US3357275A

US3357275A - Power tool

Info

Publication number: US3357275A
Application number: US467057A
Authority: US
Inventors: Jr James Green; Gresens Robert
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1965-06-25
Filing date: 1965-06-25
Publication date: 1967-12-12
Anticipated expiration: 1984-12-12
Also published as: GB1143677A; DE1552298A1; NL6608866A; ES327809A1

Description

Dec. 12, 1967- J. GREEN. JR. ET AL POWER TOOL 4 Sheets-Sheet 1 Filed June 25, 1965 five/7.0a;

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Dec. 12, 1967 J, GREEN, JR. ETAL 3,357,275

POWER TOOL Filed June 25, 1965 4 Sheets-Sheet 2 12, 1967 .J. GREEN. JR. ETAL 3,

POWER TOOL Filed June 25, 1965 4 Sheets-Sheet E- 7V///d/15-' Mamas 4/650 4 PM! I was-ms Dec. 12, 1967 J, GREEN, JR. ETAL 3,357,275

POWER TOOL Filed June 25, 1965 4 Sheets-Sheet 4 James 6/6ffl 4/4 PMs/ i f 6/6.?!5.

United States Patent 3,357,275 POWER T001. James Green, Lira, Spencerport, and Robert Gresens, Rochester, N.Y., assignors to General Electric Company, a corporation of New York Filed June 25, 1965, Ser. No. 467,057 8 Claims. (Cl. 74-606) This invention relates to the subject of hand held electrically powered tools, and more particularly to a multipurpose product concept wherein a single power unit may be utilized to drive a variety of interchangeable tool heads.

In the field of small electrical appliances, each individual product typically includes its own power source in the form of an electric motor. Since it is now very common for a single consumer to own many different small appliances, the possibility of designing appliances so that a single multipurpose power unit may be utilized to power several devices provides a potential mean of attaining lower costs for such appliances, hence making them more salable to the consumer. Hand held electrically powered tools are particularly likely prospects for such a concept since there are a wide variety of different tools currently being utilized, such as drills, sanders, and saws, to name a few; and each of these devices has certain common or similar design characteristics such as power needs.

However, when a single power unit is employed to drive several different ultimate tools, there are certain requirements which must be met in order to provide a successful multipurpose arrangement. Such units must, of course, perform as well or better than conventional power tools not having the interchangeability feature. Secondly, the power unit must be interchangeable with the various tool heads in a very convenient fashion. Also, to be successful in the market, such a tool should be more economical to the consumer than similar individual motorized appliances even though there is considerable space saving advantage in the multifunction approach.

Although the concept of employing a single power unit to perform several different tasks has been previously considered, such earlier known arrangements have all been inadequate in certain of the foregoing respects and none are believed to have been commercially successful. Accordingly, it is a primary object of this invention to provide an improved portable electric power tool having,

a single multipurpose power unit for driving a variety of tool heads and which meets the foregoing requirements.

It is another object of this invention to provide a tool having a power unit which is releasably attached to a tool head and has an accurate but simple alignment means between the power unit drive shaft and the tool head to obtain a smooth efiicient drive connection.

It is another object of this invention to provide a convenient and simple means for attaching the power unit to the tool head which does not interfere with the alignment means.

Yet another object of this invention is to provide an improved multipurpose power unit which is interchangeable with a variety of tool heads and employs a minimum number of parts such that the cost of a single power unit and several different tool heads is considerably less than the cost of comparably sized power tools having integral power units and tool heads.

It is another object of this invention to provide a unique housing for such a power unit to facilitate assembly and disassembly of the power unit to a particular tool head.

Further features, objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims appended and forming a part of this specification.

Briefly stated, in one embodiment of the invention there is provided an electrically powered tool including a power unit housing containing an electric motor having a drive shaft which extends beyond the housing with a pinion gear formed on its outer end. A bearing within the housing supporting the end of the shaft adjacent the pinion gear is formed with a hub having an outer diameter accurately concentric with the bearing inner diameter and the motor drive shaft. Also included is a tool head housing containing reduction gearing and drive means for the ultimate tool. The tool head housing has a unique rear wall which forms an intermediate plate to cooperate with the front surface of the power unit housing. An aperture formed in the intermediate plate snugly receives the power unit bearing hub. This aperture is accurately positioned such that the pinion extends into the tool head housing to properly engage the reducing gears.

Suitable fasteners are provided to releasably secure the intermediate plate of the tool head housing to the power unit housing while the motor shaft and bearing hub extend through the aperture in the intermediate plate. In order to have the plate transversely oriented only by the bearing hub and the-aperture, the fasteners utilized have loose tolerances in a plane parallel to the plate.

, Hence, to obtain accurate alignment of the power unit with respect to the tool head, it is only necessary to maintain precise tolerances between the shaft bearing and the aperture in the intermediate plate. Naturally, the reducing gearing employed in the particular tool head is also accurately positioned within the tool head with respect to the aperture and the intermediate plate. With such an arrangement, the power unit may be conveniently connected to various tool heads by simply inserting the motor shaft and bearing hub into the intermediate plate forming a wall of the particular tool head housing.

For a better understanding of this invention, reference may be made to the following description and the accompanying drawings in which:

FIG. 1 is a perspective view of the power tool of the invention showing the power unit in this instance connected to a drill head;

FIG. 2 is a side elevational view of the tool;

FIG. 3 is a top plan view of the tool;

FIG. 4 is a side elevational, sectionalize'd view of the tool shown in FIG. 1.

FIG. 5 is an exploded perspective view of the tool showing the tool head separated from the main power unit housing and showing the details of the unique power unit housing;

FIG. 6 is an end elevational view of the power unit;

FIG. 7 is an end elevational view of the drill tool head; and

FIG. 8 is a perspective view of the interchangeable power unit shown connected to a sanding tool head.

Referring now to the drawings, the power tool of the invention may be seen to include a power unit 10 connected to a tool head generally indicated at 12 which carries the tool work output means, that in this instance is a drill chuck 14. As will be eventually explained in greater detail, the power unit 10 is releasably attached to the tool head so that this same power unit may be used to drive various tool heads. Hence, while a drill is illustrated in FIGS. 1-7 to describe the'invention, it should be understood that various other tool heads may also be employed such as a sander 16 as seen in FIG. 8.

The powerunit 10 is contained within a somewhat rectangular shaped, preferably aluminum, housing 18 including a top wall 19, a bottom wall 20,

side walls

21 and 22, a rear wall 23 and, as seen in FIGS. 5 and 6, a

front, primarily open wall 24. A hollow handle 26 is formed integral with bottom wall 20. Within the power unit housing 18, there is positioned a suitable electric motor 28, which as shown is of the series type, having a field winding 30, an armature shaft 32 carried by a rear bearing 34 and a front bearing 36. These bearings are preferably of the self-centering spherical type and made of sintered bronze impregnated with lubricant. As can be seen from FIG. 4, the bearings are clamped in position on supporting lugs 91 by

suitable straps

38 and 40 attached to additional lugs 88 formed integral with housing 18.

A fan 41 cools the motor 28 with air drawn in through inlet slots 42 located in the rear portion of the

side walls

21 and 22, and exhausted through exhaust ports 46 located in the forward portion of the side walls. A suitable switch assembly 50 is positioned in the upper portion of the handle 26, and an electrical cord 52 for energizing the tool enters through the lower portion of the handle, as seen in FIG. 4. Electrical energy is of course provided to the motor 28 through leads (not shown) and a suitable pair of brushes 54.

As can be seen, the head 12, in this case a drill, but typical of many possible tool heads, includes a housing 56 having a front wall 57, a removable rear wall or plate 58, and top, bottom and side walls 59 which form a somewhat rectangular cross section to mate with that of the power unit housing. Within the housing 56 there are positioned suitable reducing

gears

60, 61 and 62 mounted on

shafts

64 and 66 supported by appropriate bearings. More specifically, shaft 64 is mounted on sleeve bearing 68 extending through an opening in the front wall 57 of housing 56 and on a sleeve and thrust bearing 70 mounted in a boss 71 in the rear wall 58 of housing 56. The rear wall 58, which is secured by screws 72 to internal lugs formed integral with the housing 56, in etfect forms an intermediate plate between the power unit housing 18 and the tool head housing 56, as will be discussed in greater detail in the following paragraphs. Shaft 66 is mounted on one end in a suitable sleeve bearing 74 located in a boss 76 formed in intermediate plate 58, and the other end of shaft 66 is mounted in a suitable bearing (not shown), supported by housing front wall 57.

As has been indicated, the power unit is removably attached to the tool head 12 so that this same power unit may be utilized to drive other tool heads. In order to have a workable arrangement of this nature, there must be an efficient means for transmitting force from the power unit to the tool head. In accordance with the present invention, the motor armature shaft 32 extends outwardly beyond the front face of housing 18 and has formed thereon a pinion gear 78. This pinion gear 78 extends through an opening 80 formed in intermediate plate 58 into the interior of the tool head to drivingly engage the mating reduction gear 62.

In order for such a direct drive arrangement to be utilized, it is necessary that the pinion gear 78 be very accurately aligned with mating gear 62 and with respect to the other gears in the tool head. In accordance with the invention, a very simple but unique means is provided employing the front motor bearing 36 and the aperture 80 in the intermediate plate 58. As seen in FIG. 4, the outer end of front bearing 36 is formed with an integral hub 37 positioned immediately adjacent pinion gear 78 and extending through the plate aperture 80 when the tool head is assembled to the power unit. To maintain the desired accuracy, the outer diameter of hub 37 must be very precisely concentric with the interior of bearing 36, and the diameter of aperture 80 is accurately controlled to snugly receive the hub 37. In this way, the exterior surface of the hub serves as a simple locating means for positioning shaft 32. As an indication of the accuracy desired to obtain proper meshing of the gears, the bearing hub was given an outside diameter of .4206 inch with a tolerance of 0005 and the aperture was given a dimension .42l8 inch with a tolerandce of i.0005 in a production version of the device. The inner diameter of bearing 36 was given a total tolerance of .0003 inch. As mentioned, one end of the

shafts

64 and 66 are mounted in bearings supported by intermediate plate 58. The tolerances between the positioning of these bearings and the aperture 80 is also precisely controlled, with center to center dimensions being accurate to i002 inch.

While the hub and aperture arrangement described provides accurate alignment of the gearing, it should be completely independent of the means for attaching the tool head to the power unit. For the attaching function, there are provided three threaded fasteners or screws 82 carried by the tool head housing 56 and threadably received Within threaded inserts 84 carried in the front wall 24 of the power unit housing 18. As can be seen from FIG. 4, screw 82 extends through an aperture 83 in the front wall 57 of houisng 56 and through an opening in intermediate plate 58 before being received in insert 84. The head 82a of screw 82 engages the front wall of housing 56 when the screw is threaded tightly into the insert, thereby clamping the tool head to the power unit.

Since the alignment of the gearing is obtained by the hub 37 and plate aperture 80, the screws 82 should not interfere with their alignment. This is quite critical since undue stresses on the gearing could overload the motor and cause undesirable gear wear. In accordance with the invention, the unthreaded portion 82b and the threaded portion 820 of the screws 82 are only loosely received within the apertures in housing 56 and intermediate plate 58. Thus, the screws 82 only provide axial force to the tool head and power unit.

To retain the screws 82 in the tool head when the tool head is disassembled from the power unit, a suitable resilient retaining ring 86 is mounted on the unthreaded portion of the screws. Since the outer diameter of retaining ring 86 is larger than the diameter of aperture 83 in the front wall 57 of housing 56, the screws will not inadvertently fall from the tool head housing. The inner diameter of the retaining ring is smaller than the diameter of the screw threaded portion 82c so that the ring must be forced over the threads to be removed and hence will not simply fall from the screw.

In the preferred embodiment of this invention, the power unit housing 18 is formed by two essentially vertically split sections 18a and 18b, as can be seen in FIG. 5. All of the motor components are uniquely mounted to section 18a to facilitate assembly operations while section 18!) serves primarily as a cover for these components. As can be seen, section 18a is formed with suitable integral lugs 88 for receiving threaded fasteners 80 securing the various elements of the motor to the housing. Also formed in section 18.11 are the integral lugs 91 having inner curved recesses for receiving the

spherical hearings

34 and 36. The lugs integral With section 18]; adjacent front wall 24 serve as locating surfaces during manufacturing operations. The two housing sections are clamped together by suitable threaded fasteners 92 extending through holes 93 in the cover section 18b and threaded into additional lugs 94 in section 18a.

To simplify the accuracy of alignment of the means for attaching the power unit to the tool head, housing section 18a is formed with a projection 96 on the top portion of its front surface which extends into a mating recess 97 in cover section 1812. Section 18a is further formed with a projection 98 on the lower portion of the front wall. The threaded inserts 84 are positioned within these projections and the other front surface of section 18a so that all of the attaching screws are threaded into the same housing section. In this fashion, tolerance and alignment problems are minimized.

From the foregoing, it can be seen that the intermediate plate is of primary importance in achieving the necessary locating accuracy to insure proper gear mesh. In addition to this function, the plate serves as a grease or lubricant seal for the tool head. It permits each tool head to be an integral sealed unit containing its own independent gear sys tem and mechanisms designed for optimum performance of the particular tool. Since the gearing required within the tool head is dependent upon the ultimate tool to be driven and the function to be performed by this tool, the main portion of the tool head housing will be somewhat different for the various tool heads to be employed. However, intermediate plate 58 has been uniquely designed to be utilized with several different tool heads.

As has been explained, bosses 71 and 75 in plate 58 are utilized to hold bearings when the tool head is a power drill. An additional boss 1% shown in FIGS. 4 and 5 is formed in the intermediate plate 58 to be utilized for receiving a suitable bearing when the tool head employed is the sander 16 shown in FIG. 8. The particular reduction gearing and other drive mechanism required for the sander is not shown in that it is not necessary to an understanding of this invention; however, it should be readily appreciated that suitable gearing may be selected to perform the function desired and to properly mate with the drive shaft pinion. Similarly, other bosses or means for receiving a hearing may be formed within the intermediate plate. Also, if the power unit is to be utilized to drive other tools, such as a saber saw, further means may be added to the intermediate plate to perform the desired function.

To enhance the convenience of the power unit when used for its various functions, a threaded hole 99 is formed in top wall 19 and

side walls

21 and 22 for receiving a suitable removable handle (not shown).

Thus, it will be seen that a particularly versatile power tool concept has been described wherein a single power unit may be employed to drive a plurality of difierent tool heads. With the unique alignment and attaching means disclosed, a highly convenient and low cost arrangement is provided.

While a particular embodiment of the invention has been illustrated and described, modifications thereof will occur to those skilled in the art. Accordingly, it should be understood that this invention is not limited to a particular arrangement disclosed and it is intended in the appended claims to cover all such modifications which do not depart from the spirit and scope of the invention.

What is claimed is:

1. An electrically powered tool comprising: a power unit housing, drive means positioned within said power unit housing including a drive shaft, a gear on one end of said shaft, a first bearing for supporting the end of said shaft which is remote from said gear, a second bearing in said power unit housing supporting the end of said shaft carrying said gear, a hub formed on the end of the second bearing adjacent the gear with the hub outer diameter being concentric with the second bearing inner diameter, means for securing said hearings in said power unit housing for supporting said shaft for rotation, a tool head housing separable from said power unit housing adapted to cooperate with said power unit housing, gear means positioned within said tool head housing to engage and be driven by said drive shaft gear, tool work output means driven by said gear means, an intermediate plate forming a wall of said tool head housing to be aligned with said power unit housing adjacent said gear, an aperture formed in said plate to snugly receive said hub, said aperture being accurately positioned so that said gear extends into said tool head housing to properly engage said gear means, and fastening means for releasably securing said intermediate plate to said power unit housing while said shaft and hub extend through said aperture, said fastening means having loose tolerances in a plane parallel to said plate so that the fastening means does not precisely position the plate in said plane and the plate is primarily located within said plane only by said hub and said aperture.

2. A hand held electrically powered tool comprising a power unit housing, an electric motor positioned within said power unit housing having a drive shaft extending beyond said housing with a pinion gear located on its outer end, a first bearing for supporting the end of said shaft which is remote from said pinion gear, a second bearing in said power unit housing supporting the end of said shaft adjacent said pinion gear, said second bearing having a hub formed thereon adjacent the pinion gear with the hub outer diameter being closely concentric with the second bearing inner diameter, means for securing said bearings in said power unit housing for supporting said shaft for rotation, a tool head housing separable from said power unit housing, gear means positioned within said tool head housing to be driven by said drive shaft, tool work output means driven by said gear means, an intermediate plate forming a wall of said tool head housing to be aligned with said power unit housing, an aperture centrally l0- cated in said plate to snugly receive said power unit hub, said aperture being accurately positioned so that said pinion extends into said tool head housing to properly engage said gear means, and fastening means releasably securing said intermediate plate and tool head housing to said power unit housing while said shaft and hub extend through said aperture, said fastening means having loose tolerances in a plane parallel to said plate so that the plate is transversely located within said plane essentially by said bearing hub and said aperture.

3. A hand held electrically powered tool comprising: a power unit housin having a top wall, a bottom wall, side walls, a rear wall, and an essentially open front wall; handle means attached to said bottom Wall; an electric motor positioned within said power unit housing having a drive shaft extending through said open front wall beyond said housing with a pinion gear located on its outer end; said housing being formed of two vertically split sections, a first bearing for supporting the end of said shaft which is remote from said gear, a second bearing for supporting the end of said shaft adjacent said pinion gear, said second bearing having a hub formed thereon adjacent the pinion gear with the hub outer diameter being closely concentric with the second bearing inner diameter; means for securing said hearings to one of said housing sections for supporting said shaft for rotation thereon; a tool head housing, gear means positioned within the tool head housing to be driven by said drive shaft; tool work output means driven by said gear means; an intermediate plate forming a wall of said tool head housing to be aligned with the open front wall of said power unit housing; an aperture formed in said plate to snugly receive said power unit hub; said aperture being accurately positioned so that said pinion extends into said tool head housing to properly engage said gear means; a plurality of fasteners carried by said tool head housing and extending through said intermediate plate; said one housing section including projections on its front wall portion extending into recesses formed in said other housing section so that the majority of the front wall surface is formed by said one housing section; means for receiving said fasteners formed at spaced locations in the front wall of said one housing section; means for selectively positioning said fasteners in one of a plurality of locations in a plane parallel to said plate so that the plate is transversely located within said plane essentially by said bearing hub and said aperture.

4. An electrically powered tool comprising: a power unit housing, drive means positioned within said power unit housing including a drive shaft having a pinion gear located on its outer end, a first bearing for supporting the end of said shaft which is remote from said pinion gear, a second bearing supporting the end of said shaft carrying said pinion gear, a hub formed integral with said second bearing adjacent the pinion with the hub outer diameter being accurately concentric with the second bearing inner diameter, means for securing said bearings in said power unit housing for supporting said shaft for rotation;

a tool head housing, gear means positioned within said tool head housing to engage and be driven by said pinion gear, an intermediate plate forming a rear wall of said tool head housing and being aligned with said power unit housing adjacent said pinion gear, an aperture formed in said plate snugly receiving said hub, said aperture being accurately positioned so that said pinion gear extends into said tool head housing to properly engage said gear means, a plurality of threaded fasteners extending through holes formed in the front wall of said tool head housing and through openings in said intermediate plate with an enlarged head on the fasteners engaging said front wall, and means formed in said power unit for threadably receiving said fasteners for releasably securing the tool head to the power unit while said pinion gear and hub extend through said plate aperture, said holes and said openings defined in said intermediate plate being somewhat larger than the diameter of said fasteners so that the plate and tool head may be moved slightly with respect to said fasteners in a plane parallel to said plate so that the plate is primarily located within said plane only by said hub and said plate aperture.

5. In an electrically powered tool, a motorized power unit, a tool head including a housing adapted to cooperate with said motorized power unit for driving the tool head, said tool head housing having top, bottom, front, and side walls together with a removably secured rear Wall, gear means positioned within said tool head housing, said gear means being mounted upon one or more shafts supported on bearings mounted on said housing front wall and on said housing rear wall, tool work output means driven by said gear means extending through the front wall of said housing, a centrally located aperture formed in said rear wall being accurately positioned with respect to said shafts and bearings, a drive shaft mounted for rotation on said power unit, a pinion gear located on the outer end of said drive shaft, said pinion gear being insertable through said aperture and accurately located by said aperture so as to properly mesh with said gear means, and fastening means carried by said tool head housing for releasably securing said housing to said power unit in a manner such that the locating function served by said aperture is not impaired by the fastening means.

6. In an electrically powered tool, a power unit, a tool head including a housing adapted to cooperate with said power unit for driving the tool head, said tool head housing having a plurality of walls including a front wall and a rear wall adapted to mate with said power unit, gear means positioned within said tool head housing, said gear means being mounted upon one or more shafts supported on bearings supported in said housing front wall and said housing rear wall, tool work output means driven by said gear means extending through a wall of said housing, a centrally located aperture formed in said rear wall being accurately located with respect to said shafts and bearings, a drive shaft supported for rotation on said power unit having a gear on its outer end for insertion through said aperture and for being accurately located by said aperture so as to properly mesh with said gear means, and fastening means carried by said tool head housing for releasably securing said housing to said power unit in a manner such that the locating function served by said aperture is not impaired by the fastening means.

7. A hand held electrically powered tool comprising a power unit housing, an electric motor positioned within said power unit housing having a drive shaft extending beyond the front wall of said housing, a gear located on the outer end of said shaft, a first bearing for supporting the end of said shaft which is remote from said gear, a second bearing mounted in said housing supporting the end of said shaft adjacent said gear, locating means formed on the end of the second bearing adjacent said gear being accurately positioned with respect to the second bearing inner diameter, means for securing said bearings in said power unit housing for supporting said shaft for rotation, a tool head housing, gear means positioned within said tool head housing to be driven by said drive shaft gear, said tool head housing having a rear wall mating with said power unit housing front wall, aperture means formed in said rear wall to snugly receive said bearing locating means, said aperture means being accurately positioned with respect to said gear means so that said gear extends into said tool head housing to properly engage said gear means, and fastening means for releasably securing said tool head housing to said power unit housing while said shaft and locating means extend through said aperture means, said fastening means having loose tolerances in a plane parallel to said rear wall so that the tool head housing is transversely located within said plane essentially by said locating means and said aperture means.

8. An electrically powered tool comprising a power unit housing, an electric motor positioned within said power unit housing including a drive shaft having one end extending beyond said housing, a gear formed on said shaft end, a first bearing for supporting the ends of said shaft which is remote from said pinion gear, a second bearing supporting the end of said shaft carrying said gear, a hub formed on the end of the second bearing adjacent the gear with the hub outer diameter being con centric with the second bearing inner diameter, means for securing said hearings in said power unit housing for supporting said shaft for rotation, a tool head housing adapted to cooperate with said power unit housing, gear means positioned within said tool head housing to engage and be driven by said drive shaft gear, tool work output means driven by said gear means, said tool head housing having one wall to mate with said power unit housing, an aperture formed in said one wall to snugly receive said hub, said aperture being accurately positioned so that said gear extends into said tool head housing to properly engage said gear means, and fastening means for releasably securing said tool head housing to said power unit housing while said shaft and hub extend through said aperture, said fastening means having loose tolerances in a plane parallel to said one wall so that the fastening means do not precisely position the housing in said plane and the housing is primarily located within said plane only by said hub and said aperture.

References Cited UNITED STATES PATENTS 1,898,945 2/ 1933 Fitzgerald 31050 1,922,258 8/1933 Packer 310-50 1,965,669 7/1934 Robb 310-83 2,350,631 6/ 1944 Mitchell 31050 2,813,432 11/ 1957 Hoskins.

3,009,493 11/1961 Dodegge 14435.1 3,070,409 12/ 1962 Jakel 3087 2 3,073,176 l/l963 Daug-irdas 74-606 3,203,742 8/ 1965 McCarty et al 74606 FRED C. MATTERN, In, Primary Examiner.

W. S. RATLIFF, Assistant Examiner.

Claims

1. AN ELECTRICALLY POWERED TOOL COMPRISING: A POWER UNIT HOUSING, DRIVE MEANS POSITIONED WITHIN SAID POWER UNIT HOUSING INCLUDING A DRIVE SHAFT, A GEAR ON ONE END OF SAID SHAFT, A FIRST BEARING FOR SUPPORTING THE END OF SAID SHAFT WHICH IS REMOTE FROM SAID GEAR, A SECOND BEARING IN SAID POWER UNIT HOUSING SUPPORTING THE END OF SAID SHAFT CARRYING SAID GEAR, A HUB FORMED ON THE END OF THE SECOND BEARING ADJACENT THE GEAR WITH THE HUB OUTER DIAMETER BEING CONCENTRIC WITH THE SECOND BEARING INNER DIAMETER, MEANS FOR SECURING SAID BEARINGS IN SAID POWER UNIT HOUSING FOR SUPPORTING SAID SHAFT FOR ROTATION, A TOOL HEAD HOUSING SEPARABLE FROM SAID POWER UNIT HOUSING ADAPTED TO COOPERATE WITH SAID POWER UNIT HOUSING, GEAR MEANS POSITIONED WITHIN SAID TOOL HEAD HOUSING TO ENGAGE AND BE DRIVEN BY SAID DRIVE SHAFT GEAR, TOOL WORK OUTPUT MEANS DRIVEN BY SAID GEAR MEANS, AN INTERMEDIATE PLATE FORMING A WALL OF SAID TOOL HEAD HOUSING TO BE ALIGNED WITH SAID POWER UNIT HOUSING ADJACENT SAID GEAR, AN APERTURE FORMED IN SAID PLATE TO SNUGLY RECEIVE SAID HUB, SAID APERTURE BEING ACCURATELY POSITIONED SO THAT SAID GEAR EXTENDS INTO SAID TOOL HEAD HOUSING TO PROPERLY ENGAGE SAID GEAR MEANS, AND FASTENING MEANS FOR RELEASABLY SECURING SAID INTERMEDIATE PLATE TO SAID POWER UNIT HOUSING WHILE SAID SHAFT AND HUB EXTEND THROUGH SAID APERTURE, SAID FESTENING MEANS HAVING LOOSE TOLERANCES IN A PLANE PARALLEL TO SAID PLATE SO THAT THE FASTENING MEANS DOES NOT PRECISELY POSITION THE PLATE IN SAID PLANE AND THE PLATE IS PRIMARILY LOCATED WITHIN SAID PLANE ONLY BY SAID HUB AND SAID APERTURE.