Abstract:
A tool carousel comprising a plurality of wheel sections which can be joined together to form a carousel wheel and a plurality of tool-holding portions. Each tool-holding portion and wheel section is configured such that each tool-holding portion can be journalled to a respective wheel section for rotation between a tool-storage position and a tool-accessing position in which a tool can be installed thereupon or removed therefrom.

Description:
This is a division, of application Ser. No. 09/150,967 filed Sep. 11, 1998 U.S. Pat. No. 6,155,961. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a tool carousel for use in a tool changer on a machining centre. 
     2. Background Art 
     A tool carousel comprises a wheel-like structure which, in use, carries a series of individually removable tools. In most cases, the carousel comprises a rotational index means, so as to enable any selected one of the tools carried by the carousel wheel to be located at a predetermined rotational position, whereat a transfer mechanism can present the selected tool to a machine tool for use in a machining operation. 
     FIG. 1 shows a typical example of a machining centre which includes a tool carousel. As can be seen, the tool carousel wheel  1  is mounted upon a horizontal rotational axis adjacent a transfer mechanism  2 . Referring to FIG. 2, it will be seen that the transfer mechanism  2  comprises a transfer arm  4  for transferring tools between the carousel wheel  1  and a machine tool spindle  3 . The transfer arm rotates about a central rotation axis  8  and has a tool grip at each end. 
     As can be best seen in FIG. 3, the carousel wheel  1  comprises a plurality of circumferentially consecutive pots  6 , each of which is used for storing a respective tool. In the storage position, each of the pots orientates its respective tool with its axis generally horizontal. However, when a tool is specified by the machine control, the carousel wheel  1  is rotated until the correct tool is located in the transfer position  5 , and the pot containing this tool is then rotated through 90° about a horizontal axis which is perpendicular to the rotational axis of the carousel wheel. As a consequence, the pot hangs vertically down and in the manner of the pot designated  7 . In this position, the tool has its axis parallel to the centre line of the machine tool spindle  3 . Once the pot is in this position, the transfer arm  4  is able to rotate about its vertical rotation axis  8  and remove the selected tool from its pot  7  whilst simultaneously removing any existing tool from the spindle nose  3 . As it continues to rotate, the position of the two tools is reversed, the selected tool is presented to the spindle nose, and the deselected tool is presented to the appropriate pot on the carousel  1 . That pot is then rotated back up through 90°, so that the deselected tool is stored with its axis inclined horizontally, in common with the other tools stored in the carousel. 
     Tools for use in a machine tool are invariably heavy, metal items and the carousel wheel of a tool carousel therefore has to be strong and sturdy enough to carry all of the tools without buckling or breaking over a long period of time during which the tool carousel is required to operate reliably, without breakdown. It has therefore been the practice to form carousel wheels of known tool carousels from metal. 
     FIG. 4 shows a vertical section through a known tool changer incorporating such a carousel. From the figure, it can be seen that a pot  105  has a generally cylindrical form and is attached to the hub  124  of a carousel wheel via a rotation axis  117 . The pot  105  is fitted with a collar  119  at an end thereof which is situated at the top of the pot when it is rotated through 90° about the axis  117  at the tool access position, as shown in dotted lines in FIG.  4 . Axially inwardly of the collar  119 , there is located a retention collar  104  that comprises a central axial bore into which a number of balls  100  are resiliently urged to project. In use, a pull-stud of a tool holder is located within the bore and gripped by the balls  100 . When the tool is to be extracted, this is achieved by axial displacement of an extractor  118  which can be axially pushed into the bore, to force the pull-stud out. 
     A dog  102  is provided in the mouth of the pot  105  for retaining the correct orientation of the tool holder. A tool identification tag is provided at  101 . 
     The pot  105  is integrally formed with an arm  103 . The arm  103  projects radially from the side of the pot body and terminates with a transversely extending section, upon which is fitted a rotatable pusher wheel  108  and a bearing  109 . The pot is retained in the storage position shown in FIG. 4 by the action of the bearing  109 , which bears against a bearing plate  125 . The plate is provided with a local slot at the pot release position, thus enabling the pot to be rotated about the axis  117 , when it is located there. The pusher wheel  108  is adapted to fit within a mouth  123  of a fork  122  mounted at the end of a rod  121  of a vertically aligned piston  120 . As the carousel wheel rotates, the pusher wheel  108  of each pot assembly consecutively enters the mouth  123  of the fork  122  from the side. When the appropriate tool holder is in position, pneumatic cylinder  120  is actuated, so as to cause the piston rod  120  to extend vertically downwards. As this happens, the pusher wheel  108  is urged downwardly by virtue of the fact that it is constrained within the mouth  123  of the fork  122 . This downward movement causes the pot  105  to rotate about the axis  117 , thereby eventually bringing the tool holder into the position shown in chain-dotted line in FIG.  4 . From this position, a transfer arm can transfer the tool to the machine head, as generally described in relation to FIGS. 1 to  3 . 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a tool carousel which is easier and less expensive to manufacture and, if necessary, repair than that of the known tool changer. 
     According to a first aspect of the invention there is provided a tool carousel according to claim  1 . Preferred features of this aspect of the invention are set out in claims  2  to  21 . According to a second aspect of the invention, claim  25  provides a lifting and lowering mechanism for a tool carousel wheel. A third aspect of the invention is set out in claim  28  and provides a drive mechanism for a tool carousel. Preferred features of the various aspects of the invention are set out in the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will be described by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic front elevation of a machining centre incorporating a tool changer; 
     FIG. 2 is a schematic plan view of the machining centre shown in FIG. 1; 
     FIG. 3 is a schematic side elevation of the machining centre shown in FIGS. 1 and 2; 
     FIG. 4 is a front elevation, partially in cross-section, showing part of a tool changer in accordance with the prior art; 
     FIG. 5 is a perspective view of thirty bracket units, assembled together to form part of a tool carousel in accordance with the invention; 
     FIG. 6 is a perspective view of one of the bracket units of FIG. 5 in conjunction with a pot unit and retention collar in accordance with the invention; 
     FIGS. 7A,  7 B and  8  are perspective views of the bracket unit of FIG. 6; 
     FIG. 9 is a plan view of the bracket unit of FIGS. 7 and 8; 
     FIGS. 10A and 10B are perspective views of the pot unit of FIG. 6; 
     FIG. 11 is a rear elevation of the pot unit of FIGS. 10A and 10B; 
     FIG. 12 is an underneath view of the pot unit of FIGS. 10 and 11; 
     FIG. 13 is a perspective view of the retention collar of FIG. 6; 
     FIG. 14 is a plan view of the retention collar of FIG. 13; 
     FIG. 15 is a ghost front elevation of a pot lifting and lowering mechanism in accordance with the invention shown in conjunction with the assembly of FIG. 6; 
     FIG. 16 is a ghost side elevation of the arrangement of FIG. 15; 
     FIG. 17 is a cross-section of the cam wheel of the pot lifting and lowering mechanism of FIGS. 15 and 16; 
     FIG. 18 is a front elevation of the cam wheel of FIG. 17; 
     FIG. 19 is a rear elevation of the cam wheel of FIGS. 17 to  19 ; 
     FIG. 20 schematically shows a positioning mechanism for use with a carousel wheel gear index; and 
    
    
     DETAILED DESCRIPTION 
     FIG. 21 is a graph of rotational speed relative to angular orientation of the shaft of the drive motor shown in FIG.  20 . 
     Referring to FIGS. 5 and 6, it can be seen that the tool carousel according to this embodiment of the invention comprises a number of bracket units  200  which are interlocked to form a carousel wheel  500 . Each bracket unit  200  is connected with a respective pot unit  300  and retention collar  400 . Each bracket unit  200 , pot unit  300  and retention collar  400  is injection moulded from a plastics material which comprises a chemical lubricant. Of course, the components could be manufactured from a different material and an alternative lubricant could be used. 
     Each of the three primary components shown in FIG. 6 will now be described in detail. 
     Where the bracket units are described, expressions such as “radial”, “axial” and “circumferential” are used with reference to the assembled carousel wheel shown in FIG.  5 . 
     One of the bracket units  200  is shown in detail in FIGS. 7A to  9 . It can be seen that the unit comprises a main bracket body portion  202 , which has a generally wedge-shaped profile when viewed in plan, such as in FIG.  9 . On a first radially extending side of the body portion  202 , there is provided a male dovetail portion  212 . On the opposite radially extending side of the body portion  202 , there is provided a female dovetail portion  214 . Each bracket unit  200  has the same configuration, so the dovetail formations from two adjacent bracket units  200  can be interlocked, so as to join them together. Due to the wedge-shape of the body portion  202 , thirty bracket unit can be joined together so as to form a complete ring, thereby defining the carousel wheel shown in FIG.  5 . Of course, it is not necessary for the carousel ring to be formed from thirty bracket assemblies. If it is intended for the carousel wheel to hold a smaller or greater number of tools, then a corresponding number of bracket units should be used. In such a case, the bracket units will need to have the radially extending sides of the body portion  202  moulded at an appropriately different angle of separation. Generally speaking, if a greater number of bracket assemblies are required in order to house a respectively larger number of tools, then the angle between the two sides of the body portion  202  will be relatively smaller. On the other hand, if fewer bracket units are required, then the angle between the two sides of the body portion  202  would be relatively greater. 
     This configuration is particularly suitable for manufacture from plastics materials, hence a lightweight, relatively inexpensive wheel can be constructed, without sacrificing strength and durability. Furthermore due to its modular construction, the wheel can easily be repaired or modified. 
     Integrally formed with the body portion  202 , there is a hinge arm  204 . The hinge arm  204  extends from the radially outer end of the body portion  202  and is inclined at an angle of 50° to the carousel wheel axis. As can be seen particularly clearly in FIGS. 8 and 9, the hinge arm  204  is defined by a generally box-like outer wall structure  216  which is strengthened by three intersecting cross-webs  218 ,  220  and  222 . An integrally formed cylinder  224  is situated at the intersection of the three cross-webs  218 ,  220  and  222 . It should be noted that the cross-webs and the cylinder all have walls which are generally parallel with the wheel axis. This feature can best be seen in FIG.  6 . 
     The box-like section of the hinge arm  204  has generally parallel sides  226  and  228 . Moving away from the junction of the hinge arm  204  with the body portion  202 , the walls  226  and  228  lead into a distal end portion of a relatively narrower width  230 , via inclined walls  232  and  234 . The distal end portion of the hinge arm  230  is provided with a transversely extending barrel  236  having a generally circular cross-section. The barrel  236  has length which is very slightly longer than the width of the distal end portion  236 . Each end face  238  of the barrel  236  is provided with a respective axle lug  240  of a generally circular cross-section. 
     The barrel  236  comprises a pair of pockets  242 , each having a rectangular cross-section. The pockets  242  extend in a direction parallel to the cross-webs  218 - 222  and the wheel axis. 
     The cross-webs,  218 - 222 , the cylinder  224  and the pockets  242  result in a strong, yet lightweight unit that can be manufactured from a relatively small amount of material. 
     As can be seen most clearly in FIGS. 7A and 7B, a flexible tongue  206  extends perpendicularly from the bracket unit body  202  from a region close to the junction of the hinge arm with the body portion  202 . The tongue  206  comprises an elongate hook portion  244  extending transversely along its distal end, so as to face generally towards the barrel  236 . The tongue  206  is provided with four integrally formed ribs  246  on the opposite side to the hook portion  244  and in the region of the end at which it is joined to the body portion  202 . The ribs  246  each have a thickness which tapers in a curved fashion from the junction of the tongue  206  and the body portion  202  towards the distal end of the tongue  206 . The ribs  246  serve to constrain the flexion of the tongue  206  in a gradually reducing fashion towards its distal end, thereby providing a precisely controlled spring characteristic. The tongue comprises a further four ribs  247  on its opposite face, these ribs being directed towards the barrel and tapering in thickness from the hook portion  244  to the axial centre of the tongue. The ribs  247  reduce stresses in this part of the unit to an acceptable level when the unit is fully loaded. 
     On a surface of the hinge arm  204  which generally faces the tongue  206 , there are provided a pair of stops  248 , one of which can be seen clearly in FIG.  7 A. Each stop is located generally towards the side of the hinge arm  204  and includes a square-section rebate  250  running in a direction generally perpendicular to the axis of the tongue  206 . 
     As can be seen from FIGS. 7A and 7B, the body portion  202  has a general box-structure which is strengthened by a pair of further cross-webs  252  and  254 . This structure also provides strength with low weight and requires a relatively small amount of material for manufacture. Extending from the tongue-side face of the body portion  202  are a pair of lugs  256  and  258 , which extend generally parallel to the tongue  206 . Each lug is generally in the form of a cylinder which extends into the box of the body portion  202 . 
     As can be seen from FIG. 9, the bores of the cylinders  256 ,  258  extend through to the opposite surface of the body portion  202 . A further generally cylindrical portion  260  is situated in a crook defined between the cross-webs  252  and  254  and generally towards the opposite end of the body portion  202 . As with cylinders  256  and  258 , the bore of cylinder  260  extends through to the opposite surface of the body portion  202 . 
     Lugs  256 ,  258  are used for radial positioning in conjunction with an annular groove in the hub upon which the carousel wheel  500  is mounted. If the annular groove is replaced by a series of accurately bored holes, the lugs can be used for circumferential as well as radial positioning of the bracket units. In such a case, the dovetail formations  212 ,  214  could be omitted. 
     The opposite surface of the body portion  202  is provided with a generally trapezoidal wall  208  that extends perpendicularly from the face of the body portion  202  in a direction parallel to the wheel axis. When the bracket units are assembled together to form a carousel wheel, the walls  208  together define a series of radial slots which are used as carousel-locator slots in a “Geneva wheel” mechanism for controlling the rotational position of the carousel wheel. 
     The end face of the body portion  202  that faces radially inwardly, when the bracket units are connected together as shown in FIG. 5, comprises a series of radially-inwardly facing teeth  210  which, in conjunction with the teeth provided on the other connected bracket units  200  define a circular gear rack. 
     In use, the gear rack is used to control the rotational orientation of the carousel wheel. Since both the Geneva mechanism and the gear rack have the same general purpose, one or the other may be deleted. However if both are provided on the bracket units, a choice of rotational position control mechanisms is provided, without requiring two different types of bracket unit to be produced. 
     Referring to FIGS. 6 and 10A to  12 , the pot units  300  will now be described. 
     Each pot unit comprises a tool cylinder  302  integrally formed with a hinge arm  304 , which extends generally radially from an outer surface of the tool cylinder  302 . The hinge arm  304  is hollow and formed from two generally planar flank walls  308  joined by a transverse end wall  310  at their distal ends. Each of the flank walls  308  comprises a circular aperture  312 . Due to the natural resilience of the flank walls  308 , the apertures  312  snap-fit over the axle lugs  240  provided on a bracket unit. The attachment of a pot unit to a bracket unit in this manner can be seen clearly in FIG.  6 . As an alternative, the hinge arm  204  could be constructed to provide the necessary resilience to enable the snap-fit. Each flank wall  308  comprises a cut-away portion  314  which has a generally V-shaped profile, with a somewhat rounded bottom. The cutaway portions  314  are set into the respective edges of the flank walls  308  which address a bracket unit when the two are connected together and arranged in the manner of FIG.  6 . The cut-away portions  314  serve to accommodate the box portion  316  of the bracket assembly. 
     The end wall  310  of the hinge arm  304  is provided with a series of axially extending ridges  316  which interlock with ribs  247  provided on the bracket unit  300 . 
     Each of the ridges  316  terminates in an undercut  322 . In use, the transversely extending hooked portion  244  provided on the tongue  206  of the bracket unit  200  snap-fittingly locates underneath the undercut  322  when the pot is rotated about the hinge  240 ,  312  in the clock-wise direction, to the position shown in FIG.  6 . This secures the position of the pot unit  300 , relative to the bracket unit  200 . Although this clipping method has been found particularly effective, other arrangements may be employed. For example, a much bigger clip, for gripping a cylinder, may be provided on each bracket unit  200 . 
     On the axially opposite side to the cut-away portions  314 , the hinge arm  304  is provided with an integrally formed barrel  318 . The barrel comprises a bore  320 , which extends in a direction perpendicular to the axial direction and the radial direction of the cylinder  302 . In use, the barrel co-operates with a lifting mechanism which comprises a fork  122  for constraining the barrel, the mechanism being used to cause rotation of the pot assembly about the hinge  312 ,  240 . Such a mechanism is described below. 
     The hinge arm  304  further comprises an internal, lateral cross member  324  for strength and stiffness. Two further, mutually parallel internal walls  325  extend perpendicular to the cross member  324 . These also enhance the stiffness of the structure. 
     The tool cylinder  302  is provided, at one axial end, with a seat portion  306  for accommodating a retention collar  400 . Referring to FIG. 10, it will be seen that the seat portion  306  takes the form of an axially extending seat cylinder  326  concentrically situated at one end of the tool cylinder  302 . Extending radially into the mouth of the seat cylinder  326 , there are provided three lugs  328  at 120° intervals. Circumferentially in line with each lug and axially inwardly of the mouth of the cylinder  326 , there is provided an elongate recess channel  329 , which has a part-circular cross-section. A similarly shaped channel  330  is provided between each pair of lugs  328  and extends from the mouth of the cylinder  326  to a shoulder  332  which faces axially back towards the mouth of the cylinder  326 . 
     Referring to FIGS. 6,  13  and  14 , it will be seen that the retention collar  400  is generally cylindrical and provided with three radially projecting lugs  402  which are located at 120° intervals about its periphery, each at an axial distance which is approximately mid-way between the two end surfaces of the holder. 
     Each lug  402  is in the form of a flexible bridge, which extends across a respective axially extending channel  403 . The radially outer surface of each lug  402  is provided with an axially extending rib  404 , mid-way between its two circumferential ends. 
     The lugs  402  co-operate with the lugs  328  provided in the cylinder  326  of the tool cylinder  302 . In use, the retention collar  400  is presented to the cylinder  326 , with the ribs  404  circumferentially aligned with the channels  330  provided on the inner face of the cylinder  326 . The retention collar is then inserted axially into the cylinder  326 , until the advancing end surface of the collar  400  abuts the shoulder  332 . At this point, the collar  400  is rotated and the lugs  402  flex radially inwardly, as the ribs  404  are urged out of the channels  330 . To secure the collar  400  in place, it is rotated until the lugs  402  are each situated behind a respective lug  328 , at which point the ribs  404  become circumferentially aligned with the channels  329  and snap into position due to the inherent flexibility of the lugs  402 . The combination of the bridge-shape of the lugs  402  and the channels  403  provides sufficient radial flexibility for this operation to be performed. Once the lugs  402  are located axially behind the lugs  328 , the collar  400  is axially secured within the tool cylinder  302 . To remove the collar  400  from the tool cylinder  302 , the collar  400  must first be rotated against the radial resilience of the lugs  402 , until the ribs  404  are once again circumferentially aligned with the channels  330 , whereupon the collar can be axially withdrawn. 
     The retention collar  400  further comprises six internal, axially extending tongues  406 , which are arranged in three groups of two, the groups being located at 120° intervals. Each tongue  406  is secured at one end to the inner wall of the collar cylinder  400 , and, at the opposite end (towards the top of FIG. 13) is unrestrained, thereby enabling each tongue  406  to flex radially. Each tongue is provided with a radially inwardly facing tool-gripping lug  408  proximate to its distal end. In use, the tongues co-operate to grip the pull-stud of a tool holder  600  to secure the tool axially within the cylinder  302 . In this regard, it can be seen in FIG. 6 that the pull-stud of the illustrated tool holder  600  comprises a radial flange  602 . In practice, the tool holder is inserted into the tool cylinder  302  from the end of the tool cylinder that is opposite to the end in which the tool holder  400  is inserted. Therefore, the pull-stud of the tool holder enters the collar  400  from the bottom of FIG.  13 . As the pull-stud moves between the lugs  408 , the tongues  406  move radially outwardly. Once the flange  602  has moved above the lugs  408 , as shown in FIG. 13, the tongues snap back into place, thereby resisting downward movement of the pull-stud  600 . 
     Radially inwardly directed struts  410  serve restrict radial displacement of the pull-stud during insertion, thereby protecting the tongues  406  from over-flexion. 
     As a consequence of the described arrangement, collars having different internal dimensions, for holding tool holders configured to different standards, may be interchangeably secured within the pot cylinder. It is even possible to configure a retention collar to be axially reversible; that is to say with means for gripping one type of pull-stud in one axial end region and different means for gripping a different type of pull-stud in the opposite axial end region. 
     In use, the bracket units are connected together in the manner shown in FIG.  5 . Each bracket unit is provided with a respective pot unit  300 , these being connected together as shown in FIG.  6 . Each pot unit  300  has a retention collar  400  fitted inside in the manner described above. 
     The assembled carousel is fitted to a tool changer of the general type shown in FIG.  1 . In this arrangement, the Geneva wheel indexing mechanism will be situated towards the right of the tool carousel, as viewed in FIG. 1, and the open, tool receiving end of each tool cylinder  302  will face towards the left of FIG.  1 . 
     The carousel is caused to rotate using either the Geneva wheel mechanism or the gear rack  210 , until the desired tool is situated at the bottom of the tool carousel. When in this position, a tool release mechanism (described below) causes the tongue  206  to lift up, thereby allowing the tool cylinder to rotate around the hinge  240 ,  312 . The rotation of the tool cylinder is controlled by a lifting/lowering mechanism (described below) which interacts with the barrel  318 . Once the pot unit has been moved into a position whereby the tool cylinder has its axis vertically aligned, the tool can be removed from the cylinder using a tool arm in the standard manner. 
     A pot lifting and lowering mechanism  700  will now be described with reference to FIGS. 15 to  19 . 
     The mechanism comprises a vertically-mounted cam wheel  702  which rotates on a horizontal axis  704 . The cam wheel comprises a radially outer geared periphery  706  which meshes with drive gearing provided on a drive motor  708 . 
     The cam wheel  702  comprises two primary camming formations. The first of these is a spiral channel  710  which is moulded into a first face of the cam wheel  720 . The spiral channel accommodates a lug (not shown) of a lifting arm  121 , which extends generally vertically, as can be seen in FIG.  16 . As the cam wheel  702  is caused to rotate by the motor  708 , the lug, which is entrained within the spiral channel and which is constrained to move only vertically, is caused to move up or down, depending upon the direction of rotation of the cam wheel  702 . This, in turn, causes the arm  121  to move up and down and, thus, the fork  122  with its mouth  123  moves up and down correspondingly. 
     On its opposite side, as can best be seen from FIGS. 17 and 20, the cam wheel  702  is provided with a generally circumferentially extending camming surface  712 . This camming surface bears against a release pin  714 , which is vertically mounted and comprises a lifting catch  716  at its axially lowermost end. The lifting catch  716  is hooked underneath he tongue  206  of the bracket unit currently in position. A helical compression spring  718  encircles the release pin  714  between its head  720  and the upper surface of a mounting bracket  722 , through which it extends. The compression spring  718  urges the release pin  714  upwards, but this action is resisted by the camming surface  712 , which bears on the head of the pin  720 . 
     Referring to FIG. 19, it will be seen that the camming surface  712  has a part-circular portion  713  that extends for 270° about the axis at a constant, maximum radial distance. Whilst this part-circular bearing surface  713  bears against the head of the release pin  714 , the pin is maintained in the lowermost position shown in FIG.  16 . However, the camming surface  712  also comprises a chamfered portion  715  defined by two flat portions  717 , each of which is radially closer to the rotation axis of the cam wheel  702  than the part-circular portion  713 . Consequently, when the cam wheel  702  is rotated to bring the chamfered portion  715  above the release pin  714 , the pin is allowed to move upwardly, under the action of the compression spring  718 , and the lifting catch  716  lifts the tongue  206  of the pot unit  200  upwards. This releases the pot unit  300  in such a manner that it can be rotated about the hinge  240 . 
     Due to the relative configurations of the spiral  710  and the cam surface  712 , at the time that the hinge is lifted upwards, the arm  121  is caused to move downwardly and the fork  122  then begins to push the barrel  318  downwardly, thereby causing the pot unit  300  to rotate about the axis. 
     More specifically, referring to FIG. 19, when the cam wheel  702  is orientated such that position A on its circumference is at the lowermost point, the arm and the release pin  714  will be positioned as shown in FIG.  16 . If the wheel  702  is then caused to rotate in the clockwise direction of FIG. 19, the arm  121  is first lifted slightly, so causing the fork  122  to take the load of the clip  206 . The release pin  714  is then allowed to spring upwards, thereby lifting the tongue  206 . Thereafter, the arm  121  is gradually lowered, until point B is lowermost, at which time the pot unit has been rotated around the axle  240  to such an extent that it will not longer be interfered with by the tongue  206 . Therefore, the cam surface  712  once again lowers the catch. As the cam wheel  702  is rotated further in the clockwise direction, the arm  121  is lowered still further until it reaches a lowest point, when circumferential point C of the cam wheel  702  is lowest. At this point, the tool cylinder  302  has its axis aligned vertically and the tool is ready for removal by the transfer arm  4 . 
     To lift the pot unit, the cam wheel  702  is merely rotated in the opposite direction, so as to move the cam wheel  702  anticlockwise as seen in FIG.  19 . 
     FIG. 20 shows a drive mechanism  800  for use in conjunction with the circular gear track  210 . The drive mechanism comprises a motor  801  fitted with a drive shaft  802  with a radial gear  804  for meshing with the drive track  210 . The drive shaft is fitted with a steel bar  806  that rotates as the drive shaft  802  rotates. Three proximity switches  808 ,  810  and  812  are provided along the rotational path of the bar  806 . These are connected with a control device  814  that controls the rotational speed of the motor  801 . 
     The operation of the mechanism will now be described with reference to FIG.  21 . 
     Upon application of a current to the motor, it ramps up to a maximum speed indicated at W in FIG.  21 . The motor continues at this speed until an end  850  of the bar  806  passes proximity switch  808 , as shown in chain-dot line in FIG.  20 . Once the proximity switch detects the presence of the iron bar, the motor is ramped down to an intermediate speed, the occurrence of which is shown at X in FIG.  21 . The motor then continues to rotate at the intermediate speed, until proximity switch  810  detects the presence of the iron bar. This event is indicated at Y in FIG.  21 . It will be seen that the controller  814  then steps the motor down to the minimum rotational speed, until the leading edge of the bar end passes proximity switch  812  and the trailing edge of the bar end simultaneously passes proximity switch  810 , when the controller sends a signal for the motor to stop, as indicated at Z in FIG.  21 . 
     This arrangement allows the rotational velocity of the motor to be arrested in a precise and controlled manner that avoids damage to any of the components of the carousel. 
     Whilst the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is to be understood that this invention is not limited to the disclosed embodiment, but is intended to cover various arrangements including within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.