the invention will be described in detail in conjunction with the accompanying drawings , in which : fig1 is a view in elevation , partly broken away and in longitudinal section , for a first embodiment ; fig2 is a view similar to fig1 for a second embodiment ; fig3 is a schematic diagram explaining disengagement movement involved in operation of the invention ; fig5 is a plan view to show an alternative of fig4 structure ; and fig6 is an enlarged view in perspective , partly broken - away and in section , to show detail of centering structure in fig2 . in fig1 the vertically displaceable spindle 1 of a coordinate measuring machine , illustratively of portal construction , carries a sensing or probe head 4 with a probe pin 5 movably mounted therein . the sensing head 4 may , for example , be a so - called measurement sensing head , as described in west german c - no . 2 , 242 , 355 . the sensing head 4 is suspended from spindle 1 by way of a carrier 6 which is secured to the upper end of head 4 . the upper part 6a of carrier 6 is hook - shaped and mounts a hemisphere 7 which is approximately concentric to the longitudinal axis ( z ) of the sensing head 4 . the hemisphere 7 of carrier 6 is suspended in a conical depression 3 in the horizontal part of an l - shaped holding plate or bracket 2 secured within and to spindle 1 . the hemisphere 7 and the conical depression 3 from the centering member of the suspension . above hemisphere 7 , a vertically directed rod 8 is rigidly secured to the carrier 6 . the upper end of rod 8 lies in the same plane as the top of the l - shaped bracket 2 mounted to spindle 1 . a leaf spring 9 lies in said plane and connects the bracket 2 to rod 8 . the leaf spring 9 is flexible about the x and y axes , which be in the horizontal plane but forms a rigid connection against torsion about the vertical axis z . the upper end of rod 8 is therefore secured against displacement in the horizontal plane , i . e ., against torsion about the z axis , as well as against translation in the x or y direction . the at - rest position of the sensing head 4 is therefore clearly defined when hemisphere 7 is engaged within the centering depression 3 , under the weight of the sensing head . however , the sensing head 4 can yield in the event of collision with a workpiece . in such case , as shown in fig3 the hemisphere 7 disengages from the depression 3 . a switch 10 mounted to the carrier part 6a has contacts which will be understood to be so connected to the emergency stop button as to stop the drive of the measurement machine . after eliminating the collision condition , ball 7 returns under the weight of the sensing head 4 into the conical depression 3 and restores the centered at - rest position of the sensing head 4 with a high degree of precision . the yieldability of the sensing head is assured with respect to all directions in space ± z , ± y and + z which enter into consideration for a collision involving the arrangement of fig1 . yieldability in the direction - z is unnecessary , since the transverse dimensions of the sensor head 4 do not exceed those of the spindle 1 from which it is suspended . in the embodiment of fig2 a sensing head is mounted in collision - protected manner to the end of the horizontal arm 11 of a measuring machine of cantilever design . in this type of machine , yieldability of the sensing head 14 in the event of a collision is required in all six directions in space ± x , ± y and ± z . in this embodiment , the upper end of a sensing head 14 is also secured to a carrier 16 having a hook - shaped part 16a which mounts a hemisphere 17 on its underside . the carrier 16 is suspended via this hemisphere 17 , in a conically tapering depression 13 . also as in fig1 an upstanding tubular rod or sleeve 18 is connected to the carrier part 16a , and , via an attachment part 30 , a leaf spring 19 connects the upper end of sleeve 18 to a holding plate 21 mounted to the end of extension arm 11 ; leaf spring 19 thus secures the sensing head 14 against torsion about and tilting with respect to the z axis . yieldability in the - z direction is obtained by hinge - mounting the l - shaped part 12 which carries the centering concavity 13 ; specifically , the l - shaped part 12 is a rocker which can swing or rock about a horizontal pin 3 . the pin 3 of part 12 locates in an inverted v - shaped yoke 22 on the holding plate 21 , and part 12 is continuously urged by a tension spring 23 ( acting between rocker 12 and holding plate 21 ), against an abutment ball 24 on the holding plate 21 . the drive - disconnect switch 25 which is carried by part 16a responds not only upon dislocating displacement of the hemisphere 17 but also upon a yielding of the sensing head in the - z direction when the rocker 12 displaces about pin 3 ( against the tension force of spring 23 ); even though ball 17 may remain seated in concavity 13 during a - z displacement , the switch 24 will nevertheless be sensitive to the - z displacement because switch 25 is at offset from ball 17 and is poised for response to the outer end of rocker 12 . the sensing head 14 can be locked in position manually by means of a hand crank 27 , thus placing the anti - collision device out of operation . for this purpose , crank 27 actuates two eccentric cams 28 and 29 on a horizontal shaft 26 , which is journaled in carrier 16 below rocker 12 . upon actuation of crank 27 , cams 28 and 29 engage the bottom of rocker 12 and thus clamp the latter to the carrier 16 . since the axis of the shaft 26 is perpendicular to the axis of tilt 3 of the rocker 12 , and in view of the stiffness of the leaf spring 19 in the y - direction , the clamp is also effective to lock against tilting movement of the rocker 12 . the plan view of fig4 clearly shows the leaf spring 19 which secures the sensing head 14 against torsion about the z axis . the leaf spring is formed of two strips of spring steel 19a and 19b arranged in a v pattern , and each strip is longitudinally stiffened by ribs 20 . as a result of the division into two leaf springs , tilting about the y - axis of symmetry is readily possible , so that disengagement of the ball 17 from the depression 13 in the event of collision in the direction of the y - axis is assured . the v - shaped arrangement , on the other hand , prevents translation of the top of the rod 18 in the y - direction and thus secures the sensing head in centered position , i . e ., with ball 17 engaged , against tilting . from the perspective view of fig6 it can be seen that three angularly spaced cylinders 13 ( a - b - c ) having their longitudinal axes inclined to the z axis and to each other are fixed in the conical concavity 13 in which the hemisphere 17 centers itself . ball 17 therefore rests on three discrete points of contact , thus offering advantages of reproducibility of the at - rest position , as compared with ball seating in a purely conical concavity . as an alternative to the leaf spring 19 in the embodiment of fig2 and 4 , the anti - torsion element of fig5 comprises three bars 119 ( a - b - c ) and a tension spring 123 . the ends of the bars 119 ( a - b - c ) are mounted via self - aligning ball bearings 122 ( a - b - c - d ) respectively at two spaced points on the carrier plate 121 and at two spaced points on a fastening part 130 at the upper end of the vertical rod 118 , the tension spring 123 assuring freedom of play . the n - shaped arrangement of the three bars 119 ( a - b - c ) also secures the top of the rod 118 against torsion about the z axis and against translation in the plane of the x and y axes , while also permitting rotation about the x and y axes , and displacement of part 118 in the z - direction .
the sensing head of a measuring machine is pendulously suspended via self - centering ball - and - socket structure which affords resilient dislocating relief from its normal precisely centered position of measurement . the resilient connection permits feeedom for such dislocation for all possible directions in spaces , while effectively anchoring the head against torsional deflection about the vertical axis . once the collision condition is removed , the self - centering suspension assures resumed precision of the at - rest condition in which measurements can be made .