Dimensional Metrology
In general, there will be errors of size in any machined work piece. This
means that the actual dimension will be different from nominal dimension.
These errors should be within certain given limits by tolerances and determined
by the dimensional measurement to guarantee the product quality. The dimensional
measurement includes:
Post-process Measurement
Traditionally, measurements have been made after the part has been produced.
It is called the post-process measurement. The post-process measurement
can be used to high production run of smaller parts. The inspection process
can be made by traditional methods. If the dimensions are not within the
given tolerance zone, a correction can be made to the next part through
the machine tool.
Block Gauge
Gauge blocks are individual square, rectangular, or round metal blocks
of various sizes. Their surfaces are lapped and are flat and parallel within
a range of 1-5 micro inch. Gage blocks are available in sets of various
sizes. The blocks can be assembled in many combinations to obtain desired
lengths. The gage block assemblies are used as an accurate reference length
to measure the part's length.
Micrometer
The micrometer is commonly used for measuring the thickness and inside
or outside diameters of parts. Micrometers are also available for measuring
depths. Micrometers can be equipped with digital readout to reduce errors
in reading.
Profile Projector
The profile projector is used for measuring two-dimensional contours of
precision specimens and other work pieces produced. The part to be measured
is magnified by an optical system and projected on a screen. The reading
on the screen gives the dimension of the part. The following is the photo
of a profile projector.
Coordinate Measurement Machine (CMM)
A coordinate measurement machine (CMM) is an advanced, multi-purpose quality
control system used to help inspection keep pace with modern production
requirements. It replaces long, complex and inefficient conventional inspection
methods with simple procedures. A CMM provides instant measurement results
without complicated setup and operating procedures. It combines surface
plate, micrometer and vernier type inspection methods into one easy to
use machine. CMM can check the dimensional and geometric accuracy of everything
from small engine blocks, to sheet metal parts, to circuit boards.
A CMM consists essentially of a probe supported on three mutually perpendicular
(X, Y & Z) axes. Each axis has a built-in reference standard.
Procedure for simple measurements on a CMM includes:
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Calibration of the probe system.
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Define datum(s) on the work piece.
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Perform measurement(s).
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Compute the required dimensions from measurements made in Step 3.
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Assess conformance to specification.
On-process Dimensional Measurement
When the manufactured parts are big, with higher material cost and longer
cycle times, on-process measurement is required to improve the productivity
and reduce the cost. In the on-process measurement, parts are measured
while they are on the machine tool.
The existing on-process measurement methods can be divided into direct
and indirect methods according to the measurement principle.
Direct methods. In direct method, the dimension of the
work piece is directly measured using an adequate instrument, while the
work piece is located on the machine tool. Therefore, the effects of tool
wear distortions and machine errors can be taken into account.
Indirect methods. The work piece accuracy can also be indirectly
evaluated from radius measurements, by monitoring the motions of the carriage,
carrying the cutting tool or by noting the position of the tip of the cutting
tool.
The on-process measurement can be implemented by several methods. Here
are several on-process dimensional measurement methods:
Mechanical Methods
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Caliper Type
A typical caliper type contact gauge consists of a simple scissors caliper
with non rotating circular contact pads. The instrument can be set to measure
over a range of diameters. The contact pads or jaws are in continual rubbing
contact with the work piece. It is attached to the machine bed on its own
slide so that it can be rapidly withdrawn and returned to the measuring
position in a repeatable manner. The rear gap of the scissors is bridged
by sensing element, which can be a pneumatic or electrical transducer.
The caliper is set with respect to a circular setting master. it is possible
to derive an electrical signal with both types of transducer, which can
be used to control the machining process such as grinding and turning.
The measured work piece diameter range with this method reaches 5-190mm
and repeatably is 0.5 um.
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Friction Roller Type
This method measures the perimeter of the work piece by counting the
number of revolutions of the measuring roller for one or more complete
revolution of the work piece as illustrated in the following figure. The
application of this method is restricted to rigid work piece, due to the
high pressure applied by the roller. This technique has been used in turning
and grinding.
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Probe Type
A probe in mechanical contact with the work piece is used to determine
the actual size of work piece. For the gauging process, the probe is moved
towards the work piece and deflected by the contact. The coordinate value
of the point of the touch makes it possible to determine the work piece
radius provided the position of the axis of rotation is known.
Optical Methods
An optical method of on-process measurement is defined as one in which
the transmitter module produces and emits a light, which is collected and
photo electrically sensed through the object to be measured, by a receiver
module. This produces the signals which can be converted into a convenient
form and displayed as dimensional information. The principal advantages
of optical methods are
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Direct mechanical contact between the sensor and the object to be measured
is not required.
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The distance from the object to be measured to the sensor can be large.
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The response time is limited only to the electronics used in the sensor.
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The light variations can be directly converted into electrical signals.
The main optical on-process measurement methods include:
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Scanning Light Beam
This technique uses laser beams for the measurement process. It employs
transmitted module which emits a high speed scanning laser beam, generally
by means of a combination of a mirror and a synchronous motor. The object
to be measured interrupts this beam, and produces a time dependent shadow.
This shadow is electrically detected by a receiver, and converted into
dimensional readings by a control unit.
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Machine Vision
The method uses a light source and the image of the work piece can
be focused on the measuring grid on the face of a television tube or CCD
(Charge coupled device). Then the diameter of the work piece is computed
in terms of the image parameters, such as the image application factor,
focal distance and the image length on CDD.
Some other optical methods exist. For example,
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Light gauging
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Light focusing
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Light-spot detection
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Light sectioning
Pneumatic Methods
This method measures a pressure drop in the gap between the air gauge and
work piece, and converts it into an electrical signal. A schematic diagram
of pneumatic method is shown as the following.
Ultrasonic Methods
In this method, ultrasound travels to the work piece, then reflects back
to the transducer which also acts as a receiver. The transit time depends
on the variation from the specified distance between work surface and transducer.
By determining the transit time, the distance can be calculated.
