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Chapter 10
D. Surface Plate Tools
and Techniques
1. SURFACE PLATE EQUIPMENT
Select and use height gages, V-blocks,
indicators, etc., to measure various types of
products. (Application)
Part II.D.1
Body of Knowledge II.D.1
Surface Plate
A surface plate provides a true reference plane from which measurement can be
made. A cast-iron surface plate is a heavy ribbed, box-like casting that stands on
three points (establishing a plane) and has a thick and well-supported flat top
plate. New plates generally have an average of 18 bearing spots on an area of 6.5
cm 2 ( 1 in. 2 ) that do not vary from a true plane by more than 0.005 mm (.0002
in.). The use of natural stones for surface plates is becoming increasingly popular
because of their hardness, resistance to corrosion, minimal response to temperature
change, and nonmagnetic qualities. Figure 10.1 shows a granite surface plate
used in inspection work.
Dial Indicators
Indicating gages and comparators magnify the amount a dimension deviates
above or below a standard to which the gage is set. Most indicate in terms of actual
units of measurement, but some show only whether a tolerance is within a given
range. The ability to measure to 25 nanometers (nm) (.00001 in.) depends on magnification,
resolution, accuracy of the setting gages, and staging of the workpiece
and instrument.
Dial indicators are simple mechanical indicating gages. An example of a dial
indicator is depicted in Figure 10.2. Movement of stem A is transmitted from the
rack to compound gear train B and C to pointer D, which moves around a dial face.
Springs exert a constant force on the mechanism and return the pointer to its original
position after the object being measured is removed.
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