Abstract:
A stackable modular cabinet having modular, interlocking side units which allow cabinet dimensions to be tailored both vertically and laterally to user needs while also providing the strength and stability to support heavy equipment such as electronic subassemblies. Each side unit has a pair of mating flanges with apertures for receiving and retaining fastening pins that interlock to the fastening pins of a different side unit when the respective flanges of the two side units mate. The interlocked fastening pin structure forms unified rods which extend the full height of the cabinet and which cooperates with the intervening side unit flanges. Each side unit further has re-enforced, double-sided bracket structures at opposing ends which, in combination with the pin structure, provides an unusual amount of strength to the modular cabinet.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a modular cabinet design having stackable modules that provide both the rack-mount support and aesthetically pleasing exterior for electrical equipment; and, more particularly, relates to a break-down cabinet design which can be constructed in incremental heights and depths as dictated by the number of stackable modules and the size of the equipment being housed. 
     2. Description of the Prior Art 
     Most electrical equipment contains individual subsystems that interconnect to one another to form the overall system. In particular, mid-size and large-scale computing systems often contain subsystems such as instruction processors, memory controllers, memory modules, input/output units, and power and cooling subsystems. Often these subsystems can be packaged together in different configurations to satisfy differing system and performance goals and requirements. 
     In large-scale and mid-size computer systems, individual subsystems are typically mounted within strong, heavy, rack-mount cabinets. These rack-mount structures must generally satisfy the mounting requirements of the Electronic Industry Association (EIA) standard RS-310. In addition, the cabinet structures must be capable of supporting a large amount of weight while withstanding torquing forces such as those imposed by earthquakes. 
     Subsystems installed within rack-mount cabinets are interconnected to form an overall system. Then metal covers called &#34;skins&#34; are installed over the rack-mount structures to enclose the system for safety and to provide an aesthetically-pleasing appearance. 
     Typically, the subsystems of a computing system can be packaged together in several combinations, often referred to as configurations. This allows systems to be tailored to individual user needs. A low-end system, for example, could be configured to include only one instruction processor and a small amount of memory, whereas another system may contain multiple instruction processors and a large memory. This flexibility allows users to expand their systems as their computing needs grow. 
     In the past, rack-mount cabinetry was available in only a limited number of sizes. Users desiring systems that did not conform to one of the available cabinet sizes were forced to purchase cabinets that were too large. In addition, users desiring system upgrades were forced to purchase entirely new cabinets if existing space did not allow for additional subsystems or modules. As a result, users of low-end machines or those anticipating upgrades were often forced to purchase cabinets larger than necessary, adding unneeded size and expense to their systems. In addition to the cabinet cost, this often required use of a larger area of floor support, thereby increasing the facility cost. 
     Manufacturers of electronic and computer systems have long been searching for a more adaptable cabinet design. Nilsson, U.S. Pat. No. 4,754,369, teaches a lightweight, stiff structure containing removable box-section end walls which are fastened to the cleats of a base plate. Although the knock-down walls make the system much more transportable, the cabinet dimensions can not be varied. Taylor, U.S. Pat. No. 4,836,626, teaches a knockdown cabinet design which can be assembled without the use of tools and with only a minimum number of plastic plugs. Like Nilsson, however, the cabinet can not be incrementally expanded. Cutright, U.S. Pat. No. 5,250,752, teaches a knockdown shielded electronic rack enclosure incorporating the use of squaring corner brackets for joining adjacent vertical side wall frames to the horizontal top and bottom frame components. These corner joints prevent torquing and provide additional stability. Although the structure provides improved rigidity, the design can not be expanded. 
     Expandable design structures have been disclosed for uses other than those associated with electronics equipment. Teranishi, U.S. Pat. No. 3,854,783, teaches a modular shelf structure which can be easily assembled and disassembled, and which can be expanded both vertically and horizontally. The Teranishi design receives much of its structural support through the use of block modules, each formed by a bottom panel, two side panels, and an interlocking top panel. While providing expandability and rigidity, this block module structure including the integrated bottom panel is not well suited to housing electronic equipment, the components of which are operationally interconnected, such as by cables, or otherwise fitted together. In the past, expandable structures formed of just expandable side panels without the intervening horizontal panels were considered much too unstable to support the weight associated with electronic equipment. 
     OBJECTS 
     It is a primary objective of this invention to provide an improved cabinet for housing electrical equipment. 
     It is a further object of this invention to provide a modular cabinet structure comprised of individual modules which can be stacked to produce enclosures of variable heights and depths. 
     Yet another object of the invention is to provide a modular cabinet structure comprised of stackable modules that can be interlocked to provide structural strength. 
     Still another object of the invention is to provide expandable modules for use in a modular cabinet. 
     Another object of the invention is to provide an interlocking mechanism for interlocking adjoining modules and providing structural strength for a modular cabinet. 
     It is still a further object of this invention to provide a stackable modular cabinet structure having the strength to support large electrical subsystems. 
     It is another object of this invention to provide a stackable modular cabinet structure conforming to the Electronic Industry Association (EIA) standard RS-310 requirements. 
     It is yet another object of this invention to combine the dual functionality of support and aesthetics into a single structure which thereby replaces both the rack mount cabinet and the skin coverings. 
     It is another object of this invention to provide a structure which can be easily and quickly constructed. 
     It is still another object of this invention to provide a structure which can be easily and quickly expanded with a minimum of system disturbance. 
     It is yet a further object of this invention to provide a cost-effective structure for housing electrical equipment. 
     Without departing from the spirit and scope of the invention, other more detailed objectives will become apparent to those skilled in the art from a consideration of the Drawings and the Detailed Description of the Preferred Embodiment. 
     SUMMARY OF THE INVENTION 
     The stackable modular cabinet disclosed herein provides a cabinet design which can be easily and quickly expanded vertically by adding modular units. In addition to providing modularity, the design exhibits strength and stability through the use of a unique module interconnection system, and a double-sided, reinforced corner design. The constructed cabinet is able to support a large amount of weight while resisting torquing forces. Moreover, the cabinet provides a cost-effective alternative to the frame-and-skins combination traditionally used to house electronic equipment. The modular side units, which are formed of a strong, light-weight material such as commercially-available cold-rolled steel, provide both the support structure and the aesthetically-pleasing coverings for the electronic subassemblies housed therein. 
     Finally, the design allows for easy lateral expansion to accommodate larger subassemblies or large cabling requirements. 
     The stackable modular cabinet consists of a base unit, one or more stackable modules stacked upon the base unit, and a cover unit. Each of the stacked modules are generally comprised of two side units, a front fascia panel, a back panel, and two optional side extension units. 
     The current modular cabinet design overcomes deficiencies in prior designs and is able to provide the needed strength through a unique pin interlocking mechanism which attaches the modular side units at selected locations, such as at the corners. 
     Mating pins having interlocking structures at each end form the basis of this interlocking mechanism. 
     The pin receiving structures are formed by cabinet support members at two opposite edges of the modular side units. In the preferred embodiment, these cabinet support members are flanges formed to provide upper and lower mating shelves. These flanges contain pin-retaining apertures into which the mating pins are inserted and retained. Side units are then interconnected by mating the flanges from first and second side units so that the pins from the first side unit may be interlocked to those of the second side unit. 
     The resulting interlocking pin structure provides strength and stability in two dimensions. When multiple side units are interconnected, the interlocked pins form unified steel rods which further interlock with the base unit and extend the full combined height of the cabinet at every corner. These rods further incorporate and interlock with the side unit flanges running approximately perpendicular to the interconnected pins The resulting support structure formed by the interlocked side units therefore consists of steel rods and intervening flanges positioned into a rigid grid structure. 
     Strength is further added by the unique side unit corner design. The side units have channels at the edges adjacent to the flanges to form double-sided end coupling structures, which in the preferred embodiment are brackets. These bracket structures are further mounted to re-enforcing stiffening members which interlock with the side unit flanges to strengthen the side units at the corner edges. In the preferred embodiment, these stiffening members are re-enforcing support joints. 
     The cabinet contains rack structures mounted to the re-enforcing support joints which provide the support for the enclosed electronic subassemblies. Although the cabinet could be used to house subassemblies without actually attaching the equipment to the cabinet structure, interlocking equipment to the cabinet frame provides an additional measure of stability. When a subassembly is in place, the corner bracket device-mounting apertures can be used to lock the subassembly into position between the two side units. By attaching the side units to the subassembly at all corners, the corner rods and side flanges are further locked into positioned with respect to one another. A cover unit and a base support unit may be interlocked with the opposing side units of the top and bottom subassembly modules, respectively, thereby even further locking the two side structures into a fixed alignment with one another. 
     The depth of any module within the stackable modular cabinet may be easily expanded by adding side extension units. The side extension units have bracket structures similar to those of the side units, and to which the displaced back panel may be remounted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a stackable modular cabinet that includes extension modules; 
     FIG. 2 is an exploded perspective view of interlocking fastening pins; 
     FIG. 3 is a cutaway perspective view of a side unit and shows a fastening pin inserted in the side unit pin holes; 
     FIG. 4 is cutaway perspective view of a side unit bracket attached to a rack-mount joint; 
     FIG. 5 is an exploded perspective view of first and second side units, and first and second side extension units, and shows how the side units attach to one another and to respective side extension units; 
     FIG. 6 is a cutaway close-up perspective view of two fastening pins interlocked to attach a first side unit to a second side unit; 
     FIG. 7 is a perspective view of a rack-mount joint; 
     FIG. 8 is a perspective view of a modular side unit including a support rack; 
     FIG. 9 is a perspective view of a subassembly supported on a support rack and attached to a side unit; and 
     FIG. 10 is an exploded perspective view of two side extension units as they attach to a back panel 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is an exploded perspective view of a stackable modular cabinet that includes side extension modules. Two stackable modules 10 and 12 are situated between a base unit 14, and a cover unit 16. Module 12 attaches to base unit 14, module 10 attaches to module 12, and cover unit 16 attaches to module 10 through a structural interlocking mechanism that will be described in more detail below. 
     Although only two stackable modules 10 and 12 are shown, it is understood that more modules may be included. The modules may be of like dimensions, or they may be of varying heights. In the preferred embodiment, the height of the modules varies in fixed increments of 1.750 inches, which is an increment well-known in the electronics cabinet industry as 1 &#34;Unit&#34; or 1 &#34;U&#34;. In FIG. 1, module 10 illustratively has a height h1 of 3 Us, whereas module 12 illustratively has a height h2 of 9 Us. It is of course understood that more or fewer modules can be utilized, and the each module can be selected to its required height. 
     Each stackable module contains similar structural components and is constructed in a similar manner. The module components are formed of a strong, light-weight, material such as commercially-available cold-rolled steel. Module components include two side units, a front fascia panel, and a back panel. In addition, each module may optionally include side extension units to increase cabinet dimensions. The stackable modules 10 and 12 are shown connected to associated extension units, though it should be understood that extensions are not required in all cases. 
     Module 10 has a side unit 18a attached to side extension unit 22a (shown dashed), and a side unit 20a attached to side extension unit 24a. The attachment of the side units to the side extension units is described in detail below. Side unit 18a interlocks to two fastening pins 56a (shown dashed) and 58a, and side unit 20a interlocks to two fastening pins 52a and 54a. Fastening pins 52a, 54a, 56a and 58a provide the means of interlocking module 10 to module 12. The means of interlocking fastening pins to a side unit to thereby attach one module to another module will be described in detail below. 
     Module 10 also has a front fascia panel 26a and a back panel 28a. The front fascia panel 26a mates with side units 18a and 20a. The attachment of the front fascia panels to the side units will be described in detail below. Back panel 28a is connected at each end to side extensions units 22a and 24a, respectively. If the optional extension units were not employed in this implementation, back panel 28a would be connected at each end to side units 18a and 20a, respectively. The attachment of the back panels to the side extension units, or alternately to the side units, is described in detail below. Finally, module 10 is shown housing electronic subassembly 30a. The attachment of an electronic subassembly to a side unit is described below. 
     Module 12 has two side units 18b and 20b. Side unit 18b has a mating surface 38, and side unit 20b has a intermediate mating surface 40. Side unit 18b interlocks to two fastening pins 56b and 58b, and side unit 20b interlocks to two fastening pins 52b and 54b. Fastening pins 52b, 54b, 56b and 58b provide the means of interlocking module 12 to base unit 14 and of further interlocking module 12 to module 10. The means of interlocking fastening pins to a side unit to thereby attach one module to another module will be described in detail below. 
     Module 12 further has side extension units 22b and 24b. Side unit 18b is attached to side extension unit 22b, and side unit 20b is attached to side extension unit 24b. The attachment of the side units to the side extension units is discussed in detail below. Module 12 houses an electronic subassembly 30b, which has a front mating surface 31. 
     Module 12 also has a front fascia panel 26b. Front fascia panels 26a and 26b provide aesthetic appeal to the stackable modules 10 and 12, respectively. The module 10 front fascia panel 26a is shown installed, whereas front fascia panel 26b is shown disconnected from module 12. 
     Front fascia panel 26b has a unit mating surface 32, a first tab surface 34, and a second tab surface 36. First tab surface 34 has two spring-loaded tabs 34a and 34b, and second tab surface has two spring-loaded tabs 36a and 36b. Spring-loaded tabs 34a, 34b, 36a, and 36b can be depressed to allow the unit mating surface 32 to mate with the front mating surface 31 of electronic subassembly 30b. When the spring-loaded tabs are then released, tabs 34a and 34b mate with and exert pressure against intermediate mating surface 40 of side unit 20b. Likewise, tabs 36a and 36b mate with and exert pressure against intermediate mating surface 38 of side unit 18b. The pressure exerted by the tabs holds the front fascia panel 26b to module 12, while allowing easy removal if desired. 
     Although the foregoing discussion describes the front fascia panel 26b of module 12, it is understood that all front fascia panels have the same attributes and attach to their respective modules in the manner described above. Finally, it should be mentioned that front fascia panels merely provide aesthetic appeal, not structural support, to the cabinet. It is understood the cabinet could be constructed without these additional components. 
     Module 12 further has a back panel 28b, which is connected at each end to side extensions units 22b and 24b, respectively. If the optional extension units were not employed in this implementation, back panel 28b would be connected to side units 18b and 20b. The attachment of the back panels to the side extension units is described in detail below. 
     The base unit 14 of the preferred embodiment is a support structure formed of a strong, sturdy material such as commercially-available cold-rolled steel. Base unit 14 has a floor structure 42 having cut-away portions 42a and 42b which allow cables or other interconnecting devices (not shown) to be connected to the electronic subassemblies 30a and 30b. The floor structure 42 is attached to four side walls 44, 46, 48, and 50. Side walls 46 and 50 are formed with support flanges 46a and 50a, respectively. Flange 46a has base holes 52 and 54 for receiving the fastening pins 52b and 54b, respectively, which attach side unit 20b to base unit 14. Likewise, flange 50a has base holes 56 and 58 for receiving the fastening pins 56b and 58b, respectively, which attach side unit 18b to base unit 14. The attachment of side units 18b and 20b to the base unit 14 will be described below. 
     In the preferred embodiment, the base unit 14 is supported by casters at the corners, illustratively shown as 60a and 60b, which are attached to the floor structure 42 to aid in cabinet mobility. Retractable stabilizing structures at the corners, illustratively shown as 62a, 62b and 62c, are used after the cabinet has been positioned to prevent inadvertent cabinet motion. It is understood that the floor structure 42 could be supported by any support structures, or alternately could rest directly on the floor. 
     As mentioned above, stackable module 12 is attached to base unit 14 with four fastening pins 52b, 54b, 56b, and 58b. Each of these fastening pins is interlocked to a respective side unit, and further inserted into a respective base hole. Fastening pins 52b and 54b interlock to side unit 20b and are further inserted into base holes 52 and 54, respectively. Fastening pins 52b and 54b are secured to flange 46a by securing devices (such as nuts) 52c and 54c, respectively. Likewise, fastening pins 56b and 58b interlock to side unit 18b and are further inserted into base holes 56 and 58, respectively. Fastening pins 56b and 58b are secured to flange 50a by securing devices 56c and 58c, respectively. The fastening pin structure, as well as the mechanism which interlocks the fastening pins to a respective side unit, will be described below. 
     Stackable module 10 is attached to stackable module 12 with four fastening pins 52a, 54a, 56a, and 58a which interlock with fastening pins 52b, 54b, 56b, and 58b, respectively. Fastening pins 52a and 54a interlock to side unit 20a and further interlock to the side unit 20b fastening pins 52b and 54b, respectively. Fastening pins 56a and 58a interlock to side unit 18a and further interlock to the side unit 18b fastening pins 56b and 58b, respectively. The fastening pin interlocking mechanism is described in detail below. 
     Cover unit 16 has a cover mating surface 64. Cover mating surface 64 has fastening pegs 66, 68, 70 and 72 for positioning and fastening cover unit 16 to stackable module 10. Fastening pegs 66 and 68 interlock with the side unit 20a cover holes 66a and 68a, respectively. Likewise, fastening pegs 70 and 72 interlock with the side unit 18a cover holes 70a and 72a, respectively. Although cover unit 16 includes pegs integrally formed thereto, it is understood that cover 16 could be fastened to a stackable module using any commercially available fastening device, including screws, which would interlock with the side unit cover holes. 
     Extension cover 74 provides the top surface for the optional extension module formed by side extension units 22a, 24a, 22b, and 24b. Extension cover 74 has a mating surface 75 for attaching on each side to side extension units 22a (shown dashed) and 24a, respectively. 
     The FIG. 1 stackable modules 10 and 12 each contain two side units of like height. For example, module 12 contains side units 18b and 20b, and each side unit is of a height h2 illustratively shown as 9 Us. It should be understood that one or both sides of module 12 could consist of multiple side units with a combined height of 9 Us interlocked with fastening pins to form a single side unit. For example, one or both sides of the module 12 could consist of three side units, each having a height h1 of 3 Us. The combined height of the three modules after being interlocked as described below would be 9 Us. 
     Furthermore, the stackable module cabinet of FIG. 1 is shown housing electronic subassemblies 30a and 30b. It is understood, however, that the stackable module cabinet disclosed herein is suitable for housing other types of equipment. 
     Finally, it should be reiterated with respect to FIG. 1, that although one stackable module is of the same construction as, and contains components that are similar to, any other stackable module, the components within a stackable module do vary with respect to height. These height variations allow each module to be constructed according to the size of the electronic subassembly to be enclosed therein. 
     FIG. 2 is an exploded perspective view of fastening pin 56a as it interlocks with fastening pin 56b. All fastening pins in a stackable modular cabinet are of similar construction, though they may be of different lengths. It is therefore understood that the following description applies to any and all fastening pins in a stackable modular cabinet. 
     Each fastening pin has a predetermined length, which is a multiple of a predetermined increment. In the preferred embodiment, the increment is 1 U. As mentioned above, the length of a fastening pin is determined by the height of the side unit with which it is associated. Fastening pin 56a has a length h1 of approximately 3 Us, whereas fastening pin 56b has a length h2 of approximately 9 Us. 
     Fastening pin 56b in the preferred embodiment is of a generally cylindrical construction. Fastening pin 56b has a receiving end 76, and an insertion end 77. Receiving end 76 has a bored and tapped receiving channel 78, and insertion end 77 has a bored and tapped insertion channel 79. Fastening pin 56b further has a threaded peg 80 inserted into tapped insertion channel 79, and which extends beyond the insertion end of fastening pin 56b. For illustrative purposes, peg 80 is shown as a separate component interlocked with tapped insertion channel 79, but it is understood peg 80 could be integrally formed and threaded as part of insertion end 77. 
     The threaded peg 80 of any fastening pin is designed to interlock with the tapped receiving channel 78 of a different associated fastening pin. As shown, peg 80 of fastening pin 56a is designed to interlock with the receiving channel 78 of fastening pin 56b. The receiving end 76 of any fastening pin further has an insertion slot 81 for receiving an insertion tool, illustratively shown in FIG. 2 as screw driver 82. The insertion tool is used to aid insertion of peg 80 of fastening pin 56a into the receiving channel 78 of fastening pin 56b. Pins 56a and 56b are illustrated as being interlocked by a threaded interlocking structure. It should be understood that other interlocking mechanisms could be used provided that the resultant interlocked pin structure is substantially rigid when in place. 
     FIG. 3 is a cutaway perspective view of side unit 18b and shows a fastening pin 58b inserted in side unit pin holes 89a and 89b. Side unit 18b has an interior surface 85, and two cabinet support members which in the preferred embodiment consist of an upper mating flange 86 and a lower mating flange 87. Both the upper mating flange 86 and the lower mating flange 87 are bent at a predetermined angle to the interior surface 85 to form upper and lower mating shelves. In the preferred embodiment, this angle is approximately 90 degrees. Upper mating flange 86 has a top mating surface 88, two upper pin holes 89a and 90a, two positioning guides 92 and 93, two cover holes 72b and 70b, and two top threaded attachment holes 94a and 96a. Lower mating flange 87 has a top support surface 98, a bottom mating surface 100, two lower pin holes 89b (shown dashed) and 90b, two bottom attachment through holes 94b and 96b, and two positioning holes 92b and 93b. 
     Fastening pin 58b is inserted into upper pin hole 89a so that insertion end 77 mates with support surface 98 and so that peg 80 extends through lower pin hole 89b. When fastening pin 58b is inserted into side unit 18b, receiving end 76 is approximately flush with top mating surface 88. Fastening pin 56b (not shown in FIG. 3) is mounted in a similar manner. While the above discussion refers to side unit 18b and the insertion of pin 58b into associated pin holes of side unit 18b, it is understood that this description applies to any side unit and to any fastening pin interlocked with a side unit. 
     FIG. 4 is a cutaway perspective view of a side unit bracket attached to a rack-mount joint. Although the following discussion uses side unit 18b and front bracket 104 for explanative purposes, the description applies to any bracket of any side unit or side extension unit in the stackable modular cabinet. Side unit 18b has a front edge 102 coupled to a first end coupling structure. In the preferred embodiment, the first end coupling structure is front bracket 104, which is integrally formed to front edge 102, although it is understood that front bracket 104 could be a separately formed component attached to side unit 18b with commercially available fasteners. In the preferred embodiment, front bracket 104 has a first transitional portion 106 which consists of an arcuate bend of approximately 90 degrees formed towards interior surface 85. Front bracket 104 further has an intermediate portion 108 which in the preferred embodiment is approximately parallel to interior surface 85. Intermediate portion 108 has a mating surface 38 for mating with second tab surface 36 of front fascia panel 26b, as explained above. Intermediate portion 108 further has a rack-mount joint mating surface 110, and two mounting holes 112a and 113a. Finally, front bracket 104 has a terminating portion 114, which in the preferred embodiment is at an approximate right angle from intermediate portion 108, and which has a plurality of device mounting holes 116. In the preferred embodiment, the device mounting holes 116 are positioned to be consistent with the Electronic Industries Association (EIA) Standard RS-310. 
     Front bracket 104 is attached to a stiffening member, which in the preferred embodiment is a rack-mount joint 118 that is fitted between upper mating flange 86 and lower mating flange 87. Rack-mount joint 118 has a bracket mating surface 120, and a rack support surface 122. Bracket mating surface 120 mates with rack-mount joint mating surface 110, thereby providing added strength and stability to front bracket 104. Bracket mating surface 120 further has two bracket mating holes 112b and 113b (shown dashed). Rack-mount joint 118 is attached to front bracket 104 by inserting fastener 112 through mounting hole 112a and through bracket mating hole 112b, and by further inserting fastener 113 through mounting hole 113a and through bracket mating hole 113b. In the preferred embodiment, fasteners 112 and 113 are screws, but other fastening mechanisms could be utilized. 
     Rack support surface 122 is at a predetermined angle from bracket mating surface 120, which in the preferred embodiment is approximately 90 degrees. Rack support surface 122 has a plurality of rack-mount slots 128 (shown dashed.) 
     FIG. 5 is an exploded perspective view of two side units and two corresponding side extension units, and shows how a side unit attaches to another side unit and to its corresponding side extension unit. Although side units 18a and 18b and the corresponding side extension units 22a and 22b are used for explanative purposes, it is understood that the following detailed description also applies to the attachment of any side unit to another side unit or to its corresponding side extension unit. 
     FIG. 5 further shows fastening pins inserted into the corresponding pin holes of the corresponding side units. Fastening pin 56a is inserted into the upper and lower pin holes in flanges 86 and 87, respectively, of side unit 18a so that threaded peg 80 of fastening pin 56a extends through flange 87. Fastening pin 58a is inserted in a like manner in side unit 18a. Fastening pins 56b and 58b are likewise inserted into pin holes of side unit 18b. 
     Side unit 18a is aligned with side unit 18b by fitting positioning extensions 92 and 93 of side unit 18b into positioning holes 92b and 93b, respectively, of side unit 18a so that top mating surface 88 of side unit 18b mates with bottom mating surface 100 of side unit 18a. To maintain this positioning, fastener 94 is inserted through bottom attachment hole 94b of side unit 18a, further inserted through top attachment hole 94a of side unit 18b, and secured by securing device 94c. Likewise, fastener 96 is inserted through bottom attachment hole 96b of side unit 18a, further inserted through top attachment hole 96a of side unit 18b, and secured by securing device 96c. In the preferred embodiment, fasteners 94 and 96 are screws. 
     After side unit 18a is positioned next to side unit 18b, fastening pins 56a and 58a may be interlocked with fastening pins 56b and 58b, respectively. As described above, peg 80 of fastening pin 56a is inserted into the receiving channel 78 of fastening pin 56b so that threaded peg 80 is screwed into and interlocked with tapped receiving channel 78, and so that the receiving end 76 of fastening pin 56b mates with bottom mating surface 100 of side unit 18a. An insertion tool may be used with insertion slot 81 of fastening pin 56a to aid in this interlocking process. Likewise, fastening pins 58a and 58b are interlocked in a similar manner. 
     It is understood that threaded pegs 80 of fastening pins 56b and 58b, respectively, could be received by the receiving channels of corresponding fastening pins inserted in an additional side unit, if bottom mating surface 100 of side unit 18b were mating with the additional side unit of yet another module. However, in the cabinet of FIG. 1, bottom mating surface 100 of side unit 18b mates with flange 50a of base unit 14. Therefore, pegs 80 of fastening pins 56b and 58b, respectively, are received by base holes 56 and 58, respectively, and secured by securing devices 56c and 58c, respectively, to fasten side unit 18b to base unit 14. 
     It is further understood that receiving channels 78 of fastening pins 56a and 58a, respectively, could receive the pegs 80 of corresponding fastening pins inserted in yet another additional side un it if top mating surface 88 of side unit 18a were mating with an additional module. However, top mating surface 88 of side unit 18a mates with cover mating surface 64, as shown in FIG. 1. Therefore receiving ends 76 of fastening pins 56a and 58a, respectively, also mate with cover mating surface 64. 
     It may be further noted that cover holes 70b and 72b of side unit 18b are unused, since top mating surface 88 of side unit 18b mates with bottom mating surface 100 of side unit 18a. However, if module 10 were removed from the cabinet of FIG. 1 so that top mating surface 88 of side unit 18b mated with cover mating surface 64, cover holes 70b and 72b would receive fastening pegs 70 and 72 respectively. 
     Side panel 18b farther has a back edge 130 coupled to a second end coupling structure. In the preferred embodiment, the second end coupling structure is back bracket 132, which is integrally formed to back edge 130, although it is understood that back bracket 132 could be a separate component attached to side panel 18b instead of being integrally formed thereto. Back bracket 132 has all attributes described above in relation to FIG. 4. 
     Side extension units 22a and 22b, which are optionally attached to respective side units 18a and 18b, are available to increase cabinet capacity. Although the following discussion illustratively uses side extension unit 22b, it is understood the description applies to any side extension unit. 
     Side extension unit 22b has a bracket mating surface 134 for mating with the mating surface 38 of back bracket 132. Bracket mating surface 134 further has two back bracket mating holes 136a and 138a for receiving fasteners 136 and 138, respectively. Fasteners 136 and 138 are further inserted into mounting holes 112a and 113a, respectively, thereby attaching side extension unit 22b to side unit 18b. 
     Side extension unit 22b has an extension edge 140 integrally formed to an extension bracket 142, although it is understood that extension bracket 142 could be a separate component attached to side panel 22b rather than being integrally formed therewith. Extension bracket 142 has all attributes of the bracket described above in relation to FIG. 4. 
     Side extension unit 22b further has a top extension edge 144, and a bottom extension edge 146. When side extension unit 22a is attached to side unit 18a, side extension unit 22b is attached to side unit 18b, and side unit 18a is attached to side unit 18b, top extension edge 144 of side unit 18b is aligned with bottom extension edge 146 of side unit 18a. This relationship is maintained by spacer 148. Spacer 148 has a top spacer groove 148a for receiving bottom extension edge 146 of side extension unit 22a, and further has a bottom spacer groove 148b for receiving top extension edge 144 of side unit 18b. A similar spacer is inserted between every stacked pair of side extension units in the stackable modular cabinet of FIG. 1 to provide alignment and added stability to the extension structure. 
     FIG. 6 is a cutaway close-up perspective view of fastening pins 58a and 58b interlocked to attach side units 18a and 18b. Insertion end 77 of fastening pin 58a mates with bottom support surface 98 of side 18a. Peg 80 of fastening pin 58a extends through lower pin hole 89b of side unit 18a, extends further through upper pin hole 89a of side unit 18b, and interlocks with tapped receiving channel 78 of fastening pin 58b. Receiving end 76 of fastening pin 58b mates with bottom mating surface 100 of side unit 18a. 
     The fastening pins are integral to the corner structure design of the stackable modular cabinet. When interlocked as shown in FIG. 6, fastening pins form four unified corner structures that extend from base unit 14 to the cover unit 16 at the corners of the cabinet. These structures interconnect adjacent modules one with another. Moreover, these unique interlocking pin structures incorporate intervening perpendicular structures created by the mating side unit flanges. The two pin structures associated with any given side unit are therefore locked into a grid-like side structure that provides strength and stability in two dimensions. When the two side structures of a cabinet are further attached to electronic subassembly frames, as will be discussed below, the subassemblies lock the two grid-like side structures into a relationship with one another. This adds strength and stability in yet another dimension. Therefore, the unique pin structure ultimately forms the basis of a very strong three-dimensional grid which is lightweight and expandable, while still being capable of withstanding large torquing forces and supporting very heavy equipment. 
     FIG. 7 is a perspective view of a rack-mount joint 118. As described above, the joint fits between upper mating flange 86 and lower mating flange 87 of an associated side unit. Rack-mount joint 118 has bracket mating surface 120, and a rack support member 122 that is at a predetermined angle from bracket mating surface 120. In the preferred embodiment, the predetermined angle is approximately 90 degrees. Rack support member 122 has a plurality of rack-mount slots 128 described further below. 
     FIG. 8 is a perspective view of modular side unit 18b including a support rack 160. Support rack 160 has a device (subassembly) support surface 162 and a side guide surface 164. Side guide surface 164 has rack-joint surface 166 for mating with rack support member 122. Rack-joint surface 166 is at a predetermined angle from side guide surface 164 which in the preferred embodiment is approximately 90 degrees. Support rack 160 further has insertion prongs 168a and 168b formed at a predetermined angle to rack-joint surface 166, which in the preferred embodiment is 90 degrees. Each insertion prong 168a and 168b is for inserting into a selected respective one of rack-mount slots 128. Support rack 160 may be mounted to side unit 18b in varying relationships depending on the rack-mount slots 128 which are selected to receive the respective insertion prongs 168a and 168b. 
     Support rack 160 further has a support bracket 172 which is integrally formed thereto at a predetermined angle, which in the preferred embodiment is 90 degrees. Support bracket 172 has a mating hole 174a. Support bracket 172 mates with terminating portion 114 of back bracket 132 of side unit 18b, and is attached thereto with fastener 174. Fastener 174 is inserted through mating hole 174a, and further inserted through a respective one of device mounting holes 116 of back bracket 132. In the preferred embodiment, fastener 174 is a screw. 
     FIG. 9 is a perspective view of subassembly 30b supported on support rack 160 and attached to side unit 18b. Subassembly 30b has a bottom surface 180 which rests upon, and is supported by, device support surface 162. Subassembly 30b further has a side surface 181 which rests against side guide surface 164, and a front mating surface 31 which mates with unit mating surface 32 (discussed above). Subassembly 30b has a bracket mating surface 184 which mates with terminating portion 114 of front bracket 104 of side unit 18b. Bracket mating surface 184 has two front mating surface holes 185 and 186. Device 30b is mounted to side unit 18b by inserting fastener 185a through front mating surface hole 185, and further through a respective one of device mounting holes 116 of front bracket 104. Likewise, fastener 186a is inserted through front mating surface hole 186, and further inserted through a different respective one of device mounting holes 116 of front bracket 104. 
     Subassembly 30b further has a side mount bracket 190. Side mount bracket 190 has a side mating surface 191, and a bracket mating surface 192. Side mating surface 191 mates with, and is attached to, side surface 181 using any commercially-available fasteners. Bracket mating surface 192 is at a predetermined angle from side mating surface 191, which in the preferred embodiment is 90 degrees. Bracket mating surface 192 has a bracket hole 194. Fastener 194a is inserted into bracket hole 194, and into one of device mounting holes 116 of back bracket 132. 
     In a like manner, subassembly 30b is attached to oppositely-positioned side unit 20b (shown in FIG. 1). Attaching subassembly 30b Side panel 18b and 20b provides an added dimension of strength and stability to module 12. Subassembly 30b holds side units 18b and 22b in a predetermined relationship to one another. Fastening pins 52b, 54b, 56b, and 58b, the side units 18b and 20b, and subassembly 30b are therefore interlocked into a very strong, stable structure which can withstand a large amount of torquing force, and can further support a large amount of weight. 
     Although it is understood that side units may be stacked without being connected to subassemblies, connecting side units to subassemblies provides a stronger module structure. 
     FIG. 10 is an exploded perspective view of two side extension units as they attach to a back panel. As discussed above, side extension units 22b and 24b have extension brackets 142 possessing all of the attributes described with respect to FIG. 4. Back panel 28b has a back panel mating surface 200 for mating with the terminating portions 114 of extension brackets 142 of side extension units 22b and 24b. Back panel mating surface 200 has four back panel pegs 202, 204, 206, and 208 for attaching back panel 28b to side extension units 22b and 24b. Back panel pegs 202 and 204 are inserted into different respective ones of device mounting holes 116 of extension bracket 142 of side extension unit 22b. Likewise, back panel pegs 206 and 208 are inserted into different respective ones of device mounting holes 116 of extension bracket 132 of side extension unit 24b. 
     Side extension units are used to extend cabinet capacity. Modules may need the added capacity to accommodate larger electronic subassemblies or to meet cabling requirements, for example. However, if the additional capacity is not required, back panels may be fastened directly to the side unit back brackets. For example, if side extension units 22b and 24b were not required in module 12, back panel mating surface 200 would mate with terminating portions 114 of back brackets 132 of side units 18b and 20b. Back panel pegs 202 and 204 would be inserted into different respective ones of device mounting holes 116 of back bracket 132 of side unit 18b. Likewise, back panel pegs 206 and 208 would be inserted into different respective ones of device mounting holes 116 of back bracket 132 of side unit 20b. Finally, it should be noted that back panels 28a and 28b merely provide aesthetic appeal, not structural support, to the cabinet. It is understood the cabinet could be constructed without these additional components. 
     The above paragraphs describe a stackable modular cabinet in which the modules are stacked in a vertical manner, with module 10 stacked upon module 12. However, the stackable modular cabinet may be positioned on one side so that cabinet expansion occurs laterally. For example, the cabinet of FIG. 1 could be positioned so side units 18a and 18b are in contact with a desktop or floor. This positioning may be useful if space constraints exist. 
     The invention has been described in its presently contemplated best mode, and it is clear that it is susceptible to various modifications, modes of operation and embodiments, all within the ability and skill of those skilled in the art and without the exercise of further invention activity. Accordingly, what is intended to be protected by Letter Patents is set forth in the appended claims.