Ring gauge for checking external dimensions

A ring gauge for checking external diametral dimensions comprising an arm-set essentially consisting of an integral member including two arc-shaped arms, two resiliently deformable portions adapted to define fulcrums for the rotation of the arms and an intermediate portion coupled to a support. Two feelers are adjustably fixed to the arms, approximately at intermediate points of their longitudinal development. The elements of a transducer are fixed at the free ends of the arms.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a ring gauge for checking external 
diametral dimensions of parts having a surface of rotation, comprising 
support means; two feelers movable with respect to the support means, the 
feelers being adapted to touch the part to be checked; two arc-shaped arms 
coupled to said support means and adapted to support said feelers; and 
transducer means adapted to provide an electric signal depending on the 
position of the feelers. 
2. Description of the Prior Art 
Known ring gauges for checking external diameters comprise a support 
structure and a ring coupled to the support structure through a flat 
spring. Fixed to the ring and passing through a hole of it is a cartridge 
gauging head having a movable feeler protruding internally with respect to 
the ring. A carbide reference contact or fixed feeler adapted to touch the 
surface of the part to be checked is fixed to the internal surface of the 
ring, at a position diametrically opposite that where the movable feeler 
is arranged. The flat spring permits centering of the ring with respect to 
the part. Other known ring gauges have a substantially U-shaped arm-set, 
housed within an annular casing and fixed to the casing so as to perform 
angular displacements about a point of the casing, for permitting 
centering of the arm-set with respect to the part. Fixed at the ends of 
the U and protruding internally with respect to the casing are, 
respectively, a cartridge type gauging head and a carbide reference 
contact. These known ring gauges are rather expensive and have large 
overall dimensions, mainly due to the use of cartridge gauging heads, 
which moreover may cause incorrect operation due to possible seizures. For 
zero setting these ring gauges it is necessary to act on the cartridge 
heads and therefore it is necessary to have skilled operators which have 
to reach components of the ring gauge not immediately accessible. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a simple and cheap ring 
gauge having small dimensions and excellent reliability and which can be 
zero set and quickly and easily used for measurements. 
This and other objects and advantages are reached by a ring gauge of the 
type set forth, wherein said arms are coupled to the support means for 
performing angular displacements, the feelers are fixed to the arms 
substantially at intermediate points of their longitudinal development and 
the arms have free ends in proximity of which there are fixed the 
transducer means.

DETAILED DESCRIPTION 
As shown in FIGS. 1 and 2, the ring gauge comprises a first support element 
1, having substantially an annular shape, i.e. the shape of a circular 
crown, with a flange 2 arranged along a direction perpendicular to the 
axis of symmetry of said circular crown. 
The arm-set of the gauge comprises an integral element 3 having the shape 
of an open ring. Element 3 defines two measuring arms 4 and 5 having 
substantially opposite semicircular shapes, an intermediate linear portion 
6, parallel to flange 2, and two portions 7 and 8 connecting relevant 
first ends of arms 4 and 5 to linear portion 6. Portions 7 and 8 have a 
reduced thickness so as to be resiliently deformable and define two 
geometric axes, perpendicular to the plane of FIG. 1. Arms 4 and 5 can 
perform rotational displacements about these axes. 
Portion 6 is fixed to flange 2 through screws 9 and in this way the arm-set 
of the gauge is secured to support 1. The arrangement of screws 9, which 
lock the arm-set at its back side, guarantees a high resistance of the 
arm-set locking with respect to possible impacts along the measurement 
direction. 
A pair of seats i.e. threaded holes having transversal axes is 
approximately midway of the longitudinal development of each arm 4 and 5; 
the holes 10 and 11 defined in arm 4 are visible in FIG. 2. In either hole 
10 or 11, e.g. in hole 10, there is screwed a feeler 12 (shown only in 
FIG. 1), frictionally fastened by a self-locking threaded insert 13. 
Another feeler 14 is frictionally and threadedly engaged, through a 
self-locking threaded insert 15, within a hole 16 of arm 5, similar to and 
coaxial with hole 10 of arm 4. 
Feelers 12 and 14 are adapted to touch the cylindrical surface of the part 
17 to be checked, along a diametral direction coinciding with the common 
geometric axis of holes 10 and 16. 
Arranged near the free ends of arms 4 and 5, which ends are close to each 
other, is located a resilient element, i.e. a return spring 18 which urges 
arms 4, 5 towards each other so as to keep feelers 12 and 14 into contact 
with part 17. 
A device for limiting the angular displacements of arms 4 and 5 is arranged 
near spring 18; this devices comprises two stop elements, i.e. dowels 19 
and 20--threadedly engaged in holes 21 and 22 of arms 4 and 5, 
respectively--which can contact the lateral surface of a pin 23 fastened 
to support 1, for adjustably limiting the displacements of arms 4 and 5 
along the measurement direction. 
Two further stop elements, i.e. pins 24 and 25 fixed to support 1 are 
arranged at relevant sides of the free ends of arms 4 and 5 for limiting 
the opening displacements of arms 4 and 5, i.e. along the direction 
opposite to the measurement direction, by contacting these free ends. 
Located within through holes 26 and 27 of arms 4 and 5 are two elements or 
supports 28 and 29, which are locked by pins housed in holes 30 and 31 
having geometric axes perpendicular to those of holes 26 and 27. Support 
28 houses the electric windings 57 of a differential transducer 52, while 
fixed to support 29 of the transducer 52 is a stem 32 carrying the 
magnetic core, which is not visible. 
An electric cable connecting transducer 52 to a power supply, processing 
and indicating unit, is not shown. 
The arm-set of the gauge is protected by two elements, i.e. a cover 
33--made of stamped plate--, which encircles integral element 3 and an 
annular element 37 which is encircled by arms 4 and 5. Cover 33, which has 
a cylindrical surface and an annular upper base, is fastened to support 1 
by screws 34, only one of which is visible in FIG. 1. 
Cover 33 has two openings 35 and 36, aligned with feelers 12 and 14, which 
permit adjustment of feelers 12 and 14, by operating from the outside. 
Annular element 37 constitutes protection and centering means because it 
protects measuring arms 4 and 5 when inserting part 17 and provides 
centering of the gauge with respect to part 17. Annular element 37 is 
fastened to support 1 by screws 38, one of which is visible in FIG. 2. 
Element 37 has two U-shaped openings 39 and 40, aligned with feelers 12 
and 14, for permitting the feelers to protrude internally with respect to 
element 37. 
Support 1 can be secured to a base 41 by screws 42, one of which is visible 
in FIG. 2. For coupling the gauge to a handle 43, handle 43 is locked to 
base 41 in a way not shown. 
In this case, the electric cable which outputs the signal detected by 
transducer 52 passes through base 41 into handle 43, via an opening 44 
defined in base 41. 
The operation of the ring gauge is now described. 
The gauge is initially zero set on a cylindrical master part having a 
diameter equal to the nominal diameter of the parts 17 to be checked. The 
mechanical zero setting operation consists of adjusting the position of 
feelers 12 and 14 with respect to arms 4 and 5 until the power supply, 
processing and indicating unit provides an indication sufficiently next to 
zero. A subsequent electrical zero setting, by means of a potentiometer of 
the power supply, processing and indicating unit, permits the obtainment 
of an indication corresponding to the zero value. 
Then the gauge is manually applied to a part 17 to be checked. 
If the gauge is not provided with handle 43 and support 1 is laid on a flat 
surface, the part 17 is manually inserted within the gauge, i.e. within 
element 37. In this case the electric cable emerges from the gauge through 
an opening, not shown, in cover 33. 
Feelers 12 and 14, by entering into contact with part 17 cause rotations of 
arms 4 and 5 about portions 7 and 8. The moving away of arms 4 and 5 
causes changing of the electric signal provided by differential transducer 
52, the changing being detected and indicated by the power supply, 
processing and indicating unit. 
As previously mentioned, there ae two pairs of holes 10, 16 and 11, 47 
(FIG. 3) which can house feelers 12 and 14. This feature permits checking 
of difficultly accessible surfaces, i.e. surfaces which are difficult to 
insert within the gauge. 
If, for example, it is desired to check, in parts 53, the diameter of 
cylindrical portions slightly protruding from the other portions of the 
parts 53, that might be impossible by using a ring gauge having a pair of 
feelers arranged approximately midway of the gauge thickness. Thus, as 
shown in FIG. 3, the feelers 12 and 14 can be engaged into holes 11 and 
47. 
However, not withstanding the possibility of two different locations for 
each feeler 12 or 14, it may happen that a cylindrical surface to be 
checked is not accessible, as shown in FIG. 4. 
The part 54 shown in FIG. 4 has a recess terminating with an annular 
shoulder. A cylindrical surface, the diameter of which must be checked, 
protrudes from the annular shoulder and is mostly arranged within the 
recess. 
This measurement problem is solved by using two extension elements 45 and 
46, which are engaged in holes 11 and 47, respectively; the extension 
elements 45 and 46 are substantially L-shaped and have threaded holes 48 
and 49 housing the stems of feelers 50 and 51 adapted to contact the 
above-mentioned cylindrical surface of part 54. Thus, feelers 50 and 51 
may be arranged at a determined distance, along a direction perpendicular 
to arms 4 and 5, from the same arms 4 and 5. 
In this modified embodiment the protection annular element 37' has a 
particular shape, because it features a hollow, annular reference jut 55, 
for preventing contact between the part 54 and extension elements 45 and 
46. The extension elements 45, 46 are partially housed within the hollow 
jut 55. 
During measurement taking, jut 55 is partially inserted within the recess 
of part 54 and the end of jut 55 is adapted to contact the annular 
shoulder of part 54, thus defining the cross-section of the cylindrical 
surface where measurement is performed.