How to check if an internal micrometers faces are parralell

To check if a micrometer is parallel—that is, whether the anvil and spindle faces are aligned properly and close evenly—you’re assessing both mechanical parallelism and flatness at the contact points. This is crucial because even small misalignments can introduce significant measurement errors. Here's a step-by-step guide, including both basic and more precise methods:

1. Clean the Measuring Faces

Assumption: Dust, oil, or grit can create false impressions of misalignment.

Use a lint-free cloth or lens paper to clean the spindle and anvil.

For stubborn dirt, use a bit of isopropyl alcohol.

Avoid touching faces with fingers after cleaning.

2. Visual Inspection

Goal: Look for gross misalignment or damage.

Close the spindle gently using the ratchet or friction thimble until it just touches the anvil.

Hold the micrometer against a light source.

Look from different angles—check for uneven light gaps between the faces.

If you see light on only one side, the faces may not be parallel.

3. Use Optical Flats and Monochromatic Light (High Precision Method)

Context: This method relies on interference fringes to detect deviations in flatness or parallelism.

Place an optical flat (a highly flat, transparent disc) across the micrometer faces.

Shine monochromatic light (commonly sodium light) through it.

Examine the interference pattern (fringes):

Straight, parallel, evenly spaced lines: surfaces are parallel.

Curved or uneven lines: indicates tilt or curvature.

Wedge-shaped fringes: faces are not parallel.

This method can detect angular errors down to a few microradians.

4. Check with Gage Blocks (Slip Gauges)

Alternative if optical flats are unavailable.

Place a calibrated gage block between the anvil and spindle.

Close the micrometer gently and take the reading.

Rotate the micrometer slightly (in-plane with the block) and check for variations in resistance or reading.

Significant changes indicate non-parallelism or worn spots on the faces.

5. Use a Micrometer Standard Rod

Assumption: You're verifying repeatability at known lengths.

Insert the calibrated standard (usually supplied with the micrometer).

Close gently and check the reading.

Rotate the standard or change its position (while still spanning the faces); any variation in reading suggests angular misalignment or non-flat faces.

6. Feel Test (Crude but Common)

Useful in field settings without tools.

Slowly close the spindle using the ratchet.

Feel for a consistent, even "click" or resistance as it closes.

If it contacts unevenly (e.g. one side touches before the other), it's a warning sign.

Notes:

Thermal expansion can affect results. Let the micrometer and tools acclimate to ambient temperature.

Face wear or burrs can cause apparent misalignment. These might need lapping or replacement.

Calibration labs use comparators or laser interferometry for tighter tolerances (<1 µm parallelism deviation).