referring firstly to fig1 the apparatus according to the invention is used for a rotatable chuck 10 mounted on the headstock 11 of a machine tool ( not shown ). the chuck has three jaw - ways 12 , 13 and 14 and the jaws are similar to those described in our european patent application no . 82306569 . 3 . in other words by rotating a release device the jaws are freed from their actuating mechanism so that the jaws can be slid out of the chuck for replacement by sliding them radially outwardly along the t - shaped slots in which they are mounted . the apparatus according to the invention comprises two principal components , namely a jaw changing apparatus 15 which also embodies a chuck location device , and a jaw release mechanism 16 . actual handling of the jaws is carried out by a conventional workpiece loading arm ( not shown ) mounted on an overhead gantry . the function of the jaw changing apparatus 15 is to guide a replacement jaw into the associated chuck jaw - way , as the jaw is moved in the requisite direction by the workpiece loader arm . the function of the jaw release mechanism 16 is to release the jaw that is to be replaced , and to locate the new jaw when it has been placed in position . turning now to fig3 to 5 , the jaw changing apparatus 15 comprises essentially a guide shoe 17 which , as best shown in fig3 defines a t - shaped slot 18 . as best seen in fig2 the side 19 of the shoe which defines the right - hand side of the slot 18 extends vertically to a point 20 . on the other hand the side 21 which defines the left - hand side of the slot 18 extends only to a significantly lower point 22 . this means that a t - shaped jaw can be located in the slot 18 by a series of three movements . firstly the jaw is moved in the direction of arrow a of fig3 until the base 23 of the jaw abuts the front face of the shoe . at this point the lowermost edge of the jaw will be at a level lying between 20 and 22 . the jaw can then be moved by the loader arm in the direction of arrow b of fig3 to cause the right - hand arm 24 of the jaw t to engage in the right - hand side of the t - shaped slot 18 . this engagement brings the jaw into full register with the t - shaped slot 18 and the jaw can now be moved downwardly , perpendicular to the directions a and b , i . e . in the direction of arrow c of fig2 . the shoe is provided with a proximity detector 25 which checks when a jaw has passed through the shoe . if , when a jaw is slid out of the shoe through its t - shaped exit mouth , the jaw is to move smoothly into the jaw - way of the chuck , it is important for the jaw - way of the chuck to be accurately aligned with the slot in the shoe . for this purpose the chuck location mechanism is used and this comprises a plunger 26 which is shown in fig4 and 5 . the plunger is mounted in a cylinder 27 attached to the rear of the shoe . the plunger has a tapered nose 28 and it can be extended by hydraulic fluid or compressed air to force the nose against the body of the chuck . as best seen in fig4 the chuck 10 is provided with a tapered recess 29 adjacent each chuck jaw - way . when it is desired to change a jaw the chuck is stopped with the desired recess 29 substantially in register with the plunger 28 and subsequent extension of the plunger 28 causes the plunger to engage in the recess 29 and accurately locate the jaw - way adjacent the guide shoe . turning now to fig6 to 8 , it will be seen that the jaw release mechanism 16 essentially comprises a plunger 30 having a screwdriver type head 31 designed to engage with the release member of the chuck jaws ( not shown ). the plunger is mounted in a cylinder 32 and is sealed thereto by means of an o - ring 33 . the plunger can be moved from its retracted position shown in fig6 and 7 to an extended position by the application of hydraulic fluid or compressed air . in the retracted position shown in fig6 and 7 , a peg 34 on the rear of the plunger engages in a socket 35 as best seen in fig7 . means are provided to rotate the plunger . the periphery of the plunger is provided with splines 36 and these engage with two other plungers 37 and 38 ( see fig8 ) each of which has rack - like teeth 39 engaging with the splines 36 . the plungers 37 and 38 are mounted in cylinders 40 and 41 which extend at right angles to the cylinder 32 . o - ring seals 42 are provided and the application of compressed air or hydraulic fluid to the cylinders 40 and 41 causes the plungers 37 and 38 to move in such a way as to rotate the plunger 30 . because of the engagement of the peg 34 in the socket 35 however , the plunger 30 can only rotate when it is in the extended position . a proximity detector 43 is arranged to detect when the plunger is in the retracted position . there is also a third proximity detector 44 ( see fig5 ) which detects when the plunger 26 of the chuck location apparatus is in the retracted position . turning now to fig9 it will be seen that the cylinder 29 of the chuck location device is connected by lines 45 , 46 respectively to a supply line 47 and a solenoid operated valve 48 . it will be seen that as a fail safe device pressure is continually applied to line 45 acting to return the plunger 26 to the fully retracted position . to extend the plunger the solenoid operated valve 48 is actuated to also apply the supply pressure to the upper face of the plunger 26 . since there is a greater exposed area at this upper face there is a nett downward force which moves the plunger to its extended position . the cylinders 32 , 40 and 41 are interconnected as shown with the supply line 47 and a valve 49 operated by two solenoids . in the position shown pressure is applied through line 50 to maintain plunger 30 in the retracted position . the rear of cylinder 32 and the cylinders 40 and 41 are connected to exhaust by the valve 49 . rotation of the chuck spindle is stopped with the chuck jaws in the workpiece loading position . the workpiece is gripped by the loading arm , the chuck jaws are opened and the workpiece is removed from the chuck by the workpiece loader . to clear any swarf from the chuck prior to removing the jaws the chuck is then rotated at maximum spindle speed with a copious supply of coolent applied over the chuck body . the coolent is then turned off and the chuck is allowed to run for a short period prior to stopping the spindle and positioning a first jaw - way , for example jaw - way 12 , substantially opposite the shoe 15 . the chuck jaws are then closed and the valve 48 is actuated causing the plunger 26 to extend and accurately position the chuck body relative to the guide shoe . valve 49 is then actuated by the right - hand solenoid causing pressure to be applied to the rear of cylinder 32 . this produces a greater force on plunger 30 than is produced by the pressure applied via line 50 and the plunger 30 is accordingly extended to engage the release member of the chuck . as soon as the peg 34 clears the socket 35 pressure applied to cylinders 40 and 41 causes the plunger to rotate through 180 ° to release the chuck jaw . the loading arm then engages the jaw ( which may be provided with pick - up grooves to facilitate this ) and extracts the jaw from the jaw - way . proximity detector 25 detects that the jaw has moved from the chuck into the guide shoe . the used jaw is then transported by the loading arm and is placed in a jaw storage area . the appropriate jaw of the replacement set of jaw is then retrieved from the jaw storage area by the loader arm and is transported by the loader arm ready for insertion at the guide shoe . the jaw is engaged with the guide shoe as described above and is moved radially into the chuck jaw - way . the detector 25 detects that the jaw has moved out of the shoe and into the chuck . the loader arm then moves to pick up the second jaw of the replacement set . valve 49 is then moved in the reverse direction by the second solenoid . this causes the plunger 30 to rotate in the opposite direction to lock the new jaw in position and once rotation has been completed the plunger 30 retracts . the fact that retraction has taken place is detected by the detector 43 . the plunger 26 is then retracted and once again retraction is sensed , this time by the proximity detector 44 . the machine spindle position system then rotates the chuck to position a second jaw - way opposite the guide shoe and the plunger 26 is extended to accurately locate the second jaw - way relative to the guide shoe , thus commencing a repeat of the above described sequence . the sequence is repeated three times until all three jaws are changed . after changing the last jaw retraction of the plungers 26 and 30 is checked and the spindle is rotated into the workpiece loading position . the chuck jaws are then opened and a new workpiece is loaded . the jaws close to grip the workpiece , the loading arm is removed and spindle rotation commences . the proximity detector 44 is interlocked with the spindle drive to ensure that the machine spindle is inhibited from rotating when the plunger 26 is extended . if a jaw is incorrectly engaged then the plunger 30 will not be able to rotate and hence will not be capable of being retracted . this will be sensed by the proximity detector 43 . to ensure that each jaw is only located in the associated jaw - way each jaw - way may be provided with a groove which is in a different position to the grooves of the other jaw - ways , and each jaw being provided with a pin which will only register with the groove of the correct jaw - way . the shoe 17 , as shown in fig3 has a slot 50 therein wide enough to accept all the pins . instead of using a loading arm mounted on an overhead gantry , the invention is equally applicable to the use of a loading arm mounted on a robot positioned adjacent to a machine tool . with such an arrangement the configuration shown in fig1 may be used . the jaw changing apparatus 15 is positioned at the side of the chuck 10 , instead of vertically above it , the jaws being withdrawn in the horizontal direction . the jaw release mechanism 16 is mounted as shown with the axis of plunger 30 vertical instead of horizontal . the operation of the second embodiment is identical to that of the first embodiment , the only difference being in the positioning of the jaw changing apparatus 15 and the jaw release mechanism 16 . the configuration of the chuck 10 is slightly different , since the release member for a given jaw must be positioned clockwise of the jaw for the embodiment shown in fig1 , and anti - clockwise of the jaw for the embodiment shown in fig1 . turning now to fig1 , there is shown an alternative embodiment in which the guide shoe 17 actually comprises part of the chuck body . the guide shoe in fact comprises a tapered jaw - way into which a correspondingly tapered base jaw 51 can be inserted by a robot arm . the jaw - way tapers in two directions so that the entry mouth 52 is wider and thicker than the leading end 53 of the base jaw . this allows the robot arm a certain amount of tolerance when inserting the base jaw into the jaw - way . the precise position of the base jaw with respect to the jaw - way is not critical on entry . once the base jaw has been located in the mouth of the jaw - way however , the jaw - way provides progressively greater guidance . although the base jaw , once inserted into the chuck bod , will undergo a certain amount of radial movement for the purpose of clamping and unclamping a workpiece , the amount of radial movement , and the extent of the tapers , is such that there is little change in the clearance between the base jaw and the jaw - way over the working range of the jaw . in each of the directions of taper , the taper is 0 . 004 inches per inch . the height of the base jaw as viewed in fig1 is 4 inches . this means that when the robot arm inserts the jaw in the jaw - way there is a clearance , in each of the two relevant directions , of 16 thousandths . once the jaw is in its working position however , the clearance may be as little as 1 thousandth . the range of working movement of the jaw in this embodiment is 1 / 4 inch and so , although the jaw is working in a tapered jaw - way , the clearance between the jaw and the jaw - way varies by only 1 thousandth over the working range of the jaw . the invention is not restricted to the details of the foregoing embodiments . for example , in the embodiment shown in fig1 , it is not essential that the taper be 0 . 004 inches per inch . the taper may for example be from 0 . 002 inches per inch to 0 . 06 inches per inch .