US3157081A

US3157081A - Method and apparatus for forming resistance elements

Info

Publication number: US3157081A
Authority: US
Publication date: 1964-11-17
Anticipated expiration: 1981-11-17

Description

3,157,081 METHOD AND. APPARATUS FOR FORMING RESISTANCE ELEMENTS Filed Feb. 23, 1961 V. J. ABRAMS Nov. 17, 1964 5 Sheets-Sheet 1 FIG.3

FIG. 2

INVENTOR.

VIRGIL J. ABRAMS ATTORNEY Nov. 17, 1964 v. J. ABRAMS 3,157,081

METHOD AND APPARATUS FOR FORMING RESISTANCE ELEMENTS Filed Feb. 25, 1961 3 Sheets-Sheet 2 IN VEN TOR.

VIRGIL JABRAMS ATTORNEY V. J. ABRAMS Nov. 17, 1964 METHOD AND APPARATUS FOR FORMING RESISTANCE ELEMENTS 3 Sheets-Sheet 15 Filed Feb'. 23, 1961 FIG. 8

FIG. 6

FIG.7

INVENTFR. BY VIRGIL J. ABRAMS ATTORNEY United States Patent 3,157,031 AND APPARATUS FER FQRMENG REEETANCE ELEMENTS Virgil .i. Abrams, Anaheim, Calif., assignor to Beclnnan instruments, Kno a corporation of California Filed Feb. 23, 196i, Ser. No. 91,080 12 filaims. (Cl. 83- 13) The present invention relates generally to resistance elements and, more particularly, to an improved method and apparatus for forming variable resistance elements.

Many potentiometers, variable resistors, rheostats and the like are presently constructed with a cavity in which the resistance element is retained in a substantially closed path, usually shaped in the form of an annulus. Such components are commonly denoted as single-turn potentiometers, variable resistors, etc. Many of such singleturn components are constructed in a Way that the movable wiper may be continuously rotated in either or both directions of rotation. In this type of component, it is conventional practice to provide a void or dead space in the resistance element of sutficient length so that the potentiometer wiper will not cause an electrical short circuit between the conductive ends of the resistive element when traversing that area. Commonly, the dead space is provided by removing the required number of turns of resistance wire at each end of the annular resistance element.

To reduce the varying torque which results when the wiper passes through this dead space, a bridge is normally formed over the dead space. One common form of this bridge comprises filling up the void with a suitable dielectric material. In this type of construction, the length of the gap left between the ends of the annular shaped resistance element is not critical since it is filled by the bridge forming material.

A preferred bridge and method for constructing same are disclosed and claimed by Wilfred McCann in the co pending application Serial No. 91,079 entitled Improved Variable Resistance Bridge and Method for Making Such Bridge, med on even date herewith and assigned to Beckman Instruments, Inc, assignee of the present invention. In this bridge, the resistance element itself, after suitable modification, serves as the bridge over the dead space. Additional dielectric material is not required in this type of construction; therefore, the length of the gap between the ends of the annular resistance element should be as small as possible to obviate any change in torque when the wiper passes over the dead space, and to elimimate the damage to the ends of the resistance element Which may result if the wiper or a portion thereof is permitted to fall into the gap. Obtaining a gap length which is very small in relation to the length of the Wiper contact presents a serious problem in the construction of potentiometers since the present manufacturing tolerance for precision potentiometer housings is such that each resistance element must be individually cut to fit a specific housing. Housings of molded plastic or die cast metal constructed according to present manufacturing techniques have .a tolerance usually substantially larger than the allowable gap in the resistance element. A representative tolerance value is i0.015 inch for the internal circumference of the housing while the allowable maximum gap may be only 0.003 inch. Thus, if all of the resistance elements are pro-formed to have the identical circumference, the ends of the resistance elements will overlap and deform the annular element when fitted into some of the housings, the ends will fit properly in others, and the ends will leave too large a gap when fitted into still others of the housings.

Heretofore, hand-tailoring each annular resistance element to a particular housing has been diiiicult and time consuming because of the extremely small gap permitted in most precision components. As an illustration of the gap dimension, the saws used to cut the resistance element are usually thicker than the desired gap between the ends of the resistance element. Very precise cutting of the resistance element has in the past usually required the Worker to observe his work through a microscope while using precision techniques and skills. Moreover, the cutting of the resistance elements to such fine tolerances often has required that the element be cut and/ or filed several times before a proper fit is achieved; oftentimes this procedure results in an annular element having too small a circumference which must then be discarded and a new resistance element constructed.

One precedure proposed in the past for accomplishing a suitable fit between the resistance element and potentiometer housing is that of cutting a number of annular elements to different circumferential lengths, measuring the circumference of each individual housing, and selecting an annular element cut to the appropriate length. Although this procedure might be suitable under certain conditions, it would not be feasible for the manufacture of most precision potentiometers because of the large variation in types of resistance elements used. For example, many potentiometers, variable resistors, rheostats, and the like employ resistance wire wound about a support or mandrel as it is known in the art. The physical and electrical characteristics of resistance elements so constructed may vary 'by using resistance wire of difierent resistivity, size, and temperature coeflicient; using mandrels having different cross-section dimensions; and varying the pitch at which the resistance wire is wound upon the mandrel. This substantial number of variables would require an inordinate number of annular resistance elements to be prepared and accessible for the operation since a plurality of different sized elements would have to be prepared for each particular type of resistance element. For example, using the representative housing tolerances heretofore noted, some ten different sized anuular elements would have to be prepared for each type of resistance element in order to maintain a minimum gap of 0.003 inch when the total variation in housing size is 0.030 inch. Also, this procedure would be quite wasteful and thereby additionally expensive since invariably a large number of the pre-cut elements would not be utilized.

Not only do potentiometer housings have a dimension tolerance as presently manufactured, but also the resistance elements themselves vary in circumferential length depending upon mandrel size, resistance wire diameter, and thickness of enamel or like coatings. Accordingly, even if housings were available to the tolerance required to maintain the minimum gap width, this variation in resistance elements would still require that the element be fitted to a specific housing. Accordingly, it is the principal object of the present invention to provide an improved method and apparatus for easily and quickly forming with minimum Waste a resistance element for a specific potentiometer housing.

Another object of this invention is to provide a method and apparatus for cutting annular resistance elements to fit specific potentiometer housings with a very fine gap between the ends of the element which does not require either that the operator be skilled in precision techniques or that the operator observe the resistance element through a microscope.

Still another object of this invention is to provide a method and apparatus for cutting annular resistance elements to fit specific potentiometer housings with a very fine gap between the ends of the element in which a single cut will produce a correctly fitted element.

It is a further object of this invention to provide a method and apparatus for cutting annular resistance elements to precisely fit specific potentiometer housi gs which does not require that the operator measure either the inside circumference of the housing or the outside circumference of the resistance element with a micrometer or like instrument.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

Briefly, in accordance with a preferred form of the present invention, there is provided a gauge for cutting a resistance element to fit the cavity of a particular nousing. This gauge includes a bifurcated plate having caliper jaws respectively formed upon one face of the plate. Upon the other face of the plate are located cavities which mate to substantially define the peripheral surface of the housing cavity. The gauge further includes a means for varying the spacing of said caliper jaws.

Initially, the spacing of the caliper jaws is decreased and the jaws inserted into the housing cavity. The jaws are then moved further apart to abut the housings peripheral surface. A resistance element is then placed within the gauges mating cavities so that ends of the element overla the gap between the caliper jaws. The resistance element is then cut by suitable cutting means extended into said gap.

A substantially exact fit of the resistance element in the housing may be achieved by the apparatus and method of this invention by relating the dimensions of the caliper jaws and cavities of the gauge plate to compensate for the Width of the cutting means as described hereinafte A more thorough understanding of the invention may be obtained by the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a perspective view of a representative housing for a variable resistance element;

FIG. 2 is a perspective view of a representative resistance element which must be precisely cut to properly fit the housing of FIG. 1;

FIG. 3 is a cross-sectional view taken along the lines 33 of FIG. 2;

FIG. 4 is a perspective view of an apparatus for forming resistance elements constructed in accordance with the present invention;

FIG. 5 is a perspective view of a gauging plate constructed in accordance with this invention;

FIG. 6 is a cross-sectional view taken along the line 6--6 of FIG. 5;

FIGS. 7a and 7b are cross-sectional views taken along the line 77 of FIG. 5 while forming a resistance element; and

FIG. 8 is a cut-away view of the gauging plate showing the details of the ejector pin.

Referring now to FIG. 1, there is shown in perspective View a representative housing Ittl for a single-turn potentiometer, variable resistor, rheostat, and the like. As is customary in the art, this housing comprises a cylindrical shell portion 11 and a back plate 12 forming a cylindrical cavity 21. As described hereinbefore, the present techniques utilized in constructing these housings such as molding and casting have a tolerance substantially larger than the tolerable gap between the ends of the resistance element.

A representative annular resistance element 13 to be fitted within the housing of FIG. 1 is shown in the perspective view of FIG. 2 and in the cross-sectional view of FIG. 3. This element may, for example, comprise an annular support, or mandrel 14 as it is known the art, having a plurality of turns of resistance wire 15 wound thereon to a predetermined pitch. As shown, the element is cut with overlapping

terminal portions

16 and 7.

The apparatus of this invention designed to precisely Cir fit the resistance element 13 to the cavity 21 of a particular housing 16 is illustrated in FIGS. 4, 5, 6, 7a, 7b and 8. Referring to FIG. 4-, a base plate 18 is provided with a rectangular groove 19 in which a carriage 20 is free to slide upon a mating dove-tail 3t) affixed to the base plate. A carriage actuating arm 31. is supplied with a handle 32 at one end and is pivoted a; the other end on pivot pin 33 which is rigidly attached to the base plate 13. Ann 31 is adapted to actuate the carriage 29 by carriage drive pin 34 which is rigidly attached to the carriage and pivotably mounted to the arm at a predetermined position between the end pivot pin 33 and handle 32. The forward movement of the carriage is restricted by carriage stop ill.

A gauging assembly comprises a base 35 rigidly attached to the carriage 2t? and having

upstanding standards

36 and 37 at the opposite ends of the gauging assembly base. A hinge flap 38 is rotatably mounted to the standar s 36 and 37 by a hinge pin 39 which passes through both the standards and the hinge flap. As shown in FIG. 5, the hinge flap is provided with a groove 4-0 into which is rigidly mounted a gauging plate 5 3 by suitable fastening screws 51 or like means. Hinge pin 39 and hinge flap 38 permit the gauging plate to be rotated approximately 180.

The gauging plate 59 is a bifurcated member having left and right-

hand segments

52 and 53 separated by a vertical gap 54 which extends a substantial portion of the length of the gauging plate. This gap permits the left and right-hand sections to be drawn closer together or further apart; also, the gap permits a saw blade 78 to enter between the gauging

plate sections

52, 53 to cut the resistance element as hereinafter described. Each of the left and right-

hand plate segments

52, 53 include respective caliper jaws on one face and integral support means comprising mating depressions or cavities on the other face. The caliper jaws may simply comprise juxtaposed tips (not shown) or preferably the semicircular raised shoulders as 57 and 58 shown in FIG. 5. The segment cavities define the peripheral surface of the housing cavity and, as shown in FIG. 4, preferably comprise semicircular

cylindrical cavities

55 and 56. As described hereinafter, the dimensions of the

cavities

55, 56 and jaws 5'7, 58 are related to each other and the particular model of housing to be fitted with a resistance element. Preferably, the hinge pin 39 is removable so that several gauging plates having depressions and shoulders of different dimension may be utilized in conjunction with a unitary apparatus.

The gap 54 between the left and right sides of the gauging plate is varied by means of a lever 59 and a collar 6t) attached to one end of a circular rod 61. Righthand threads 7% at the other end of this rod are retained in mating threads 71 in the right-hand segment 53 of the gauging late (FIG. 6). A compression coil spring '72 is positioned around rod 61 in a cavity 73 formed by mating

cylindrical cavities

7 and 75 in the respective left and right-hand segments of the gauging plate. As shown in FIG. 6, rod 61 and spring 72 are positioned nearest the side of the gauging plate provided with caliper jaws so that the saw blade '78 may freely enter the groove 54 adjacent the

cavities

55 and 56. As the lever 59 is rotated in a clockwise direction (arrow 76 of FIG. 4), the right-hand threads 70 of the rod cause the collar 69 to exert a force on the left-hand segments 52 so as to draw the left and right-hand segments together and diminish the dimension of gap 54. Conversely, a counterclockwise rotation of the lever handle 18 (arrow 77 of FIG. 4) diminishes the force caused by the collar the force exerted by the coil spring 72 then causes the left and right-hand segments to move apart and increase the dimension of the gap 54. The pitch of the threads '79, 71 must be such that the gap 54 is varied over a somewhat larger range than the variation in inside diameter of the cavity 21 of housing 10 due to the tol- .5 erances of the molding, casting or like methods of construction. Ordinarily, the pitch of these threads may be made sufiiciently large that less than a complete rtation of the lever 59 will provide the required gap variation. Accordingly, a stop 62 is preferably attached to the left-hand section 52 of the gauging plate to so limit the rotation of lever handle 59. This stop also prevents the gap from being inadvertently decreased to a smaller width than the width of the saw blade 78.

Saw blade 78 is fixedly mounted to a shaft 79 rotatably mounted in shaft support 80. Saw blade 78 is so mounted that its plane coincides with the vertical plane of the vertical gap 54 of the gauging plate. A motor 99 fixedly attached to the base plate 18 drives the opposite end of shaft 79 by a suitable belt 91 and pulley 92. Preferably, the saw blade includes a hinged saW guard assembly 93 for protection of the operator. An On-Off switch 94 for the saw is conveniently located in carriage actuating handle 32.

The method of forming resistance elements according to this invention is as follows: The gauging plate is initially rotated to the position shown in FIG. so as to expose the caliper jaw shoulders 57, 53. The gauging plate used is determined by the particular model of potentiometer or like housing to be fitted with a resistance element. In each instance, the minimum diameter formed by the jaws 57, 58 labeled dimension B in FIGS. 7:: and 7b is somewhat smaller than the inside diameter of the smallest housing to be fitted and the maximum length of dimension B is somewhat larger than the inside diameter of the largest housing to be fitted. With the lever 59 rotated to its extreme clockwise position against stop 62 to reduce the vertical gap 54 to its minimum width, a housing 10 is placed face down with the caliper jaws inside the housing cavity 21 as shown in FIG. 5. Lever 59 is then rotated in a counterclockwise direction until it meets interference with the stop 62. Referring now to FIG. 7b, this movement of the lever 59 diminishes the force exerted by the collar 60 and causes the vertical gap 54 in the gauging plate 56 to expand due to the biasing force of the coil spring 72. The magnitude of expansion which can occur is limited by the abutting of the caliper jaws against the peripheral surface of the housing cavity. The gauging plate is then rotated approximately 180 to expose the cylindrical cavity 95 formed by the semicircular

cylindrical cavities

55 and 56 in the left and right-hand segments of the gauging plate (FIG. 4). The annular resistance element 13 is then compressed to a diameter slightly less than that of cavity 95, and placed therein in a position which brings the overlapped terminal ends 16 and 17 approximately equidistant from the vertical gap 54. Upon release, the resilience of the element will tend to return it to its free diameter, but its expansion will be restricted to the inside diameter of the cavity. Switch 94 on the carriage actuating handle 32 is then actuated to cause the saw blade 78 to rotate and the arm 31 moved forward to advance the carriage and gauging plate toward the saw blade. As the saw blade 78 passes through the portion of the gap 54 adjacent the cavity 95, the annular resistance element 13 is cut at a point approximately equi distant from the

ends

16 and 17 thereof. After the re sistance element has been sawed it is removed from the coil well by actuating an ejector pin 96 in the direction shown in the. cutaway view of FIG. 8. As shown in FIGS. 4 and 8, this pin is slidably retained in an opening 97 adjacent the side and bottom of depression 56. The gauging plate is then rotated to its original position in which the potentiometer housing is exposed. The housing is easily removed after lever 59 is rotated in a clockwise direction thereby decreasing the width of the vertical gap 54 and releasing the biasing force on the peripheral surface of the housing cavity 21.

A substantially exact fit of the resistance element in the housing may be achieved by the apparatus and method of this invention by relating the diameter of cavity (dimension A of FIGS. 7a and 7b) to the diameter formed by the shoulders 57, 58 (dimension B) in the following manner: The width of the saw blade 78 will normally be greater than the allowable gap between the ends of the cut resistance element. Thus, if dimension A of the cavity 95 were made identical to dimension B of the shoulders (which assumes the dimension of the peripheral surface of the housing cavity to be fitted) each resistance element would have a gap W equal to the width of the saw blade W. The width of this blade may, however, be easily compensated for by making the circumference C of the cavity larger than the circumference of the housing and shoulders by the dimension W or:

it has been found that resistance elements having the required minimum gap are achieved by use of the apparatus and method of this invention for housings having an inside cavity which is slightly out of round. It will be apparent that the shoulders 57 and 58 of the gauging plate will be restrained by the minimum inside diameter of the housing. This procedure results in an appropriately sized resistance element since in practice the rigidity of the annular resistance element prevents it from assuming exactly the curvature of the housing walls. Thus, the resistance element, when placed within the out of round housing, retains its own original annular shape with a diameter equal to the minimum diameter of the housing.

Because of this rigidity of the resistance element, it will be apparent that the face of the gauging plate opposite the caliper jaws is not necessarily required to contain a cavity. For example, a support means comprising a plurality of upstanding projections may under some circumstances be employed so long as the peripheral shape of the housing cavity is defined thereby.

Although the invention has been shown and described by reference to housings having cylindrical cavities since the vast majority of sin gle-turn variable resistance elements are so constructed, it will be apparent that this invention will also be applicable for housings lnavin other shapes such as elliptical or even rectangular cross-sections. In these instances, the gauging plate segments would be provided with appropriate support means and caliper jaw means defining the required cross-sectional geometric figure be it an ellipse, rectangle or the like.

It will further be clear to those skilled in the art that other variations may be made in the structural design of the apparatus herein shown without departing from the principles and teachings of the present invention. By way of example, the gauging plate and saw blade may be so arranged that the saw blade will attack the resistance element in a vertical rather than a horizontal plane. Thus, although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

1 claim:

1. A method or" cutting a resistance element to fit the cavity of a particular housing comprising the steps of inserting a pair of caliper jaws separated by a gap within said cavity, increasing said gap until said jaws abut the peripheral surface of said housing cavity, said caliper jaws having support means integral therewith substantially defining the peripheral surface of said housing cavity, placing a resistance element upon said support means so that ends of said element overlap each other on said gap, and cutting said resistance element by suitable cutting means extended into said gap.

2. A gauge in combination with a cutting means for cutting to length of coiled cylindrical resistance element to fit the cavity of a particular housing comprising a pair of caliper jaws, said jaws having means adapted to extend into and measure the internal peripheral surface of the cavity of said housing, said caliper jaws having integral support means adapted to receive the resistance element, said support means substantially defining the peripheral shape of said housing cavity as determined by the means for measuring the internal surface thereof.

3. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a pair of caliper jaws adapted to extend into the cavity of said housing and including means for vary the spacing between the jaws to abut the internal peripheral surface of said cavity, said jaws having support means integrally formed thereon for the reception of the resistance element, said support means substantially defining the internal peripheral surface of said housing cavity as determined by the position of said jaws.

4. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a pair of caliper jaws having means for varying the spacing between said jaws and having an integral cavity for the reception of the resistance element and a means which cooperate together with said means for varying the spacing between said jaws to measure the internal peripheral surface of the cavity of said housing, said resistance element supporting cavity substantially defining the internal peripheral surface of said housing cavity as determined by the means for measuring the internal surface thereof.

5. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a pair of caliper jaws having means for varying the spacing between said jaws, each of said caliper jaws having integral therewith a support .ieans for the reception of the resistance element and a means which cooperates together with said means for varying the spacing between said jaws to measure the internal peripheral surface of the housing, said support means substantially defining the internal peripheral surface of said housing cavity as determined by said caliper measuring means, said cutting means adapted to extend through the space between said caliper jaws for cutting the resistance element substantially to the desired length.

6. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a bifurcated plate having two sections, caliper jaws respectively formed upon the plate sections on one face thereof, means attached to said jaws adapted to enter into the cavity of a housing to measure the internal peripheral surface of the cavity, and support means on the other face of each section of said plate for the reception of the resistance element, said support means being adapted to adjust the relative size of said supporting structure in accordance with the measurement of the internal peripheral surface of said housing as determined by the caliper jaw measuring means.

7. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a pair of caliper jaws having means for extending into said cavity to measure the internal peripheral surface thereof, said caliper jaws being separated by a gap of variable width and having means for varying the space therebetween, cutting means having a width W, means for moving said cutting means through said gap, and support means for the reception of the resistance element integral with each of said caliper jaws, said support means substantially defining the internal peripheral surface of said housing cavity as defined by said caliper jaws, the circumferential distance defined by said support means being substantially W larger than the circumferential distance defined by said caliper jaws.

8. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a pair of caliper jaws spanning a distance B and separated by a gap of variable width, said caliper jaws each having means adapted to extend into and measure the internal peripheral surface of the cavity of said housing and including means for varying the spacing between said jaws, cutting means having a width W, means for moving said cutting means through said gap, and support means integral with each of said caliper jaws and defining a cavity substantially conforming to said housing cavity as determined by said caliper jaws thereby to receive the resistance element, the diameter A of the cavity formed by said support means being related to B and W by the equation A=B+ g 9. A gauge in combination with a cutting means for cutting to length a resistance element to fit within the cylindrical cavity of a particular housing including a pair of caliper jaws separated by a gap of variable width and including means for varying the gap therebetween, each of said jaws having an upstanding substantially semicircular shaped shoulder adapted to extend into the cavity of the housing, a pair of support means integral with said caliper jaws, said support means cooperatively forming a substantially circular cavity for receiving the resistance element, and means for varying the gap between said caliper jaws to cause said jaws to abut against the cavity walls of said particular housing thereby to define the dimensions of said cavity of said support means in accordance with the internal peripheral surface dimensions of the cavity of said housing.

10. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a base, a bifurcated plate having two sections separated by a gap of variable width mounted upon said base, caliper jaws respectively formed upon said plate sections on one side thereof and adapted to extend into the cavity of said housing, means for varying the spacing between said two sections of said bifurcated plate thereby to vary the relative position of said caliper jaws, support means formed upon thet plate sections on the other side thereof for receiving the resistance element so that the relative dimensions of said support means are varied in accordance wih variation in spacing between said bifurcated plate sections, said cutting means comprising a blade supported upon said base, and means for extending said blade into said gap.

11. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to the cavity of a particular housing comprising a base, a bifurcated plate having two esctions separated by a gap of variable width, means for rotatably mounting said plate upon said base so that said plate is rotatable about an axis perpendicular to the plane defined by said gap, caliper jaws respectively formed upon said plate sections on one side thereof, said caliper jaws adapted to extend into the cavity of a particular housing, means for varying the spacing of said gap thereby to adjust the position of said caliper jaws, support means formed upon the plate sections on the other side thereof, said support means adapted to receive said resistance element and to define the relative size of said resistance element as determined by the means for varying the gap between said two sections of said bifurcated plate, said cutting means comprising a cutting blade supported upon said base, and means for extending said blade into said gap.

12. A gauge in combination with a cutting means for cutting to length a coiled cylindrical resistance element to fit the cavity of a particular housing comprising a base, a

carriage slidably mounted upon said base, a bifurcated plate having two sections separated by a gap of variable Width mounted upon said carriage, means for Varying the width between said two sections of said bifurcated plate, caliper jaws formed upon the respective plate sections on one side thereof, said caliper jaws adapted to extend into the cavity of a particular housing and to move adjacent the surface of said cavity upon expanding the width of said gap between said two sections of said bifurcated plate, support means formed upon the plate sections on the other side thereof for receiving the resistance element, said support means substantially defining the internal peripheral shape of said housing cavity as determined by it? said caliper jaws, a cutting blade supported upon said base in the plane defined by said gap, and means for translating said carriage so that the blade extends into said gap to sever said resistance element.

References Eited in the file of this patent UNITED STATES PATENTS 252,247 Olmsted Ian. 10, 1882 1,611,960 Thomas Dec. 28, 1926 1,651,980 Teetor Dec. 6, 1927 2,503,353 Pugh Apr. 11, 1950 2,516,602 Snyder July 25, 1950

Claims

1. A METHOD OF CUTTING A RESISTANCE ELEMENT TO FIT THE CAVITY OF A PARTICULAR HOUSING COMPRISING THE STEPS OF INSERTING A PAIR OF CALIPER JAWS SEPARATED BY A GAP WITHIN SAID CAVITY, INCREASING SAID GAP UNTIL SAID JAWS ABUT THE PERIPHERAL SURFACE OF SAID HOUSING CAVITY, SAID CALIPER JAWS HAVING SUPPORT MEANS INTEGRAL THEREWITH SUBSTANTIALLY DEFINING THE PERIPHERAL SURFACE OF SAID HOUSING CAVITY, PLACING A RESISTANCE ELEMENT UPON SAID SUPPORT MEANS SO THAT ENDS OF SAID ELEMENT OVERLAP EACH OTHER ON SAID GAP, AND CUTTING SAID RESISTANCE ELEMENT BY SUITABLE CUTTING MEANS EXTENDED INTO SAID GAP.