Dial indicator

How it works

The contact point is carried on a spindle or, in the lever type, on a pivoting arm. Starrett’s account of the drop type is that the movement of the spindle is multiplied, through a pinion and a train of gears, to a hand that sweeps the dial, so that a small movement of the contact is shown large and can be read to a thousandth of an inch or finer.

An indicator does not measure a size by itself. It is usually held by the stem in a fixture or on a stand, set to zero on a known surface or master, and then shows the deviation of the work from that zero. Starrett lists its uses as determining runout, height, flatness and distance.

Anatomy

Contact point: Replaceable; Starrett lists points of extra length, special form, tapered and roller types.
Spindle (plunger) or lever arm: The moving member. A plunger moves in a straight line; the finger of a test indicator moves in an arc.
Gear train: The pinion and gears that multiply the movement. Starrett describes a hardened stainless steel train with jeweled bearings on its watchmaker-style indicators.
Dial and bezel: The graduated face. Starrett color-codes dials, white for inch and yellow for metric.
Pointer (hand): Sweeps the dial; long-range indicators add a count hand and a double dial.
Stem or lug: The mount by which the indicator is clamped; the AGD size classes standardize case diameters.

Variants

Drop (plunger) indicator: The first type introduced, according to Starrett; the spindle moves along its own axis.
Back plunger indicator: The plunger is centered at the back of the case, for use in small spaces; Starrett says it is easier to read there than a drop type.
Test (lever) indicator: A finger or lever contact that moves in an arc. Its range is smaller and Starrett says the user must watch for cosine error.
Dial test indicator on a stand: Mounted at the end of a horizontal arm on an upright post; a universal version with attachments checks lathe spindle runout.
Dovetail mount indicator: A test indicator whose angled head carries three dovetail mounts.
Digital and wireless indicator: Uses an LED, a glass scale and a reticle, or similar encoders, in place of gears; Starrett says they hold their accuracy over longer ranges.
Non-shock indicator: A mechanism that isolates the gear train from blows, or a shock-absorbing anvil in place of the regular contact.

Specifications

Graduations, inch · in: 0.001Starrett lists graduations of 0.001, 0.0005, 0.00025, 0.0001 and 0.00005 inch.
Graduations, metric · mm: Starrett lists 0.01, 0.002 and 0.001 mm, with ranges up to 125 mm.
Range · in: Starrett lists ranges from 12 inches down to 0.006 inch.
Case size classes · AGD size 0–4: Sizes 1 to 4 follow the American Gage Design dimensions, essentially the same for all manufacturers; size 0 is smaller and has its own dimensions.
Stated accuracy · graduations: ±1Starrett says most accuracies are specified as plus or minus one graduation over the full 2½-turn range.

Made to

ASME B89.1.10m
ISO 463
DIN 878

Tips

Preload the indicator before zeroing
Starrett says an indicator should be “loaded” by some part of a turn before testing or measuring. For an AGD indicator with a range of 2⅓ turns, it advises loading to about 1⅓ turns and setting zero on the master, so that the indicator then shows the deviation for a full revolution plus or minus.1
Turn the bezel to put zero where the hand is
Starrett’s Catalogue No. 17 describes an adjustable dial: by turning the knurled rim the dial can be moved so the zero mark sits wherever desired in relation to the hand. The catalog says the contact points can be changed for different work and that the indicator can be set in any position on a friction joint.1
Watch cosine error on a lever indicator
A test indicator’s finger moves in an arc, not a straight line, so the reading depends on the angle the lever makes with the surface. Starrett says that the user must be aware of cosine error when setting it up because everything depends on the position of the lever.1 2
Choose the resolution the job needs
Starrett’s selection advice is not to pick a 0.0001-inch readout if 0.001 inch will do. It adds that electronic or digital indicators are the better choice when the measurement data must be collected, printed or stored.1
Read the dial color
Starrett color-codes the faces to avoid errors: a white dial for inch graduations and a yellow face for metric. Indicators may have balanced graduations (zero in the middle) or continuous graduations.1
Fit a shock-absorbing anvil where blows are likely
Starrett says a regular indicator subject to sudden blows can be converted to a non-shock type by replacing the contact with a shock-absorbing anvil unit, or a non-shock mechanism can be substituted so that impact is isolated from the gear movement.1

History

Earliest documented

15 May 1883 (application); 21 August 1883 (issue)

Waltham, Massachusetts, United States

US Patent 283,627, “Gage,” issued to John Logan of Waltham, Massachusetts. The specification describes a case with a dial and pointer whose movement is multiplied, a gaging bar sliding in the case and moving the pointer through a chain wound on an arbor, a spring that returns the bar, and jaws that let the same body act as a caliper gage and a depth gage.1

The dial indicator came from the watchmaking trade. Starrett’s history says it was introduced in the late nineteenth century as “an improvement in gages,” by John Logan, a watchmaker from Waltham, Massachusetts, who saw the need for an instrument to measure small distances and angles with an uncomplicated value reading. Starrett describes its own indicators as manufactured to fine watchmaking standards, with jeweled bearings and precisely finished gears and pinions.

Logan’s patent shows the first form. Its stated object was a simple and efficient device for gaging the distance between two surfaces, such as the depth of a shoulder, groove or hole, and the diameter or thickness of an article, with the movement of the pointer multiplied for greater delicacy. The movement was transmitted by a chain wound on the pointer arbor and returned by a spring, and the same body carried jaws that made it a caliper. It was a self-contained gage, not yet a comparator on a stand.

The indicator then divided into types. In Starrett’s sequence the drop indicator came first and the back plunger followed as a variant for tight spaces, then the test indicator, whose finger moves in an arc. Its Catalogue No. 17 lists a universal dial test indicator, so the lever type was in the company’s line by about 1902 on the dating given by the Internet Archive.

Interchangeability followed. Starrett says that in 1945 NIST issued the American Gage Design classes for case diameter so the user could choose an indicator by the space available, and that sizes 1 through 4 are essentially the same across manufacturers. This is the company’s statement of the history; the NIST document itself has not been read for this entry.

Electronic scales then displaced gears in many applications. Starrett says mechanical indicators, built from gears, pinions, rubies and racks, wear with time and lose accuracy, and that digital indicators built on a linear encoder avoid that wear and keep their accuracy over a longer range. These claims come from the manufacturer; no independent comparison was located.

Timeline

15 May 1883
John Logan of Waltham files the application for what is issued as US Patent 283,627, “Improvement in Gages.” Starrett dates the first indicator patent to this day.1 2
21 August 1883
US Patent 283,627 is issued. Logan’s gage multiplies the movement of a longitudinal bar through a chain and arbor to a pointer on a dial, for measuring depth of shoulders, grooves and holes and diameter or thickness.1
1883
Mitutoyo’s timeline lists “John Logan (U.S.) invented a dial gage” in this year.1
about 1902
Starrett Catalogue No. 17, dated 1902 by the Internet Archive, lists a Universal Dial Test Indicator with an adjustable dial, a friction joint and interchangeable hardened and ground contact points, for use on a surface gage, on a lathe or planer and in setting up machinery.1
1945
According to Starrett, the National Institute of Standards and Technology sets out the American Gage Design (AGD) classifications for the outside diameter of indicators.1 2
1975
Starrett’s Bulletin 1203 describes the Starrett “Last Word” Indicator as a replacement for the scriber on a vernier height gage.1
2025
Starrett describes electronic and wireless indicators using a linear encoder (LED, glass scale and reticle), with IP67 sealing and optional data collection, as having replaced mechanical types in many instances.1

Where the sources disagree

What does “the first patent dated May 15, 1883” refer to?

Starrett says the first indicator patent is dated May 15, 1883.1 2
The L. S. Starrett Company
May 15, 1883 is the date of the application; the patent, No. 283,627, was issued on August 21, 1883.1
The patent document

Gaps in the record (7)

The pre-1883 history of dial or lever gages (watchmakers’ and clockmakers’ instruments) is not covered by any source read.
Logan’s chain-and-drum mechanism was replaced at some point by a rack and pinion; no source read dates or attributes the change.
The NIST 1945 American Gage Design is known here only through Starrett; the NIST publication has not been located.
The date of Starrett’s “Last Word” indicator and of the earliest test indicators is not given in any source read.
Starrett’s Catalogue No. 17 scan is poorly OCR-recognized; the wording cited is limited to what could be read.
Care and safety have no sourced entries.
Standards: the slugs listed are candidates to be resolved and none was confirmed from a source.