What Are CNC Locating Pins?

A CNC locating pin is a precision mechanical feature used to establish the position of a workpiece relative to a fixture. The pin does not automatically control every degree of freedom. Its actual function depends on its geometry, position, mating hole and relationship with the rest of the locating system.

Six Degrees of Freedom

A rigid workpiece has six degrees of freedom:

The fixture must constrain the movements that matter for the machining operation without creating unnecessary redundant constraints that make loading difficult or distort the part.

Round Locating Pin vs Diamond Pin

Why Two Full Round Pins Can Bind

Consider two holes in a machined plate. The fixture also has two locating pins. Neither the part hole spacing nor the fixture pin spacing is mathematically exact. Each has manufacturing tolerance.

Two full round pins attempt to impose a rigid relationship in both radial directions at both holes. The result can be binding, difficult loading or forced seating.

How Tight Should a Locating Pin Fit Be?

There is no single clearance value that is correct for every locating-pin application.

The fit must account for pin tolerance, hole tolerance, repeatability requirements, loading method, material, temperature, engagement length and production frequency.

A production fixture should never depend on an operator forcing a component onto a locating pin.

Design the Fixture Around the Locator

A locating pin is only one part of the dimensional chain. Its accuracy depends on the fixture body, installation method, mounting interface, part hole and the machine reference.

For the broader fixture architecture, see CNC Fixture Design .

Material Considerations

How to Inspect a Located Part

Inspection should reproduce the engineering characteristic being controlled. A CMM is useful for many positional relationships, but it is not automatically the best instrument for every feature.

When NOT to Use Locating Pins

Locating pins are useful, but they are not automatically the best solution for every component.

• The part has no suitable controlled hole.
• The locating hole is a critical finished functional bore.
• The workpiece is too flexible around the hole.
• The hole can be easily damaged by repeated loading.
• A broad nest would provide substantially better support.
• The fixture blocks the required cutter or holder access.
• The part is a one-off and a standard vice provides sufficient control.
• Thermal expansion makes a tight locating arrangement unsuitable.

Practical Engineering Examples

Example 1 — Two-Hole Aluminium Plate

A plate contains two controlled holes used to establish the manufacturing location.

Using two full round pins can create a binding condition because the actual hole-to-hole distance contains manufacturing variation. A round primary pin plus a correctly oriented relieved secondary pin allows the fixture to establish position and orientation without demanding impossible geometric coincidence.

Example 2 — Thin-Wall Housing

The locating holes may establish XY position correctly, but aggressive clamping around a flexible housing can distort the walls.

The better solution is to combine hole location with distributed support and controlled clamp loading. Location accuracy alone does not guarantee geometric accuracy if the workpiece bends.

Example 3 — Recurring Production Fixture

A recurring component may justify dedicated locating hardware when repeated manual indication is consuming setup time or when operator-to-operator loading variation is creating rework.

The engineering case should consider setup time, scrap, inspection, fixture cost, maintenance and annual production volume rather than quantity alone.

Common Locating-Pin Mistakes

DFM Recommendations for Locating Features

• Use controlled locating holes when the design genuinely benefits from repeatable hole-based location.
• Relate locating features to functional drawing datums.
• Avoid unnecessary precision on holes that have no functional or manufacturing requirement.
• Provide accessible surfaces for fixture supports.
• Avoid placing critical features where clamps or locators block tool access.
• Consider inspection access during the fixture-design stage.
• For recurring production, consider how the locator will be maintained and replaced.

For broader machining DFM considerations, see the CNC DFM Checklist .

CNC Locating Pin FAQ

What are CNC locating pins used for?

They establish the physical position of a workpiece relative to a fixture and help make repeated loading predictable.

Why use one round pin and one diamond pin?

A round pin can provide two-axis location from the primary hole, while a diamond pin can provide the required secondary constraint without unnecessarily constraining the hole in both directions.

Can I use two round pins?

It can be appropriate in some engineered applications, but two full round pins can over-constrain a two-hole location scheme when hole spacing and fixture spacing contain manufacturing variation.

How tight should a locating pin fit be?

There is no universal clearance. The fit should provide the required repeatability while allowing reliable loading under actual tolerance, material and temperature conditions.

Are locating pins the same as clamps?

No. Locators establish position. Clamps primarily maintain the workpiece against the locating and supporting surfaces.

Can locating pins distort a part?

Yes. Excessive interference, poor support, unsuitable hole geometry or aggressive clamp loading can deform the workpiece.

Are locating pins useful for 5-axis machining?

Yes. They can establish a repeatable datum while the machine accesses multiple faces, provided the fixture permits the required tool and holder movement.

Should every CNC fixture use locating pins?

No. Vises, stops, nests, soft jaws and machined fixture surfaces can be better choices depending on geometry, tolerance, quantity and production requirements.