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Locating & Datums

Correct locating fixes the workpiece in space so that each feature is machined from a stable datum. All locating and clamping decisions start from the six degrees of freedom.

1Degrees of Freedom (6-DOF)

A free body has six degrees of freedom: three translations and three rotations. Locating must constrain the degrees of freedom relevant to the operation.

  • Translation along X, Y and Z axes.
  • Rotation about X axis (roll), Y axis (pitch) and Z axis (yaw).
  • Each locating point removes one degree of freedom.

23-2-1 (Six-Point) Locating Principle

For a rectangular block, six locating points arranged on three mutually perpendicular planes fully constrain the part:

  • Primary plane (bottom): 3 points — constrains Z translation and rotation about X and Y.
  • Secondary plane (side): 2 points — constrains Y translation and rotation about Z.
  • Tertiary plane (end): 1 point — constrains X translation.
C (1) B (2) A (3) 3-2-1 locating
Over-Locating Warning More than 3 points on one plane, or a round pin plus a second round pin on a long hole axis, creates redundant constraints. The part then 'chooses' which locators to touch, causing unpredictable position error. Replace the second round pin with a diamond pin or reduce the points.

Common Locator Types

Flat plate / support pads

Large plane support for the primary datum. Use three or four pads; avoid one large continuous surface that may trap chips and dirt.

Adjustable support pin

For workpieces with uneven or cast surfaces. Adjustable rest pins let you level the part against a rough surface.

Round locator pin

Engages a reamed hole in the workpiece to fix position in two directions. Ground and hardened.

Diamond (relieved) pin

Used together with a round pin on two holes. The relieved sides allow for center-distance error and locate in only one direction.

V-block

Locates cylindrical workpieces on the O.D. Self-centering for rounds; works for varying diameters.

⬩D 90° V-block

Conical locator

Centers parts using a hole or center. Used on lathe centers and for location on machined center holes.

Locating Rules

  • Locate from machined surfaces whenever possible; cast or forged surfaces are inaccurate.
  • Choose a datum consistent with the drawing dimensions and the inspection datum.
  • Keep locators hard (55–60 HRC) and ground to resist wear.
  • Allow chips and dirt to escape; use relieved or discontinuous locating surfaces.
  • Never clamp in a way that lifts the part off its locators.
  • Keep the number of locating points minimal to avoid over-constraint.

Positioning Error Sources

  • Wear of locators increases clearance and shifts the workpiece.
  • Dirt, chips or burrs under a locator lift the part.
  • Clearance between locator pin and workpiece hole.
  • Inconsistent clamp force deforming or shifting the part.
Do not place more locating points than necessary on the same surface — this over-constrains the part and causes position error or distortion.