Geometrical instruments – Gauge – Movable contact probe – per se
Reexamination Certificate
2000-06-16
2003-02-25
Gibson, Randy W. (Department: 2841)
Geometrical instruments
Gauge
Movable contact probe, per se
C033S561000
Reexamination Certificate
active
06523273
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reseat system as a component of a touch signal probe installed in a coordinates measuring machine. More specifically, it relates to an arrangement of a reseat system for improving reproducibility of the return position of the reseat system during the return movement after the escape movement of the reseat system.
2. Description of Related Art
In a coordinates measuring machine, a probe movable in three-dimensional directions touches a workpiece on a fixed table and coordinate values of the respective axes (respective axes in the three-dimensional directions) when the probe touches the workpiece are read so that the dimensions and configuration of the workpiece are measured based on the coordinate values. Accordingly, a touch signal probe capable of indicating contact between a probe and a workpiece as an electric touch signal is widely used.
FIG. 6
shows a conventional touch signal probe. In the figure a stylus
1
is fixed to a movable component
2
. A contact ball
4
is provided at a distal end of the stylus
1
. Three cylindrical bodies
3
radially project at 120 degree intervals around an axis of the stylus
1
from the periphery of the movable component
2
on a plane perpendicular to the axis of the stylus
1
. On the other hand, a fixed component
5
has three pairs of V-shaped holding portions
6
positioned corresponding to the cylindrical bodies
3
of the movable component
2
. The cylindrical body
3
and the V-shaped holding portions
6
constitute a reseat component for defining the relative position of the fixed component
5
and the movable component
2
at one place.
According to the above arrangement, the movable component
2
is pressed to the fixed component
5
by virtue of a biasing force F of a biasing component (not shown) and the movable component
2
is forcibly brought into contact with the fixed component
5
through the reseat component. When pressing force from the workpiece is not applied to the distal end of the stylus
1
, the movable component
2
rests on the fixed component at six contact points. In other words, respective cylindrical bodies
3
of the movable component
2
each rest on the respective cylindrical bodies
6
at two points for a total of six points. Accordingly, the reseat system is called as a six-point contact reseat system.
According to the six-point contact reseat system, the reseat position of the movable component after an escape movement can be located at only one place. In other words, assuming that the stylus
1
is displaced parallel to the axial direction at the rest position of the stylus
1
while maintaining contact between the reseat component on the movable component side and the reseat component on the fixed component side toward respective contact points, respective loci drawn by the tip of the stylus cross the axis of the stylus at the rest position. According to this arrangement, the stylus
1
returns to a unique rest position by restoring contact with the respective contact points by the biasing force F from a biasing component (not shown). When a pressing force from a workpiece is not applied to the distal end of the stylus
1
, the movable component
2
rests on the fixed component
5
at six contact points. In other words, respective cylindrical bodies
3
of the movable component
2
rest on the V-shaped holding portions
6
at two points for a total six points. Accordingly, such arrangement is called a “six-point contact reseat system ”. The six-point contact reseat system can use a pair of hard balls
7
as reseat component on the fixed component side (FIG.
7
). Alternatively, one hard ball
8
may be used as a reseat component on the movable component side with a V-shape groove
9
provided on a surface of the fixed component as the reseat component on the fixed component side (FIG.
8
).
According to the six-point contact reseat system, the return position after an escape movement of the fixed component can be defined at one place. Specifically, assuming that the stylus
1
displaces parallel to an axial direction of the stylus
1
at rest position toward respective contact points while keeping contact between the reseat component on the movable component side and the reseat component on the fixed component side, respective loci drawn by the tip of the stylus crosses the axis of the stylus at the rest position. According to the above arrangement, the stylus
1
returns to a uniquely defined rest position only by restoring contact between the respective contact points by the biasing force F during return movement of the movable component
2
after the escape movement by virtue of the pressing force from the workpiece, so that the rest position of the stylus
1
can be maintained at one place.
Since the position of the movable component relative to the fixed component can be set unique by the six-point contact reseat system., the six-point contact reseat system has high anti-vibration rigidity. Further, irrespective of the direction of the outside pressing force, the six-point contact reseat system has high reseat ability in a relatively rough unit of, for instance, 10 &mgr;m.
However, the above-described six-point contact reseat system causes an error (“reseat shift error”) in a further fine unit of, for instance, 1 &mgr;m observed in the return movement after contact, the error being caused because the movable component is pushed by the workpiece during the escape movement of the movable component to cause displacement relative to the fixed component.
Specifically, as shown in FIG.
9
(A), when the contact ball
4
of the conventional reseat system touches the workpiece W, the stylus
1
moves in left direction in the figure as shown in FIG.
9
(B). At this time, a small reaction force is caused between the movable component
2
and the fixed component
5
, so that the movable component
2
slightly slides in the left direction in the figure. When the workpiece W and the stylus
1
connected to the movable component
2
are no more in contact with each other as shown in FIG.
9
(C), the movable component
2
conducts the return movement by virtue of the biasing force F, where the axial position of the movable component
2
is shifted on account of the aforesaid slide movement. The shift directly affects on measurement accuracy of the probe.
The Applicants of the present invention have proposed a reseat system capable of correcting the reseat position shift after return movement (European Patent Publication No. 0764827 A2), whereby an electric voltage is applied to a piezoelectric element provided on the fixed component to administer directions of frictional force applied to a reseat system.
However, according to the above arrangement, the reproducibility of final return position is improved only by actuating the piezoelectric element after causing the reseat shift. Therefore, a measurement wait time is necessary before correction, thus hindering the efficiency of measurement work.
Further, since parts costs are increased for the piezoelectric element and an alignment process is necessary for attaching the piezoelectric element, the total production cost increases.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a reseat system capable of restraining reseat shift upon returning from an escape movement to obtain an accurate return to the reseat position without adding a separate mechanism.
A reseat system according to the present invention includes: a fixed component; a movable component having a stylus to be settled at a predetermined rest position; first reseat components provided at three places on the fixed component mutually spaced apart; and second reseat components provided on the movable component to be in contact with the first reseat components at predetermined contact points, where, assuming that a first reseat component and a second reseat component corresponding thereto keep contact with each other and the stylus moves while keeping an axis of the stylus substantially
Koga Satoshi
Nishioki Nobuhisa
Gibson Randy W.
Mitutoyo Corporation
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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