes or not. It is preferred, however, at least for the present, that a'surface be chosen, and a positioning-or the drill thereon, such as. will approximately reproduce conditions now more or less regarded as standard.

For the convenience of those "wishing "to practice the invention, one cone size anddrill positioning thereon that "have been found satisfactory will be described, but it should-be recognized that this is just one of theman'y ways in which the resulting shape can be produced or closely approximated.

One cone size which has been "found very suitable is cone having a vertex angle of -82 degrees. In other words, an element of the cone (a line extending from the vertex along the cone surface in an axial plane) makes an-angle with the axis of ll'degrees.

Although'cones of almost any angle can pro- Vide some of the advantages of this invention, those between approximtely 59 and approximatelyllS" have some advantage for drills having lip-angles of 59. A smaller angle than 59 at the vertex'will lcause the body of the drill to strike the come at its eriphery in moving the. drill point close to the vertex. A larger angle than 118 may not closely approximate the conventional surfaces.

The proper positioning of the drill on such a surface will depend somewhat'on the size of the drill. It has been determined, however, that when only one type of point is desired, good results can be obtained-by keeping all adjustments constant except the distance ofthe drill from the axis of the cone. Hence the only variation in positioning that needs to --be -made for difierent sizes of drills is to move the dead'center of "the drill along an element of the coneand hence the chuck may be mounted to move m The accuracy a direction parallel with the cone element .on' which the drill is centered, and .no..other adjustmentwill ordinarily be necessary. This cone element of the cone maybe referred to as the chosen element.

With the conventional surfaceinwhich the lip has an apparent angle of approximately 59 with the axis of the drill, the lip can be placed approximately parallel to the chosen element of the grinding cone and the drill positioned with its axis forming an angle of 59 with said element- The lip is then bound to have the desired angularity with the axis ofthe drill. Speaking more accurately, it should be noted that a line or element of the heel surface extending from the dead center and which lies against the chosen element of the cone, is bound to make an angle with conventional practice.

The angle of clearance is determined in part by the angle which the axis of the drill makes with that plane which includes both the axis of the drill and the chosen element; it being assumed again that the lip of the drill is approxi mately in this plane. In Fig. 6, this'plane'has been identified as axial plane A and its intersections with the cone have been shown. The drillaxis has been shown in Fig. 6 intersecting the chosen element 4| at a point'C. This is the point at which the dead center of the drill is placed. One approximate position of the surface being ground has been shown in dotted lines. The lip 38 is here represented as lying approximately along the element 4| from slightly above point C outwardly on the cone to the point where it intersects theelement.

As seen in Fig. 6, the drill axis makes a, minimum angle of approximately six and one-half degrees with axial plane A. In other words, if the axis is projected perpendicularly onto plane A, the projection will form an angle of (S with the drill axis. Surprisingly, this 6 /2 inclination of the drill axis gives a much larger apparent clearance angle, and it isthe apparent clearance angle (measured with reference to point A in Fig. 4) which is customarily made 12 for the so-called 12 clearance. Hence the surface of a drill positioned as described will very closely match the conventional surfaces.

This positioning of the drill does not make the central angle shown in Fig. 5 quite as large as is generally desired. Hence it is at present preferred to rotate the drill about its axis until this angle is the desired size. 120-125 is at present preferred for this angle because it gives the best drilling results. i

For very close matching of conventional surfaces, the drills dead center C should be very carefully positioned along the chosen element 4|. Analysis has indicated that a one-inch drill should have its dead center C positioned apto which is to be ground, it is sometimes desirable topreliminarily increase the distance from the vertex so that the drill will not on account of its odd original shape, strike the far side of the cone. I

Although the positioning of the drill described has been chosen for giving close approximation to given surfaces, it should be recognized that some variations in positioning may later be preferred in order to match more closely some one part of conventional surfaces or some one part of a theoretically perfect surface.

Drill holding unit The general natural of the drill holding unit 2| has already been described. However, it has a number of special features which warrant further attention.

It will be observed that the cradle ring or carriage 25 is pivoted at 45 about "a pin 41 whose axis intersects the axis of chuck 24 and is parallel with, and during grinding coincides with, the chosen element All of the cone. Of course the axis of pin 41 could be alined with any element of the cone and whatever element it is alined with would become the chosen element, the axis of the chuck 24 intersecting the axis of pin ll. The pin 41 is preferably inclined at an inclination of about 16 degrees, thus choosing the element of the cone-which has the same inclination. The purpose of this is merely to make the axis of the chuck and hence the axis of the drill approximately horizontal as a matter of convenience.

It will be observed that the pin l'l is carried by bracket 48 forming an extension of pedestal 49. Secured at the opposite side of pedestal 49 is an arcuate brace 5| which is shaped concentrically about the axis of the pin W. This arcuate brace is seen best in Fig. 8. As seen in Figs. 7 and 8, the cradle or carriage 25 is provided with an extension 52 straddling the arcuate brace 5| and having a lock screw 53 for locking the cradle at any pivoted position about the pin 41. This adjustment determines the clearance angle of the drill and in each shop will ordinarily be set for the clearance angle desired by that shop. The brace 5| may be calibrated with the actual or apparent clearance angles which will result from each setting.

The pedestal 49 is slidably carried by. slide plate 56, seen best in Fig. 8. The slideway of this plate is parallel to the chosen element of the cone and hence parallel to the pin 41. The result is that movement of the pedestal 49 on slide plate 56 adjusts the position of the drill along the element of the cone without changing anything else. The position of the drill may be further adjusted by movement of crank 85. I

The slide plate 56 is in turn slidably carried by the base. 21 with a movement which is axial of the grindstone. The movement of the unit toward the grindstone is limited by stop screw 58 which is adjustable to make accommodation for variations in thickness of the grindstone H and for wear of its grinding surface. Of course a lock nut 59 or other such device is provided.

Some means should also be provided for hold ing slide plate 56 against stop screw 58. Any biasing or clamping device could be used. In the illustrated form, it is a handle-operated clamp which is loosened by a'movement away from stone I so that a continuation of this movement will move the unit away from the stone for removing one drill and inserting another.

Chuck Because this apparatus is adapted to grind a wldevanety of drill izes. it is desirable to have,

atlases a: chuck-which: will? be capable of: holding such a wide variety-of i izes'. Although: the. chuck illustrated in. Figs-.9? and has'beenx specially devised. for the purposeof'the present invention, it is alsosuitable'fon numerous other purposes and in: fact is highly advantageouswhenever a wide variety of sizes or'rodlike membersis to be held.

The accommodation ofa wide variety of drill sizeswas complicated by two factors. One is the irregularity in the shaperof the drills, not only due to the presence offiutes thereon but also because each flute is provided along one edge with a rib E! which leaves-clearance between the rest of the drill and the ides of the hole which has been drilled; The other factor which complicates the problem is the vital necessity of exactly centering all of the drills in order that when it is turned 180"degrees:about its axis, the second heel will be ground exactly as was the first According to the present invention, the chuck requirements have been solved b using two long sliding jaws which have staggered interfitting v -notch'ed teeth 62 and which slide in a V-shaped recess in the chuck. cylinder 63 which supports the jaws-ififi-andpresses therntoward one another along a'substantial portion of their length.

The staggered 'positioning'of the teeth of one jaw with'respect to: those of the other is best seen-in 7'; These teethare preferably long enough' to accommodate whatever maximum size of drill is to behandled; say one and one-half inches. The recesses between the teeth are preferably deep enough to accommodate the teeth of the opposing jaw until the jaws have been completely closed-so that they'can' hold a very small drilL. Of course this depth; may-be slightly less if, say, nothing smaller than zone+eighth inch is to be ground with a particular machine.

Of course the jaws may he slid in the chuck cylinder'53 in any convenient manner. illustrated forms; each jaw is moved by a pin or key plate 65 which ispressed in end portions of aslot (ii-in a-bloclz-tfi. To the block 68 in Figs. 9 and 10 is swivelled-a rod (is which is threaded to a sta-tionaryblock H sothat'turning the handle 12 moves the slid-ing'block- -63-- forwardly and rearwardly which in turn moves the jaws 26 forwardly or rearwardly in their V-shaped recess. The parts are held in proper position by cover plate It which also serves to brace the sides of the chuck cylinder 63. vOn rearward movement, jaws 26 are drawn apart by interaction between splines 84 and grooves 16.

The movement of block 58 is accomplished slightly diiferently in the other figures, a Worm rack 15 being secured thereto and being moved by an internally threaded ring 11 which is journaled at the end of chuck block 83.

After the chuck cylinder 53 is slid into the cradle 25, the handle 23 is screwed into the chuck cylinder 63 for operating the same.

The drill is accurately positioned within the chuck by latching the chuck in the position shown in Fig. 1 and holding the drill lip against the corner 18 of gauge 22 while the chuck is tightened. This determines both the axial position of the drill and its angular position about its axis. For the latter purpose, the sense of touch may easily be relied upon for getting the lip flat against the underface 19 of the gauge. It will be noticed that underface i9 is very narrow at one end thereof. The reason for this is so that in small drills, it will not strike the other land than that against which the gauge is being applied. It is not necessary to provide a stop for the gauge 1.0 in its active positionrasit willndropxdovm on'th'e 1ip:of the drill.

The depth of cut-is determined by-thumbscrew 8| carried by a bracket 82.1 on cradle 25. which limits the forward .positionof chuck-24 as the handle 231 is pressed down in curved: end portion 3!)"0f 'slot29i- Duringpositioning of the drill in the chuck, thumb screw: 8| 'is-turned'to its zero position; after which it isturned to the: depth of grinding desired. Alternatively, the latch 28 'could be made self-sufficient for holding handle 23 in the zero position and-stop screw fi l could then be left in a-..de'sired position for depth of grind.

The face 19 of gauge 22 determines the final position of the lip ofthe -drill. At: the present time, it is preferred-thatthis*facewill 'lie at an angle which, as viewed iroma direction parallel with theaxis of the' drill, appears to -slope 12 abovethe horizontal upwardly in adirection away from the axis ofthe drill. Since the gaugepivots to slightly difie'rent positions for'difierent siz'esof drill, there will be slight variations in thisslopa but not enough to-b'e-objectionable. This slope makes the angle-shown in Fig.- 5' approximately 123 and-the slight variations due to size of the drill do not carry this beyond satisfactory limits. Anything'from to125 is believedto be highly satisfactory and many people believe that the angle can be as large as Although this chuck and the cradle'orcarriage and slide arrangements described: are-especially suitable for the purposes of the present invention,

it is recognizedthat inpressing the drill against the internal grinding surface in accordance with this-inventionza wide variety of holding-devices may beused, and-"additional adjustments: may be desired. For example, it may be desirable to have an adjustment :for the angle between the axis of the drill and the axis of the'pin: 4 1i; Adjustments may also be provided for position:ofth'e-motor. For example, it may be:found tdbe'desirable to raise and lower the motor, or-move it in'ad-i-. rection perpendicular 'to'the chosen element and tangent to the cone in order to adjustably-position the lip ofthe drill closerto the chosen element; or slightlytherebelow: to give a-greater increase of the clearance angle inwardly in the region close to the cutting lip.

It is quite possible that other positions for the pin 4! and perhaps other inclinations for the sliding action of bracket 49 will be found desirable. For example, there would be some advantage in shifting the pin 41 to a position approximately coinciding with an extension of the corner 78 so that change of clearance angle could be made about an axis approximately coinciding with the lip of the drill, as can be done when the drill is in the position shown in Fig. 6.

It is desirable that the grinding surface be kept clean during grinding. This requires special attension because the stone is not self-cleaning inasmuch as the internal angle is such that centrifugal force tends to cause the debris to cling against the rough conical wall instead of being thrown out from the stone. The stone may be kept clean by a continuous blast of air liberated near the apex. It is preferred to use a vacuum cleaner with a nozzle extending along an element of the cone and reaching close to the apex by passing between the drill and the cone surface. The wear on the stones is not excessive. Usually, various sizes of drills will be ground and their wear will be spread over different areas of the cone surface. Slight departures of a true cone are not In iterially objectionable and do not cause any noticeable difference in the ground surface. As often as may be desired, however, the stone maybe dressed to an accurate conical surface by inserting a diamond-tipped dressing tool in the chuck, adjusting the holding unit for the proper depth of cut in the stone and turning the handle 85 rapidly as the motor drives the stone to move the diamond tip along the length of an element of the new surface. It will be observed that this jaccurate parallelism between the chosen element :of the cone and the movement of the tips of the drills held in the chuck so that turning the bandle 85 to position the chuck properly for difierent drill sizes will merely move the drill along this element without altering the depth of grind :for which the thumb screw 8| has been set.

Analysis has indicated that departures of shape of the surfaces ground in the foregoing manner are extremely slight as compared with a theoretical surface which wouldbe generated by movement of the cutting lip along a helix of the desired pitch. For example with the grinding position shown in Fig. 6, calculations have indicated that the maximum departure from the theoretical surf-ace is only .03 inch while the departure along the cutting edge is even less. Of course the ultimate test is in satisfactory drilling and that test has been met by the products of this jticing' this method in a simple manner and with adequate adjustability, but with extreme uniformity for any adjustment. I claim as my invention:

1. Drill sharpening apparatus including a grindstone having an internal conical surface centered about its axis and a drill holder including a carriage, a chuck slidably and' rotatably mounted in the carriage, means for initially correctly positioning the drill in the chuck coaxially therewith, means for swinging the chuck about the common axis to different positions to place first one heel of the drill and then the other in grinding position against the said internal surface of the grindstone with the center of the drill point displaced from the axis of the stone, and for advancing the chuck axially to a predetermined final position, and means for mounting the carriage tiltably about an axis approximately coinciding with the cutting edge of the drill in the position to which it is finally ground whereby the clearance angles behind said cutting edge may-be adjusted while leaving other factors unchanged.

2. Drill sharpening apparatus including a grindstone having an internal conical surface centered about its axis and a drill holder including a carriage, a chuck slidably and rotatably mounted in the carriage, means for initially correctly positioning the drill in the chuck coaxially therewith, means for swinging the chuck about the common axis to different positions to place first one heel of the drill and then the other in grinding position against the said internal surface of the grindstone with the center of the drill point displaced from the axis of the stone, and for advancing the chuck axially to a predetermined final position, and means for mounting the carriage tiltably about an axis approximately'parallel with the cutting edge'of the drill in the position to which it is finally ground whereby the clearance angles behind said cutting edge may be adjusted.

MELVIN DOUGLAS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 282,364 Pickering July 31, 1883 1,994,975 Williams Mar. 19, 1935 2,017,532 Elter et a1 Oct. 15, 1935 2,068,915 Hardin Jan. 26, 1937 2,245,858 Hornberger June 17, 1941 2,305,230 Allen Dec. 15, 1942

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