US2285361A - Electric reciprocating tool - Google Patents

Description

June 2 1942- H. c. Ron-:Rs :TAL I2,285,361

ELECTRIC REC IPROCATING TOOL Filed July 6, 1939 2 Sheets-Sheet 1 INVENTOR HERBERT Q ROTERS ROLD C. PASI ATTORN EY June 2, 1942. H. C. ROTERS Em 2.285,361

ELECTRIC RECIPROCATING TOOL Filed July 6, 1959 2 Sheets-Sheet 2 VENTOR RBERT C. ROTERS ROLD C. PASINI ATTORNEY Patented June 2, 1942 ELECTRIC RECIIROCATING TOOL Herbert C. Roters and Harold C. Pasini, Hoboken, N. J., assignors to Standard Electric Hammer Corporation, a corporation of New Jersey Application July 6, 1939, Serial No. 282,965

Claims.

This invention relates to electric reciprocating tools and, more particularly, to such tools comprising two relatively reciprocable magnetic cores.

Heretofore there have been devised numerous reciprocating electric tools which may be divided into two general classes: rst, that in which the reciprocation is effected by a double-acting electroniagnet actuating a single movable magnetic working plunger; second, that in which one direction of motion of the working plunger relative to a cooperating plunger is elected by an electromagnet and the return motion by a spring which is stressed during the working stroke of the electromagnet. The present inventionA relates to electric tools of the latter type. In electric tools of this type, the working plunger carrying the tool striking member is normally separated from a cooperating plunger, which may be either iixed or reciprocable, by some form of spring device. When the electromagnet is energized, the magnetic force causes the working plunger to move toward the other plunger closing the working air gap and striking the tool. At the same time the spring which acts to maintain the plungers in their normal separated position is stressed so that it is effective to return the working plunger to its normal position when the winding is de-energized. The constants of the spring and the mass of the reciprocating member are so chosen that the natural period of mechanical oscillation is substantially equal to the period at which the electromagnet is intermittently energized. The working plunger is generally mounted for reciprocatory movement in a cylindrical shell or bearing of nonmagnetic material, for example, brass.

However, the electric tools of the prior art, particularly of the type just described, have a number .of disadvantages which tend substantially to decrease their eiiiciency and increase their cost. Among these disadvantages are eddy currents of substantial amplitude set up in the conductive parts of the unit in, or in the vicinity of, the magnetic iield; for example, in the brass shell in which the striking plunger is mounted. The losses due to such eddy currents considerably increase the power requirements for a given mechanical power output and also increase the required radiation for dissipating the generated heat.

Furthermore, the electric tools of the prior art have generally comprised an ineicient magnetic circuit in which a substantial part of the reluctance is distributed outside of the working air gap between the cooperating plungers while, in the ideal case, all of the reluctance of the magnetic circuit should be concentrated in this air gap. For example, the brass shell in which the plunger is mounted for reciprocation produces an effective air gap where the shell intersects the magnetic circuit and across which appears a substantial part of the total reluctance of the magnetic circuit. The effect 0f this inelilcient design of a magnetic circuit is substantially to increase the required magnetomotive force, that is, the ampere-turns, of the electromagnetic winding for a given power output, which still further increases the requirements of radiation to dissipate the increased losses. Both of the factors described, in addition to increasing the cost and decreasing the efficiency of the unit, cause it to become bulky and unwieldy.

` Furthermore, many of the electric reciprocating tools of the prior art have the disadvantage that they tend to be destroyed by excessive vibration if, for any reason, the tool is removed from the unit or the load is removed from the too1. Furthermore, it has not generally been possible to control the intensity of the blow of the striking tool in accordance with the character of the work being performed.

It is an object of the present invention, therefore, to provide an improved electric reciprocating tool which will overcome one or more of the above-mentioned disadvantages of the arrangements of the prior art.

It is another object of the invention to provide an improved electric reciprocating too1 0f the type described, in which the efficiency is increased by concentrating the reluctance of the magnetic circuit substantially solely in the working air gap between the two plungers.

It is another object of the invention to provide an improved reciprocating electric tool in which the efficiency is improved by substantially removing from the magnetic circuit all elements having substantial conductance for eddy currents, that is, conductive elements which are not readily laminated or slotted to avoid eddy currents.

It is a further object of the invention to pro-V vide an improved electric reciprocating tool including provisions for cushioning the blow of thel tool striking plunger in the absence of a tool in the unit or upon removal of the load from the tool.

It is still a further object of the invention to provide an improved electric reciprocating too1 including provisions for controlling the intensity of the blow of the striking member in acco-rdance with the character of the work being performed.

In accordance with the invention, a reciprocating electric tool comprises a pair of'coaxial relatively reciprocable magnetic cores or plungers together with means for biasing the cores to normal relative positions, when deenergized, in which they are separated by a predetermined working air gap. An exciting winding surrounds the cores and cooperates with transversely or radially extending magnetic members disposed at either end of the winding and cooperating with the cores. The unit is so assembled and proportioned that the clearances between the radial `magnetic members and the cores are the minimum requiredA by manufacturing tolerance to avoid actual frictional Contact. There is also provided a return magnetic circuit between the transversely extending magnetic members, gerierally comprising the casing of the unit, sc that substantially all of the reluctance of the magnetic circuit of the exciting winding is concen-V trated in theV air gap between the cores when they are in their normal relative positions.

In accordance with another feature of the in- Vention,the reciprocatory bearing or guide for the cores is located completely external to the magnetic circuit so that these members do not effectively increase thereluctance of the magnetic circuit or provide a path for the flow of eddy currents.

In accordance with another feature of the invention, one of the cores is resiliently biased to a predetermined position in the path of movement of the working core, so that it constitutes pri-A marily a resilient stop for the working core and is effective to cushion its blow in the absence of a tool in the unit, or upon the removal of the load from the tool.

In accordance with a further feature of the invention, the working plunger is 'provided with a striking member projecting through the other core, while the working tool is inserted in the unit and biased to such a position that the spac-y ing between the striking member is normally slightly greater than the normal working' air gap` between the two cores so that the tool'is movable by pressure on the unit to a position in which the spacing is slightly less than the normal working air gap. With such an arrangement, when only light pressure is exerted on the working tool, a portion of the energy of the working core isr dissipated in striking the `cooperating core and absorbed in its biasing spring before" the striking member actually contacts the tool. However, for heavier loads'on the working tool, the striking member attached to the working core strikes the tool before the working air gap is completely closed so that the full force of the working `core is transmitted directly to the working tool for maximum intensity of the blow.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying vdrawings and its scope will be pointed out in the appended claims. I

In the accompanying drawings, Fig. l is a longitudinal sectional view of a. complete reciprocating electric tool constructed in` accordance with the present invention; Fig. 2a is an elevation and Fig. 2b a cross-section of the relatively stationary magnetic core of the unit of Fig. 1; Fig. Bais an elevation and Fig. 3b a cross-section of the working magnetic coreof the unit of Fig. l; Figs. 4a and 4b are plan and end views, respectively,y of each of the transversely extending magnetic members of the unit of Fig. 1; Fig. 5 is a cross-sectional detail of a modified form of support for the working tool and the relatively stationary core of the unit of Fig. l; Figs. 6a and 6b are a longitudinal sectional view and a croisssectional view, respectively, of a modified form of electric tool embodying the invention and utilizing magnetic cores of square cross-section; Fig. 7a is an elevation and Fig. 7b a cross-section of the relatively stationary magnetic core of the unit of Figs. y6a and 6b; and Fig. 8a. is an elevation and Fig. 8b a cross-section of the working magnetic VcoreV of the unit of Figs. 6a and 6b.

Referring now more particularly to the drawings, the reciprocating electric toolV illustrated in cross-section in Fig. l comprises a casing formed in three'sections, a section III comprising the body of the casing and supporting the active elements of the tool, a section I I comprising a shank or tool supporting member, and a section I2 comprisingprimarily a handle Vfor the unit and a support for the electric control mechanism of the unit. The section Iii comprises an annular ring of magnetic material provided with a plurality of longitudinal slots Ilia for minimizing eddy currents. Supported within and closely fitting into the cylindrical shell IU is a spool or form I3 of some suitable insulating material, such as Bakelite, on which is wound the actuatingfor'exciting winding I4.of the unit. The winding I4 is provided at either end with transversely'or radially extending magnetic members I5 and IG which are clamped firmly in place by end rings II and I8, respectively, threaded into opposite ends of the cylindrical shellrlll. ,For maximum effectiveness in reducing eddy currents, the slots lila extend between the magnetic members I5 and I6 over substantially the entire length of the i magnetic circuit of winding I4. The magnetic members I5 and IIiare closely fitted into eXtensions or skirts ofthe end rings I'I and I8, respectively, so that the whole stationary assembly is accurately aligned. The skirts 0f the end rings I'I and I8 are also preferably slotted radially to aid in reducing eddy currents. The shank section I I of the casing may be secured to the central section III by means of machine screws 9 threaded into the end'ring I'I; similarly, the handle section I2 may be secured to the central section I0 by means of machine screws 9 threaded into end ring I3.Y

Supported within the assembly just described, is a pair of relatively reciprocable plungers or cores I9 and 2i! maintained in axialfalignment by means of a striking member 2I which is rigidly secured to the core 2li, as by welding at 20a, and which projects entirely through the core I9 an-d is supported therein for relative reciprocation by means of a sleeve bearing 22. The core I9, is, in turn, supported within a sleeve'bearing 23 mounted in the end ring Il, while the end of the striking member 2|, projecting through the opposite end of the core 20, is supported in a sleeve bearing 2li mounted in an annular extension ISa of the end ring I8. The striking member is preferably, but not necessarily, of nonmagnetic material, for example, a. high chromium alloy stainless steel, in order to avoid any shunting of the normal air gap between cores I9 and 20.

As shown more clearly in Figs. 2a and 2b, the core IS is provided with a large numberojf longitudinal milled slots, shown as spaced at 30,-

degree intervals and extending entirely through the core throughout that portion of its length which normally lies within the magnetic circuit of the winding I4. The longitudinal sectional view of core I9 in Fig. l is taken along the line I-I of Fig. 2b through two diametrically opposed slots, while the cross-sectional View of Fig. 2h is taken along the line 2b-2b of Fig. 2a. The sleeve bearing 22 supported within core I9 does not extend entirely through the core but is terminated short of the main portion of the magnetic circuit of the winding I4. Similarly, as shown more clearly in Figs. 3a and 3b, the core 20 is provided with a large number of longitudinal milled slots 20h, also shown as spaced at 30- degree intervals and extending nearly to the central bore through which the striking member 2| extends, alternate slots being deeper than the others and leaving only a suflicient amount of material near the central bore to maintain a rigid self-supporting structure. The longitudinal sectional view of core 29 in Fig. l is taken along the line I-I of Fig. 3b through two diametrically opposed ones of the more shallow slots, while the cross-sectional View of Fig. 3b is taken along the line 3b-3b of Fig. 3a. The cores I9 and 20 are preferably of a high permeability, high resistivity, alloy steel, for example, a high silicon steel, in order to reduce the magnetomotive force required in winding I4 for a given mechanical power output and to aid in reducing eddy-current losses. A detail of each of the radial magnetic members is shown in Figs. 4a and 4b, from which it is seen that each of these members comprises a stock oi laminations or punchings arranged to provide a radial slot to minimize eddy currents due to any axial component of magnetic ux flowing through these members.

In the assembly just described, the clearances between the radial magnetic members I5 and I6 and the cooperating magnetic cores I9 and 20, respectively, are the minimum permitted by manufacturing tolerances to avoid actual frictional contact between these members and the reciprocating cores. In practice, it has been found feasible to operate with clearance between these members of 0.005 inch. Furthermore, it is noted that all of the bearing members of the unit, that is, the bearing 23 between the end ring I'I and the core I9, the bearing 24 between the end ring I8 and the projection of the striking member 2I, as well as the bearing 22 between the core I9 and the striking member 2I, are substantially completely outside of the magnetic circuit of the winding I 4. Therefore, these bearings, usually of nonmagnetic materials such as brass, bronze, or the like, do not constitute effective air gaps in the magnetic circuit. Furthermore, since they are outside the magnetic circuit, no eddy currents are induced therein which would otherwise tend to decrease the efliciency of the unit. Since the magnetic reluctance of the casing section I and the radial magnetic members I and I 6 is only an extremely small fraction of the reluctance of the normal air gap between the cores I9 and 20 and since the air gap required by the clearances between the radial magnetic members I 5 and I6 and the cores I9 and 20, respectively, is also such an extremely small fraction of the reluctance of the normal air gap between the cores, substantially the entire reluctance of the magnetic circuit of the winding I4 is concentrated in the air gap between the cores I9 and 2D, which is substantially the ideal relationship. By this provision the magnetomotive force, that is, the ampere-turns,

of 'the electromagnetic winding I4 required for any given mechanical power output of the unit is reduced to a minimum.

The core 20 is provided with an end washer or plate 25 secured thereto by a pin 26 and this plate 25 is acted upon by two opposing springs, a spring 21 acting against the extension of the end ring I8 and a spring 28 acting against a washer 29 supported on a bolt 30 threaded into a partition I2a of the casing section I2 and provided with a lock nut 3|. By this means the core 20 is efiectively floating in a deenergized or normal position which may be adjusted by adjustment of the bolt 38. The normal relative positions of the cores I9 and 20 are such as to provide a normal working gap therebetween of substantial value; in a small portable tool a Value of the order of 1A inch has been found to be satisfactory. The'constants of the springs 21 and 28 are so adjusted relative to the mass of the core 20 and striking member 2l that the natural period of mechanical oscillation of this system is substantially equal to the period of the intermittent current normally supplied to the winding I 4 which, it is understood, is generally half-wave rectified 60 cycle alternating current.

Threaded into the nose of the shank section I I of the casing is a bearing 32, held in place by a lock nut 32a, in which a working tool 33 is adapted to reciprocate. The shank 33a of the tool 33 is provided with a shoulder 33h which normally presses against a washer 34 seated on a flange IIa formed in the nose of the shank II. A spring 35 normally presses the washer 34 against the shoulder IIa and engages the end of the core I9 to bias it to the position shown in Fig. 1 in which its flange I9a seats against the end of the bearing 23. The end of the striking member 2| is tapered at 2Ia and seated into a hardened annular ring 36 having an internal taper complementary to that of striking member 2|, so that the two may be held in position by frictional engagement. The provision of this hardened ring 36 minimizes the 11p-setting of the end of the striking member 2I in the normal operation of the system.

The cooperating cores i9 and 29 and their associated parts are thus so related that, in the absence of a working tool 33, the active reciprocation of the core 20 by the intermittent excitation of the winding I4 causes it to strike the core I9 and the force of this blow is dissipated in the spring 35, thus preventing destruction of the unit by excessive vibration, as is the tendency in certain of the arrangements of the prior art in which the reciprocating plunger abuts against a rigidly xed cooperating plunger. Preferably, the bearing 23 may be designed to provide a considerable friction between it and the core I9 so that, under the circumstances described, a portion of the energy of the reciprocating core 29 is dissipated in the bearing 23.

Further, the cores I9 and 28 and their associated parts are so proportioned that the normal separation between the shank of the tool 33 and the striking element 36 of the striking member 2I is slightly greater than the normal separation between the cores I9 and 20 at the working air gap. With this arrangement, if little or no pressure is exerted on the tool 33, the core 20 in reciprocating strikes the cooperating core I9 a slight interval before the striking element 36 engages the shank of the tool so that a portion of the energy of the reciprocating core 20 is dissipated in the blow on the core I9 which, in part,

is dissipated in the spring 35 and in the friction at the bearing 23. However, if the unit is pressed against the work for heavier-blows, the shoulder 33h of the tool 53 engages the washer 34 and compresses the spring 35 until the displacement between the shank of the tool 33 and the striking element 36 is less than the normal air gap between the cores I9 and Z. Under these circumstances, the striking element 36 strikes the tool before the core 29 strikes the core I9, so that the full blow of the striking member is imparted directly to the tool. If the depth of movement of the Working tool is small, substantially the entire energy or" the reciprocating member 20 is transferred to the working tool. However, if the depth of stroke of the tool 23 is substantial, a portion of the energy of the reciprocating f member may be transferred to the core I9 and dissipated thereby during the final portion of the Working stroke` Thus, by the provisions just described, the blow of the striking member is cushioned in the absence of a tool in the shank I I or in case the load is completely removed from the tool, while the intensity of the blow on the tool may be readily controlled by adjusting the manual pressure with which the unit is pressed against the work.

The exciting winding i4 may be energized in any manner well known in the art. To this end the spool I3 is provided with an extension I3a which projects through'an aperture IGZ) provided in the casing section IB and terminalsV 31 are mounted on the projection lila and connect with the terminals of the winding I4, which are preferably in the form of `brass strips (not shown) or other equally rigid construction to withstand vibration. The terminals 3l are connected by a pair of conductors 38 and a switch 39 manually Y operated by a trigger 4i) with an external cable 4I projecting into the handle of the casing I2 through a bushing 42. It is understood that the switch 39 and the trigger 4t form no part of vthe present invention and may be of any suitable well-known construction.

In Fig. 5 therel is shown a cross-sectional detail of a modified form of the shank element II of the casing. Elements in this gure corresponding to similar elements of Fig. 1 are given the same reference numerals. In this modification the nose of the shank II is provided with a cylindrical recess IIb separated by the flange I la from the central portion ofV the unit. Within this recess Ilb is mounted a spring 35a bearing on the flange IIa and a washer 33h to bias the working tool 33 to the position illustratedV in Fig. l. In this modification, the spring 35 serves only to bias the core iQ so that the flange ld rests against the bearing 23. As in the arrangement of Fig. l., in the normal position of the elements, the spacing between the shank of the tool 33 and the striking element 2l is slightly greater than that between the cores I9 and 2i) which comprises the working air gap. The principles of operation of the modification of Fig, 2 are in all respects similar to those described above in connection with Fig. l, but the use of separate springs and 35a for biasing the members I9 and 33, respectively, makes possible the selection of diierent spring constants, each of which may be selected for optimum operating characteristics of the element biased thereby.

, In Figs. 6a and 6h there is shown a modification of the reciprocating electric tool of Fig. l, in whichthe stationary and movable magnetic core members may be made substantially entirely of laminations of magnetic material, such as silicon steel. Referring to these iigures, a central section 4I is made up of two opposed stacks of U- shaped laminations held between a handle section V42, shown in fragmentary section, and a shank section 43 by channel-shaped side plates 44 having end lugs 45 lto which the end sections 42, 43 are securedl by 'machine screws 46. Clamped between the stacks of laminations 4I and each of the side plates 44 are plates 41 of somewhat heavier gauge non-magnetic metal to which are secured by means of rivets 48 the channel-shaped bearing guides 43 of nonmagnetic bearing material such as bronze, which extend the entire length of the magnetic circuit;

The actuating winding 55 of the device is discontinuous rectangular space betweeny the opposed magnetic coremembers.

Mounted in the rectangular space between cores members 4I is a relatively stationary magnetic core member 52, shown in more detail in Figs. 7a, and 7b, rectangular in cross-section and made up of a stack of high-permeability laminations clamped between a pair of end plates 52a of nonmagnetic bearing material by means of rivets 52h so located as to minimize eddy currents therein. The end plates 52a are formed with guide bearing surfaces complementary to the bearing surfaces of the channel-shaped bearing guides 49. The core member 52 is also previded with a shoulder 52e adapted to be maintained in registry with the ends of the core members 4I by means of a spring 53. Also disposed in the rectangular space between the core members 4I is a relatively reciprocable magnetic core member 54, shown in detail in Figs. 8d and 8b, which also comprises a stack of high-permeability laminations clamped between a pair of end plates 54a of non-magnetic bearing material by means of a series of rivets 51 passing through the holes 54h. The end plates 54a are also provided with slide bearing surfaces complementary to those of the bearing guides 49.

The cross-sectional areas of the relatively stationary core member 52 and the reciprocable core member 54 are such as substantially to fill the space between the magnetic core members 4I and, when suitably mounted in the ,bearing guides 49, leave an air gap 35 comprising only the minimum yclearance required by manufacturing tolerances, whichv may be only a few thousandths of an inch. Therefore, in this construction, as in that of Fig. l, substantiallyV the entire reluctance of the magnetic circuit of the device is concentrated in the normal working air gap between the relatively reciprocable cores 52 and 54.

The cores 52 and 54 are provided with longitudinal apertures 52d and 54d, respectively, approximately square in cross-section, through which project a striking member 55 of nonmagnetic material carrying at its striking Vend a hardened steel collar 56 having a taper fit on the member 55. The striking member 55 may be secured to theA relatively reciprocable core 54 by means of the rivets 5l. which serve also to clamp the laminations between the end plates 54a and,

additionally, by a llet welding 58 at the remote end of core member 54.

The spring 53 which retains the core 52 in engagement with the core membersl 4I bears against an annular disc 59 secured to the striking member 55. There is also provided a spring 60 bearing against an internal flange 43a of the shank 43 which biases the flange 59 to retain the core member 54 in its normal position and to return it to that position after each working stroke. In this construction the spring 53 serves also as a cushion to the return stroke of the core 54 under the action of the spring 60. The shank section 43 is provided with a suitable sleeve bearing El for the tool 33 and a lock nut 62 for retaining the bearing 6l in position. As in the arrangement of Fig. 1, the relations of the parts is such that the striking member 55 engages the tool 33 during the working stroke before the core member 54 engages the core member 52.

It is thus seen that the magnetic circuit of the reciprocating electric tool of Figs. 6a and 6b comprises the essential advantages of that of Fig. l, in that the magnetic circuit of the device is not interrupted by any bearing members; substantially the entire reluctance of the magnetic circuit is concentrated in the normal air gap between the cooperating core members 52 and 54; and the air gap in the magnetic circuit outside of the working stroke is only that required for manufacturing tolerances to avoid a frictional contact between the relatively movable members, which may be only a few thousandths of an inch. In addition, in the structure just described, the entire magnetic circuit is fully laminated, thus further improving the efciency of the unit. While the features of the. cushioned blow and the controlled blow have not been illustrated in connection with the apparatus of Figs. 6a and 6b, it will be readily understood that this feature may be applied to the apparatus in a manner entirely similar to that of the apparatus of Fig. 1.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modications may be made therein Without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is 1. An electric tool unit comprising a casing, a tool shank disposed at one end of said casing, a pair of coaxial reciprocable magnetic cores normally relatively biased to provide a predetermined air gap therebetween, an exciting winding surrounding said cores, means providing a magnetic circuit for said winding including said cores, a striking member rigidly secured to one of said cores and projecting through the other of said cores into said shank, and means for resiliently biasing a tool inserted in said shank to a given position such that the spacing between said striking member and the tool is normally slightly greater than said air gap, whereby at least a portion of the energy of said striking core is dissipated in striking said other is in its normal position.

2. An electric tool unit, comprising a. casing,

core when the tool a tool shank disposed at one end of said casing, a pair of coaxial reciprocable magnetic cores nor mally relatively biased to provide a predetermined air gap therebetween, an exciting winding surrounding said cores, means providing a magnetic circuit for said winding including said cores, a striking member rigidly secured to one of said cores and projecting through the other of said cores into said shank, and means for resiliently biasing a tool inserted in said shank to a normal position such that the spacing between said striking member and the tool is normally slightly greater than said air gap, said tool being movable by pressure on the unit to a position in which said spacing is slightly less than said air gap, whereby the energy of said striking core is transferred substantially completely to the tool.

3. An electric tool unit comprising a casing, a tool shank disposed at one end of said casing, a pair of coaxial reciprocable magnetic cores normally relatively biased to provide a predetermined air gap therebetween, an exciting winding surrounding said cores, means providing a magnetic circuit for said winding including said cores, a striking member rigidly secured to one of said cores and projecting through the other of said cores into said shank, xed stops for said other of said cores and for a tool inserted in said shank, and resilient means for biasing said other of said cores and the tool against their respective stops, the spacing between said striking member and the tool being slightly greater than said air gap when the elements are in their respective biased positions.

4. An electric tool unit comprising a casing, a tool shank disposed at one end of said casing, a pair of coaxial reciprocable magnetic cores normally relatively biased to provide a predetermined air gap therebetween, an exciting winding surrounding said cores, means providing a magnetic circuit for said winding including said cores, a striking member rigidly secured to one of said cores and projecting through the other of said cores into said shank, fixed stops for said other of said cores and for a tool inserted in said shank, and a single spring for biasing said other of said cores and the tool against their respective stops, the spacing between said striking member and the tool being slightly greater than said air gap when the elements are in their respective biased positions.

5. An electric tool unit comprising a casing, a tool shank disposed at one end of said casing, a pair of coaxial reciprocable magnetic cores normally relatively biased to provide a predetermined air gap therebetween, an exciting winding surrounding said cores, means providing a magnetic circuit for said winding including said cores, a striking member rigidly secured to one of said cores and projecting through the other of said cores into said shank, xed stops for said other of said cores and for a tool inserted in said shank, and separate springs for individually biasing said other of said cores and the tool against their respective stops, the spacing between said striking member and the tool being slightly greater than said air gap when the elements are in their respective biased positions.

HERBERT C. ROTERS. HAROLD C. PASINI.

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