DIE HEAD BACKGROUND OF THE INVENTION Automatic screw machines typically employ die heads that are tripped and reset, at the completion and commencement of each cutting operation, by camming arrangements. They also typically employ a lead screw for advancing the die head at a pitch that corresponds the thread being cut. These features tend to make conventional screw machines relatively expensive to manufacture and inconvenient to employ, while at the same time affording levels of precision that are inadequate for certain applications. In particular, automatic screw machines having such features are not well adapted for ultra-precision cutting operations, subject to tight tolerances, nor are they generally suitable for use with small or miniature workpieces. A self- opening die head is described in Breitenstein U. S. patent No.

1,639,719, which employs chasers that are provided with pins upon which spring-actuated plungers bear to urge the chasers to radially outward positions. As a practical matter, such a construction is feasible only where the dimensions of the chasers (and consequently of the die head) are sufficiently large to permit ready assembly and to afford adequate structural strength to the parts. The Breitenstein construction also limits the length of the bevelled camming surface that is presented on the chaser, in turn limiting the capacity of the die head to accommo- date a wide range of workpiece sizes.

SUMMARY OF THE INVENTION Accordingly, it is a broad object of the, present invention to provide a fast-acting die head and, more particularly, such a die head that is adapted for use with small size workpieces.

It is also a broad object of the invention to provide a thread-cutting system that incorporates a die head having the foregoing features.

Additional objects of the invention are to provide such a die head and system which are highly effective in operation, and which are, in addition, relatively facile, incomplex, and inexpensive to use and produce.

Another object of the invention is to provide a chaser that is particularly adapted for use in a die head of the character described.

It has now been found that certain of the foregoing and related objects of the invention are attained by the provision of a fast-acting die head, constructed for engagement of a work- piece on an axis thereof, comprising: a body, a plurality of chasers, biasing means, and operating means. The chasers are assembled with the body for movement, in effective opposition to one another, between open and closed positions in which they are disposed, respectively, relatively close to and relatively distant from the axis. The biasing means is operatively inter- posed between the die head body and the chasers for resiliently biasing the chasers toward either the closed or (in most embodi- ments) the open position, and the operating means comprises an operating member that is reciprocally movable, relative to the body, along the axis of the die head. At least one camming surface is provided on the operating member for engagement with the chasers, to force them (against the biasing means) toward the open or the closed position (as the case may be) during move- ment of the operating member in the"one"direction. Each chaser comprises a workpiece-engaging portion and at least two force- receiving elements, one such element being a bearing surface, for engagement by the biasing means, and the other element being a cam-follower surface disposed for engagement by the camming surface of the operating member.

More particularly, each chaser comprises an elongate member having opposite end portions, one of which portions provides the workpiece-engaging portion and the other of which provides the force-receiving elements. The"other"end portion has forward and rearward lengthwise margins, and terminates in a forwardly inclined outer surface, constituting the cam-follower surface.

A nose element projects beyond the rearward margin of the chaser body and provides both the bearing surface against which the biasing means acts, and also a portion of the cam-follower sur- face. The bearing surface and the cam-follower surface portion are axially aligned, along an axis parallel to the longitudinal

axis of the chaser member, and in most embodiments the bearing surface will be in an underlying relationship to the cam-follower surface portion.

The body of the die head will usually be so constructed as to receive a workpiece inserted at least partially into it, with the chasers being disposed relatively close to the die head axis in their closed position. The chasers will generally move in planes that include the die head axis, as well as in planes to which the axis is normal; most often, the chasers will move in a translational manner. The workpiece engaging portion of each of chaser will, in most instances, comprise a thread-cutting element.

Other objects of the invention are attained by the provision of a thread-cutting system, which includes a fast-acting die head, as herein described, in combination with actuation means.

In such a system, the workpiece-engaging portion of each chaser comprises thread-cutting means, and the die head operating means includes an actuating member that is reciprocally movable, rela- tive to the operating member, and is constructed for operative engagement to move the operating member in the"one"direction as well as for independent movement in the opposite direction.

The actuating means of the system comprises: an engagement compo- nent, the actuating member of the die head, a supporting component on which the engagement component is mounted, and limiting means.

The supporting component is movable relative to the die head and is so constructed as to advance the die head to a position of initial engagement with a workpiece, acting through the engage- ment component and the actuating member; the structure of the supporting component also permits free movement of the engagement component beyond the position of initial engagement. The limiting means is so disposed as to engage the engagement component, at a point beyond its position of initial engagement, so as to thereby limit the distance that the engagement component can travel.

In operation, the supporting component of the system is moved relative to the die head so as to advance the latter to a position of initial engagement with the workpiece. The die

head and the engagement component advance together, as the thread is being cut, until they reach the point at which the engagement component engages the limiting means. Continued advancement of the components of the die head (separately from the actuating member) causes the operating member to disengage from the chasers, thereby permitting the biasing means to bias the chasers toward their open or closed positions (again, as the case may be).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a fragmentary isometric view of a thread-cutting system embodying the present invention; Figure 2 is a forward end elevational view of the die head utilized in the thread-cutting system, also embodying the present invention; Figure 3 is an exploded isometric view, in partial section, showing components of the die head; Figures 4,4a, and 4b are, respectively, side elevational, front and rear views of the camming ring employed in the die head; Figures 5,5a and 5b are similar views of the actuating collar employed in the die head; Figures 6,6a and 6b are similar views of the mounting collar of the die head; Figure 7 is an exploded sectional view depicting assembly of the camming ring and actuating collar; Figure 8 is a sectional view showing assembly of the mounting collar and the locking ring segments with the camming ring/actuating collar subassembly; Figure 9 is a sectional view showing the components depicted in Figure 8 in assembled relationship, and showing the camming ring and mounting collar locked against relative axial movement by the ring segments; Figure 10 is a similar view showing disengagement of the mounting collar from the camming ring/actuating collar subas- sembly; Figure 11 is a fragmentary sectional view of the die head, drawn to an enlarged scale and showing the cutting dies (chasers) in their open positions; and

Figure 12 is a view similar to Figure 11, showing the dies in closed positions for cutting threads in a workpiece (not shown).

DETAILED DESCRIPTION OF THE PREFERRED AND ILLUSTRATED EMBODIMENT Turning now in cletail to the appended drawings, therein illustrated is a thread-cutting system that incorporates a fast- acting die head, which in turn incorporates a unique chaser, all embodying the present invention. As best seen in Figures 3,11, and 12, the die head consists of a cylindrical body, generally designated by the numeral 10, having a forwardly opening cavity 15 and a threaded rearward end portion 12, from which threaded portion projects a shaft 14. Head structure, generally designated by the numeral 16, is disposed forwardly on the body 10 and consists of four sectors 18 spaced circumferentially by intervening radial slots 20 (of constant width), the inner ends of the slots 20 being defined by connecting web elements 22.

Four chasers, each generally designated by the numeral 24, are slidably seated within the slots 20 and against the web ele- ments 22. They are retained by four annulus sections 26, which are secured against the end surfaces of the head sectors 18 by set screws 28 engaged in threaded apertures 30. It will be noted that the annulus sections are of such arcuate length as to project beyond only one side margin of each slot 20, thereby serving to prevent inadvertent forward release of the chasers 24 while, at the same time, permitting a full range of radial movement.

Each chaser 24 has a thread-cutting element 32 on a head portion 33 thereof, formed at one end of the integral body portion 34. An integral nose element 36 projects beyond the rearward, lengthwise margin 37 of the body 34; it provides a portion of the inclined outer bearing surface 35, as well as an underlying bearing surface 38 extending at a right angle to the margin 37.

A plunger 40, and associated coil spring 44, are mounted in a blind aperture 42 that extends radially into each web element 22, and the nose element 36 of the adjacent chaser 24 projects over the web element 22 for contact by the head 39 of the corre- sponding plunger 40. A pair of parallel mounting rods 46 extend

rearwardly from the head portion 16 at diametrically opposed positions and in an axial direction.

A mounting collar, generally designated by the numeral 70, ccnsists of an inner cylindrical portion 72, which is internally threaded at 74, and an outer annular portion 76, between which portions 72,76 is defined a circumferential channel 78. The threaded portion 74 of the collar 70 engages the threaded portion 12 of the cylindrical body 10 to establish a desired relative axial position, which position is secured by tightening the set screw 77, received in the threaded radial aperture 75, against the portion 12. It will be appreciated that the axial position of collar 70 determines the diameter of the thread that is cut by the chasers 24 in operation of the cutting head. A secantal slot 92 is formed adjacent the inner end of the cylindrical portion 72 of the collar 70, and serves to seat a locking pawl 94, the pawl 94 being pivotably mounted in the slot 92 on pin 96 and being biased outwardly by coil spring 98.

Upper and lower semicircular ring segments 80 and 82, re- spectively, are seated in the circumferential channel 78 of the mounting collar 70, the upper segment 80 having an axial key 88 on its outer surface. The two segments are biased apart by springs 84, the opposite ends of which are received in small holes 86 formed into the confronting end surfaces of the segments.

A camming ring, generally designated by the numeral 48, is coaxially mounted for slidable axial movement relative to the mounting collar 70. The ring 48 consists of an inner cylin- drical portion 50 and an outer cylindrical portion 52, which intersect to form an internal annular shoulder 54 and an external annular shoulder 55 therebetween. Apertures 56 extend axially into the internal shoulder 54, and serve to receive the parallel mounting rods 46 on the head portion 16, thereby permitting non rotatable, relative axial movement tlnerebetween. Four inclined camming surfaces 60 are formed into the inside surface of the lip of the outer cylindrical portion 52 of the camming ring 48, at equiangular ly spaced circumferential locations. The surfaces 60 are dimensioned and oriente to slidabl. y engage the inclined bearing surfaces 35 of the chasers 24, to thereby exert a inward

radial closing force upon all four chasers simultaneously when the camming ring 48 is moved in a forward, axial direction there- over.

Rearward movement of the ring 48, relative to the collar 70, is limited by the extended locking pawl 94. Depressing the pawl 94 permits the ring 48 to be displaced rearwardly to a clearance position in which the chasers 24, and the associated biasing elements, can be removed from the head.

A semicircular groove 62 is formed into the interior surface oftheinnercylindricalportion 50 of the ring 48, eccentrically to the longitudinal axis (center line) of the die head, and a small aperture 64 extends through tlleportion0 incommunication with the groove 62. An axial keyway 66 is also formed into the interior surface of the cylindrical portion 50, which engages the key 88 on the upper semicircular ring segment 80, and an axial key 68 is formed on the exterior surface. (It is to be noted that the keyway 66 is shown displaced slightly from its correct position, which is diametric to the aperture 64; this was done for clarity and ease of illustration).

The operating collar, generally designated by the numeral 100, comprises a forward cylindrical portion 102 and a rearward cylindrical portion 104, between which is formed an internal annular shoulder 105. A circumferential channel 106 extends about the rearward cylindrical portion 104, and an axially extend- ing keyway is provided in the internal surface of the portion 104; the keyway 108 engages the key 68 on the camming ring 48.

A small recess 110 is formed into the interior surface of the rearward cylindrical portion 104, the forward edge of which is relieved to provide a ramp surface 112. The recess 110 serves to receive an operating ball 114, the function of which will be described presently.

As depicted in Figures 1,2 and 11 1 of the drawings, the die head is in its open configuration and the yoke by which it is operated, generally designated by the numeral 116 (and seen only in Figure 1), is in a retracted position on the rod 118.

Movement of the rod 118 to shift the yoke 116 slightly forwardly (i. e., to the left in Figure 1) will permit the ball 114 to drop

into the recess 110 of the operating collar 100, and will also cause the external shoulder 105 on the collar 100 to engage the shoulder 55 on the camming ring 48, thereby moving the latter forwardly against the force of the springs 58. Such movement will in turn cause the camming ring 48 to urge the chasers 24 radially inwardly, with the camming surfaces 60 acting upon the bearing surfaces 35, ultimately to attain the closed configuration depicted in Figure 12.

As can be seen, in the closed position the spacing x'between the diametrically opposed points of contact on the cutting ele- ments 32 is substantially less than the corresponding distance x between the elements 32 in the open configuration of Figure 11. As can also be seen, in the closed configuration the lower semicircular ring section 82 is engaged within the semicircular eccentric groove 62 of the camming ring 48, coupling the ring 48 and the mounting collar 70 against relative axial movement and thereby maintaining the chasers 24 in their closed positions.

Although not illustrated, it will readily be appreciated that the cutting elements 32 engage the surface of a workpiece (a length of screw stock, received axially therebetween) so as to cut a thread therein as the die head 16 is rotated by the spindle 120 (attached to the shaft 14) and as the workpiece proceeds into the recess 15 of the head.

With continued cutting of the thread, the entire die head, and the operating collar 100 (being locked in axially fixed positions relative to one another), are drawn forwardly. When the yoke 116 encounters the collar 122 affixed on the end of the rod 118 (the position of which is adjusted to vary the thread length), forward movement of the yoke 116, and consequently of the collar 100, is arrested. As the coupled die head and camming ring continue to advance, contact against the trailing edge of the hole 64 urges the ball 114 up the ramp surface 112 and more deeply into the hole 64, ultimately to protrude slightly and into contact against the surface of the lower ring segment 82.

This will in turn force the ring segment 82 out of the slot 62, thereby decoupling the camming ring 48 from its axially affixed position on the mounting collar 70 and permitting the compressed

springs 58 to thrust the camming ring rearwardly (i. e., to the right in Figure 12). The spring-driven plungers 40 will simulta- neously thrust the chasers 24 radially outwardly, causing the die head to quickly resume the configuration and position depicted in Figures 1 and 11.

Although the foregoing description and the appended drawings are directed to a die head for cutting threads, it will be appre- ciated that the die head of the invention may take other forms.

For example, rather than closing upon a workpiece inserted between the chasers the die head can function with the chasers moving outwardly into engagement with a surrounding workpiece surface.

The chasers will of course be so constructed as to render them most suitable for a particular purpose that the die head is intended to serve, but in any event they will preferably (if not necessarily) be integrally formed as a single piece (i. e., they will each be of unitary construction), and normally they will be machined from tool steel. Merely by way of illustration, and not limitation, a typical chaser will measure about 1/2 inch in length (i. e., in the radial dimension of the installed piece), about 1/8 inch in width, and about 7/32 inch in depth (i. e., from front to back), of which depth about 2/32 inch comprises the nose element; thus, it will be appreciated that the chasers may be made in very small sizes.

The rearwardly projecting nose element is an especially important feature of the chaser. Not only does it provide the two required force-receiving surfaces in optimal positions, on an element that is structurally strong and readily fabricated (even in small dimensions), but since the nose element also provides a portion of the cam follower surface it permits that surface to be longer that it would otherwise be. This extends the range of workpiece sizes that are accommodated by the die head, as compared to prior art chasers in which the ends of the camming surface do not extend beyond the front and rear margins of the chaser body (see for example the bevel shown at 17 in Figure 4 of the above-identified Breitenstein patent).

Thus, it can be seen that the present invention provides a novel fast-acting die head, and a novel thread-cutting system

that incorporates the same, which are highly effective in oper- ation and are particularly adapted for construction and use for machining small-size workpieces. The advantages realized are attributable in large part to the unique, one-piece chasers that are employed in the die head; utilization and manufacture of the described system and components are relatively facile, incomp- lex, and inexpensive.