Depth micrometers

A depth micrometer measures from the base reference face to the rod or spindle contact. Both seating surfaces matter.

Illustrative arithmetic: on an inch scale that reads from the selected rod’s range start, a 1–2 inch rod and sleeve/thimble reading of 0.237 inch represent 1.237 inch, before applicable calibration corrections. Reporting only 0.237 inch loses the rod range. Conversely, adding an offset to a display already preset to report total depth counts it twice.

Measurement geometry and access

Define the required depth from the drawing: identify the reference surface, bottom surface and measurement location. A flat-ended rod touching a drill-point cone does not directly establish the depth of the cylindrical, full-diameter portion of a blind hole. Likewise, a local reading on a rough or sloping bottom does not establish the depth everywhere. These are geometric limitations of the setup, so specify an alternative contact or measurement method when the required surface cannot be reached.

Check three independent dimensions before choosing a tool: the base span needed to bridge the opening, the rod diameter that can enter it, and the usable depth range. For example, Starrett's published 440 range uses a 63.5 mm base and 3.2 mm diameter rods; its catalogue lists 440MZ-75RL with three rods for 0–75 mm. These dimensions describe that configuration, not clearance requirements for every depth micrometer. Base support on a narrow land still needs assessment. Starrett, Precision Hand Tools catalogue, depth-micrometer section, undated online edition

A broader base can bridge a wider feature but may also rest on an unintended raised surface. Plan the support footprint rather than assuming a longer base is always better. Inspect for burrs, trapped chips and edge damage before interpreting a changing reading as a changing part.

Seating, force and rod changes

Keep the base fully seated while bringing the measuring end gently into contact. Mitutoyo's 128/129 manual No. 99MAA024M specifies use of the ratchet for consistent force, the same orientation and conditions during setting and measurement, and datum checks before measurement and after changing range. It also requires clean contact surfaces at the rod collar and spindle; contamination there changes the effective rod position. Those checks address both the base-to-part interface and the internal rod seat. Mitutoyo 128/129 user manual, undated manufacturer PDF

For the Series 129, that manual says to check the body reference with the 0–25 mm rod before evaluating another rod's setting. Rods are adjusted before shipment; adjustment is conditional on the prescribed check finding a deviation. Follow the matching manual rather than turning rod collars to make an unexplained reading disappear. Mitutoyo, datum adjustment and range-changing instructions

Mitutoyo's separate usage guidance calls for checking smooth travel throughout the stroke, removing dirt from base and measuring faces, and using a surface with assured flatness for the reference check. For reference settings of 25 mm or greater, it specifies gauge blocks. It also recommends periodic reference checks during extended use because temperature changes can shift the setting. These are instructions for its interchangeable-rod instruments, not a universal ratchet-turn recipe. Mitutoyo, interchangeable-rod depth-micrometer usage guidance, undated Japanese page

Reading the scale and retaining the range

Do not transfer an outside-micrometer reading habit without checking the depth instrument's scale. Starrett's Tools & Rules explains that sleeve graduations run in the opposite direction and that a longer rod introduces a range offset: its 1–2 inch rod requires adding 1 inch to the sleeve/thimble reading. Electronic preset and zero functions can change what the display represents, so confirm the reference on a known depth after configuration. The cited Bulletin 1211 copy carries the print code 06/15 and copyright 2012; it is manufacturer-authored guidance hosted by a distributor. Starrett, Tools & Rules for Precision Measuring, printed page 26

Correction sign needs a separate check. Suppose a known depth of 40.000 mm produces 40.006 mm. The indication error at that point is indicated − reference = +0.006 mm; the corresponding correction is −0.006 mm. Applying it to an illustrative 42.318 mm reading gives 42.312 mm, but only if the calibration supports using that correction there. One reference point does not establish a constant error over the full stroke.

Setting, calibration and uncertainty

Setting establishes the instrument's reference for use. Calibration determines its relationship to reference values under stated conditions; it does not become complete simply because the zero aligns. Check relevant lengths and rod configurations through an appropriate calibration procedure. Mitutoyo's calibration-course documentation identifies comparison with gauge blocks as a method for inside and depth micrometers. Mitutoyo, Gage Calibration course questionnaire, undated

A bounded specification example is Mitutoyo's 329-250-10, listed on catalogue page D-50 with 0–150 mm range, six rods and a 101.6 × 16 mm base. The page gives digital resolution as 0.001 mm and separately specifies ±3 µm for micrometer-head feed, rod accuracy and geometric characteristics. It lists base flatness as 2 µm for the 101.6 mm base. These separate specifications must not be collapsed into a claim that every 150 mm depth measurement is accurate to the display increment or head-feed figure. The standalone page carries no publication date; confirm the supplied tool's specification. Mitutoyo, Series 329/129 catalogue, D-50

Evaluate uncertainty for the actual task: reference calibration, indication errors, repeated seating, alignment, contact force, surface condition and thermal effects can matter. Include the way the base realizes the reference surface. NPL distinguishes resolution, repeatability, accuracy and uncertainty and explains uncertainty evaluation; agreement between repeated readings alone does not establish correctness. NPL, Fundamental Good Practice in Dimensional Metrology, GPG80, July 2005, corrected reprint October 2012

NIST identifies 20 °C as the reference temperature for dimensional measurements. Account for temperatures and material expansion when significant; there is no universal stabilization time for all tools and parts. NIST Engineering Metrology Toolbox FAQ, undated

Record the tool and rod, reference/check results, location and supporting surfaces, individual readings, corrections, conditions and acceptance rule. Preserve that context when comparing measurements between operators or deciding conformance near a limit.

Related reference

Inspection and quality documents