US3915242A

US3915242A - Fastener driving power tool

Info

Publication number: US3915242A
Application number: US469832A
Authority: US
Inventors: Walter Bell
Original assignee: Star Expansion Industries Corp
Current assignee: Star Expansion Industries Corp
Priority date: 1974-05-14
Filing date: 1974-05-14
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28

Abstract

A power tool for driving fasteners by the explosion of a charge which propels a piston within a bore of a barrel includes a conical shoulder in a pin guide bushing which terminates the forward end of the bore and a complementary conical surface on an enlarged piston head facing the conical shoulder and matable therewith in an overdriven position of the piston. In this manner, the piston is brought to a standstill in the overdriven position by engagement of the conical shoulder and conical surface. A pivotally mounted latch is provided which is adapted to resiliently engage the piston for returning the same to the initial position while frictionally sliding thereagainst without moving the piston when the barrel of the tool containing the piston is returned to its cocked position.

Description

United States Patent Oct. 28, 1975 Bell [ FASTENER DRIVING POWER TOOL [75] Inventor: Walter Bell, Monroe, NY.

[73] Assignee: Star Expansion Industries Corporation, Mountainville, NY.

[22] Filed: May 14, 1974 [21] Appl. No.: 469,832

[52] US. Cl. 173/134; 60/635; 173/139;

227/ 10 [51] Int. Cl. B25C 1/14 [58] Field of Search 173/134, 139; 60/635, 636,

Primary ExaminerErnest R. Purser Attorney, Agent, or Firm-Friedman & Goodman, Esqs.

[5 7] ABSTRACT A power tool for driving fasteners by the explosion of a charge which propels a piston within a bore of a barrel includes a conical shoulder in a pin guide bushing which terminates the forward end of the bore and a complementary conical surface on an enlarged piston head facing the conical shoulder and matable therewith in an overdriven position of the piston. In this manner, the piston is brought to a standstill in the overdriven position by engagement of the conical shoulder and conical surface. A pivotally mounted [56 References Cited latch is provided which is adapted to resiliently engage UNITED STATES PATENTS the piston for returning the same to the initial position 3 563 439 2,197] P 227,10 while frictionally sliding thereagainst without moving omeroy 3,690,536 9/1972 Bakoledis 227/10 plsPon r z Barrel E comdmmg the 3,820,703 6/1974 Rangger 227/10 re O 1 S we 6 14 Claims, 12 Drawing Figures 36b 12 38 36 36a 42 24 52 so 22 22a es 56 58 7o 66 72 4 4o pix-------w //i/////// /////////y/ V l l 6 9 69 30. |I 3| f I T 2 i 96 i I T/A/A A 26 u. 46 l 44 4a g I E 32 I60. 54 so 62 34 64 as I y US. Patent 0a. 28, 1975 Sheet 1 of 3 3,915,242

mm ww vm mm Ow mm op mm nwm US. Patent 0a. 28, 1975 Sheet 2 of 3 FASTENER DRIVING POWER TOOL BACKGROUND OF THE INVENTION This invention generally relates to power actuated tools, and more particularly to an improvement in an explosive-actuated power tool for driving fasteners and the like.

Explosive-actuated tools for driving fasteners are well known in the art and are widely used. In general, such tools comprise a housing of pistol-like form having a grip with an associated trigger and a generally tubular forward portion, a barrel mounted within the tubular formed portion of the housing and adapted to move axially therein, and a piston contained within the barrel and adapted to move axially with respect to the barrel.

In the usual form of such tools, the breech end of the barrel is recessed to form a chamber for an explosive cartridge, the chamber communicating with the boreso that the gasses generated by the explosive charges are transmitted to the head of the piston, anda more or less conventional firing mechanism including a firing pin is located in the housing.

In the operation of the tool, the barrel is moved forward with respect to the housing, and a cartridge is inserted in the chamber. A fastener or the like is positioned in the muzzle end of a barrel extension, forward of the piston. The barrel is then moved rearwardly with respect to the housing, bringing the chamber into proximity with the firing pin, and also, by way of suitable operating rods, cams, or the like, cocking the firing mechanism.

While tools of this type have generally been effective and have obtained wide acceptance, they have nevertheless exhibited certain disadvantages to which the present invention is directed. One disadvantage of the prior art tools has been the configuration of the piston heads and the complementary surfaces of the bore with which the piston is engageable to stop the piston in the event of overdriving the piston. This is particularly important when a fastener is not inserted into the tool prior to firing or if the driving of a fastener is attempted into an unacceptably soft work surface. The prior art piston heads have generally been cylindrical in configuration and have exhibited limited, predominantly planar surfaces which are substantially normal to the axis of the piston. The engagement of the piston with the complementary barrel surfaces has resulted in substantial shock impulses and frequent breakage of the pistons. One attempt to remedy this problem has been the utilization of resilient buffers which are disposed within the barrel for engagement with the piston head. However, the use of such buffers has decreased the effective maximum advancement or penetration of the piston and thereby the ability to overdrive to some extent without damaging the tool.

Another disadvantage of prior art power tools has been the resetting means for resetting the piston to the initial position aftter firing. This is typically accomplished by forwardly moving the barrel of the tool with respect to the housing while suitable means engages the piston and maintains the same fixed with respect to the housing. Prior art resetting means have typically included resiliently biassed pistons which are urged through a slot in the barrel to at least partially extend into the bore in which the piston is housed. The use of such resiliency biassed pistonshas presented a problem in that these pistons are slidably mounted and this brings about frictional forces which are frequently excessive and undesirably urge the piston to slightly advance within the barrel when the latter is returned to its cocked position. For this reason, the precise clearance forming the explosion chamber is not predictable and the initial velocity of the piston upon firing of the cartridge is unknown and similarly unpredictable.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a fastener driving power tool which is not possessed of the above described disadvantages inherent in comparable prior art tools.

It is another object of the present invention to provide a fastener driving power tool as described above which is simple in construction and economical to manufacture.

It is still another object of the present invention to provide a power tool of the type above suggested which absorbs shock impulses which result from the stopping of a propelled piston when the latter reaches an overdriven position to thereby decrease breakage of the piston and members engaging therewith.

' It is yet another object of the present invention to provide a power tool of the type above described which absorbs shock impulses without the utilization of resilient buffers and thereby increases the permissible overdrive of the piston.

It is a further object of the present invention to provide a power tool of the type generally under discussion which includes pivotally mounted and resiliently biassed latch means which engages the piston for returning the same to an initial position within the barrel but which exhibits low enough friction to thereby prevent advancement of the piston when the barrel is returned to its cocked position.

It is yet a further object of the present invention to provide a pivotally mounted and resiliently biassed latch which includes two spaced surfaces respectively adapted to engage the piston and a bearing surface at the breech end of a slot of the barrel, the two latch surfaces being selectively spaced to regulate the extent to which the piston is returned into the breech end of the barrel. In this manner, a.clearance forming the explosion chamber and the initial velocity of the piston may be controlled.

It is still a further object of the present invention to provide a power tool of the type generally under discussion which provides a large area of contact between a shoulder or abutment surfaceat the forward end of the bore of the tool and a corresponding surface on the head of the driving piston, by making the shoulder and corresponding surface conical, so that pressure per unit area is kept low and little deformation results when the piston is overdriven and engagement between the piston and shoulder results.

It is an additional object of the present invention to provide a power tool of the type under consideration which includes a driving piston slidable within the bore of the tool, the piston having three axially spaced areas of support for the piston head. Two areas of support are cylindrical and are spaced by a friction ring groove, and the third area of support is formed at the juncture of two base-to-base cones. The provision of axially spaced support areas in this manner stabilizes the piston while the same is propelled within the bore to drive the fastener.

In order to achieve the above objects, as well as others which will become apparent hereafter, an explosive-actuated fastener driving power tool is characterized by a discontinuity in the barrel housing the piston, the discontinuity being in the form of a conical shoulder in the pin guide bushing coaxially disposed in relation to the bore and terminating said bore, and having an increasing diameter in the direction of the breech end of the barrel. The piston head is provided with a complementary conical surface facing the conical shoulder and matable therewith in an overdriven position of the piston. In this manner, the piston is brought to a stanstill, if overdriven, by engagement of the conical shoulder and the conical surface.

In the presently preferred embodiment, the barrel is provided with a longitudinal slot and resetting means is provided in the form of a latch pivotally mounted on the housing. Biassing means are provided for resiliently urging the latch to pivot and bring at least a portion thereof through the slot into the bore of the barrel. In this manner,the latch can engage the head of the piston during movement of the barrel. However, the latch exhibits substantially less friction during pivoting than the prior art slidably mounted latches to thereby permit sliding engagement of the latch with conical surfaces of the piston head without moving the piston from its desired initial set position.

BRIEF DESCRIPTION OF THE DRAWINGS With the above and additional objects and advantages in view, as will hereinafter appear, this invention comprises the devices, combinations and arrangements of parts hereinafter described by way of example and illustrated in the accompanying drawings of a preferred embodiment in which:

FIG. 1 is a side elevational view of a fastener driving power tool in accordance with the present invention, the power tool being shown partly in dashed outline and only the portions of the housing and the barrel which cooperate with the piston are shown in cross section;

FIG. 2 is a side elevational view of the pin guide bushing body of the power tool shown in FIG. 1, supported by the housing and adapted to house a fastener, showing the conical shoulder which is adapted to engage with a corresponding conical surface of the piston;

FIG. 3 is a rear elevational view of the pin guide bushing body shown in FIG. 2;

FIG. 4 is a fragmented top plan view of the bushing body shown in FIG. 3;

FIG. 5 is a rear elevational view of the pin guide bushing body shown in FIG. 1;

FIG. 6 is a side elevational view of a piston in accordance with the present invention, shown prior to grinding of the piston head to make the same receivable within the bore of the tool barrel;

FIG. 7 is similar to FIG. 6, but shown subsequent to grinding of the piston head;

FIG. 8 is a side elevational view of the latch shown in FIG. 1;

FIG. 9 is a bottom plan view of the latch shown in FIG. 8;

FIG. 10 is a fragmented cross sectional view of the power tool shown in FIG. 1, shown with the piston in the fully overdriven position;

FIG. 11 is similar to FIG. 10, showing the manner in which the latch engages the piston to retain the same fixed in position relative to the housing to reset the po- 4 sition of the piston while the barrel is slidably moved from the cocked to a fully extended position; and

FIG. 12 is a projection of the latch shown in FIG. 8, taken along the direction of the arrow A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring specifically t0 the drawings, in which identical or similar parts have been designated by the same reference numerals throughout, and first referring to FIG. 1, the explosive-actuated fastener driving power tool incorporating the important features of the present invention is generally designated by the reference numeral 10. The general operation of the power tool 10 is similar to that disclosed in US. Pat. Nos. 3,423,001; 3,468,465; and 3,548,590. The descriptions in these latter patents are hereby incorporated by reference into this application for the teachings therein of the structures and operation of the power tool which are conventional and known in the art. Accordingly, the structures associated with the loading, firing and extracting the explosive charges will not be described in this application.

In FIG. 1, the power tool 10 includes a housing assembly 12, which is shown fragmented. The housing 12 has a breech portion 14 and a tubular forward portion 16 having an elongate bore 16a.

Disposed in the breech portion 14 of the housing is a breech block 18 and a extractor hook plate 20 which are more fully described in the aforementioned patents.

A barrel 22 is provided which has a muzzle end 24 and a breech end 26. The barrel is telescopically mounted in the tubular forward portion and axially movable therein between a cocked position, shown in FIG. 1, and a fully extended position, shown in FIG. 11.

The breech end 26 of the barrel 22 includes a cartridge chamber 28 which is adapted to receive an explosive cartridge 30. The cartridge chamber 28 is in communication with a bore 22a of the barrel 22 so that when the cartridge 30 is fired the expanding gasses are transmitted into the bore 22a.

The barrel 22 of the present invention includes a longitudinal slot 32 along the substantial length thereof. The slot has a bearing surface 34 at the breech end 26 thereof.

Retaining means in the form of a pin guide bushing body 36 is mounted on the muzzle end 24 of the barrel 22 for retaining the fastener (not shown) in position for being driven into a surface in a manner to be described. The bushing body 36 includes a longitudinal bore 36a which is coaxial with the bore 22a of the barrel and has a diameter smaller than the latter bore. In the prior art, the difference in diameters between the bores 22a and 36a represents an abrupt discontinuity in the form of a planar surface normal to the axis of the bores. An important feature of the present invention is that the discontinuity between the bores 22a and 36a is in the form of a conical shoulder 36b, as shown, having increasing diameters in the direction of the breech end 26 of the barrel 22. One end of the guide body 36 is provided with an annular projection 38 and the other end of the body 36 is provided with a screw thread 40 which is adapted to extend interiorly of the muzzle end 24 of the barrel 22 and be threadedly engaged therewith. To lock the position of the guide body 36 with respect, to the barrel 22, a ring nut 42, which is engaged with the thread 40, is tightened against the barrel 22. This prevents undesired rotation of the guide body 36 with respect to the barrel 22 and also locates the guide body axially with respect to the barrel.

Extending along the substantial length of the guide body 36, between the annular projection 38 and the ring nut 42, is a pin guide bushing sleeve 44 which is mounted for sliding movement on the guide body. The bushing sleeve 44 is provided with a rearwardly enlarged collar 46 which encloses the ring nut 42 on the thread 40. A pin guide bushing helical spring 48 is disposed in a state of compression between the barrel 22 and the enlarged collar 46 to resiliently bias the sleeve 44 into abutment against the annular projection 38. In FIG. 1, the tool is shown in a cocked, ready to fire condition, wherein the guide body 36 is forced against a surface into which the fastener is to be driven. If an attempt is made to cock the firing mechanism other than by pressing the muzzle against a rigid surface, such as by grasping the sleeve 44 and pulling it rearwardly, the sleeve moves rearwardly to a limited extent, against the action of the spring 48, into abutment against a shoulder of the housing. This constitutes a safety feature more fully described in my US. Pat. No. 3,423,001.

A piston generally designated by the reference numeral 50 is provided with a leading elongate ram or shaft 52 and a trailing enlarged head generally indicated by the reference numeral 54. The diameter of the ram 52 is smaller than that of the bore 22a and corresponds to the diameter of the bore 36a while the outer diameter of the head 54 is substantially equal in diameter to that of the bore 22a, with some clearance to permit the piston 50 to axially move therein between an initial position shown in FIGS. 1 and l l and a final fired position shown in FIG. 10.

Referring to FIGS. 6 and 7, the piston 50 is shown to include a leading conical portion 54a and a trailing cylindrical portion 54b. The cylindrical portion 54a includes a leading tapered conical surface 56 which faces and is complementary to the conical shoulder 36b and matable therewith in the overdriven position of the piston 50. The piston 50 is brought to a standstill in the overdriven position as shown in FIG. by engagement of the conical shoulder 36b and the conical surface 56, with substantial absorption of shock impulses which would tend to increase breakage of the piston and members engaging therewith.

The cylindrical and conical portions 54b and 54a are separated by an annular groove adapted to receive a latch, as to be described hereafter. The groove is formed by a further conical surface 58 on the conical portion 54a of decreasing diameter in the direction of the cylindrical portion 54b. Accordingly, the conical portion 54a includes two opposing

tapers

56 and 58. The piston is shown in FIG. 6 before the head of the same is ground to the dimensions of the bore 22a. When first formed, the opposing

tapered surfaces

56 and 58 join at an annular peaked edge. Subsequent to grinding, a cylindrical ground bearing surface 60 is formed which is axially spaced from the cylindrical surfaces 62 of the piston head portion 54b. Such separation provides stability of the piston ram 52 while the same is propelled through the barrel 22.

As suggested above, the piston is brought to a standstill, particularly when a fastener is not placed in the bore 36a, or a too-soft surface is penetrated by the fastener, by the engagement of the conical shoulder 36b with the tapered surface 56. To prevent jamming of the piston with the pin guide bushing body 36, it is an important feature of the present invention that the tapers represented by the conical shoulder 36b and the conical surface 56 be complementary and define non-locking tapers. Such tapers advantageously form an angle of 15 with the axial direction of the elongate ram 52 of the piston. However, other non-locking tapers may be utilized, as well known 'to those skilled in the art.

The piston cylindrical portion 54b is advantageously provided with an annular slot 64 in which a friction ring 66 is disposed for frictionally engaging the internal surface of the bore 22a of the barrel 22. The purpose of the friction ring 66 will be described hereafter.

To further strengthen the piston 50 against breakage. the piston rod or ram 52 is connected to the conical portion 54a by means of a blend radius 68. Similarly, the conical surface 58 is blended with the planar surface 69 of the cylindrical portion 54b by means of a rounded annular surface 70. The

surfaces

58, 69 and 70 define an annular groove which is adapted to receive a portion of a latch, as to be described hereafter.

The piston 50, as shown in FIG. 1, is disposed in its initial position prior to explosion of the cartridge 30. The piston is shown extending into the breech end of the barrel 26 to form a clearance therewith in the form of an explosion chamber 72. The axial dimension of the clearance 72, and the means for controlling the same. will be described hereafter.

In FIGS. 6 and 7, the diametrical dimension of the piston portions 54a and 54b prior to grinding is designated by the referencenumeral 76 and the corresponding dimensions subsequent to grinding by the reference numeral 78. Clearly, the dimensions 78 are substan' tially equal to the internal diameter of the bore 22a, with some clearance to permit slidable movement as suggested above, and as to be fully described hereafter. Advantageously, the free ends of the rams 52 are pro vided with chamfers 80 which assure that the ram is properly guided into the bore 360 of the fastener supporting guide body 36.

Resetting means are known in the prior art for engaging the piston and returning the same to the breech end of the barrel. An important feature of the present invention is the specific nature of the latching device, which is generally designated in FIG. 1 by the reference numeral 82. The latching assembly 82 includes a latching housing 84 fixed on the tubular forward portion 16 of the housing 12. A latch 86 is pivotally mounted on a pivot pin 88 fixed to the housing 84.

Referring to FIGS. 8, 9 and 12, the latch 86 includes a substantially cylindrical surface 90 normally disposed within the slot 32 and a planar and rounded surface 92 which normally projects through the slot 32 into the bore 22a.

Surfaces

90 and 92 are spaced from each other in the axial direction of the barrel by a length 91a, best depicted in FIG. 12, corresponding to a surface 91 of the latch.

A dowel stop pin 94 is provided at the other end of the housing 84 positioned to abut against a stop portion 96 of the latch 86 to prevent excessive counter-clockwise pivotal motion about the pivot pin 88, as viewed in FIG. 1. The pin 94 positions latch 86 in position to serve its function, but prevents the latch from dragging constantly along the surface of piston rod 52.

The latch 86 is resiliently urged for pivoting in a counter-clockwise direction to the position shown in FIG. 1 by a spring 98 whose restoring forces are maintained and adjusted by means of a jammed nut retainer assembly 100. The spring 98 maintains the latch 86 in the position shown in FIG. 1 when neither of the

conical surfaces

56 and 58 of the piston displace or depress the latch inwardly into the slot 32 against the action of the spring 98. Referring to FIGS. 8 and 9, the latch 86 includes a pivot hole 88a adapted to receive the pivot pin 88. It should also be pointed out that the surface 90 is engageable with the bearing surface 34 of the barrel when the barrel is moved to a fully extended condition, as shown in FIG. 11, and the rounded and planar surface 92 is complementary and matable with the planar surface 69 and rounded surface 70 within the groove formed by the two piston portions 54a and 54b to thereby increase the surface areas which engage between the piston and the latch. With the above described arrangement, it should be clear that the spring 98 resiliently urges the latch 86 to pivot and bring at least a portion thereof through the slot 32 and into the bore 22a of the barrel to thereby be disposed in the path of the head 54 of the piston during movement of the barrel, as to be described.

The operation of the power tool will now be described. First referring to FIG. 1, the tool is shown in a cocked, ready to fire position. In this condition, the pin guide bushing body 36 is urged against the surface into which the fastener is to be drivento thereby cock the trigger mechanism. The piston 50 is here shown in an initial position to provide a predetermined desired axial clearance of the explosion chamber 72. The rarn or rod 52 is maintained in the coaxial position shown due to the stabilizing effect of the spaced

cylindrical surfaces

60 and 62. Also, in this condition, the latch 86 is disposed with the rounded surface 92 projecting into the bore 22a, as shown in FIG. 1.

When the cartridge 30 is exploded, the expanding gasses fill the explosion chamber 72 and enter the bore 22a and propel the piston 50 towards the muzzle end 24 of the barrel 22, with the ram or rod 52 of the piston entering into the bore 36a and thereby driving the fastener originally disposed in the bore 360 into the surface of interest. In normal use, the small or leading end of the piston stops about flush with the muzzle of the tool, because at that position the fastener is fully installed.

When the piston 50 is overdriven, conical surface moves past the rounded surface 92, the surface 56 pivots the latch in a clockwise direction and gradually depresses the latch 86 into the slot 32 against the action of the spring 98 while the piston 50 continues to advance without significant resistance of the latch 86. Once the cylindrical surface 60 is past the rounded surface 92 of the latch, the latter pivots in a counter-clockwise direction to thereby enter the groove formed by the piston portions 54a and 54b. This is indicated in FIG. 10, wherein the piston is overdriven and stopped on the conical shoulder 36b.

It is important that the spring 98 have sufficient resiliency to urge the latch 86 in the counter-clockwise direction with sufficient force to assure that the piston head does not pass the rounded surface 92 of the latch without engaging the latter but that the latch enter into the groove between the two pistons to thereby engage the planar surface 69 and rounded surface 70. This happens when the barrel is manually pulled outwardly.

As suggested above, in normal use, the piston drives a fastener down flush with the penetrated work surface, and thus the end of the piston does not extend beyond the muzzle end of the tool. The two conical surfaces mate only if a fastener is driven into an unacceptably soft work surface, and especially if the user has forgotten to insert a fastener into the tool before firing it. An important feature of the present invention is the use of mating conical surfaces as described which substantially reduce the unit pressures which normally tend to increase breakage of the piston and members engaging therewith. The reduction in unit pressures is primarily due to the fact that the energy dissipating forces are distributed over greater areas of tapered surfaces. In the prior art, the surfaces which characterized the discontinuities in the two bore diameters were normally planar and normal to the axis of the bores and consequently exhibited smaller areas of engaging surfaces.

To insure that the conical shoulder 36!) engages the conical surface 56 on overdrive prior to engagement between the piston surface 69 and the latch surface 92, the distance between the blend radius 68 and the surface 69 is selected to be greater than the distance between blend radius 36c and the rounded surface 92 in the position of the barrel 22, as shown in FIG. 10. This prevents possible damage to the latching assembly 82 or to the piston itself.

In FIG. 10, the piston 50 is shown in an overdriven condition and stopped on the conical shoulder 36b. In this condition, the latch 86 is shown in a slightly depressed condition against the action of the biasing spring 98.

To reset the position of the piston from the overdriven position shown in FIG. 10 to an initial position, shown in FIGS. 1 and 11, the barrel 22 is moved from the cocked position shown in FIG. 1 to a fully extended position shown in FIG. 11. The barrel 22 can be brought to the fully extended position by snapping the tool and the barrel 22 outwardly, or in the muzzle direction of the tool, or by pulling the barrel forward by hand. In any event, the spring 98 causes the latch 86 to engage the planar surface 69 and rounded surface of the piston of thereby fix the position of the piston 50 relative to the housing 12 while the barrel 22 moves to the fully extended position. Effectively, this returns the piston to the initial position within the barrel, as shown in FIG. 11. The barrel continues to move outwardly until the surface 90 of the latch 86 engages the bearing surface 34 of the barrel slot. Accordingly, the'latch 86 limits the extent to which the barrel may move out.- wardly, in addition to maintaining the fixed position of the piston.

An important feature of the present invention is the spacing of the surface 90 and the rounded surface 92 by a distance corresponding to the length 91a of the surface 91. In this manner, as clearly shown in FIG. 11, engagement of the surface 90 with the bearing surface 34 of the slot positions the piston 50 a predetermined distance from the breech end 26 of the bore 22a to provide the desired clearance of the explosion chamber 72.

An important characteristic of the latching assembly 82 of the present invention is that returning of the barre] 22 to the cocked position does not change the position of the piston within the bore 22a. Upon slidable movement of the barrel 22 through the tubular forward portion 16, the portion of the latch 86 engaging the conical surface58 is again depressed into the slot 32 while pivoting in a clockwise direction about the pin 88. The latch 86 abuts successively on the

surface

58, 60 and 56 while these surfaces depress the latch against the action of the biassing spring 98 It is important that the rigidity of the spring 98 by sufficiently small so that the latch can ride on these camming surfaces without 9 imparting substantial friction thereagainst and thereby avoid movement of the piston towards the muzzle end 24. The latch assembly 82 of the present invention exhibits low frictional forces upon engagement with the conical portion 54a of the piston due to the pivot arrangement of the latch 86. When the barrel 22 is moved rearwardly, the piston 50 remains fixed in position within the bore 22a while the latch 86 rides over the conical portion and returns to its normal position shown in FIG. 1. To further eliminate the possibility of forward movement of the piston 50 with rearward movement of the barrel 22 due to engagement with the latch 86, suitable friction means between the piston.

and the barrel 22 may be provided. In the presently preferred embodiment, a friction ring 66 is disposed within the slot 64, with the friction ring 66 engaging the internal surface of the bore 22a to thereby impart additional friction which counteracts any possible frictional forces imparted to the piston by the resiliently actuated latch 86. Thus, the latch of the present invention is designed to provide very low frictional forces on the piston while being able to engage the piston in looking relation when the same tends to move in the forward direction by a positive engagement ofa latch portion with a corresponding or mating surface of the piston head. The latch of the present invention has less inertia and exhibits less sliding friction about the pivot 88, which makes this latch particularly suitable for this application.

Once the barrel 22 and the piston 50 are returned to their respective cocked and initial positions, as shown in FIG. 1, the spring 98 reverts the latch 86 to its normal position shown in FIG. 1 with a portion of the latch within the bore 22a in position to again intercept the piston by engagement with the planar surface 69 and the rounded surface 70.

When the conical surface 56 engages the conical shoulder 36b, outward forces are generated which act on the latter. As is clearly evident from the FIGURES, the muzzle end 24 of the barrel 22, which receives the tapered portion of the pin guide bushing body 36 in the region of the conical shoulder 36b, reinforces the bushing body to help the latter withstand the outward forces.

The power tool described above represents a simplifled design which can be more economically manufactured. The resulting construction permits a greater permissible overdrive of the piston for equal length barrels due to the tapered nature of the piston head and due to the elimination of resilient buffers within the barrel structure. The resulting construction additionally reduces pressures per unit area and absorbs shock impulses which tend to increase breakage of the piston, a common problem with prior art constructions. The latch of the present invention performs the desired functions while minimizing the amount of friction which the latch exerts upon the piston when the barrel 22 is returned to its cocked position with the piston in its initial position. For this reason, and due to the novel construction of the latch, the axial position of the piston can be accurately determined to thereby form an explosion chamber having desired dimensions and permit close regulation of the initial velocity of the piston. The latch also controls the rotational position of the barrel, acting as a key in the slot.

Numerous alterations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present dis- 10 closure relates to a preferred embodiment of the invention which is for purposes of illustration only and is not to be construed as a limitation of the invention.

What is claimed is:

1. In an explosive-actuated fastener driving power tool having a housing having a breech portion and a generally tubular forward portion, a barrel having a bore and telescopically mounted in said tubular forward portion and axially movable therein between a cocked position and a fully extended position, said barrel having a breech end and a muzzle end, a piston having a trailing enlarged head dimensioned to be received in and slidably movable in said bore and a leading elongate ram having a smaller diameter than that of said bore and axially movable therein between an initial position and a final fired position; means for explosively driving said piston from said initial position to said final position when said barrel is disposed in said cocked po.- sition; resetting means responsive to movement of said barrel from said cocked towards said fully extended positions for returning said piston from said final to said initial positions relative to said barrel; retaining means disposed on said muzzle end of said barrel-for retaining a fastener in position for driving by said piston as the latter moves from said initial to said final positions, said retaining means having a bore coaxial with said bore of said barrel and having a smaller diameter than that of said latter bore and corresponding to the diameter of said piston ram to form a discontinuity in bore diameters to permit only said piston ram to extend through said bore of said retaining means, the improvement comprising said discontinuity being in the form of a conical shoulder coaxially disposed in relation to said bores and having increasing diameters in the direction of said breech end, said trailing enlarged piston head having a complementary conical surface facing said conical shoulder and matable therewith in an overdriven position of said piston beyond said final position, whereby said piston is brought to a standstill in said overdriven position by engagement of said conical shoulder and conical surface with substantial elimination of shock impulses which tend to increase breakage of said piston and members engaging therewith; said breech end of said barrel being movable between said breech portion and said tubular forward portion of said housing as said barrel moves between said cocked and fully extended positions, said resetting means comprising latching means on said housing for latching said enlarged head of said piston and maintaining the latter substantially fixed in position relative to said housing during movement of said barrel from said cocked to said fully extended positions; and barrel being provided with a longitudinal slot, and said latching means comprises a latch pivotally mounted on said housing; biassing means for resiliently urging said latch to pivot and bring at least a portion thereof through said slot and into said bore of said barrel, whereby said latch can engage said enlarged head'of said piston during movement of said barrel; said head of said piston including a trailing cylindrical portion and a leading conical portion, said cylindrical and conical portions of said piston being separated by an annular groove adapted to receive said portion of said latch.

2. In a driving power tool as defined in claim 1, wherein said conical shoulder and said conical surface each define complementary non-locking tapers.

3. In a driving power tool as defined in claim 2, wherein each taper defines an angle of 15 with the 1 1 axial direction of said elongate ram of said piston.

4. In a driving power tool as defined in claim 1, further comprising stop means for preventing excessive movement of said latch when the latter engages said piston during movement of said barrel from said cocked to said fully extended positions.

5. In a driving power tool as defined in claim 4, wherein said stop means comprise a stop pin arranged to engage and limit the pivoting of said resiliently biassed latch.

6. In a driving power tool as defined in claim 1, wherein said groove is formed by a further conical surface on said conical portion of decreasing diameter in the direction of said cylindrical portion, and a planar surface on said cylindrical portion substantially normal to the axis of said piston, whereby said latch is pivoted against the action of said biassing means when displaced by said conical surfaces on said conical portion and returned to a normal position of said latch when received in said groove to abut against said planar surface.

7. In a driving power tool as defined in claim 6, wherein said further conical surface and said planar surface are connected by a rounded annular surface to increase the strength of said piston head.

8. In a driving power tool as defined in claim 7, wherein said portion of said latch receivable within said groove is provided with complementary surfaces to said planar and rounded surfaces on said piston head to increase the engaging surfaces between said piston and said latch.

9. In a driving power tool as defined in claim 6, wherein the resiliency of said biassing means is selected to permit said barrel and said piston to return to said cocked and initial positions respectively from said extended position by sliding engagement of said latch with said conical surfaces without displacing said piston with respect to said barrel.

10. In a driving power tool as defined in claim 9, further comprising friction means on said piston for maintaining said piston substantially fixed with respect to said barrel against the action of said latch when the latter engages and moves relative to said piston conical surfaces.

11. In a driving power tool as defined in claim 10, wherein said friction means comprises an annular slot in said cylindrical portion of said piston, and a friction ring disposed within said slot extending beyond the latter for engaging said barrel.

12. In a driving power tool as defined in claim 6, wherein said conical surface is axially spaced from said planar surface a distance greater than the corresponding distance between said conical shoulder and said latch portion engageable with said planar surface in a substantially cocked condition of said barrel, whereby engagement on overdrive between said conical surface and conical shoulder prior to engagement between said latch portion and said planar surface is assured to thereby avoid possible damage to said latch or said piston.

13. In a driving power tool as defined in claim 1, wherein said cylindrical and conical portions of said piston engage said barrel and are spaced along the axis of the latter to stabilize said piston when the latter is propelled from said initial to said fixed positions.-

14. In an explosive-actuated fastener driving power tool having a housing having a breech portion and a 12 generally tubular forward portion, a barrel having a bore and telescopically mounted in said tubular forward portion and axially movable therein between a cocked position and a fully extended position, said barrel having a breech end and a muzzle end, a piston having a trailing enlarged head dimensioned to be received in and slidably movable in said bore and a leading elongate ram having a smaller diameter than that of said bore and axially movable therein between an initial position and a final fired position; means for explosively driving said piston from said initial position to said final position when said barrel is disposed in said cocked position; resetting means responsive to movement of said barrel from said cocked towards said fully extended positions for returning said piston from said final to said initial positions relative to said barrel; retaining means disposed on said muzzle end of said barrel for retaining a fastener in position for driving by said piston as the latter moves from said initial to said final positions, said retaining means having a bore coaxial with said bore of said barrel and having a smaller diameter than that of said latter bore and corresponding to the diameter of said piston ram to form a discontinuity in bore diameters to permit only said piston ram to extend through said bore of said retaining means, the improvement comprising said discontinuity being in the form of a conical shoulder coaxially disposed in relation to said bores and having increasing diameters in the direction of said breech end, said trailing enlarged piston head having a complementary conical surface facing said conical shoulder and matable therewith in an overdriven position of said piston beyond said final position, whereby said piston is brought to a standstill in said overdriven position by engagement of said conical shoulder and conical surface with substantial elimination of shock impulses which tend to increase breakage of said piston and members engaging therewith; said breech end of said barrel being movable between said breech portion and said tubular forward portion of said housing as said barrel moves between said cocked and fully extended positions, said resetting means comprising latching means on said housing for latching said enlarged head of said piston and maintaining the latter substantially fixed in position relative to said housing during movement of said barrel from said cocked to said fully extended positions; said barrel being provided with a longitudinal slot, and said latching means comprises a latch pivotally mounted on said housing; biassing means for resiliently urging said latch to pivot and bring at least a portion thereof through said slot and into said bore of said barrel, whereby said latch can engage said enlarged head of said piston during movement of said barrel; said breech end of said barrel defining an explosion chamber with said slot of said breech having a bearing surface at the breech end thereof, said latch having two surfaces spaced from each other in the axial direction of said barrel for respectively engaging said piston and said bearing surface of said slot, the spacing of said two latch surfaces being selected to position said piston head within said explosion chamber to provide a predetermined amount of clearance therein when the respective latch engages said slot bearing surface, whereby the clearance within the explosion chamber and the initial velocity of said piston may be controlled by selection of the spacing between said two latch surfaces.

Claims

1. In an explosive-actuated fastener driving power tool having a housing having a breech portion and a generally tubular forward portion, a barrel having a bore and telescopically mounted in said tubular forward portion and axially movable therein between a cocked position and a fully extended position, said barrel having a breech end and a muzzle end, a piston having a trailing enlarged head dimensioned to be received in and slidably movable in said bore and a leading elongate ram having a smaller diameter than that of said bore and axially movable therein between an initial position and a final fired position; means for explosively driving said piston from said initial position to said final position when said barrel is disposed in said cocked position; resetting means responsive to movement of said barrel from said cocked towards said fully extended positions for returning said piston from said final to said initial positions relative to said barrel; retaining means disposed on said muzzle end of said barrel for retaining a fastener in position for driving by said piston as the latter moves from said initial to said final positions, said retaining means having a bore coaxial with said bore of said barrel and having a smaller diameter than that of said latter bore and corresponding to the diameter of said piston ram to form a discontinuity in bore diameters to permit only said piston ram to extend through said bore of said retaining means, the improvement comprising said discontinuity being in the form of a conical shoulder coaxially disposed in relation to said bores and having increasing diameters in the direction of said breech end, said trailing enlarged piston head having a complementary conical surface facing said conical shoulder and matable therewith in an overdriven position of said piston beyond said final position, whereby said piston is brought to a standstill in said overdriven position by engagement of said conical shoulder and conical surface with substantial elimination of shock impulses which tend to increase breakage of said piston and members engaging therewith; said breech end of said barrel being movable between said breech portion and said tubular forward portion of said housing as said barrel moves between said cocked and fully extended positions, said resetting means comprising latching means on said housing for latching said enlarged head of said piston and maintaining the latter substantially fixed in position relative to said housing during movement of said barrel from said cocked to said fully extended positions; and barrel being provided with a longitudinal slot, and said latching means comprises a latch pivotally mounted on said housing; biassing means for resIliently urging said latch to pivot and bring at least a portion thereof through said slot and into said bore of said barrel, whereby said latch can engage said enlarged head of said piston during movement of said barrel; said head of said piston including a trailing cylindrical portion and a leading conical portion, said cylindrical and conical portions of said piston being separated by an annular groove adapted to receive said portion of said latch.

3. In a driving power tool as defined in claim 2, wherein each taper defines an angle of 15* with the axial direction of said elongate ram of said piston.

13. In a driving power tool as defined in claim 1, wherein said cylindrical and conical portions of said piston engage said barrel and are spaced along the axis of the latter to stabilize said piston when the latter is propelled from said initial to said fixed positions.

14. In an explosive-actuated fastener driving power tooL having a housing having a breech portion and a generally tubular forward portion, a barrel having a bore and telescopically mounted in said tubular forward portion and axially movable therein between a cocked position and a fully extended position, said barrel having a breech end and a muzzle end, a piston having a trailing enlarged head dimensioned to be received in and slidably movable in said bore and a leading elongate ram having a smaller diameter than that of said bore and axially movable therein between an initial position and a final fired position; means for explosively driving said piston from said initial position to said final position when said barrel is disposed in said cocked position; resetting means responsive to movement of said barrel from said cocked towards said fully extended positions for returning said piston from said final to said initial positions relative to said barrel; retaining means disposed on said muzzle end of said barrel for retaining a fastener in position for driving by said piston as the latter moves from said initial to said final positions, said retaining means having a bore coaxial with said bore of said barrel and having a smaller diameter than that of said latter bore and corresponding to the diameter of said piston ram to form a discontinuity in bore diameters to permit only said piston ram to extend through said bore of said retaining means, the improvement comprising said discontinuity being in the form of a conical shoulder coaxially disposed in relation to said bores and having increasing diameters in the direction of said breech end, said trailing enlarged piston head having a complementary conical surface facing said conical shoulder and matable therewith in an overdriven position of said piston beyond said final position, whereby said piston is brought to a standstill in said overdriven position by engagement of said conical shoulder and conical surface with substantial elimination of shock impulses which tend to increase breakage of said piston and members engaging therewith; said breech end of said barrel being movable between said breech portion and said tubular forward portion of said housing as said barrel moves between said cocked and fully extended positions, said resetting means comprising latching means on said housing for latching said enlarged head of said piston and maintaining the latter substantially fixed in position relative to said housing during movement of said barrel from said cocked to said fully extended positions; said barrel being provided with a longitudinal slot, and said latching means comprises a latch pivotally mounted on said housing; biassing means for resiliently urging said latch to pivot and bring at least a portion thereof through said slot and into said bore of said barrel, whereby said latch can engage said enlarged head of said piston during movement of said barrel; said breech end of said barrel defining an explosion chamber with said slot of said breech having a bearing surface at the breech end thereof, said latch having two surfaces spaced from each other in the axial direction of said barrel for respectively engaging said piston and said bearing surface of said slot, the spacing of said two latch surfaces being selected to position said piston head within said explosion chamber to provide a predetermined amount of clearance therein when the respective latch engages said slot bearing surface, whereby the clearance within the explosion chamber and the initial velocity of said piston may be controlled by selection of the spacing between said two latch surfaces.