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Two instruments get called an "indicator" and they are not interchangeable.
A dial indicator has a plunger that moves along its own axis. A dial test indicator, or DTI, has a pivoting lever arm with a stylus on the end. The lever geometry lets you reach into a bore, behind a shoulder, or against a face a plunger cannot approach. That reach is the whole reason the type exists, and it comes with a cost that most users never account for.
I work in quality control at a Japanese automotive manufacturer. This follows on from my earlier articles on micrometer model numbers and caliper accuracy.
The series numbers
- Series 513 — dial test indicators, lever type. Available in horizontal, vertical, universal and pocket configurations. Jewelled bearings, carbide or ruby contact points
- Series 2 — standard plunger-type dial indicators. The familiar round-face instrument on a magnetic base
- Series 543 — Digimatic indicators. Plunger type with a digital readout and data output
As with the rest of the Mitutoyo catalog, the first three digits are the series and the rest identifies the specific model. Configurations within Series 513 differ mainly in the orientation of the dial face relative to the stylus, which is a purely practical question of what you can see from where you stand.
Cosine error, with the actual numbers
This is the part worth reading twice.
An indicator only reads correctly when its direction of travel lines up with the direction you are trying to measure. When it does not, the reading is reduced by the cosine of the angle between them:
displayed = actual × cos θ
Which means the correction is to divide, not multiply. Here is what the angle costs you:
- 10° — you are reading about 1.5% low
- 20° — about 6% low
- 30° — about 13% low
Ten degrees is a small enough angle that it does not look wrong when you set the instrument up. On a 0.05mm runout measurement, 1.5% is nothing. On a 0.5mm reading it starts to matter, and at 30° you are throwing away an eighth of your measurement.
For a plunger indicator, Mitutoyo's own guidance is direct: mount it with the spindle in line with the intended measurement direction. There is no trick to it beyond taking the setup seriously.
For a test indicator, the angle that matters is between the stylus and the surface, not between the stylus and the body. This trips people up constantly. You can rotate the body to wherever you can read the dial from; what you must control is the stylus sitting close to parallel with the surface it is sweeping. Mitutoyo also offers a special involute-form contact point that compensates automatically, which exists precisely because this error is so common.
One important exception: if you are only looking for zero, cosine error does not matter. Tramming a mill head, dialling in a chuck, sweeping a bore for centre — you are chasing a null, and cos θ multiplied by zero is still zero. That is comparative work and the angle is irrelevant. It only bites when you read an actual magnitude off the dial.
The mistake that destroys test indicators
Test indicators come in two mechanically different kinds, and they look the same.
Some are auto-reversing, using a clutch that adapts to the measuring direction on contact. You can push the stylus either way and the instrument handles it.
Others read in one direction only and have a small lever on the side to switch which. If you force the stylus against a workpiece in the wrong direction on one of these, you damage the movement. It is not a robust mechanism being asked to do something awkward; it is simply the wrong direction for the gear train.
Before the instrument touches anything, find out which kind you have. If there is a switching lever on the body, it is the second kind and the lever position is not decorative.
Measuring force changes with orientation
A detail from Mitutoyo's own documentation that almost never appears in buying guides: measuring force is lower when the spindle is horizontal or the contact point is facing up than when the contact point is facing down. Gravity is acting on the plunger.
The practical consequence is that a setup you have verified in one orientation is not verified in another. If you mount an indicator sideways or inverted, check the repeatability in that orientation before you trust it. Move the plunger and let it return several times and see whether it comes back to the same number. It usually does. When it does not, you have found the problem before it found you.
Your stand is probably the error source
Here is where I would spend attention if I were setting up a measurement today.
An indicator is a comparison instrument. It reports how far its contact point moved relative to the thing it is mounted to. If the mounting deflects, the instrument faithfully reports the deflection and you interpret it as the part.
A magnetic base with a long articulated arm, extended most of the way out, with two or three joints between the base and the indicator, will deflect by more than the resolution you are reading. Every joint is a spring. Nobody notices, because the deflection is repeatable enough to look like real data.
Three things help more than buying a better indicator:
- Keep the arm short. Bring the base close to the work rather than reaching for it
- Tighten every joint before you zero. A joint that is finger-tight is a hinge
- Push gently on the indicator body and watch the dial. If the needle moves and does not return to where it was, your stand is not rigid enough for the measurement you are attempting
That last test takes five seconds. It has saved me from reporting nonsense more than once.
Which one to buy
Comparative work — tramming, centring, sweeping, alignment: a Series 513 test indicator. The reach is what you are paying for, and cosine error does not apply when you are hunting for zero.
Reading actual displacement over a longer range: a plunger dial indicator. The travel is greater and the geometry is honest.
Recording data: a Series 543 Digimatic indicator with output. The same argument applies as with calipers and micrometers — if a human is reading a dial and typing the number into a spreadsheet, you are manufacturing transcription errors you will never detect.
Whichever you buy, budget for the stand as part of the purchase rather than an afterthought. A modest indicator on a rigid stand beats an excellent one on a flexible arm, every time.
→ Look up Series 513 dial test indicators on Amazon
→ Look up dial indicators and magnetic stands on Amazon
Specifications and included accessories change between model revisions, so confirm the details on the current spec sheet before ordering.
