A vernier caliper reads to 0.02 mm because its sliding scale has 50 divisions spanning 49 mm, so each vernier division is 0.98 mm and the line that aligns with a main-scale line gives hundredths directly. That 0.02 mm is the smallest step it shows, not a guarantee of accuracy; to know its accuracy, close the jaws to check zero and compare it with gauge blocks.
Part of Precision Tool Making
Building and reading a caliper whose sliding vernier scale interpolates between the lines of a millimetre scale.
The Vernier Turns A Coarse Scale Into A Fine One By Mismatching The Spacing
Two scales with slightly different spacing drift in and out of alignment as one slides along the other. On the carrier’s design the main scale has 1 mm lines and the vernier has 50 divisions in 49 mm, so each vernier division is 49 / 50 = 0.98 mm, 0.02 mm shorter than a main division. When the jaw has opened by 0.02 mm, vernier line 1 lines up with a main line; at 0.04 mm, line 2; and so on. The number of the aligned line, multiplied by 0.02, is the fraction of a millimetre.
Vernier Calipers builds the same principle with a 0.05 mm step (20 divisions in 19 mm), and Precision Measurement with a 0.1 mm step (10 divisions in 9 mm). The ladder is 1 mm divided by the number of vernier divisions: 1/10 = 0.1, 1/20 = 0.05, 1/50 = 0.02. The subtitle’s 0.02-0.05 mm span is those two finer steps.
What You Need
- A main beam of hardened steel bar, 200-300 mm long, with millimetre lines engraved on one edge (the carrier’s specification)
- A sliding jaw riding on the beam with light friction, and a locking screw
- A scribing point and a way of spacing the lines accurately (Dividing and Indexing)
- A magnifier to read the lines (Magnifying Glass)
- Gauge blocks or a gauge block stack to check against, once you have one
Method: Build, Read And Check
Step 1: Make the beam and jaw with a flat, straight bearing edge (Straight Edges and Surface Plates).
Step 2: Engrave the main scale with a sharp scribing point on the lathe or dividing head.
Step 3: Engrave the vernier with 50 divisions over 49 mm. The spacing must be engraved more accurately than the 0.02 mm it reads.
Step 4: Read. Read whole millimetres at the vernier zero, then find the vernier line that aligns exactly with a main line; that line’s number times 0.02 mm is the fraction.
Step 5: Check zero. With the jaws closed, the reading must be exactly 0.00.
Step 6: Check against a reference. Measure gauge blocks, or a bar of known length, at several sizes. The wiki gives no accuracy for a home-built vernier, so the check, not a number from this article, tells you what yours can do.
Worked Example
Main scale shows 23 mm at the vernier zero, and vernier line 14 is the one that aligns. Reading = 23 + 14 x 0.02 = 23.28 mm.
Proof of the reading: the vernier zero sits at 23.28 mm, and vernier line 14 is 14 x 0.98 = 13.72 mm further along, at 23.28 + 13.72 = 37.00 mm, which is exactly a main-scale line. That alignment is what you saw, so the reading is consistent.
If the engraved vernier were out by 0.01 mm per division, the aligned line would shift by about one division after 2 divisions, and readings in the middle of the scale would be wrong by a visible amount. That is the practical limit: the 0.02 mm step is only trustworthy if the engraving was done to a finer standard than the step.
Common Mistakes
- Reading the vernier on a skewed jaw. Jaws must close square and parallel; hold up to the light.
- Skipping the zero check. An offset affects every reading.
- Quoting the resolution as the accuracy.
- Measuring warm work. A hand-warm part reads large.
Safety
Sharp scribing and hardened edges
Engraving uses a sharp point and often a lathe: clamp work, keep sleeves clear of the chuck, and know where the stop is. Hardened jaws chip if dropped and sharp edges cut; deburr them.
What’s Next
- Learn next: Micrometer Construction - the next finer instrument
- Combine with: Gauge Blocks - the reference that checks it