Abstract:
A screen printing apparatus having a rotatable turret with a plurality of work supports is indexed by a curved geneva mechanism to each of a plurality of index positions. The curved geneva mechanism includes a plurality of curved slots having surfaces engaged by the driver with the curved slots designed to minimize the maximum amount of inertia and to maximize the available time for deceleration of the turret to provide a smoother and slower stopping of the turret and registration without banging and jarring of the apparatus. The preferred curved slots have a modified sine wave characteristic to provide slower changes in velocity during the stopping. The amount of inertia change during the indexing motion is analyzed and the curved slots are generated to eliminate abrupt changes in inertia that would result in a rough movement.

Description:
This invention relates to a rotary indexing mechanism for machines using a Geneva mechanism for indexing and, more particularly, to a screen printing apparatus having a turret or turntable which is indexed by a Geneva mechanism. 
     BACKGROUND OF THE INVENTION 
     A conventional screen printing apparatus for multi-color printing on cut piece textile goods or garments is disclosed in U.S. Pat. No. 4,099,460, which is hereby incorporated by reference as if fully reproduced herein. As disclosed in this patent, the multi-color, textile screen printing apparatus has a number of arms or spiders each of which carries a platen supporting the textile piece for travel in a circular path past each of a plurality of printing machines each of which has a screen and squeegee for applying a coating of a different color or impression to the workpiece when the textile piece is at a given station. Typically, the number of printing machines varies from about four to eight. In order for each of the different colors to be registered with a preceding or succeeding color or impression, the turntable is precisely located and stopped in a registered position by a registering means such as a fork which engages a locking pin on the turntable so that the turntable is precisely locked in position at the termination of each of the arcuate indexing movement of the turntable. 
     In the aforementioned patent, the indexing mechanism which turns the turntable includes a conventional Geneva mechanism which has a wheel or indexer having a plurality of straight radially extending slots therein to receive a drive roller or pin mounted on the end of a rotatable drive arm which is driven by an electric motor through a gear box mechanism. A cam operated switch means cooperates in conjunction with the movement of the turntable to operate limit switches to control the printing cycle of the printing machines after the turntable has been indexed and registered. 
     While the aforementioned conventional Geneva mechanism works satisfactorily, particularly for smaller sizes of printing machines and for turntables of a relatively small diameter, the momentum of the larger diameter turntables makes it difficult to stop the indexing movement with conventional sizes of motors and gear boxes. The load on the motors and gear boxes to stop the turntable becomes excessive. For instance, the typical sizes os screens printed with the systems of the aforementioned patent had platens with textile supported thereon which were printed by screens having a maximum size of 22×22 inches. In many instances, it is desired, but it is not economically feasible at this time, to have larger radius arms and to have larger size platens which will cooperate with printers having screens of 25×38 inches, 30×40 inches, and even larger. Of course, with the bigger screens and bigger arms for supporting the larger sizes of textile goods, the momentum and the amount of energy to be dissipated during deceleration is increased particularly with the machines printing four to eight different colors. 
     The turntables are indexed through relatively small increments usually ranging from 90° for a four-color machine having four indexes per revolution to 45° for an eight color machine having eight indexing increments per revolution. For each of the indexing movements, there must be an initial acceleration from a dead stop and then a deceleration to again a dead stop. The controlling of the stopping momentum of the arms and turntable has been a problem which has been addressed in different manners including the use of mechanical brakes, which are very hard to set and to adjust properly, so that the brakes wil dissipate the energy needed to decelerate the indexer and to stop it precisely. Other approaches for smaller size machines have used electrical controls for the motor but this has been found not to be satisfactory for larger sizes of turntables. 
     U.S. Pat. No. 4,099,460 uses the conventional Geneva mechanism having the straight line slots and driver roller on the crank arm in which the first half of the roller engagement with the slot in the wheel of the indexing cycle is used to accelerate the turntable from a dead stop to its maximum velocity and then the second half of the indexing cycle is used by the drive roller and slot to slow down and stop the turntable movement. Thus, there is fifty per cent division of time and movement for starting and stopping with the conventional Geneva mechanism. During this deceleration, a relatively high torque, for example, 20,000 inch-pounds is generated on the gear box for small presses. The usual limitation of the size of the screen printing apparatus is the maximum inertia that will be generated and then dissipated by the gear box and motor when stopping the indexing. 
     With the fine registration needed for multi-color screen printing, the turntable needs to be decelerated slowly as it arrives at the stop position and then it needs to be stopped precisely at the stop position. If the turntable is not decelerated slowly or if the turntable is not precisely stopped, the registering means is often subjected to shock and jarring which leads to maintenance problems and breakage if the stopping is with a hard jarring and from high force engagement of the registering fork and pin. Also such hard, fast stops having a tendency to throw off the registration. 
     In addition to solving the problem of dissipating the momentum forces used for large size turntables in screen printing machines, it is preferred that the apparatus be very predictable first, as to the position of the turntable at all times; secondly, as to maximum torques generated for a wide variety of sizes of spider arms; and thirdly, as to a number of different variables including the size of the printing screens and the number of different printing stations ranging from four to eight. 
     In accordance with the present invention, there has been provided a new and improved Geneva mechanism for use with screen printing apparatus having a rotatable turntable in which the stopping is controlled slowly and over a longer period of time than with a conventional Geneva mechanism. 
     As will be described herein in greater detail, the preferred apparatus accelerates the turntable more quickly and displaces the turntable through one half of its angular movement much earlier in the indexing cycle and then uses the second half of the indexing cycle period to decelerate the turntable more slowly and over a smaller displacement to a slow stop. 
     Also, as will be explained hereinafter, the indexing means is very predictable and reproducable and allows computation of the maximum momentum and torque loads as well as an X by Y displacement location of the indexer and its velocity during the indexing cycle. 
     Accordingly, a general object of the present invention is to provide a new and improved screen printing apparatus having an indexable turntable which must be accelerated and stopped a plurality of times through each revolution. 
     Another and more specific object of the invention is to provide a multi-color textile printer with an improved indexing control mechanism having a curved slot Geneva mechanism operable to provide a longer deceleration time for the turntable. 
     These and other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings in which: 
    
    
     DETAILED DESCRIPTION OF THE DRAWING 
     FIG. 1 is a partial view of a multi-color printing apparatus having a turntable movable past screen printing machines. 
     FIG. 2 is a fragmentary side elevational view of a Geneva mechanism used to drive a turntable. 
     FIG. 3 is a reduced size sectional view taken substantially along the line 3--3 of FIG. 2 showing a curved slot Geneva mechanism. 
     FIG. 4 is a view illustrating the curved slots in the indexer. 
     FIG. 5 is a fragmentary view of a fork moving into locking engagement with a registering pin on the turntable. 
     FIG. 6 is a sectional viewing showing a cam and switch operated thereby. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in the drawings for purposes of illustration, the invention is embodied in a screen printing apparatus of a multi-color kind which includes a turret or turntable assembly 12 having a turntable 24 mounted for turning about a central shaft or axis 14 which is disposed vertically in this instance. The turntable has work carriers or platens 28 rotated consecutively past each of a series of silk screen printing machines 15a, 15b, 15c and 15d. Each of the illustrated printing machines has a printing screen 16 and a reciprocating squeegee mechanism 21 for printing onto the textile workpiece which will be mounted on the platens. As best seen in FIG. 2, the turntable assembly 12 includes a large central drive wheel 20 fixed to an upper end of the central shaft 14 at a location below the turntable 25. Thus, the turntable and wheel are fixed to and rotate with the shaft 14 about the vertical axis through the shaft which is journaled for rotation in bearings 29 held on base 22 for the machine. 
     As best seen in FIG. 3, the wheel 20 is provided with a plurality of slots 26 and the slots 26 receive therein a driver or driver roller 32 which is mounted on a rotatable drive arm 30. Herein, a pair of drive rollers 32 are mounted 180° apart on the drive arm 30. The latter is mounted for rotation about a vertical drive shaft 34 to bring the drive roller 32 into driving engagement with sidewalls 27 of the slots 26 on the wheel. Herein, the drive pin or drive rollers 32 project upwardly from the drive arm 30 to seat within the slot 26 of the wheel 20, as best seen in FIG. 2. The top of the wheel 20 includes a planar surface which closes the top of the slots 26. As shown in FIG. 3, the drive arm 30 rotates in a clockwise direction and the roller 32 enters the slot 26 engages the slot wall 27 and drives the wheel 20 in a counterclockwise direction. The illustrated drive arm 30 is driven by a varible speed motor 36 which includes a gear box 37 kind of speed reduction device which produces the power output at the shaft 34 at the selected speed. 
     In FIG. 6 of U.S. Pat. No. 4,099,460, the slots shown in the wheel are each straight line slots which are radially directed toward the central rotating axis of the shaft 14. In the conventional Geneva mechanism, the dirve roller engages slot walls and turns the wheel with the wheel receiving an initial acceleration through the first one half of the turning of the wheel 20 which will be 30°, in this instance, where there are six slots. Then, in the remaining 30° of engagement of the drive roller 32 with the slot walls, the turntable is decelerated from its maximum velocity which occurs when the driver 32 is directly aligned on a theoretical line extending between the rotational axes of the shaft 14 and the shaft 34. The nature of the slow-down deceleration through this 30° is particularly accompanied by a very rapid deceleration which sometimes causes a banging or a jarring which is hard on the equipment and which sometimes causes a loss of registration or preventing the registering mechanism from operating effectively so as to assure a precise registration at each of the stations. In this conventional Geneva mechanism, the drive roller enters and exits through a common slot end on the periphery of the indexer wheel. 
     In accordance with the present invention, there is provided a new and improved Geneva mechanism which has a curved slot 26 which is precisely dimensioned and controlled such that the wheel 20 is accelerated faster to its maximum velocity in less than 50 percent of the indexing movement and over a shorter period of time i.e., less than 50 per cent of the indexing time period leaving the deceleration to occur with a substantially less displacement of the wheel 20 during the second half of the driver&#39;s travel through the curved slot. Because the displacement of the wheel may be, for example, 80% during the first half of the driver&#39;s movement into the curved slot thereby leaving only 20% of the wheel&#39;s displacement to occur over the second half of driver&#39;s movement, there can be a slower deceleration of the wheel to avoid the quick stop and jarring of the straight slot type of Geneva mechanism. Preferably, the displacement of the wheel terminates before the driver 32 leaves the curved slot 26 as will be explained hereinafter. 
     In order to compare the movement of the curved slot Geneva mechanism with the straight radial slot Geneva mechanism of the prior art, straight line slots 126 have been drawn in phantom lines on the Geneva wheel 20 in FIG. 4. The slots 26 are formed with curved walls 27 extending inwardly to an inner slot end wall 28 and the slots 126 have straight walls 127 extending radially inwardly to inner slot end walls 128. It will be seen that the end wall 28 trails the end wal 128, i.e., the end wall is displaced rearwardly in the counterclockwise direction of wheel travel by a displacement distance &#34;X&#34;. When the driver 32 has moved through one half of its movement in a slot 26 or 126, the driver 32 is aligned on a straight line &#34;L&#34; shown in FIG. 3 between the axes of the wheel shaft 14 and the driver shaft 34 and the driver 32 has penetrated to its deepest position in the slot which is a position closely adjacent the end wall 28 for this curved slot Geneva mechanism and which is a position closely adjacent the end wall 128 in the straight line conventional Geneva. With curved slots 26 in the wheel 20, the wheel will, however, have been turned through an additional incremental movement in the clockwise direction equal to the displacement &#34;X&#34; than will the conventional Geneva mechanism having straight line radial slots. It will be seen that looking at the line &#34;L&#34; in FIG. 4, that the curved slot 26 slopes away from the driver 32 when it is at its deepest penetration. That is, the slot curves forwardly in the direction of rotation such that while the driver is traveling outwardly toward the periphery of the wheel, it wil be camming against the walls 27 and displacing the wheel 20 significantly less than it would for a straight line slot 126. Because the total displacement for the straight line slot 126 and for the curved slot 26 in turning the wheel 20 through 60° of travel are equal, then during this second half of travel of the driver 32 in an outward direction in the straight slot 126, the driver must push the wheel 20 through a displacement distance &#34;X&#34; greater than does the driver in the curved slot 26. Also, in the straight line slot 126, the displacement is substantially linear and occurring right up to the moment that the driver leaves the slot 126 whereas in the curved slot the driver may not be doing any displacement of the wheel for the last few degrees of its travel as the driver leaves the curved slot 26. Thus, the wheel may be decelerated and coast to a stop with the curved slot Geneva mechanism. 
     The curved slot 26 may be precisely computed and drafted and curved in accordance with the crank rotation so that the displacement of the inertia wheel 20 is moved through the desired displacements to provide the faster acceleration and the longer deceleration. As will be explained, the preferred movement of the turntable through its indexing positions is like that of a sine wave. 
     The curve for the slot 26 is generated by assumptions such as what percentage of the wheel displacement should occur by the time the driver 32 is aligned with the axes of the shafts 14 and 34 and is in a straight line position with them. Present experience has shown that the displacement of the wheel may be as much as 80% during the first half of the driver movement in the curved slot. The maximum velocity is achieved when the crank driven roller 32 is aligned in a straight line with the axes of the shafts 14 and 34, as shown in dotted lines in FIG. 3. The crank position for each of a number of positions in X and Y coordinates is generated and the inertia change and the indexer wheel movement and the momemtum of the indexer wheel are calculated. An analysis of the inertia change will give an understanding of what size or power the electric motor 37 must be and the mnomemtum will show the maximum forces that will be applied to the gear box and the energy load that must be dissipated to stop the indexer wheel turning movement. 
     In accordance with an important aspect of the present invention, the maximum momentum can be readily calculated and can be limited by changing the various variables so that the gear box 37 is not overloaded as to cause its failure or excessive wear due to high loads applied thereto during the stopping of the indexing movement. By way of example, there is provided in Table 1, a samaple of a curved slot plotted for a machine of the type shown herein but which has eight arms and eight indexes per revolution. Each spider arm has a length of 60 inches and the bearing diameter at the central shaft 14 being 1.25 inches. The number of indexes per hour is 1,000, and the radius of the wheel is 12 inches and the radius of rotation for the crank arm and its drive roller 32 is 4.97 inches. The difference between the centerlines of the crank shaft 14 and the driver shaft 34 is 12.9887 inches. In the example given in Table 1, the percentage of the indexing movement during the inward travel of the driver to the slot end eall 28 is 60 percent. Table 1 lists in &#34;X and Y&#34; coordinates the location of crank roller 32, and hence, the point on the slot wall 27 at which the roller 32 is in engagement for each of a large number of successive positions labeled from 69 through 135. Table 1 also lists the &#34;inertia change&#34;, the &#34;indexer movement&#34;, and, &#34;momentum&#34;, at each crank rotation position. At crank rotation position 89, the inertia change reaches 2093. Beginning at crank rotation position 125, it will be seen that the slow down of the wheel is taking place very quickly and that through the last positions of 128-135 the wheel 20 is essentially at its registered position even though the roller 32 has not yet left the slot 26. The &#34;zero&#34; in the several columns at the end of the Table 1 shows almost no movement of the wheel 20 at the end as it coasts into the registering position. 
     
                                           TABLE 1__________________________________________________________________________Spider Arm: Length 60 inchesBearing Diameter + 1.25 inch% of Index to Stop = .6Number of Indexes Per Hour = 1000Radius of the Geneva = 12 inchesRadius of the Crank = 4.97055809 inchesCrank Centerline To Shaft Centerline = 12.9887046 inches           INERTIA                  INDEXERCRANK           CHANGE MOVEMENT                          MOMENTUMROTATION  X   Y    (inch lbs)                  (inch)  (inch lbs.sup.2)__________________________________________________________________________ 69    7.407      -1.505           -214291                  .76     214191 70    7.419      -1.394           146081 .429    68210 71    7.433      -1.369           2196   .422    66014 72    7.449      -1.344           1595   .417    64419 73    7.468      -1.319           1535   .412    62884 74    7.49      -1.294           1576   .407    61308 75    7.514      -1.268           2387   .399    58921 76    7.54      -1.243           1652   .393    57269 77    7.569      -1.218           1657   .387    55612 78    7.6 -1.192           1793   .381    53819 79    7.633      -1.167           2386   .372    51433 80    7.668      -1.141           1765   .366    496680 81    7.706      -1.116           1702   .359    47966 82    7.746      -1.09           1848   .352    46118 83    7.789      -1.065           2321   .344    43797 84    7.833      -1.04           1731   .337    42066 85    7.88      -1.015           1696   .33     40370 86    7.928      -.99 1996   .322    38374 87    7.979      -.965           2262   .312    36112 88    8.032      -.94 1608   .305    34504 89    8.086      -.915           1571   .298    32933 90    8.143      -.891           2093   .288    30840 91    8.201      -.866           1907   .279    28933 92    8.262      -.842           1565   .271    27368 93    8.324      -.818           1547   .264    25821 94    8.387      -.794           2069   .253    23752 95    8.453      -.77 1612   .244    22140 96    8.52      -.746           1399   .236    20741 97    8.588      -.723           1381   .228    19360 98    8.659      -.699           1932   .216    17428 99    8.73      -.675           1360   .208    16068100    8.803      -.652           1230   .2      14838101    8.878      -.629           1195   .191    13643102    8.954      -.606           1696   .179    11947103    9.031      -.583           1070   .171    10877104    9.109      -.56 1032   .163     9845105    9.188      -.537           1068   .153     8777106    9.269      -.514           1484   .14      7293107    9.351      -.491            641   .134     6652108    9.433      -.468            706   .126     5946109    9.517      -.446            966   .116    49800110    9.602      -.422            954   .104     4026111    9.688      -.399            600   .096     3426112    9.774      -.377            547   .088     2879113    9.861      -.354            608   .078     2271114    9.95      -.33  539   .068     1732115    10.038      -.308            348   .068     1732116    10.128      -.285            301   .054     1083117    10.218      -.262            323   .045     760118    10.309      -.24  237   .037     523119    10.4      -.218            146   .031     377120    10.492      -.196            116   .026     261121    10.585      -.175            118   .019     143122    10.678      -.155            58    .015      85123    10.771      -.135            37    .011      48124    10.865      -.116            26    7E-03     22125    10.959      -.099            16    4E-03     6126    11.053      -.082             4    2E-03     2127    11.148      -.067             2    1E-03     0128    11.243      -.053             0    0         0129    11.338      -.041             0    0         0130    11.434      -.03   0    0         0131    11.529      -.021             0    0         0132    11.625      -.014             0    0         0133    11.721      -8E-03             0    0         0134    11.817      -4E-03             0    0         0135    11.913      -1E-03             0    0         0__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Spider Arm: Length 60 inchesBearing Diameter + 1.25 inch% of Index to Stop + .5Number of Indexes Per Hour + 1000Radius of the Geneva = 12 inchesRadius of the Crank = 4.97055809 inchesCrank Centerline To Shaft Centerline = 12.9887046 inches           INERTIA                  INDEXERCRANK           CHANGE MOVEMENT                          MOMENTUMROTATION  X   Y    (inch lbs)                  (inch)  (inch lbs.sup.2)__________________________________________________________________________ 69    7.407      -.167           286254 .879    286254 70    7.419      -.076           159290 .585    126964 71    7.433      -.071            477   .584    126487 72    7.449      -.066            605   .583    125882 73    7.468      -.062            862   .581    125020 74    7.49      -.057           1052   .578    123968 75    7.514      -.052           2451   .573    121517 76    7.54      -.048           -403   .573    121920 77    7.569      -.045           1080   .571    120840 78    7.6 -.041           2462   .565    118378 79    7.633      -.038           1731   .561    116647 80    7.668      -.034           2027   .556    114620 81    7.706      -.031           2010   .551    112610 82    7.746      -.029           2598   .545    110012 83    7.789      -.026           2248   .539    107764 84    7.833      -.024           2581   .533    105183 85    7.88      -.023           2608   .526    102575 86    7.928      -.021           2652   .519    99923 87    7.979      -.02 2788   .512     97.35 88    8.032      -.02 2898   .504    94237 89    8.086      -.019           2891   .496    91346 90    8.143      -.02 3063   .488    88283 91    8.201      -.02 3436   .478    84847 92    8.262      -.021           3195   .469    81652 93    8.324      -.022           3235   .46     78417 94    8.387      -.023           3187   .45     75230 95    8.453      -.025           3316   .44     71914 96    8.52      -.027           3290   .43     68624 97    8.588      -.03 3151   .42     65473 98    8.659      -.033           3258   .41     62215 99    8.73      -.036           3353   .398    58862100    8.803      -.039           3603   .386    55259101    8.878      -.042           3352   .374    51907102    8.954      -.045           3085   .363    48822103    9.031      -.048           3095   .351    45727104    9.109      -.052           3043   .339    42684105    9.188      -.056           3012   .327    39672106    9.269      -.059           2995   .314    36677107    9.351      -.063           2866   .302    33811108    9.433      -.066           2685   .289    31126109    9.517      -.069           3096   .275    28030110    9.602      -.072           2576   .262    25454111    9.688      -.075           2452   .249    23002112    9.774      -.077           2371   .236    20631113    9.861      -.079           2226   .222    18405114    9.95      -.081           2099   .209    16306115    10.038      -.082           2016   .196    14290116    10.128      -.082           1943   .182    12347117    10.218      -.082           1951   .167    10396118    10.309      -.082           1343   .156     9053119    10.4      -.08 1659   .141     7394120    10.492      -.078           1378   .127     6016121    10.585      -.075           1177   .114     4839122    10.678      -.072           1034   .101     3805123    10.771      -.067            890   .088     2915124    10.865      -.062            738   .076     2177125    10.959      -.057            602   .065     1575126    11.053      -.051            480   .054     1095127    11.148      -.044            418   .042     677128    11.243      -.037            248   .034     429129    11.338      -.031            181   .025     248130    11.434      -.024            113   .019     135131    11.529      -.018            73    .012      62132    11.625      -.012            39    7E-03     23133    11.721      -&amp;E-03            16    4E-03     7134    11.817      -4E-03             6    1E-03     1135    11.913      -1E-03             1    0         0__________________________________________________________________________ 
    
     This should be contrasted with the data shown in Table 2, in which the same parameters are used except that the wheel has eight straight line, radial Geneva slots 126 such as shown in U.S. Pat. No. 4,099,460 and that 50 percent of the wheel movement occurs during the inward travel of the drive to the inner slot end wall 128 rather than 60 percent wheel movement as described above in connection with Table 1. 
     It will be seen that the motor size for the straight line Geneva should be higher as the inertia change at crank rotation position 90 is 3.063 inch-pounds versus the 2,093 inch-pounds of inertia change for the curved slot Geneva mechanism of Table 1. Contrasting positions of the crank position 128 in Table 2 with the crank position 128 of Table 1, it will be seen that the indexer crank driver and wheel still have 0.034 inches to travel whereas thet indexer crank driver and wheel for the curved slot mechanism have zero inches to travel showing that travel is already completed. This shows the significantly earlier stopping of the indexer movement which allows the registration mechanism to register even before the roller 32 leaves the slot 26. 
     The illustrated and preferred registering mechanism is that which is described in U.S. Pat. No. 4,099,460 and which comprises a locking fork 66 having a recess 68 to receive the locking pin 64 which is fixed to the turntable 25. The machine illustrated in FIG. 1 is a six color apparatus, with two of the six printing machines being not shown, to allow a better view of the turntable assembly 12. Thus, in the illustrated apparatus of FIG. 1, there will be six locking pins 64 each at 60° increments about the turntable 25. The fork is guided for vertical movement upwardly to the locking position by guide rollers 70. The fork is mounted on the upper end of the vertically movable locking bar 58 which is pivoted at its lower end to an actuating rod 56. The actuating rod 56 is pivoted at its rear end to a pivot pin 56a fastened to the base 22. The registering arm is actuated by a vertical arm 54 which carries a cam follower roller 52 which is movable by a cam means 50 which has cam rises 48 and cam falls 46 curved in a manner that the guide rod is lifted as the crank pin reaches position 128 in the Table 1 to provide the early locking and registering without the banging accompanying the use of the straight line Geneva mechanism of the prior art. Manifestly, there may be no registering means used or the registering means may be of various kinds of mechanism such as a detent mechanism and the screen printing apparatus will still fall within the purview of the appended claims. 
     When the turntable 25 stops at each index location, each of the printing machines is automatically actuated in a conventional manner, as disclosed in the aforementioned patent to begin a screen printing cycle. More specifically, the driver shaft 34 operates a switch means which includes limit switches 40 and 42 controlled by a cam 44 fixed to the shaft 34. The cam 44 has recesses 45 thereon which are followed by cam rollers 41 mounted on each of the switches 40 and 42 to operate the switches when the shaft and cam follower arrive at the termination of the indexing movement. As explained in the aforementioned patent, the operation of the limit switches controls the starting and stopping of the printing machines 15A-15D as well as the starting and stopping of the electric motor to drive the indexer. 
     From the foregoing, it will be seen that the present invention provides a new and improved curved slot Geneva mechanism for indexing the turntable in a screen printing apparatus. By having more than fifty percent of the total indexing movement occur during the first half of the crank and driver movement into the slot, there is less indexing movement to be made over the last half of the crank and driver movement outward from the slot. Because the slot is curved in the direction of wheel and driver travel, the wheel may have completed its travel before the driver exits the curved slot. This allows more time for stopping and for the engagement of the registering mechanism thereby eliminating the abrupt stopping or jarring often accompanied by the use of straight slot Geneva mechanism in the conventional apparatus. 
     While a preferred embodiment has been shown and described, it will be understood that there is no intent to limit the invention by such disclosure but, rather, it is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention as defined in the appended claims.