Comparative measurement with a dial indicator

The idea in one pass

A dial indicator does not read a size; it reads how far its plunger moved (GB/T 1219-2008). Used alone, that movement is only a difference. Comparative measurement gives the difference a meaning: press the plunger onto a reference of known size, set the dial to zero, and the same plunger on a part shows how the part deviates from the reference. Add the deviation to the reference and the size is there.

A gauge-block stack is the usual reference: blocks wrung to the nominal size, counted at their nominal length when used by grade (GB/T 6093-2001, and the gauge blocks page). The stack carries the accuracy of the calibration laboratory into the workshop, and the indicator only has to resolve the small difference around it.

What zeroing removes — and what stays

Zeroing cancels the indicator’s own zero error: whatever the dial reads at the reference is defined as zero, so a plunger that starts 0.01 mm off no longer shifts the result. The same cancellation works for any error that is identical at the reference and at the part, such as a small stand deflection that repeats in both positions.

What zeroing cannot remove stays in the result: the uncertainty of the block stack itself, the temperature difference from 20 °C, a tilted lever-stylus (cosine error), and positioning errors — plunger not square to the measured direction, contact not on the measured point. Comparative measurement is accurate exactly to the extent that these are controlled (see measurement conditions).

Doing it

1. Build the reference stack at the nominal size, with as few blocks as possible (see gauge blocks).
2. Mount the indicator on a stand with the plunger along the measured direction, and press in a preload past first contact, far enough that the pointer can move either way.
3. Set the dial to zero on the stack, turning the bezel until the pointer meets the zero line.
4. Move the part under the plunger, in the same direction, with the same contact.
5. Read the difference with its sign: pointer right of zero, the part is larger.
6. Part size = stack size + reading.

Keep the setup unchanged between reference and part. Every change — stand moved, plunger re-angled, part lifted with the hand still on it — writes a fresh error into the difference that zeroing cannot know about.

A worked example

Reference stack 32.45 mm (1.45 + 1 + 30). Zeroed on the stack, the indicator on the part reads +0.03 mm: part size = 32.45 + 0.03 = 32.48 mm. A reading of −0.02 mm would give 32.43 mm — the sign decides which side of the tolerance the part lands on, so record it every time.

Standards

Dial indicators are covered by GB/T 1219-2008 (current); gauge blocks by GB/T 6093-2001 (current). The same zero-and-read-difference workflow underlies the bore gauge and the runout checks in this course.

Planned for the lab

Dial indicators and gauge blocks are planned for KELIBRON (intake group G2). Until then, practise the shared habits on the caliper scenes below: check the zero first, keep the gauge square to the measured direction, and read to the resolution of the instrument.