Locating Pin Alignment: How to Prevent Part Binding in Jigs and Fixtures (Video)

Quick answer: Parts bind on locating pins when two pins constrain the same direction, so any variation in hole spacing has nowhere to go. The most common fix is to pair a round pin with a diamond (relieved) pin, with the diamond’s flats set across the line between the pins. The pins then control position and rotation without fighting each other. Tapered or bullet-nose pin tips and chamfered holes make loading easier.

This episode covers the most common solutions to locating pin alignment problems in jigs and fixtures.

What is part binding?

Binding happens when a workpiece sticks or jams as it is placed on, or removed from, a pair of locating pins. The part ends up cocked or out of alignment, so it can’t seat or release cleanly.

Why binding matters

• Slower cycle times as operators wiggle parts on and off
• Reject parts from improperly assembled or machined components
• Faster pin and hole wear
• Inconsistent, non-repeatable location

Common causes

• Two full-diameter round pins: hole-to-hole spacing tolerance plus pin-to-pin spacing tolerance leaves no room to engage both pins.
• Too little clearance between the pin and the part’s hole.
• No lead-in: blunt pin tips or sharp hole edges catch.
• Angled loading or removal, where the part tilts instead of moving straight along the pin axis.

Solutions that reduce binding

1. Use a round pin plus a diamond pin. The round pin fixes X and Y position. The diamond pin is relieved on two sides and is oriented so its flats face across the line connecting the pins, which stops rotation without over-constraining the part. This is the standard answer to two-pin binding.
2. Add lead-in. Tapered or bullet-nose tips help the part find the pin. Chamfered holes help too.
3. Set pin heights. Make one pin taller so the part engages one pin first and then the second.
4. Check the fit. Choose pin and hole tolerances so the part slides on freely but stays repeatable. Consult a standard fit table for your hole tolerance.
5. Use a slot instead of a second round hole, where the part design allows it.
6. Load and unload along the pin axis. Add guides or stops that encourage straight motion.

Watch: Locating Pins Pt. 4: Alignment Concerns

FAQ

What causes parts to bind on locating pins?
Two round pins constrain the part in the same direction, so normal tolerance variation in hole spacing prevents both pins from engaging cleanly.

What is a diamond pin?
A locating pin relieved on two sides so it contacts the hole at only two points. It resists rotation about the round pin without adding a second position constraint.

How do I orient a diamond pin?
Place its flats perpendicular to the line between the two pin centers, so the contact surfaces sit across that line.

Can I eliminate binding with looser fits alone?
Looser clearance helps, but it reduces repeatability. A round-plus-diamond pair addresses the cause and keeps accuracy.