Mechanism For Driving Reciprocating Tool

Abstract

A reciprocating, air-driven mechanism comprising, in one embodiment, a center-seeking piston connected to an oscillatably pivoted linkage which drives a material working tool. Air is delivered to the opposite ends of the piston at different stages of its reciprocation through the use of peripheral grooves on the piston and selected passages in the surrounding cylinder. An operator-actuated valve includes passages through which air may initially be delivered to one end of the piston so as to move it off center when operation of the tool is initiated. In a second embodiment, the air may be delivered to the piston by means of a single valve which is actuated by a piston-driven system to alternately communicate each piston-end with incoming and exhaust air passages. In either embodiment, as the piston is reciprocated, air exhausted from the non-driving end of the piston flows between a bearing plate and a shoe bar to create an air bearing and to pressurize the interior of the tool to prevent contamination thereof by particles of the material upon which the tool is working.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 84,786, filed Oct. 28, 1970, now abandoned and assigned to the Assignee hereof.

Description

BACKGROUND OF THE INVENTION

Many present day reciprocating hand tools such as sanders, hacksaws, etc., are driven by means of linkage which include one or more rack and gear drives which convert rotary electric or reciprocating pneumatic motion into a reciprocating motion of the material working part of the tool. Such tools have proven to be so noisy that they are actually detrimental to the hearing of the operator.

As a further disadvantage, normal usage of the tools creates wear in the mechanical drive linkages, such as rack and gear drives, so that a relatively short tool life is produced, requiring repair or replacement of the tool after a much shorter period of time than desired by purchasers.

Presently available hand power tools of this type are also susceptible to wear across the large bearing surface between the stationary and reciprocating portions of the tool. Wear is further increased because such tools have proven to be highly susceptible to internal contamination by airborne particles of foreign matter removed from the material upon which the tool is working. Such particles can get into the reciprocating mechanisms within the tool, allowing the internal parts to become fouled, corroded, scored, etc.

Repair and maintenance of the tools of this type is a sufficiently serious problem that in shops in which the tools are used extensively, they must either be replaced on a periodic basis or a maintenance program must be followed which requires a large enough inventory of such tools to allow "down" time for cleaning and repair of some of them while others are being used.

Therefore, it has been desirable to provide a relatively maintenance-free reciprocating tool, the speed of which may be controlled by the operator. Such a tool must be relatively simple and easy to manufacture so that its purchase cost is not prohibitive to purchasers. The tool should be provided with means to prevent contamination of the interior of the tool body by small particles of the material being worked on by the tool and should be so structured that a minimum of wear occurs within the tool so that extensive maintenance and repair is not required.

SUMMARY OF THE INVENTION

The present invention relates to such a reciprocating tool and, in the following description, has been applied to a reciprocating sander for purposes of illustration since such sanders usually produce a maximum amount of airborne particles from the workpiece. It is to be understood, however, that the principles of the invention could easily be utilized in a reciprocating hacksaw or other tool, merely by making design changes which might be considered to be within the skill of the art when the principles of the present invention are understood.

One form of tool embodying the present invention may comprise a reciprocating, center-seeking piston to which one end of a linkage may be attached. The opposite end of the linkage may be fastened to the reciprocating portion of the tool so that a suitable motion may be imparted to a work-contacting portion thereof. In this embodiment, the linkage can be pivoted at an intermediate point to minimize the tool vibration and the force moments generated within the tool. In other words, the pivot within the linkage allows the reciprocating piston to be moving in one direction at the same time that the working tool moves in the opposite direction. This feature may be used to prevent the creation of forces which would cause the operator to be unable to control the precise positioning of the tool. If the reciprocating piston is formed as a center-seeking device, even as it is moving in one direction, forces can be created which will tend to move it in the opposite direction and prevent the generation of unwieldy end-of-travel forces within the body of the tool.

In view of the center-seeking effect of the piston, it is considered to be quite likely that when the tool is stopped, the final position of the piston will be at "center." Peripheral grooves may be formed in one embodiment of the piston; the grooves may be located so as to cooperate with passages in a surrounding cylinder block and thus provide for the flow of air to opposite ends of the piston for distribution of the driving force. In this embodiment, when the piston is in the center position, the passages and grooves will probably not be aligned and no driving can occur. In that event, a device may be provided for creating an initial pressure at one end of the piston to move it off center a small amount so that driving can begin.

In this preferred embodiment, the off-centering or starting mechanism may comprise a trigger-actuated pin having an air passage system which temporarily allows air to be delivered to one end of the piston as trigger actuation is initiated by the operator. As the trigger actuation is continued still further, the starting air passage may be closed and driving air is then delivered to the opposite ends of the piston through the cylinder block passages and piston grooves.

As air is delivered to each end of the piston, the air previously delivered to the opposite end must be exhausted so that it will not prevent movement of the piston. In this form of the present invention, the exhaust air is passed between the stationary and moving portions of the tool so as to create an air bearing therebetween which greatly reduces tool wear. The passages through which the exhausting air moves may be so arranged that the exhaust air then enters the tool body about the cylinder block and linkage and the interior of the tool can thus be pressurized. Airborne particles are thus prevented from entering the tool body and fouling the linkages. The exhausting air is allowed to escape from the body through the normal openings at the mating surfaces of the various portions thereof.

In another preferred embodiment of the invention, a reciprocating piston may be utilized via a similarly pivoted linkage to actuate the working tool in a similar manner; i.e., simultaneous motion of the piston and working tool may be in opposite directions to prevent the generation of unmanageable forces. Additionally, the piston may also be suitably connected to a movable element which may be utilized to oscillate a valve spool about its axis. In one embodiment, the valve spool may comprise a cylinder-like member having a pair of blind bores extending inwardly along the axis from either end thereof. Each bore may be formed so as to be in communication with a radial slot, the slots being located at substantially 180.degree. from one another relative to the spool axis. The valve spool may be oscillated within a valve body which is so formed that one side of the body is in communication with air passages leading to one end of the cylinder and the other side of the valve body is in communication with passages leading to the rear of the cylinder.

With structure of this type, air may enter one end of the spool along the blind bore and leave the spool along the radial slot which may be temporarily located on one side of the valve body. Thus, the incoming air will be delivered to the air passages leading to the end of the piston thus communicated therewith. Simultaneously, the opposite end of the piston will be in communication with the second blind bore of the spool via suitable air passages and the radial slot which communicates with the second blind bore. The second blind axial bore of the valve spool may be located so as to be in constant communication with exhaust air passages which deliver the air being exhausted from the end of the cylinder toward which the piston is being driven in order to pressurize the sander housing and create the air bearing therein in a manner similar to that previously described.

With structure of this type, such a tool can be driven reciprocally by means of a signle valve spool which is oscillated through a linkage connected to the piston. The structure is relatively simple and only a few relatively close tolerances need be machined to achieve the desired results.

Thus, the present invention may be embodied in a variety of reciprocating tools which are easy and inexpensive to produce, are quiet, have a relatively maintenance-free long life, and provide for the distribution of forces in such a way that tool position on the workpiece is easily and accurately controlled by a tool operator.

Other objects, advantages, modes, and embodiments of the present invention will be understood by those skilled in the art when the following Detailed Description has been read with reference to the accompanying drawings. It is to be understood that the scope of the present invention includes all modes and embodiments thereof which are defined by the accompanying claims and is not restricted to the preferred embodiments shown and described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial vertical section of the tool, taken along a plane passing along the longitudinal dimension thereof;

FIG. 2 is a partial vertical section of the tool as seen along a line II--II of FIG. 1;

FIG. 3 is a horizontal section of the tool of FIG. 1, as seen along the multiple plane defined by the line III--III therein;

FIG. 4 is a bottom view of a bearing plate used with the preferred embodiment of the tool, as seen along the line IV--IV of FIG. 1;

FIG. 5 is a partial vertical section of a second preferred embodiment of a tool embodying the present invention, taken along a plane passing along the longitudinal dimensions thereof;

FIG. 6 is a partial horizontal section of the tool shown in FIG. 5, as seen along a line VI--VI therein;

FIGS. 7 and 8 are partial sectional views of the tool of FIG. 5, as seen along lines VII--VII and VIII--VIII respectively; and

FIGS. 9 and 10 are schematic illustrations of an oscillatable valve structure which may be used to actuate a tool such as that shown in FIG. 5 .

DETAILED DESCRIPTION

Referring now to the drawings in greater detail, a tool body 11 may be provided with a forward handle 13 and a rear handle 15 by means of which an operator can handle and control the tool. A cylinder block 17, having an end cover 19, can be mounted within the body 11 and formed so that a piston 21 may reciprocate therein.

A piston clevis 23 may be fastened to the piston so as to be reciprocated therewith, and a first link 25 may be suitably attached to the clevis 23 at one end thereof and to a pivotable link 27 at the opposite end thereof. The pivotable link, which can be located to pivot about a pin 29 mounted in the housing 11, may be fastened to a second link 31 which may be also connected to a shoe clevis 33.

Reciprocating movement of the piston 21 will thus cause the shoe clevis to be reciprocatably moved. However, the provision of the pivot 29 in the interconnecting linkage causes the piston and the shoe clevis to move in opposite directions.

With structure of the type thus described, it will be quickly realized that two significant accomplishments may be achieved therewith. First, the degree of wear in the moving parts is significantly reduced over the prior art devices, particularly those devices employing a rack and pinion gear to transmit the forces from the drive system to the working tool, since, historically, such racks and pinions are highly susceptible to fast wear. Additionally, a device formed in accordance with this description will be significantly quieter than the prior art devices since the gearing slap and play are completely eliminated. Secondly, proper design selection of the piston and the movable tool parts will allow them to be of substantially equal weight. Since the piston and the movable tool parts are traveling in opposite directions at any given instant, the resisting forces which must be sustained by the tool operator are significantly reduced since moments which tend to "throw" the tool from his hands, are substantially eliminated. Consequently, the operator can position the tool relative to the workpiece with a greater degree of accuracy than heretofore possible since his position control will be significantly increased.

In other words, a tool formed in accordance with this invention may thus be provided in such a manner as to be easy to handle, relatively quiet, and relatively free from wear.

Shoe clevis 33 may be formed and located so as to move within a slot 37 suitably formed in a bearing plate 39 which may be suitably fastened to the bottom of the housing 11. A shoe bar 41, carrying a sander plate 43 to which a cushioning pad 45 and sandpaper 47 may be suitably attached, is fastened to the clevis 33 by any suitable means, such as screws 49 and/or rivets 51'.

As stated previously, design of the tool may be such as to cause the mass of the piston 21 to be substantially equal to that of the shoe bar 41, sander plate 43, and cushioning pad 45, so that the moments generated within the tool are substantially equal, allowing the operator to easily control the precise position of the tool on the workpiece.

A valve body of any desired type, such as that shown at 51, may be suitably attached to the tool by any means such as bolts 53. A suitable valve seat 55 may be formed within the body, against which a spherical closure ball 57 may be biased by a spring 59 contained within a plug 61 removably fastened in the body. With the valve illustrated, air will enter the valve body by means of a suitable fitting 63 to which a hose (not shown) may be attached. In the position shown in FIG. 1, air is prevented from reaching the body of the tool when the sphere 57 is thus seated against the valve seat 55.

In this exemplary valve structure, when the operator actuates a trigger 67 in the handle 15 by pressing the palm of his hand against it, a lever 69 properly located thereon may be used to exert a force against a valve pin 71, driving the ball 57 downwardly against the force of spring 59. During the initial part of the downward motion of the valve pin 71 and sphere 57, air from the fitting 63 may be directed between the sphere and the valve seat 55 into a radial slot 73 which can be suitably formed in the lower end of the pin 71. Air entering the slot 73 may then pass into an axial passage 75 in the valve pin, a radial passage 77, peripheral groove 79 therein, and then into a passage 81 in the valve body. Comparing FIGS. 1 and 2, it will be seen that air in the passage 81 will be delivered to a cylinder compartment 83 at one end of the piston 21, causing the piston to move toward the left within the cylinder block 17.

As the operator continues to depress the trigger 67, if the peripheral groove 79 is suitably located on the valve pin 71, the continued downward movement of the valve pin will shut off communication between the peripheral groove 79 and the passage 81. Consequently, the air passing between the sphere and the valve seat 55 may then enter a longitudinal passage 85 suitably extending through the valve body and the cylinder block.

When piston 21 has been moved toward the left in this exemplary structure, the air in passage 85 may be delivered to a radial passage 87 in the cylinder block 17, enter a peripheral groove 89 in the piston, and be transferred to a second radial passage 91 in the block.

As shown in FIG. 3, the outer ends of all of the radial passages at the upper portion of the cylinder block may be suitably closed to prevent the escape of air in that area by any desired means such as set screws, or even by the inner surface of body 11. For clarity of illustration, however, the set screws have not been shown in FIG. 1.

Referring now to FIG. 3, air transferred to the radial passage 91 may be transferred through a second longitudinal passage 93 to a forward radial passage 95 for delivery to a cylinder chamber 97 at the forward end, as viewed in FIG. 1, of piston 21. Thus, the air may be caused to act against the forward end of the piston, driving it toward the rear, or right, as viewed in FIG. 1.

As the piston 21 is thus moved toward the rear of the tool, air in the cylinder chamber 83 may be exhausted through a rear radial passage 101, a third longitudinal passage 103, and a radial passage 105. Air in the passage 105 may then be exhausted through a suitable peripheral groove 107 in the piston 21 and a forward exhaust port 109 in the cylinder block.

As the piston reaches the center position shown in FIG. 1, the delivery of air to the forward cylinder chamber 97 may be discontinued and the exhausting of air from chamber 83 may also be shut off by means of the illustrated structure. Continued movement of the piston toward the right may be cushioned by the compression of the remaining air in chamber 83. As the piston crosses over center, air being delivered through the radial passage 87 may enter the peripheral groove 107 and pass through the radial passage 105, longitudinal passage 103, and the radial passage 101 into the chamber 83 to drive the piston 21 toward the front of the tool. At the time that this occurs, air in the chamber 97 may be exhausted through the radial passage 95, longitudinal passage 93, radial passage 91, peripheral groove 89, and a second exhaust passage 111 in the cylinder block.

Thus, air delivered to the tool may be forced to flow substantially continuously to one of the two cylinder chambers 83 and 97, causing the piston 21 to seek a central, balanced position. The addition of the pressurized air to the chamber into which the piston is being driven and the compression of the air remaining in that chamber can cause the piston 21 to continue to reciprocate so that the force developed is transferred through the linkage to the work-contacting portion of the tool.

It will be realized, of course, that although the above-described structure may be expedient in some instances, other structures may be provided which will produce substantially the same results as defined by the present invention. Since the piston of this device has been illustrated, in this embodiment, as being center-seeking, the device is also quieter and subjected to less wear since the piston will act as a brake by the compression of the air at the end of the cylinder toward which it is being driven. In other words, as the piston is being driven toward one end of the cylinder, it can be prevented from colliding with that cylinder end as a result of the biasing force generated by the compression of air remaining between the cylinder and the piston. It will be realized, of course, that since the piston velocity will thus decrease at each end of its travel at a rate which is less sudden than if the piston were allowed to collide with the cylinder end, the material working structure of the tool will be similarly controlled. Therefore, the sanding shoe, in the illustrated embodiment, will be operated through a smoother stroke and will be less susceptible to damaging or marring the finish of the workpiece.

Referring now to FIG. 4, it can be seen that in this embodiment air exhausting through conveniently located ports 109 and 111 in the valve body can be directed to enter a longitudinal groove 113 in the bearing plate which may be located so as to be in communication with the slot 37 in which the shoe clevis 33 reciprocates. If this expediency is utilized, the air can enter the housing 11 in the volume surrounding the linkage 25, 27, and 31, so that the interior of the body may thus be maintained under pressure by air which is as clean as that delivered by the compressor.

As the internal tool body pressure builds up, air pressure may also be exerted along a horizontal plane 115 (FIG. 2) between the bearing plate 39 and shoe bar 41, thereby creating an air bearing between the stationary plate 39 and the reciprocating shoe bar 41. The air bearing may also serve to keep workpiece particles from getting between the elements 39 and 41, thereby reducing tool wear, if it is properly sized. The separation of the plate and bar may, for example, be limited by a pair of brackets 117 on either side of the tool, thereby allowing the internal pressure in the tool housing and the air bearing to be controlled by the initial tool design.

Referring now to FIGS. 5-10, a second mode of a device embodying the present invention has been illustrated. In those figures, those elements which are substantially identical to elements in the above-described embodiment have been provided with similar identification numerals and no further description of their functions is necessary here.

Referring now to FIG. 5, a piston 211 may be suitably positioned within a convenient cylinder so as to form chambers 213 and 215 at opposite ends of the piston. Any convenient clevis 217 may be fastened to the piston 211 and suitably connected to the linkage 25, 27, and 31 so as to activate the shoe 33 in the manner previously described. Thus, when the piston is driven in one direction, the shoe will move in the opposite direction, thereby moving the shoe bar 41 in that direction, as a result of the pivoting of link 27 about the pin 29. Consequently, the advantage of this invention which allows the cancelation of the force moments so as to prevent the tool from being uncontrollable may thus be utilized in different embodiments.

Any expedient device, such as a link 219, may be fastened to the clevis 217 and provided with suitable rack-like teeth 221 adjacent one end thereof. Thus, as the piston reciprocates, the link or rack 219 will similarly reciprocate. The rack 219 may be utilized to operate a conveniently located pinion gear 222. The pinion gear may be suitably fastened to a valve spool 223 so that as the pinion gear is rotated, the valve spool is similarly rotated.

In this exemplary embodiment, the pinion gear 222 is formed in the shape of a ring, i.e., with a bore through the axis thereof so that air may pass into the valve spool 223 in a manner to be described. Although air may be delivered to the bore of the pinion gear 222 by any suitable structure, in the illustrated embodiment the lever 69 on trigger 67 may be utilized to actuate a spring biased pin 225 to drive the valve ball or sphere 57 away from the valve seat 55. When this is accomplished, air from a source (not shown) may be passed into the tool via the fitting 63, past the valve ball 57 and pin 225 and into a vertical passage 227 which is in communication with the central bore of the pinion gear 222. The ends of the passage 227 are suitably closed off by any desired means such as cap screws 229 and 231.

It should be borne in mind that the specific structure of the valve may, if desired, be replaced by any other suitable valve structure. However, it will be realized that the novel valve structure to be described will produce a highly desirable result while, at the same time, being relatively simple.

Valve spool 223 is rotatable within a valve body 235 which is suitably positioned within the rear end of the housing 11 and substantially coaxial with the passage 227. Thus, as seen in FIG. 5, the valve spool and valve body are located within a coaxial counterbore of the passage 227.

Referring now to FIGS. 9 and 10, it is seen that the valve spool 223 may be formed so as to have an upper or first blind bore 237 and a lower or second blind bore 239. Each bore may be formed so as to end at an end wall 241 which is substantially centrally located relative to the axis of the spool. One end of the spool is provided with a radial passage 245 which provides communication between the external periphery of the spool and the first blind bore 237. Similarly, the opposite end of the valve spool is provided with a radial slot 249 which provides communication between the spool periphery and the second blind bore 239.

Referring now to FIG. 9 specifically, it will be seen that air entering the upper blind bore 237 will pass through the radial passage 245 and into a bore within the housing 11 schematically illustrated at 251. As shown in FIG. 5, the passage 251 is in communication with the forward chamber 215 so as to drive piston 211 into the rear chamber 213. Referring again to FIG. 9, it will be seen that air from the chamber 213 will enter a series of vertical and horizontal passages 253 located on the opposite side of the valve body 235. If the valve body is formed with expediently located dividers 257, the air from the upper blind bore 237 will enter the passages 251, while at the same time, air being exhausted through the passages 253 will be communicated to the lower radial slot 249 and the second blind bore 239. Consequently, as the air is exhausted from the lower blind bore 239, it will enter a series of passages 259 so as to be exhausted into the interior of the body 11, thereby pressurizing the interior of the body and causing the formation of the air bearing in a manner described relative to the first embodiment.

Comparison of FIGS. 9 and 10 will now clearly reveal to those skilled in the art that when the piston 211 is reciprocated nearly to the rear of chamber 213, it will cause the valve spool 223 to oscillate about its axis, thereby causing the upper radial slot 245 and the lower radial slot 249 to move from the positions illustrated in FIG. 9 to that of FIG. 10. Consequently, air entering the upper blind bore 237 will be moved through passages 253 and into the rear chamber 213. Simultaneously, the air driven from the chamber 215 by movement of the piston 211 toward the front of the tool will be driven through passages 251, the radial slot 249, the second blind bore 239, and the passages 259 into the interior of the body 11.

With this description, it will now be realized by those skilled in the art that the objects and advantages of the present invention can be achieved by the use of a wide variety of structures of widely differing designs. For example, in the second embodiment, a simple valve structure has been provided in which a single movable element, the oscillatable valve spool 223, quickly and easily controls the direction of piston movement, the pressurization of the interior of the housing 11 and the formation of the air bearing between the bearing plate and shoe bar.

Consequently, with these structures, the applicant has disclosed novel concepts which may be embodied in inexpensive, maintenance-free hand tools which are easy to handle, which inherently prevent the fouling of their internal mechanisms, and which prevent the wear of their bearing surfaces by the controlled exhaust of tool-driving air.

Claims

Other embodiments and modes of the present invention will now become apparent to those skilled in the art without exceeding the scope of the invention as defined by the claims of this application, wherefore what is claimed is:

1. In a hand tool,

a cylinder block having

a first chamber and

a second chamber,

a piston, reciprocally positioned in said cylinder block between said first and second chambers, having

a plurality of peripheral grooves thereon and

a movable work-performing tool attached thereto,

means for delivering a fluid under pressure to a selected one of said peripheral grooves,

means for transferring the fluid from said selected one of said peripheral grooves to one of said first and second chambers,

means for exhausting the fluid from the other of said first and second chambers, and

means positioned between said cylinder block and said tool for creating a fluid bearing therebetween by the fluid being exhausted.

2. The hand tool of claim 1 including

means connecting said piston and said work-performing tool together such that movement of said piston in one direction causes movement of said work-performing tool in the opposite direction.

3. The hand tool of claim 1 wherein

said fluid transferring means and said peripheral grooves are so arranged as to cause said piston to seek an equilibrium position between said first and second chambers.

4. The tool of claim 3 including

means for dislodging said piston from the equilibrium position to start the reciprocating movement thereof.

5. The hand tool of claim 1 including

linkage means interconnecting said work-performing tool and said piston and having

means therein causing simultaneous opposed movement of said work-performing tool and said piston.

6. In a hand tool,

a cylinder block having

a first chamber,

a second chamber, and

a substantially planar surface having an aperture therein,

a piston positioned in said cylinder block for reciprocation between said first and second chambers,

tool means mounted on said cylinder block adjacent said substantially planar surface for reciprocation relative to said cylinder block,

means in said cylinder block linking said piston and said tool means whereby the latter is actuated by the former but in the opposite direction,

means for actuating said piston means by selective delivery of air to said first and second chambers,

means for directing air being exhausted from said first and second chambers to said aperture, and

means fixed to said tool means and operatively connected to said cylinder block and spaced therefrom parallel to said substantially planar surface for forming an air bearing therebetween.

7. In a hand tool,

a housing including

a substantially planar surface having

an aperture therein,

an air motor mounted in said housing,

means for directing compressed air delivered to said housing into said air motor,

means for exhausting compressed air from said air motor to the interior of said housing for pressurization thereof,

flat plate means mounted on said housing in parallel relationship with said substantially planar surface,

means interconnecting said air motor and said flat plate for reciprocation of the latter, and

means for holding said flat plate in a predetermined spatial relationship with said substantially planar surface whereby air being exhausted from said housing through said aperture forms a bearing between said housing and said flat plate.

8. The hand tool of claim 7 including

tool means fixed to said reciprocatable flat plate means for reciprocation of said tool means relative to a workpiece.

9. The hand tool of claim 7 wherein

said air motor comprises

a reciprocatable piston having

a plurality of peripheral grooves thereon,

means for delivering compressed air to one of said peripheral grooves, and

means for transferring the compressed air from said one of said peripheral grooves to a location in which it acts against a face on said piston.

10. The hand tool of claim 7 wherein

said air motor comprises

a first chamber,

a second chamber, and

piston means mounted for reciprocation between said first and second chambers upon the direction of compressed air thereinto, and

means for selectively delivering compressed air to one of said chambers and conducting air away from the other of said chambers including

a valve body mounted within said housing and having

a dividing means thereon,

a valve spool movably mounted in said valve body to opposite sides of said divider and having

air direction means therein for delivering said compressed air to one side of said divider and for receiving air from the other side of said divider for passage thereof to the interior of said housing, and

means for moving said valve spool relative to said valve body.

11. The tool of claim 10 wherein

said valve spool is oscillatable about an axis common to said valve body and comprises

a blind bore in each end thereof and

a pair of radial apertures, each connecting a blind bore with the periphery of said valve spool and located on substantially opposite sides of said valve spool.

12. In a hand tool,

a cylinder having

a first chamber and

a second chamber

a center-seeking reciprocatable piston movable in said first and second chambers,

means for selectively delivering pressurized fluid to one of said chambers to drive said piston into the other of said chambers,

means for selectively exhausting the other of said chambers,

a work-performing tool,

means interconnecting said piston and said work-performing tool for opposed reciprocation of said work-performing tool by said piston, and

means for moving said piston away from its centered position between said first and second chambers when the tool is started by an operator.

13. The tool of claim 12 wherein said piston and work-performing tool are of substantially equal weight to balance forces generated by said hand tool.

14. The tool of claim 12 including

means for mounting said work-performing tool relative to said cylinder as to form an air bearing therebetween by air being exhausted by said other of said chambers.

15. In a tool,

a relatively stationary housing having

a first and

second cylinder chambers fixed therein,

a reciprocatable piston movable in said first and second chambers,

a work-performing tool attached to said piston extending exterior of said housing to act upon a workpiece

means for selectively delivering pressurized fluid to one of said first and second cylinder chambers to drive said piston into the other of said first and second cylinder chambers,

means for selectively exhausting the other of said first and second cylinder chambers as said piston is driven into it,

linkage means interconnecting said work-performing tool and said piston and having

means therein causing simultaneous opposed movement of said work-performing tool and said piston relative to said housing, and

means for controlling the flow of fluid as it is being exhausted from the other of said first and second chambers to form a fluid bearing between said work-performing and said housing.

16. In a tool,

a relatively stationary housing having

first and

second cylinder chambers fixed therein,

a reciprocatable piston movable in said first and second chambers,

a work-performing tool attached to said piston extending exterior of said housing to act upon a workpiece,

means for selectively delivering pressurized fluid to one of said first and second cylinder chambers to drive said piston into the other of said first and second cylinder chambers,

means for selectively exhausting the other of said first and second cylinder chambers as said piston is driven into it,

said fluid delivery and exhaust means being so positioned as to cause said piston to seek an equilibrium position in said first and second cylinder chambers,

linkage means interconnecting said work-performing tool and said piston and having

means therein causing simultaneous opposed movement of said work-performing tool and said piston relative to said housing,

means for moving said piston away from said equilibrium position when said tool is started by an operator comprising

normally closed valve means for controlling the flow of the pressurized fluid,

a preliminary distribution system for delivering pressurized fluid from said valve means to one of said first and second cylinder chambers,

means for delivering pressurized fluid to said preliminary distribution system only during initial actuation of said valve means and then only to said selective delivering means during additional actuation of said valve means, and

means for actuating said valve means.

17. The tool of claim 16 including

means for controlling the flow of fluid as it is being exhausted from the other of said first and second chambers to form a fluid bearing between said work-performing tool and said housing.

18. The tool of claim 16 wherein

said delivering means comprises

a valve-actuating means,

fluid passage means in said valve-actuating means for alignment and communication thereof with said preliminary distribution means, and

means for discontinuing communication between said fluid passage means and said preliminary distribution means.

19. The tool of claim 18 including

means for controlling the flow of fluid as it is being exhausted from the other of said first and second chambers to form a fluid bearing between said work-performing tool and said housing.

20. The tool of claim 16 including

means for delivering the fluid being exhausted from the other of said first and second cylinder chambers to the interior of said housing to prevent the entry of particles therein from a workpiece being acted upon by said tool.

21. The tool of claim 20 including

means for controlling the flow of fluid as it is being exhausted from the other of said first and second chambers to form a fluid bearing between said work-performing tool and said housing.