Mechanically indexed mask stretching apparatus

Method and apparatus are disclosed for tensioning uniform foil shadow masks by clamping the edges of a mask and moving the clamps through a preselected fixed displacement, thereby obviating the need for extensive controls on the tensioning process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to tensioned shadow masks for color cathode 
ray tubes. More specifically, the present invention relates to methods and 
apparatuses for producing interchangeable shadow masks by tensioning the 
masks by stretching them to a fixed displacement. 
2. Discussion of the Related Art 
U.S. Pat. No. 4,902,257, ('257), issued Feb. 20, 1990, and assigned to the 
same assignee as the present invention, describes apparatus and process 
for manufacturing color cathode ray tubes of the flat tension mask type 
wherein shadow masks and front panels are respectively interchangeable 
during mask-panel assembly. According to the cited patent, position 
sensing means and a feedback control system are provided for applying 
controlled forces at a plurality of locations about the periphery of the 
mask, for the purpose of moving the mask to a desired position and 
stretching it to a desired size and shape for registration and assembly 
with the screened faceplate. 
FIG. 1, corresponding to FIG. 12 of the '257 patent, depicts a prior art 
machine for applying controlled forces to a plurality of clamps gripping 
peripheral portions of the flat tension mask. The machine carries optical 
markers which cooperate with position-sensing apertures in the mask. It is 
the task of this machine to apply a distribution of forces to the mask 
such as to bring all position-sensing apertures in the mask into 
coincidence with their corresponding optical markers. This is achieved by 
feeding position error signals to a computer which calculates the required 
forces and feeds appropriate pulses to the stepping motors. Springs 
inserted between motors and clamps convert motor displacements into well 
defined forces. 
The '257 patent also points out that if the photoetched masks were all 
exactly alike in thickness, elastic properties and detailed geometry, the 
forces to be applied to them to obtain a standard shape would always be 
the same, and no feedback would be required; but in practice there are 
unavoidable variations in thickness of the masks. To compensate for these 
variations, force adjustments are necessary, and these are controlled by a 
feedback loop operated by a computer system connected to each clamp. 
It has now been found that a different, simpler mask stretching process is 
also capable of achieving the goal of interchangeability. This process 
eliminates the need for a feedback control system. Thus, position-sensing 
apertures in the mask, optical markers and photosensors in the assembly 
machine are no longer needed. The concept of applying feedback-controlled 
forces to the mask edges, embodied in the apparatus and method taught by 
the '257 patent, is set aside, and the concept of imposing a predetermined 
fixed displacement upon the mask edges, regardless of the required forces, 
is put in its place. 
Forces acting on an elastic body are related to the displacement of its 
boundaries by Hooke's law. So long as the elastic limit is not exceeded, 
the two are proportional to each other. It may therefore seem, at first 
glance, that there is no important difference between controlling force 
and controlling displacement. If all masks were exactly alike, this would 
indeed be true. It is the nature of the variations between masks actually 
encountered which makes the distinction between force and displacement 
important and renders the process according to this invention suitable for 
large-scale production of flat tension mask cathode ray tubes. 
Tension masks for mass-produced cathode ray tubes are presently made of 
steel. Nickel-iron alloys such as molybdenum permalloy may be used instead 
because of their superior mechanical and magnetic properties; such masks 
are described in U.S. Pat. No. 4,900,976 assigned to the same assignee. 
Typically, tension masks are 1 mil (one-thousandth inch) thick. 
Manufacturing tolerance on the thickness is typically plus or minus 10%. 
Experience shows that there are small thickness variations--considerably 
less than 10%--within each mask, as well as within one production lot of 
masks. In addition, there are significant thickness variations between 
different production runs which show up as corresponding variations in 
mask stiffness. 
The variations just discussed change the forces that must be applied to the 
mask to stretch it to the desired shape and size. Therefore if, for 
example, electric stepping motors are used to produce the required forces, 
and springs are inserted between motors and clamps to produce an 
adjustable force, as suggested in the '257 patent, rather than a 
controlled displacement, then masks from different production runs will 
require different numbers of motor steps. A thicker mask, for example, 
requires the stepping motor to stretch its spring further so as to produce 
the extra force required by the extra thickness. Similarly, a small 
thickness variation between top and bottom halves of the mask will require 
different forces to be applied to the top and bottom clamps along the 
vertical edges of the mask. Therefore, a feedback system to control the 
stepping motors as described in '257 becomes necessary. 
Other patents cited as background in the art include: U.S. Pat. Nos. 
4,942,333; 3,573,528; 4,555,034; and 4,748,370. 
This invention arrives at the desired result--to stretch a mask to the 
desired size and shape, and position it correctly, by a different, more 
straightforward procedure. Taking advantage of the fact that the 
unstretched photoetched masks are very much alike except for minor 
variations in thickness, in a preferred arrangement to be described, the 
initial position of the mask upon insertion into the assembly machine can 
readily be defined with great precision by retractable alignment pins 
cooperating with photoetched apertures in the mask. Means for selectively 
gripping a mask edge, e.g. clamps similar to those shown in the '257 
patent but having hard-surface jaws (no elastomeric coating) are precisely 
positioned before the jaws are closed. These clamps are connected to a 
motive means, e.g. stepping motors, by links which are as rigid as 
possible in the direction of the pulling force exerted upon the mask, so 
that a given or fixed displacement of each stepping motor is transferred 
unchanged from the motor to the corresponding clamp and thus to the mask 
edge. 
With this arrangement, the stepping motors are programmed to advance by a 
predetermined number of steps after the clamps are closed, thereby 
producing a predetermined fixed displacement of the mask edge around the 
circumference of the mask and stretching the mask to a predetermined size 
and shape. Since the mask was initially correctly positioned by means of 
the alignment pins and since all displacements from that initial position 
are made in accordance with a predetermined data file, the stretched mask 
not only has the correct size and shape, but is also located at the 
correct position.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
FIGS. 2A and 2B schematically illustrate one form of apparatus 100 
according to the invention. Mask 102 is placed on a flat table 103 
equipped with precisely shaped alignment pins 104, 108 which are 
retractable into the table 103. The alignment pins 104, 108 mate with a 
round aperture 106 and a slot aperture 110, respectively, which are 
photoetched into the mask. The pins 104, 108 and apertures 106, 110 
thereby determine the initial position of the mask. 
Multiple clamps 112, located about the periphery of the mask 102, are 
powered to be opened or closed by air cylinders 113, or the like, attached 
thereto. As is known, a plurality of clamps is required along each of the 
four mask edges to allow for tangential stretching along that edge. As all 
four edge systems are substantially equivalent, the clamping arrangement 
for one edge will be described. The clamps 112 are individually mounted on 
long levers 114. The several levers 114 are mounted on a common lever 
plate 121 through pivots 116, allowing for lateral movement of the clamps 
112. The lever plate 121 is, in turn, carried on a slide 118. Retractable 
pins 120 carried in the lever plate 121 cooperate with a slot 122 in each 
lever 114 to ensure lateral alignment of the levers before the clamps 112 
close. 
The slide 118 moves in a straight line along a precision guideway 124 
equipped with roller bearings 126. The slide 118 is linked to a motive 
means, such as stepping motor 128, through a threaded post 129 and a lead 
screw 132, which is seated in a thrust bearing 131. Stepping motor 128 
receives preprogrammed control pulses from a data file 134 through a power 
amplifier 136. Referring now to the whole apparatus 100, the table 103, 
guideways 124, thrust bearings 131 and stepping motors 128 are all mounted 
on a baseplate 140. 
In operation, each mask edge stepping motor 128 is advanced sufficiently to 
pull the clamps 112 out of the way so that a mask 102 can be positioned on 
the table 103 over the alignment pins 104, 108. Optionally, a top plate 
(not shown) may be placed over the mask 102 to ensure flatness, or other 
means operable at ambient temperatures such as a vacuum pump 130 beneath 
the table 103 may be used for this purpose. Next, the stepping motor 128 
is backed up to the desired starting point, with the clamps 112 guided by 
pins 120. The exact position of each clamp is fully defined by its pin 
120, the length of its lever 114 and the position of slide 118 which in 
turn is controlled by stepping motor 128. 
All clamps are then closed, preferably by pneumatic action of air cylinders 
113, and the alignment pins 104, 108 and 120 are withdrawn. Each stepping 
motor 128 is then advanced by a predetermined increment to achieve a fixed 
displacement, stretching the mask 102 to the desired size and shape. In 
the process, levers 114 along each edge spread apart from each other by 
pivoting laterally at the pivot 116 by a small amount, typically in the 
order of one-thousandth of the dimensions of the active area of the mask, 
in order to accommodate stretching of the mask along the orthogonal axis. 
Once the mask 102 is stretched, remaining manufacturing operations such as 
insertion and positioning of a screened faceplate equipped with a mask 
support structure, welding the mask to the support structure, cutting the 
mask along the outside of the support structure, extracting the 
faceplate-mask assembly, opening the clamps and removing the unused 
peripheral portion of the mask may be carried out as described in the 
aforementioned '257 patent. 
The procedure just described presupposes that the screened faceplate which 
is to be joined to the stretched mask is correctly positioned, so that 
after welding the mask to the support structure mounted on the faceplate, 
mask and screen are properly registered. U.S. patent application Ser. No. 
370,204, now U.S. Pat. No. 4,973,280, which is a continuation-in-part of 
the application now constituting the '257 patent, describes means and 
procedures for measuring the location of the screen pattern on the 
faceplate and for displacing the faceplate if need be so that the screen 
pattern is correctly positioned. The application also shows that it is not 
necessary for the mask to be correctly positioned, so long as it is 
stretched to the correct size and shape. Inspection means, for example 
optical devices, may then be used to determine the position of the mask, 
and appropriate corrections may be applied to the displacement of the 
faceplate so as to compensate for the original position errors of the mask 
as well as the screen. 
This same principle may be applied to the present invention. Accordingly, 
it is not necessary for the mask to be placed with extreme accuracy in 
position for registration with the screened faceplate before the clamps 
close; an original positioning error may later be compensated by the 
procedure just described. This possibility is important if the mask, for 
example, is extremely thin (for example, less than 0.001") so that 
positioning by tightly fitting registration pins could warp or tear it. 
It will be understood that the essence of the invention is to stretch a 
mask by a fixed displacement to a predetermined size and shape by 
displacing a plurality of points on its periphery outward by predetermined 
increments. This may be done not only by means of the apparatus described 
in FIGS. 2A and 2B, but also in other ways. 
FIGS. 3A and 3B illustrate schematically an alternative fixed displacement 
mask-stretcher. Only the mechanism 200 associated with one clamp 202 is 
described in detail, it being understood that any desired number of clamps 
may be used, equipped with similar mechanisms. 
The mechanism 200 described hereinafter is supported by a movable carrier 
plate 226 which serves to move the mechanism 200 out of the way for mask 
insertion. One such carrier plate 226 is used for each of the four mask 
edges. FIGS. 3A and 3B show part of the carrier plate 226 serving the 
right vertical edge of the mask 102. At all times except during mask 
insertion and during removal of the unused peripheral portion of the mask 
at the end of the process, i.e. when the mechanism is in the mask loading 
position, the carrier plate 226 is pressed by carrier plate driver 227 
firmly against stops 228 which are mounted on the main frame 229 of the 
assembly machine. This latter position is denominated as the "operational 
position" of the mechanism 200. 
A clamp 202 is laterally guided on either side thereof by leaf springs 204 
which in turn are attached to posts 206 mounted on carrier plate 226. The 
leaf springs 204 ensure correct lateral placement of the clamp 202 when 
the clamp is open, but permit the small lateral movement which occurs when 
the clamp is closed and the mask is being stretched. 
Attached to the clamp 202 is a "U"-shaped rigid frame 208 which carries 
therein a bar 210. In the rest position of the clamp, the bar 210 is 
pushed firmly against pins 212 which are mounted interiorly of the rigid 
frame 208 on carrier plate 226. The pushing is accomplished by the action 
of a resilient link, or spring 220, then under compression, attached to 
the bight 209 of each frame. All springs 220 associated with one edge of 
the mask are connected to a common yoke 222 which is attached to a driver, 
such as pneumatic driver 224, the latter also mounted on the carrier plate 
226. 
In the following description, the terms "right" and "left", used for 
greater clarity, refer specifically to FIGS. 3A and 3B, which show 
mechanism 200 located to the right of the mask 102. It will be understood 
that more generally, "right" means "away from the mask" and "left" means 
"toward the mask." 
When the pneumatic driver 224 is activated, yoke 222 is pulled to the 
right. The spring 220 is stretched, and instead of pushing the clamp 202 
to the left as before, it now pulls the clamp to the right, stretching the 
mask. However, the displacement of the clamp 220 is limited by a hard 
physical barrier, such as an adjustable stop 216 which is comprised of a 
micrometric screw 217 set into a block 218 which is mounted on the carrier 
plate 226. The screw 217 makes contact with a half-ball 214 which is 
attached to the bar 210 on its right side. The setting of the stop 216, 
and not the spring force, thus determines the displacement of each clamp. 
Each spring 220 must be stiff enough to produce the force required for 
stretching the mask with a reasonable displacement of the pneumatic driver 
224. However, there is no need for precisely adjusted spring constants or 
for precisely defined displacement of the pneumatic driver. All the 
required precision is lodged in the setting of the adjustable stop 216. 
In operation, carrier plate 226 is moved to the right by carrier plate 
driver 227 to displace the entire mechanism 200 by an amount sufficient to 
permit inserting a mask, i.e., into the mask loading position. During this 
time, the clamps 202 are open and in their rest position, i.e., the bars 
210 make firm contact with the pins 212. The mask is positioned by means 
of retractable alignment pins such as described in connection with FIGS. 
2A & 2B. Once the mask is in place, the carrier plate 226 is moved back to 
the left and pressed firmly against the stops 228, i.e., into its 
operational position. It will be noted that because the carrier plate 226 
is moved only when the clamps 202 are open, very little force is required 
to move it. 
The clamps 202 are now closed, the mask alignment pins withdrawn and the 
pneumatic drivers 224 (one for each mask edge) activated. All clamps move 
until the half balls 214 are seated against their respective stops 216. 
The mask is thereby stretched to the correct size and shape, and it is in 
the correct position; welding and cutting may now be done as previously 
described. Finally, the clamps 202 are opened, the pneumatic drivers 224 
are de-activated and carrier plates 226 are moved away from the mask to 
permit easier removal of the peripheral portion of the mask. The machine 
is then ready to accept a new mask. 
The details and the timing of faceplate insertion and removal of the 
completed faceplate-mask assembly have not been described here. They do 
not form part of this invention and may be carried out in any convenient 
manner. 
The form of the invention just described has the advantage that only four 
mechanical drivers are required, regardless of the number of points along 
the edges whose displacements one wishes to specify; and even those four 
drivers, one for each edge, may be relatively crude pneumatic drivers 
rather than precisely made, computer-driven stepping motors. The required 
precision is attained by accurately fixing the displacement of each clamp 
by means of mechanical stops such as pins 212 and adjustable stops 216. 
FIGS. 4A & 4B show an alternative form embodying the same principle as 
FIGS. 3A and 3B. As in FIGS. 3A & 3B, only the mechanism serving the right 
side of the mask 102 is illustrated, it being understood that similar 
mechanisms serve the other three sides. Again, "right" and "left" mean 
more generally "away from the mask" and "toward the mask" respectively. In 
FIGS. 4A & 4B each mask clamp 112 is independently driven during operation 
rather than being connected to a common yoke or carrier, as further 
explained below. Clamps 112, with levers 114 attached to platforms, or 
lever plates 121, through pivots 116; function as described in connection 
with FIGS. 2A and 2B. Before clamping, the lateral position of levers 114 
are standardized by retractable alignment pins 120. 
Lever plates 121 are mounted on slides 402. Each slide 402 thus carries one 
clamp 112 independent of the other clamps. The bases 404 of these slides 
are themselves fastened to a common, movable carrier plate 226. The common 
carrier plate 226 is slidably mounted on a fixed base 140. The carrier 
plate 226 also supports several first posts 406, each first post 406 being 
proximal to its associated slide 402. Attached to slide 402 is a 
"U"-shaped plate control structure 410. A double-acting air cylinder 408 
mounted to each post 406, and attached to the plate control structure 410 
through a piston rod 407, drives the plate control structure 410 to the 
left or right as desired. Motion to the left or right is limited by stops 
412, 414, respectively, which are affixed to the arms of the plate control 
structure 410. All stops 412, 414 may be adjusted, for example, by means 
of shims 413, to independently control the movement of each clamp 112, as 
further explained below. 
Attached to the fixed base 140 at the right end, is a single second post 
415. A single third post 419, intermediately placed between the first and 
second posts is mounted on the movable carrier plate 226. 
The carrier plate 226 may be moved to the right by a distance sufficient to 
permit mask insertion as well as the removal of a finished mask-faceplate 
assembly. A return spring 416 connected between the second and third posts 
415, 419 respectively, provides the small required force for this 
movement. Once a new mask 102 is in place but before the clamps 112 are 
closed, an air cylinder 418, attached to the 2nd post 415 and to the 3rd 
post 419 through a piston rod 422, is activated and moves carrier plate 
226 to the left to a position determined by a stop 420. The stop 420 is 
cantilevered from the second post 415 at a point intermediate the first 
and second posts 406, 419 respectively. The double acting air cylinders 
408 then move the slides 402 to the left against their respective stops 
412. Next, air cylinders 113 are activated to close the clamps 112 onto 
the mask 102. The air supply to each air cylinder 408 is then reversed to 
pull the slides 402 to the right against their respective stops 414, 
thereby stretching the mask 102. 
While the operation of only one carrier plate 226 has been described, it 
will be understood that in practice there is one carrier plate 226 and one 
air cylinder 418 for each of the four sides of the mask, and there is one 
slide 402, one plate 121, one post 406, one double action air cylinder 
408, and one plate control structure 410 for each of the several clamps 
112 on each side of the mask 102. 
The embodiment just described shares with the one described in connection 
with FIGS. 3A and B the advantage that the displacement of each clamp 112, 
and thus the displacement of the corresponding portions of the mask edge, 
may be individually fixed by adjusting the setting of stops 216 (FIGS. 3A 
and B) and stops 412 and/or 414 (FIGS. 4A and B). The last described 
embodiment has the further advantage that none of the drivers--in this 
case, air cylinders 408--need produce a larger force than what is required 
for a single clamp. The sum of the forces acting on all clamps along one 
mask edge, typically amounting to several hundred pounds, is exerted 
against stop 420 but need not be generated by any single driver. 
The need for a plurality of clamps along each edge arises, as previously 
explained, because each edge must be free to stretch as the entire mask is 
stretched. It is possible, however, to use just one broad clamp along each 
edge if the clamps are sufficiently far away from the central, apertured 
portion of the mask. FIG. 5 illustrates such an arrangement. 
As seen in FIG. 5, mask 302 has a central apertured portion 304 and four 
appendages 306. The appendages are of sufficient length L to permit the 
edges 305 of the apertured portion 304 to stretch even though the outer 
edges of appendages 306 are clamped, without producing excessive stress in 
the mask material. If desired, portions of appendages 306 may be 
perforated in order to improve the stress distribution. 
A clamp 308 is attached to each appendage 306 and to a stepping motor 310. 
Operation of this embodiment is the same as that described in connection 
with FIG. 2A and 2B. It will be seen that the mechanisms described in 
connection with FIGS. 3A and B and 4A and B, with the displacement of the 
clamps controlled by mechanical stops, may be substituted for stepping 
motors 310 whose displacement is controlled by a data file 134 through the 
power amplifier 136. 
It will be understood that in a machine constructed according to FIG. 5, 
control over the size and shape of the apertured portion 304 is not as 
close as with machines built according to FIGS. 2A, 2B, 3A, 3B and 4A &B. 
Appendages 306, made long enough in dimension L to permit transverse 
stretching of the aperture portion edges 305 are also long enough to 
permit certain geometrical distortions to occur. These distortions, 
however, are generally the same with each mask and thus are of little 
consequence. 
It is further understood that a machine according to FIG. 5 wastes a larger 
percentage of mask material than the machines shown in the earlier 
figures. However, a machine constructed according to FIG. 5 has the 
important advantage of low complexity, e.g.. absence of pivots, 
retractable pins, springs, etc., resulting in less down time and reduced 
maintenance, yet is capable of stretching masks with high uniformity 
provided that the unstretched masks are sufficiently uniform. 
While the present invention has been illustrated and described in 
connection with the preferred embodiments, it is not to be limited to the 
particular structure shown, because many variations thereof will be 
evident to one skilled in the art and are intended to be encompassed in 
the present invention as set forth in the following claims.