Locating Pins for Fixtures: Material, Tolerance and Heat Treatment You Need to Specify
A locating pin is the smallest part of a fixture and the one that decides whether the part sits where it should. This guide explains why a round pin and a diamond pin are used together, what fit to call with the bushing, which steel and hardness make sense, and when to replace them as a set.
A locating pin is the smallest part of a fixture and, at the same time, the one that decides whether the part sits where it should. When it wears, the part starts to "float" a few hundredths and nobody notices until an inspection station rejects a lot. When it is replaced badly, the part does not go in, goes in by force, or ends up rotated.
The problem is almost never the machining; it is what was ordered: a diameter with no tolerance class, a steel with no hardness, a round pin where a diamond pin belonged. This guide is for the manufacturing engineer or toolmaker who replaces or specifies fixture pins: what to order so the pin locates correctly from day one and lasts as long as it should.
In Summary
- A round pin and a diamond pin work together: the round pin fixes position, the diamond pin only controls rotation. Two round pins over-constrain the part.
- The fit with the bushing is called with a tolerance class on both sides, for example H7/g6 for a sliding pin or H7/n6 for a fixed one.
- D2, 4140 or 1045, depending on wear, toughness and cost, always with the target hardness on the drawing and its heat-treatment certificate.
- The final diameter comes off the grinder, after hardening. A pin that was only turned does not locate.
- At Radii, pins and locators are part of custom tooling and MRO for plants, with dimensional inspection before delivery.
1. What a locating pin does (and what it does not)
In a fixture, the part is placed following the 3-2-1 principle: three support points define a plane, two more define a line within that plane, and a final one fixes the position. In practice, many parts are located by two holes and a face: the face rests on the base plate and two pins go into two holes in the part.
The locating pin does one thing only: give position. It should not clamp or carry machining or assembly loads; that is what clamps, jaws and supports are for. A pin that both locates and takes load wears quickly and loses accuracy exactly where it matters most.
The same logic applies to any locating element: the pins of a dimensional inspection fixture, the nests of an assembly jig or the fingers of a gripper. If the locator is wrong, everything downstream inherits the error.
2. Round pin and diamond pin: why they go together
The most common mistake when replacing pins is installing two round pins where the original design had one round pin and one diamond pin (also called a relieved pin).
Why two round pins do not work: the distance between the two holes in the part has a tolerance, and so does the distance between the two fixture pins. They never match exactly. With two tight round pins, a part at the edge of its tolerance does not go in, goes in with force, or deforms on the way in.
How the diamond pin solves it: the diamond pin has two relieved flats, so it only makes contact in two narrow zones perpendicular to the line between the two pins. It controls the part's rotation but leaves free the direction in which the hole spacing varies.
| Element | What it controls | What it leaves free |
|---|---|---|
| Round pin | Position in X and Y | Nothing in the plane; only rotation around it |
| Diamond pin | Part rotation around the round pin | Movement along the line between the pins |
| Support face | Height and flatness (Z and two tilts) | The plane of the base plate |
What the drawing has to say:
- Which pin is round and which is diamond, and the diamond's orientation: its contact flats must be perpendicular to the line between the pins. A diamond rotated 90° controls nothing.
- The distance between pins with its tolerance, referenced to the same datums the part drawing uses.
- The height of each pin above the plate, with a conical tip or a lead-in radius. The round pin is usually taller than the diamond so the part engages one first and then rotates onto the other.
3. The fit with the bushing
In heavily used fixtures, the pin does not go directly into the base plate: it goes into a replaceable hardened bushing. When the hole wears, the bushing is changed, not the plate.
The fit between pin and bushing, and between bushing and plate, is called with standard tolerance classes from the ISO 286 system. You can calculate the exact limits for each combination with the site's ISO 286 fits calculator.
| Case | Fit type | Example classes |
|---|---|---|
| Pin that slides in the bushing or is changed often | Clearance | H7/g6 |
| Pin fixed in the base plate, removed only during maintenance | Transition | H7/m6 or H7/k6 |
| Pin or bushing that must not move | Interference | H7/n6 or H7/p6 |
Three things that prevent trouble:
- Call the class on both sides. "10 mm pin" is not a specification. "Ø10 g6 pin in Ø10 H7 bushing" is.
- Choose the fit by function, not habit. An interference-fit pin that has to be changed every month ends up damaging the plate when it is pulled.
- Think about total clearance. The clearance between pin and bushing, plus the clearance between the pin and the part hole, adds to position uncertainty. If the station needs a very precise position, that total clearance has to fit within what the part tolerates.
For standard catalog dowel pins, the reference standard is ISO 8734 (parallel pins of hardened steel). A custom locating pin, with a conical tip, diamond flats or a shoulder, is specified with the same logic: diameter with tolerance class, hardness and finish.
4. Material and heat treatment
The pin works through repeated contact with the part. The material is chosen by how much wear it takes, how much toughness it needs (whether it can take an impact) and how much it costs.
| Steel | Typical treatment | When it fits | When it does not |
|---|---|---|---|
| D2 (tool steel) | Hardened and tempered; high hardness, on the order of 58–62 HRC as common practice | Heavy wear, many cycles, abrasive or burred parts | Pins that can take hard impacts: it is hard but less tough |
| 4140 | Quenched and tempered; medium hardness adjustable through tempering | Pins that need toughness and good strength | Constant abrasive wear |
| 1045 | Induction or flame surface hardening | Moderate use at lower cost; tough core with a hard surface | Sustained high precision with heavy wear |
| Martensitic stainless | Hardened | Environments with coolant or humidity where carbon steel rusts | When corrosion is not an issue and D2 can be used |
Tool steels such as D2 are specified under standards like ASTM A681; requiring the material certificate to cite it is how you verify the steel is what you ordered.
What the drawing or order has to say:
- The steel, by designation (D2, 4140, 1045), not just "tool steel."
- The target hardness with its range, for example "58–62 HRC", and for surface hardening the case depth.
- The material certificate and the heat-treatment certificate. Without them there is no way to verify hardness short of measuring it again.
5. Grinding: where the final size comes from
Hardening distorts the part: it grows, goes out of round or bows slightly. That is why a locating pin is made in stages:
- Machined with stock (extra material) on the working diameter.
- Hardened and tempered.
- Ground to final size on the working diameter, along with roundness and finish.
A pin that was only turned and then hardened cannot hold a class like g6: hardening distortion has already eaten the tolerance. If a supplier quoting a hardened pin with a tight fit does not mention grinding, ask how they will reach size.
What to ask for besides the diameter:
- Surface finish on the working zone (grinding leaves a fine finish that reduces wear on the bushing and the part).
- Roundness or cylindricity if the part drawing requires very precise position.
- A dimensional report with the measured diameter of each pin. It is what lets you reorder the next set without measuring anything again.
For tolerances below what a lathe delivers, the criterion is the same as in any high-precision machining job: the operation that closes the dimension and the method that verifies it have to be clear before quoting.
6. Spares: as a set and with the bushing
Pins wear, and on a critical line you cannot wait to quote when one fails.
What makes sense:
- Replace the round and diamond pins together. Changing only one shifts the part reference slightly from how the fixture was validated.
- Check the bushings in the same change. A new pin in an oval bushing still has play.
- Keep a spare set on the shelf for critical-line fixtures: round pin, diamond pin and bushings, from the same drawing and with their dimensional report.
- Mark the pins with their drawing or set number, especially if the plant has several similar fixtures.
- Define when they are replaced: by periodic play inspection, by visible wear or after any crash, according to the fixture maintenance plan.
7. What to send to quote pins and locators
So the supplier quotes without guessing:
| If you have… | Send |
|---|---|
| The fixture drawing | Pin and bushing drawing with diameters, tolerance classes, steel, hardness and finish |
| Only the worn pin | The pin, the model or drawing of the part it locates and, if possible, the hole or bushing size |
| A new fixture | The part CAD with its datums and the fixture's function (inspection, machining or assembly) |
And in every case: the number of sets, whether you need material and heat-treatment certificates, and whether the pins must be interchangeable with the ones already in the plant.
Frequently Asked Questions
Why does a fixture use one round pin and one diamond pin instead of two round pins?
Because two round pins over-constrain the part: the distance between the two holes in the part and the distance between the two pins are never exactly equal, so the part jams or does not go in. The round pin fixes the position in X and Y; the diamond pin, with two relieved flats, only controls rotation and leaves free the direction along the line between the pins. That way the part goes in even when the hole spacing varies within its tolerance.
What fit should be called between a locating pin and its bushing?
It depends on whether the pin has to slide or stay fixed. For a pin that goes in and out of the bushing or is changed often, use a clearance fit, typically H7/g6. For a pin that stays fixed in the base plate, use a transition or interference fit, such as H7/m6 or H7/n6. What matters is that the drawing states the tolerance class on both sides, not just the nominal diameter.
Which steel is best for locating pins?
Hardened D2 tool steel when there is heavy wear or many cycles; quenched and tempered 4140 when toughness with good strength is needed; surface-hardened 1045 for moderate use at lower cost. In every case the pin is ground after heat treatment, because hardening distorts the part and the final diameter has to come off the grinder, not the lathe.
What hardness should a locating pin have?
As a common industry reference, hardened D2 usually ends up around 58 to 62 HRC, quenched and tempered 4140 in a medium range that depends on tempering, and induction-hardened 1045 reaches high hardness only at the surface. The drawing should state the target hardness with its range and require the heat-treatment certificate; without it there is no way to verify it.
When should fixture pins be replaced?
When the part starts to show play, when the pin shows visible wear or marks, after a crash, or according to the fixture maintenance plan. Replace the round and diamond pins together, along with their bushings if those are worn too, and keep a spare set on the shelf if the fixture serves a critical line.
Conclusion: the cheapest pin is the one that locates right the first time
A locating pin costs little compared with a lot rejected because a part was mispositioned. The difference between a pin that works and one that does not is almost never the machine: it is an order that says whether it is round or diamond, what fit it has with its bushing, which steel, which hardness, and that the final diameter is ground after hardening.
Radii makes pins, bushings and locators through its network of audited shops in Mexico, with grinding and dimensional inspection of the set before delivery. If you have the fixture drawing or only the worn pin, upload it to Radii and get a quote.