Geometrical instruments – Gauge – With support for gauged article
Reexamination Certificate
2001-02-27
2003-04-01
Bennett, G. Bradley (Department: 2859)
Geometrical instruments
Gauge
With support for gauged article
C033S559000
Reexamination Certificate
active
06539642
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a probe type shape measuring sensor, and a NC processing equipment and a shape measuring method using the sensor.
2. Prior Art
When a workpiece must be manufactured precisely it is essential to have a technology for measuring the shape of the workpiece on the processing machine, i.e. so-called on-machine measurement technology. Such an on-machine measurement technology can improve the accuracy of the processing by eliminating the positioning errors which occur when the workpiece is removed and replaced, and at the same time the processing efficiency can be improved and the measurements can be automated and the manpower required for preparations can be saved.
Apparatus for measuring the shape of a workpiece, known in the prior art, include a probe type shape measuring sensor wherein the tip of a measurement probe touches the surface of the workpiece and measures the shape thereof. Such probe type shape measuring sensors are classified generally into analog and digital types depending on the means of detecting the position of the measurement probe.
In an analog-type shape measuring sensor, for example in an electric micrometer, displacements in the position of the probe are converted into analog electric values by detecting variations in the voltage of a differential transformer, electrostatic capacitance, resistance of a strain gauge, etc. However, because the system uses analog signals, large drifts in the output occur and the detected output is not very linear, so sub-micron accuracy such as about 0.1 &mgr;m cannot possibly be obtained if the overall distance moved by the sensor is about 100 &mgr;m.
Conversely, a digital-type shape measuring sensor, such as for example a digital micrometer, measures displacements in the position of the measurement probe, digitally using an optical scale, magnetic scale or a length measuring system using optical interferometer, therefore a maximum resolution of about 10 nm can be achieved.
However, even with the digital-type shape measuring sensor, the measurement probe must be supported by a linear ball bearing or an air slide, to enable it to move in the axial direction, and a spring or air pressure is used to press the probe onto the workpiece. Consequently, the measurement pressure fluctuates as the probe moves, and a large pressure is needed for the measurement and the pressure cannot be controlled freely, and this is a practical problem.
More explicitly, a spring applies a minimum pressure of about 10 grams which is too large to obtain a high accuracy, and because the spring force varies depending on the displacement of the probe resulting in variations in the pressure, there are large measurement errors, which is also another problem. When air pressure is applied, although the measurement pressure can be reduced by using a low air pressure, the minimum is still about 1 gram. There is another problem that if the air pressure is reduced, the stiffness of the air slide is also reduced, allowing the probe to tilt excessively, and the measurement errors are increased. Therefore, even with a digital system, a sub-micron accuracy of about 0.1 &mgr;m cannot be achieved.
To obtain a high accuracy of the sub-micron order, it is desirable that the measurement pressure should be as low as possible (preferably, about 500 milligrams or less). And to prevent a deterioration in the measurement accuracy caused by sideways displacements of the probe during a measurement, the measurement pressure should preferably be freely adjustable. These requirements have been shown by analysis.
To satisfy these requirements, a high-accuracy shape measuring device typically as shown in
FIG. 1
has been developed. This shape measuring device uses a minimum measuring force as small as about 50 milligrams, and measures the displacement of the measurement probe with a laser interferometer, thereby achieving a measurement accuracy of about 0.1 &mgr;m. However, with the device shown in
FIG. 1
, many optical elements such as moving mirrors and prisms are required, so the device itself becomes very large and delicate, therefore the device has the problem that it cannot be installed on a processing machine for making measurements on the machine.
When the aforementioned probe type shape measuring sensor is installed on a conventional NC processing device, a personal computer etc. is used to output a command to define the position of each point to be measured, to the NC control device. And the probe is stopped for a predetermined time at the defined point, and when the position of the probe is considered to have stabilized, the output from the shape measuring sensor is saved to determine the shape of a workpiece. However, according to this means, the times required to move the probe to the defined positions and the waiting times during which the probe is stopped accumulate, a long time is required. In addtion as intermediate points between points cannot be measured, a large number of defined points are required, so causing the problem that very long time is required to complete the measurements.
SUMMARY OF THE INVENTION
The present invention is aimed at solving the various problems mentioned above. That is, an object of the present invention is to provide a probe type shape measuring sensor with a small electric drift, excellent linearity of the output, small variations in measurement pressures during changes in the position of the probe, without decreasing the stiffness of the probe bearing, measurement pressures that can be adjusted to a constant very small load and changed freely, thus a sub-micron accuracy of about 0.1 &mgr;m can be obtained, and also capable of being made compact, and easily applied to on-machine measurements, and an NC processing apparatus and a shape measuring method using the sensor.
Another object of the present invention is to offer an NC processing apparatus and a shape measuring method using the aforementioned probe type shape measuring sensor, in which the waiting time is reduced, and the shape between defined points can be measured, thereby enabling the number of necessary defined points to be reduced and the measuring time shortened.
According to the present invention, a probe type shape measuring sensor is provided and characterized to be composed of a probe head (
10
) that supports a probe (
2
) that contacts a workpiece (
1
) in such a way that the probe can move towards the workpiece with an extremely low resistance to sliding and drives the probe towards the workpiece with a very low force, and a displacement measuring device (
20
) that measures the displacement of the probe, very accuracy and without contact.
Because the probe (
2
) is supported by the probe head (
10
) so that it can move with an extremely low resistance to sliding and is driven towards the workpiece, the probe can trace the surface of the workpiece precisely while contacting the surface of the workpiece with a very low load (about 500 mgf or less). Furthermore, by measuring the displacement of the probe with the displacement measuring device (
20
) which is very accurate and requires no contact, a sub-micron accuracy of about 0.1 &mgr;m can be achieved.
According to a preferred embodiment of the present invention, the aforementioned probe head (
10
) is provided with a long thin probe shaft (
12
) with the probe installed at one end (
12
a
) thereof and a step in cross section (
11
a,
11
b
) at an intermediate portion thereof, air bearing (
14
a,
14
b
) that are disposed at each side of the above-mentioned step and support the probe shaft, and a means (
16
) of feeding air that supplies gas at the first pressure to the location of the aforementioned step; the above-mentioned air bearings have a high stiffness in the radial direction and are disposed in such a way that the gas at the first pressure causes the probe shaft to float to reduce its resistance to sliding; the aforementioned gas feeding means keeps the pressure or pressures of gas or gasses at second and/or thir
Morita Shinya
Moriyasu Sei
Ohmori Hitoshi
Yamagata Yutaka
Bennett G. Bradley
Griffin & Szipl, P.C.
Riken
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