Thickness Responsive Variable Position Die Set

This invention relates to cutting dies and more particularly to a unique variable position die set adapted for precisely cutting and trimming metal blanks subsequently wrapped to cylindrical shape.

In the manufacture of electric motor shells, it has been general industry practice to roll precut lengths of sheet metal into a cylindrical form. The ends of the rolled sheet metal are then welded. The internal diameter of the resulting cylindrical motor shell must be maintained within exact tolerances. This is necessary since it is common manufacturing practice to press fit the field laminations for the electric motors into place within the shell. If the motor shells are not formed with accurate circularity and with a constant, predetermined internal diameter, distortion of the field laminations may result. Such distortion presents variations in the air gap between the field and the rotor resulting in defective motor operation.

One apparatus and method for precisely forming cylindrical motor shells from flat blanks of sheet metal is disclosed in U.S. Pat. No. 3,732,614 to Boutell, entitled METHOD FOR MAKING MOTOR SHELLS AND THE LIKE, issued May 15, 1973, and assigned in common herewith. As shown therein, a precisely dimensioned and precision-ground forming arbor or mandrel is employed to produce the motor shells, by wrapping individual, precut lengths of sheet metal of an appropriate width around the forming arbor, using pivotally mounted jaw-like elements. Since the forming arbor has a diameter equal to the diameter of the field laminations for the electric motor, the resulting cylindrically shaped motor shell will possess the required internal diameter. Further, the shell will have accurate circularity, and therefore distortion of the field laminations of the motor is obviated. In the preferred present practice, the sheet metal blanks are precut in a cutoff and pierce die of the general type disclosed in the aforementioned U.S. patent. Although the sheet metal stock is typically supplied by its manufacturers with a specified nominal thickness, in stated ranges, the actual thickness of the individual cut blanks usually varies within the tolerances allowed by sheet metal manufacturers. In many industrial applications this variance in thickness is inconsequential and is generally ignored. However, in the manufacture of cylindrical sleeves or the like with controlled inside diameters, as in the case of motor shells, variances in the thickness of the sheet metal blanks result in changes in the internal diameter of the resulting shell if the shell is rolled so that the ends of the blank abut. This variation in the internal diameter of the shell causes serious quality control problems for their manufacturers, including a higher incidence of rejections and scrapped units as well as warranty claims for motors which fail prematurely.

If the shell manufacturer employs an apparatus of the type disclosed in the above-mentioned U.S. patent, as the thickness of the sheet stock varies the weld gap between the ends of the rolled or wrapped blank will likewise vary. For example, if the thickness of the blank is less than nominal stock thickness, the gap will decrease, and in fact it would not be unexpected for the ends of the wrapped blank to overlap, thus resulting in a defective shell. Further, if the thickness of the blank should be greater than the nominal stock thickness, the gap between the ends of the blank will increase. The increase may be sufficient to prevent effective welding of the ends of the wrapped blank. In either case, as the actual thickness of the sheet metal blanks varies from the nominal thickness, as is routinely the case, the resulting motor shell varies between two extremes, either of which is unacceptable.

A need therefore exists for an apparatus for precisely producing a sheet metal blank wherein the cut length may be varied as a function of the actual thickness of the sheet metal so that a cylinder of constant internal diameter having a constant weld gap may be produced.

A variable position die set is provided for precisely cutting metal blanks whereby the problems heretofore experienced in the manufacture of cylindrical shells from flat metal stock are substantially alleviated. Essentially, the variable position die set includes a lower actuating die plate, an upper actuating die plate superimposed on the lower die plate, a stationary cutting block arrangement fixably positioned between the die plates and a movable cutting block arrangement slidably positioned in spaced relationship from the stationary cutting arrangement between the actuating die plates. Provision is made for sensing the actual thickness of the individual, precut blanks as they are positioned between the actuating die plates and within the stationary and movable cutting block arrangements. Provision is made for shifting the position of the movable cutting block arrangement as a function of the sensed blank thickness.

Initially, the movable cutting block arrangement is positioned so that the cut length of the flat stock is equal to pi times the sum of the desired internal diameter and the actual blank thickness. As the sensed blank thickness varies, the movable cutting block arrangement is shifted a distance equal to pi times the change in thickness. As the thickness decreases, the movable block arrangement shifts to reduce the cut length from the initial preset or nominal length. As the thickness of the stock increases, the movable cutting block arrangement shifts so that the cutting length is greater than the nominal length.

Therefore, the variable position die set in accordance with the present invention is capable of precisely cutting metal blanks which may be subsequently formed into cylindrical shells having an internal diameter maintained within precise tolerances and a precisely dimensioned welding gap.

FIG. 1 is a front, elevational view of a variable position die set in accordance with the present invention;

FIG. 2 is a perspective view of the variable position die set shown with the upper actuating plate removed;

FIG. 3 is a fragmentary plan view of the variable position die set shown with portions of the upper plate removed;

FIG. 4 is a fragmentary cross-sectional elevational view taken generally through section line IV--IV of FIG. 1; and

FIG. 5 is a fragmentary, cross-sectional, elevational view taken generally through section line V--V of FIG. 3.

The preferred embodiment of the variable position die set in accordance with the present invention is illustrated in the drawings and generally designated 10. As shown in FIGS. 1 and 2, the die set includes a lower actuating die plate 12 and an upper actuating die plate 14. Plate 14 is superimposed on plate 12. A pair of centrally positioned, spaced guide pins or posts 16, 18 extend upwardly from the lower plate 12 through apertures formed in the upper plate 14. Suitable sleeves 20 may be received within apertures formed in the upper actuating die plate 14 to provide a guide structure. The upper actuating die plate 14 is therefore mounted for vertical motion relative to the lower actuting die plate 12.

Positioned between the die plates 12 and 14 so as to be actuated thereby are a stationary cutting block assembly 22 and a movable cutting block assembly 24. Each cutting block arrangement 22, 24 includes shearing blocks 26, 28 and a vertically movable forming (or preforming) block 30 suspended and preloaded by springs 32.

With stationary assembly 22, the shearing block 26 is fixed to the lower die 12. The movable forming block 30 is suspended from the upper plate 14 in direct vertical alignment with the fixed block 26. The shearing block 28 is fixedly secured to the upper plate 14 adjacent to and in sliding contact with the movable forming block 30.

The laterally shiftable or variable position cutting block assembly 24 has the lower fixed shearing block 26 fixedly secured to lower inner die plate 34 while the vertically movable forming block 30 and the fixed upper shearing block 28 are secured to an upper, inner die plate 36.

The inner die plates 34, and 36 are mounted between the actuating die plates 12, 14 for limited, longitudinal movement. As best seen in FIGS. 1 and 2, a pair of transversely spaced guide pins 40, 42 extend upwardly through guides 41, 43 in the lower inner die plate 34 and pass through guides in the upper inner die plate 36 and through slots the upper actuating die plate 14. As a result, the lower inner plate 34 may be shifted either to the right or to the left as viewed in FIG. 1. The pins maintain the vertical alignment between the various cutting blocks of the shiftable assembly 24. As shown in FIG. 1, a pair of coil springs 46 may be positioned between the plates 34, 36. These coil springs open the cutting block arrangement after the cutting operation.

As best seen in FIG. 1, the shiftable forming blocks 30 and the fixed lower shearing blocks 26 may be relieved or varied in thickness along a radius of curvature at their cutting or shearing edges 47. This radius of curvature may approximate that of a wrapping arbor or mandrel if the precision cut sheet metal blanks are to be wrapped in a cylindrical form. When a sheet metal blank is placed within the die arrangement and the upper die plate 14 is moved downwardly towards the lower die plate 12 the springs 32 are compressed and the shiftable upper blocks 30 will grip the sheet stock against the lower blocks 26. Upon further downward movement of the upper plate 14, the shearing blocks 28 and 26 will shear or cut the sheet metal blank. During the shearing step, the sheet of stock becomes bent downwardly along the edge by the lip of forming block 30. Bending of the sheet stock will typically occur after the shearing, and the edges of the sheared stock will therefore be severed at right angles. However, when the sheet stock is wrapped about the forming arbor, the free end of the blank will define an outwardly-widening V-shaped configuration suitable for the welding process. The forming of the edges eliminates an undesirable "roof" effect upon wrapping of the blank because of the inability of clamping the systems marginal edges.

As previously set forth, in order for a cylindrical shell to be efficiently formed on a mass-produced basis wherein each shell has a constant internal diameter or the gap between the free edges of the blank upon wrapping remains constant, the cut length of the sheet must be varied as a function of the thickness of the sheet. The cut length L will be equal to the internal circumference of the resulting cylinder plus pi times the thickness T of the sheet. Since the internal circumference of the cylinder must be capable of receiving a particular size of electric motor, having a given external diameter D, the cut length may be defined by the following equation:

since the initial thickness T of any sheet metal stock will vary and the internal circumference of the resultant shell must be maintained constant, the relationship between the initial cut length L and subsequent cut lengths may be defined as follows:

where .DELTA. T equals the change in thickness between the different blanks.

Therefore, in order to obtain proper results provision must be made for shifting of the die set 24 a distance proportional to pi times the change in thickness of subsequent sheets. One such arrangement is illustrated in the drawings as including a mechanical sheet-thickness sensing arrangement 50 and a pi-multiplier 51 (FIG. 1).

As best seen in FIG. 4, the thickness sensor assembly 50 includes a spring-loaded sensor block 60 suspended from the upper actuating die plate 14 and preloaded by a pair of coil springs, or the like, 62. Intermediate the sides of the sensor block and adjacent the trailing edge thereof, the block includes a depending finger or projection 64. Mounted directly below the sensor block 60 is a sensor base block 66. The base block 66 is secured to a plate 67 which in turn is secured to the lower actuating die plate 12. The face 69 of base block 66 is angled to assist in placement of a blank within the die. As best seen in FIGS. 3 and 5, directly below the depending finger 64, the base block 66 is formed with a bore 68 within which is disposed a sensor pad 70 which is mounted within the bore for vertical movement by a locator pin and coil spring 72 (FIG. 5).

The pi multiplier 52 includes a cam link rod 74, a bell crank or lever 76, and a die set push arm assembly 78. The bell crank 76 is pivotally mounted to the lower actuating die plate 12 by a pin 80. The cam link rod 74 is pivotally secured to the short arm 82 of the bell crank by a pin assembly 84. The die set push arm assembly 78 includes a thrust block 86 pivotally secured to the end of the long arm 88 of the bell crank by a pin assembly 90. A thrust bolt 92 is secured to the thrust block 86 and extends through an aperture formed in a retainer block 94. The retainer block 94 is fixedly secured to the lower shiftable die plate 34 of assembly 24 (FIGS. 1 and 3). A pair of shiftable die assembly return springs 96, 98 are positioned between retainer blocks 100, 102 and 104, 106, respectively. The spring retainer blocks 100 and 104 are fixedly secured to the lower actuating die plate 12 while the retainer blocks 102, 106 are fixedly secured to the shiftable die plate 34. These return springs serve to bias the variable position or movable die assembly 24 against a pair of stop block assemblies 110, 112. As best seen in FIGS. 1 and 3, the stop block assemblies include upstanding blocks 114, each having an internally threaded aperture for receiving an adjustment bolt and stop nut 116. It should also be noted that a pair of adjustment nuts 118, 120 are threadably received on thrust bolt 92 of the die set push arm assembly 78, on opposite sides of retainer block 94. As a result of this arrangement, the initial positioning of the shiftable die assembly 24 may be set by adjustment of the stop bolt assemblies 116 and the adjustment nuts 118, 120 on the thrust bolt 92.

As best seen in FIG. 5, the lower end 130 of the sensor pad 70 is beveled along an angle of 45.degree.. This beveled edge defines a camming surface which meets with or engages a cam follower surface 132 on the cam link rod 74. The cam follower surface 132 is also cut along an angle of 45.degree.. As a result, there is a one-to-one correspondence between the movement of the sensor block and the movement of the cam link rod 74 when the thickness-sensing assembly is pressed down on a sheet metal blank.

The bell crank 76 is dimensioned so that any change in the sensed thickness of each sheet metal blank will result in a shift in the position of the variable position die set 24 equal to pi times the change in thickness. This shift in position is readily obtained by making the bell crank 76 so that the ratio of the length of the short arm 82 over the length of the long arm 88 is equal to the reciprocal of pi.

As best seen in FIGS. 1 and 2, back stops 140, 142 are provided adjacent the rear edge of the lower actuating die plate 12. Further, a leaf spring type stop or edge guide 144 is secured to the lower actuating die plate 12 adjacent the fixed shearing block 26. Also, gibs or slide track ways 150, 152 may be positioned adjacent the lateral edges of the lower plate 12 to slidably receive the variable position, lower die plate 34. A front stop 134 (FIG. 2) is provided to insure proper positioning of each blank within the die assembly. The back stops 140, 142, edge guide 144 and front stop 134 fixably position the blanks within the die arrangement.

The mode of operation of the above described arrangement will now be more fully discussed. If the die set is to be employed in the manufacture of electric motor shells, the sheet stock is initially precut to the desired width and preferably to a length equal to the product of the constant pi times the sum of the diameter of the motor and the maximum stock thickness, plus four times the maximum stock thickness. Such precut lengths insure sufficient material for the final trimming operation in the variable position die set and reduce the amount of unnecessary material waste. Next, one of the precut blanks is positioned within the die set and between the shearing blocks. The blank is pushed forward until it abuts the back stops 140, 142. The sensor block base 66 is beveled along its forward edge for ease of insertion of the blank. Also, blanks for use in forming motor shells may be prepunched at various locations. Pilot pins or locating pins may be positioned on the lower actuating die plate 12 to assist in locating the blank within the die set. The thickness of the first sheet is determined and the variable-position cutting block arrangement 24 is positioned so that the desired wrapped dimensions will be obtained. This positioning is accomplished through the adjustment nuts 118, 120 provided on the thrust bolt and also through positioning of the end stop bolts. The actuating dies may then be pressed together thereby severing and preforming the edges of the blank.

As subsequent precut blanks are positioned within the die set arrangement, and the upper actuating die plate is pressed down toward the lower die 12, the sensing block 60 will initially contact the blank 154, as shown in FIG. 4. Depending finger 64 will then contact the sensor pad 70 and force the pad 70 downwardly an amount dependent upon the change in the thickness of the blank from the initial preset position. Movement of the sensor pad 70 will be transmitted through the cam link rod 74 to the bell crank 76. As a result, the bell crank 76 will move the lower die plate 34 a distance equal to pi times the change in sensed blank thickness.

As will be readily apparent to those skilled in the pertinent art, various modifications may be made to the die set as described and illustrated. For example, other forms of pi multipliers may be employed to obtain the desired positioning of the variable die structure. Such arrangements could be electrical, hydraulic or other forms of mechanical linkages. Also, the present arrangement may be adapted to vary the cut length as a function of thickness other than by positioning in a manner proportional to pi. The sensing and shifting arrangement are adaptable to any use wherein the ultimate cut length of a sheet should be made dependent upon the thickness in some proportional relationship.

When the apparatus is employed for precision trimming of blanks which are subsequently rolled or wrapped into a cylindrical configuration as in the manufacture of motor shells, the resultant shell will have a constant internal diameter and/or a constant gap between the adjoining edges of the wrapped blank. As a result, the quality control involved in this manufacturing process is substantially increased. The resulting welds which bond the joining edges of the blank may be of uniform quality since the gap will be maintained constant. Also, since the internal diameter may be directly controlled in this manner any distortion to the motor field laminations subsequently press fit into the shell will be avoided. It can, therefore, be seen that the problems heretofore experienced by prior art methods and apparatus have been substantially alleviated by the present invention. It is expressly intended, therefore, that the above description be considered as that of the preferred embodiment only. The true spirit and scope of the present invention should be determined by contemplation of its underlying concept, and by reference to the appended claims.