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# Spline Measurement - The Three Minute Metrology Guide
- URL: https://www.metrology.com/metrology-guide-to-spline-measurement/
- Published: 2026-07-13T07:05:13.000Z
- Updated: 2026-07-20T01:37:49.000Z
- Author: The Datum
- Tags: Three Minute Metrology Guide, The Datum

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## **The issue**

Ask what diameter an internal spline is and you've asked three questions. Major, minor and pitch circle diameter each govern a different aspect of fit — and the PCD, the one that controls load sharing and backlash, is a geometric construction no surface of the part physically presents. Meanwhile the tool that cuts the spline is wearing from its first stroke: PCD drift is the earliest warning a broach gives, and in most shops nobody is positioned to hear it until the scrap bin does.

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## **The problems with the usual answers** 

Measurement between pins with calculation is accurate and slow — a bench exercise, not a production check, and error-prone on odd tooth counts where the pins don't sit opposite each other. Gear-checking machines and CMMs give the full answer at the cost of a queue: the part leaves the machine, the number arrives an hour later, and the broach keeps cutting in between. Plug-style spline gauges answer go/no-go only — no value, no trend, no warning. None of these puts the PCD, as a number, in an operator's hand at the spindle.

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## **The three-point advantage**

A spline-application head carries contacts machined to your tooth form — line or point contacts seating in the tooth spaces at the pitch line (the best-wire position) for PCD, line contacts for major diameter, extended contacts reaching the root for minor. Three-point contact self-centres in the spline, odd and even tooth counts each get their own contact arrangement, and the head presets against a calibrated setting ring so every reading is a deviation from a certified reference — repeatable to 0.002 mm (0.0001") in any operator's hands, in seconds, at the machine. If your drawing states a measurement-over-pins figure, the contacts are designed to reproduce that exact convention, so your gauge and your customer's calculation agree by construction. Standard coverage 6–200 mm (0.236"–7.87"); specials from 2 to 500 mm (0.079"–19.69").

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## **How to specify**

Send the spline profile drawing with full design details — it's the one application where the drawing isn't optional, because the contact geometry is machined to the tooth form. From it we take: number of teeth (N), pitch circle diameter (PCD), major (MAJ) and minor (MIN) diameters, and, if specified, best wire size (BW) and distance between pins (DBP) — plus which diameter or diameters you need to read. Odd counts, even counts, and combined-diameter heads on one handle are all standard design work. Send the profile drawing to the design office and a designed solution — head, contacts, setting ring — comes back within 24 hours.

![](https://storage.ghost.io/c/97/2a/972a49a2-bf56-480d-bfc6-da726d4924be/content/images/2026/07/MET-GDE-005B-spline-specification-1.svg)

MET-BRF-001

Design Office

## Send us the drawing.

A design engineer responds within 24 hours. No drawing yet? You can describe the part instead.

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