Printer with interchangeable paper-feed modules
BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of a printer in accordance with the invention embodying a tractor-type paper-feed mechanism; 
     FIG. 2 is a front perspective view similar to FIG. 1 but showing the printer with a pin-feed-type paper-feed module in operative position; 
     FIG. 3 is fragmentary perspective view of the embodiment of FIG. 1 shown without the outer housing; 
     FIG. 4 is a front elevational view of the pin-feed module; 
     FIG. 5 is an enlarged, fragmentary side elevational view taken along the plane V--V of FIG. 1, showing the guideways for placement and seating of the paper feed modules, as well as other details; 
     FIG. 6 is an enlarged, fragmentary side elevational view showing the worm-gear drive train; 
     FIG. 7 is an enlarged, central sectional elevation of the worm gear, with the worm diameter shown in phantom; 
     FIG. 8 is an enlarged, fragmentary sectional elevation taken through the plane VIII--VIII of FIG. 7, showing the tooth profile of the gear; and 
     FIG. 9 is an enlarged, fragmentary side elevational view showing the engagement of the worm and worm gear. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now in more detail to the drawings, the printer 10 has an outer housing 12 with a removable upper portion 12a which encloses the end portions of a removable and interchangeable paper-feed modules, shown in FIG. 1 to embody a tractor-type paper-feed mechanism 14 and in FIG. 2 to embody a pin-feed type mechanism 16, each to be described more particularly hereinafter. 
     The tractor module 14 is seen in more detail in FIG. 3, as is part of the structure of the printer apparatus. Preferably, this module (like the others) has a basic structure comprising a pair of spaced upright side plates which are joined into a generally rigid unit by elongated interconnecting members. In the case of the tractor module 14, side plates 18 are joined together by an upper guide bar 20 of straight, round stock, and a generally L-shaped or other such reinforcing interconnecting support 22, which may be extruded aluminum. The side plates carry a rotatably-mounted, splined driving shaft 24, and a platen 26, which also is preferably rotatable (but not necessarily driven). A print head 28 may be moved laterally across the front of the platen so as to print characters on paper stock fed between the print head and the platen, the paper being pulled upwardly by the tractor devices 30. The paper preferably passes through and is guided by an elongated opening 32 defined by the internal frame work of the printer, which for present purposes may be regarded generally as a rigid, supportive base structure upon which the outer housing 12 is mounted as a protective enclosure which also gives shape and appearance to the device generally. It is to be understood that the tractor devices 30 are well known commercially available paper-handing components, obtainable from a number of different manufacturers, including for example Kidder-Machinery Div., Moore Business Firms Co. 
     The paper-feed modules are preferably mounted upon spaced side plates 34 comprising part of the internal support structure or frame of the printer, between which extend such elements as a rigid guide bar 36 and a rotatably mounted lead screw 38 which may be used to move the print head structure 28, all as disclosed in more detail in copending application Ser. No. 725,690, Filed Sept. 23, 1976, entitled Carriage Suspension And Frame Aseembly For Impact Printers And The Like. The side plate 34 illustrated at the right side in FIG. 3 also mounts a generally vertically-disposed drive motor 40, preferably a stepping motor, which is used to drive the paper-feed module, e.g. in the embodiment of FIG. 3 to rotate a cluster gear 42 and thereby drive the belt 44, which turns the driving shaft 24 through a toothed pulley 46 attached to such shaft. Rotation of the latter drives the tractors 30. Also, the upright side plates 34 of the printer define a supportive mounting bed for the interchangeable paper feed modules 14 or 16, each of which is removably mounted on such support bed. In the case of the tractor module 14 shown in FIG. 3, the drive gear 42 is preferably rotatably carried on a fixed shaft 27, on which the platen 26 is also rotatably mounted, for random (undriven) rotation during paper advancement. 
     One example of a pin-feed-type module 16 is seen in more detail in FIG. 4. As already stated, this module is preferably similar in a basic structural sense to the tractor module 14, and it has the same length, as well as a pair of similar side plates 181, between which extend lateral supports analogous to the guide bar 20, and rigidifying support 22 of FIG. 3, together with a platen 26&#39; mounted on a shaft 27&#39;. In this module, the lateral support members preferably mount a downwardly-depending sheet metal or like paper guide 48 which lies behind the platen 26&#39; and generally parallel to the guideways 60 (FIG. 5) (to be discussed subsequently), for the purpose of guiding paper through a space between the guide and the platen. The platen itself is of a known type, being a hard but resilient rubber-like roller element which in this embodiment is secured to its support shaft for rotation therewith, upon rotation of drive gear 42&#39; secured to shaft 27&#39;. 
     The platen has a pair of end portions 50 which define pin-feed mechanisms, comprising in effect a dish-like structure having an apertured tubular periphery through which protrude drive pins 52, which engage holes along the edge of the paper to be printed, to thereby draw it upwardly. The pin feed elements 50 are known commercially available mechanisms, which preferably have an internal eccentric cam apparatus which actuates the pins 52 outwardly at the point where they must project to engage the holes in the paper to be fed, but which retracts the pins during other portions of each revolution of the platen. Attached to the platen and pin feed mechanism at each end is a bail support member 54, which may hook over or hang upon the upper edge of the paper guide 48, and which has arms 56 supporting a paper hold-down bail 58 having a series of cylindrical friction rollers 59. Also, the support members 54 include dependent arms having guide finger portions 53 which hold and guide the paper adjacent the feed pins 52 for trouble-free feeding. The pin-feed module may if desired include friction-feed roller elements such as those indicated at 90 and 92, which may be of a generally known nature analogous to those usually found on typewriters or the like. While the specific nature of such mechanism is not of itself a central aspect of this invention, it may be noted that the rollers 92 may conveniently be mounted along a common support shaft and positioned in contact with the platen, to be rotated by the latter, while the rollers 90 are preferably arranged in sets (e.g., groups of two) which are pivotally mounted independently from one another so as to be self-aligning with the rollers 92, to feed paper therebetween upon rotation of the latter by the platen. It is also to be noted that, in accordance with the concept hereof, such a friction-feed mechanism may be implemented as a wholly separated module, distinct from the pin feed (or tractor) module, but based on the same underlying structural concept and also interchangeable with the other modules. 
     Each of the paper-feed modules just described thus comprises a light weight, self-contained paper feed mechanism which can readily be removed from or placed into position upon the support bed defined by the frame of the printer, as mentioned previously. Each such module has its own different type of paper-feed mechanism, as well as its own platen, (although it may be desired to practice the basic concept thereof by using a common platen, which remains mounted upon the printer frame rather than being a part of the paper feed modules and removable therewith), together with sufficient structural support to provide a relatively rigid structure suitable for functional operation when used upon a supportive frame which helps it maintain its shape, without substantial load or stress application. The providing of such an apparatus, including the basic printer mechanism with its supportive main frame, with light-weight, modularized, interchangeable paper-feed modules of a simple structural nature, is an important aspect and objective of the invention. In this connection, it is to be noted that to enchance removability and interchangeability, and the real practicality of such features, it is desirable to exclude from the removable structure many components which are usually mounted upon conventional fixed paper feed structure, including for example electronic components and circuitry in general, controls, sensors, tab or margin mechanisms, motors, encoders, paper switches, and the like, all of which when present as part of the paper-feed module serve to make removability a very complicated and difficult matter, and which need not be made part of the paper-feed apparatus, or module, it being entirely feasible, and indeed within the concept of this invention, to embody such elements in, or on, the main frame of the printer. 
     The manner in which the paper-feed modules fit into and are held in mounted position is seen in more detail in FIG. 5, in which the tractor module 14 is illustrated as typical of either such module in this respect. First, the side plates 34 of the printer frame are shaped to define a pair of adjacent cut-away or recessed portions separated by an elongated, upstanding, rearwardly-angled guide tongue 60, whose two opposite elongated sides thus define guide surfaces 60a and 60b, as do edge portions 34a, 34b, 34c of the cutouts in the side plates 34. Mounted on the inwardly-facing side of each of the paper feed module side plates 18 are a series of guide blocks 62, which in effect define a guide channel or path for receiving guide tongue 60, together with guide blocks 64, 65 and 66, the first of which guides along and rests against the upright guide surface 34a, while the other two of which seat downwardly against the top edges 34b and 34c of the side plates 34, to index the paper-feed module in seated position thereupon. When so seated, a toggle-type or other such latching element 66, which is secured to the upper edge of each main frame side plate 34, is actuated by its respective handle 67 to bring a resilient locking pad 68 into locking contact against the upper edge of the paper feed module side plate 18, thereby locking the entire paper feed module into place. As illustrated, in this position the platen 26, mounted on and carried by the paper feed module, is positioned immediately adjacent and in direct alignment with the print head 28, which is carried by the printer main frame. The tractor mechanisms 30 are thus disposed above the platen, as already shown and described, slidably mounted upon the upper guide bar 20 and the splined driving shaft 24. 
     Accordingly, it may be seen that the entire paper feed module may be readily removed or placed into position, or interchanged, by merely actuating each of the taggle clamps to release them, which swings their locking head portions 68 clear of the rear edge of the module side plates, and then lifting the entire module upwardly and out of the printer, during which the guide blocks 62 slide upwardly along guide surfaces 60a and 60b, until the module is clear. 
     FIG. 6 shows the mounting and positioning of the components which drive the paper-feed module, in relation to other parts of the apparatus already discussed. Thus, the drive motor 40 is seen in its position relative to the printer main frame side plate 34, and the guide tongue 60 thereof, discussed just above. It will be noted that the drive motor 40 (preferably, a stepper motor) is attached to side plate 34 with the motor output shaft 70, having its axis of rotation 72 oriented at a rearward inclination (relative to the front of the printer), in the same general direction as the angle of the guide tongue 60, represented by reference line 74, but at a greater acute angle relative to true vertical, (with the printer in its normal operating position), as illustrated, true vertical being represented by the reference line designated 76. 
     Rotational drive from motor 40 is most preferably imparted to the paper feed module by a worm-gear drive since this provides certain advantages over other potentially useable drive systems such as a spur gear train. For example, a spur gear system would require multiple gears, thereby adding considerable expense by way of additional parts and machining during manufacture, as well as additional skilled labor for assembly; furthermore, a spur gear train would require that the axis of the drive motor be parallel to that of the platen, which can lead to packaging problems. In the preferred worm gear drive illustrated in the figures, the motor shaft 70 has a drive worm element 78 secured thereto. Worm 78 engages the teeth of a mating gear 42 carried on the paper feed module (designated 42&#39; on the pin-feed module), to effect a drive coupling between the printer main frame and the paper-feed modules. In the case of the tractor module such rotary motion is coupled by the belt 44 to the splined tractor-drive shaft 24, as pointed out above. 
     With the foregoing structure, as either of the paper-feed modules is slid along the guide paths, or guideways, discussed above, into seated relation upon the printer main frame bed, the worm gear 42 mounted on the platen shaft 27 is moved downwardly along a path parallel to the guide path reference line 74, with the tips of the gear teeth following such reference line. Thus, the worm gear moves along a path which converges with the axis of rotation 72 of the worm, thereby bring the tips of the worm gear teeth gradually into engagement with the thread of the worm. This is an important aspect of the paper-feed modules interchangeability, particularly since it is important that easy and trouble free interchangeability and gear meshing always occur, without any prospect of gear interference or tooth tip binding. 
     The desired type of gear closure and mating is most preferably achieved in accordance with the invention by disposing the axis of rotation 72 for the worm and drive motor at an angle of approximately fifteen degrees relative to the vertical, i.e., reference line 76, while disposing the guide path reference line 74 at a smaller acute angle, preferably on the order of about 10°, relative to the vertical reference line. Thus, a small acute angle, preferably on the order of about 5°, but at least within the range of about 3 or 4° to about 20°, exists between the axis of the worm and that of the worm gear, and it is along one of those two mutually converging axes that the worm gear is moved into mating relation with the worm as the paper-feed module is moved into position on the printer. 
     The angularly-disposed relationship just described is provided to help insure consistent, trouble-free mating engagement of the worm and gear when the paper feed modules are placed in position upon the printer. Furthermore, the relative size and shape of the worm and its thread, and that of the worm gear, are preferably as illustrated in FIGS. 7-9 inclusive. First, the worm gear preferably has an overall diameter on the order of two or three times that of the worm, but with each having the rounded tooth or thread profile shown in FIG. 8, i.e., having straight, angularly-disposed sides 80 defining a pressure angle of about twenty degrees, and with a rounded nose section 82. For a specific set of values corresponding to the angular converging axial relationships set forth above, the worm gear may have an O.D. of 1.521 inch, a pitch diameter of 1.375 inch, with a pitch of 24, preferably in a throated-type spur gear, while the worm may have an O. D. of 0.583 inch, a pitch circle diameter of 0.5 inch, a normal diametral pitch of 24 and an axial pitch of 0.13 inch. This produces the mating relationship from FIG. 9, from which it may be appreciated that it is virtually impossible for the teeth of the worm gear to be wedged tip-to-tip against the teeth of the worm when the paper-feed module is slid into position and seated, since the closing angle and the shape and size of the gear teeth and worm thread will always cause the worm gear (which is freely rotatable with the platen shaft during mounting of the module) to make first contact with the worm either along their sides, in mating relation, or at the curved nose sections of the gear teeth and worm thread, at any possible position of relative rotation between the two where initial contact may be made. Such that the worm gear merely rotates whatever small extent may be necessary to smoothly engage its teeth into the worm thread. 
     It is entirely conceivable that upon examining the foregoing disclosure, those skilled in the art may devise embodiments of the concept involved which differ somewhat from the embodiments shown and described herein, or may make various changes in structural details to the present embodiment. Consequently, all such changed embodiments or variations in structure which utilize the concepts of the invention and clearly incorporate the spirit thereof are to be considered within the scope of the claims appended herebelow, unless these claims by their language specifically state otherwise.