Vernier Calipers

Part of Precision Measurement

Using and making vernier calipers to measure external dimensions, internal dimensions, and depths to 0.02 mm accuracy.

Why This Matters

The vernier caliper is the most versatile precision measuring instrument in the workshop. A single instrument measures outside dimensions, inside dimensions, depths, and step heights. Its range covers from 0 to 150 mm (or larger) without changing instruments. It reads to 0.02 mm (or 0.05 mm for simpler versions) — accuracy sufficient for the vast majority of fitting and machining work.

More than any other instrument, the vernier caliper is the one that every machinist keeps at hand throughout the day. It is the first thing picked up to check a dimension and the last thing put down. The ability to make one from scratch — and to read one accurately — is fundamental knowledge for any precision workshop.

The principle behind the vernier scale is beautiful in its simplicity: two scales that are nearly the same length, slightly offset, create a beat frequency that provides much finer resolution than either scale alone.

The Vernier Principle

A main scale has 1 mm divisions. A vernier scale is exactly 19 mm long but divided into 20 equal parts — so each vernier division is 19/20 = 0.95 mm.

The difference between one main scale division and one vernier division is: 1.00 - 0.95 = 0.05 mm.

When the caliper jaws are closed (zero), vernier division 0 aligns with main scale 0.

When the caliper is opened by 0.05 mm, vernier division 1 aligns with the next main scale mark. When opened by 0.10 mm, vernier division 2 aligns. … When opened by 1.00 mm, vernier division 20 (= division 0 of the next cycle) aligns.

Reading: Find which vernier division aligns exactly with a main scale line. The vernier reading × 0.05 mm is the fractional part.

For 0.02 mm resolution: Vernier has 50 divisions spanning 49 mm. Each division = 0.98 mm. Difference from 1 mm = 0.02 mm.

For 0.025” (imperial) resolution: Vernier has 25 divisions spanning 0.600”. Each division = 0.024”. Difference = 0.001”. Each vernier division = 0.001”.

Reading a Vernier Caliper

Step 1: Read the main scale — the last complete millimeter mark before the zero of the vernier scale. Example: 23 mm.

Step 2: Read the vernier — find the vernier division that most nearly coincides with any main scale line. Example: division 3 coincides.

Step 3: Multiply vernier reading by the resolution: 3 × 0.05 mm = 0.15 mm.

Step 4: Add: 23 + 0.15 = 23.15 mm

Parallax Error

Look straight across the scale, not at an angle. The lines on the main and vernier scales are not in the same plane — reading at an angle shifts the apparent alignment. Always position your eye directly above the scale and perpendicular to it.

Parts of a Vernier Caliper

Part	Function
Main scale (beam)	Fixed reference, 1 mm divisions
Sliding jaw	Moves to contact the workpiece
Vernier scale	Mounted on sliding jaw, subdivides main scale
Outside jaws	Measure external dimensions (placed outside the work)
Inside jaws	Measure internal dimensions (placed inside bores)
Depth rod	Extends from end of beam; measures depths and steps
Locking screw	Locks jaw in position for reading or transfer

Using Each Feature

Outside measurement:

1. Open jaws wider than the workpiece
2. Slide gently onto workpiece until both jaws contact
3. Do not tilt — the beam must be square to the measurement direction
4. Apply consistent, gentle pressure — do not force
5. Read and record

Inside measurement:

1. Close inside jaws fully
2. Insert into bore
3. Open until jaws contact the bore walls with slight pressure
4. Read — the reading is the bore diameter minus the jaw thickness (usually zero for well-made calipers, but check)

Depth measurement:

1. Rest the beam end flat on the reference surface
2. Slide the depth rod down until it contacts the bottom
3. Read the scale

Tilting Error

If the caliper is tilted when measuring a cylinder, the reading is artificially large (you are measuring a chord across the cylinder, not the diameter). Always keep the beam perpendicular to the cylinder axis.

Making a Vernier Caliper

A vernier caliper can be made in a workshop with:

• A flat beam of steel (20 mm wide, 3 mm thick, length as desired)
• A sliding carriage with a clamping screw
• Hardened steel jaw inserts

Critical dimensions:

The vernier scale must be exactly the right length. For a 0.05 mm resolution, 20 divisions must span exactly 19 mm. If the vernier is even 0.05 mm off, the resolution is wrong.

To lay out the vernier:

1. Scribe a line 19 mm long on the vernier carriage
2. Divide this line into 20 equal parts using the geometric bisection and trisection methods
3. Or use dividers: set to 0.95 mm (if you have a known 1 mm reference) and step off 20 times

Jaw making:

• Jaws must be the same thickness (or their difference added to the reading as a constant)
• Jaw faces must be flat and parallel to each other when fully closed
• The zero reading when closed must be exactly 0.00

Testing:

1. Close on a known gauge block and verify reading
2. Open and close several times; verify repeatability
3. Check zero when fully closed
4. Check inside jaws against an outside measurement of known size

Care and Maintenance

Daily:

• Wipe jaws clean before and after use
• Check that the locking screw releases smoothly
• Look for chips lodged between beam and carriage (cause false readings)

Periodic:

• Check jaw flatness: blue the outside jaws and check contact on a surface plate
• Verify zero setting
• Clean the sliding fit — grit causes wear and uneven sliding

Damage to avoid:

• Dropping onto a hard surface (bends or chips the jaws)
• Forcing open jaws by prying with a screwdriver
• Using as a clamp (springing the beam)

A good vernier caliper is accurate to 0.02 mm or better, covers a range of 150 mm or more, and measures in three modes. Few instruments offer more capability for their size. Making one is a statement that your workshop has reached a true precision capability.