Dial indicators
A dial indicator does not tell you how big a part is. It tells you how far it deviates from a reference. It is the instrument that centres a part in the chuck, checks whether a milling table is parallel, and finds the wear on a shaft. The choice comes down to a few things: the type, the readout, the resolution, and the base that will hold it still.
The plunger travels in a straight line inside the body and the movement is read off the dial. Long travel (typically 0-10 mm, up to 0-100 mm) for flatness, parallelism and wear checks. The default choice for general use.
Instead of a plunger they use a pivoting lever stylus. Short travel (±0.1 to ±0.8 mm), but they reach where nothing else fits: inside bores, under overhangs, for centring in the chuck and measuring run-out.
A dial and a needle, no battery. For centring they win outright: your eye picks up instantly which way the needle is going and how fast, which is something a flickering number does not show.
A display with no reading mistakes, zeroing at any point, mm/inch switching and resolution down to 0.0005 mm. Choose digital when you need data output (Bluetooth, USB, Digimatic) or MAX/MIN functions for logging.
Resolution is not the instrument’s accuracy, it is the smallest difference you can read. And the finer the reading, the shorter the travel: 0.01 mm reaches up to 100 mm, 0.001 mm typically stops at 5 mm, and 0.0005 mm leaves you about 0.05 mm. Pick the travel you need to cover first.
The machine shop default. It covers the longest travel, up to 100 mm, and is all you need for flatness, parallelism, wear checks and centring on the lathe.
Ten times finer, at the cost of travel: typically up to 5 mm. It makes sense on bearings, precision shafts and QC work, provided your setup and temperature can support it.
High-resolution digital, down to 0.0001 mm. Laboratory work: at this scale the instrument picks up everything, including the irregularities of the very surface it sits on.
The contact point is a consumable and screws onto a standard M2.5 thread. Spherical for general use, flat for soft materials, disc for grooves. In carbide or ruby when wear matters. On dial test indicators, never fit a stylus longer than the standard one: it changes the lever ratio and the indicator starts telling lies.
The indicator's back plate is interchangeable and gives you another way to mount it: flat, lug or magnetic, so the indicator sits where you want it without a stand.
They screw in between the plunger and the contact point and reach up to 100 mm deeper. Useful in deep bores, at the cost of added flex with every extra length.
Ø8 mm stem
Dial indicators mount by a Ø8 mm stem, the market standard, and almost every magnetic base accepts it. Dial test indicators usually mount by a dovetail instead.
An indicator needs a base
An indicator without rigid support measures nothing: every movement of the base passes straight into the reading. See the magnetic bases buying guide.
Accuracy is about ±1 graduation
The practical rule: an indicator is accurate to within roughly one graduation of its own dial. On a 0.01 mm indicator that means about ±0.01 mm. The decimals on a display are not a promise of accuracy.
Always read in one direction
Analogue indicators run on gears, and a gear train hesitates when it reverses (backlash). Let the needle move continuously in one direction and the error disappears. Digital indicators with a glass scale have no gears at all, so no backlash.
Never upside-down
An indicator works upright or horizontally, but never upside-down: the spring is not strong enough to lift the weight of the spindle itself.
It measures difference, not size
An indicator is a comparative instrument: it shows deviation from wherever you zeroed it. For an absolute dimension you need a caliper or a micrometer.
Should I just buy the most "accurate" indicator to be safe?+
Not necessarily. Mitutoyo puts it the other way round: why throw away 97% of the dial face? A 70 µm (.003 in) deviation on an indicator with 25 mm (1 in) of travel uses just 3% of the dial and 0.3% of the range. The indicator reads it, but the needle barely moves. The check is done by eye, and the eye reads the angle of the needle, not the number. Choose the travel so that the deviation you are checking sweeps a good arc of the dial.
Dial indicator or dial test indicator?+
It comes down to access and travel. If the measuring point is open and the deviation could be millimetres, take a dial indicator. If you are centring in the chuck, checking run-out or measuring inside a bore, a dial test indicator is the only option, but its travel runs out in less than a millimetre. Most shops keep both.
Why does my dial test indicator read less than expected?+
Because the stylus is not parallel to the surface. The larger the angle, the smaller the reading you get from the real movement. This is cosine error. Keep the stylus as parallel to the surface as you can: at 5° you read 99.6% of the real movement, at 10% you read 98.5%, and at 15° you read 96.6%. What counts is the angle between the stylus and the surface, not how steeply the body of the instrument is tilted.
Do I definitely need a magnetic base?+
You need some rigid support. A magnetic base is the most flexible on a lathe or mill, but it only grips steel and cast iron. The alternatives are a comparator stand with a column for the bench, or mounting through a back plate.
Digital or analogue for centring?+
For centring, most people prefer analogue: you do not care about the number, you care how much the needle swings per revolution, and you see that at a glance. Digital wins when you are logging values, need MAX/MIN, or send readings to a computer.
Will it survive coolant and swarf?+
Only if the indicator is sealed. On the machine, pick a digital one rated IP65 or IP67. An unsealed indicator needs a clean, dry plunger, and never oil the plunger, it collects dust and makes it stick.
How do I know my indicator is telling the truth?+
Through calibration at regular intervals. A quick bench check: press and release the plunger ten times and see whether the needle always returns to the same zero. That is repeatability. If it does not, the indicator needs a service.
Want the technical background (parts, types, errors and maintenance)? See the dial indicators wiki.