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Quick Guide to Precision Contracer (Contour Measuring Instruments)
Measuring Instruments
Traceable Angle Accuracy
Down slope As the detector units of the X and Z axes incorporate scales, the
magnification accuracy is displayed not as a percentage but as the
linear displacement accuracy for each axis.
Up slope 77 or less 87 or less Overload Safety Cutout
The maximum angle at which a stylus can trace upwards or downwards If an excessive force (overload) is exerted on the stylus tip due, perhaps,
along the contour of a workpiece, in the stylus travel direction, is to the tip encountering a too-steep slope on a workpiece feature, or
referred to as the traceable angle. A one-sided sharp stylus with a tip a burr, etc., a safety device automatically stops operation and sounds
angle of 12° (as in the above figure) can trace a maximum 77° of up an alarm buzzer. This type of instrument is commonly equipped with
slope and a maximum 87° of down slope. For a conical stylus (30° cone), separate safety devices for the tracing direction (X axis) load and vertical
the traceable angle is smaller. An up slope with an angle of 77° or less direction (Y axis) load.
overall may actually include an angle of more than 77° due to the effect For model CV-3200/4500, a safety device functions if the arm comes off
of surface roughness. Surface roughness also affects the measuring the detector mount.
force.
For model CV-3200/4500, the same type of stylus (SPH-71: one-sided Simple or Complex Arm Guidance
sharp stylus with a tip angle of 12°) can trace a maximum 77° of up
slope and a maximum 83° of down slope. In the case of a simple pivoted arm, the locus that the stylus tip traces
during vertical movement (Z direction) is a circular arc that results in an
Compensating for Stylus Tip Radius unwanted offset in X, for which compensation has to be made. The
larger the arc movement, the larger is the unwanted X displacement
A recorded profile represents the locus of the centre of the ball tip () that has to be compensated. (See figure, lower left.) The alternative
rolling on a workpiece surface. (A typical radius is 0.025 mm.) Obviously is to use a complex mechanical linkage arrangement to obtain a linear
this is not the same as the true surface profile so, in order to obtain an translation locus in Z, and therefore avoid the need to compensate in X.
accurate profile record, it is necessary to compensate for the effect of
the tip radius through data processing. Z axis Measurement Methods
Stylus Though the X axis measurement method commonly adopted is by
RxM means of a digital scale, the Z axis measurement divides into analog
methods (using a differential transformer, etc.) and digital scale
Recorded profile RxM methods.
RxM Analog methods vary in Z-axis resolution depending on the
Workpiece contour measurement magnification and measuring range. Digital scale
R: Stylus tip radius methods have fixed resolution.
M: Measurement magnification Generally, a digital scale method provides higher accuracy than an
analog method.
If a profile is read from the recorder through a template or scale, it is
necessary to compensate for the stylus tip radius beforehand according
to the applied measurement magnification.
L Compensating for Arm Rotation
The stylus is carried on a pivoted arm so it rotates as the surface is
traced and the contact tip does not track purely in the Z direction.
Therefore it is necessary to apply compensation in the X direction to
ensure accuracy. There are three methods of compensating for arm
rotation.
1: Mechanical compensation
2: Electrical compensation
3: Software processing. To measure a workpiece contour that involves a
large displacement in the vertical direction with high accuracy, one of
these compensation methods needs to be implemented.
L-43
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