Armature of an electromagnet-coil/armature system for dot matrix print heads, and method of manufacturing same

In a method for the manufacture of armatures from sheet-metal parts stacked in the manner of laminated inductors, predeterminately shaped openings or indentations (6) and corresponding coaxially opposite projections (8) are defined in and during the formation of the sheet-metal parts (1, 2, 3), the sheet-metal parts are stacked so that the projections (8) nestedly engage the shaped openings or indentations (6) of the adjacent sheet-metal part (1, 2, 3) and thereby provide form-locked mechanical connections between the parts, and the stack (11) of sheet-metal shapes is secured against loosening and disengagement of the form-locked mechanical connections (10) by cementing or thermal bonding or the like of the sheet-metal parts one to another.

FIELD OF THE INVENTION 
The present invention relates to a method for producing armatures of the 
electromagnet-coil/armature system for dot matrix print heads, and to an 
improved armature, particularly but not exclusively of the flap armature 
type, formed of shaped sheet metal parts or elements which are stacked in 
the manner of a laminated inductor. 
BACKGROUND OF THE INVENTION 
Dot matrix print heads form a central assembly on which the performance of 
a dot matrix printer depends. The type of construction and materials 
employed and the resulting performance characteristics of the print head 
determine the endurance, and thus the life, of the printer. Dot matrix 
print heads are typically replaceable in the printer and are accordingly 
available as spare parts therefor. The performance characteristics of the 
print head are substantially determined by the system or arrangement of 
the magnetic circuit, i.e. of the electromagnet-coil/armature system. In 
this connection a distinction may be drawn between so-called pretensioned 
magnet systems, in which the armature consists of a spring, and 
electromagnet-coil/armature systems of the flap armature type. 
The present invention relates to the construction and production of either 
of these two types of armature, although its preferred application--and 
that to which the present disclosure is specifically directed--is to 
systems of the flap armature type. In any event, the magnetic induction 
produced by the electromagnet coil in the magnet core passes in a 
pulsating manner through the armature. In armatures consisting of a 
homogeneous ferrous material, the induced voltages thereby produce 
eddy-current losses since the armature acts in the manner of a 
short-circuited winding. These eddy-current losses contribute to 
significant heating of the armature, causing undesired heating of the dot 
matrix print head and, in addition, to less efficient and effective 
utilization of energy. This is particularly disadvantageous in that other 
components or elements present in the dot matrix print head assembly, 
including the print head itself, are also heated. Not infrequently the 
resulting temperatures amount to more than 100.degree. C. 
It is known that such eddy-current losses can be minimized, or at least 
significantly reduced, by subdividing the iron body into a multiplicity of 
thin sheets of material which are insulated from each other (as, for 
example, by a unilateral paper coating or by surface lacquering) and 
through the use of silicone-containing iron which has a higher electrical 
resistance (e.g. alloy dynamo sheets). 
When using such laminations on armatures or flap armatures for dot matrix 
print heads, the relatively small dimensions--such, for example, as an 
armature length of about 25 mm and an armature thickness of about 6 mm--of 
the parts necessarily require new manufacturing techniques. Thus, the use 
of laminated armatures requires a secure connection of the lamellae since 
the resulting structure, in contradistinction to stationary devices or 
machines such as transformers or to the stators of electrical machines or 
of uniformly moving electrical apparati, operatively undergo relatively 
high forces of acceleration and deceleration in each cycle of armature 
action. 
It has heretofore been proposed, for example in European Patent Application 
No. A1-0 152 117, that the stacked sheet-metal parts or elements be 
connected to each other by way of rivets. Because of the small dimensions 
of the armature assembly, however, rivets are difficult to handle, add 
excessive weight to the armature and add substantial expense to unit 
manufacturing costs. 
SUMMARY OF THE INVENTION 
It is accordingly the object of the invention to provide a laminated-type 
armature and a method of producing such armatures, incorporating between 
its sheet-like parts or elements a substantially permanent connection 
having an effective lifetime of, by way of example, as much a 500 million 
work cycles with the armature being of light weight and relatively smooth 
along its exterior surface. 
These and other objects are achieved in accordance with the invention by 
practicing the successive method steps of: 
a) forming, simultaneously or substantially concurrently with stamping of 
the plural sheet-metal elements or armature parts, at least one 
predeterminately shaped opening or indentation in the outer surface of the 
first and each succeeding sheet-metal part, and at least one projection, 
opposite said opening or indentation, in the outer surface of each of the 
second and subsequent sheet-metal parts, each such projection on a 
sheet-metal part being predeterminately shaped so as to fit nestedly into 
the shaped opening of an adjacently-disposed sheet-metal part; 
b) stacking the sheet-metal parts such that the shaped opening or 
indentation in each part and the shaped projection of the 
adjacently-disposed sheet-metal part in each case define a form-locked 
mechanical connection between those adjacent parts; and 
c) securing together the sheet-metal elements or parts in the stack against 
loosening of the formed-locked mechanical connection by cementing or 
thermal bonding or welding or the like. 
The predeterminately-shaped openings, indentations and projections are 
effective to assure and maintain precise relative positioning of the 
plural sheet-metal parts and thus the contour of the entire armature. In 
addition, an interlocking form-locked connection is produced by a 
continuous process which includes all manufacturing steps. The mechanical 
form-lock is thereupon secured against disengagement or release in the 
finished armature by a nondetachable connection of all of the sheet-metal 
parts. The resulting armature accordingly possesses the advantageous 
properties of a stack and is, advantageously, not unduly subject to the 
heretofore common heating resulting from eddy-current losses. Furthermore, 
the armature of the invention has a relatively smooth or unbroken pole 
surface. 
As a further improvement in accordance with the invention the thermal 
bonding between each two adjacently-disposed sheet-metal parts may be 
effected spot-wise--i.e. by spot welding or the like--in the region of the 
outer surfaces of the adjoining sheet-metal parts. In providing the 
substantially permanent or nondetachable connections the insulated or 
uninsulated sheet-metal parts may be separated by a narrow air gap. 
Another improvement of the invention contemplates the placement of the 
thermal bonding or cementing at surface areas which, during operation of 
the electromagnet-coil/armature system, lie outside of areas of contact 
with one or more electromagnet coil cores. The provision of such 
nondetachable connection does not, therefore, disturb the contacting 
function of the armature and neither does it negatively influence or 
interfere with the path of armature movement. 
The armature thus manufactured in accordance with the invention may 
furthermore be shaped so that the section of the armature opposite one 
pole surface of an associated electromagnet coil core is thicker, as 
compared to another section facing or carrying a print element, and so 
that the form-locked mechanical connection between adjacently-disposed 
parts is provided in this thicker section. At least one or two of the 
sheet-metal parts may form the armature arms on which the print element is 
carried or fastened. 
The armature produced as herein described may, with respect to a limiting 
sheet-metal shape which keeps the armature relatively smooth or unbroken 
on its exteriorly-disposed surface, be entirely punched through at its 
shaped openings in such manner that the outermost sheet-metal shape has 
continuous shaped throughbores or openings into which the projections of 
the next inner or immediately adjacent sheet-metal part in the stack 
engage. 
Further objects, features and advantages of the present invention will be 
more fully appreciated by reference to the following detailed description 
of presently preferred, but nevertheless illustrative, embodiments in 
accordance with the invention when taken in conjunction with the 
accompanying drawings. It is to be understood, however, that the drawings 
are designed solely for the purpose of illustration and not as a 
definition of the limits of the invention, for which reference should be 
made to the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The individual sheet-metal parts or elements or members 1, 2, 3 having the 
appropriate outer shapes or contours 4 such, by way of example, as are 
depicted in the drawings herein are punched out or otherwise formed by, 
for example, a sheet metal stamping tool. At the same time, one or more 
predeterminately shaped openings or indentations 6 are formed or defined 
in a first face or surface 5 of each sheet-metal part. The indentations 6 
in the part 3 may comprise a continuous throughbore extending between the 
opposite faces thereof. In each of the parts 1 and 2, on the other hand, 
each opening 6 is defined not as a continuous throughbore but, rather, as 
an indentation or depression or concavity 6 in the face 5. Also formed in 
each part 1, 2--preferably also simultaneously or substantially 
concurrently with the stamping of those parts--is at least one projection 
8 which extends outwardly from the second or opposite face or surface 7 of 
the sheet-metal part aligned substantially coaxially with a corresponding 
indentation 6. In size, cross-sectional configuration and manufacturing 
tolerances each projection 8 is such that it fits nestedly into a shaped 
opening or indentation 6 of an adjacently-disposed sheet-metal part 1, 2 
or 3, thereby creating a form-locked mechanical connection between each 
two adjacently-disposed sheet-metal parts (FIGS. 1 and 2). This 
element-to-element mechanical connection of the plural sheet-metal parts 
1, 2, 3 results, when the elements are so stacked atop each other, in a 
complete armature 9 (FIG. 3). 
It should be understood that although the openings or indentations 6 and 
the projections 8 are illustrated in the drawings as having generally 
circular cross-sectional configurations, such configurations are by way of 
example only and are not intended to be construed as a limitation on the 
scope of the invention. Thus, the shapes of the indentations or openings 6 
and of the projections 8 may be predetermined or selected as a matter of 
design choice. 
In a presently preferred construction, each part or element 1, 2, 3 
includes two projections 8 and, correspondingly, two shaped openings or 
indentations 6 in the longitudinal direction of the armature 9. The 
totality of openings or indentations 6 and of projections 8 results in an 
integrated form-locked mechanical connection 10 (FIG. 2) between the 
plural parts in the stack and provides precise relative alignment and 
positioning of the parts and, therefore, of the overall contour of the 
armature 9. 
The armature 9 (FIG. 3) accordingly consists of a stack 11 of sheet-like 
parts--much in the manner of a body formed of a series of parallel 
slices--which is held together by, for example, adhesive or cement, 
thermal bonding or welding or other thermal method. Thermal bonding may, 
for example, be effected spot-wise (i.e. by spot welds) between each two 
adjacent sheet-metal parts 1 and 2, or 2 and 2, or 2 and 1, or 1 and 3, 
respectively. This procedure may result in the provision and maintenance 
of an air gap 12 between portions of adjacent-disposed parts; such air 
gap, while present in practice, is negligibly small. The successive 
bonding spots or points in the region of the outer surfaces 5 and 7 of the 
sheet-metal parts 1, 2, 3 produces the row of melt spots 13 visible in 
FIG. 3, which row of melt spots may be compared to a weld seam. 
The row of melt points 13 is advantageously situate between the mechanical 
form-locked connections 10 in a surface area 14 which, in operation, lies 
outside or spaced from the contact surfaces 15 and 16 of the yoke surfaces 
of corresponding or associated electromagnet coil cores. The armature 9 
includes, opposite such pole surfaces (not shown) of electromagnet coil 
cores, a section 17. The armature section 17 is thicker than another 
section 9a, the thickness of the section 9a being selected so as to 
enable, if desired, the attachment thereto of a print element, as for 
example a print pin 18. 
The outermost sheet-metal part 3 preferably carries no projections 8 for 
which purpose, as previously noted, the shaped openings 6 in the first 
part 3 comprise continuous throughbores into which the projections 8 of 
the next inner or adjacently-disposed sheet-metal part 1 nestedly engage 
without protruding beyond the outer face thereof. 
While there have been shown and described and pointed out fundamental novel 
features of the invention as applied to preferred embodiments thereof, it 
will be understood that various omissions and substitutions and changes in 
the form and details of the device illustrated, and in its method of 
manufacture, may be made by those skilled in the art without departing 
from the spirit of the invention. It is the intention, therefore, to be 
limited only as indicated by the scope of the claims appended hereto.