Plug comparator for checking the diameter of holes

A plug comparator for checking the diameter of holes of mechanical parts comprising a support, a movable measurement unit and a protection casing. The movable measurement unit comprises two movable arms bearing respective feeler elements which come into contact with the part to be checked. A position transducer detects the position of the movable arms and is connected by an electric cord to an electric supply and indication group. The casing has a lower base with a frustoconical seat for the fitted housing of a closure plate. The closure plate has a cut in diametral direction which subdivides it into two adjacent portions connected by a third portion which is elastically yieldable so as to permit fitting the closure plate in the frustoconical seat.

The present invention relates to a plug comparator for checking the 
diameter of holes in mechanical parts, comprising support and protection 
means which define an outer housing and a casing or nose with a 
substantially circular base plate; a seat for the plate; two movable arms 
supported by the support and protection means and housed within the 
housing; two feeler elements fastened respectively to the movable arms and 
passing through respective openings of the nose to come into contact with 
diametrically opposite points of the part to be checked; and transducer 
means adapted to detect the position of the movable arms. 
BACKGROUND OF THE INVENTION 
The traditional plug comparators have a protection and support structure 
including a casing or nose which has openings for the passage of the 
feeler elements. The feeler elements are fastened in an adjustable manner 
to the arms in order to permit the use of the comparator in different 
measurement ranges. This adjustability may also require the use of 
different casings. 
To the base of the nose there is usually fastened by screws a closure plate 
which has the two-fold purpose of mechanically protecting the inner 
elements of the comparator and of reducing the possibility of foreign 
matter, for instance, chips, powder and cooling liquids used in the 
machining of the parts, from accidentally entering the comparator. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a plug comparator which involves 
a reduced cost of manufacture as compared with that of the traditional 
comparators and which makes it possible to simplify the assembly and 
maintenance operations. 
This object is achieved by means of a plug comparator of the type indicated 
at the start of the present specification in which, in accordance with the 
invention, the base plate has a cut substantially in diametral direction 
which divides it into two adjacent portions connected by a third portion, 
the third portion being elastically yieldable so as to permit bringing the 
first two portions together in order to insert the plate into the seat and 
effect a fitted connection of the plate to the seat.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 to 6, the manual plug comparator comprises a handle 1, 
preferably of plastic material, having an upper base 2 which is 
perpendicular to the longitudinal axis of the comparator and has a central 
aperture or hole 3 (FIG. 5) for the passage of an electrical cord 4. The 
handle 1, which is hollow on the inside, has substantially cylindrical 
inner and outer surfaces coaxial with the longitudinal axis of the 
comparator. The lower part of the handle 1 terminates in a flange 5 which 
serves to connect the handle to a support element consisting essentially 
of a perforated plate 6. The connection between the flange 5 and the plate 
6 is effected by means of two screws 7, 8 which pass through holes 9, 10 
in the flange 5 (FIG. 4) and are threaded in threaded holes 11, 12 in the 
plate 6 (FIG. 3) so as to clamp the lower face of the flange 5 to the 
upper face of the plate 6. 
The plate 6 furthermore supports a protection and centering casing 13 by 
means of four screws, not visible in the drawing, which pass through holes 
14, 15, 16, 17 of the plate 6 and are threaded in threaded holes, not 
visible in the drawing, provided in a flange 18 of the casing 13 in such a 
manner that the upper face of the flange 18 is clamped to the lower face 
of the plate 6. The four fastening holes of the flange 18 are accessible 
through openings 19 in the flange 5 (FIG. 4). 
The plate 6 furthermore acts as a support element for the movable unit of 
the comparator. To the upper face of the plate 6 there are connected the 
lower faces of two flanges 20, 21 (FIG. 2) which form a single body 22 
with the movable unit. The connection is effected by means of two screws, 
one of which, 23, is visible in FIG. 1, passing through holes 24, 25 in 
the flanges 20, 21 and which are threaded into threaded holes 26, 27 of 
the plate 6. In the flange 5 there are provided seats 28, 29 for housing 
the flanges 20, 21 and the screws 23 (FIG. 4). 
The flanges 20, 21, shown partially in dashed line in FIG. 3, are opposite 
each other with respect to the vertical plane passing through the axis of 
the comparator and through the axes of the holes 11 and 12. 
The body 22, which is obtained by bending a plate of spring steel, 
furthermore comprises an upper base 30 of substantially rectangular 
perimeter and with a hole 31 for the passage of the electrical cord 4, two 
connecting parts 32, 33 between the flanges 20, 21 and the base 30, and 
two movable arms 34, 35. The arms 34, 35 are obtained by bending two 
flanges from the steel plate and are connected with the upper base 30 by 
two portions 36, 37 which are lightened by chamfers, as can be noted from 
FIG. 1, which form two fulcrums for the rotation of the arms 34, 35 around 
respective axes perpendicular to the arms. 
The structure of the arms 34, 35 is substantially similar and therefore 
only arm 34 will be described, with reference to FIGS. 1 and 2. The arm 34 
is of a C-shaped cross section so as to present high resistance to flexure 
and has a threaded hole 38 in which there is threaded a screw 39 (FIG. 1) 
which supports one end of a spring 40, the other end of which is duly 
fastened to the arm 35. The spring 40 works in compression and therefore 
tends to open arms 34, 35. The mutual position of the two arms 34, 35 is 
detected by means of a position transducer 41 with a differential 
transformer. 
The transducer 41 comprises a cylindrical housing 42 inserted into a hole 
43 of the arm 34 and fastened to arm 34. 
The housing 42 contains within it the primary and secondary windings of the 
differential transformer. Within the windings 44 there is movable, as a 
result of the mutual displacements of the arms 34, 35, a magnetic core 45 
supported by a shaft 46 integral with a cylinder 47 fastened to the arm 
35. The cord 4, which is connected to the windings 44, emerges laterally 
from the housing 42 and passes through an opening 48 in the arm 34 
communicating with the hole 43. An elastic bellows gasket 49 having its 
ends fastened to the cylinder 47 and to the housing 42 assures tightness 
within the transducer 41. 
The arms 34, 35, as already stated, are of a C-shaped cross section and 
therefore have bent flanges 50, 51. The arms 34, 35 house at their free 
ends, in suitable holes 52 passing through further bent flanges 53, 
cylinders 54, 55 which bear feelers 56, 57, respectively. The cylinders 
54, 55 are locked by screws 58 with hollow hexagonal heads which make it 
possible to adjust the radial position of the cylinders and therefore of 
the feelers 56, 57. 
The casing 13 has holes 59, 60 for the passage of the feelers 56, 57 and is 
closed on the bottom by a circular base or plate 61 perpendicular to the 
geometrical axis of the comparator. The plate 61, which is fastened to the 
casing 13 in a manner which will be described further below, has a central 
hole 62. 
The plate 6 (FIG. 3) is of substantially ring shape with a central hole 63 
and has two inwardly protruding parts 64, 65 having approximately the 
shape of triangular prisms with chamfered vertices 66, 67. The two parts 
64, 65, which are diametrically opposite each other in a direction 
perpendicular to that in which the two flanges 20, 21 are arranged, form 
stop surfaces adapted to limit the opening stroke of the arms 34, 35. As a 
matter of fact, as can be noted also from FIG. 1, the arms 34, 35 pass 
through the hole 63 since the base 30 and the free ends of the arms are on 
opposite sides of the plate 6. At the level of the plate 6, the central 
part of the C-shaped section of the arms 34, 35 is adjacent the vertices 
66, 67. 
Therefore, if the part 68 to be checked shown in FIG. 1 is removed, the 
opening movement determined by the spring 40 is limited due to the contact 
of the central part of the C-shaped section of the arms 34, 35 with the 
vertices 66, 67. 
On the other hand, when the casing 13 is introduced into a hole of a part 
68 to be checked, the feelers 56, 57, which in condition of rest protrude 
with respect to the surface of the casing 13 by a predetermined amount, 
cause the arms 34, 35 to close, bringing them into measurement position, 
and it is possible to read from an instrument of a supply and detection 
group 69 connected to a connector 70 fastened to the end of the electrical 
cord 4, the difference between the diameter of the hole and the value of 
the diameter of a sample part by means of which, as is customary, the zero 
setting of the comparator is effected. 
From the above description it can be noted that a single element, namely 
the plate 6, acts both as a central support to which there are connected 
the handle 1, the arms 34, 35 (via the intermediate support element 
comprising the flanges 20, 21, the base 30 and the lightened sections 36, 
37) and the casing 13 and as a limiter of the opening stroke of the arms 
34, 35. 
The limiting of the opening stroke is obtained, as described above, by two 
parts of the plate or ring 6 which protrude towards the inside and which, 
terminating with substantially sharp-edged surfaces, define stop surfaces 
of small size, so as to eliminate the influence of foreign elements which 
may tend to deposit on the arms 34, 35 or on the stop surfaces 66, 67 and 
to assure a limiting of the opening stroke to a constant value. The 
vertices which constitute the stop surfaces may be more or less beveled or 
sharp, depending on the shape of the arms 34, 35. 
The handle 1, above the upper base 2, has two tangs 71, 72 of substantially 
prismatic shape but with two faces having the shape parts of cylindrical 
surfaces of the same diameter as the base 2 of the handle 1. In the lower 
part of the tang 71 there are provided two opposite undercuts 73, 74 
(FIGS. 5, 6). Similar undercuts 75, 76 are provided in the tang 72 (FIGS. 
1 and 6). 
The comparator furthermore comprises a cord-holder device consisting of two 
elements 77, 78 which are clamped by screws 79, 80 against the tangs 71, 
72, as is visible in FIG. 1. The screw 79 passes through a hole 81 in the 
element 77 (FIGS. 8, 9) a hole 82 in the tang 71 and is screwed into a 
threaded hole of the element 78. In a similar manner, the screw 80 passes 
through a hole in the element 78 and a hole 83 in the tang 72 and is 
screwed into a threaded hole 84 of the element 77. 
The element 77, which is identical to the element 78, comprises a central 
body 85, a semitubular part 86 and two lateral arms 87, 88. The arms 87, 
88 in which holes 81, 84 are present have two teeth 89, 90 adapted to be 
inserted into the undercuts 73, 75 of the tangs 71, 72 which define stop 
surfaces cooperating with the teeth 89, 90, so as to prevent displacement 
of the element 77 in the axial direction of the comparator. Within the 
central body 85 there is provided a semicylindrically threaded seat 91 
coaxial with the semitubular part 86. 
When the cord-holder device is mounted, the threaded seat 91 and the 
corresponding seat of the element 78 grip onto the jacket of the cord 4, 
which is thus locked in a threaded hole. The crests of the threads 
penetrate into the jacket so as to prevent displacement of the cord 4 with 
respect to the cord-holder device in the handle 1. The lower part 92 of 
the body 85 and the corresponding part of the body of the element 78 
remain housed within the hole 3 of the handle while the lower faces of the 
lateral arms 87, 88 of the element 77 and the corresponding ones of the 
element 78 rest against the base 2 of the handle 1. 
The semitubular part 86 and the corresponding part of the element 78 are 
placed together so as to form an inner hole aligned with the threaded hole 
formed by the threaded seats of the elements 77, 78. This inner hole has a 
diameter such that its surface does not grip the jacket of the cord 4. The 
upper end of the semitubular part 86 terminates with an outer surface 93 
having the form of a semifrustoconical surface, with a lower base 
protruding with respect to the outer underlying surface of the semitubular 
part 86. Over the outer surface 93 defined by the semitubular part 86 and 
by the corresponding semitubular part of the element 78 there is placed a 
flexible rubber tube 94 which is locked by the frustoconical surface 
defined by the elements 77, 78. The tube 94 serves to rigidify the cord 4, 
avoiding possible breaks of its jacket in the immediate vicinity of the 
cord-holder device. 
The structure of the cord-holder device is such as to prevent damage to the 
wires of the cord 4 and to prevent mechanical stresses exerted on the cord 
from the outside of the comparator from being transmitted to the part of 
the cord contained within the handle 1 and from this part of the cord to 
the movable arm 34. 
As a matter of fact the two elements 77, 78 of the cord-holder device 
cannot move either axially, due to the teeth 89, 90, or transversely, due 
to being abutted against the tangs 71, 72. 
The element 77 (and thus also the element 78) has an outer surface 95 which 
constitutes part of a cylindrical surface of a diameter corresponding to 
that of the base 2 of the handle, so that the transverse size of the 
cord-holder device and of the tangs 71, 72 is equal to that of the base 2. 
In the element 77 there is provided a hole 96 of substantially elliptical 
section, which communicates with a hole 97 provided in the upper base 2 of 
the handle 1. Another hole 98 provided in th base 2, symmetrical with 
respect to the hole 97 and adjacent, like hole 97, to the central hole 3 
communicates with the hole provided in the element 78 corresponding to 
hole 96. 
The presence of the holes 62, 96, 97, 98 makes it possible to clean the 
inside of the comparator without it being necessary to remove any part. As 
a matter of fact, by means of a nozzle inserted through the hole 62 it is 
possible to introduce air or other suitable fluid under pressure within 
the protective and support housing formed by the casing 13 with the plate 
61, plate 6 and handle 1. The air flows within the plug substantially from 
the bottom to the top passing through the inside of the casing 13, the 
hole 63 and handle 1. The air flow removes any foreign matter and 
discharges towards the outside together with the foreign matter through 
the holes 96, 97, 98. The air discharges in part also through the free 
annular spaces defined by the cylinders 54, 55 and by the surfaces of the 
holes 59, 60. The simplicity of this cleaning operation is such that it 
can be repeated frequently in such a manner as to have assurance that the 
parts thereof critical for the operation, such as the cylinders 54, 55, 
the stop surfaces 66, 67, the arms 34, 35, the spring 40 and the portions 
36, 37, do not accumulate foreign matter in such an amount as to 
jeopardize the precision and the reproducibility of the measurements, or 
cause oxidation of the surfaces. The transducer 41, being tightly closed 
by the gasket 49, does not suffer from this cleaning operation with 
compressed air. 
The hole 3 in the handle 1 has a diameter such as to permit, after the 
disconnection of the connector 70 from the group 69, the dismounting of 
the cord-holder device and the unscrewing of the screws 7, 8, the 
withdrawal of the handle 1 by passing over the cord 4 and the connector 
70. In this way the assembly and the disassembly of the comparator are 
facilitated and it is possible to assemble the comparator by easily and 
rapidly connecting its various assembled components together, one of which 
is the handle and another of which is the movable unit provided with 
transducer, cord and connector. 
If mechanical stresses are applied to the handle 1 or to the part of the 
cord 4 which emerges from same through the cord-holder device, these 
stresses are not transmitted to the moving measurement unit since the 
stresses act only on the plate 6 and on the cord-holder device. This also 
is important from the standpoint of the reproducibility of the 
measurements effected by the comparator. 
Referring to FIGS. 1, 10 and 11, there will now be described in detail the 
structure of the plate 61 and its connection to the casing 13. 
The casing 13 is provided at its lower base with a frustoconical seat 99 
above which there is an annular stop surface 100. The plate 61 has a cut 
101 in diametral direction passing through the hole 62. The cut 101 
divides the plate 61 into two adjacent portions 102, 103, connected by a 
third portion 104 having a length which is sufficiently limited so that 
the third portion 104 is elastically yieldable to permit bringing the 
adjacent portions 102, 103 together, thereby reducing the width of the 
free zone produced by the cut 101. The side surface 105 of the plate is of 
a frustoconical shape with minimum and maximum diameters slightly greater 
(in condition of rest of the portion 104) than the corresponding ones of 
the frustoconical seat 99. With a fork-shaped device, not shown, having 
two tines which are inserted into two holes 106, 107 provided in the two 
portions 102, 103 in the vicinity of the free end of the cut 101, the two 
portions 102, 103 can be brought together so as to be able to insert the 
plate 61 within the seat 99. Thereupon the device is acted on in such a 
manner as to permit the two portions 102, 103 to move away from each 
other. The device then is removed from the holes 106, 107, thus leaving 
the plate 61 clamped in the seat 99. 
In the same manner it is possible to remove the plate 61 from the casing 
13. 
The structure of the plate 61 and the system for the fitted connection 
between the casing 13 and the plate 61 are therefore very simple and 
economical and make it possible to effect the assembly and disassembly 
operations with great speed, facilitating access to the inside of the 
casing 13 and in particular to the screws 58. The width of the cut 101 may 
be sufficiently small so that the presence of the cut 101 does not 
appreciably affect the cleaning operation described above.