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How to Use a Tube Micrometer for Wall Thickness
Guide · Updated October 28, 2026
A tube micrometer has a round or cylindrical anvil that fits inside the bore of a tube so the curved wall touches at one point. A flat-anvil micrometer would bridge the curve of the inside wall and read high.
Check the mic at size
Check the mic against a gage block stack near the wall size. The free Gage Block Calculator builds the stack.
Step by step
- Pick the anvil: Spherical (ball) anvils suit small bores and curved walls; cylindrical anvils suit larger tubes and are less sensitive to tilt along the tube axis.
- Check the bore fits: The anvil and its frame must reach into the tube far enough to measure where you need.
- Zero the mic: Close the spindle on the anvil with the ratchet and check zero.
- Check at size: Measure a gage block near the expected wall.
- Measure: Slide the anvil inside, close the spindle on the outside wall with the ratchet.
- Rock to find the minimum: Rock the mic slightly around the tube; the true wall is the smallest steady reading.
- Measure around: Take readings at 4 or more positions around the tube; walls are seldom uniform.
Why a flat anvil reads high
The inside wall of a tube is concave. A flat anvil wider than a point touches at its two edges and leaves a gap at the center, so it reads the wall plus that gap. A ball or cylinder touches at one point or line and reads the true wall.
Add-on ball attachments
A clip-on ball attachment on a standard mic adds its diameter to the reading. With a 0.2000 in ball, a reading of 0.3650 in means a wall of 0.3650 − 0.2000 = 0.1650 in. Measure the ball itself first and use its actual size, not the nominal.
Worked example: wall variation (eccentricity)
A 1.500 in OD tube with nominal 0.120 in wall reads 0.1185, 0.1210, 0.1225 and 0.1200 in at 0°, 90°, 180° and 270°. Thinnest to thickest is 0.1225 − 0.1185 = 0.0040 in. Opposite readings 0.1185 and 0.1225 average 0.1205, so the bore is offset about (0.1225 − 0.1185) ÷ 2 = 0.0020 in from the OD. Compare the minimum, 0.1185 in, with the drawing's minimum wall.
Checking with gage blocks
For a ball-anvil mic, close on a single gage block laid flat on the ball: the reading should equal the block size. For an add-on ball, the reading should equal block + ball.
Common problems
Reading high: mic tilted so the ball contacts off the high point; rock to the minimum. Reading low: burr at a cut tube end; measure away from the end. Inconsistent: dirt inside the tube.
Tips
- Mic basics: reading an inch micrometer.
- Check: check with gage blocks.
- Zero: zero a digital mic.
FAQ
Why not a normal mic?
A flat anvil bridges the curved inside wall and reads high.
Ball or cylindrical anvil?
Ball for small bores; cylindrical for larger tubes.
What about clip-on balls?
Subtract the measured ball diameter from the reading.
How many readings?
At least four around the tube; report the minimum.
How do I check it?
Measure a gage block laid on the ball.
Related guides and calculators
- How to Read an Inch Micrometer
- How to Check a Micrometer with Gage Blocks
- How to Zero a Digital Caliper and Micrometer
- How to Use a Blade Micrometer
- How to Use a Depth Micrometer
- How to Use a V-Anvil Micrometer on 3- and 5-Flute Tools
- How to Use 1-2-3 Blocks for Setups
- How to Use an Ultrasonic Thickness Gage
- All free machinist calculators
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