Abstract:
A thread checking apparatus and method for threaded bores in small plastic parts, in which a threaded tap is mounted to rotate on a fixed axis at a workstation, the workstation having guide structure associated with the rotating tap to slidingly engage and guide a part with a threaded bore onto a free outer end of the rotating tap. The rotating tap draws the part down the length of the tap in sliding, rotation-preventing engagement with the guide structure, until the part&#39;s threaded bore is fully verified and chased, at which point the part engages a reversing limit switch to automatically reverse the rotation of the tap and drive the chased and verified part back off the tap.

Description:
FIELD OF THE INVENTION 
   The invention is in the field of apparatus and methods for verifying and chasing threaded bores in small parts. 
   BACKGROUND OF THE INVENTION AND DESCRIPTION OF RELATED ART 
   Molded plastic parts with molded-in threaded bores often require the thread/bore diameter to be verified for consistency before the molded parts are shipped for assembly with mating threaded parts. Bore and thread gauges are known, but while useful for gauging the open ends of the bore, are generally not practical for measuring inconsistencies along the full length of relatively small diameter bores in large numbers of parts. Molded bores in both thermoplastic and thermosetting plastic materials often tend to “hourglass” or constrict adjacent thicker areas of the part that create heat sinks affecting the cooling rate along the bore. If tolerances are strict, the need for full-length bore verification in parts prone to this problem becomes important. Moreover, bore gauges only diagnose the problem. 
   It has been known to use a specially machined threaded tap (or one of the mating parts, modified to cut) to verify and “chase” or remove extra material from the bore by attaching the tap to a hand drill and running it manually through the bore. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is an automated thread-verification and thread-chasing apparatus in which a tap rotates about a fixed axis on a base such as a worktable, driven for example by an electric motor. A fixed, stable, passive guide structure associated with the rotating tap receives the part in sliding fashion to guide the part&#39;s threaded bore onto the free outer end of the rotating tap. The parallel guide structure maintains the part&#39;s bore in alignment with the tap&#39;s axis, prevents the part from rotating, and allows the part to be drawn smoothly and evenly down over the length of the rotating tap. Bores with reparable hour-glassing are automatically chased; parts with irreparably hour-glassed bores and parts with oversized bores will not be drawn fully or smoothly over the rotating tap, and can be discarded. 
   In a further form of the invention, the tap rotates by default in a reverse mode that drives the part off the end of the tap. The apparatus is provided with a motor-reversing switch operated when the part is loaded into the apparatus to reverse the tap and draw the part down over the tap. A reversing limit switch is operated automatically via a cam associated with the guide structure when the part bottoms out on the tap, switching the tap rotation back into default mode and automatically driving the part back off the free end of the tap. The person checking the bores simply places the part on the guide structure, presses the motor-reversing switch (if operated manually), watches while the part is verified, chased, and driven back off the tap, and then removes the part from the guides. In the preferred form the reversing limit switch is manually accessible to the workstation operator, and is used to initially reverse the motor when the part is first loaded. 
   In yet a further form of the invention, the rotating tap has an adjustable cutting diameter. The tap is cut or split over a portion of its length into multiple cantilevered sections with spaced cutting flutes separated by relief areas. The cantilevered sections are internally tapped to form an internally threaded center bore capable of accepting a tapered screw. The tapered screw is used to adjust the radial spacing of the cantilevered sections, and when properly adjusted the sections are locked in place, for example with a removable spot weld on the screw. 
   The invention also includes the method of running a fixed, table-mounted tap in a default reverse (part-ejecting) rotational direction in the confines of a parallel passive guide structure; loading a part onto the guide structure to cause a threaded bore in the part to be guided onto and mated with the end of the tap; operating a motor-reversing switch to change the rotation of the tap to engage and draw the part down over the tap via its engagement with the threaded bore; and, automatically operating a reversing limit switch with the moving part when the part bottoms out on the guide structure, returning the tap to its default, part-ejecting rotational mode and automatically driving the chased/verified part back off the tap where it can be removed from the guide structure. 
   These and other features and advantages of the invention will become apparent upon further reading in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an apparatus according to the present invention, in which a molded plastic part with a threaded bore is about to be engaged with a guide portion of the apparatus that guides the part onto a rotating tap. 
       FIG. 2  is similar to  FIG. 1 , but the molded part has been placed on the guide and engaged by the rotating tap, and is being drawn down the tap and guide structure. 
       FIG. 3  is similar to  FIG. 1 , but shows the molded part hitting a reversing limit switch associated with the guide, momentarily stopping and then reversing to rotate in the opposite direction to drive the part up off the tap and guide structure. 
       FIG. 4  is similar to  FIG. 1 , but shows the molded part being driven off the tap where it can be removed from the guide structure. 
       FIG. 5  is a perspective view of the apparatus of  FIG. 1 , with the tap exploded off a chuck in the apparatus. 
       FIG. 6  is a detailed perspective view of a split, adjustable tap for use in the apparatus of  FIG. 1 . 
       FIG. 7  is a perspective view of an alternate apparatus according to the invention, using the same rotating tap as in  FIG. 1 , but with a different guide structure for a different part. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIG. 1 , a molded part  10  made from a thermosetting or thermoplastic polymer material of known type is shown elevated above an automated thread verification and chase apparatus  12  (hereafter “thread checker”). Thread checker  12  is securely mounted on a stable base  14  such as a workstation table in an assembly shop, for example with steel baseplates  14   a  and bolts  14   b . Thread checker  12  has a rotating tap  16  threaded to mate with threaded bore  10   a  in part  10 . Tap  16  is rotatably mounted on base  14  to spin on a fixed axis, driven for example by an electric motor  13  mounted underneath the table. The tap is connected to the motor through a rotatable chuck  13   a  adapted to securely receive a lower shank portion  16   a  of tap  16 . In the illustrated embodiment tap  16  is made from tool steel, but it will be understood that other materials could be used depending on the material which forms the bore in part  10 . 
   Tap  16  has an external thread-form  16   b , interrupted by cutting flutes  16   c , that mates with bore  10   a  in part  10 . Tap  16  is either a specially formed tool designed to mimic a mating threaded part that will later be assembled with molded part  10 ; or, tap  16  is a modified mating threaded part adapted to be mounted in thread checker  12  and rotatably driven by motor  13  in chuck  13   a . For example, a mating threaded part can be modified by machining flutes  16   c  into the thread-form to provide cutting and work-relief faces needed to remove excess material from the threaded bore. The modified mating threaded part might be ready to connect to motor  13 , or it might need a lower portion ground off to form a shank, or a shank might be added to one end to be grasped in chuck  13   a . These and other modifications to mating threaded parts for use as the rotating tap  16  will be apparent to those skilled in the art, and will depend on the type of part. 
   It will also be understood that while tap  16  is shown driven by an in-line electric motor to which it is connected by a chuck, other known apparatus for spinning the tap on a fixed, stable axis can be used without departing from the scope of the invention. Further, while the illustrated thread checker  12  is shown with a vertical motor drive axis and a vertical axis tap  16  relative to the workstation, other drive-axis and tap-axis orientations, including horizontal and inverted, are possible. 
   Still referring to  FIG. 1 , thread checker  12  has a passive parallel guide structure  18  (rods  18   b  and walls  18   c ) associated with tap  16 . Guide structure  18  is shaped and arranged around the tap to slidingly engage portions of part  10 , permitting part  10  to travel along the length of tap  16  in parallel alignment with the tap axis as the tap threads  16   b  engage bore  10   a , and to prevent the part from rotating while doing so. The illustrated example of  FIG. 1  shows both part-internal and part-external guides  18   b  and  18   c . The part-internal guides  18   b  are a pair of vertical rods adapted to slidingly engage holes  10   b  in the body of part  10 , and the part-external guides  18   c  are a pair of wall members adapted to slidingly engage exterior side surfaces  10   c  of part  10  in a close sliding fit. It will be understood that whether to use part-internal guides that go through the part, part-external guides that engage outer surfaces or edges of the part, or a combination of both will depend on the nature and shape of the part whose threads are being checked. It will also be understood that while pairs of guides for preventing rotation are shown, single guides whose shape presents a non-rotatable sliding engagement with some interior or exterior portion of the part could also be used. For example, a single rectangular internal guide rod  18   b  might be used if part  10  has a conveniently located rectangular hole through its body. 
   Thread checker  12  preferably includes a protective housing  20  surrounding tap  16  and part  10  and guides  18 , to prevent accidental contact with the cutting flutes and/or interference with the part as the part is moving up and down the tap. In the illustrated embodiment, housing  20  is formed in part by the exteriors  20   a  of guide walls  18   c , and in part by a clear front wall or window  20   b  connected to walls  18   c  and allowing the operator to watch the part&#39;s progress. It is also preferred that the rear of the housing  20  (opposite the operator) is left open for manual access to a limit switch (discussed below), for routine cleaning, and for access to the tap for adjustments or replacement. 
   Thread checker  12  includes a reversing limit switch mechanism  22  placed for activation by some portion of part  10  at the part&#39;s lower limit of travel. Limit switch  22  is connected by wiring  24  to motor  13 , and/or to a motor power source to reverse the motor in response to momentary operation of switch button  22   a . Suitable circuitry or control structure for reversing the motor in response to switch activation can be located in the switch housing, between the motor and the switch, or in the motor itself, and will be apparent to those skilled in the art. 
   Referring to the arrow R in  FIG. 1 , tap  16  normally rotates in a direction that would disengage the tap threads  16   a  from the part&#39;s threaded bore  10   a . This is the default mode of rotation. Motor  13  preferably drives tap  16  in constant fashion, such that the tap is always rotating as long as power is supplied to thread checker  12 . 
   Referring to  FIG. 2 , the operator has manually reversed the rotation direction of tap  16 , as shown by the arrow F. The manual reverse can be accomplished using any known switch mechanism for reversing the direction of motor  13 , including but not limited to a separate hand-operated switch (not shown) on the motor housing or workstation, or a foot-operated switch (not shown) on the floor or a lower part of the workstation. In the preferred, illustrated embodiment, however, the workstation operator uses the reversing limit switch  22 , which is easily accessed through the open rear of housing  20 . A simple press with a finger on switch  22  is sufficient to reverse the direction of rotation from R in  FIG. 1  to F in  FIG. 2 . Manual activation of switch  22  by the workstation operator is made easier by a limit-activating cam strip  23  extending below switch button  22   a  and described further below. 
   Although a manually activated reverse of motor  13  for initially engaging part  10  is the preferred, illustrated example, those skilled in the art will recognize that switches operated in a more automatic fashion are possible. By way of non-limiting example, a photo-optical switch that senses the approach or initial engagement of part  10  with guide structure  18  could be used, or a mechanical switch triggered by the initial engagement of the part with guide structure  18  could be used. 
     FIG. 3  shows part  10  having bottomed out on tap  16 , bore  10   a  having traveled fully over the tap, and an external portion of part  10  displacing the lower end of cam strip  23  into switch-activating engagement with switch button  22   a.    
   The manually-activated reversing limit switch mechanism  22 ,  23 , coupled with the default tap rotation direction R, results in a simple, inexpensive, and nearly foolproof part-feed and switching operation, particularly where a human workstation operator is desirable or preferred. The need for conscious activation of a switch in close physical and visual association with the guide and tap structures ensures the operator&#39;s careful attention to the part-engaging step, since the switching and part-engaging steps must be coordinated. The automatic limit-reversing activation of switch  22  via cam strip  23  helps keep the operator&#39;s hands away from the tap and the part while the part is moving over the tap. And, finally, as shown in  FIG. 4 , the part is automatically driven back off the tap even if the workstation operator is inattentive, and will float harmlessly on top of the reverse-rotating end of the tap, restrained by the guides  18 , until removed by the workstation operator. It is preferable to make one of the guide rods  18   b  slightly shorter than the other to aid in part removal. 
   Referring next to  FIG. 5 , tap  16  is shown removed from chuck  13   a  for replacement, maintenance, or adjustment. Any known chuck apparatus can be used to hold tap  16  in an operative, rotating connection to motor  13 . 
     FIG. 6  shows a modified tap  116 , split through cutting flutes  116   c  into four cantilevered cutting sections  116   d , and having a threaded central bore  116   e  that accepts a tapered adjustment screw  117 . Threading screw  117  into bore  116   e  spreads cutting sections  116   d  apart for a greater cutting diameter; backing screw  117  out of bore  116   e  reduces the tap&#39;s cutting diameter. This allows the operator to make very fine adjustments to tap  116  to increase or reduce the amount of material removed by the tap from bore  10   a  in part  10 . The proper adjustment can be gauged by hand using a test part, and then screw  117  can be temporarily secured in place to lock the adjustment, for example with a spot weld or thread-locking adhesive. It will be understood that while a tapered screw is illustrated, other devices for adjusting the cutting diameter of the modified tap  116  can be used, for example a screw with a tapered bushing, or a screw with a tapered cone. 
   It will be understood that while adjustable tap  116  is shown split into four equal sections, it may be split into more or fewer adjustable sections as desired. It will also be understood that the tapered screw will spread the upper ends of the sections  116   d  very slightly farther apart than their lower portions, but that this will not affect the quality of the verifying and chasing operation. For example, the cutting diameter of the upper portion of the tap in the illustrated embodiment can be modified up to 0.050 inches in diameter. 
   Referring next to  FIG. 7 , a modified thread checker  112  with a part-external guide structure  120  is illustrated for a part  110  that is verified/chased by the same tap  16  illustrated in  FIGS. 1-5 . Although part  110  has a threaded bore  110   a  identical in diameter and pitch to  10   a , it does not have through-holes such as  10   b  that allowed part  10  to be guided on rods  18   b . Accordingly, the housing  120  around the tap is sized with interior guide wall surfaces  118   c  that maintain a close sliding fit with at least two exterior side surfaces  110   c  of part  110 , preventing the part from rotating as it is drawn down tap  16 . It will be understood that a purely part-external guide structure such as  120  can be configured to guide many different shapes of part having two or more exterior side surfaces which when held will prevent the part from rotating. 
   It will finally be understood that the disclosed embodiments are representative of presently preferred forms of the invention, but are intended to be illustrative rather than definitive of the invention. The scope of the invention is defined by the following claims.