| The depth micrometer is an accurate and reliable tool to use
for depth measurement (Figure 1). The depths of holes, slots,
shoulders, and projections can be measured accurately to within
0.001 of an inch. |
Figure 1 Typical depth
micrometer
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| When using a depth micrometer, two points must be kept in
mind. Depth micrometers measure from a reference plane to a point.
The large base of the depth micrometer (Figure 2) makes up the
reference plane. The very small area of the measuring rod makes up
the point of contact. |
Figure 2 Measuring distance is
from the reference plane to the contact point.
|
|
Figure 3 Depth micrometers read from the right to the
left.
|
With a depth micrometer it is important that the area in which
the reference base of the depth micrometer makes contact with the
workpiece is clean and free of dirt or burrs.
The other aspect of the depth micrometer, which must always be
kept in consideration, is that it reads in reverse from other
micrometers (Figure 3).
 |
| Depth micrometers can be purchased with a number of different
length rods, allowing the measuring tool to be used over a broader
range of depths (Figure 4). |
Figure 4 Additional rods can be purchased in a variety of
lengths. |
| The head movement of the depth micrometer is one inch.
Whenever you use the micrometer, with the existing rod or to different length rods, check the accuracy of the depth
micrometer against a known standard (Figure 5). |
Figure 5 The depth micrometer is checked for accuracy
before every use. |
When using the depth micrometer, there is no need to move the
micrometer around to attain the proper feel. In fact, sliding the
micrometer should be avoided. The small rods will wear very quickly and
the accuracy of the micrometer may be lost. For greater accuracy, take
several readings at slightly different positions.
Measuring Drilled Holes
| When measuring the depth of a drilled hole, it is important to
measure at the outside wall of the hole to obtain the depth of the
full diameter portion of the hole (Figure 6).
Hold the measuring rod next to the wall of the drilled hole to
assure an accurate full diameter depth measurement. |
Figure 6 Drilled holes are
typically measured to full diameter depth
|
Types of Depth Micrometers
| Depth micrometers come in a variety of styles. Each style is
designed for a certain set of circumstances. The most common depth
micrometer has a 2-1/2 inch long base with a 1/8 inch or 5/32 inch
diameter rotating rod (shown in Figure 1). Some depth micrometers
come with non-rotating rods. The non-rotating rod is ideal for
measuring a narrow shoulder without the rod rolling off. Depth
micrometers can also be purchased with very small rods. Rods as
small as 0.045" inch in diameter are available for measuring very narrow
slots, recesses, or the depths of small holes. The half-base depth
micrometer (Figure 7) will allow you to measure depths of holes
and slots close to shoulders or between obstructions. |
Figure 7 Half
base depth micrometer |
|