How to Use Round and Diamond Locating Pins

A removable plate can fit the prototype fixture and still bind on a second fixture built to the same drawings. When two close-fitting round pins locate the plate, small differences in the distance between the pins and the distance between the holes can prevent both pins from entering. One round pin and one diamond pin can accommodate that spacing variation while still locating the plate and controlling its rotation. Carr Lane guide to locating pins

Use the round pin to establish the plate's position in its mounting plane. At the second hole, orient the diamond pin so its relief gives clearance along the line between the two pin centers. Its locating faces act across that line, limiting rotation about the round pin. If the diamond pin is installed a quarter turn out of position, the extra clearance is in the wrong direction. The plate can still bind on hole spacing and may have more angular play than intended.

Separate seating, locating and clamping

The mounting face or support pads establish where the plate sits. The round pin limits movement in two directions parallel to that face. The diamond pin limits the remaining in-plane rotation. Screws or clamps hold the plate against its seating surfaces. Check these functions separately in the assembly so a bolt shank, a shoulder or a third pin does not accidentally become another tight locator.

Choose the primary hole near the feature whose lateral position matters most, such as a connector, a probe target or a mating bore. Place the secondary locator where it provides enough leverage to control rotation. A longer distance between pins reduces angular movement for a given amount of lateral play, but it can increase spacing variation from temperature differences or the way the parts are made.

For parts loaded by hand, a taller round pin can engage first and guide the plate onto the shorter diamond pin. A lead-in chamfer can help with entry. Check the usable engagement below the chamfer and verify that neither pin bottoms in a blind hole before the plate seats. MISUMI guide to positioning with locating pins

Check the fit in two directions

Along the pin centerline, compare the possible differences in center spacing between the fixture and the plate with the clearance at the relieved pin. Use the smallest finished holes and the largest applicable pin dimensions when checking whether the parts can assemble. Include the location of the installed pins, especially if they are pressed into separate mounting bores. Check the supplier's profile drawing or the actual CAD geometry. The clearance between two flats does not by itself describe how an offset circular hole clears the rest of the diamond profile.

Across the centerline, check how much movement the assembly can tolerate. Both locators can contribute. For an illustrative layout with pins 100 mm apart, suppose the round pin allows 0.03 mm of one-sided transverse movement and the diamond pin allows 0.02 mm. If the two shifts act in opposite directions, the relative transverse shift is about 0.05 mm. Dividing by 100 mm gives approximately 0.0005 radians, or 0.029° of one-sided angular play. At a feature 200 mm from the primary pin, that rotation alone can move the feature about 0.10 mm. This example isolates clearance. Add the actual hole and pin location variation and evaluate translation at the critical feature separately.

Show the relief direction on the drawing

Identify the seating face and the hole that acts as the primary locator. For the diamond pin, show the installed orientation in a top view or section and dimension the locating width and relieved profile as needed. If the shank is round, give the assembly team a way to set and verify the orientation. A keyed mounting feature or a visible orientation mark can help, depending on how the pin is installed and serviced.

Distinguish the fit of the pin shank in its holder from the fit of the head in the removable plate. The head needs to meet the assembly's entry and repeatability requirements after finishing. If the pin or the mating bushing will wear in service, document how it is replaced and how the installed location is checked afterward. Carr Lane diamond locating pin dimensions and fits

Verify interchangeability on the first build

Consider a machined adapter plate that moves between two benches. Put the round pin near its connector and the diamond pin toward the far end of the plate. During sample approval, try each available plate on both benches and confirm that it drops fully onto the seating surfaces by hand before tightening. Then check connector alignment at the same defined point after repeated removal and installation.

Record assembly effort, seating and measured alignment separately. If both benches accept the plate but the connector position varies, investigate clearance, pin locations and the clamping sequence. If the connector stays within its allowed range but one combination binds, focus on the fit check along the line between pins. That separation helps the team decide which tolerance to adjust.

For a prototype or production quote, provide the mating CAD models, pin or bushing details, finish requirements and the allowed movement at the critical interface. Note which components must interchange and which features are measured after finishing. Request a project review from Aluprototype

Frequently asked questions

Can I use a slot with a second round pin?

Yes. A slot running along the pin centerline can provide spacing clearance while its sides limit transverse movement. Check the slot width, end clearance, wear and finished dimensions. A diamond pin is useful when the mating plate already has two round holes or when a replaceable locator is preferred.

Should the locating pins carry the operating load?

Define how the assembly reacts to the actual load. Depending on the design, clamp friction, a mechanical stop or a pin designed for shear may carry it. Verify pin strength, bearing stress and movement if the locating pins are part of that load path.