Die set with sunken load cells

An instrumentation system for measuring forces during tool and die forming operations including a load cell positioned in a well bored into a base plate. The plate includes a series of channels that carry wiring from the load cell to the periphery of the base plate. The load cell is preferably mounted flush with a supporting surface of the base plate to facilitate periodic cleaning. The load cell includes sensors generating a signal corresponding to the force or compression on the load cell. The load cell also includes apertures extending therethrough to allow existing tooling to be mounted on the load cell to the base plate via fasteners and pins extending through the load cell to the base plate. An universal load cell embodiment includes a pattern of apertures allowing two or more tools having unique hole patterns to be mounted on the load cell with fasteners and pins extending therethrough.

FIELD OF THE INVENTION 
This invention relates generally to instrumentation of stamping or forming 
die sets, and more particularly to measurement of loads on tools of die 
sets during stamping or forming operations. 
BACKGROUND OF THE INVENTION 
In the formation of metal parts, one very common forming technique is a 
stamping operation utilizing die and tool sets. Frequently, the forming 
operation includes a series of dies or tools through which the part passes 
successively to move from a blank form to a final form, with each stamping 
operation excessively deforming or performing some other operation on the 
blank part until it reaches its final desired configuration. 
During stamping operations, in many cases it is desirable to measure the 
load being applied to the tool of the die set during certain stamping 
operation. The load is monitored on various forms of monitoring equipment, 
such as oscillographs or recorders, or analyzed using various electronic 
systems to determine whether the process is being carried out within 
pre-selected limits or parameters and give an indication of any problems 
which are developing or have developed. 
One common technique for measuring the force during stamping operations or 
blanking operations is by the use of various types of load cells. These 
load cells can take several different forms. One form utilizes piezo 
electric crystals which often are embedded in the tooling and are 
monitored to give an indication of the force by measuring electrical 
output as a function of the compression of the crystals. 
This is a relatively inefficient technique and does not give high precision 
required for many operations. 
Another technique which is utilized during blanking or forming operations, 
especially of can lids, is by the use of strain gauge load cells. In one 
particular prior art application, the load cell utilizes a spool member 
having strain gauges arranged around the periphery of this spool between 
the flanges at the opposite ends of the spool. The strain gauges are then 
potted with epoxy potting material. Wiring extends from the strain gauges 
to sense the movement or compression of the spool during forming 
operations. The load cell, is in turn connected between a tool and a 
tooling base. 
The various prior art systems of interfacing the load cell to the tooling 
have drawbacks. In many, the wiring from the sensor is exposed. This 
exposure often results in premature failure of the sensor due to 
mishandling or rough treatment of the product during the course of normal 
maintenance. For example, die stations tend to accumulate metal fragments 
and other debris, which must be cleaned periodically. It is not uncommon 
for a screwdriver or putty knife to be hastily scraped between tooling to 
remove such debris, which scraping can sever wiring or cut into wiring 
insulation. 
In other prior art systems, a specialized mounting piece is used, which 
requires a differently configured mounting piece for each tool. This adds 
cost to a system. In still other prior art systems, the tool itself 
contains the strain gauges and thus becomes the load cell; this adds cost, 
since the entire unit might need to be scrapped if either the tool or the 
sensor fails. Thus, there is a need for an improved system of interfacing 
load cells to tooling. 
SUMMARY OF THE INVENTION 
According to the present invention, an improved die assembly incorporating 
sunken load cells is provided. The die assembly includes a tooling base 
having a support surface thereon and an outer surface. At least one, and 
usually a plurality, of wells, preferably cylindrical in shape, are 
provided in the tooling base extending from said support surface to a 
bottom wall. A load cell insert, preferably in the shape of a spool, is 
provided for insertion in each well. The inserts each include one, and 
preferably several, strain gauges. The strain gauges of each insert are 
attached to wiring, which wiring is disposed in channels associated with 
each well when the insert is disposed in the well. The channels extend to 
the outer surface and preferably are open at the support surface of the 
tooling base. The wiring extends to the outer surface and is potted in its 
respective channel. Wider channels accommodating wiring from a number of 
load cells are also potted. When the load cell inserts are in the wells, a 
supporting surface on each load cell preferably is flush with the support 
surface on the tooling base. Appropriate tooling is mounted on the 
supporting surface of each load cell insert for the particular tooling 
stages, and during operation of the tool the strain at each tool location 
is measured and transmitted by the wiring which can be translated into 
force on each tool at various stages of operation. Also, the load cell has 
a number of holes extending therethrough to allow existing tooling to be 
mounted with fasteners and pins that extent through the load cell into the 
base plate. Since different tools can have fasteners and pins located in 
different arrangements, preferably the load cell is configured for 
universal use at a plurality of stages, with hole patterns that allow a 
number of different tools having differently arranged fasteners and pins 
to be mounted thereon. In the alternative, the load cell can have a single 
hole pattern thereby allowing only tooling unique to one stage or location 
to be mounted thereon. 
It is therefor an advantage of the present invention to provide a load cell 
that mounts to tooling with the fasteners used to secure the tooling to a 
tool base. 
It is also an advantage of the present invention to provide an instrumented 
die assembly with wiring that is protected from periodic cleanings of 
accumulated debris. 
It is another advantage of the present invention to provide an instrumented 
die assembly having a load cell mounted flush with the supporting surface 
of the tooling base to facilitate cleaning. 
It is yet another advantage of the present invention to provide an 
instrumented die assembly than can be easily retrofitted to instrument 
existing tooling. 
It is still another advantage to provide a universal load cell that can 
have more than one tool having fasteners and pins positioned in different 
arrangements mounted thereon. 
These and other advantages will become more apparent from a detailed 
description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the present invention will be described more fully hereinafter with 
reference to the accompanying drawings, in which a preferred embodiment of 
the present invention is shown, it is to be understood at the outset of 
the description which follows that persons of skill in the appropriate 
arts may modify the invention here described while still achieving the 
favorable results of this invention. Accordingly, the description which 
follows is to be understood as being a broad, teaching disclosure directed 
to persons of skill in the appropriate arts, and not as limiting upon the 
present invention. 
Referring now to the drawings, and for the present to FIGS. 1-4, a die set 
10 for stamping or blanking can lids from preformed stock is shown. The 
particular die set illustrated is for three parallel blanking lines, each 
having a series of blanking or stamping stations through which preformed 
stock is passed successively, and at each stop an operation is formed 
thereon, such as initial forming, continuing forming, forming a tab area 
and mounting a tab. In this particular embodiment, each successive station 
of the three lanes is identical so that three identical end products 
emerge after passing through the succession of stations. It should be 
understood, however, that this particular arrangement of three lanes of 
several stations is merely illustrative, and that either one or two lanes 
could be used as well as more than three lanes. Also, the particular 
stations used are not material, the invention being applicable to any 
number of stations, including a single station and can be used for various 
stamping or blanking operations, not just for can lids. 
The invention itself is directed to the structure and mounting of a load 
cell in a tooling base for the support of a tool member thereon, and thus 
the particular configuration of the die set, including the tooling 
configuration, at the various stations is immaterial. In the disclosed 
embodiment, there is a plurality of stamping or blanking stations 12, 12A, 
12B, 12C, 12D, 12E, 12F, 12G, 12H and 12I, in each lane, these lanes being 
designated by the suffice -1, -2 and -3 where applicable. (The parts at 
the various stations 12, 12A, etc., will be described hereinafter without 
their suffixes unless necessary to differentiate between stations.) 
The die set includes a tooling base 14 which preferably is formed in 
multiple sections, but which could be formed as a single plate member. For 
clarity of description, the tooling base 14 will be treated as if it were 
a single member. 
The tooling base 14 has a support surface 16 with a plurality of wells 18 
formed therein. There are some stations wherein force measurements are not 
required and thus no load cell is needed; therefore, no well need be, or 
is, provided at these stations. 
Each of the wells 18 is preferably cylindrical in shape and defined by a 
side wall 20 and a bottom wall 22. Each of the bottom walls 22 have 
through threaded bores 24 and essentially smooth pin-receiving bores 26. 
The particular pattern of the threaded bores 24 and smooth bores 26 for 
each of the series of stamping stations 12A, 12B, 12C, etc., may vary, in 
a manner which will be indicated presently, to prevent the insertion of an 
improper tool at that station. A series of channels 28 are provided which 
communicate with each of the wells 18. The series of channels 28 open at 
the support surface 16 and extend to the outer periphery 31 of the tooling 
base 14. Preferably narrower channels 29 extend from each well, which 
narrower channels 29 connect with each other and with wider channels 30 to 
form a sunken, continuous path from each well 18 to the periphery 31 (FIG. 
3). 
As can best be seen in FIGS. 2 and 4, a plurality of load cells 34 are 
provided, one for each of the wells 18. The load cells 34 preferably have 
the same cross-sectional shape as the wells 18, in this case circular. 
Load cells of different shapes can be used, e.g., a square load cell. As 
shown in FIGS. 5, 6, and 8, the load cells 34 can be in the form of a 
spool 36 having flanges 38 at the opposite ends thereof separated by a 
depressed central portion 40. A series of strain gauges 42 are secured in 
the central portion 40 of the spool 36 between the flanges 38 and potted 
therein by potting compound 44 (not shown in FIG. 6). The strain gauges 42 
are connected to wiring 46 which extends through a central aperture 48 
(see FIG. 8) which extends transversely through the spool 36 and allows 
the escape of the wire from the spool. The wiring 46 extending from the 
load cell 34 is protected by a protective sheath 47, which is disposed in 
the series of channels 28, and thus disposed beneath the support surface 
16. The wires 46 within the protective sheath 47 may be secured and 
protected in the series of channels 28 by a protective compound such as an 
epoxy potting compound (not shown) that completely or substantially fills 
the channels and that completely or substantially covers the wiring. The 
load cells 34 also have a plurality of openings 50 which correspond to the 
position of the threaded bores 24 and a plurality of openings 52 which 
correspond to the position of the smooth bores 26. 
A blanking or forming tool 56A-56I is provided at each station 12A, 12B, 
etc., and where there is a load cell 34 at that station, the tool 56 rests 
on the load cell 34. As known to those skilled in the art, each tool 
56A-56I is configured to perform a stamping, forming, or other operation 
on the piece being formed, and hence each has a different surface 
configuration depending upon the operation, although adjacent stations 
might have a similar or identical surface configuration to restrike the 
work piece. Each tool 56 has openings 58 corresponding to openings 50 in 
the load cell 34 and openings 60 corresponding to openings 52 in load cell 
34. A plurality of threaded bolts 62 are provided which extend through the 
openings 58 in tool 56 and openings 50 in the load cells 34 and threadably 
engage the threaded bores 24 of base 14 to secure each tool and each load 
cell 34 in its respective well 18. Smooth pins 64 are inserted through 
openings 60 in tool 56 and openings 52 in the load cell 34 and through the 
smooth bores 26 in the tooling base 14. The pins 64 are a very tight fit, 
such as press fit in the openings 60 and 26 to allow precise positioning 
and alignment of the tools 56 and precise positioning and alignment of the 
load cells 34 at the particular stages. As indicated earlier, the pattern 
of the threaded bores 24 and smooth bores 26 for each station in the die 
set 10 are not all identical and only a load cell 34 and tool 56 having 
that particular configuration of openings can be secured thereon. The load 
cells are preferably made of A2 tool steel or, in the alternative, can be 
made of AISI 4130 steel. The upper and lower surfaces of the load cell 34 
must be very flat and parallel to each other. Similarly, the bottom of the 
well 18 must be very flat and parallel to the upper surface 16 of the base 
plate 14. The clearance between the inside wall surface 20 of the well 18 
the outside surface of the load cell 34 can be about 0.0004 inches or 
more. The depth of the well 18 and the overall height of the load cell 34 
are matched so that the load cell does not protrude above the surface of 
the tool base more than about 0.0002 inches. The bottom surface 22 of the 
well 18 and the upper and lower surfaces of the load cell 34 must be flat 
to within .+-.0.0002 inches and cannot be larger than the diameter of the 
receiving pocket. The load cell 34 can be smaller than the pocket but not 
smaller than the tool mounted above it. This sensor 34 is designed to last 
the lifetime of the tool set as a minimum and is expected to last a 
minimum of five years. 
Each load cell 34 has associated therewith a sensor, e.g., strain gages, 
used to generate an electrical signal corresponding to the amount of force 
applied to the load cell. Four, eight, or more sensors can be used with 
each load cell 34, as known to those skilled in the art. FIG. 7 is a 
schematic wiring diagram of the wiring configuration of a four strain gage 
load cell showing strain gages R1, R2, R3 and R4 operating in a Wheatstone 
bridge configuration. Voltages of +10 VDC and -10 VDC are applied to the 
+V and -V terminals, respectively, and the signal output (potential 
difference between +SIGNAL and -SIGNAL) corresponds to the load on the 
strain gauges 42. A total load sensitivity of 3 of signal per volt of 
excitation is acceptable. The strain gages are placed in circuit 
communication with an electronic device (not shown) used to display, 
record, and/or analyze the signal output and, optionally, determine the 
force applied to the load cell or determine another parameter related to 
the force applied to the load cell. Load cells having suitable sensors 
therein are available as Model No. SS1133 from Toledo Transducers of 6834 
Spring Valley Drive, Holland, Ohio 43528. 
As known to those skilled in the art, as shown in FIGS. 1, 2 and 4, an 
annular ring 66 is typically provided around each tool 56. This ring is 
optional depending on the preference of the manufacturer, and is used to 
protect the tools from damage. 
Referring now to FIG. 8, another embodiment of a load cell 70 is shown. The 
load cell 70 of FIG. 8 is a universal load cell 70 in that it has a 
pattern of openings allowing use of the load cell 70 with a plurality of 
tools having different hole patterns. The load cell 70 is similar in 
configuration to the load cell 34 in that it is formed of a spool 36 
having flanges 38, a central portion 40 with strain gauges potted thereon 
and an aperture 48 extending therethrough. However, in this embodiment, a 
series of openings 72 and 74 are provided which allow the strain gauge to 
be utilized in more than one of the wells 18 of the tooling base 14 having 
different patterns of holes, i.e., more than one configuration of holes 58 
and 60 in tool 56 that match with corresponding bores 24 and holes 26 in a 
well 18 can be accommodated by a single load cell 70. Expressed another 
way, the openings 58 and 60 in the tool and bores 24 and holes 26 are 
specific to the specific tool and will be accommodated by the load cell 
70. Thus, the load cell is specific to a particular tool when load cell 34 
is used, whereas load cell 70 is a universal load cell which can be used 
at more than one station with the location of the threaded bores 24 and 
the smooth bores 26 determining which tool, 58A, 58B, 58C, etc., will fit 
thereon. 
The specific universal load cell 70 shown in FIG. 8 is universal to all of 
the stations associated with the plate 14 shown in FIG. 3. As is apparent 
from that figure, wells having two different hole patterns are shown in 
the plate 14 of FIG. 3: (imagining each well as corresponding to the face 
of a clock and using the center of the bottom 22 of each well as a common 
reference point) six wells having holes at about 3 o'clock (94), 6 o'clock 
(95), 9 o'clock (96), 12 o'clock (97) and three wells having holes at 
about 1:30 (98), 2:30 (99), 4 o'clock (100), 8 o'clock (101), 9:30 (102), 
and 10:30 (103). The universal load cell of FIG. 8 can be used in wells 
having either hole pattern because the universal load cell 70 has holes at 
about 1:30 (104), 2:30 (105), 3 o'clock (106), 4 o'clock (107), 6 o'clock 
(108), 8 o'clock (109), 9 o'clock (110), 9:30 (111), 10:30 (112), and 12 
o'clock (113). The load cell 70 of FIG. 8 also shows optional threaded 
openings 90, 92 used to retain temporary jacking screws (not shown) to 
facilitate removing the load cells 34, 70 from the wells 18. 
With the present invention, the load cell 34 or 70 is separate from the 
tool 56 and tool base 12, and it can easily be inserted and removed from 
the base 12 for repair or replacement. Moreover, the wiring is protected 
in the channels. Additionally, with the embodiment of FIG. 8, a universal 
load cell is provided so that load cells specific to each station need not 
be stocked. 
Accordingly, the preferred embodiment of the present invention has been 
described. With the foregoing description in mind, however, it is 
understood that this description is made only by way of example, that the 
invention is not limited to the particular embodiments described herein, 
and that various rearrangements, modifications, and substitutions may be 
implemented without departing from the true spirit of the invention as 
hereinafter claimed.