Abstract:
A method for post-fabrication modification of the snap actuation properties of a thermally responsive bimetallic actuator by exposing a pre-formed bimetallic actuator to laser energy, thereby permanently altering the thermal response properties of the bimetallic actuator, and a thermally responsive bimetallic actuator having snap actuation properties developed according to the method.

Description:
This application is a division of application Ser. No. 09/976,388 filed Oct. 11, 2001 now U.S. Pat. No. 6,580,351, which claims the benefit of U.S. Provisional Application Ser. No. 60/240,482, filed in the names of Robert F. Jordan and George D. Davis on Oct. 13, 2000, the complete disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to methods for manufacturing thermally responsive bimetallic members, and in particular to methods for permanently compensating thermal response characteristics of snap-action bimetallic members. 
   BACKGROUND OF THE INVENTION 
   Thermally responsive bimetallic members that exhibit a snap-action response are commonly utilized to actuate overheat protection and thermostatic switching mechanisms. One type of such switching mechanisms is a thermostatic switch that utilizes an actuator formed of a bimetallic material having materials of relatively low and high thermal expansion coefficients joined together along a common interface. The bimetallic actuators that drive such switching mechanisms typically exhibit a forceful snapping action between two states of stability with each of these states being responsive to a predetermined threshold or set-point temperature. When the switching mechanism senses a temperature that is below a first lower of these predetermined set-point temperatures, the thermally responsive member, i.e. the bimetallic actuator, is in one of the two stable states. Accordingly, when the sensed temperature is above a second higher predetermined set-point temperature, the thermally responsive member forcefully snaps to a second of the two stable states and remains in this second state while the sensed temperature remains above the first lower set-point temperature. Should the sensed temperature be reduced to the first lower temperature, the temperature of the member is lowered correspondingly. As a result, the thermally responsive member forcefully snaps back to the first lower temperature state. The difference between the two predetermined set-point temperatures corresponding to the respective first and second states of stability is known as the “differential temperature” of the thermally responsive member. 
   A known method of manufacturing thermally responsive snap-action switches of the variety described above has included a forming operation in which a pre-sized blank of thermally responsive bimetallic material is positioned between two opposingly positioned shaping or die members. The shaping members are actuated to engage the blank, thereby forming a bimetallic disc having a configuration that achieves forceful snap-action at each of the two predetermined set-point temperatures. Such a configuration usually consists of a knee and/or corresponding bowed portion, a dimpled portion or portions, or a series of ridges. Examples of such of formations are described in U.S. Pat. Nos. 3,748,888 and 3,933,022, each of which is incorporated herein by reference in its entirety, wherein a thermally responsive snap-action bimetallic disc is provided. 
   U.S. Pat. No. 3,748,888 also describes a smoothly formed prior art disk-shaped snap-action bimetallic member, as illustrated in side view in  FIG. 1 . A bimetallic member  1  is formed using a disc of material formed of two materials  2 ,  3  having different thermal expansion coefficients and joined together along contiguous surfaces. One of the members  2  is formed of a material having a relatively high coefficient or rate of thermal expansion, while the other member  3  is formed of a material having a low rate of thermal expansion relative to that of the first member  2 . The difference in thermal expansion coefficients between the two conjoined members  2 ,  3  is a factor in determining the set-point temperature at which the resulting bimetallic disc actuator  1  operates and in the force F produced by the snap-action. The disk-shaped bimetallic member  1  is often circular and, in some instances, is provided with a small, centrally located aperture therethrough (not shown). Bimetallic discs of this type are generally formed by “bumping” a flat circular disc blank with a punch-and-die set to stretch the bimetallic material of the disc into a concave structure having a depth H 1 , as illustrated by full line  4  in  FIG. 1 . The bimetallic disc  1  is formed, for example, with a substantially planar peripheral hoop portion  5  surrounding a central portion  6  that is stretched into a concave configuration. The set-point operation temperature of the snap-action and the force F applied thereby are thus physical characteristics of the two members  2 ,  3  that form the bimetallic member  1 . 
   Generally, when the bimetallic disc  1  is intended to operate at a temperature above ambient temperature, the disc  1  is bumped on the high expansion side  2  to form the central stretched portion  6 , whereby the central portion  6  is stretched to space the inner concave surface thereof to the depth H 1  away from the plane P of the peripheral hoop portion  5 , as illustrated by the full line configuration  4 . The depth of penetration of the punch during the bumping operation determines the depth H 1  and thus is another factor in determining both the upper set-point temperature and the force F applied by the snap-action operation of the disc  1 . The set-point operation temperature and the force F applied by the snap-action arc thus also structural characteristics of the bimetallic member  1 , as is also described in above-incorporated U.S. Pat. No. 3,748,888. 
   In  FIG. 1 , the full line  4  illustrates the bimetallic disc  1  in one of its two states of stability. Assuming the bimetallic disc  1  is intended for operation at a set-point temperature above ambient temperature, the high expansion rate side is located on the surface  2  and the low expansion rate side is along the surface  3 . If the bimetallic disc  1  is intended for operation at a set-point temperature below ambient temperature, the bimetallic disc  1  is formed in the opposite shape with the low expansion rate side located on the surface  2  and the high expansion rate side along the surface  3 . For purposes of explanation only, the bimetallic disc  1  shown in  FIG. 1  is assumed to be intended for operation at a set-point temperature above ambient temperature. Accordingly, at a temperature well below the upper set-point temperature the bimetallic disc  1  is configured with the central stretched portion  6  in an upwardly concave state, as shown by the upper dotted line  7 . 
   As the temperature of the bimetallic disc  1  is raised to approach its upper set-point operating temperature, the high expansion rate material  2  begins to stretch, while the lower expansion rate material  3  remains relatively stable. As the high expansion rate material  2  expands or grows, it is restrained by the relatively more slowly changing lower expansion rate material  3 . Both the higher and lower expansion rate sides  2 ,  3  become distorted by the thermally induced stresses, and the bimetallic disc  1  changes configuration with a slow movement or “creep” action from the upper dotted line configuration  7  to the fill line configuration  4  with the inner concave surface of the central concave portion  6  spaced the depth H 1  away from the plane P of the peripheral hoop portion  5 . The full line configuration  4  is considered herein to be a first state of stability. 
   As soon as the temperature of the bimetallic disc  1  reaches its upper predetermined set-point temperature of operation, the central stretched portion  6  of the disc  1  moves with a forceful snap-action downward through the unstretched hoop portion  5  to the second state of stability with the inner concave surface of the central concave portion  6  spaced a distance H 2  away from the plane P of the peripheral hoop portion  5 , as shown by the phantom line  8 . If the temperature of the bimetallic disc  1  is raised to a still higher temperature, the high expansion rate material  2  continues to expand at a greater rate than the relatively lower expansion rate material  3  joined thereto. As a result of this continued differential expansion, the bimetallic disc  1  creeps toward a state of even greater downward concavity, as shown by the second lower dotted line configuration  9 . 
   As the temperature of the bimetallic disc member  1  is reduced form the high temperature toward the lower predetermined set-point temperature of operation, the bimetallic disc  1  moves from the state of extreme concavity, as shown by the lower dotted line  9 , toward the second state of stability indicated in phantom  8 . As the temperature of the bimetallic disc  1  is reduced below the second or lower predetermined set-point temperature of operation, the material  2  having the relatively larger thermal coefficient also contracts or shrinks more rapidly than the other material  3  having the relatively smaller thermal coefficient. The bimetallic disc  1  changes configuration with a similar slow movement or creep action from the state of greatest downward concavity toward the second state of stability indicated in phantom  8 . As the bimetallic disc  1  reaches the lower set-point temperature, the central stretched portion  6  forcefully snaps back through the unstretched hoop portion to the first state of stability, as shown by the upper full line  4 . If the temperature is decreased still further, the differential expansion between the high and low rate materials  2 ,  3  causes the bimetallic disc  1  to continue to creep toward the state of greatest upward concavity, as shown by the upper dotted line  7 . 
   The manufacture of snap-action bimetallic discs  1  results in set-point temperatures that vary with only slight differences in the fabricated thicknesses of each of the materials  2 ,  3 . Material fabrication parameters, such as inconsistencies in the alloy content, and rolling temperatures and pressures also affect set-point temperatures, as do internal material stresses induced both during original forming and during joining together of the individual materials  2 ,  3 . Inconsistencies in the depth of penetration of the punch during the bumping operation that determines the depth H 1  introduce more variation in the set-point temperatures, as do time and temperature variations during heat treatment and thermal cycling operations. Other factors also cause variations in the set-point operation temperatures of the finished discs  1 . 
   Thus, tolerance in the set-point operation temperature in many switching mechanisms often exceeds the ability of the fabrication process to reliably reproduce a disc  1  that satisfies the tolerance required by specific applications. The process variations often result in yields below acceptable limits and cause the disc manufacturer to individually screen the manufactured discs at a cost of significant time and effort. Uncertainty in the final yield also upsets the production planning process. 
   Furthermore, many thermal switch designs use one of the bimetallic discs  1  that snap into a different state of concavity at a predetermined threshold or set-point temperature, thereby closing a contact or other indicator to signal that the set-point has been reached. A minimum force F is required to actuate the switch or indicator. As described above, the force F is thermally induced in the bimetallic disc  1  as the result of both the depth H 1  of the concavity formed in the disc  1 , and the differential thermal expansion between the high and low expansion sides  2 ,  3  thereof. The force F produced during transition from one state of stability to the other state must be sufficient to overcome the restoring force in the switch or indicator device in order to actuate the device. If a bimetallic disc  1  with insufficient snap force F is installed into a thermal switch or other indicator device, the switch or device may fail prematurely, requiring replacement of the bimetal disc. 
   Currently, the force F produced during the snap is tested in situ by placing the disc  1  in the intended device and testing the fully assembled thermal switch or other indicator mechanism. This measurement technique is preceded by pre-screening of the individual bimetallic elements  1  capable of generating a sufficiently powerful snap force F to overcome the restoring forces of the device. For example, the bimetallic discs  1  are pre-tested to ensure that each exerts sufficient snap force F at temperature application rates of about 1 degree per minute or less to overcome a restoring spring force in a flexible switch contact. The testing process is thus cumbersome and time consuming. Furthermore, the present testing process is a simple go/no-go test in which marginally-performing bimetallic discs  1  may remain undiscovered. The manufacturer may thus be forced to employ excessively conservative quality control measures. 
   Therefore, the manufacture of snap-action bimetallic discs is currently less than optimal, and improved methods of manufacture having more consistent product, and thus higher yields, are desirable. 
   SUMMARY OF THE INVENTION 
   The present invention is a means of delicately adjusting the physical properties of a thermally responsive bimetallic actuator by exposing a pre-formed bimetallic actuator to laser energy, thereby permanently altering the thermal response properties of the bimetallic actuator. The present invention thus provides post-fabrication modification of the snap actuation temperature set-points, thereby increasing predictability of temperature set-point and producibility of the bimetallic actuator. 
   The present invention includes the bimetallic actuator having delicately adjusted physical properties that result in permanently altered thermal response properties. 
   According to one aspect of the invention, a thermally responsive bimetallic member is provided that exhibits a snap-action response, the bimetallic member including a bimetallic material fabricated of two materials having different coefficients of thermal expansion and formed in a predetermined non-planar shape to achieve a snap-action between first and second stable states as a function of temperature, and an artifact formed in a first of the two materials and cooperating with the non-planar shape to achieve the snap-action. 
   According to another aspect of the invention, the artifact is a pattern of localized surface heat-treated areas or grooves that cooperates with the non-planar shape to achieve the snap-action of the bimetallic member within a predetermined range of temperatures. 
   According to another aspect of the invention, the snap-action of the bimetallic member is achieved within a predetermined range of temperatures that is a function at least one of: a value of the coefficient of thermal expansion of the first of the two materials relative to the coefficient of thermal expansion of a second of the two materials, and a physical parameter of the artifact. For example, the physical parameter of the artifact includes one or more of a shape and a position of the artifact. 
   According to various other aspects of the invention, the snap-action of the bimetallic member achieved by the cooperating non-planar shape and artifact exerts a predetermined force, i.e., the bimetallic member exerts a predetermined amount of energy during the snap-action transition between the first and second stable states. For example, the force exerted by the snap-action is a function of at least a shape and a position of the artifact. 
   According to yet another aspect of the invention, the predetermined non-planar shape of the bimetallic material is a dish-shape formed centrally of a substantially planar peripheral edge portion. 
   According to still other aspects of the invention, the bimetallic member is coupled with a pair of relatively movable contacts that are positioned relative to the thermally responsive bimetallic member such that the thermally responsive bimetallic member is positioned to actuate one of the pair of relatively movable contacts. For example, the thermally responsive bimetallic member is positioned to actuate the relatively movable contact by transitioning between one and another of the first and second stable states. 
   According to yet other aspects of the invention, a method for forming a thermally responsive bimetallic actuator is provided, the method including forming a blank of bimetallic material into a predetermined non-planar shape having a substantially round and planar peripheral edge portion to achieve a snap-action transition between first and second stable states at an initial set-point temperature; and laser treating one surface of the bimetallic material to form a predetermined pattern therein. The method may also include determining the initial set-point temperature prior to the laser treating, and the laser treating results in the snap-action transition being achieved at a set-point temperature that is different from the initial set-point temperature. 
   According to another aspect of the method of the invention, the laser treating the surface includes treating the surface in a prescribed manner as a function of a predetermined influence of one or more predetermined parameters on the set-point temperature. The prescribed manner of treating the surface may include reference to a representation of influences of predetermined parameters on the set-point temperature. Furthermore, the representation of influences of the parameters may be a graphical representation. For example, the representation may be a nomogram. 
   According to another aspect of the method of the invention, the method may include determining prior to the laser treating an initial energy exerted by the bimetallic actuator during the snap-action transition, and the laser treating preferably results in the energy exerted by the bimetallic actuator during the snap-action transition being substantially optimized. For example, the energy exerted by the bimetallic actuator during the snap-action transition from the first stable state to the second stable state is made substantially the same as the energy exerted during the snap-action transition from the second stable state to the first stable state. 
   According to still other aspects of the method of the invention, the pattern formed in the bimetallic material by the laser treating influences the set-point temperature at which the snap-action transition is achieved. The snap-action transition is thus a function of temperature, and the pattern formed by the laser treating. 
   According to various other aspects of the method of the invention, the thermally responsive bimetallic actuator is formed as a disk and the pattern formed by the laser treating is an annular area of localized surface heat treatment applied, for example, by a low power laser, and being positioned adjacent to the peripheral edge of the disk. Extensive laser treating may remove material thereby inscribing or cutting an annular groove adjacent to the peripheral edge of the disk. Alternatively, the pattern is an annular surface laser-treated area, including a groove, being spaced inwardly of the peripheral edge of the disk. The pattern may also be an annular surface laser-treated area, including an annular groove, being positioned near to the center of the disk of bimetallic material. 
   According to still another aspect of the method of the invention, the pattern formed by the laser treating influences the energy generated by the bimetallic actuator during the snap-action transition. According to various aspects of the invention, the pattern formed by the laser treating is a plurality of surface heat-treated areas or grooves formed radially to the center of the disk of bimetallic material. Alternatively, the pattern is a plurality of surface heat-treated areas or grooves positioned at an angle to a grain in the surface of the bimetallic material. For example, the heat-treated pattern is positioned substantially crosswise to the grain in the surface of the bimetallic material. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  illustrates a known bimetallic actuator disc; 
       FIG. 2  is a top plan view of the thermally responsive device of the present invention embodied as a snap-action thermal switch including a thermally responsive snap-action actuator of the present invention embodied as a disc-shaped actuator; 
       FIG. 3  is a cross-sectional view of the thermally responsive device illustrated in  FIG. 2 , wherein the thermally responsive snap-action actuator is shown spaced away from an intermediary striker pin, whereby the actuator force F is removed from an armature containing a moveable electrical contact; 
       FIG. 4  is another cross-sectional view of the thermally responsive device illustrated in  FIG. 2 , wherein the thermally responsive snap-action actuator is shown exerting a force on an intermediary striker pin, whereby the actuator force F is transmitted to the armature containing the moveable electrical contact; 
       FIG. 5  illustrates the thermally responsive bimetallic member of the invention embodied as the bimetallic disc having a set-point temperature adjusted using a laser surface treatment performed in a prescribed manner according to a method of the invention; 
       FIG. 6  illustrates in flat pattern the thermally responsive bimetallic member of the invention shown in  FIG. 5 ; 
       FIG. 7  illustrates the artifact pattern of the invention applied to the bimetallic disc of the invention as a smaller diameter annular artifact pattern positioned part way between the peripheral edge and the center of the bimetallic disc; 
       FIG. 8  illustrates the artifact pattern of the invention applied to the bimetallic disc of the invention as a still smaller diameter annular groove positioned at the center of the bimetallic disc; 
       FIG. 9  illustrates the artifact pattern of the invention applied to the bimetallic disc as a quantity of radial heat-treated areas or grooves having a predetermined depth and width; and 
       FIG. 10  illustrates the thermally responsive bimetallic member of the invention embodied as a laser adjusted bimetallic disc having a the laser surface treatment applied to inscribe the artifact pattern embodied as a quantity of cross-grain artifacts, wherein the artifact pattern is inscribed at an angle to the rolled grain of one of the first and second materials of the bimetallic member, as indicated by the arrow. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   In the Figures, like numerals indicate like elements. 
   The present invention is a compensation method that provides for delicately adjusting one or both of the set-point temperature and the snap force F by using a laser to physically alter the bimetallic snap-action actuator element. The present invention includes the bimetallic snap-action actuator element resulting from the compensation method as well as the thermostatic switching mechanisms and other indicators that utilize the bimetallic snap-action actuator element of the invention to signal that the set-point has been obtained. 
     FIG. 2  is a top plan view and  FIGS. 3 and 4  are cross-sectional views of the thermally responsive device of the present invention embodied as a snap-action thermal switch  10  including a thermally responsive snap-action actuator of the present invention embodied as a snap-action bimetallic disc actuator  12 . The thermal switch  10  also includes a pair of electrical contacts  14 ,  16  that are relatively movable under the control of the disc actuator  12 . The electrical contacts  14 ,  16  are mounted on the ends of a pair of spaced-apart, electrically conductive terminal posts  20 ,  22  mounted in a header  24  such that they are electrically isolated from one another. For example, terminal posts  20 ,  22  are mounted in the metallic header  24  using a glass or epoxy electrical isolator  26 . 
   As illustrated in  FIGS. 3 and 4 , the electrical contacts  14 ,  16  are moveable relative to one another between an open state ( FIG. 4 ) and a closed state ( FIG. 3 ). For example, the movable contact  16  is affixed to an electrically conductive carrier  28  that is embodied as an armature formed of an electrically conductive spring material. The armature  28  is affixed in turn in a cantilever fashion to the electrically conductive terminal post  22 , such that the spring pressure S of the armature  28  operates to bias the movable contact  16  toward the fixed contact  14  to make electrical contact therewith, as shown in  FIG. 3 . The electrical contacts  14 ,  16  thus provide an electrically conductive path between the terminal posts  20 ,  22  such that the terminal posts  20 ,  22  are shorted together. 
   The disc actuator  12  is spaced away from the header  24  by a spacer ring  30  interfitted with a peripheral groove  32 . A cylindrical case  34  fits over the spacer ring  30 , thereby enclosing the terminal posts  20 ,  22 , the electrical contacts  14 ,  16 , and the disc actuator  12 . The case  34  includes a base  36  with a pair of annular steps or lands  38  and  40  around the interior thereof and spaced above the base  36 . The lower edge of the spacer ring  30  abuts the upper case land  40 . A peripheral edge portion  42  of the disc actuator  12  is captured within an annular groove created between the lower end of the spacer ring  30  and the lower case land  38 . The disc actuator  12  operates the armature spring  28  to separate the contacts  14 ,  16  through the distal end  44  of intermediary of a striker pin  46  fixed to the armature spring  28 . Separation of the contacts  14 ,  16  creates an open circuit condition. 
   As shown in  FIG. 3 , while the thermal switch  10  is maintained below a predetermined set-point temperature, the disc actuator  12  is maintained in a first state with the bimetallic disc actuator  12  withdrawn into a space between the lower case land  38  and the case end  36 . In this first state, an inner concave surface  48  of the bimetallic disc actuator  12  is spaced away from the intermediary striker pin  46 , whereby the actuator force F is removed from the armature  28 . The relatively moveable electrical contacts  14 ,  16  are moved together under the spring pressure S supplied by the armature  28  and thereby form a closed circuit. The spacing between the inner concave surface  48  of the bimetallic disc actuator  12  and the distal end  44  of the striker pin  46  is sufficient to prevent slight movement of the actuator disc  12  effecting contact engagement. 
   The armature  28  operates under the control of the bimetallic disc actuator  12 , which inverts with a snap-action as a function of a predetermined set-point temperature between two stable states of opposite concavity. As shown in  FIG. 4 , in response to an increase in the sensed ambient temperature above the predetermined set-point, the disc actuator  12  inverts in a forceful snap-action into a loaded relationship with the electrical contacts  14 ,  16 , whereby the inner concave surface  48  is inverted into an outer convex surface  48  that rapidly engages the distal end  44  of the intermediary striker pin  46 . The snap-action of the bimetallic disc actuator  12  operates with sufficient force F to overcome the spring pressure S of the armature  28  and flex the movable contact  16  away from the fixed contact  14 . The disc actuator  12  pivots the armature spring  28  upwardly to separate the contacts  14 ,  16  through the intermediary striker pin  46  fixed to the armature spring  28 . Separation of the contacts  14 ,  16  creates an open circuit condition. 
   The speed at which the bimetallic disc actuator  12  changes state is commonly known as the “creep rate.” As the term implies, the change from one stable state to the other is not normally instantaneous, but is measurable. A high creep rate means that the state change occurs at a low rate of speed, while a low creep rate means that the state change occurs at a high rate of speed. High creep rate results in arcing between the contacts  14 ,  16 . High creep rate thus limits the current carrying capacity of the thermal switch  10 . In contrast, a low creep rate means that the change in state occurs rapidly, which increases the amount of current the thermal switch  10  can carry without arcing. 
   According to one embodiment of the invention, the bimetallic disc actuator  12  is fabricated with a low creep rate. Accordingly, the snap-action of the bimetallic disc actuator  12  changes state within about 1 millisecond while exerting sufficient force F to overcome the spring pressure S of the armature  28 . The movable contact  16  is thus flexed away from the fixed contact  14  rapidly, so that the current carrying capacity of the thermal switch  10  is maximized. 
   When the ambient temperature sensed by the bimetallic disc actuator  12  is reduced below the predetermined set-point, the disc actuator  12  is rapidly returned to the spaced-away, noninterference relationship with the electrical contacts  14 ,  16 , as shown in  FIG. 3 . The relatively moveable electrical contacts  14 ,  16  are rapidly moved together again under the spring pressure S of the armature  28  and thereby form a closed circuit between the two terminal posts  20 ,  22 . Accordingly, one embodiment of the invention provides a snap-action that changes state of the bimetallic disc actuator  12  within about 1 millisecond. The spring pressure S of the armature  28  causes the movable contact to follow the retreating disc actuator  12 . The movable contact  16  is thus flexed into contact with the fixed contact  14  rapidly, so that the current carrying capacity of the thermal switch  10  is maximized. 
   The thermal switch  10  is sealed to provide protection from physical damage. The thermal switch  10  is optionally hermetically scaled with a dry Nitrogen gas atmosphere having trace Helium gas to provide leak detection, thereby providing the contacts  14 ,  16  with a clean, safe operating environment. 
     FIG. 5  illustrates the thermally responsive bimetallic member of the invention embodied as the bimetallic disc  12  having a set-point temperature adjusted using the laser surface treatment performed in a prescribed manner according to the method of the invention. The bimetallic disc actuator  12  according to the invention is initially fabricated according to generally known methods, as described in connection with  FIG. 1 . For example, a thermally responsive bimetallic material  50 , such as ASTM-1, is selected according to known criteria for forming a bimetallic actuator. Such thermally responsive bimetallic material includes a first metallic material  52  having a first coefficient of thermal expansion and a second metallic material  54  having a second relatively higher coefficient of thermal expansion. The first and second metallic materials  52 ,  54  of the thermally responsive bimetallic material  50  are bonded together along one contiguous surface  56 . 
   The bimetallic material  50  is formed into a blank of desired shape and size. For example, a flat, round disk-shaped blank is formed having a diameter D sized to move freely within the annular groove created in the thermal switch assembly  10  between the lower end of the spacer ring  30  and the lower case land  38 . 
   The disk-shaped blank is subjected to a forming or “bumping” operation in which the blank of thermally responsive bimetallic material is positioned between two opposingly positioned shaping members (not shown). The shaping members are actuated to engage the disk-shaped blank of bimetallic material  50 , thereby forming bimetallic disc having a configuration that achieves forceful snap-action at each of the two predetermined set-point temperatures. For example, the disk-shaped blank is placed in a female die which supports the blank along its peripheral edge portion  42 . A male punch having a spherical end is pressed against the center of the disc to stretch the metal and form the inner dish-shaped concave surface  48 . The peripheral edge portion  42  either retains its substantially planar initial shape, or is formed by the shaping members with a substantially planar shape. Examples of such dish-shaped discs are illustrated in U.S. Pat. Nos. 2,717,936 and 2,954,447, each of which is incorporated herein in its entirety by reference. The formed bimetallic disc may be subsequently subjected to a conventional oven heat treatment operation in order to achieve forceful snap-action at each of the two predetermined set-point temperatures. 
   The dish-shaped bimetallic discs are subjected to thermal testing, which determines the actuation or set-point temperature of each individual disc  12 , and the discs  12  are categorized according to a predetermined methodology. For example, the tested discs  12  are separated by material type into categories defined by low set-point temperature ranges of about 1 to 2 degrees Fahrenheit with predetermined differential temperatures. 
   According to the invention, the categorized bimetallic discs  12  are subjected to a laser surface treatment performed in a prescribed manner, whereby the laser treated bimetallic disc  12  of the invention is formed. The laser surface treatment accurately adjusts the set-point temperature of the bimetallic disc  12  upwardly or downwardly in a predictable manner. Variations in the manufacturing parameters of the disc  12  are used to predictably cause different upward and downward changes in the high and low set point temperatures. The manufacturing parameters so varied include, for example, laser intensity, i.e., power and dwell time; location of the localized heat-treated pattern; combinations of different localized surface treatments applied to the high and low expansion sides of the disc  12 ; forming the bimetallic disc  12  using different types of first and second metallic materials  52 ,  54 ; and other parameters. 
   According to the method of the invention, each bimetallic disc  12  is pre-tested to determine its initial set-point temperature and differential temperature. For example, the bimetallic disc  12  is pre-tested to determine both its initial low set-point temperature and its differential temperature. 
   One of the first and second materials  52 ,  54  is inscribed or cut in a predetermined pattern  56  of artifacts, which is a function of the particular bimetallic material  50  and the amount of change required of the particular bimetallic disc  12  to move the set-point to the temperature desired for a particular application. For example, the pattern  56  is inscribed in one of the first and second materials  52 ,  54  using a laser to generate controlled, isolated heat in a predetermined position. The laser may be any laser operated in a controlled manner to produce the predetermined pattern  56  in the desired position with the desired depth and width to change the set-point to the desired temperature. For example, the laser may be a low-power YAG laser embodied as a conventional laser part marker or scribe. 
   The parameters that affect the set-point temperature of the bimetallic disc  12  are categorized as the type of bimetal material  50 , the physical parameters of the predetermined pattern  56  of one or more artifacts, and the laser power used to inscribe the pattern  56 . The type of bimetal material  50  includes the type of the first and second materials  52 ,  54 . The physical parameters of the predetermined artifact pattern  56  include the shape of the pattern  56 , i.e., its depth, width, and length; the positioning of the pattern  56  on the bimetallic disc  12 ; and which of the first and second materials  52 ,  54  is inscribed with the pattern  56 . The laser power used to inscribe the pattern  56  includes the power and speed of the laser during inscription. All of these parameters that influence the degree to which the laser inscription affect the set-point temperature of the bimetallic disc  12 . The manner in which the bimetallic disc  12  is subjected to a laser surface treatment is thus a function of these parameters. According to one embodiment of the invention, a nomogram is formulated that quantifies the amount of influence of each of the parameters has on the set-point temperature, including combinations of the parameters. The nomogram is consulted to determine the manner in which the bimetallic disc  12  is subjected to laser surface treatment to change the set-point to the desired temperature. Other representations of the amount of influence of the parameters on the set-point temperature, such as tables, are considered equivalent to the nomogram and are similarly contemplated by the invention. 
   The nomogram, or other representation of the influence of the parameters on the set-point temperature, is developed using empirical data based upon pre-treatment and post-treatment testing of set-point temperature. For example, a design of experiments (DOE) is developed that efficiently quantifies the amount of influence of the parameters, both individually and in combinations. A statistically significant quantity of the bimetallic discs  12  are fabricated of a predetermined bimetallic material  50 , less the laser surface treatment of the invention. The set-point temperatures of the bimetallic discs  12  are pre-tested using conventional methods, and the pre-tested bimetallic discs  12  are categorized accordingly. Optionally, the differential temperatures of the bimetallic discs  12  are pre-tested with the set-point temperatures and the categorizing of the bimetallic discs  12  accounts for variations in differential temperatures. 
   The pre-tested bimetallic discs  12  are subjected to the laser surface treatment of the invention according to the DOE. The laser surface treated bimetallic discs  12  are post-tested for set-point temperature, and optionally, for differential temperature. The empirical data developed is used to generate the nomogram, or other representation of the influence of the parameters on the set-point temperature. 
   The nomogram is used to adjust the set-point temperature of bimetallic discs  12  into specific ranges of set-point temperature determined to satisfy a particular application. For example, the set-point temperature of bimetallic discs  12  are adjusted using the laser surface treatment of the invention to adjust the set-point temperature of one or more bimetallic discs  12  by 1 to 10 degrees F. into compliance with a predetermined set-point temperature range required by a particular application. 
   According to one embodiment of the invention, the DOE is performed according to the type of bimetallic material  50 , and includes using different laser power settings for applying different shapes of the pattern  56  to both of the first and second materials  52 ,  54 . For example, the artifact pattern  56  is applied to the first material  52  as an annular area of localized surface laser heat-treated material positioned at a short distance from the peripheral edge  48  of the bimetallic disc  12 , as illustrated in  FIG. 6 , where the bimetallic disc  12  is shown in flat pattern. Alternatively, the localized laser treatment is applied with sufficient energy that material is removed and the artifact  56  is embodied as a an annular groove having a predetermined width and depth and positioned at a short distance from the peripheral edge  48  of the bimetallic disc  12 , as illustrated in  FIG. 5 . The grooved artifact  56  is applied in an annular pattern as illustrated in  FIG. 6 . 
     FIG. 7  illustrates the artifact pattern  56  is applied to the bimetallic disc  12  as a smaller diameter annular artifact pattern  56  positioned part way between the peripheral edge  48  and the center of the bimetallic disc  12 . 
     FIG. 8  illustrates the artifact pattern  56  is applied to the bimetallic disc  12  as a still smaller diameter annular artifact pattern  56  positioned at the center of the bimetallic disc  12 . The annular artifact pattern  56  is optionally placed at other positions on the bimetallic disc  12  during the DOE to generate empirical data for the nomogram. Other shapes and locations for the artifact pattern  56  are also optional in generating the empirical data. 
   According to the invention, the bimetallic disc  12  is subjected to laser surface treatment according to the manner prescribed by the nomogram, or other representation of the influence of the parameters on the set-point temperature. The set-point temperature of the bimetallic disc  12  is thereby adjusted upwardly or downwardly by 1 to about 10 degrees F. to comply with a predetermined set-point temperature range required by a particular application. 
   According to other embodiments of the invention illustrated in  FIGS. 9 and 10 , the laser surface treatment is utilized to adjust the force F with which the bimetallic disc  12  changes state upon sensing its set-point temperature.  FIG. 9  illustrates the artifact pattern  56  is applied to the bimetallic disc  12  as a quantity of radial artifacts  56 . The laser energy may be applied in a manner that removes material, whereby the artifact pattern  56  is embodied as radial grooves having a predetermined depth and width. 
     FIG. 10  illustrates the thermally responsive bimetallic member of the invention embodied as a laser adjusted bimetallic disc  12  having a the laser surface treatment applied to inscribe the artifact pattern  56  embodied as a quantity of cross-grain artifacts, wherein the artifact pattern  56  is inscribed at an angle to the rolled grain of one of the first and second materials  52 ,  54 , as indicated by the arrow  58 . According to one embodiment of the invention, the artifact pattern  56  is inscribed substantially perpendicular to the rolled grain of the material  52 ,  54 . The artifact pattern  56  is applied as radial ( FIG. 9 ) or cross-grain ( FIG. 10 ) artifacts in one surface of the bimetallic disc  12  to optimize the energy or force F with which the bimetallic disc  12  changes state. The artifact pattern  56  is applied to alter the force F exerted by the stronger change of state by adjusting the tension in the material  52 ,  54 . 
   The above method of determining the proper combination of parameters to be applied to the artifact pattern  56  is performed using the transition force F as the target characteristic, instead of the set-point temperature. 
   A representation of the influence of the parameters on the transition force F, such as a nomogram or table, is developed using empirical data based upon pre-treatment and post-treatment testing of transition force F. For example, a design of experiments (DOE) is developed that efficiently quantifies the amount of influence of the parameters, both individually and in combinations. The DOE is used to generate empirical data for the nomogram. 
   According to the invention, the bimetallic disc  12  is subjected to laser surface treatment according to the manner prescribed by the nomogram, or other representation of the influence of the parameters on the set-point temperature. The transition force F in the snap-action of the stronger side  52 ,  54  of the bimetallic disc  12  is thereby adjusted downwardly to optimize the transition force F of the snap-action during transition from the first state to the second state and from the second state back to the first state. Generally, the transition force F is optimized to maximize the current carrying capability of the thermal switch  10 , as illustrated in  FIGS. 2 through 4 , when the bimetallic disc  12  is used to open and close the contacts  14 ,  16  of the thermal switch  10 . For example, the transition force F is substantially equalized between the transition from the first state to the second state and the transition from the second state back to the first state. 
   Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.