Abstract:
A press mounted cam has a slide guided on an adapter by structural features including as one or more T blocks fixed to the slide and received in a respective channel formed in the adapter. In an aerial cam embodiment, a vertical clearance space is provided between the T block and adapter channel such that positive driving engagement between cam surfaces on the slide and the adapter is momentarily delayed after the initial engagement of the slide and driver surfaces to let the momentum of the slide be absorbed prior to the beginning of positive driving by the press motion, reducing shock and noise. The slide is laterally located with respect to the driver by an upwardly projecting locator-guide key on the driver received in a central channel in the slide. A positive return is provided by engagement of a driver key having T features received in a T-shaped slide channel and captured therein when the slide is driven laterally. This engagement creates a positive return of the slide when the press upper platen is raised.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. provisional Ser. No. 60/629,147, filed Nov. 18, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention concerns press mounted cams, which are mechanisms installed in forming presses to produce a feature on a workpiece being formed within the press by die having an upper part installed on an upper platen of the press and a lower die part installed on a lower press platen. The cam is used to form a punched or tapped hole where the feature is located such that it must be formed by tool motion along a direction at a working angle across the direction of press movement. The press mounted cams are used to produce such crossing tool motion.  
         [0003]     These cams are comprised of a “slide”, carrying the tool, a “body” or “adapter” affixed to one of the die parts or press platens on which the slide is slidably mounted, and a separate “driver” mounted on the other of the die parts or press platen. The driver engages the slide and drives the same by engagement of cam surfaces when the press is operated.  
         [0004]     In an “aerial” cam shown in  FIG. 1 , a slide  12  is suspended on a body or adapter  10  either directly mounted to the upper platen  2  or more typically to an upper part  6  of a forming die. A driver  14  is likewise either directly mounted to the lower platen  4  or more typically to a lower part  8  of a forming die and has fixed inclined cam surfaces  16  extending parallel to the working angle, typically defined by wear plates affixed to parallel faces on the driver  14  and slide  12 .  
         [0005]     As the upper platen  2  descends, a resulting cam action causes the slide  12  to be advanced along the working angle against the resistance of one or more springs  15 , with tooling T projecting from the slide  12  driven in that direction. The horizontal component of the motion requires that the working slide  12  also move laterally on the adapter  10 . Engaged horizontal bearing surfaces  18 ,  19  are provided on the top of the slide  12  and the bottom of the adapter  10  respectively for this purpose. In other configurations, an angled surface may be on the adapter, and a horizontal surface on the driver as in the embodiment seen in  FIG. 12 .  
         [0006]     In a “die mounted” cam (shown in  FIG. 2 ) the slide  12  and adapter  10  are both mounted to the lower platen  4  (or die part  8 ) which does not move, but rather the driver  14  is mounted to the moving upper platen  2  (or die part  6 ) and descends with the press upper platen  2  to engage the slide  12 .  
         [0007]     The die mounted cam thus does not result in vertical movement of the relatively heavy slide  12  with the upper platen  2 , as occurs in an aerial cams. This vertical movement of the slide can cause problems as described below, but aerial cams are often used nonetheless since they create a clearance space to allow transfer of the workpieces into and out of the die and press.  
         [0008]     In either cam mount design, in order to accurately locate the tooling T with respect to the workpiece W, the slide  12  must be accurately located laterally when being driven, and to achieve this, the practice heretofore has been to form the lower cam surfaces  16 A in a V-shape so as to provide a lateral location of the slide on the driver as well as a camming surface as the slide  12  engages the driver  14  as seen in  FIGS. 3 and 4 .  
         [0009]     Additional flat surfaces  16 B are sometimes required for larger sized cams to provide adequate area to distribute the stresses imposed on the slider  12  by the press. Precision machining of the V-shaped surfaces is difficult and adds substantially to the cost of making the slide  12  and driver  14 .  
         [0010]     The slide  12  is suspended on the adapter  10  by means of side plates  20  engaged with hook over plates  22  attached to the sides of the slide  12 . The slide  12  is guided along the plates  20 ,  22  when being advanced by the camming action on the slide  12  caused by the descent of the press upper platen.  
         [0011]     The plates  22  are confined between side walls  24  to be laterally guided. A vertical hooked bar  26  is mounted on each side to reinforce the fixing of the plates  20 .  
         [0012]     Particularly in larger sizes, the need to machine features on the adapter  10  and slide  12  at locations on the outside of these components requires the use of large size machining centers, adding to substantially to the manufacturing costs.  
         [0013]     In aerial cams, due to the large mass of the slider  12 , an auxiliary roller cam  28  is provided to initiate and assist cammed lateral slide motion by engagement with a machined slot  30  on the driver  14 , just prior to engagement of the cam surfaces. This helps to assist in redirection of the motion of slide  12  laterally to reduce peak stresses and consequent noise, shock, and wear of the cam surfaces. However, the roller cam  28  also adds substantially to the cost of such aerial cams.  
         [0014]     A positive retraction auxiliary cam comprised of cam bars  32  and  34  is also provided to insure return movement of the slide  14  if return springs  38  in pockets  36  should fail due to excessive shock loading or are unable to withdrawn the tool for some other reason such as a severely jammed tool.  
         [0015]     The retraction cam bars  32 ,  34  are located at the outboard ends of the slide  12  and driver  14  and thus are difficult to machine especially in the larger cam sizes as described above. Also, the area of engagement therebetween is limited to the stroke of the slide  10 , and the bars  32 ,  34  are subject to failure since a large force may be necessary to retract the slide  12  if a tool is severely hung up.  
         [0016]     It is an object of the present invention to provide aerial cams which impose less shock on the mating components and to eliminate the need for auxiliary roller cams.  
         [0017]     It is a further object to eliminate difficult to machine retention and locating features in both aerial and die mounted cams.  
         [0018]     It is a further object to provide a more robust and durable positive retraction mechanism for both aerial and die mount cams.  
       SUMMARY OF THE INVENTION  
       [0019]     The above objects and other objects which will become apparent upon a reading of the following specification and claims are achieved by suspending the slide on the adapter by structural features establishing a predetermined clearance space which must be taken up before the press can act to cam the slide laterally to be driven along its working angle. This momentarily delays the driving engagement of cam surfaces by the press until some brief time interval after initial contact of the slide with the driver has occurred. This allows the downward momentum of the slide to first be absorbed by the driver before the slide is forcibly cammed along the working angle by the press motion to reduce noise and the peak loading imposed on the slide-driver mating surfaces.  
         [0020]     The slide is preferably suspended on the adapter using one or more T blocks fixed to the slide and captured in corresponding T channel slots at an inboard location on the adapter with a head portion of the T blocks resting on surfaces on each side of the slots. The use of inboard located T blocks and eliminates the outboard located retention plates on the sides of the slide previously used.  
         [0021]     The T blocks and channels may comprise the structural features mentioned above establishing a predetermined clearance space between the adapter and slide mating bearing surfaces so that when the slide first contacts the driver, the clearance space must first be taken up before the press motion itself will cause camming of the slide laterally. The presence of this clearance momentarily delays the driving engagement between the adapter and slide mating bearing surfaces until the clearance space is taken up. Auxiliary roller cams are thus not necessary.  
         [0022]     In both aerial and die mounted cams, one or more in board locator-guide key projects from the driver and moves into a guide slot in the slide for lateral location and guidance as the slide cam surfaces approaches engagement with the inclined cam surface of the driver. These are much easier to machine laterally locator-guide surfaces. The in board locator-guide key also eliminates the need for the precision machining of guide surfaces on the ends of the slide.  
         [0023]     Also, in both aerial and die mount cams, a positive retraction mechanism may be incorporated, using a pair of spaced apart T heads affixed to the driver locator-guide key and moving into a mating T channel formed in the slide when engaging the slide through respective spaced apart openings in the T channel and captured as the slide is advanced along the stroke. This effectively doubles the length of mating engagement surfaces on the driver and slide when a positive retraction is necessitated to substantially increase the forces able to be exerted to positively retract the slide. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a partially sectional elevational view of a typical aerial cam and die parts installed in a press, shown in fragmentary form.  
         [0025]      FIG. 2  is a similar view of a die mounted cam installed in a press.  
         [0026]      FIG. 3  is an exploded pictorial view of an aerial cam of a prior design.  
         [0027]      FIG. 4  is a pictorial exploded reverse view of the prior aerial cam shown in  FIG. 3 .  
         [0028]      FIG. 5  is an exploded pictorial view of an aerial cam according to the present invention.  
         [0029]      FIG. 6  is an exploded reverse pictorial view of the aerial cam shown in  FIG. 5 .  
         [0030]      FIG. 7  is a pictorial partially sectioned view of the aerial cam shown in  FIGS. 5 and 6 .  
         [0031]      FIGS. 8A-8D  are reduced size simplified views of an aerial cam according to the invention, showing successive stages in the work cycle.  
         [0032]      FIG. 9  is a diagrammatic view of the aerial cam shown in  FIGS. 8A-8D , in the fully advanced position.  
         [0033]      FIG. 10  is a side elevational view in partial section of another configuration of an aerial cam according to the present invention of the configuration.  
         [0034]      FIGS. 11 and 12  are enlarged fragmentary sectional views showing the relationship of T blocks and channel block supporting and guiding the slide on the adapter as the press is cycled with an exaggerated illustration of the clearance spaces used to effect a delay in positive engagement between the driver and slide.  
         [0035]      FIG. 13  is a partially exploded view of a second embodiment of an aerial cam according to the present invention.  
         [0036]      FIG. 14  is a view of a lengthwise section taken through the aerial cam shown in  FIG. 12 .  
         [0037]      FIG. 15  is a view of the transversely sectioned aerial cam shown in  FIGS. 12 and 13 .  
         [0038]      FIGS. 16A, 16B , and  16 C are side elevational views of the second embodiment of an aerial cam shown in  FIGS. 13-15  in partial section installed in a press, shown in fragmentary form, in successive positions occurring during operation of the press.  
         [0039]      FIG. 17  is a partially sectional side elevational view of an aerial cam according to the invention having a positive retraction mechanism incorporated therein according to a further feature of the invention.  
         [0040]      FIG. 18  is a fragmentary transverse sectional view through the cam shown in  FIG. 17 .  
         [0041]      FIG. 19  is a pictorial view from below of the channel block attached to the slide shown in  FIGS. 17 and 18 .  
         [0042]      FIG. 20  is a pictorial view from above of the key attached to the driver of the cam shown in  FIGS. 17 and 18 .  
         [0043]      FIG. 21  is an exploded pictorial view of a die mounted cam according to the present invention.  
         [0044]      FIG. 22  is a side elevational view of the components of the cam shown in  FIG. 21  prior to engagement of the driver with the slide.  
         [0045]      FIG. 23  is a side elevational view of the components of the cam shown in  FIG. 22  with the driver in initial engagement with the slide.  
         [0046]      FIG. 24  is a side elevational view of the components of the cam shown in  FIG. 23  with the driver fully descended.  
         [0047]      FIG. 25  is a fragmentary pictorial view of the portions of the adapter and slide in partial section. 
     
    
     DETAILED DESCRIPTION  
       [0048]     In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims.  
         [0049]     Referring to  FIGS. 5-7 , an aerial cam  38  according to the present invention includes an adapter  40 , a slide  42 , and a driver  44 .  
         [0050]     The adapter  40  is affixed to an upper die part or press upper platen (neither shown) using keys  46  to be accurately and securely positioned thereon.  
         [0051]     The driver  44  is mounted to a lower die part or directly to a press lower platen (neither shown) with upper surfaces of wear plates  76  and lower surfaces of wear plates  78  aligned to become engaged upon continued descent of the upper platen.  
         [0052]     The slide  42  is suspended on the adapter  40  by a pair of inboard located T blocks  48  affixed to the upper side of the slide, and passing through complementary slots or channels  50  in a base plate  52  of the adapter  40 . As seen in  FIGS. 7, 11  and  12 , the base plate  52  has an underplate  54  affixed thereto, with slots  56  formed therein freely receiving the narrow lower part  49  of a respective T block  48 , the head portion  51  resting on surfaces adjacent the respective slot  56 .  
         [0053]     A predetermined clearance space  58  ( FIGS. 8A and 11 ) exists between the mating bearing surfaces on slide  42  and adapter  40  when the slide  42  is suspended from the adapter  40 , but is taken up after a momentary delay when the press motion causes the initial engagement of the slide  42  with the driver  44  as described below. The clearance space  58  is shown greatly exaggerated and can be relatively slight, i.e., a space on the order of 0.001-0.002 inches would normally be sufficient. Thereafter, the press motion causes forcible engagement of the various mating surfaces to drive the slide  12  along the working angle.  
         [0054]     The slide  42  moves laterally on the adapter  40  in the embodiment shown in  FIGS. 5-7  when the slide  42  is shifted by engagement with the driver  44  as the upper press platen descends as seen in  FIGS. 8B-8D .  
         [0055]     A set of wear plates  60  mounted on the surface of the adapter  40  rides on a mating set of wear plates  62  on the upper side of the slide  42 .  
         [0056]     As will be discussed below, the angle of the engagement surface on the adapter  40  changes with the angle of the driver  44  since the included working angle of the slide  42  typically remains constant with changes in the working angle. Thus, the wear plates and the surfaces on the adapter  40  will be inclined down from horizontal as the working angle becomes shallower.  
         [0057]     The lateral component of the motion of the slide  42  relative the adapter  40  proceeds against the resistance of a series of compression springs  68  in pockets  64  formed in the slide  42 , the springs  68  projecting out against end wall  66  of the adapter  40 .  
         [0058]     A closure lock as described in copending U.S. application Ser. No. 10/954,960, filed on Sep. 29, 2004 may be employed particularly if nitrogen springs are used.  
         [0059]     A combination mechanical spring may be used instead of nitrogen springs as described in U.S. application Ser. No. 10/936,213, filed on Sep, 7, 2004.  
         [0060]     The driver  44  is preferably of a segmented built up construction comprised of a flat base plate  70 , having a side by side series of parallel upright flat plates  72  affixed to the upper surface. The length and working angles are easily varied by changing the configuration and number of plates  72  and the size of the base plate  70 . This is much cheaper than producing a new casting for each configuration particularly considering that a separate mold for each configuration is necessary as described in copending application Ser. No. ______, filed on ______, attorney docket DNY-122.  
         [0061]     A support plate  74  is affixed to the upright plates  72  held at the working angle by the angled upper ends of the upright plates  72 .  
         [0062]     Cam wear plates  76  are secured to the support plate  74 .  
         [0063]     The inclined lower side of the slide  42  is provided with mating cam wear plates  78 .  
         [0064]     An upwardly projecting central locator-guide key  80  is affixed to the driver  44 , aligned with a central slot  82  in the lower side of the slide  42 .  
         [0065]     The locator key  80  is placed and configured to move into the slot  82  as the upper platen lowers the slide  42  into engagement with the driver  44  but before engagement of the cam wear plates  76 ,  78 . This laterally locates the slide  42  and guides it after the slide  42  is advanced along the working angle by the platen motion and engagement of the cam wear plates  76 ,  78 .  
         [0066]      FIGS. 8A-8D , and  11 , illustrate the successive stages of movement of the first embodiment of the aerial cam according to the invention.  
         [0067]     In the initial condition shown in  FIGS. 8A , the slide  42  is suspended below the adapter  40  by the T blocks  48  and channels  50 , with the predetermined clearance space  58  therebetween.  
         [0068]     As the upper platen  84  descends towards the lower platen  86 , the locator key  80  enters the slot  82  to provide lateral location and guidance, as seen in  FIG. 8B .  
         [0069]     The clearance space  58  is then still present, and the surfaces of the wear plates  76 ,  78  have not yet engaged.  
         [0070]     Continued descent of the upper platen  84  brings the surfaces of the wear plates  76 ,  78  into initial contact as seen in  FIG. 8C . The clearance space  58  still exists, although now being reduced.  
         [0071]     This initial contact of the wear plates  76 ,  78  allows the downward momentum of the slide  44  to be absorbed by driver  44  and redirected to cause lateral motion of the slide  42  to be initiated as suggested by the partial compression of the springs  68  shown.  
         [0072]     It should be noted that the extent of this motion and the size of the clearance space  58  is shown in an exaggerated form in order to be readily visible in the drawings.  
         [0073]     In the next stage, shown in  FIGS. 8D and 12 , the clearance space  58  has now been completely taken up, and the press upper platen  84  forcibly causes continued camming advance of the slide  42  laterally along the working angle. This drives the tooling  88  into contact with a workpiece W, fully compressing the springs  68  in the advanced position, as indicated diagrammatically in  FIG. 9 .  
         [0074]     Thus, in the initial engagement of the wear plates  76 ,  78  only the downward momentum of the slide  42  is absorbed by the driver  44 , and the positive press drive is momentarily delayed until the clearance space  58  is taken up. This reduces shock and noise, and obviates the need for auxiliary cam rollers, formerly used.  
         [0075]     As noted, if the working angle is shallower, the adapter  40  will have an inclined surface on which the wear plates are mounted as seen in another embodiment shown in  FIG. 10 .  
         [0076]     In this case, the adapter  40 A may also be constructed using a parallel series of plates  92  each mounted to a base plate  93  cut at an angle to incline base plate  93 , in similar fashion to the driver  44 A. The T block  96  passes through a slot in the support plate  94  and has wings which ride on the upper surface  95  of the support plate  96 . The same initial clearance between wear plates  97 ,  99  is provided as indicated.  
         [0077]     Referring to  FIGS. 13-15  and  16 A,  16 B,  16 C, a second embodiment of a simpler aerial cam  100  according to the invention is shown, of a much smaller size.  
         [0078]     In this embodiment, the driver  130  has a horizontal slide surface and the adapter  102  is formed with a sloping cam surface engaging a complementary surface on the slide  106 .  
         [0079]     The adapter  102  is mounted on an upper platen  104  (FIGS.  16 A-C) of a press. A slide  106  is suspended on the adapter  102  by means of a single centrally located T block  108  secured to an upper sloping surface  110  of a slide block  112  by screws  114 .  
         [0080]     A T guide  116  is attached to the adapter body  118  by screws  120 , and is formed with a T-shaped channel  125  defined by surfaces  122  and  124  configured to slidably receive the T block  108 . The weight of the slide  106  is supported on surfaces  124  by the wings of the T block  108  before the adapter  102  forcibly engages the slide  106  after the slide  106  engages the driver  130  fixed to the lower platen  132  ( FIG. 16A ). The channel  125  is elongated to accommodate lateral movement of the slide  106 .  
         [0081]     Downwardly facing inclined cam surfaces  126  on the T guide  116  have wear plates  128  secured thereto with screws (not shown) abutting an inclined cam surface on the upper sloping surface  110  on the slide block  112  when the slide  106  is engaged by the driver  130 .  
         [0082]     An end face  134  of the slide block  112  is aligned with a facing surface  136  of the adapter body  118 .  
         [0083]     A single combination spring  138  is received in a bore  140  in the slide block  112  and is compressed against the surface  136  when the slide block  112  is advanced towards the surface  136 . The combination spring  138  abuts a closure plug or a snap ring  142  adjacent the end of the bore  140  to allow compression thereof.  
         [0084]     A variable spring rate is produced by the combination spring  138  as described in detail in copending U.S. application Ser. No. 10/936,213, filed on Sep. 7, 2004, attorney docket no. DNY-119.  
         [0085]     A retainer-stop plate  144  is secured to the stepped front face  148  of the T guide  116  with a screw  146  to keep the T block  108  within the T shaped channel  125 .  
         [0086]     The slide  106  is equipped with a tooling plate  150  to allow adjustment of the location of the tool T as described in U.S. application Ser. No. 11/027,494, filed on Dec. 30, 2004, attorney docket no. DNY-120.  
         [0087]     The driver  130  has a central locator-guide key  152  attached to a base  154  with screws  156  fixed to the press lower platen  132 , aligned with a mating central slot  158  in the slide block  112 .  
         [0088]     A pair of wear plates  160  is attached to under surfaces  162  on the slide block  112  with screws (not shown). The surfaces  161  of the wear plates  160  engage aligned surfaces  164  on the driver base  154  on either side of the locator key  152 .  
         [0089]     Referring to  FIGS. 16A, 16B  and  16 C, at the start of a cycle the upper platen  104  is elevated so that the slide  106  is spaced above the driver  130 . The slide  106  is suspended on the adapter  102  such that a predetermined clearance space “A” exists between the top of the T-shaped channel  125  and top of the T block  108 . The undersides of the T block  108  rests on the surfaces  124 .  
         [0090]     A predetermined clearance space “B” also exists between wear plates  128  and surfaces  110 .  
         [0091]     As the upper platen  104  is lowered, the locator-guide key  152  enters the slot  158  as seen in  FIG. 16B , and the surface  164  and wear plates  160  engage.  
         [0092]     The spaces A, B thus are eliminated, and the wings of the T block  108  lift off surfaces  124 . Wear plates  128  engage surface  110  at the same time. The presence of the gaps A, B delays the forcible engagement between the wear plates  128  and surface  110  by the press motion, such that the momentum of the slider  106  is first absorbed by the driver  130  by engagement of the surfaces  162 ,  164 , as in the above described embodiment.  
         [0093]     Forcible engagement therebetween by the press is thus momentarily delayed. Thereafter, the camming action proceeds due to the engagement of the angled wear plates  128  and surface  110 , driving the slide  106  horizontally fully to right as seen in  FIG. 16C .  
         [0094]     Thus, shock loading is reduced without the use of auxiliary rollers, etc. to reduce the manufacturing costs.  
         [0095]     The locator-guide key  152  and guide T block  108  and T-shaped channel  125  are centrally located and thus easy to machine to reduce costs to achieve the objects of the invention.  
         [0096]     Referring to  FIGS. 17-20 , a positive return interengagement between the driver  176  and slide  178  is shown in another embodiment of an aerial cam  116  according to the invention incorporated in the driver  176 , slide  178 , locator-guide key  168  and channel  174 .  
         [0097]     The aerial cam  166  is similar to the embodiment shown in  FIGS. 14 and 15 .  
         [0098]     However, the locator-guide key  168  attached to the driver  176  has a pair of T heads  170  fixed thereto, creating a localized T block shape along the length thereof.  
         [0099]     A channel block  172  affixed to the slide  178  is formed with a T-shaped channel  174  mating with locator-guide key  168 .  
         [0100]     A pair of openings  178 A,  178 B are aligned with the T heads  170  when the slide  178  is in the retracted position so that the T heads  170  can enter the channel  174  as the press is operated. The T heads  170  slide along the channel  174  as the driver  176  cams the slide  178  laterally as the press motion continues capturing the T heads  170  therein.  
         [0101]     Upon retraction of the upper platen (not shown), if the compressed return spring  180  is not sufficiently strong to drive the slide  178  back to its start position, the T heads  170  forcibly engage the wings  182  of the channel  174 , positively camming the slide  178  back to its start position. The openings  178 A,  178 B are then aligned with the T heads  170  and this allows separation of the driver  176  and slide  178 , and continued upward movement of the slide  178 .  
         [0102]     The use of two spaced apart T heads  170  doubles the length of structure exerting the positive return forces over the bars previously used to greatly strengthens the mechanism and avoid serious damage when a tool is severely hung up.  
         [0103]     A die mounted embodiment of a press mounted cam  184  according to the present invention is shown in  FIGS. 21-25 .  
         [0104]     The die mounted cam  184  has the driver  186  mounted on an upper press platen or die part (not shown) above a slide  188  mounted to an adapter  190  mounted to a lower platen or die part (not shown).  
         [0105]     A locator-guide key  192  is centrally affixed to the driver  186  of cam surfaces  194 , aligned with a channel  196  defined in an angled cam plate  198  forming a part of slide  188 .  
         [0106]     The slide  188  is movably mounted on the adapter  190 , guided by a centrally located T block  200  ( FIG. 25 ) affixed to the bottom of the slide  188 . A T shaped channel  202  formed in the adapter  190  guides lateral movement of the slide  188  on the adapter  190  on surfaces  191 ,  193 .  
         [0107]     A return spring  204  received in a cavity  206  engages an opposing plate  208  of the adapter  190  to be compressed as the slide  188  is cammed laterally by the driver  186 .  
         [0108]     The locator-guide key  192  has a pair of T heads  210 A,  210 B creating localized T shapes on the key  192 .  
         [0109]     A slot  212  interrupts the wings  214  of the channel at a point aligned with one of the T heads  210 A,  210 B with the slide  188  in a retracted start position as seen in  FIG. 22 .  
         [0110]     As the driver  186  descends, the lead T head  210 A passes through the slot  212  to enter the channel  196 . The main body of the locator-guide key  192  locates on the sides of wings  214  of the channel  196  and guides the slide  188  laterally as the camming surfaces  194 ,  195  are engaged to drive the slide  188  to the left to the fully advanced position shown in  FIG. 24 .  
         [0111]     Upon reversal, the T heads  210 A, B engage the underside of the channel wings  214  to positively retract the slide  188  if the compressed return spring  206  is unable to do this.  
         [0112]     Upon reaching the full retracted position of the slide  188 , the slot  212  is again aligned with element  210 A and the other element  210 B clears the channel  196  to allow separation of the driver  186  from the slide  188 .