GB2092753A - Contour Measuring Instrument - Google Patents

Abstract

An instrument for measuring the contour of the outer surface of a workpiece 23 has a stylus 22 secured at one end of a counterweight arm (6) which is pivoted about an axis (33) so that the stylus rests in contact with the workpiece. The arm is moved in the longitudinal direction thereof, and the vertical displacement of the stylus, tracing the irregularities of the outer surface of the workpiece, is measured by a differential transformer (69). The arm (6) consists of two parts (18, 20) which are urged to a relative position by means of springs (29, 30). A mechanism (39) detects relative movement between the arms so that when a force exceeding a predetermined value applied to the stylus is detected the movement of the arms is stopped. The fulcrum (33) for the arm is movable in the longitudinal direction of the arm and a pivotal connecting link (78) is connected to the arm, so that errors in circular arcs of the stylus due to the rotational movement of the arm can be corrected. <IMAGE>

Description

SPECIFICATION Contour Measuring Instrument The present invention relates to a contour measuring instrument of the type which takes measurement of the contour of an object to be measured by tracing its outer surface with a stylus secured to one end of an arm.

In general, in such a contour measuring instrument, a stylus to be brought into contact with the outer surface of an object to be measured is secured to one end of an arm, the other end portion of this arm is rotationally supported by an arm support member, and the centre of gravity of the arm is positioned to the stylus side of the arm axis so that the stylus constantly rests in abutting contact with the outer surface of the object to be measured.In measuring the contour of the outer surface of the object by use of the contour measuring instrument of this type, the arm support member is moved in the longitudinal direction of the arm, the stylus is displaced in accordance with the contour of the outer surface of the object to be measured due to the movement of this arm support member so as to rotate the arm about its fulcrum where the arm is pivoted, the rotation of the arm is detected by use of contour signal detecting means such as a differential transformer, and the signal detected by this contour signal detecting means is combined with a signal corresponding to the feeding of the arm support member, to carry out the measurement of the contour of the object.

However, with such a conventional contour measuring instrument commonly used, a problem arises in that, when the stylus falls e.g. into a deep groove the stylus may be broken and the object to be measured may be damaged. More specifically, since no damage preventing mechanism for protecting the stylus and the like is provided in the conventional instrument, when the stylus falls into the deep groove the arm support member continues to be moved, and the stylus is brought into abutting contact with the wall surface of the groove under a strong pressing force, thus resulting in breakage of the stylus.

Furthermore, with such a conventional contour measuring instrument commonly used, due to rotational movement about the fulcrum on which the arm is pivoted, the stylus follows circular arcs in motion, thus moving not only in the vertical direction but also in the feeding direction of the arm support member. Consequently, the movements in this feeding direction may form errors in measurement (errors in circular arcs), and, when there are considerable irregularities on the outer surface of the object to be measured, the errors may increase accordingly, hence rendering accurate measurement of the contour of the object with the stylus impracticable.

To overcome this problem, there has been proposed a mechanism wherein one end of a link bar is rotatably mounted on an end portion of an arm opposite to the side, to which the stylus is secured, through a cross spring, and the other end of the link bar is supported on an arm support member (slide plate) through a cross spring, whereby, when the arm is rotated, the linkbar is rotated about the point supported on the arm support member, and due to this rotation, the arm is moved in a direction of correcting errors in the circular arcs, so that, finally, the tip of the stylus can move substantially vertically, thereby enabling the avoidance of errors in the circular arcs; or a mechanism similar to the above wherein parallel links are used in place of the link bar described above (see Japanese published Patent Application No. 53352/78).

However, in the conventional mechanism as described above, the link bar or parallel links for supporting the arm are required, and this has involved such a disadvantage that the arm is not necessarily provided at the most preferable position, so that a force of an excessively high value may act upon the arm.

Further, in the conventional contour measuring instrument, a differential transformer as contour signal detecting means is mounted at a position spaced apart a predetermined distance from a plane incorporating therein the centre of rotation of the arm and parallel to the feeding direction of an arm support member (hereinafter referred to as a "reference plane"). More specifically, a core (magnet) of the differential transformer provided projecting in a direction perpendicular to the arm, and a coil of the differential transformer is fixed on the arm support member in a manner to include the core at the centre thereof, whereby the control position of the core and coil in the axial direction thereof is necessarily spaced apart a predetermined distance from the abovedescribed reference plane incorporating therein the centre of rotation of the arm.With this arrangement, there is such a disadvantage that the inclination value of the core in the coil is different depending on whether the arm moves upwards above the reference plane or downwards below the reference plane, thereby causing errors due to the difference. There has been developed such a mechanism wherein the core is secured to the arm not directly but through parallel links, so that, when the arm is rotated, the core can move vertically without being inclined in the coil. With such an instrument, errors due to the direction of rotation of the arm can be prevented. However, such disadvantages result that the resultant complex construction involves accumulated errors due to looseness of connecting portions, and increased costs due to the complex construction.

In accordance with the present invention, we provide a contour measuring instrument wherein one end portion of an arm, which is provided at its other end thereof with a stylus, is rotatably supported on an arm support member and displacement of the arm due to abutment of the stylus against an object to be measured is measured by contour signal detecting means while the arm support member is being fed in a predetermined direction, characterized in that damage preventing means is provided which detects a force exceeding a predetermined value applied to the arm in the longitudinal direction thereof so as to prevent the stylus or the object from being damaged.

The preferred damage preventing means for the stylus and/or for the object to be measured is constituted by a pair of arms slidable on each other in the longitudinal direction thereof, and detecting means for detecting relative movement between the pair of arms when a force exceeding a predetermined value acts on one of the pair of arms to which the stylus is secured, whereby a force of an unusually high value acting on the stylus is detected, so that the movement of an arm support member can be stopped manually or automatically to remove the force of the unusually high value acting on the stylus.

Preferably, the contour signal detecting means is a differential transformer and the central position of a coil and of a core of the differential transformer is set on a reference plane intersecting the axis of rotation of the arm and parallel to the feeding direction of the arm support member, when the tip of the stylus is set on the reference plane. When the arm rotates in any direction, the core is thus arranged to move symmetrically so that the difference in the rotating direction thereof can be eliminated and complex construction can be avoided.

Errors in the measured value caused by shifts in position of the stylus in the feeding direction of the arm support member may be corrected by circular arc error correcting means when the stylus is displaced upon following the contour of the object to be measured.

Preferably, the fulcrum for rotatably supporting the arm in relation to the arm support member is movable in a direction parallel to the feeding direction of the arm support member; and there is a connecting link, one end of which is rotatably supported by the arm and the other end of which is rotatably supported by a fixed fulcrum on the arm support member so that when the tip of the stylus is positioned on a reference plane incorporating the axis on which the arm is pivoted,and the fixed fulcrum of the connecting link, at least the length of the arm between its fulcrum and the fixed fulcrum of the connecting link is substantially aligned with the reference plane. As a result only a force substantially in the longitudinal direction acts on the arm.

An example of an instrument constructed in accordance with the invention is illustrated in the accompanying drawings, in which: Figure 1 is a front view showing the general arrangement of the instrument; Figure 2 is an enlarged sectional view of the essential portions of Figure 1; Figure 3 is a section taken on the line Ill-Ill in Figure 2; Figure 4 is a sectional view taken along substantially the axial line of the arm in Figure 2; Figures 5 to 7 are sections taken on the lines V-V, VI--Vl.and VIl-VIl in Figure 2, respectively; Figure 8 is a perspective view showing the connecting link in Figure 2; and, Figure 9 is an enlarged section taken on the line IX-IX in Figure 3.

As shown in Figure 1, a support pole 2 is mounted on a base 1 and secured thereto with an X-direction drive detecting mechanism 3 which is vertically movable along the support pole 2 by driving a handle 4. This X-direction drive detecting mechanism 3 is adapted to move a Y-direction detecting mechanism 5 in the direction X, i.e., in the longitudinal direction of an arm or an arm assembly 6, which is hereinafter referred to briefly as an "arm 6", provided in the Y-direction detecting mechanism 5, and to detect the movement thereof. The Direction drive detecting mechanism 3 is provided therein with a driving motor 7, rotation of which is imparted to a feed screwshaft 9 through a gearing mechanism 8, and detected by a rotation detector 10 such as a rotary encoder.A nut member 11 is screw threadably coupled onto the feed screwshaft 9 and movable in the direction X by rotation of the feed screwshaft 9. Connected to this nut member 11 through a connector 12 is the Y-direction detecting mechanism 5 which is movable in the direction X along with the nut member 11.

Figures 2 through 9 show in detail the construction of the Y-direction detecting mechanism 5. In these drawings, the Y-direction detecting mechanism 5 is provided with an arm support member 13 made of a casting; the longitudinal opposite surfaces and the upper surface of this arm support member 13 being covered with a cover 14 being of a letter 'C' shape in cross section; and the end faces perpendicularly intersecting the longitudinal direction thereof being covered with side covers 15,16. Disposed at the centre of the upper surface of this cover 14 is the aforesaid connector 12 which is rigidly secured to the arm support member 13 through three stud pins 17 (refer to Figure 3).

As shown in Figures 2 and 4, the composite arm 6 consists of a pair of arms including a first arm 1 8 of a substantially tubular shape with a small diameter, and a second arm 20 of a substantially tubular shape having a larger diameter and adapted to receive therein the first arm 18 through a bush 19. The first arm 18 includes a proximal arm 1 8A inserted into the second arm 20 and a distal arm 1 8B extending from the forward end of the proximal arm 1 8A through a joint 21. One end of the proximal arm 1 8A projecting from the second arm 20 and the distal arm 1 8B are formed into thin wall pipes for the purpose of rendering them light in weight, and a stylus 22 is rigidly secured to the forward end of the distal arm 1 8B in a manner to project in the transverse or radial direction. Furthermore, the distal arm 1 8B can be inverted in the vertical direction at the portion of the joint 21, so that the stylus 22 can be also directed upwards and not only downwards as shown, thereby enabling the measurement not only of the upper surface of an object 23 to be measured but also the upper wall in a transverse hole thereof. In this case, the joint 21 is provided with a detent mechanism 24 including e.g. spring-biased balls and sockets, so that the joint 21 can be accurately located at a position inverted through 1 800. Further, the joint 21 is adapted to rotate on the central axis of the second arm 20 so that, even if the stylus 22 is inverted in its direction, the tip end of the stylus 22 can face in the same position as before the inversion.

The first arm 18 and the second arm 20 are adapted to slide on each other in the axial direction thereof, and normally both arms are adapted to keep a predetermined positional relationship therebetween through the agency of energizing means 25 which includes first and second compression coil springs 29,30 to be described hereinafter.More specifically, a setscrew 26 is solidly secured to the right end of the first arm 1 8 as shown in Figure 2, a sliding disc 27 is slidably coupled onto the outer periphery of the first arm 18 at a position to the left of the setscrew 26, and the first compression coil spring 29, constituting a portion of the aforesaid energizing means 25, is confined between the sliding disc 27 and a flange 28 integrally formed on the first arm 18, thereby constantly bringing the sliding disc 27 into abutting contact with the head of the set-screw 26.Furthermore, the outer periphery of the sliding disc 27 is made slidable on the inner periphery of the second arm 20, and the second compression coil spring 30, constituting the remaining portion of the energizing means 25, is confined between the sliding disc 27 and an inner diameter stepped portion of the second arm 20, whereby the sliding disc 27 is brought into abutting contact with the inner end face of a box nut 31 screwed into the right end of the second arm 20 through the agency of the second compression coil spring 30.

Formed on the inner end face of this box nut 31 is a recess 32, within which the head of the setscrew 26 is movable in the axial direction of the set-screw 26. With the arrangement as described above, the sliding disc 27 is pressed against the inner end face of the box nut 31 through the agency of the second compression coil spring 30, while, the set-screw 26 is pressed against the right end face of the sliding disc 27 through the agency of the first compression coil spring 29, whereby the first and second arms 18,20 are normally held in the condition as shown.If, in this condition, the first arm 1 8 is pressed rightward in Figure 2 by a force higher in value than the force of the first compression coil spring 29, then the first arm 1 8 moves rightward against the force of the first compression coil spring 29, whereby the sliding disc 27 is brought out of abutting contact with the set-screw 26. Whereas, if the first arm 1 8 is pulled leftward in Figure 2 by a force higher in value than the force of the second compression coil spring 30, then the sliding disc 27 is pressed leftward by the head of the set-screw 26 rigidly secured to the right end of the first arm 18, whereby the first arm 1 8 moves leftward along with the sliding disc 27 against the force of the second compression coil spring 30.

Inserted through the flange 28 of the first arm 1 8 in the horizontal direction, i.e. in a direction perpendicularly intersecting the paper surface in Figure 2 is a fulcrum shaft 33, opposite end portions of which project from the second arm 20 through a slot 34 formed in the second arm 20, and bearings 35 are mounted on the abovedescribed projecting portions, respectively.

These bearings 35 are brought into abutting contact with a pair of needles 36 (refer to Figure 5), which constitute an imaginary straightened guide surface and are solidly secured to the arm support member 13, whereby the arm 6 constituted by the pair of first and second arms 18,20 is rotatably and axially slidably supported on the fulcrum where the arm 6 is pivoted.

Furthermore, the central portion of the fulcrum shaft 33 is fixed by means of a screw shaft 38 inserted through a slit 37 (Figure 2) in the upper surface of the second arm 20. Rigidly secured to the upper end portion of this screw shaft 38 is an actuating member 39, against which abuts a working arm 42 of a microswitch 41 serving as detecting means and secured to the outer periphery of the second arm 20 through a bracket 40. With the abovedescribed arrangement, if the first arm 18 moves against the energizing means 25 to make a movement relative to the second arm 20, the relative movement can be detected by the microswitch 41, which is connected to a power source circuit of the driving motor 7 described in Figure 1, and, when actuated by the actuating member 39, the microswitch 41 turns 'OFF' the power source circuit of the driving motor 7.

Rigidly secured to a core portion of the box nut 31 mounted on the right end of the second arm 20 is one end of a balance shaft 43 extending in the axial direction of the arm 20, and the other end of the balance shaft 43 extends outwardly from the side cover 1 6. A balance weight 44 is mounted on the extended portion of the balance shaft 43 in a manner to be movable in the axial direction of the balance shaft 43, and may be fixed in place by means of a bolt 45.In the case of normal measurement where the stylus 22 is directed downwards as shown in Figure 2, the position of the balance weight 44 is set such that the weight of the arm 6 and associated parts from the fulcrum shaft 33 to the end of stylus 22, on one side of the fulcrum shaft 33 is slightly heavier than the weight of the parts from the fulcrum shaft 33 to the end of the balance weight 44.

With the above described arrangement, the arm 6 on the side of stylus 22 tends to fall in the free condition of the arm 6, and the stylus 22 is adapted to be brought into abutting contact with the outer surface of the object 23 to be measured with a slight contacting force.

Extending up from an intermediate portion of the balance shaft 43 is a pin 46 which is opposed to an eccentric cam 48 fixed on an output shaft of a DC motor 47. With this arrangement, if the motor 47 is driven and the eccentric cam 48 is brought into abutting contact with the pin 46, then the balance weight side of the arm 6 is pressed down and the stylus side of the arm 6 is raised, whereby the stylus 22 is brought out of abutting contact with the object 23 to be measured. If the-motor 47 is stopped with the largest eccentric portion (the position of the largest diameter) of the eccentric cam 48 operative, the arm 6 is stopped at a position in which the stylus 22 is disposed at a position farthest from the object 23 to be measured.

Further, if the motor 47 is driven again from the stopped position as described above, then the pin 46 is slowly allowed to rise to the unbalanced force of the arm 6 while remaining in abutting contact with a cam surface of the eccentric cam 48, so that the stylus 22 can be slowly brought into abutting contact with the outer surface of the object 23 to be measured, thereby preventing the stylus 22 from being broken upon sudden contact. Furthermore, the DC motor 47 is rigidly secured by screws to a stand 49 integrally formed with the arm support member 13, and the balance shaft 43 extends through a hole 50 formed in the stand 49.Further, as shown in Figure 7, limit switches 51,52 are secured to opposite sides of the upper portion of the stand 49, respectively, and working arms 53,54 of these limit switches 51,52 can be brought into abutting contact with an actuating pin 55 mounted on a hub portion of the eccentric cam 48. These limit switches 51,52 are connected to the power source circuit of the DC motor 47, and, when actuated by the actuating pin 55, are arranged to turn 'OFF' the power source for the DC motor 47 so as to stop the DC motor 47. With the abovedescribed arrangement, if the limit switch 51 is actuated, then the motor 47 is stopped in operation with the largest diameter portion of the eccentric cam 48 disposed at a position furthest from the pin 46 of the balance shaft 43 as shown in Figure 7.While, if the limit switch 52 is actuated, then the largest diameter portion of the eccentric cam 48 comes into abutting contact with the pin 46, whereby the motor 47 is stopped at the elevated position of the stylus 22.

A spring engaging pin 56 is horizontaily planted in the actuating member 39 fixed to the upper end of the screw shaft 38 for fixing the fulcrum shaft 33 to the first arm 18, and a coil spring 59 is stretched between this spring engaging pin 56 and a spring engaging pin 58 provided on a rotary member 57. As shown in Figure 6, this rotary member 57 is rigidly secured to one end of a rotary shaft 61 rotatably supported by a pair of stands 60 integrally mounted on the arm support member 13.

-Screw threadedly coupled into the other end of this rotary shaft 61 is a finger grip 62 which is non-rotatably coupled to the rotary shaft 61 by means of a pin 63. A large diameter operating portion of this finger grip 62 projects outwardly from the cover 14, and, rotation of the finger grip 62 externally of the cover 14 causes the rotary member 57 to be rotated and moved from a position indicated by a broken line to a position indicated by a chain line in Figure 2. A click motion detent mechanism 64 is provided between one side surface of this rotary member 57 and one of the stands 60 for regulating the anguiar rotation of the rotary member 57.This click motion mechanism 64 includes a socket 65 secured to the stand 60, a compression spring 66 is received in the socket 65 and a ball 67 is constantly pressed against one side surface of the rotary member 57. The ball 67, when engaged in one of recesses formed at one side surface of the rotary member 57, provides an intermittent and positive motion.

Furthermore, the coil spring 59 stretched between the spring engaging pins 56 and 58 is in a slackened condition with no biasing force when it is in position indicated by a broken line in Figure 2, i.e., in the normal condition. Whereas, if the finger grip 62 is rotated to bring the rotary member 57 into the position indicated by the chain line in Figure 2, then a tensile force is generated in the coil spring 59 which provides a turning force to the arm 6 through the screw shaft 38 to cause it to rotate about the fulcrum shaft 33 in the clockwise direction in Figure 2.With the abovedescribed arrangement, when the rotary member 57 is turned to the position indicated by the chain line, if the left end portion of the first arm 1 8 shown in Figure 2 is inverted at the joint 21 through 1800 to direct the stylus 22 upwards, then a contacting force of a predetermined value is applied to the tip end of the stylus 22, so that the stylus 22 can be brought into abutting contact with e.g. the upper surface of a transverse hole formed in the object 23 to be measured under the contacting force of the predetermined value, thereby enabling measurement to be carried out without changing the position of the balance weight 44.

As shown in Figures 3 and 4, a cutout 68 is formed at one side of the central portion of the second arm 20, and a differential transformer 69 serving as contour signal detecting means is mounted on the arm support member 13 within the cutout 68. A core shaft 70 of this differential transformer 69 is secured to the outer periphery of the second arm 20 through a bracket 71, so that the value of rotation of the second arm 20 about the fulcrum shaft 33 can be measured.

Furthermore, as shown to a larger scale in Figure 9, a coil 73 is housed in a casing 72 of the differential transformer 69, and the centre of this coil 73 in the vertical direction in Figure 9 intersects plane P incorporating the central axis of rotation of the arm 6, i.e., the central axis of the fulcrum shaft 33, and parallel to the feeding direction of the arm support member 13 as indicated by a doubleheaded arrow X in Figure 1, whereby distances L from a point, where the reference plane P intersects the coil 73, to the upper and lower opposite end faces of the coil 73 are equal to one another. Furthermore, the core shaft 70 extends into a central hole of the coil 73, and is rigidly secured to a core 74 in the central hole.The position, where the core 74 is secured, is so determined that, when the arm 6 is positioned in such a manner that the tip end of the stylus 22 is set on the reference plane P (herein referred to as an "arm reference position"), the centre of the core 74 is set on the reference plane P, whereby the distances L from the point, where the reference plane P intersects the core 74, to the upper and lower opposite end faces of the core 74 are equal to one another.

A support link 75 of an inverted letter 'C' shape in plan view is rigidly secured to the left end of the second arm 20 as seen in Figures 2 to 4.

Rotatably supported across opposite ends of the 'C' shaped opening of this support link 75 through bearings 76 and a pivot 77 is one end of a connecting link 78. The other end of this connecting link 78 is rotatably supported by a pair of stands 81 integrally mounted on the arm support member 13 through a pivot 79 and bearings 80, and this supporting portion serves as a fixed fulcrum for the connecting link 78. As shown in Figure 8, this connecting link 79 is formed such that opposite sides of a cylinder, having a through-hole 82 of an elliptic shape in cross section in the axial direction, thereof are flattened off, and the pivots 77,79 project from these flat surfaces. The first arm 1 8 extends through the through-hole 82 with a clearance of a predetermined value from the wall of the throughhole 82.Circular arc errors correcting means of the arm 6 includes the support link 75, bearings 76,80, pivots 77,79 connecting link 78 and stands 81 of the arm support member 13.

Positioned on the reference plane P incorporating therein the fulcrum where the arm 6 is pivoted, i.e., the central axis of the fulcrum shaft 33 and in parallel to the feeding direction of the arm support member 13, i.e., the direction indicated by the arrow X, is the fixed fulcrum for the connecting link 78, i.e., the centre line of the pivot 79.

Furthermore, when the tip end of the stylus 22 is positioned on the reference plane P, i.e., at the arm reference position, at least a section of the axial line of the arm 6 disposed between the fulcrum where the arm 6 is pivoted and the fixed fulcrum of the connecting link 78 is adapted to be positioned substantially on this reference plane P, so that the force applied to the arm 6 can be a tension or compression substantially in the axial direction of the arm 6.

In addition, Figure 1 shows a recorder 90, which is a so-called X-Y recorder, for receiving a signal of rotational value of the feed screwshaft 9 from the rotation detector 10, i.e., a signal of movement value of the Y-direction detecting mechanism 5 in the direction X and a signal of movement value in the direction Y (feed signal) of the arm 6 from the differential tranformer 69 of the Y-direction detecting mechanism 5, and combining the both signals, thereby providing and enlarging illustration of the contour of the outer surface of the object 23 to be measured on a record paper.

A description will now be given of action of this apparatus.

The object 23 to be measured is rested on the base 1 directly or via a predetermined mounting.

Subsequently, the handle 4 is rotated to set the stylus 22 in the vertical direction (direction Y) through the X-direction drive detecting mechanism 3 and the Y-direction detecting mechanism 5, and the driving motor 7 of the Xdirection drive detecting mechanism 3 is operated to set the stylus 22 in the lateral direction (direction X), so that the stylus 22 can be positioned at a starting position of a portion of the object 23 to be measured with a suitable gap. In this case, the DC motor 47 for driving the eccentric cam 48 is stopped in the condition with the largest diameter portion of the eccentric cam 48 in abutting contact with the top of the pin 46.

The stylus 22 is thus held at an elevated position above the measuring position, and, by the position setting as described above, the stylus 22 is positioned slightly above the left end portion of the object 23 to be measured shown in Figure 2.

Then, if the DC motor 47 is driven to rotate the eccentric cam 48, because the weight of the portion of the arm 6 from the fulcrum shaft 33 to the end of the stylus 22 is slightly heavier than the weight of the portion of the arm 6 from the fulcrum shaft 33 to the end of the balance weight 44, the upper end of the pin 46 slowly ascends while abutting against the cam face of the eccentric cam 48. Meanwhile, the stylus 22 slowly descends to abut against the outer surface of the object 23 to be measured without any impact.In this condition, if the driving motor 7 is driven again to rotate the feed screwshaft 9 through the gearing mechanism 8 and move the Y-direction detecting mechanism 5 through the nut member 11 and connector 12 rightward in Figure 1, the stylus 22 is moved to the right, moving vertically as it follows the irregularities of the contour of the outer surface of the object 23 to be measured. This vertical motion of the stylus 22 is converted into rotations of the arms 6, i.e., of the first and second arms 1 8,20 about the fulcrum shaft 33. The rotations of the first and second arms 18,20 are converted into a motion of the core shaft 70 secured to the second arm 20 through the bracket 71, and the value of displacement of the core shaft 70 is detected by the differential transformer 69 serving as the contour signal detecting means.In this case, the centres of the coil 73 and core 74 of the differential transformer 69 are positioned on the reference plane P when the arm 6 is in the arm reference position, and hence, when the arm 6 rotates in any direction, the core 74 and coil 73 are adapted to change the positional relationship therebetween in a completely symmetrical manner. Meanwhile, the displacement of the Ydirection detecting mechanism 5 in the direction X is detected by detecting rotation of the feed screwshaft 9 by means of the rotation detector 10, consisting of the rotary encoder, and the displacement in this direction X, i.e., the feeding, and the displacement in the aforesaid direction Y are detected and recorded on a record paper by the recorder 90, so that the measurement of the contour of the outer surface of the object 23 to be measured can be carried out.

In measuring the object 23, when there are no large irregularities on the contour of the outer surface of the object 23, there arises little problem. However, if there are large irregularities, the so-called errors in circular arcs may raise problems. More specifically, it 5 ideal that the stylus 22 normally moves in the vertical direction (direction Y). However, the motion of the stylus 22 actually is the rotational movement about the fulcrum shaft 33. Consequently, if the vertical movement of the stylus 22, i.e., the rotational angle of the arm 6 exceeds a certain level in relation to the distance between the fulcrum shaft 33 and the stylus 22, then the component in the direction X of the displacement due to the elevation of the stylus 22 becomes large to a not negligible extent, which results in errors in measurement requiring some correction.In this embodiment, this correction is carried out by the circular arc error correcting means including the support link 75 and connecting link 78. Now, if it is assumed that the stylus 22 is moved upwards, then the first arm 18 rotates in the clockwise direction, and the second arm 20 and the support link 75 integrally formed on the second arm 20 also rotate in the clockwise direction. The connecting link 78 is connected to this support link 75 through the bearing 76 and the pivot 77 is connected to the stands 81 of the arm support member 13 through the pivot 79 and the bearing 80, and hence, this connecting link 78 is rotated in the counter-clockwise direction, and the connecting link 78 and the second arm 20 tend to be displaced and bent upwardly at the position of the pivot 77.By this, the distance between the pivot 79 fixedly supported by the stand 81 and the fulcrum shaft 33 tends to be contracted.

However, due to the fixed supporting of the pivot 79, the fulcrum shaft 33 moves on the needles 36 to the left in Figure 2 through the agency of the bearing 35, whereby the first arm 18 and the stylus 22 move leftward. Consequently, the higher the stylus 22 ascends, the more leftward the stylus 22 is moved through the action of the correcting means. Whereas, as viewed from the rotation of the stylus 22 about the fulcrum shaft 33, due to the component in the direction X of the displacement by the turning action of the arm 6 in accordance with the elevation of the stylus 22, the higher the stylus 22 ascends, the more to the right is the stylus 22 moved. The abovedescribed both actions offset each other, and finally, the stylus 22 ascends substantially vertically, thus the preventing errors in circular arcs from occurring.

Description will now be given of the damage preventing means for preventing the stylus 22 and the object 23 to be measured from being damaged in case the stylus 22 falls into a deep groove formed in the object 23 or the stylus 22 is caught at the inlet of the transverse hole formed in the object 23 when inserted. Now, if it is supposed that, while moving rightward in Figure 2, the stylus 22 falls into a deep groove, the stylus 22 is caught by the inner wall of the deep groove, while, the Y-direction detecting mechanism 5 tends to continue rightward moving, whereby a force of a high value acts on the stylus 22. This force is imparted to the second compression coil spring 30 of the energizing means 25 through the first arm 18 secured to the stylus 22 and the sliding disc 27 unlodgingly secured to the arm 18 by the set-screw 26.With this arrangement, if the force exceeds a predetermined value, the second compression coil spring 30 is flexed, the first arm 18 and the second arm 20 slide on each other in the axial direction. This sliding of the first arm 18 on the second arm 20 is imparted through the screw shaft 38 to the actuating member 39, which actuates the microswitch 41 serving as the detecting means, whereby the driving motor 7 is stopped in operation. This stops the Y-direction detecting mechanism 5 in movement, so that the stylus 22 and the object 23 to be measured can be prevented from being broken or damaged. In this case, in the second compression coil spring 30, the spring constant is determined such that the stylus 22 is compressed by a force giving no damage to the stylus 22.Furthermore, if the stylus 22 or the left end face of the first arm 18 comes into abutting contact with the inlet of the transverse hole while the stylus 22 moves to the left in Figure 2, the first arm 18 is subjected to a force pressing the first arm 18 rightward. By this, the first arm 18 projects from the sliding disc 27 against the first compression coil spring 29 confined between the flange 28 of the first arm 18 and the sliding dise 27, and the first arm 18 and the second arm 20 slide on each other in the axial direction thereof as aforesaid. This sliding actuates the microswitch 41 which stops the driving motor 7, thereby preventing the stylus 22 from being broken or the first arm 18 being deformed.

Description will now be given of the case in which the distal arm 1 8B is rotated at the joint 21 to direct the stylus 22 upwards and measure the contour of the upper surface of the inner wall of the transverse hole formed in the object 23 to be measured. In order to direct the stylus 22 upwards for measurement, it is necessary to take the measurement in such a condition that, in contrast to the above, an upwardly force is applied to the stylus 22, whereby the stylus 22 comes into contact with the object 23 to be measured under a force of a comparatively low value. To do this, the balance weight 44 may be moved for adjustment, which is, however, troublesome because, when the stylus 22 is restored tobe directed downward, the balance weight 44 has to be moved again for readjustment. Therefore, to apply the upward force to the stylus 22 without moving the balance weight 44, there are provided the rotary member 57 and the coil spring 59. More specifically, when the rotary member 57 is positioned as indicated by a broken line in Figure 2, the coil spring 59 is in the slackened condition and no force acts on the spring engaging pin 56, and consequently, no force is applied to the arm 6. However, if the finger grip 62 is rotated to set the rotary member 57 at the position indicated by a chain line in Figure 2 through the rotary shaft 61, then the coil spring 59 is stretched, whereby a force pulling rightward in Figure 2 is applied to the spring engaging pin 56. By this, the screw shaft 38 is urged in the clockwise direction about the fulcrum shaft 33, and the arm 6 is urged in the clockwise direction as well.If this force applied by the coil spring 59 is properly selected, then the upwardly directed force of a predetermined value can be applied to the stylus 22 without moving the balance weight 44, and the measurement can be effected on the upper surface of the inner wall of the transverse hole formed in the object 23 to be measured.

In this embodiment as described above, there is provided damage preventing means, wherein the arm 6 is constituted by a pair of arms, i.e. the first and second arms 18,20 which are slidable on each other, these first and second arms 18,20 are held in place by the energizing means 25, and the relative movement between the first and second arms 18,20 is detected by the microswitch 41 serving as the detecting mechanism, which stops the driving motor 7, so that the stylus 22 can be effectively protected. Furthermore, the cylindrical connecting link 78 is used as the circular arc error correcting means and the arm 6 extends through this connecting link 78, so that the instrument can be rendered compact in size. Further, the connecting link 78 is formed to have a comparatively thin wall thickness.Consequently, in assembling the bearings 76,80 with the pivots 77,79, if the bearings 76,80 are slightly excessively driven into the pivots 77,79, then the connecting link 78 is flexed accordingly, with the result that the looseness generated between the bearings 76,80 and the pivots 77,79 due to the long term service is absorbed by the connecting link 78, whereby no looseness is generated in long-term service. The core shaft 70 is fixed on the periphery of the second arm 20 through the bracket 71, so that the construction of mounting the core shaft 70 can be simplified, and moreover, the movement of the arm 6 can be accurately imparted.The arm 6 is supported by the bearings 35 and the needles 36 through the fulcrum shaft 33 in order to follow the movements of the first and second arms 18,20 in the axial direction due to the provisions of the damage preventing means for the stylus 22 and the object 23 to be measured, and of the circular arc error correcting means, so that the contact between the bearings 35 and the needle 36 is made at a point. Smooth motion can thus be effected, and moveover, a flat and smooth surface, with which the bearings 35 come into contact, can be provided by use of the needles 36, inexpensively.Further, the movement of this fulcrum shaft 33 is made in the condition where the bearings 35 are in abutting contact with the needles 36 due to the weight of the arm 6 and the balance weight 44 so that the fulcrum shaft 33 can be moved with low friction and the movement of the fulcrum shaft 33 does not change the arm 6 in its height, which can remain constant. Moreover, the construction can be rendered simple and compact in size as compared with other constructions in which the arm is suspended by use of a link or a cross spring.

Furthermore, the differential transformer 69 is used as the contour signal detecting means and the centre of this differential transformer 69 is set on the reference plane P, so that the errors due to the turning direction of the arm 6 can be avoided.

Furthermore, this differential transformer 69 does not require any new link and its construction is simplified, so that it can be manufactured inexpensively. Further, as the differential transformer 69 does not project in the direction from the reference plane P in relation to the arm 6, the instrument as a whole can be rendered compact in size. The arm 6 is provided such that, when the stylus 22 is set on the reference plane P, i.e., the arm 6 is at the arm reference position, at least a section of the axial line of the arm 6 disposed between the fulcrum where the arm 6 is pivoted and the fixed fulcrum of the connecting link 78 is adapted to be set substantially on the reference plane P.A force of an excessively high value such as a bending moment is thus rarely to be applied to the arm 6, and distortion can be prevented from being generated in the arm 6, thereby minimising the errors in measurement.

Further, the arm 6 and the parts associated therewith are set substantially on the reference plane P when the arm 6 is set at the arm reference position, so that accumulation of errors can be minimized. Furthermore, there are provided the rotary member 57 positioned by means of the click motion mechanism 64 and the coil spring 59 connected at one end thereof to this rotary member 57 and the rotary member 57 is made rotatable with the finger grip 62, so that the direction of applying the measuring force to the stylus 22 can be readily changed over without needing to adjust the balance weight 44.

In addition, to the above described construction, constructions wherein both arms 18 and 20 are slidable on each other in the axial direction may be adopted such that the first arm 1 8 receives therein the second arm 20 or both arms are not formed into tubular shapes and are disposed in parallel to each other. Further, the arm 6 may be formed into a rod shape connected to the arm support member 1 3. In this case, the detecting means for the damage preventing means may detect the relative displacement between the arm 6 and the arm support member 1 3. Further, the energizing means 25 is not limited to the first and second compression coil springs 29,30 which are dually disposed, and any other construction may be adopted.Furthermore, construction is not limited to one wherein, when a force of an excessively high value is applied tQ the stylus 22, the driving motor 7 is stopped in operation as in the abovedescribed embodiment, but a construction may be adopted in which the stylus 22 is protected by any other means such as a warning buzzer. Furthermore, in the abovedescribed embodiment, the differential transformer 69 is interposed between the fulcrum shaft 33 and the pivot bearing 79. However, the present invention is not limited to this, but, for example, the differential transformer 69 may be provided to the right of the fulcrum shaft 33 or to the left of the pivot bearing 79 in Figure 3. In short, it suffices that the central position of the coil 73 and core 74 of the differential transformer 69 is on the reference plane P.Further, in the abovedescribed embodiment, the coil 73 of the differential transformer 69 is secured to the side of the arm support member 13 and the core 74 is secured to the side of the arm 6. However, the present invention is not limited to the above, but, the coil 73 may be secured to the side of the arm 6 and the core 74 may be secured to the arm support member 13. Furthermore the contour signal detecting means is not limited to the differential transformer 69, but other ordinary means such as the difference in electrostatic capacity may be used. Further, the circular arc error correcting means is not limited to one for mechanical correcting as in the abovedescribed embodiment, but one, in which a signal from the contour signal detecting means is electrically corrected, may be adopted.Furthermore, the connecting link 78 is not limited to one being of a ring shape in cross section, but may be of a letter 'U' shape in cross section with one side of the ring being open, in which case, the through-hole 82 is replaced with an extended groove. Further, the connecting link 78 may be divided into two parts disposed at opposite sides of the arm 6 and may be supported at the sides of the turning surface of the arm 6. In short, it suffices that the connecting link 78 is one through which the arm 6 can pass in substantially a straight line. Furthermore, in the abovedescribed embodiment, the arm 6 is supported by two bearings 35 and two needles 36 through the fulcrum shaft 33. However, the present invention is not limited to this, but an arrangement may be adopted in which the bearings 35 and the needles 36 may be omitted, a pair of ridges integrally formed on the arm support member 13 being provided in the axial direction of the arm 6. The upper surfaces of these ridges are finished to provide flat, smooth and straightened surfaces and the fulcrum shaft 33 is directly supported by these ridges, or one bearing 35 and also one needle 36 may be used, the arm 6 being supported by the bearing 35 disposed in a direction perpendicularly intersecting the axis of the arm 6 at a position passing through the axis of the arm 6.

Claims (8)

Claims

1. A contour measuring instrument wherein one end portion of an arm, which is provided at its other end thereof with a stylus, is rotatably supported on an arm support member and displacement of the arm due to abutment of the stylus against an object to be measured is measured by contour signal detecting means while the arm support member is being fed in a predetermined direction, characterized in that damage preventing means is provided which detects a force exceeding a predetermined value applied to the arm in the longitudinal direction thereof so as to prevent the stylus on the object from being damaged.

2. An instrument according to claim 1, wherein the arm comprises a pair of arms slidable on each other in the longitudinal direction thereof, and there are provided energizing means for urging -the arms to a predetermined positional relationship in the normal condition and detecting means for detecting relative movement between the pair of arms against the energizing means.

3. An instrument according to claim 1 or claim 2, wherein the contour signal detecting means is a differential transformer and the central position of a coil and of a core of the differential transformer is set on a reference plane intersecting the axis of rotation of the arm and parallel-to the feeding direction of the arm support member, when the tip of the stylus is set on the reference plane.

4. An instrument according to any one of the preceding claims, wherein errors in the measured value caused by shifts in position of the stylus in the feeding direction of the arm support member is, corrected by circular arc error correcting means when the stylus is displaced upon following the contour of the object to be measured.

5. An instrument according to claim 4, wherein the fulcrum for rotatably supporting the arm in relation to the arm support member is movable in a direction parallel to the feeding direction of the arm support member; and there is a connecting link, one end of which is rotatably supported by the arm and the other end of which is rotatably supported by a fixed fulcrum on the arm support member so that when the tip of the stylus is positioned on a reference plane incorporating the axis on which the arm is pivoted and the fixed fulcrum of the connecting link, at least the length of the arm between its fulcrum and the fixed fulcrum of the connecting link is substantially aligned with the reference plane.

6. An instrument according to claim 5, wherein the connecting link has a through-hole or an extended groove, and the arm extends through the through-hole or extended groove, and outwardly therefrom.

7. An instrument according to claim 5 or claim 6, wherein the fulcrum for the arm is arranged to move in a direction parallel to the feeding direction of the arm support member while abutting against a straight guide surface provided on the arm support member.

8. An instrument according to claim 1, substantially as described with reference to the accompanying drawings.