Gauge Block Guide: Wringing, Error & Uncertainty

A gauge block is the most fundamental standard in precision measurement, used to calibrate instruments and establish a length reference; its accuracy depends on wringing quality, surface cleanliness and temperature stability. This guide covers wringing, burr handling, interference-fringe reading, and the size-error and uncertainty concepts on a calibration certificate.

1. What a Gauge Block Is and Selection

A gauge block establishes standard length, used to calibrate calipers, micrometers and equipment, for precise size comparison and length reference. When combining, minimise the number of blocks, prefer thicker blocks and start from the smallest unit — to reduce accumulated error and wringing risk.

2. Wringing Principle and Steps

Wringing joins two blocks by molecular adhesion: surfaces must be extremely flat and clean, with a very thin oil film, and adhesion forms by sliding. Standard flow:

3. Three Wringing Methods

4. Interference-Fringe Reading (Key)

Observe with an optical flat: no fringes means clean and burr-free; fringes disappearing means good contact; local fringes mean a burr or contamination. This is a key way to judge block quality. Burr handling: hone with a fine-grit stone, rub back and forth lightly, then re-check with the flat; if the burr is large, replace the block.

5. Temperature Effect

Block size is affected by temperature: handling causes expansion, so wait for it to stabilise. Use in a temperature-controlled room, avoid long hand-holding, and let the block reach thermal equilibrium with the environment before measuring.

6. Calibration Certificate Parameters

Calibration includes a five-point measurement (centre and four corners):

7. Uncertainty

A measurement result is not an absolute value but an interval: standard uncertainty is 1σ, expanded uncertainty is 2σ (about 95% confidence). For example "value ±0.06 μm" means a 95% chance of falling in that range. For overall system uncertainty see the CMM Accuracy & Uncertainty Guide.

8. Choosing a Grade: Use Chart

The sections above cover how to use gauge blocks. This one covers which grade to buy. Blocks come in grades K, 0, 1 and 2, and the wrong choice is not only wasted money — using an under-specified block to calibrate an instrument breaks the traceability chain.

Start from the use. The principle is one sentence: the further upstream the block sits (the closer it is to calibrating other blocks or instruments), the higher the grade it needs; the further downstream, on the shop floor, the looser the grade can be.

9. The JIS B 7506 Tolerance Tables: Grades K/0 and 1/2

Tolerance widens in the order K, 0, 1, 2. The two tables below follow JIS B 7506, the Japanese Industrial Standard for gauge block accuracy, in micrometres. "Deviation" is the permitted departure of the length from nominal, and "variation" is the spread within one block.

Table 1: grades K and 0 (standard and reference use)

Table 2: grades 1 and 2 (inspection and working use)

The lower bound on the first row is printed as in the JIS table. These values are taken from JIS B 7506 and are not measured by this site; for a purchase, go by the inspection certificate the supplier provides. A grade states how far upstream a block can work, not how good it is: grade 2 is entirely correct for setting cutting tools, and using grade K for the same job only pays for accuracy nobody will use.

This article is part of Precision Measurement Complete Guide: Ask What You Are Measuring First, Then Pick the Instrument; that guide shows how the whole topic fits together.

Published: 2026-03-23 | Last updated: 2026-07-25