Abstract:
A coordinate measurement machine 10 has a movable head 20. A probe 12 and arm 14 are attached via a disconnectable joint to the head. An exchange rack 30 is provided for exchanging one probe/arm, for another. A plate 50 acts to disconnect and reconnect the joint and also to support the probe/arm whilst disconnected. Plate 50 actuates a toggle mechanism 42 when the head is moved relative to the plate and causes a plunger 26 to engage/disengage with a peg 24 thereby securing or releasing the joint during the said movement.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a disconnectable joint particularly but not necessarily for use with measurement components having two joinable elements.  
           [0003]    2. Description of the Prior Art  
           [0004]    There are many ways of providing a disconnectable joint, i.e. one which can be repeatedly made and unmade. For measurement components at least, it is often desirable to repeatedly make and break a joint but to have the components returned to the same relative position every time the joint is remade. The need for automatic connection and disconnection of parts is also desirable. As a result of these requirements some known joints are complicated and unreliable.  
           [0005]    Examples of joints are disclosed in EuropeAn Patent No. 0142373 and United Kingdom Patent No. 1,599,751.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention, according to a first aspect, provides a disconnectable joint comprising first and second connectable parts, a peg resiliently mounted to the first part and a pin slidably mounted to the second part and engageable with the peg when the first and second parts are connected wherein the peg has an enlarged end and the pin is slidably engageable with the enlarged end to keep the parts connected. The pin may be urged into engagement with the peg by a spring force.  
           [0007]    Preferably the pin is slidable in use by means of a toggle mechanism. More preferably there is a toggle mechanism on each side of the pin.  
           [0008]    Preferably the joint includes an actuation plate having a cam profile, the plate being adapted for causing the toggle mechanism to move during relative displacement of the plate and the toggle mechanism thereby to disconnect the two parts.  
           [0009]    Preferably the plate is adapted to support the second part when the second part is disconnected from the first part.  
           [0010]    The invention provides also, according to a second aspect, a coordinate measurement machine having a measurement device connected to a mounting head by a joint according to the first aspect, or its preferred features.  
           [0011]    Preferably an actuation plate adapted for disconnecting the joint, provides support for the probe when disconnected from the mounting head. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will now be described with reference to the drawings, wherein:  
         [0013]    [0013]FIG. 1 shows an embodiment of the invention used on measuring equipment;  
         [0014]    [0014]FIG. 2 shows a cut-away view of the joint shown in FIG. 1;  
         [0015]    [0015]FIG. 3 shows a partially cut-away component of the joint shown in FIG. 2;  
         [0016]    [0016]FIG. 4 shows another view of components of the joint shown in FIG. 2;  
         [0017]    [0017]FIGS. 5 a , 5   b  and  5   c  are a schematic representation of the toggle mechanism used in the joint shown in FIG. 2; and  
         [0018]    [0018]FIG. 6 is an alternative embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    Referring to FIG. 1 there is shown an quill  10  of a coordinate measurement machine (CMM) having a movable measurement head  20  mounted thereto. Attached to the head is a measurement device in the form of a probe  12  and in this instance an extension arm  14  with a base  22 , The probe has a stylus tip  16  used to obtain workpiece dimensional information. The arbor is capable of translational movement in x,y and z directions and the head  20  can cause the stylus to move in rotation about axes rx and ry. Thus the probe  12  and stylus  16  can be moved in a multitude of directions. However, to increase the range of measurement that the CMM can carry out, different probes, styli and extensions can be used. An alternative stylus, in this case a star stylus  18  for bore measurement, is supported on a rack  30  and this alternative stylus together with its arm and base can be exchanged for the probe  12 , stylus  16 , arm  14  and base  22  shown attached to the head  20 . Exchange of styli and extension arms takes place by removing the presently attached probe and extension ant and replacing it with another. This is carried out either manually or (preferably and in this example) automatically. Automatic exchange is carried out by moving the head  20  to the rack  30 , Goading into the rack  30 , a probe/arm  12 / 14  and then moving to another position to reload the head with another probe/arm.  
         [0020]    Automatic changing of the stylus is made possible by the provision of a disconnectable joint  40  at the base  22 , one embodiment of which is shown in FIGS.  2 - 4  and described below.  
         [0021]    The joint  40  makes use of an actuator plate  50  held on the rack  30 . In use the joint  40  is moved to the plate  50  and the plate is caused to be driven into the joint, resulting in disconnection of the joint and separation of the head  20  from the base  22 . The plate subsequently provides support for the dieconnected probe/arm.  
         [0022]    The head, without the probe/arm attached, is then moved to a new position adjacent the alternative probe/arm. The head is moved away from the new position, pulling with it the alternative base with its probe/arm. When the head has moved sufficiently far and an actuation plate is withdrawn from the joint  40  then the disconnectable joint  40  between the head  20  and the alternative base is remade.  
         [0023]    [0023]FIG. 2 shows the joint  40  and a plate  50  in more detail. A portion only of head  20  is shown as a cut-away view. A portion only of base  22  is shown. For, clarity the portion of the base shown has been hatched, whereas the portion of the head shown has not been hatched. The view is substantially a half-section and is substantially mirrored by the parts not shown.  
         [0024]    In this embodiment the head  20  forms a first part of the joint and the base  22  forms a second part. The head has a resiliently mounted peg  24 , an: enlarged end  28  of which is capturable by a slidable pin or plunger  26  of the base.  
         [0025]    Plunger  26  is movable away from the peg by means of a toggle mechanism  42 , described in detail below. The toggle mechanism  42  is actuated by plate  50  as it moves into a slot  44  formed in the base  22 . When the plate  50  is in the slot the joint can be disconnected by withdrawal of the peg  24  from the base  22 , principally in the y direction.  
         [0026]    Removal of the plate  50  either with or, without the base  22  being connected to the head  20 , actuates the toggle mechanism  42  and forces plunger  26  to slide toward and engage peg  24  or, if no peg is present then in the direction of an access hole ( 46  FIG. 4.) in which the peg would sit.  
         [0027]    [0027]FIG. 3 shows the peg assembly in more detail. The peg  24  is mounted for resilient movement. A conical portion  52  is urged by coiled spring  54  against three static balls  56 . In use the peg may be forced from its urged position in order to enter. or leave hole  46 . Plunger  26  in the base  22  is pushed towards a face  48  of the enlarged end of the peg. A sprung, ball  65  is captively housed within the plunger  26  (as detailed in FIG. 5 c ). When the plunger is pushed by the toggle mechanism  42  toward the peg  24  then the ball  65  contacts the face  48  of the peg and is urged by a spring  72  inside the plunger so as to retain contact with the face  48 . This action brings together and holds the head and the base.  
         [0028]    [0028]FIG. 3 shows also the two peg location balls  64 , again mounted in the base, which stop the plunger from pushing the peg  24  too far, and provide together with ball  65 , three points of contact on the face  48  of the peg. Plunger  26  and the two balls  64  are illustrated in their operative position in FIG. 3. When the plunger  4 .  26  is moved away from the peg  24  by the, toggle mechanism, the peg, by virtue of its resilient mounting, is capable of sliding over the two balls  64  whilst head  20  is moved away from the now disconnected base  22 .  
         [0029]    [0029]FIG. 4 shows a base  22  and arm  14 ,supported by a plate  50 . This view shows three pairs of balls  58  which in use each come together with one of three further balls  4  ( 62  FIG. 3, only one of which can be seen) fixed to head  20 . These ball locations form a kinematic location which allows repeatable positioning of the joint parts each time the joint is broken and remade. To aid initial alignment of the base and head three pins  59  in the head are acceptable within three holes  61  in the base one of which only is shown in FIG. 3.  
         [0030]    Rack  30  may be provided with a plurality of plates so o so that various bases can be supported thereon.  
         [0031]    [0031]FIGS. 5 a  and  5   b  show two positions of the toggle mechanism  42  shown in FIG. 2. FIG. 5 a  is an unlocked position where the plunger  26  is withdrawn from access hole  46  and is the state in which plate So holds the toggle mechanism  42  when it supports a base  22 . A lug  68  of the toggle mechanism FIG. 5 b  is captured and caused to move in a cam slot  66  in the plate. Relative displacement of the slot  66  and lug  68  causes actuation of the toggle mechanism  42 . FIG. 5 b  shows a locked position at which ball  65  in plunger  26  is engaged with peg  24 . In use this position will normally only be occasioned when the plate So is not engaged with the toggle mechanism  42 , i.e. when the joint  40  is made and the head  20  and the base  22  are adjoined. Therefore peg  24  is omitted from the hole  46  for clarity. Lug  68  has been moved by the plate  50  into a position whereby the plunger  26  is in its operative position and is urging ball  65  into engagement with peg  24 .  
         [0032]    [0032]FIG. 5 c  shows a section through plunger  26 . Captively held ball  65  is loaded by means of captive spring  72 .  
         [0033]    [0033]FIG. 6 shows an alternative embodiment of the invention. A half-section is shown in this Figure tibrough head  20  and part of base  22   a . These parts are similar in construction to the head  20  and base  22  described above. However, a different toggle mechanism  42 ′ is shown in this embodiment. The toggle mechanism  42 ′ has levers to one side only of plunger  26  and  30  is not mirrored in the missing half of the head  20  and base  22 ′. Rather than an actuator plate, plunger  26  is caused to move by the manual operation of a lever so between the extremities of movement shown by arrow M. The position shown is the locked/connected position of the joint  40 . Moving the lever will cause the plunger  26  to withdraw from hole  46  and the head and base can then be manually separated.  
         [0034]    Many variants and modifications of the construction shown will be apparent to the skilled addressee. In particular the manner in which the plunger  26  may be moved, rather than by means of the toggle mechanism illustrated, could be by means of any displacement device, powered or manually operated e.g. a solenoid. Plate or plates  50  may be supported in any manner and in any orientation. Likewise the base(s) may be supported in any orientation on the plate(s), e.g. a plate may have its slots facing upwardly and the base may be supportable on such a plate with its arm  14  extending upwardly.  
         [0035]    Moreover, whilst the invention is illustrated with reference to a joint between two parts of a measurement apparatus, the invention is not intended to be so restricted, and can find application in many other fields.