This print hammer assembly method utilizes an elongated thin metal
strip punched to form repetitive flag panels each having a flag
region non-symmetrically linked to a surrounding frame, and a connector
arm extending obliquely between the frame and flag. All assembly
steps are performed while maintaining the panels connected in the
strip. A flat coil, centered by a boss on the flag region, is sandwiched
between and adhesively bonded to the strip and a second flag panel
severed from another like strip. Spring wires are attached to the
connector arms of both panels before sandwiching, and the coil leads
are subsequently attached to the connector arms. Simultaneous soldering
of the coil leads and spring wires to the connector arms is carried
out from one side of the assembly strip, advantageously using solder
paste and focused infrared heating. After discarding the excess
frame portions, the hammers are embedded in a molding compound while
still linked together in a strip. After molding, the links are severed
to separate the individual hammers.
1. A flag strip for use in the fabrication of print hammers, comprising:
an elongated, unitary, planar thin strip of metal having a plurality
of identical flag panels formed therein, each flag panel including:
a generally rectangular flag region,
a frame substantially surrounding said flag region,
a plurality of links connecting said frame to said flag region,
a connector arm extending from said frame at one side of a centerline
through said flag region.
2. A flag strip according to claim 1 wherein the links on one side
of said centerline are offset with respect to the links on the other
side of said centerline, so that when one of said panels is separated
from said strip and superimposed atop and in facing relationship
with another panel of said strip, the links of said other panel
will not be covered by the links of said one panel.
3. A flag strip according to claim 1 wherein said connector arm
includes a spring wire connection region, and wherein each flag
panel includes an opening between said frame and said flag panel
situated symmetrically opposite the spring wire connection region
on the opposite side of said centerline from said connector arm,
so that when one of said panels is separated from said strip and
superimposed atop and in facing relationship with another panel
of said strip, the spring wire connection region of the connector
arm of said other strip will be exposed through the symmetrically
situated opening of said one panel.
4. A flat strip according to claim 3 wherein each flag region includes
a central opening and a slot leading thereto, and wherein each connector
arm extends obliquely from a corner of said frame towards said slot,
the distal end of said arm extending through said slot and terminating
within said opening.
5. A flat strip according to claim 4 wherein said spring wire connection
region of each connector arm is offset with respect to the plane
of said frame and flag region to form a seat for a spring wire,
and wherein said connector arm is provided with crimp ears extending
laterally from said connection region for crimped engagement of
a spring wire.
6. A flag strip according to claim 1 wherein index holes are situated
along the frame portion of each panel parallel to one elongated
edge of said strip, and wherein the frames of adjacent panels are
connected to tabs.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a print hammer for use in a printing
mechanism, and to a method of making the same.
2. Description of the Prior Art
In high speed, moving type, impact printers of the kind typically
employed in data processing systems, a separate print hammer is
situated at each print position across a row of type. Hammer bank
assemblies for this purpose are described in the U.S. Pat. Nos.
3643595 to Helms et al and 3983806 to Ishi, both assigned to
Dataproducts Corporation, the assignee of this case.
Such hammer bank assemblies utilize a plurality of print hammers
each having a flat, electrically conductive coil disposed in a generally
rigid housing having an impact tip at one end, and supported by
a pair of crossed springs at the other end. The springs provide
electrical contact to the coil, and aid in restoring the hammer
to its rest position subsequent to impact. In the hammer bank assembly,
each hammer is situated between a pair of stationarily mounted flat
permanent magnets. When a current flows through the coil, the resultant
magnetic field interacts with the field of the permanent magnet,
resulting in a force which propels the hammer toward the type font
and the medium being printed. The basic configuration and operation
of such a printing hammer is set forth in the U.S. Pat. No. 3279362
to Helms, also assigned to Dataproducts Corporation.
Print hammers and hammer bank assemblies of the type disclosed
in the above cited patents have proven to be very effective in use,
have exhibited high reliability and long lifetime as measured in
number of impact operations between replacement, and have gained
widespread acceptance. However, the manufacture of the print hammers
themselves is a rather complex process. Typically, it involves the
fabrication of ten or twelve fairly complicated individual parts
which must be accurately located and assembled in a multi-step process
requiring considerable manual labor and expensive fixtures. A principal
objective of the present invention is to provide a print hammer
having substantially fewer components, and to provide a method of
fabrication that is substantially simpler than the prior art.
In a prior art print hammer, a flat coil is sandwiched between
a pair of rectangular, rigid, anodized metal plates called "flags".
A separate core member was situated in the center of the coil between
the flags. A separate terminal board was mounted at one end of the
flags. Individual flat wire springs were hooked to terminals on
this board. Hard anodizing was used to prevent electrical conduction
between the coil, which was electrically connected to the spring
wires, and the flags. A complex jig arrangement was necessary accurately
to locate the individual flags and associated components while these
were joined by appropriate adhesive. Considerable hand labor was
necessary to install the individual components in the jig, and to
make the requisite mechanical and electrical connections. Once this
partial assembly was complete, the individual units were molded
to secure in place an impact tip, to embed the terminal board area
and its components in a potting compound, and to form a support
foot for the flat wire springs.
By contrast, it is an object of the present invention to provide
a hammer assembly method in which the unassembled flags, instead
of being separate, individual items, are themselves part of an elongated,
punched metal strip. The flags are maintained in strip configuration
throughout most of the assembly process, thereby facilitating automation
of most of the assembly process.
Further objectives of the present invention include the following:
1. the elimination of manual handling of individual hammers;
2. the elimination of components such as the core and terminal
board elements of prior art hammers;
3. the elimination of flat wire, formed springs;
4. the elimination of hard anodizing of flags;
5. the replacement of ultrasonic or resistance welding of coil
leads by a more controllable joining method;
6. the simplification of production equipment;
7. the elimination of close manufacturing tolerances of certain
8. facilitating assembly of a symmetrical hammer from one side
9. the reduction of manual finishing operations;
10. reduction of tooling maintenance; and
11. the overall automation of the hammer fabrication process.
SUMMARY OF THE INVENTION
These and other objectives are achieved by a print hammer manufacturing
process in which the principal component is an elongated thin metal
strip which is punched to form separate flag panels connected by
tabs. During substantially the entire manufacturing operation, a
plurality of hammers are assembled in this strip format, the hammers
being separated as the final manufacturing step. By using this strip
approach, the manual handling of individual hammers is virtually
The pattern which is punched into the flat strip forms a series
of separate flag panels which are connected by tabs. Each panel
itself includes a flag connected to a surrounding frame by means
of links. A spring wire connector arm also is formed in each panel.
This connector arm extends obliquely from a corner of the frame
and includes crimp ears for attachment of a round spring wire. The
connector arm also includes a section which projects into a slot
region of the flag, so as to provide a connection point for the
inner lead of the hammer coil. Of particular importance is the arrangement
of the flag, frame, links and connector arm in each flag panel.
The arrangement is such that the same configuration can be used
as either the bottom or the top flag for the hammer.
One flag strip is used to form the bottoms of the hammers being
assembled. This strip remains substantially intact throughout the
assembly process. A second, like strip is used to form the tops
of the hammers. At an appropriate step in the process, individual
flags are severed from this strip and attached to the hammers being
assembled on the bottom flag strip.
In the process, the inner surfaces of both top and bottom flag
strips may be coated with a thermoplastic adhesive. The flags of
the bottom strip are embossed to form a central, coil-locating region.
Straight spring wires of round cross-section are attached to the
connector arms of both the upper and lower flag strips. The wires
are held in place by crimping. A flat coil is placed on the bottom
flag, located by the boss. The top flag, severed from the second
flag strip, is placed face down atop the coil so that the connector
arms of the bottom and top flags are situated on opposite sides
of a centerline through the hammer being formed. Localized heating
is used to bond together the bottom and top flags in the boss region,
by means of the pre-coated adhesive.
With the assembly so held together, the inner lead wire of the
coil is connected to the end portion of the connector arm of one
flag, and the outer coil lead is attached to the connector arm of
the other flag at the point of spring wire attachment to the bottom
flag. The entire hammer assembly is heated between plates to adhesively
tack the coil to the flags. A soldering compound is applied to both
connector arms at the point of spring wire attachment, and to the
connector arm end to which the inner coil lead is attached. Focused
infrared heating, or other heating technique is used from the top
side only of the assembly to solder all three coil lead and spring
wire connection points. The electrical insulation of both coil leads
is penetrated by the thermo-chemical action of the soldering compound
and by the high energy concentration of the localized heating.
Next, excess portions of the frames surrounding both the bottom
and top flags are severed and discarded, leaving the partially assembled
hammers connected to one another by the link and tab members of
the original flag strip. While still so connected, a group of the
hammers simultaneously are embedded in a molding compound such as
a structural epoxy. A hammer tip also may be mounted in this embedding
process. A single multicavity mold may be used simultaneously to
form a plurality of hammers. For each hammer, the hammer housing
and the separate spring-wire foot assembly are formed simultaneously.
Finally, the connecting links are cut away to separate the hammers
and complete the assembly process.
BRIEF DESCRIPTION OF THE DRAWINGS
The detailed description of the invention will be made with reference
to the accompanying drawings, wherein like numerals designate corresponding
parts in the several figures.
FIG. 1 is a perspective view of a print hammer manufactured in
accordance with the present invention.
FIG. 2 is a flow chart of the assembly steps in the inventive print
hammer manufacturing process.
FIG. 3 is a top plan view of a flag strip from which hammers like
that of FIG. 1 are assembled.
FIG. 4 is a perspective view of a flat coil utilized in each print
FIGS. 5 through 15 illustrate successive steps in the hammer assembly
process and are individually identified in the flow chart of FIG.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The following detailed description is of the best presently contemplated
mode of carrying out the invention. This description is not to be
taken in a limiting sense but is made merely for the purpose of
illustrating the general principles of the invention since the scope
of the invention best is defined by the appended claims.
A typical print hammer 20 manufactured in accordance with the present
invention is shown in FIG. 1. The hammer 20 consists of a coil housing
21 containing a flat coil 22 (FIG. 4) sandwiched between a pair
of flags 23B, 23T (described below) and embedded in a molding compound
such as a structural epoxy. The main portion of the coil housing
21 may have a typical thickness on the order of 0.035 inches. The
impact end 21a has an enlarged thickness, typically on the order
of 0.10 inches, and includes a forwardly projecting impact tip 24.
Projecting diagonally in opposite directions from an enlarged bottom
end 21b of the housing 21 are a pair of spring wires 25 26 each
having a round cross-section. The spring wires 25 26 are laterally
separated from one another so that they do not touch at the apparent
cross-over location shown at 27 in FIG. 1. The spring wires 25
26 pass through, and are embedded in a foot member 28 formed of
the same molding compound as the coil housing 21. The spring wire
lower ends 25a, 26a project below the foot member 28 and are available
for electrical connection to the hammer 20.
A principal component utilized in the mass production of print
hammers 20 is a "flag strip" 30 (FIG. 3) consisting of
an elongated thin metal strip that is punched to form a plurality
of separate flag panels 31 connected by tabs 32. By way of example,
the flag strip 30 may be formed of aluminum having a thickness of
0.004 inches and a width of 3 inches. The strip 30 is of arbitrary
length. In a preferred embodiment, two identical flag strips 30
are used to provide respectively the bottom and top flags 23B, 23T.
Initially, a set of index holes 33 may be punched at regular intervals
along a longitudinal border region 30a of the strip 30. These index
holes 33 then may be used both to locate and to transfer the strip
30 during subsequent assembly steps.
The first of these steps is the successive punching of the strip
30 to form the individual flag panels 31. In the preferred design
illustrated in FIG. 3 each panel 31 includes a flag 23 which is
connected to a surrounding frame 34 by a set of links 35a-35d (collectively
referred to as links 35). Extending from a corner of the frame 34
is a spring wire connector wire 36 having a diagonal spring wire-receiving
section 36a from which project laterally a pair of crimp ears 36b.
The free end of the connector arm 36 forms a wire terminal 36c.
This terminal end 36c is situated within a slot 37 which leads to
a central opening 38 in the flag 23. Each of the panels 31 is of
As part of the initial fabrication procedure (step 40 FIG. 2)
one face of each flag 23 is coated with a thermoplastic adhesive.
This adhesive coating may be done either before or after the flag
panels 31 are punched. The steps of index hole perforation, flag
panel punching and epoxy coating may be carried out in automated
fashion on a strip 30 which is wound from reel to reel during the
As a next assembly step 41 a coil centering boss 42 is formed
in each panel 31 of a first flag strip 30B that is used to provide
the bottom flags 23B (FIGS. 5 and 7). This flag strip 30B is maintained
intact as a continuous strip throughout most of the assembly process.
As illustrated in FIGS. 5 and 6 the coil centering boss 42 surrounds
the central flag opening 38 and conforms in shape to the central
opening 22a of the coil 22 (FIG. 4). Advantageously, the height
of the boss 42 corresponds to the thickness of the coil 22. The
same embossing operation that is used to form the coil centering
boss 42 also may be used to offset or depress the spring wire-receiving
section 36a of the connector arm 36 (FIGS. 5 and 6). Also, the terminal
end 36c of the connector arm 36 may be severed so as to leave a
short stub 36d at the end of the arm 36. Also, the crimp ears 36b
may be folded upward to facilitate insertion of the spring wire
Next, (step 43 FIG. 7) a straight spring wire 25 is pierced through
the upstanding ears 36b of the connector arm 36. The ears 36b then
may be folded over and crimped to hold the spring wire 25 in place.
Advantageously, the spring wire 25 itself may have a circular cross-section,
with a typical diameter on the order of 0.029 inches.
Next, a coil 22 is placed onto the bottom flag 23B (step 44 FIG.
10) and centered with respect to the boss 42. The coil itself may
be wound (step 45) of 5.5 mil diameter aluminum wire arranged in
four or five layers so as to form a flat coil having the overall
shape shown in FIG. 4. Advantageously, the coil wire may be insulated
and have an overcoat of a thermoplastic adhesive, typically the
same type of adhesive which is used to coat the faces of the flags
As separate operations which may be carried out concurrently with
the process steps 41 through 45 a second flag strip 30T is processed
(FIGS. 8 and 9) to form the top flags 23T. First (step 46) a flag
strip 30 (FIG. 3) is turned "upside down", so that when
viewed from the aspect of FIG. 8 the connector arm 36 appears to
project from the lower right hand corner of the frame 34 rather
than the lower left hand corner of the bottom strip 30B. The spring
wire receiving section 36a' is offset upwardly (as viewed in FIG.
8) to provide a seat for the spring wire 26. This wire 26 is situated
"above" the frame 34 and is crimped in place by the ears
36b' which are folded back "under" the offset region 36a'.
As a next step 47 individual top flag panels 23T' (FIG. 9) are
formed by severing the flag panels 31 of the top strip 30T along
the broken line 48 shown in FIG. 8. The upper two links 35a, 35c
are severed, while the lower links 35b and 35d are left in place.
These keep the lower frame portion 34' and the connector arm 36
attached to the top flag 23T'.
The severed top flag panel 23T' so formed (FIG. 9) next is mounted
(step 49 and FIG. 11) atop the coil 22 that is mounted on the bottom
flag strip 30B. Alignment can readily be accomplished by matching
the central opening 38' in the top flag 23T with the corresponding
opening 38 in the boss 42 region of the bottom flag 23B.
The top flag 23T immediately is bonded in place by bringing a heating
element 51 into contact with the portion of the top flag 23T that
is aligned with the boss 42. This is illustrated in FIG. 12. Optionally,
a mating heating element 52 may be disposed beneath the lower flag
23B within the boss 42 region. With this arrangement, the boss region
is heated to a temperature at which the adhesive on the mating faces
of the flags 23B and 23T is brought to a plastic or tacky state.
Typically this may take about two seconds. The heating elements
51 52 then are withdrawn. The adhesive quickly cools and solidifies,
thereby effectively bonding together the top and bottom flags 23B
and 23T in the region of the boss 42. The resultant bond is sufficient
to maintain the top flag 23T in place during the next assembly operations.
A permanent mechanical bond between the hammer components is not
required at this step.
Next, the electrical leads 22b, 22c (FIG. 4) of the coil 22 are
mechanically attached to the connector arm terminal 36c of the top
flag 23T and to the spring wire receiving region 36a of the bottom
Next, (step 53 and FIG. 13), the entire hammer assembly is sandwiched
between two hot plates 54 55. This heating makes the adhesive on
the inner faces of both the bottom and top flags 23B, 23T and on
the coil 22 become tacky. When the heating plates 54 55 then are
removed, the adhesive solidifies so as to bond firmly the sandwich
consisting of the bottom flag 23B, the coil 22 and the top flag
The next step 56 is to solder the two spring wire connector regions
36a, 36a' and the terminal 36c from the same side of the hammer
assembly. This is illustrated in FIGS. 11 and 14. First, a measured
amount of solder paste is deposited on each of the regions 36a,
36a' and 36c. The paste may include appropriate flux and solder.
Then, three infrared radiation sources 57 (mounted on an appropriate
jig, not shown) are focused respectively on the three areas 36a,
36a' and 36c. The wavelength, amount of radiated energy, and time
parameters are appropriately adjusted so that the incident, focused
infrared radiation will melt the solder, cause the solder to penetrate
the coil wire insulation, and accomplish the requisite soldering
operation. These parameters may be different for the three regions.
For example, a shorter time period, or a lower energy level may
be sufficient to solder the inner coil lead 22b to the terminal
36c than is required to solder both the spring wire 25 and the outer
coil lead 22c to the connector arm region 36a. Soldering of the
spring wire 26 to the connector arm region 36a' may be aided by
the presence of a small opening 36e (FIGS. 3 8 and 9) which is
formed in the flag strip 30 during the initial punching operation
Now (step 58 and FIG. 15) the lower frame portions 34 of both the
bottom and top flags 23B and 23T are severed from the assembly by
cutting where indicated by the broken line 59 in FIG. 11. This results
in a strip 60 (FIG. 15) of partially completed hammers that are
still connected by the links 35a-35d and tabs 32 of the bottom frame
30B. The top flag 23T and the connector arm 36' are attached to
the structure 60 solely by the adhesive which bonds them to the
boss 42 and the coil 22. The top connector arm 36' is totally separated
from the top flag 23T. Thus there is no electrical connection between
the connector arm 36' and the top flag 23T. Electrical isolation
of the inner coil lead 22b from the flag 23T thus is achieved. Similarly,
beneath the coil 22 the lower connector arm 36 to which the outer
coil lead 22c is attached, is both physically and electrically isolated
from the bottom flag 23B.
Next, a group of the attached incomplete hammer assemblies 60 are
simultaneously molded to form the coil housing 21 and the foot member
28 (step 61). This may be accomplished by separating the assembly
60 into strips each of which contains say a dozen of the connected
hammer assemblies. Each such strip then may be placed in a multiple
cavity mold appropriately configured to form both the coil housings
21 and the foot members 28 for all dozen of the hammers in the strip.
A generally U-shaped metal hammer tip 62 (FIG. 15) may be appropriately
positioned in th mold, or alternatively, may be clipped to the upper
corner of the sandwiched flags 23B, 23T to provide a rigid font-striking
surface at the impact tip 24 of the hammer 20.
An appropriate molding compound, such as a structural epoxy, then
is used to embed the assembly 60 thereby to form the coil housing
21 and the foot member 28. When the embedding operation is complete,
the assembly 60 is removed from the mold and the links 35 are severed
to separate the now completed hammers 20.
After the molding step 61 the spring wires 25 26 may be bent
so that they project downwardly in parallel fashion from the foot
member 28 as shown in FIG. 1. The portions of the spring wires
25 26 which are embedded in the foot member 28 may be coated with
an elastomer. Alternatively, part of the foot itself may be coated
with an elastomer. Such elastomer may have the benefit of damping
ultrasonic vibrations of the foot during hammer operation when the
hammers are installed in an appropriate hammer bank assembly.
The spring wires 25 26 may be of any appropriate material such
as berrylium copper or steel. Alternatively, the spring wires 25
26 could even be formed of a conductive plastic, or of a plastic
that was coated or inserted with metal so as to provide electrical
conduction to the coil 22.
Since each flat 23B and 23T is substantially U-shaped, induced
eddy currents are substantially reduced or eliminated. The slots
37 leading to the central openings 36 yield this result, by eliminating
a current path around each opening 36.
From the foregoing description, it will be appreciated that one
of the features of the flag strip 30 which makes the present invention
workable is the offset arrangement of the links 35. This offset
arrangement permits (step 58 and FIGS. 11 and 15) the lower links
35b, 35d of the top flag 23T to be severed, while still maintaining
the integrity of the links 35 that connect adjacent flag panels
in the bottom strip 30B. Thus, the staggered link arrangement permits
the entire mass production hammer assembly operation to be carried
out atop one flag strip 30B which maintains its integrity as a continuous
strip throughout the entire fabrication process.