Abstract:
A method and apparatus for automatically installing in tandem pairs of protective covers to adjacent corners of a rectangular picture frame, to protect the corner during shipment, includes a pair, e.g., right and left of substantially identical corner protector installation mechanisms oriented at ninety degrees with respect to one another and mounted on a supporting frame laterally spaced apart at a distance adjustable to accommodate frames of various widths. Each mechanism has a work table which supports a pair of mutually perpendicular, non-intersecting, vertically disposed, longitudinally elongated guide plates which support on the upper surfaces thereof, near inner diagonally cut transverse vertical end walls of the plates, a clamp support beam member, a lower surface of which define with inner sides of the guide plates and the work table surface a diagonally disposed frame corner entrance opening. Sensors within each of the guide plates, near the inner ends thereof, provide a logic signal which indicates when both adjacent corners of a frame inserted horizontally into the apparatus have been fully inserted into and correctly oriented with respect to the guide plates, output signals from the sensors being logically ANDED to produce a command signal initiating simultaneous automatic operation of both mechanisms. Each mechanism includes a feeder mechanism which loads an individual, pre-cut cardboard preform into position on the work table inwardly of the opening through the guide plates. Also each mechanism includes folder mechanisms including flap folder arms which fit into workable recesses, and which bend a side cover flap and securement flap of a preform into a vertical position adjacent a first side of the frame corner and perpendicularly inwardly to overlie the first frame member, bend a spine flap and right-hand triangular cover flap of the preform into a vertical position adjacent the second frame member, and the right-hand triangular flap perpendicularly inwards from the spine flap to overlie the second and first frame members. An automatic staple gun then inserts a staple downwardly through the right-triangular cover flap into the securement flap and left-hand frame member, securing the corner protector in a folded disposition over the frame corner.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to methods and apparatus for protecting the corners of rectangularly-shaped frames, particularly picture frames, from impact damage during shipping. More particularly, the invention relates to a method and apparatus for attaching protective corners to two adjacent corners of a picture frame in tandem. 
     B. Description of Background Art 
     Picture frames for holding and displaying paintings, photographs, diplomas, certificates and similar flat articles are manufactured in prodigious quantities worldwide. Although picture frames vary in shape and construction details, most frames have a rectangular plan view shape. Moreover, the structure of most picture frames consists essentially of four straight channel members or moldings, each having an inner channel that intersects channels of adjacent members at a ninety degree angle. Four such channel members fastened together form a rectangular ring-shaped frame. The channels or recesses in the moldings comprise spaces for receiving a rectangularly-shaped flat display piece such as a photograph or painting, and sometimes, a backing panel, mat and protective cover glass. 
     Picture frames of the type described above are made from a variety of materials including, wood, metal and plastic. Whatever material the frame is made of, the geometry of a rectangular picture frame dictates that it have four peripheral members of generally uniform thickness which are joined at each other at forty-five degree mitre angles to form ninety-degree corners. These corners are sharp, and are therefore highly subject to breakage, denting or cosmetic damage during shipment. Accordingly, most picture frames, whether empty or occupied, are fitted with some sort of corner protectors during shipment. Such corner protectors are usually made of a relatively inexpensive recyclable material such as cardboard or polystyrene foam. Typical corner protectors of this type are disclosed in U.S. Pat. Nos. 3,955,677, 4,598,825, and 5,447,233. Other patents, related to protecting corners of objects during shipping include U.S. Pat. No. 4,407,898. Also, U.S. Pat. No. 5,255,458 discloses a three-dimensional picture corner and U.S. Pat. No. 4,787,553 discloses a corner fastening device. 
     In addition to the above-referenced patents related to corner protectors and the like, a variety of machines for bending sheet metal or cardboard of the type used for corner projectors have been disclosed in the following U.S. Pat. Nos.: 4,132,102, 4,585,432, 4,713,957, 4,857,038, 4,956,961, and 5,184,998. 
     None of the aforementioned references disclose a machine for automatically attaching corner protectors to picture frames. Accordingly, the task of attaching corner protectors to picture frames was formerly labor intensive and time consuming. In response to those limitations of the prior art, the present inventor disclosed an automatic Method and Apparatus For Attaching Corner Protectors to Picture Frames, in U.S. Pat. No. 6,018,934. In that patent, the present inventor disclosed a method and apparatus for installing covers to protect picture frame corners from damage during shipment, utilizing thin cardboard preforms having symmetric, left and right-hand, right-triangular cover flaps joined at vertical sides thereof to opposite vertical sides of a vertically elongated, rectangular spine flap, the left-hand triangular cover flap having depending downwardly from its base a horizontally elongated rectangular side cover flap, and depending downwardly from the lower lateral edge of the side cover flap a trapezoidally-shaped securement flap. The preform is positioned below a picture frame corner, with intersecting side members of the frame vertically aligned with the sides of the left-hand right-triangular cover flap, which serves as a lower face cover flap. The apparatus includes folder mechanisms including flap folder arms which are retractable into recesses provided in a work table, and which are extendible and rotatable to thereby bend the side cover flap and securement flap into a vertical position adjacent a first side of the frame corner and perpendicularly inwardly to overlie the first frame member, bend the spine flap and right-hand triangular cover flap into a vertical position adjacent the second frame member, and bend the right-hand triangular flap perpendicularly inwards from the spine flap to overlie the second and first frame members. An automatic staple gun then inserts a staple downwardly through the right-triangular cover flap into the securement flap and left-hand frame member, securing the corner protector in a folded disposition over the frame corner. 
     The present invention was conceived of to provide an automatic tandem corner protector attachment method and apparatus characterized by increased throughput rate, and a further enhanced degree of automation over the prior art. 
     OBJECT OF THE INVENTION 
     An object of the present invention is to provide a method for attaching in tandem a pair of protective covers to adjacent corners of a picture frame, to protect the corners from damage during transit. 
     Another object of the invention is to provide an apparatus for automatically attaching in tandem a pair of protective covers to adjacent corners of a picture frame. 
     Another object of the invention is to provide an apparatus for folding a pair of flat sheets of flexible material over two adjacent corners of a picture frame, and securing the folds of each sheet together to form a pair of separate protective covers, one for each of the two adjacent corners of the picture frame. 
     Another object of the invention is to provide an apparatus for bending a pair of flat cardboard preforms cut to a pre-determined shape over a pair of adjacent corners of a picture frame, and securing the folded portion of each cardboard preform to thereby form a pair of corner protectors for the picture frame. 
     Another object of the invention is to provide a picture frame corner protector attaching apparatus which includes means for clamping and holding a pair of adjacent corners of the frame, folding over each corner a separate one of a pair of preform sheets, each having the shape of a truncated isosceles triangle with a trapezoidally-shaped tab depending downwardly from one side of the base of the triangle, into three separate flaps which overlay each of the two corners of the frame, and securing the flaps of each preform sheet together and to a frame molding to form a pair of corner protectors covering the two corners of the frame. 
     Another object of the invention is to provide a picture frame corner protector attaching apparatus which includes a pair of adjustably spaced apart, mutually perpendicular holding jigs adapted to receive a pair of adjacent corners of picture frames of various sizes, the apparatus including sensor means associated with each of the two holding jigs to provide an index signal indicating that a picture frame corner has been fully inserted into a holding jig in a properly aligned disposition, and logic means responsive to the simultaneous presence of index signals from both holding jig sensors in producing a command signal to initiate attachment of a pair of corner protectors to the adjacent corners of the picture frame. 
     Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims. 
     It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, I do not intend that the scope of my exclusive rights and privileges in the invention be limited to details of the embodiments described. I do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention comprehends a method and apparatus for automatically installing in tandem pairs of protective cardboard covers to adjacent corners of rectangularly-shaped frames, particularly those used to hold pictures, documents and the like, the covers protecting the corners of the frame from damage during shipment. 
     The automatic tandem corner protector attachment method and apparatus according to the present invention preferably utilizes corner protectors made from thin sheets of cardboard, each pre-cut, by die cutting, for example, into a generally triangular shape which is modified to adapt the preform to be folded over and enclose a corner and adjacent portions of the side channel members of a conventional rectangularly-shaped picture frame. Four such corner protectors are used to cover the four corners of the frame. In a preferred embodiment of the invention, the overall plan view shape of the corner protector preform is that of two mirror symmetric, left and right, right-triangularly shaped flaps having collinear horizontal bases joined to opposite vertical sides of a vertically elongated rectangular spine flap. Thus shaped, the preform has the appearance of an isosceles triangle whose vertex is horizontally truncated. The preform preferably includes a horizontally elongated, rectangularly shaped lower cover flap which depends downwardly from the base of one of the right-triangle flaps, the left one, for example, and has the same width as the left triangular cover flap. A trapezoidally-shaped securement flap depends downwardly from the bottom edge of the lower cover flap, the securement flap having a lower edge wall parallel to but shorter than that of the lower cover flap. In a preferred embodiment, the horizontal width of the vertically elongated rectangular spine flap and the vertical height of the horizontally elongated lower cover flap have a common value which is slightly greater than the thickness of a picture frame which is to protected, e.g., about {fraction (15/16)} inch for a frame thickness of ⅝ inch. Preferably, the vertical side walls of the vertical spine flap and the horizontally disposed upper and lower edges of the lower cover flap are scored to facilitate folding those respective flaps to contact adjacent perpendicular channel members or moldings of a picture frame. 
     According to the method of the present invention, a pair of preforms, score lines up, are positioned below two adjacent corners of a picture frame. Each of the two adjacent corners of the frame is oriented with respect to a preform with a pair of perpendicularly intersecting adjacent side walls defining a corner of the frame vertically aligned with the perpendicular sides of the left-hand right-triangular flap having the downwardly depending lower cover and securement flaps. The lower cover and securement flaps are then folded vertically upwards as a planar unit along the upper or inner score line defining the junction between the base of the left right-triangular cover flap and the lower cover flap. Next, the securement flap is folded downwardly and inwardly along the lower or outer score line towards a horizontal position contacting the upper surface of the frame member overlying the left triangularly-shaped cover flap. Then, the second, right-hand, right-triangularly-shaped cover flap and spine flap are bent upwards as a unit along the innermost vertical fold line, i.e., the left-hand vertical edge of the spine flap, to a vertical position. The right-hand triangular cover flap is then bent downwardly and inwardly along the outermost, right-hand vertical fold line of the spine flap to a horizontal position overlying and contacting the securement flap. Finally, the overlying right-hand triangular cover flap is fastened to the securement flap, by stapling, for example. In a preferred embodiment of the invention, using a wooden picture frame, a staple is driven through both the overlying right-hand triangular cover flap and the securement flap and into the underlying wooden frame member, thereby securing the corner protector to the frame. 
     With a preform attached to a corner of a picture frame as described above, the first, left-hand triangular cover flap covers a right triangular portion of the lower side of the frame corner, and the lower, rectangularly-shaped cover flap covers a portion of the outer, vertical side wall of a first side channel member of the frame, e.g., the “left-hand” side channel member. Also, the second, right-hand triangular cover flap covers a right triangular portion of the upper side of the frame cover overlying the lower triangular cover flap, while the spine flap covers a portion of the outer, vertical side wall of the second, “right-hand” side channel member. 
     The apparatus according to the present invention includes a base plate or work table having a flat upper surface for supporting a pair of adjacent corners of a picture frame, and a pair of frame holder clamping jigs for receiving and holding the adjacent corners of the picture frame on the surface of the table. In a preferred embodiment, the apparatus includes a pair of substantially identical corner protector installation mechanisms oriented at ninety degrees with respect to one another, each including a clamping jig and laterally movably mounted on a supporting frame. This construction allows the lateral spacing between the two clamping jigs to be adjusted to accommodate picture frames of various widths. 
     Each of the two tandem corner protector installation mechanisms includes a pair of corner protector flap folder actuators positioned on opposite sides of the corner of a picture frame held in the clamping jig. Each corner folder actuator includes a flat, rectangularly-shaped flap-contacting arm mounted near a longitudinal end thereof on the upper end of a post which is actuable by a linear actuator from a lower inactive position in which the arm resides within a rectangularly-shaped recess in the upper surface of the base plate, with the upper surface of the arm flush with the upper surface of the base plate, to an upper, active position. Each post also includes a 90-degree rotary actuator means, which is effective in rotating a flap contacting arm, when raised, from an outer position contacting the outer vertically disposed side of a flap to an inner position effective in exerting an inwardly and downwardly directed folding force on the flap. 
     In operation, the first corner folder actuator arm is extended vertically, folding the lower cover flap and securement flap upwardly into a vertical position adjacent one side of a frame. The arm is then rotated inwards, bending the securement flap inwardly and downwardly into contact with the upper surface of the first frame molding strip. 
     Next, the actuator arm of the second corner folder actuator is elevated, folding the upper triangular cover protector flap upwardly to a position vertically adjacent the outer edge of the second frame molding strip. The arm of the first corner folder actuator is then rotated outwardly from its position overlying and holding down the securement flap, while the arm of the second actuator is simultaneously rotated inwardly, bending the triangular flap down into contact with the securement flap. An automatic stapling machine then inserts a staple through the upper triangular flap, underlying securement flap, and underlying frame molding, thereby securing the corner protector to the frame. Both actuator arms are then automatically rotated and retracted to their recessed positions, flush with the work table surface. At the same time, the frame holding clamp is also automatically released, allowing the frame with attached corner protector to be removed from the clamp. 
     The apparatus according to the present invention includes means for automatically and substantially simultaneously initiating operation of two corner protector installation mechanisms of the type described above, each time two adjacent corners of a picture frame are fully inserted into and correctly oriented with respect to clamping jigs of the mechanisms. In a preferred embodiment, apparatus components for implementing this automatic operation include a pair of guide plates in each clamping jig that have inner vertically disposed wall surfaces that define therebetween a ninety degree intersection angle or vortex, which is cut off or truncated by a plane oriented at forty-five degrees to the guide bars, to thereby form an opening for insertably receiving the corner of picture frame, with the intersecting channel moldings of the frame aligned with the guide plates. 
     Each guide plate is provided, near the opening between the inner longitudinal ends of the guide plate, with a sensor which provides an electrical signal indicating that a frame corner molding has been fully inserted into the clamping jig. The electrical signal from each pair of sensors for each of the two clamping jigs are logically ANDed, producing a command signal to initiate automatic operation of the machine only when all three outer sides of the frame channel moldings defining two adjacent corners of the frame are fully inserted into and properly aligned with the guide plates of the two clamping jigs for proper operation of the corner protector folding and attaching operation. This command signal initiates a sequence of operations by which a pair of corner protectors are automatically attached in tandem to the two corners of a picture frame. Upon completion of this automatic operation, the holding clamps of both clamping jigs are automatically released, allowing the frame with a pair of attached corner protectors installed to be withdrawn from engagement with the apparatus. The frame may then be rotated one hundred and eighty degrees in a horizontal plane, and re-inserted into the apparatus to initiate attachment of a pair of corner protectors to the remaining two corners of the picture frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of an automatic tandem corner protector attachment apparatus for picture frames and the like according to the present invention. 
     FIG. 2 is a view similar to that of FIG. 1, but showing a guide plate of one of two installation mechanisms of the apparatus removed, and the two mechanisms slid laterally together to accommodate narrower frames than those which the configuration of FIG. 1 is adapted to accommodate. 
     FIG. 3 is an upper plan view of a corner protector preform or blank usable with the apparatus of FIG.  1 . 
     FIG. 4 is an upper perspective view of one of two tandem installation mechanisms comprising parts of the apparatus of FIG. 1, showing preforms loaded into a protective cover installed on the mechanism. 
     FIG. 5 is an upper plan view of the apparatus of FIG. 2, showing two corner protector preforms advanced to two separate clamping locations provided for two adjacent corners of a picture frame. 
     FIG. 6 is an upper plan view of the apparatus of FIG. 2, showing a picture frame inserted partially into the apparatus. 
     FIG. 7 is an upper plan view similar to that of FIG. 6, but showing a picture frame fully inserted into the apparatus, thereby producing sensor signals which initiate operation of the apparatus. 
     FIGS. 8A,  8 B are a segmented oblique transverse sectional view of the apparatus of of FIG. 7, showing clamp bars of the left and right installation mechanisms brought down into compressive contact with the upper surface of the frame at left and right corners thereof. 
     FIG. 9 is a view similar to that of FIG. 7, but showing a first, corner flap actuator of each of the two installation mechanisms in an upwardly actuated position. 
     FIG. 10 is a view similar to that of FIG. 9, but showing a fold-effecting arm of the first corner flap actuator of each of the two left and right mechanisms rotated to contact and fold down a securement flap of the corner protector preform into a position overlying the picture frame. 
     FIG. 11 is a fragmentary rear perspective view of the left-hand mechanism of the apparatus of FIG. 1, disposed as shown in FIG.  10 . 
     FIG. 12 is a view similar to that of FIG. 10, but showing a second corner flap actuator of each mechanism of the apparatus in an upwardly actuated position. 
     FIG. 13 is a view similar to that of FIG. 11, but showing a fold effecting arm of the second lower flap actuator of each of the two installation mechanisms rotated to contact and fold down a second triangular cover flap of the corner protector preform into a position overlying the securement flap of the corner protector preform. 
     FIG. 14 is a view similar to that of FIG. 12, but showing the fold-effecting arm of the first corner flap actuator of each mechanism rotated back into an inactive, home position, and showing a stapler comprising part of each mechanism inserting a staple through the second triangular cover flap and securement flap and into the picture frame. 
     FIGS. 15A,  15 B are segmented transverse sectional views of the apparatus of FIG. 7, showing staplers thereof in active, downward positions to insert staples. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-15 depict an automatic tandem corner protector attachment method and apparatus for picture frames and the like, according to the present invention. The structure and function of certain portions of the apparatus according to the present invention are described in detail in the present inventor&#39;s U.S. Pat. No. 6,018,934, the entire specification, claims, abstract and drawings of which are hereby incorporated by reference into the present specification. Accordingly, a complete understanding of the aforementioned structure and function of those portions may be obtained by reference to that patent. 
     Referring now to FIGS. 1 and 2, an automatic tandem corner protector attachment apparatus  10  for picture frames according to the present invention may be seen to include a pair of substantially identical corner protector installation mechanisms  20 , designated  20 A,  20 B, for convenience when necessary to distinguish between the two. Mechanisms  20 A,  20 B are each substantially similar in structure and function to the apparatus disclosed in U.S. Pat. No. 6,018,934. In apparatus  10  according to the present invention, mechanisms  20 A and  20 B are spaced laterally apart, with their respective horizontal longitudinal axes inclined at ninety degrees with respect to one another, and plus and minus forty-five degrees, respectively to a laterally disposed axis between the two mechanisms. As shown in FIGS. 1 and 2, installation mechanisms  20 A and  20 B are preferably laterally slidably mounted on a laterally elongated, rectangular box-shaped support frame  11 . Thus, as shown in FIGS. 1 and 2, each installation mechanism  20 A,  20 B is slidably mounted on a plurality of laterally disposed, parallel, front and rear horizontal rails  12 R,  12 L forming part of support frame  11 . Each mechanism is securable at a desired adjustable lateral position on front and rear rails  12 F,  12 R by clamps  13 , e.g., toggle clamps. 
     As shown in FIGS. 1 and 2, each corner protector installation mechanism  20  includes a generally rectangular box-shaped enclosure  21  surmounted by a generally flat, upper base plate or work table  22 . As may be seen best by referring to FIG. 1, apparatus  20  includes an elongated, generally straight frame holder/clamping jig  23  mounted on the upper surface  24  of work table  22 . Preferably, clamping jig  23  is disposed obliquely, e.g., at 45 degrees, with respect to the rectangular plan-view shape of work table  22 . As shown in FIGS. 1,  2 ,  5  and  8 , clamping jig  23  is spaced above upper surface  24  of table  22 , and includes an elongated, horizontally disposed upper straight beam member  25 , supported at opposite lateral ends thereof by a pair of laterally opposed, left and right guide plates  26 L,  26 R, respectively. Guide plates  26  have flat and parallel lower and upper surfaces,  226  and  227 , these surfaces supporting clamp beam member  25  so that its lower surface  27  is parallel to and spaced above upper surface  24  of work table  22 . 
     As shown in FIGS. 1,  4 , and  5 , enclosure  21  of each mechanism  20  has parallel, vertically disposed left and right side panels  62  and  63 , which are perpendicular to front and rear vertically disposed panels  65 ,  64 . Front panel  65  is joined to left and right side panels  62 ,  63  by left and right vertically disposed transition panels  18 ,  19 . The latter are obliquely angled to front panel  65 , e.g., at 45 degrees. This construction enables pairs of mechanisms  20  to be oriented with longitudinal axes of the mechanism, defined by a horizontal line centered in work table  22  and perpendicular to front panel  65 , perpendicular to one another, with transition panels  19 ,  18  of mechanisms  20  spaced closely next to one another, as shown in FIG. 1, to accommodate narrow picture frames. The spacing between adjacent mechanisms  20 A and  20 B indicated in FIG. 5 can further be decreased to accommodate even narrower picture frames by reducing the length of guide plate  26 AR-BL, which serves as a common right-hand guide plate  26 R for mechanism  20 A, and left-hand guide plate  36 L for mechanism  20 B, to a value which results in right transition panel  19  of left-hand mechanism  20 A contacting left-hand transition panel  18  of right-hand mechanism  20 B. 
     As shown in FIGS. 1 and 6, left and right guide plates  26 L and  26 R have inner longitudinally disposed vertical wall surfaces  228 L and  228 R, respectively. If extended inwardly or rearwardly towards the center of work table  22 , inner vertical surfaces  228 L and  228 R of guide plates  26 L and  26 R would intersect to form a right angle corner. However, as shown in FIGS. 1-5, guide plates  26 L and  26 R are provided with obliquely disposed inner vertical transverse face walls  229 L and  229 R, respectively, that are coplanar, and angled at 45 degrees to inner vertical side walls  288 L and  228 R. Thus arranged, inner vertical side walls  228 L and  228 R form guide surfaces along which may be slid the outer side walls of side channel members of a rectangular picture frame that intersect each other at a ninety-degree corner. A laterally disposed space  230  between opposed inner corners  231 L and  231 R of guide plates  26 L and  26 R allows the corner of a picture frame to produce inwardly beyond inner face walls  229 L and  229 R of the guide plates. 
     As may be seen best by referring to FIGS. 1 and 8, clamping jig  23  includes a horizontally disposed, elongated straight clamp bar  28  located below and in vertical alignment with clamp beam member  25 . Clamp bar  28  is vertically movably held with respect to clamp beam member  25 . Thus, as shown in FIGS. 1 and 6, clamping jig  23  includes a linear actuator comprising a double acting pneumatic cylinder  30 , for raising and lowering clamp bar  28 . Clamp bar cylinder  30  is mounted on the upper surface  31  of clamping jig support beam  25 , and has a piston rod  32  which protrudes vertically downwards in a bore  33  provided through the thickness dimension of the support beam. The lower end of piston rod  32  has a square transverse sectional shape which protrudes through a similarly shaped bore through clamp bar  28 , and is secured to the clamp bar by a screw  29 . Since pneumatic cylinder  30  is of the type in which piston rod  32  is non-rotatable, clamp bar  28  is maintained in a fixed horizontal orientation when moved vertically with respect to work table  22 . 
     As shown in FIG. 1, clamp bar cylinder  30  has an upper pressurized “down” air inlet port  34  connected to a first “down” air inlet hose  35 , and a second, lower, “up” air inlet port  36  connected to a second, “up” air inlet hose  37 . Hoses  35  and  37  connect to tubular fittings  38  and  39  which protrude downwards through upper surface  24  of work table  22 , to control valves and a source of pressurized air, as will be discussed in detail below. When pressurized air is supplied to “up” inlet port  36  of clamp air cylinder  30 , pressurized air on the lower side of a piston within the cylinder forces the piston, attached piston rod  32 , and attached clamp bar  28  upwards to an upper, unlocked position, as shown in FIG.  1 . Conversely, when pressurized air is supplied to “down” inlet port  34  of clamp air cylinder  30 , pressurized air on the upper side of the piston within the cylinder forces the piston and clamp bar  28  downwards to a lower position in clamping contact with a picture frame A, as shown in FIG.  8 . 
     A preferred embodiment of mechanism  20  according to the present invention includes a feeder mechanism  60  for automatically feeding one corner protector preform  40  at a time to a pre-determined position beneath clamping bar  28  of clamping jig  23 , prior to inserting a frame to be protected into the clamping jig. The structure and function of feeder mechanism  60  may be best understood by first reviewing the construction of corner protector preforms  40  used with apparatus  20 . FIG. 3, shows a corner protector preform  40  of the type the apparatus  20  is intended to be used with. 
     As shown in FIG. 3, preform  40  is made of a thin sheet of readily bendable material, such as corrugated cardboard. Preform  40  has two laterally mirror symmetric, right-triangular shaped flaps  41  and  42 , respectively. In the orientation of preform  40  shown in FIG. 4, flap  41  is on the left-hand side of the figure, and will be referred to as the left-hand flap as a matter of convenience throughout the ensuing discussion. Left and right triangular flaps  41  and  42  have collinear horizontal bases  43  and  44  joined at their respective ninety degree corner angles to opposite vertical sides  45  and  46  of a vertically elongated rectangular spine flap  47 , which has a base  48  collinear with the bases of the triangles. Thus shaped, preform  40  has an appearance approximating that of an isosceles triangle having a horizontal base  43 - 48 - 44  and an upper vertex truncated by a horizontal edge  49  parallel to the base. 
     Referring still to FIG. 3, preform  40  may be seen to include a horizontally elongated rectangularly shaped lower cover flap  50  which depends downwardly from base  43  of left-hand triangular flap  41 , and has the same width as the flap. A trapezoidally-shaped securement flap  51  depends downwardly from the bottom edge wall  52  of lower rectangular cover flap  50 , the securement flap having an upper edge wall coextensive with bottom edge wall  52  of the rectangular cover flap, and a lower edge wall  53  parallel to but shorter than edge wall  52 . 
     In a preferred embodiment of corner protector preform  40 , the width of spine flap  47  and the height of lower cover flap  50  are each equal to a common value which is slightly greater than the thickness of a typical picture frame, e.g., about {fraction (15/16)} inch, for a frame thickness of ⅞ inch. As shown in FIG. 4, preform  40  is preferably scored along edge walls  45  and  46  of vertical spine flap  47 , and along edge walls  43  and  52  of lower rectangular cover flap  50 . These score lines are located along boundaries between adjacent flaps, and facilitate folding the flaps around a picture frame corner, as will be explained below. 
     Referring again to FIGS. 1 and 2, corner protector feeder mechanism  60  may be seen to include a longitudinally elongated, rectangularly shaped guide plate  61  having a generally flat upper surface mounted above and parallel to upper surface  24  of work table  22 . Guide plate  61  is oriented with its long axis parallel to left and right side walls  62  and  63  of enclosure  21 , and protrudes rearwardly beyond rear side wall  64  of the enclosure. As shown in FIGS. 1,  5  and  6 , guide plate  61  of feeder mechanism  60  has through its thickness dimension a perforation  66  having substantially the same outline shape as corner protector preform  40 . Guide plate  61  is spaced above upper surface  24  of table  22  by longitudinally disposed support ribs  72 , at a distance slightly greater than the thickness of corner protector preform  40 , e.g., {fraction (5/16)} inch for preforms having a thickness of about ¼ inch. Thus, a corner protector preform  40  placed conformally over perforation  66  will drop through the aperture onto work table surface  22 , allowing the preform to be slid into operating position under clamping jig  23 , as will be described in detail below. 
     In a preferred embodiment of apparatus  10 , as shown in FIGS. 1 and 2, each corner protector installation mechanism  20  includes a plurality of vertically disposed guide rods  67  which protrude perpendicularly upwards from upper surface  24  of work table  22 . Guide rods  67  define between tangents to the outer wall surfaces of inner facing sides of the rods a uniform transverse cross-section, vertically disposed bore  88  which is vertically aligned with guide plate perforation  66  and which is adapted to receive a vertical stack of preforms  40 . 
     Referring still to FIGS. 1 and 8, it may be seen that each corner protector installation mechanism  20  includes a mechanism for fastening corner protectors  40  onto a picture frame, such as an automatic stapler mechanism  91 , for stapling a corner protector  40  which has been folded around the corner of a picture frame. As shown in FIG. 1, stapler mechanism  91  includes a pneumatic stapler gun  92  such as a Josef Kohlberg, Model 680-6-46, which has a horizontally elongated body  93 , a front vertically oriented staple-inserting head  94 , and a rear air inlet port  95 . Stapler gun  92  has an internal pneumatic actuator mechanism (not shown) which causes a jaw  97  at the lower end of stapler gun head  94  to eject a staple forcibly outwards from the jaw into an object contacted by the jaw, when pressurized air is supplied through air supply hose  96  to air inlet port  95  of the stapler gun, manufactured by Josef Kohlberg. Box  126 , HJO, Sweden. 
     As may be seen best by referring to FIGS. 1 and 6, stapler mechanism  91  includes an adjustable support structure  98  for holding and supporting at an adjustable longitudinal distance, or throat depth, jaw  97  of stapler gun  92  relative to a picture frame held in clamp jig  23 . 
     Support structure  98  is mounted on the upper end of a compressor/elevator actuator mechanism  99  which lowers stapler gun  92  to force jaw  97  into compressive contact with a corner protector-covered picture frame corner to insert a staple therein, and raises the jaw to an upper rest position upon completion of the stapling operation. 
     As may be seen best by referring to FIGS. 1 and 2, compressor/elevator actuator  99  mechanism of stapler mechanism  91  includes a pair of vertically disposed, laterally spaced apart support rods  100  fastened at the upper ends thereof to the lower portion of stapler gun support structure  98 . Support rods  100  are vertically slidably supported in sleeve bearings  101  located below an elongated rectangular base plate  102 . Base plate  102  is fastened to a structural member  103  within enclosure  21 , and is located below a similarly-shaped aperture  104  through work table  22 . 
     Referring now to FIGS. 1-6, corner protector apparatus  20  may be seen to include a pair of corner flap folder actuators  120 L and  120 R located on the left and right sides, respectively, of a longitudinal center line of work table  22  of the apparatus. As shown in FIG. 1, flap folder actuators  120 L and  120 R are located longitudinally inwards of clamping jig  23 , in a location that places the actuators adjacent to the sides of a frame inserted inwardly under clamp bar  28  of clamping jig  23 . Flap folder actuators  120 L and  120 R are identical in structure and function. Therefore, in the ensuing description, the flap folder actuators will be referred to generally without a subscript. 
     As shown in FIGS. 1 and 11, each flap folder  120 , such as left-hand folder actuator  120 L, may be seen to include a flat, rectangularly-shaped flap folder arm  121  which fits into a complementarily shaped recess  119  provided in upper surface  24  of work table  22 , flush with the upper surface. Arm  121  is oriented with a long, inner longitudinal edge  122  thereof parallel and adjacent to a side of a picture frame protruding diagonally inwardly from beneath clamp bar  28  of clamping jig  23 . 
     Each flap holder arm  121  is mounted on the upper transversely disposed horizontal end all  123  of a vertically disposed, cylindrically-shaped support post  124 . Also, each corner protector installation mechanism  20  includes a linear actuator for elevating supporting post  124  along its longitudinal axis, to thereby elevate flap folder arm  121  attached to the upper end of the support post upwardly from its rest position recessed in upper surface  24  of work table  21 , and a rotary actuator for reciprocally rotating elevator support post  124  about its longitudinal axis, to thereby rotate the flap folder arm ninety degrees away from and back to its rest position, where the linear actuator retracts the flap folder arm into recess  119 . 
     Each corner protector installation mechanism  20  according to the present invention also includes sensor means which provides a signal indicating that a corner of a picture frame has been fully inserted into clamping jig  23 , with the moldings adjacent the corner parallel to and proximate inner vertical wall surfaces  228 R,  228 L of guide plates  26 R,  26 L. 
     Various types of proximity or optical sensors suitable for that purpose are well known to those skilled in the art. In an example embodiment of apparatus  10  tested by the present inventor, each sensor  160  consisted of a UZJ-111, fixed-focus type micro-photo sensor, manufactured by Matsushita Electric Works, Ltd., Automation Controls Group, 1048 Kadoma, Osaka 571, Japan. That sensor contains an infrared light emitting diode and phototransistor arranged within a circuit module with their optical axes generally parallel, and their respective exit and entrance pupils adjacent each other behind an infrared window comprising the outer face of the module. When a surface such as the outer surface of a picture frame molding is positioned within about 5 mm±2.5 mm of the outer face of the sensor, an output transistor within the sensor is turned on, providing a logic TRUE current through a load resistor connected in series with a collector supply voltage, and the emitter and collector output terminals of the sensor. 
     As shown in FIGS. 1 and 2, a pair of sensors  160  of the type described above with their outer faces installed in guide plates  26 R,  26 L, flush with the inner surfaces  228 R,  228 L of guide plates  26 R,  26 L, respectively, near the inner transverse vertical end walls  161 R,  161 L thereof, and near upper surface  24  of work table  22 . Each sensor  160  produces a logic TRUE signal when a frame molding is parallel to and a predetermined distance from inner surface  228 R,  228 L adjacent the outer face of the sensor. Moreover, apparatus  10  includes an electronic AND circuit  162  for logically ANDING signals from the output terminals of all four sensors  160  of left and right corner protector installation mechanisms  20 A and  20 B, e.g., sensors  160 AL,  160 AR,  160 BL,  160 BR. The output signal from the AND circuit is false when any of the 4 sensors  160  outputs a false signal, indicating that a picture frame molding near that sensor is not substantially close to and parallel to the sensor, such as shown in FIG.  6 . However, when the two adjacent corners of a frame have been fully inserted into and properly aligned with clamping jigs  23  of left and right corner protector installation mechanisms  20 A,  20 B of apparatus  10 , as shown in FIG. 7, the logical AND circuit produces a TRUE output signal which initiates simultaneous operation of each corner protector installation mechanism. Electronic circuitry for performing the foregoing functions is well known to those skilled in the art, and therefore need not be described in further detail here. For example, such circuitry can be implemented in a Programmable Logic Controller, PLC, or a general purpose microprocessor or micro controller. 
     OPERATION OF THE INVENTION 
     In response to a TRUE output signal from AND circuit  162  when all four sensors  160 AR,  160 AL,  160 BR,  160 BL produce TRUE output signals, Programmable Logic Controller (PLC)  113  issues in sequence drive signals to the various actuators of both corner protector installer mechanisms  20 A,  20 B of apparatus  10  according to the present invention, as follows: 
     1. Vertical stacks of corner protector preforms  40  are loaded into bores  68  between guide rods  67  of feeder mechanisms  60  of both left and right tandem installer mechanism  20 A,  20 B. 
     2. Power is turned on and PLC  113  resets and checks both mechanisms  20 A,  20 B of apparatus  10 , readying the PLC for outputting sequential drive signal commands to the various actuators of the apparatus. A program for the operational sequence of apparatus  10  is stored in electronic memory means in PLC  113 . This program can be executed via an internal or external computer. 
     3. Actuator cylinders  82  of feeder mechanism  60  in each installer mechanism  20 A,  20 B are actuated to reciprocate piston rod  84 , thereby moving an individual corrugated corner protector blank preform  40  forward into position below clamp bar  28  of each installer mechanism. (FIG. 5) 
     4. Adjacent corners C 1 , C 2  of a picture frame A are inserted into position beneath clamp bars  28  and over blanks  40  of left and right installer mechanisms  20 A,  20 B, respectively. (FIG. 6) 
     5. Frame A is positioned relative to apparatus  10  so that corners C 1 , C 2  of the frame are fully inserted into clamping jigs  23  of mechanisms  20 A,  20 B, respectively. (FIG. 7) 
     6. Sensors  160 AL,  160 AR,  160 BL,  160 BR of installer mechanisms  20 A,  20 B output TRUE signals, causing AND circuit  162  to output a TRUE signal, in turn causing PLC  113  to simultaneously issue drive signals to both installer mechanisms, as follows: (FIG. 7) 
     7. Hold down clamp bars  28  of clamping jigs  23  of mechanisms  20 A,  20 B are forced downward onto the channels of frame A defining corners C 1 , C 2 , respectively. (FIG. 8) 
     8. Blank preforms  40  under each corner C 1 , C 2  of frame A are automatically manipulated as follows: First, rectangular cover flap  50  and securement flap  51  of each preform  40  are bent inwardly in unison by elevating left-hand flap folder arm  121 L of each mechanism  20 A,  20 B from its recess  119 L, with the flap folder arm in its clockwise position. (FIG. 9) 
     9. Securement flap  51  of each preform  40  is bent inwardly and downwardly over a left frame channel by rotating left-hand flap folder arm  121 L counterclockwise. (FIG. 10) 
     10. Right-hand triangular cover flap  42  and spine flap  47  of each preform  40  are bent as a unit into a vertically disposed position by extending right-hand flap folder arm  142 R upwardly from recess  119 R. (FIG. 11) 
     11. Cover flap  42  of each preform  40  is bent inwardly and downwardly over a right-hand frame channel by rotating flap folder arm  124 R ninety degrees clockwise. (FIG. 11) 
     12. Left flap folder arm  121 L is rotated ninety degrees clockwise to its angular rest position, and retracted downwardly into recess  119 L to its vertical rest position. (FIG. 12) 
     13. Staple gun  92  of each mechanism  20 A,  20 B is lowered toward frame A. Each staple gun  92  has an automatic firing trigger that discharges a staple through cover flap  42 , securement flap  51 , and frame A, upon compressive contact of the staple gun with the upper surface of cover flap  42 . (FIG. 13) 
     14. Right flap folder arm  121 R of each mechanism  20 A,  20 B is rotated ninety degrees couterclockwise to its angular rest position. (FIG. 12) 
     15. Right flap folder arm  121 R of each mechanism  20 A,  20 B is retracted downwardly into recess  119 R to its vertical rest position. (FIG. 11) 
     16. Hold down clamp bars  28  of clamping jigs  23  of each mechanism  20 A,  20 B are retracted vertically upwards to their rest positions, unclamping corners C 1 , C 2  of frame A. 
     17. Frame A is retracted from apparatus  10 , disengaging corners C 1 , C 2  from clamping jig  23 . 
     18. Frame A is rotated one hundred and eighty degrees in a horizontal plane, and steps 4-16 repeated to attach corner protectors  40  to the remaining two corners C 3 , C 4  of frame A. 
     19. Step 17 is repeated.