Mastering Micrometer Measurements: Metric and Imperial

Constant Force Micrometer Mechanisms

Figure 3.34 illustrates a constant force mechanism designed to ensure measurements remain unaffected by variations in pressure. This system utilizes two constant force components to maintain measurement consistency.

Metric Micrometer Measurements

The thread pitch is 0.5 mm, meaning a full turn of the screw increases or decreases the distance between the measuring stops by 0.5 mm. The graduations on the sleeve above the reference line represent millimeters, while those below represent half-millimeters, corresponding to the spindle's movement. To achieve an accuracy of 0.01 mm, the drum scale must contain 50 divisions.

Reading a Metric Micrometer

To obtain a reading, follow these steps:

• Note the last number visible on the sleeve and multiply by 1 mm.
• Count the number of lines (above and below the baseline) visible beyond the last number and multiply by 0.5 mm.
• Add the drum line number that aligns with the reference line (each division equals 0.01 mm).

Example Calculation (Figure 3.36)

Based on Figure 3.36:

• Last number on the sleeve is 10: 10 × 1 mm = 10 mm.
• Two divisions beyond the number line: 2 × 0.5 mm = 1 mm.
• Line 36 on the drum aligns with the reference: 36 × 0.01 mm = 0.36 mm.
• Final reading: 11.36 mm.

Micrometer in Inches

The threaded end of an inch-based micrometer has 40 threads per inch, resulting in a pitch of 0.025". The distance of one inch on the sleeve is divided into 40 equal parts, each equal to 0.025".

• When closed, the drum zero line must align with the reference line.
• Each line on the sleeve equals 0.025".
• Every fourth line is longer and numbered, representing 0.100".
• The drum has 25 divisions, providing an accuracy of 0.001".

Reading an Inch Micrometer

1. Note the last number on the sleeve and multiply by 0.100".
2. Count the visible lines beyond the numbers and multiply by 0.025".
3. Add the drum division that coincides with the reference line (each division equals 0.001").

Example Calculation

For a reading where the sleeve shows 2, one extra line, and drum line 18:

• Sleeve number: 2 × 0.100" = 0.200"
• Visible lines: 1 × 0.025" = 0.025"
• Drum line: 18 × 0.001" = 0.018"
• Total reading: 0.243"