Cutting – Processes
Reexamination Certificate
1999-07-12
2001-02-06
Bray, W. Donald (Department: 3725)
Cutting
Processes
C083S689000, C083S698910, C279S028000, C279S076000, C403S409100
Reexamination Certificate
active
06182545
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improvement in a retainer such as is conventionally used to secure a tool such as a punch, or, a die bushing (or die or die button), or forming tool, removably in a die shoe.
BACKGROUND OF THE INVENTION
A retainer for a punch (punch retainer) secures the punch held within it to a die shoe, usually the upper, of a punch press so that the punch may be moved downwards into a die bushing with precision, over and over again so that stringent specifications of a punched sheet may be maintained. The die bushing, in turn, is held in a retainer (die bushing retainer) and secured to an opposed die shoe of the punch press. Typically both the retainers are removably secured to their respective die shoes; and the punch and the die bushing are also removably secured in their respective retainers.
For several decades a “ball lock punch retainer” has been used to secure the punch, and in fewer instances, also the die bushing which is more often clamped to the lower die shoe of the press, or tightly fitted into a recess therein. Despite the many problems associated with the use of a spring-biased retaining ball biased against a helical spring held in an angulated elongated passage within the retainer, this is the industrially favored mechanism because of the relatively low cost of manufacturing its components. However, aside from the relatively poor precision with which the shank (upper portion) of such a punch can be positioned, and the tolerable accuracy with which the point (lower portion) of the punch makes a through-passage (“hole” for brevity) of arbitrary cross-section in a sheet of stock being punched, a serious problem is that it is routinely an arduous and frustrating task to release a punch when it is to be replaced. One of the reasons is that repeated operation of the punch distorts the shape of the ball, which then becomes immovably lodged against the punch or against a helical spring against which the ball is biased. The problem of replacing the punch is worse when the ball is sheared, which typically happens when the stripping force exceeds that which the ball can withstand. In operation, punches are routinely subjected to unexpectedly large stripping forces typically cause by galling of the point.
An inherent result of using a ball seat or pocket in the shank of a punch to lock it with a ball is that, the shank of the punch is of necessity, cylindrical. If the point of the punch is non-circular in lateral cross-section, it can be sharpened only until the point is used up and the shank is reached. Moreover, by reason of the clearances required between the pocket and the ball, and the relatively small force exerted by the spring against the ball, it is difficult to maintain concentricity with tolerance less than 0.001 inch. Particularly when the shape of the hole to be punched is other than circular, the shank is not held tightly and non-rotatably in its elongated passage with the result that the play between the ball and the pocket results in slight but unacceptable variations in orientation of the hole punched. These problems are more readily envisioned by reference to
FIGS. 1 and 2
in which the prior art mechanism is briefly described. Moreover, the structural differences and their effect on the forces exerted on a tool to be replaced, when compared to those of the present invention, will more readily be appreciated.
Similar considerations apply to securing a forming tool which operates in a forming press and which forming tool is typically secured in a manner analogous to a punch. A commonly used forming punch has a point for making the desired hole in a sheet of stock, and has an upwardly flared conical portion directly above the tip of the point. The flared portion serves to provide desired concavity. Hereafter, for brevity and convenience, a punch and a forming tool or forming punch, and a die bushing are together referred to by the term “tool”, and are identified individually when specifically referred to.
Referring to
FIGS. 1 and 2
, there is illustrated a retainer block indicated generally by reference numeral
10
and a conventional punch
20
held therein. A forming toot if used, would be analogously held. The retainer block
10
includes a through-hardened backing plate
12
conforming to the upper surface of the retainer block, both being adapted to be secured to an upper die shoe of a punch press or other machine with a punching or forming function by suitable fastening means such as Allen-head screws (not shown). Since a tool (punch or forming) is generally used in a vertical attitude in a punch or forming press, the description herein refers to upper and lower in relation to such attitude. The retainer block
10
is provided with a cylindrical bore or tool socket
14
in which is slidably inserted and removably secured the shank (upper portion)
22
of the punch
20
, the lower portion of which is an oval-shaped point
24
. Block
10
is also provided with a cylindrical bore
15
which is angularly disposed relative to the bore
14
and which extends inwardly and downwardly into the retainer block
10
so as to partially intersect socket
14
. The partial intersection occurs because the lower end of the bore
15
is provided with a stepped surface forming ball seat
13
.
A retainer ball
16
is movably disposed in bore
15
, and a helical compression spring
18
is snugly held in the bore
15
with one end abutting the backing plate
12
so as to urge the ball
16
outwardly of the intersecting portion of bore
15
. Though the ball projects into the socket
14
the ball cannot escape (into the socket
14
). The retainer block is also provided with a through-passage or release-bole
17
through which a thin rod or drift pin is inserted to push the ball upward and move it out of the ball seat
13
when the punch
20
is to be removed. To replace the ball
16
when it gets distorted or damaged, the retainer block
10
is removed from the backing plate
12
and the spring and ball removed through the top of bore
15
.
The shank
22
is provided with a semi-pocket or ball seat
25
shaped generally like a one-half of a falling tear drop viewed in longitudinal elevation, and which is adapted to receive locking ball
16
to releasably lock the punch
20
in the bore
14
. The pocket's upper portion
26
appears as a straight section forming a continuation of the bore
15
; and the lower portion is provided with a return section
28
which is curved upon a radius greater than the radius of the ball
16
so as to connect the deepest part of the pocket
25
to the surface of the shank. When the ball
16
is held in pocket
25
its bottom may be in contact with the ball if the radius of section
28
is substantially greater than that of the ball; or, if the radius of the ball is substantially greater than that of the return section
28
, the extreme edges
34
,
35
of the pocket
25
will contact the ball.
To appreciate the advantage of locking a punch precisely positioned in the retainer block, the problem with using a pocket and retaining ball is illustratively presented in
FIGS. 3 and 4
so it may be more readily visualized. Both problems, namely of securing the tool to the die shoe, and positioning the punch (and die bushing) precisely, is particularly severe with relatively small diameter punches having a shank less than about 7.6 cm (3 ins.) in diameter. A larger diameter shank may be secured and precisely positioned with screws and dowels through the shank and die shoe. In
FIG. 3
is shown a shank
22
A having a pocket
25
A with an arcuate section having a radius substantially greater than that of ball
16
A, allowing the punch to rotate slightly in either direction, as shown by the arcuate double-headed arrow, so that accurate alignment between a non-circular punch and its corresponding die bushing cannot be maintained. In
FIG. 4
, in shank
22
B, the arcuate section of pocket
25
B has a radius smaller than that of ball
16
B so that it engages the corner portions
34
B,
35
B of the pocket in
Bray W. Donald
Lobo Alfred D.
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