US2398250A - Drilling machine - Google Patents

Description

Ap 9, 1946. L. RoBBlNs DRILLING MACHINE Filedoct. 12, 1942 Mmmm l Arm/waff April 9, 1946. l.. RoBBlNs 2,398,250

I DRILLING MACHINE Filed oct. 12, 1942 5 sheets-Sheet 2 ENToR.'

v `BY 9, 1946i L.. RoBBlNs 2,398,250

DRILLING MACHNE Filed oct. 12, 1942 5 sheets-sheet@ x IN V EN TOR.

[en: @ha I April'9, 1946. L. RoBBlNs DRILLING MACHINE Filed 1.12, 1942 5 Sheets-Sheet 5 INVENTOR. [ed Ruhm BY Z A fr0/Mfr! Patented Apr. 9, 1946 UNITED STATES PATENT oFFicE 2,398,250 n u DRILLING MACHINE f Leo Robbins, New York, N. Y., assignor, by mesne assignments, to Reconstruction Finance Corporation, a corporation of the United States Application October 12, 1942, Serial No. 461,723

14 claims. (c1. 12S-3b) This invention relates to the art of `making wire-drawing dies and in particular to the dirill- 'ing of holes in diamonds used in said dies.

The general object of the present invention is to devise automatic means for drilling the holes in the diamonds.

A further object is to .devise automatic means for drilling extremely fine holes.

The principles of the invention will be defined in the claims and certain embodiments intendedfor the purpose of illustrating these principles will be described in connection with the accompanying drawings, in which:

Fig. 1 is an end elevationof apparatus embodying the invention; Y

Fig. 2 is a plan view;

Fig. 3 is a front elevation;

Fig. 4 is a section on the line 4-4 of Fig. 3;

Fig. 5 is a fragmentary view partly in section showing a lever arml and mounting therefor;

Fig. 6 is an elevational view partly in section showing the drill spindle and bearings therefor, and the housing for the drill spindle and bearings; l

Fig. 7 is a section on the line 'l--l of Fig. 6;

Fig 8 is an isometric and partly diagrammatic view showing the drill, the work holder and associated elements;

Fig. 9 is an enlarged fragmentary View of cer'- tain parts of the bracket in which the Work holder is mounted;

Fig. 10 is a front elevational view partly in section of a modification showing a modified form of means for actuating the work holder;

Fig. 11 is a plan view partly in section, the upper part of Fig. 11 being taken on the line IIa- IIa of Fig. 10 and the lower part of Fig. 11 being taken on the line IIb-I Ib of Fig. 10;

Fig. l2 is a fragmentary View on the line I2 I2 of Fig. 11;

Fig. 13 is a front elevational View showing a miscroscope mounting for the inspection of the contact between the diamond and the drill point;

and

Fig. 14 is a side View of the mounting shown in Fig. 13.

In accordance with the invention and as shown diagrammatically in Fig, 8, the drill I may consist of a fine 'wire having a vertical axis and mounted in a drill spindle 2 which likewise has a vertical axis and' is mounted to rotate about that axis. A work holder 3 having a vertical longitudinal axis is positioned below the drill. This Work holder may take the form of a rod.

,On the upper end of the rod 3 the diamond to be drilled. is `held in what may be a conventional .formof die mounting 5. In mounting-the diamond in the die the diamond 4 is embedded in a matrix of relatively soft metalwithin aring 6 and parts of the mounting'are 'then ground away and provided with conicalpdepressions 8 (note Fig. 6) so as to'expose a predetermined point of the surface of the jdiamond to the action of the drill and to assist in confining the action of the drill to that point-j Means are provided to impart a rapid up and 'down motion to the work holder and to subject the diamond to a series of rapidly recurring upward thrusts toward the drill I and in contact with the point thereof; The conical depression in the die is providedwith a suitable diamond cutting abrasive, e. g. .diamond dust, and it is essentially the abrasive which effects the cutting action rather than the point of the drill, thelatter serving `toactuatje the cutting action of the abrasive.4 Thejrapid up and down movement ofthe ,workholdver 3 not only brings the diamond into Vcutting relation with the point of the drill I but also withdraws the diamond away from that point and permits the abrasive to enter the depression o r hole`made by the drill so that the hole is always'thereby replenished with the abrasive. n 1

` The vertical axis of the workholder during its movement is carefully and accurately controlled in relation to the axis of theV drill, so that the action of the drill is confined to the predetermined path through the diamond which it is desired to create and maintain.

In drilling extremely fine holes by means of a line wire, the action is preferably character- Iized by an innite number of 4relatively gentle impacts of the diamond and abrasive against the point of the rotating drill, and the invention inoludes means for creating and maintaining these relatively gentle contacts. However, in the use of an extremelyne wire serving as drill there is a tendency for the impact to bendthe wire and therefore to cause distortion,` and in accordance with the invention the excess energy of impast which tends to cause this distortion is absorbed by suitable means which will be illus-` fected xby means of a pulley driven by theV belt 2l from a driving pulley 22 which in turn is driven by a belt 2,4 and pulley xedto the shaft 26 of a motor.

Referring now to the work holder 3Y which holds the diamond and the `movement of the work holder in relation to the drill, one Way ofV securing the desired up and down movement of thework holder while maintaining the delicacy of cuttingcontact relation between the diamond andthe .drill is to suspend the work holder below l andin spaced relation to the drill by means of l a strand .30' (noteFig. 8) and to apply and maintain on this strand a predetermined tension urging the workholder upwardly andto bring the diamond on the work holder into cutting relationwith the rotating point of the' drill. Then by imparting a series of rapidly recurring downwardly' directed impulses ordisplacements to the work holder thelatter is. given an upV and down movement', the upward movement being controlled bythe tension on'the strand 30.

In order. to properly confine or define the up and down movementof the work holder so that the longitudinaland vertical axes thereof will be properly maintained in a predetermined position in f relation Ito the axis of the drill, the work holder is guided by suitable bearings and bearing mountings which may be provided with suitablelateral adjusting means so that the position yof the vertical axis of the work holder can be 3D- which at one end thereof is operatively secured to the work holder by means of the hook 32. The other end of the strand passes over a pulley 33 supported on the upright partition Il and terminates in a bucket or container 35 which maybeweighted to a desired degree, as for example by means of shot 36. The strand is inclined as shown and thereby not only servesl to provide a vertical biasing support for the work holder but also to maintain lateral pressure or bias4 of the work holder against the rollers 38 serving as guide rollers or bearings for the work holder. The bucket and shot constitute, therefore, a counter weightwhich provides the desired delicate upwardvurgeor bias ofthe work holder toxxbring the diamond thereon into cutting l.relation with the drill, anda desired prede-v termined tension on the strand may be maintained-by controlling the mass of the counter weight. Other equivalent means of providing and 'maintainingthe desired upwardly and laterallyy urging force or bias vto the work holder may be. employed, the one specifically shownV having been proved to be effective. for the ipurpose. i 'Y Referring-'again'.to Figc-,Vit will be-seen that the lever arm indicated generally at 40 and pivoted on the pivot 4I is rocked by means of the eccentric 42 mounted on shaft 43 and provides means for imparting to the work holder a rapid series of downwardly directed thrusts or displacements by Contact of the arm 45 with the stop 46 on the work holder 3 which is normally biased upwardly against said arm 45. Following each downward thrust,tl'refsaidv arm `45,;` of course moves upwardlyy and releases-the-work holder, whereupon the tension on the strand 30 urges the work holder upwardly and brings the diamond at the top of the work holder into cutting relation withthe rotating point of the drill. This upwardurge results in an upward impact of the 1 diamond and cutting abrasive against the point of the drill and tends to produce a certain amount of shock which. tends to bend the drill (when the latter consists of a ne wire) and thereby to` cause distortion. 1n order to prevent such distortion the excess energy of this impact, which is the cause of. the distortion; may be absorbed by :providing suitablel shockI absorbing means. One of suchmeansmay'be provided by equipping the rod supporting strandwith a resilient member,` as a tension Vspring 48 which imparts resilient qualities to the strand sothat the resiliency thus provided will absorb the shock and prevent the bending orv distortion mentioned.

Referring more indetail to the means for imparting a series ofrapidly recurring downward thrusts tothe work holder, this, as already indicated, may takethe form of a lever-arm 40. One form of such arm is illustrated in Figs. l, 2, 3 and 8, and this together with its associated mechanism will now be described in detail. This arm comprises an elongated leverbar provided intermediate the ends thereof and nearer one end than the other with a pivot 4 i j At the endv ofthe short arm portion of said lever 40 there is provided a right angled bracket 5D, including the arm 45 secured to the lever by means of screw 5| and positioned thereon with theaid of a flat. spring 52 secured to the short lever arm by screw 53. The other end of the lever arm is given an up and down movement. by means lof an eccentric 42 mounted on a shaft 43 driven bya pulley 55 which in turn is driven by a belt 56 from a pulley 5l' on the shaft 58. The pivot 4|.is pivoted in a housing having therein a hole 6| through which the lever arm extends. This housing is supported from a bracket E2 by means of a threaded member 64 which passes vertically through the housing in spaced relation to thelever arm, 40 and terminates at one end in the adjusting knob 6.5 and at the other end is provided with a lock nut 65. In addition to this threaded member 64 a dowel pin 68 extends through the bracket B2 and housing 6| to assist in guiding said housing in its movement. The housing is therefore vertically adjustable. The reason for this is to permit raising of the work holder in relation to the point of the drill as the drilling operation progresses so asto permit the drill to penetrate the diamond to the desired extent.

Since the drill is fixed and does not move vertically it is necessary to move the diamond upwardlyin order to eflectthel desired` penetration of the-drll. Since in the form shown in Fig. 3 the upward movement. of the work. holder is limited bythe arm 45 which acts as a stop, raising of the pivot 4I vwillrase the upwardly limited positionof the arm 45; and therefore permitthe strand 3Q. to raise the Work holder to aA corresponding. degree..

An alternative means of providingthe desired rapidly recurring downward thrusts on theiwork holderis illustrated in Figs. 10, 11 and 12.` Here insteadof rocking in a vertical plane, the levers 40` rockin a horizontal plane and this horizontal motion is converted into vertical -downward thrusts in a manner which will be shown.

Referring to'Figs. 10, 11 and 12, there is shown in Fig. 11 a group of two units including the horizontal levers 40, it being iunderstood that any number of units may be employed. Referring to Fig. 11, the levers 40 are pivoted in the housing 60 by means of verticallyaligned pivots 10 (corresponding to the horizontallyaligned.pivots 4| shown in Fig. 8), the said'levers 40 being'pivoted intermediate the ends thereof and nearer one end.

than the other. VTo the shaft 43thei-e is secured a collar 1| and an eccentric 12. Cooperating with thiskeccentric there is a bevelled T-shaped link ing motion is, of course, communicated to the short arms 11 of the levers 40 `which are provided with off-set bars 18 and a threaded contact member 19 which extends through said bars and bears against lever springs 80 `secured at one end to the bracket 82 carried by the upright partition .and provided at the other end with a shoe 83 which may be made of any relatively soft material as, for example, leather, rubber or thelike. Y As the short ends 10 of the levers 40 rock, the spring 80 on each of the levers which may be normally slightly arcuate as shown is flexed `or flattened out and thus imparts a downwardimpulse to the work holder 3, thus performing a function analogous to that performed by the vertically rocking lever 40 illustrated in Fig. 8. v

Referring now to the work holder mounting and the means for maintaining the vertical axis of the work holder in an adjustably predetermined position, during its movement, in relation to the axis of the drill, reference may be had to Figs. l, 2, 3, 4, 9, 10 and 11. The structureillustrated (note particularly Fig. 4) includes a pair of vertically aligned and spaced guide rollers 38. As specically illustrated, each member of the pair consists of two discs 85 in spaced relation rotatably mounted between needle points 88 which extend through holes in the yoke 90 and are xedly positioned therein by means of set screws 9|. The yoke 90 is supported by a bracket 92 secured to the front upright wall The internal spacing of the discs 85 in relation to each other is such that the work holder rod 3 rides on the peripheral edges of the discs 85. A horizontal threaded member 94 extends through the yoke 90 intermediate the rollers and bears against 4the surface of the `upright wall at one end, the other end of the threaded member terminating in a screw head 95. `This threaded member therefore provides lateral adjustment of the yoke 90 and rollers 38 in one direction. Adjustment in another lateral direction perpendicular tothe first mentioned direction is provided by an interiorly` threaded bushing 98 which engages the threaded member 93, as shown more clearlyfin Fig. 19. The threaded member is engaged in a threaded socket in the yoke and it will be :clear `thatgturning the adjusting nut |00on the threaded bushing 98 vprovides for lateral movement ofthe yoke 90carrying the rollers 38 in the direction of the axis of the threaded member 99. Therefore thetwo threaded members 94 and 99, the axes of which are atright angles to each other, provide for lateral adjustment of the yoke 90 in diierentjdirections and therefore enable an accurate control over the positionof the axis of the -work holder 3 in relation Ato the axis of the drill I.; Set screws |02 maintain the yoke 90` in its adjusted-position. "Now referring back to the` spindle mounting and yhousing therefor,4 as previouslyl stated the spindle isprovided with upperl and lower ball bearings I5. These ball bearingsare mounted in upper and lower housings|04 connected by an oil conduit |05 terminating at each end in threaded portions |06 engaging corresponding threaded portions in thehousings |04. From the bottom of the, lowerhousing `|04 another conduit |08 leads. to an oil collector |09 which may be an oillter. Theupper portion of the upper housing |04 communicates with an oil reservoir ||0. The `lower endof the spindle 2 carries a collar ||2, the upper `end ofthe collar being flanged and acting as an oil delector.

Asimilar oil delector ||4 is provided in the upper housing. The oil reservoir ||0 is secured to the upper housing by `means of a threaded nipple llfih engaging acorresponding socket in the-housing, By means ofthe` construction shown, oil may .fbe continuously passed in contact with the bearings and ltered and then re-used. Theupper and lower bearing housings |04 `are separated-by a cylindrical shell 6 having an opening inthe front thereof for the passage of the driving belt.` The upper part of the `cylindrical shell |16 carries threaded memberslIB and |811, one of which is provided with a threaded shankl 9 having a knob |20 at the end thereof and a brake shoe |22 at the other end secured to the threaded shank ||9 by means of a screw |23. vThe othervthreaded member ||8a is provided on the inside thereof with a smooth bore through which extends apin |24 likewise carrying a brake shoe |22a secured to the end of the pin |24 by means of a screw |25. The pin is normally held out of engagement with the pulley by means of a, spring |21. Each of the brake shoes may be brought to bear against the driven pulley to control the speed thereof and the pin |24 with the brake shoe |22a at the end may be employed as a brake to `stop the rotation ofthe spindle, if desired.

In order to observe thedrilling operation a magnifying device is desirable, one form of which is shown in Figs. 13 and 14, comprising a microscope barrel |28 supported in bracket |29 by means of the screw |30, the bracket |29 being supported upon a rod|3| which in turn is supported in a socket |32 at the top of an adjusting screw |33 provided with a knob |35. The top of the bracket is provided with a member |36 carrying depending bars |31 secured therein by means of set-screws |38.

The magnifying unit thus described can be transferred from one drilling unit to the other to observe-the drilling operation. For this purpose on each drilling unit there may be provided, as illustrated at the upper right hand corner of Fig. 11 and in Fig. 13, an adjusting and supporting member for the magnifying device. This adjusting and supporting member may comprise a bar |40 adjustably. supported in relation tothe housing |04 by means of adjusting screws |4|.

4 assetato The bar |40 is provided withholes ,Midori-receiving the bars l31 on thebracketlZS.' Thevertical position of the magnifying device can be adjusted by means of the adjusting-screw. |33.

What is claimed is:

l. Apparatus for drilling-a hole in a diamond, or'the like, comprising, in combination with a drill mounted'tov rotate about a vertical axis, a rodpositioned below said Adrill and in vertical co-axialalignment therewith and having awork holder adapted to carrya diamond'at the-upper end thereof and in spacedrelation to saidi drill; a pair of vertically spaced guide rollers; a-flexible resilient strand; a pulley; oneend of said strand being operatively securedl to the rod intermediate the ends thereof "andiintermediate the said guide rollers, the otherl end of the strand being looped over said pulley and carrying a counter weight, said strand and counter weight suspending said rod below and in spaced relation to said drill and in yielding contact withsaid guide rollers and biasing saidv work holderv upsaidyguide means comprising a pair of vertically spaced. guide rollers, each, having vertically aligned peripheral parts positioned in rolling contactwith and at one sidev of said rod in the direction of its lateral bias and thereby maintaining said rod ina predetermined vertical position in relation tothe axis of the drill, said spaced guide rollers-being mounted in a bracket and said bracket being provided with means for adjustment thereof in a horizontal plane toward and away from said rod.

4. Apparatus for drilling :a hole in a diamond comprising, in combination with a drill mounted to rotate about a vertical axis, a rod positioned below said drill and having a work holder adapted to carry a diamond at the upper end thereof :and in spaced relation to vsaid drill; a resilient and flexible strand having one end operatively sewardly to bring said diamond into cutting relation with the rotating point of said drill; means operatively connected with said rod and arranged and adapted to subject saidv rod to a series of rapidly recurring downwardly directed displacements in opposition to said upward bias, whereby a rapid up and dawn movement is-` imparted to the rod and the diamond is subjected to av series of rapidly recurring upwardly directed diamond cutting contactsrwith the point of said rotating drill. Y

2. Apparatus for drilling a hole ina diamond comprising, in combination with a drill mounted to rotate about a verticalaxis, a rod'positioned below said drill and having awork holder adapted to carry a diamond in spaced relation to said drill; counterbalanced suspension means suspending said rod in counterbalfanced condition below and in spaced relation to said drill, and arranged and adapted to bias said rod upwardly and laterally; means operatively arranged 'in relation to said rod and adapted to subject said rod to a seriesV of rapidly recurring downwadly directed displacements in opposition-to said-upward bias, whereby a rapid up and down movement is imparted to the rod; and guide means-to determine the path of said up and down movement of the rod so that the axis thereof moves through a predetermined vertical path in relation tothe'axis of the drill, said guide means-comprising a pair of vertically spaced guide rollers, each having vertically aligned parts positionedk inrolling-contact with and at one side of'said rod'in the direction of its lateralbias and thereby` maintaining said rod in a predetermined'vertical'position in relation to the axis of the drill.

3. Apparatus for'drilling al hole in adiamond comprising, in combination with a drill mounted' to rotate about a vertical'axis, a-.rod positioned below said drill andv having a work holder adapted to carry al diamondin spaced relation to said drill; suspension means suspending said rod below and in spaced relation to said-drill, and-tending to bias said rod upwardly and laterally; means operatively arranged in relation toY said-froci and adapted to subject said rod to a series Aofrapidly recurring downwardly directed displacements in opposition to said upward bias, whereby a rapid up and down movement isl imparted to `the rod; and guide meansto determine the path of said upand downV movement of thek rod so that the axis-thereof moves through apredetermined verticalpathfin relation to the axis ofthe'- drill,

cured to said rod, counterweight means secured to the other end of the strand arranged and adapted to bias said rod axially upward and laterally; means operatively arranged in relation to said rod` and adapted to subject said rod to a series of rapidly recurring downwardly and axially directed displacements in opposition to said upward bias whereby a rapid up and down movement is imparted to the rod and whereby the diamond is. subjected to a series of rapidly recurring upwardly directed diamond cutting contacts with the point of said rotating drill, the resiliency of the strand absorbing shock resulting from said upwardly directed movements of the rod; and guide means arranged and adapted to laterally engage said rod to define the path of movement of the rod in a predetermined vertical path in relation to the axis of the drill.

5. Apparatus for drillingl arhole in a diamond comprising, lin combination with a drill mounted to rotate about a vertical axis, a rod positioned below said drill and having a work holder adapted to carrya diamond at the upper end thereof and in spaced relation to said drill; a flexible and extendable rod supporting strand having one end operatively secured to said rod, counter weight means secured to the other Vend of said strand to bias said rod axially upward and laterally; a pivotally mounted rocker arm; means to rock said armvat a predetermined rate; means at one end of said rocker arm and operatively connected to said rod to deliver a series of rapidly recurring downwardly and axially directed displacements to said rod in opposition to said upward bias, whereby a rapid up and down movement is imparted to the rod and the diamond is subjected to a series of rapidly recurring upwardly directed diamond cutting contacts with the point of said rotating drill; and guide means arranged and adapted to laterally engage said rod to define the path of movement ofV the rodin a predetermined vertical path in relation to the axis of the drill.

6. In apparatus `for drilling a hole in a diamond or the like, whereina drill carrying spindle is supported at a xed operating level and awork holder andl a vertically disposed work holder supporting rod are arranged beneath said spindle with said rod in vertical co-axial alignment therewith, the combination of vertically spacedy guide rollers laterally engaging said rod at one side thereof, 'a pulley, a flexible strand having one end portion vsecured to said rod and the other endportion passing over said pulley, acounterweight secured to said other end portion of said strandand arranged and adapted-to substantially counterbalance the weight' of said rod and work holder and to bias the same upwardly, and a tension spring interposed in said strand between said pulley and the rod, said pulley, said strand, said counterweight and said spring being positioned at one side of said rod and arranged and adapted to bias said rod laterally toward said guide rollers. A

7. In apparatus for drilling a hole in a diamond or the like, wherein a drill carrying spindle is supported at a iixed operating level and a work porting rod are arranged beneath said spindle with said rod in vertical co-axial alignment therewith, the combination of vertically spaced guide rollers laterally engaging said rod at one side thereof, a pulley, a iiexiblestrand having one end portion secured to said rod and the other end portion passing over said pulley, a counterweight secured to said other end portion of said strand and arranged and adapted to substantially counterbalance the weight of said rod and work holder and to bias the same upwardly, a tension spring interposed in said strand between said pulley and the rod, said pulley, said strand, said counterweight and said spring being positioned at one side of said rod and arranged and adapted to bias said rod laterally toward said guide rollers, a stop extending laterally from said rod, an oscillating arm having an end prtion normally positioned to engage and limit the extent of upward movement of said stop, and means operatively connected to said arm and arranged and adapted to rapidly actuate said arm and thereby impart rapid vertical oscillation to said stop and said rod.

8. Apparatus for use in drilling line wire dies comprising in combination, a die holder, means supporting the die holder for movement inr a drilling axis to and from a drilling position, a counterweight connected to the die holder and support and arranged to give slight bias of the die holder toward drilling position, means to impart to the counterweighted die holder recurring impulses to move the die holder repeatedly away from drilling position, and means in addition to the counterweight acting on the die holder to absorb and dampen said repeated movement.

9. Apparatus for use in drilling line wire dies comprising in combination, a die holder, means supporting the die holder for movement in a drilling axis to and from a drilling position, a counterbalance comprising the die holder and support and comprising a counterweight movable as a whole and arranged to bias the die holder and support toward drilling position, means to impart to the counterbalance recurring impulses to move the die holder repeatedly away from drilling position, and means in addition to the counterweight acting on the die holder to absorb and dampen said repeated movement.

10. Apparatus for use in drilling fine wire dies comprising in combination, a die holder, means supporting the die holder for movement in a drilling axis to and from a drilling position, the die holder being arranged for repeated movement away from drilling position under recurring imholder and a vertically disposed work holder suppulses, a counterweight adapted to give slight bias of the die holder toward drilling position, connecting means between the counterweight and the dieholder comprising resilient means to absorb and dampen said repeated movement of the die holder under recurring impulses.

11. Apparatus for use in drilling fine wire dies comprising in combination, a die holder, means supporting the die holder for movement in a drilling axis to and `from a drilling position, the die holder being arranged for repeated movenient away from drilling position under recurring impulses, a counterweight adapted to give slight bias of the die holder toward drilling position,

connecting means between the counterweight and the die holder comprising yielding means to absorb and dampen said repeated movement of the die holder under recurring impulses.

12. Apparatus for use in drilling ne wire dies comprising in combination, a die holder, means supporting the die holder for movement in a drilling axis to and from a drilling position, the die holder being arranged for repeated movement under recurring impulses away from drilling position, a counterweight adapted to give slight bias of the die holder toward drilling position, connecting means between the counterweight and the die holder comprising a flexible strand and spring to absorb and dampen said repeated movement of the die holder under recurring impulses.

13. Apparatus for use in drilling fine Wire die comprising in combination, a die holder, means supporting the die holder for movement in a drilling axis to and from a drilling position, the die holder being arranged for repeated movement under recurring impulses away from drilling position, a counterweight adapted to bias the die holder toward drilling position, connecting means between the counterweight and the die holder comprising a flexible strand, the strand being mounted over a pivot, the connecting means also comprising a tension spring to absorb and dampen said repeated movement of the die holder under recurring impulses.

14. Apparatus for use in drilling ne Wire dies, comprising in combination a die holder, means supporting the die holder for movement in a drilling axis to and from a drilling position, the die holder being arranged for repeated movement under recurring impulses away from drilling position, means for imposing a substantially constant counterbalancing force on the die holder and resilient means lesser in degree than the counterbalancing force between the die holder and the counterbalancing means to absorb and dampen said repeated movement of the die holder under recurring impulses, and means to impose said recurring impulses on the die holde1` between the die holder and the counterbalancing means, whereby the counterbalancing means remains in substantially constant position during said repeated movement of the die holder.

LEO ROBBINS.

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