Instrument Usage Rules

Without Measurement, There Is No Quality

"In this case, it should be okay" doesn't work in manufacturing. If the drawing says φ20 ±0.03, it must fall within 19.97 ~ 20.03. 0.01mm is 1/8 the thickness of a hair. You can never see it with your eyes.

Vernier Caliper — The Most Common Measuring Tool

It has a wide measuring range and can measure outer diameter, inner diameter, depth, and step difference in one go.

How to Read (Based on 0.05mm Scale)

1. Read the left scale mark of the main scale (fixed jaw) where the 0 of the sliding jaw is → whole mm
2. Find the mark on the sliding scale (moving jaw) that aligns straight with the main scale → decimal

The 0 of the sliding jaw is between 32 and 33 on the main scale → 32mm
The 7th mark on the sliding scale aligns perfectly → 0.35mm  (7 × 0.05)
                                   ───────
                          Measured value = 32.35mm

Precautions

• Keep the measuring force consistent. Too tight gives a smaller value, too loose gives a larger value
• Apply pressure to the root of the jaw for accuracy. Applying pressure to the end causes it to open
• Check the zero point before use — close the jaws and if it's not zero, adjust accordingly or request calibration

Micrometer — Precision of 0.01mm

It is 10 times more precise than a caliper. However, the measuring range is divided into 25mm units (0~25, 25~50...).

How to Read

Sleeve (fixed) scale: 7.5mm  (top scale 7 + bottom 0.5)
Thimble (rotating) scale: 0.28mm (28th mark)
                     ─────────
            Measured value = 7.78mm

Key Point — Always Use the Ratchet

If you manually tighten the thimble, the value varies by person. Turn the ratchet until it clicks 3 times to ensure consistent measuring force. Just following this rule significantly reduces measurement deviation.

Dial Indicator — Measures Differences

It doesn't measure absolute dimensions, but displacement relative to a reference surface.

• Roundness, wobble (runout), parallelism
• Eccentricity check of material clamped in a lathe chuck
• Axial clearance of spindle

The measuring tip (spindle) must be perpendicular to the surface being measured. If it's tilted, a cosine error occurs.

Height Gauge

It is used on a surface plate for height and scribing. Since the flatness of the surface plate is the standard, if the surface plate is damaged, all measurements become incorrect. Do not drop tools on the surface plate.

Measuring Instrument Management — Common Rules

• Check zero point before use
• Apply oil and store in case after use (to prevent rust)
• Report immediately if dropped — even if it looks fine, it may be damaged
• Check the validity period of the calibration sticker (usually 1 year)
• Wipe off cutting fluid and chips before measuring
• Measure after cooling hot parts immediately after machining (thermal expansion)

Which Measuring Instrument to Use — The 1/10 Rule

> The minimum scale of the measuring instrument must be less than 1/10 of the tolerance.

Tolerance ±0.03  →  Total tolerance 0.06
0.06 ÷ 10 = 0.006  →  0.01mm scale (micrometer) is barely sufficient
                      → 0.001mm scale measuring instrument is recommended

Tolerance ±0.5   →  Total 1.0 → 0.1 or less → Caliper (0.05) is sufficient

If you are measuring a tolerance of ±0.01 with a caliper, that data is not reliable.