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Datum Alignment Help

Every engineering drawing defines its own datum structure, and the correct CMM alignment follows directly from it — there is no single alignment routine that works for every part. The examples below show common datum structures alongside the alignment sequence each one requires.

Steps are described using MCOSMOS terminology — Align Base Plane, Create Origin, Align Axis Through Point. If you use different CMM software the menu names will differ, but the underlying logic is the same.

0.05 A Ø25 0.1 A B Ø10 Ø0.05 A B C SIDE VIEW — FLANGE-MOUNT CYLINDER

Example 01 — Flange Face / Main Bore / Bolt Hole

Flange-Mount Cylinder — Face, Bore & Bolt Circle

Datum A — Primary

Measure datum A as a plane. Use datum A to Align Base Plane, with the origin in the element.

Datum B — Secondary

Measure datum B. Use datum B to Create Origin.

Datum C — Tertiary

Measure datum C. Align Axis Through Point, using datum C as the point.

A 0.05 Ø20 Ø0.1 A B 0.1 A C FRONT VIEW — RECTANGULAR BLOCK

Example 02 — Base Plane / Central Bore / Side Face

Rectangular Block — Base, Bore & Side Face

Datum A — Primary

Measure datum A as a plane. Use datum A to Align Base Plane, with the origin in the element.

Datum B — Secondary

Measure datum B as a cylinder. Use datum B to Create Origin, placing the datum origin in the element.

Datum C — Tertiary

Measure datum C as a plane. Create a line at the intersection of datum A and datum C. Use that line to Align Axis Parallel to Axis.

0.05 A Ø40 Ø0.1 A B 2× Ø10 Ø0.1 A B C END VIEW — CIRCULAR DISK

Example 03 — Flat Face / Central Bore / Bolt Hole Pair

Circular Disk — Face, Bore & Hole Pattern

Datum A — Primary

Measure datum A as a plane. Use datum A to Align Base Plane, with the origin in the element.

Datum B — Secondary

Measure datum B as a cylinder. Use datum B to Create Origin, placing the datum origin in the element.

Datum C — Tertiary

Measure both datum C holes as circles. Construct a line between the two hole centres. Use that line to Align Axis Parallel to Axis.

0.05 A 0.05 A B 0.05 A B C OBLIQUE VIEW — RECTANGULAR BLOCK

Example 04 — Three Perpendicular Planes

Rectangular Block — 3-2-1 Datum Alignment

Datum A — Primary

Measure datum A as a plane. Use datum A to Align Base Plane, with the origin in the element.

Datum B — Secondary

Measure datum B as a plane. Create a line at the intersection of datum A and datum B. Use that line to Align Axis Parallel to Axis.

Datum C — Tertiary

Measure datum C as a plane. Use datum C to Create Origin, completing the 3-2-1 alignment.

0.05 A 0.05 A B FRONT VIEW — CYLINDER

Example 05 — Cylinder and End Face

Turned Part — No Datum C

Datum A — Primary

Measure datum A as a cylinder. Use datum A to Align Base Plane, with the origin in the element.

Datum B — Secondary

Measure datum B as a plane. Use datum B to Create Origin.

Note — Unconstrained Rotation

With only datums A and B, rotation about the cylinder axis is deliberately left free — one degree of freedom remains unconstrained. This is normal for turned parts, which are rotationally symmetric and require no fixed angular reference. To complete the alignment, construct a theoretical line perpendicular to datum A and use it to Align Axis Parallel to Axis.

Diagram coming soon

More examples

More Examples Coming Soon

Additional datum structures and alignment sequences will be added here. If there's a specific part type or datum configuration you'd like to see covered, get in touch via the contact page.