US2472968A

US2472968A - Drilling machine

Info

Publication number: US2472968A
Application number: US448554A
Authority: US
Inventors: Goldberg Herman
Original assignee: Individual
Current assignee: Individual
Priority date: 1942-06-26
Filing date: 1942-06-26
Publication date: 1949-06-14
Anticipated expiration: 1966-06-14

Description

/ June 14,1949. H, GQLDBERG 2,472,968

DRILLING MACHINE Filed June 26, 1942 15 Sheets-Sheet 1y @7&9 72

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DRILLING MACHINE Filed June 26, 1942 15 Sheets-Sheet 2 June 14, 1949. H. GOLDBERG 2,472,968

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Filed June 26, 1942 June 14, 1949. H. GoLDBERG DRILLING MACHINE gli..

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DRILLING MACHINE Filed June 26, 1942 15 Sheets-Sheet 10 |27 |b |95 ,99a Cjgg oe June 14, 1949.

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DRILLING MACHINE Filed June 2e,- 1942 15 Sheets-Sheet 11 June 14, 1949. H. GOLDBERG 2,472,968

DRILLING MACHINE Filed June 26, 1942 15 Sheets-Sheet 12 June 14, 1949. H. G0L-DEER@ 2,472,968

DRILLING MACHINE Filed June 26, 1942 15 Sheets-Sheet 13 d, 28o mb 60 2&0/

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UNITED STATES PATENT OFFICE DRILLING MACHINE Herman Goldberg, Chicago, lll. Application June 26, 1942, Serial No. 448,554 7 claims. (cmi-32)' This invention relates to improvements in drilling machines and other machine tools in which a rotatable tool, such as a drill or a tap, has relative axial movement with respect to the work during the operation of drilling, tapping orthe like. The present invention is an improvement upon that described and claimed in my copending application, Serial No. 323,648, illed March 13, 1940.

The principal object of the present invention is to provide an improved machine of the type referred to in which the axial movements of the rotating tool with respect to the work are effected, preferably, 'by compressed yair or the like and controlled by the movement of the work to the position where it is to be en'gaged by the tool. Thus, in one embodiment of the invention, the tool is rotated by an electric motor and is moved axially with respect to the work by compressed air which is controlled by the movement of the work piece, to the position where it is to rbe engaged by the tool, to initiate the axial movement of the tool, means being provided for cushioning and automatically arresting the tool at the end of its working stroke and for promptly and automatically reversing the directionof its axial movement. This control of the initiation of the work cycle by the movement of the work piece is termed piece part control and it may supplement or be replaced by a manual operation, herein'termed foot pedal control. It is also possible to adjust the apparatus so that after its operation has been initiated by either one of the methods just referred to, it will perform a continuing series of work cycles without interruption.

In this improved machine. the movement of the tool to and through the work is eifected by a resilient yielding pressure which is capable of permitting the movement of the tool to be entirely arrested in the event that unusual resistance is encountered during the drilling, tapping or other work operation, thereby substantially eliminating danger of breakage of the tool. The tool is returned aft-er each forward stroke by a constantly applied/air pressure which is overcome,

circuit which is associated with the air pressure system by which the tool is actuated in such a manner that manual control, piece -part control, or continuous automatic operation, of the apparatus may readily be brought into use at the will of the operator. This electric circuit may include a starting switch which is normally closed in one position by a part of the machine when the tool is fully retracted and which is then connected in a branch circuit adapted to be closed either by the temporary actuationof a foot pedal control switch or of a piece' part control switch to energize a solenoid for opening a valve in the compressed air system and thereby initiating the forward stroke of the tool. After a predetermined limited movement of the tool, the starting switch moves automatically to another position, thereby shunting the foot pedal control and piece part control switches and continuing the -circuit of the solenoid through an operating switch which maintains one closed position until the tool reaches the end of its forward stroke whereupon the operating switch is moved automatically to another position, thus deenergizing the solenoid and closing the circuit. of an exhaust solenoid which then opens an exhaust valve and permits the air which produced the forward stroke to exhaust to the atmosphere while allowing the continuously applied back air pressure to produce a return stroke of the tool. At the end of the return stroke bothv the starting switch and the operating switch are returned to their initial positions bythe movement of a part of the machine so that conditions are then in readiness for another forward stroke when the foot pedal control switch or the piece part control switch is again Y actuated. The circuit may preferably include a safety switch controlled by convenient means, such as the foot pedaLfor permitting the operator in case of emergency to open the circuit of the operating solenoid and temporarily establish the circuit of the exhaust solenoid, thereby restoring all of the parts to their initial positions. The circuit also includes branches containing solenoids for controlling the actuation by compressed air of a work holder or fixture and comprising, switches for causing these solenoids to be actuated in response to the movement of the tool so that the work piece is automatically gripped in advance of the commencement of the working stroke ofthe tooland is released when the work is iinished Another object is to provide an improved l work holder having means for gripping the work,

a switch actuated by the work piece for controlling the operating circuit of the machine and means for electing the work piece when the work is completed. Other objects of the invention are to provide various improved features of construction and arrangement which will appear more fully hereinafter.

The nature of the invention will be understood from the following speclcation taken with the accompanying drawings in which one embodiment of the invention and modifications of certain of its parts are illustrated. JIn the drawings,

Figure 1 shows a front elevation of a drilling machine embodying the present invention;

Fig. 2 shows a side elevation of the machine illustrated in Fig, 1, looking toward the left as viewed in Fig. l;

Fig. 3 shows an enlarged side elevation oi a portion of the upper part of the machine illustrated in Fig. 1, with parts thereof shown in vertical section in a plane at right angles to the view of Fig. 1; v

Fig. 4 shows a vertical section on the line 4 4 of Fig. 3;

Fig. 5 shows a horizontal section taken on the line 5--5 of Fig. 3;

Fig. 6 shows a horizontal section taken on the line 6-6 of Fig. 3;

Fig. '7 shows a horizontal section taken on the line 1 1 of Fig. 3;

Fig. 8 shows a vertical section on the line 8-8 of Fig. 5, illustrating the dash-pot retarding mechanism in its retracted condition;

Fig. 9 shows a horizontal section on the line 9-9 of Fig. 8;

Fig. 10 shows a vertical section taken on the line lll-l0 of Fig. 7;

Fig, l1 is a partial sectional view similar to that of Fig. 8, showing the dash-pot retarding mechanism after it has been partially actuated at the end of a working stroke;

Fig. 12 is a partial front elevation of the upper part of the machine shown in Flg. 1, with the front cover plate removed to reveal the starting and operating switches and the dash-pot mechanism, and with the working cylinders shown in vertical axial section;

Fig. 13 shows a top plan view of one of the double contact switches, such as the starting switch or the operating switch;

Fig. 14 shows a vertical longitudinal section taken on the line II-'II of Fig. 13;

Fig. 15 shows a bottom plan view of the casing and operating parts of the switch illustrated in Fig. 14 with the lower cover plate removed;

Fig. 16 shows a vertical section taken on the lineVIB-IG of Fig. 14;

Fig. 17 shows an enlarged front elevation of the switches and dash-pot retardlng mechanism shown in Figs. 5 and 12, a portion of the casing being broken away to show a safety relief valve connected with the upper ends of`the Working cylinders;

Fig. 18 shows a side elevation of the structure illustrated in Fig. 17, looking toward the right as viewed in Fig. 17;

Fig. 19 is a detail vertical section taken on the line lil-I9 of Fig. 17;

Fig. 20 is a diagrammatic view showing the principal parts of the machine and their operating connections with the compressed air system and with the electric circuit by which the operation of the machine is controlled;

Fig. 21 is a vertical section through the side wall of the upper part of the housing shown in Fig. 2, illustrating in side elevation the pressure regulators, pressure gaugesl and valve mechanism for controlling the admission of compressed air to the upper and lower ends ci the working cylinders;

Fig. 22 shows a front elevation of the apparatus shown in Fig. 21 with the side wall of the housing removed;

Fig. 23 shows a top plan view of the apparatus shown in Figs. 21 and 22 with the housing illustrated in horizontal section; A

Fig. 24 shows a vertical section taken on the line 24-24 of Fig. 23;

Fig. 25 shows a rear elevation of the apparatus which is shown in front elevation in Fig. 22;

Fig. 26 shows a sectional view taken on line 26-26 of Fig. 24;

Fig. 27 shows a sectional view taken line 21-21 of Fig. 2l;

Fig. 28 shows a sectional view taken line 28-28 of Fig. 29;

Fig. 29 shows a sectional view taken line 29-29 of Fig. 26;

Fig. 30 shows a sectional view taken line 30-30 of Fig. 31; 1

Fig. 3l shows a. sectional view taken line 3I-3I of Fig. 26;

Fig. 32 shows a vertical section taken line 32-32 of Fig. 3l;

Fig. 33 shows a vertical section taken line 33-33 of Fig. 31;

Fig. 34 shows a vertical section taken line 34-34 of Fig. 33;

Fig. 35 shows a vertical section taken on thc line :i5-35 of Fig. 22, illustrating the regulating valve in the operating pressure line;

Fig. 36 is a horizontal section through a casing carried by the main frame of the machine, illustrating top plan view of the solenoid valves for controlling the exhaust of compressedl air from the tool operating cylinders and for controlling the admission of compressed air to and the exhaust from a work holder or xture;

Fig. 37 shows a side elevation of the foot pedal switch and its connections to the solenoid casing, with parts of the foot pedal structure broken away and illustrated in vertical longitudinal section;

Fig. 38 shows a vertical section taken on the line 38-38 of Fig. 37;

Fig. 39 shows a vertical section through the lower portions of the solenoid valves for controlling the work holder or fixture, the section being taken on the line 39-39 of Fig. 36;

Fig. 40 shows a vertical section through the lower portion of the solenoid valve for controlling the exhaust from the tool operating cylinders, the section being taken on the line 40--40 of Fig. 36;

Fig. 41 shows a sectional view taken on the line Il -llof Fig. 39;

Fig. A42 shows a partial top plan view of the form of work holder or ixture shown on the platen of the drilling machine illustrated in Figs. 1 and 2, with the work piece fully inserted to the position where it is to be engaged by the drill and wherein it has closed the operating circuit of the tool cylinders;

Fig. 43 shows a complete top plan view similar to that of Fig. 42, showing the work piece partially inserted and before it has actuated the switch to close the starting circuit;

Fig. 44 shows a partial front elevation of the work holder shown in Figs. 1, 42 and 43 with the tool in its elevated position;

the

on the on the on the on the on the on the on the on the Fig. 45 shows a vertical section taken on the line 45-45 of Fig. 43:

Fig. 46 shows a vertical section, similar to that of Fig. 45, through a modified form of work holder or fixture in which the work piece is adapted to be gripped and held by the action of compressed air or the like during the drilling operation;

Fig. 47 is an elevation of the chuck or cradle in which the work piece is mounted in the structure illustrated in Fig. 46, the view being taken in the direction of the arrows 41-41 in Fig. 46;

Fig. 48 shows a vertical section taken on the line 48-48 of Fig. 46;

Fig. 49 shows a front elevation of another form of work holder or fixture in which the work piece is held by pneumatic pressure and in which provision is also made for controlling the electric circuit of the tool operating system in response to the movement of the work piece;`

Fig. 50 shows a sectional view taken on the line 58-58 of Fig. 49;

Fig. -1 is an enlarged sectional view similar to that of the upper portion of Fig. 50 but with the work piece inserted to close the circuit, and illustrating by dotted lines the position of the work piece after it has reached its nal position wherein it is engaged by the tool; and

Fig. 52 is a vertical section taken on the line 52-52 of Fig. 50.

The invention is illustrated in the drawings as being embodied in a drilling machine comprising a hollow cast metal housing 55 having an enlarged base 55B adapted to be secured upon a floor or other support. The front of the casing 55 is provided with a vertical slotted guideway 55h on which there is mounted a vertically adjustable bracket 56 formed integrally with the work table 51 on which the piece of work 58 to-be drilled is supported by a work holder or fixture 68 secured in adjusted position on the work table by cap screws 6I. The bracket 56 and the table 51 are adapted to be adjusted vertically by a worm and worm wheel operated by a crank 62 and when the table 51 has reached the desired level, it is secured in adjusted position by a clamp 63. The work piece 58 is adapted to be engaged and acted upon by a drill 65 carried by a pressure platen I I8 which, in turn, is actuated by compressed air or the like acting through fluid pressure mechanism carried by a cylinder block 61 which is secured to the front face of the upper part 55c of the housing 55. The cylinder block 61 has mounted on the front thereof a valve block 66 which carries the dash-pot mechanism by which the downward stroke of the drill 65 is retarded and arrested at the end of the stroke and which has mounted therein the starting and

operating switches

68 and 69, respectively, which control the circuit of the solenoid switches by which the operation of the cylinders is controlled. These switches and the major portion of the block are normally enclosed by a casing 18 which is attached to the block by screws 1 I.

As shown in Fig. 2, the upper end of the cylinder block is closed by a pulley casing 12 which is secured to and also encloses the upper end of the housing 55. Within the upper portion 55 of the housing 55 there is mounted an electric motor 13 which is connected to elect the rotation of the drill 65 through the mechanism hereinafter described- The electric connections for operating this motor are brought in through the rear of the housing and the starting and stopping of the motor are controlled by push buttons 14 and located in the side of the hou/sing. The circuit f this motor is so arranged that lt may be operated at two diierent speeds which may be regulatedby the push buttons 16 and 11, also located in the side of the housing. The electric. connections thus brought into the housing may also serve to supply another electric motor 18 mounted outside of and adjacent the lower end 'of the housing which drives a pump 19, mounted in the base portion 55sl of the housing, by which oil or the like is elevated through a pipe 8 8 and discharged through a nozzle 8| onto the work piece 58 at the point where it is engaged by the tool 65. The

circuit of the pump motor 18 is controlled by a switch comprising the

push buttons

82 and 83 mounted in the side wall of the housing.

At one side of the base of the housing 55 there is mounted a foot pedal de vice 85 which includes the foot pedal switch and the safety switch and which has its connections extending through a flexible conduit 86 to the interior of the` housing where connections are made with other parts of the operating circuit including the starting and

operating switches

68 and 69, respectively, and the solenoid valves which control the admission to and exhaust from the cylinders in the block 61 of the compressed air by which the tool 65 is moved toward and from the work. The air pressure for effecting the downward or working stroke of the tool pistons is admitted to the upper ends of the cylinders in the block 61 through a pressure regulator 88 located in the housing and operated by a handle 88a projecting through the side wall of the housing as shown in Fig. 2. The compressed air which is constantly applied for effecting an upward stroke of the tool pistons when the down pressure is released flows to the lower ends of the cylinders through a pressure regulator 89, also located in the housing and having an operating handle 898L projecting through the side wall of the housing adjacent the handle 88a. The pressure in the line in which the regulator 88 is connected is indicated by a gauge 90 and the pressure in the line in which the regulator 89 is connected is indicated by a gauge 9 I, these gauges being located in apertures in the side wall of the housing as shown in Fig. 2. The rates ofiiow of compressed air through the

regulators

88 and 89 are regulated by metering valves having stems 92 and 93, respectively, projecting through the housing wall below the

handles

88a and 89% and adapted to be engaged by wrenches or the like. The Aconnections of the compressed air lines for operating the tool and` for effecting other functions are shown diagrammatically inv Fig. 20 and will be explained more fully hereinafter.

The foregoing description has been intended to give a general view of the principal parts -of the machine before describing `the details of any of these parts and reference will now be made to the driving connections between the motor 13 and the drill 65 which are best illustrated in Figs. 2, 3 and 4. As there shown the motor is suspended in an aperture in the top wall of the housing with its outwardly turned anges 138L resting upon the flanges 551 of the housing and provided with slots engaged by studs secured in the flanges 55d. These studs are engaged by nuts 94 which secure the motor in the desired position.. By this arrangement, the motor may be shifted in the housing in order to tighten the belt 95 which extends around a pulley 96 fixed on the shaft 13b of the motor and another pulley 91 which is fixed upon a vertical hollow pulley shaft 98. The

pulleys

96 and 91 are constructed in the form of truncated cones being provided with oppositely disposed series of grooves 86a and 91 of different diameters, the larger grooves of each pulley being located opposite to the smaller grooves oi' the other pulley so that by shifting the belt 95 to different pairs of grooves the speed of the pulley shaft may be varied to cause a corresponding variation in the speed of rotation of the tool 65, in` addition to the variation which may be obtained by regulating the speed of the motor through the electrical connections controlled by the push buttons 16 and 11. The shifting of the belt may be effected by moving the motor on its support and then restoring the motor to the proper position.

As shown in Fig. 4, the pulley 91 islsecured by a key 99 upon the upper end of the pulley shaft 68 and it is arranged to seat upon an annular shoulder 08a formed on an enlargement of the shaft. The upper end of the shaft is threaded for engagement by a nut which is adapted to seat in a recess formed in the upper end of the pulley 91 and which holds the pulley against endwise displacement on the shaft. The upper end of the cylinder block 61 is providedv with a central bore 61a in which the pulley shaft 68 is journaled through the intermediate sealed ball bearing unit |0|. The upper end of the bore 61* is closed by a cover plate |02 secured to the upper end of the cylinder block and the lower end of the pulley shaft 98 is threaded for engagement by a pair of lock nuts |03 which hold the pulley shaft against vertical displacement in its bearing. The cylinder block is secured to the iace ofthe housing 55c by studs |04, shown in Fig. 5, which pass through apertures inthe cylinder block and engage threaded holes in the bosses 55 which are formed on the upper end of the housing. The cylinder block is held against lateral displacement by a. vertical key engaging keyways in the cylinder block and in the housing and held against vertical displacement by a screw |0511. In this manner, the cylinder block and the pulley shaft are mounted in relatively flxed positions and the shaft is adapted to be operated continuously, when the motor 13 is in operation, for driving a vertically adjustable drill spindle |01 which has a splined engagement with the lower end of the shaft 98, as shown at sab in Fig. 4.

The drill spindle through the central tubular bore of a pressure platen I|0 which is adapted to be actuated for moving the drill spindle |01 longitudinally and thereby moving the drill 65 toward and from the work. 'I'he pressure platen ||0 comprises an upwardly extending sleeve ||0 adapted to slide vertically in a bushing mounted in the central bore' 61h of the cylinder block, and the drill spindle |01 is journaled in a ball bearing unit |'2 secured in the intermediate part of the sleeve portion H0 of the pressure platen. This ball bearing unit ||2 seats against a shoulder at the end of the lower enlarged portion |01a of the drill spindle and is held in position against that shoulder by a nut ||3 engaging a threaded portion |01b of the spindle |01. The pressure platen ||0 is also provided with a downwardly extending sleeve I'llb and the lower part of the drill spindle |01 is journaled in another ball bearing unit ||4 which is mounted in the lower end of the sleeve H0". The outer ring of this ball bearing unit seats against an annular shoulder formed on and extending inwardly from the surrounding sleeves and it is held in position against that shoulder by a nut ||5 engaging the internal threads on the lower end of .the sleeve, as illustrated in |01 extends downwardly` Fig. 4. An annular shoulder |01d is formed on the lower end of the spindle |01 within the nut I5 and the inner ring of the ball bearing unit is clamped against this shoulder by a pair of nuts 5 I6 engaging the threaded portion |'01 of the drill spindle. A standard drill chuck |08 is mounted upon the tapered lower end or the drill spindle for detachably securing in place, in'axial alignment with the spindle, the previously described drill 65. A nut |08 is mounted on the threaded portion |01f o f the spindle and may be rotated to force the chuck off of the spindle when desired.

The pressure platen' ||0 is provided with two similar oppositely extending arms ||0d which are apertured for engagement by the reduced extremities of piston rods |20, the rods being secured in fixed relation to the arms of the pressure platen by nuts |2| which are secured on the threaded ends of the piston rods. The piston rods |20 slidably engage apertures in cylinder heads |'22 mounted in the lower ends of the cylinders 61c which are formed in the cylinder blocks 61 at 'opposite sides of the central bore 61h, the cylinders being equidistant from the axis of the drill spindle |01. At their upper ends, the piston rods |20 are secured to pistons |23 which have sliding ts within the cylinders 61. The upper ends of the cylinders 61 are closed by detachable threaded plugs |24 and these plugs are accessible through the chambers 61d which are formed in the upper end of the cylinder blocks and throughv which access may be had to the plugs |24 when the pulley casing 12 is removed. The pistons |23 are normally moved to their upper positions, as shown in Fig. 4, by the action of compressed air which is admitted to the lower ends of the pistons from a pipe |25, shown in Fig. 20, which communicates with branch pipes |25, shown particularly in Fig. 6, connected by threaded nipples |26 with passages 61 extending through the cylinder blocks 61 and connecting with the lower ends of the cylinders 61, as illustrated in Fig. 3. The upward movement of the pressure platen I0 is limited by the engagement of the annular shoulder ||0e of the pressure platen with the lower end of the cylinder block, as shown in Fig. 4, and when the pressure platen is in this upper position the pistons |23 are spaced slightly below the plugs |24 which close the upper ends of the cylinders.

In order to move the pistons |23 downwardly against the constantly acting pressure of the compressed air admitted to the lower ends of the cylinders 61 through the pipe |25, compressed air having a higher pressure than that in the lower ends of the cylinders is admitted at intervals to the space between the pistons |23 and the plugs |24 at the upper ends of the cylinders and this air at a higher pressure is supplied through a pipe |21, shown in Fig. 20. As shown in Figs. 7, 10 and 12, this pipe is connected through a nipple |28 with a passage 66a extending vertically in the Valve block 66. This passage communicates with an inclined passage 66h, shown in Fig. '1, which in turn communicates with a passage |1t formed in the face of the block 61 behind the block 68. The passage 61f communicates with branch passages 61g which lead to the upper ends of the cylinders 61 so that 70 when compressed air is admitted to the valve block 68 through the pipe |21 the pistons |23 are actuated to move the pressure platen ||0 downwardly and thus carry the rotating drill 65 into engagement with the work piece 58. As the pistons |23 move downwardly, the sleeve lli)a of the pressure platen slides in the bushing and the drill spindle |01 slides downwardly within the hollow pulley shaft 98 while maintaining its splined connection therewith.

Having described the principal structural features ofthe drilling machine mechanism, reference will now be made to the mode of operation of the machine and to-the general characteristics of the controlling system before describing the structural features of the individual parts which are embodied in the controlling system. The circuit connections of the compressed air or other fluid pressure system by which the pistons |23 are actuated and by which the tool holder may be operated, as well as the electrical con- ;nections by which the fluid pressure system is controlled, are shown diagrammatically in Fig. where the branch pipes |25a are shown as extending directly from the pipe to the lower ends ofthe cylinders 61 while the pipe |21 leading to the upper ends of the cylinders is shown as being connected by `branch pipes directly tol the upper ends of the cylinders, these branch pipes correspondingto the passages 61g and other associated passages which are illustrated in Figs. 7 and 10. The compressed air for effecting the forward and reverse strokes of the pistons |23 is supplied through a pipe which leads from an air compressor or other source of supply to a metering block |3|. From this metering block |3I, compressed air flows through the pressure regulator 88 and gauge 90and then through the pipe |52 and back through the metering block and metering passages to the pipe |21 by whichI the compressed air is supplied to the upper ends of the cylinders 61C. By adjusting the regulator 88, the pressure of this 'air may be caused to assume any desired value, as indicated by the gauge 90, and the rate of flow may be regulated by adjusting the metering screw 92 in order to cause the desired rate of ow of the air and thereby regulate the speed of the downward strokes of the pistons |23. Compressed air also flows from the passages of the metering block |3| through the pressure regulator 89 and the gauge 9| at a lower pressure to the pipe |25 which leads through branch passages to the lower ends of the cylinders 61, for retracting the pistons and for normally holding them in their retracted positions.

On the top of the metering block |3|, there is mounted a solenoid-operated valve |33 having its stem extending into the chamber of the metering block for controlling the admission of compressed air through the passages of the block to the passages which lead to the pipe |21 by which compressed air is supplied to the upperJ ends of the cylinders. This solenoid-operated valve is normally closed but when it is opened, due to the energizatlon of the solenoid, compressed air, having a higher pressure than that which is constantly applied to the lower ends of the pistons, is admitted to the upper ends of the cylinders to effect a downward stroke of the pistons |23and'the corresponding downward movement of the rotating drill 65. This action takes place against the lower pressure which exists in the lower endsof the cylinders 61s, the

pressure regulators

88 and 89 being so adjusted that the air pressure in the ylowerends of the cylinders will be overcome by thehigher air pressures in the upper ends of the cylinders during the downe their upper positions,

ward strokes ofthe pistons so thatthe lpistons are enabled to-complete their full forward strokes during which there may be further compression 'and also to permit some .this switch is normally |48, a conductor of the low pressure air which is trapped in the Y lower ends of the cylinders. During this downward stroke the compressed air in the upper ends of th'e cylinders moves the tool 65 with a yielding pressure and maybe further compressed by the retardation of the tool lf unusual resistance is encountered. It may be desirable to permit some leakage of air around the pistons |23 from the lower sides of the pistons to their upper sides leakage of air around the piston rods |20 where they pass through the cylinder heads |22, by omitting the use of packing, which will facilitate the operation of the machine with comparatively low air pressures on the upper sides of the pistons. At the end of the downward strokes of the pistons, the air in the upper ends of the cylinders which produced the downward strokes is permitted to exhaust through the pipe |21 and metering block |3| and through an exhaust pipe |34- whichleads from the block |3| to the block |6| of an exhaust solenoid valve |35, the rate of the exhaust and the speed of the return stroke being regulated by the metering screw 93, which may be adjusted to control the ow of the exhaust air through a restricted passage. When the pistons |23 have reached the lower or forward ends of their strokes, the solenoid |35 is actuated to open a valve in the block |35a and thus permit the air to exhaust to the atmosphere through a pipe |36 so that the back pressure continually supplied to the lower ends of the cylinders 61 through the pipe |25 may then effect an upward or reverse stroke of the pressure platen ||0 of the pistons connected thereto.

The operation of the apparatus may be initiated either by closing a switch |31 which forms a part of the foot pedal device or by the closing of a switch |38 which forms a part of the work holder 60 by which the work piece is supported during the drilling operation. The rst of these methods may be termed foot pedal control while the latter method constitutes the "piece part control previously referred to. In order that the method of control may be selected as desired, the switches 31 and |38 are connected in a circuit which includes a controller |39 having a contact arm |39a and three contacts, |39", |39 and |39d. When the arm |39a is on the middle contact |39c, the foot pedal switch |31 is included in the operative circuit and when this normally open switch is closed by pressing onl the foot pedal the conductor |40 leading from the contact |39c is connected to another conductor |4| which leads to the upper contact 68b of the starting switch 68 previously referred to. The movable arm 68B of in its upper position enwhen the pistons |23 are in as shown in Fig. 20, so that the circuit is then continued from the switch 68 through the conductor |42 to one terminal of the solenoid-operated switch |33. The other terminal of this solenoid-operated switch |33 is connected through a conductor |43 to one of the line conductors |44 leading from a source of supply of electric current. The other supply conductor |45 has connected therein a manually operated switch |46 and, assuming this switch to be closed, the circuit is then completed from the other line conductor through a branch conductor |41 to a safetyswitch |48 which also forms a part of the foot pedal device 85. From this safety switch |49 leads to the movable arm 69 of the operating switch 69 which arm engages the upper contact |59b when the pistons gaging the contact 68b