A fixture that went out of alignment after 50 setups. The fixture used two Ø16 mm dowel pins to locate a workpiece on a base plate. After 6 months, the workpiece shifted 0.1 mm. The customer thought the pins wore. They were fine — but the holes in the workpiece had been re-bored by a previous operator. The dowel pin fit changed. This is about locating pin design and the tolerance stack-up that determines repeatability in fixtures.
The two-pin layout
A fixture locates a workpiece with two pins. One pin is a round dowel (locates in X and Y). The other is a diamond pin (locates in Y only, allows X sliding). This prevents the “locating over-constraint” problem: two round pins in two holes can’t be assembled if the hole spacing varies. The diamond pin absorbs the tolerance in the hole spacing direction.
The round pin fits in a reamed hole (H7). The pin is precision ground (h6 or g6). The clearance between pin and hole is 0-0.02 mm. The workpiece locates repeatably to ±0.01 mm. If the hole is over-reamed (H8), the clearance grows to 0.05 mm. The workpiece shifts. The machining tolerance blows out.
What went wrong
The workpiece holes were originally reamed to H7 (Ø16 +0.018/0 mm). The dowel pin was h6 (Ø16 -0.011/-0.024 mm). The clearance was 0.011 to 0.042 mm. After a tooling change, an operator reamed the holes with a twist drill (not a reamer) to clear a chip. The holes became Ø16.1 mm. The clearance grew to 0.09 mm. The workpiece could shift 0.09 mm in X or Y. The machined features were 0.1 mm off.
The fix
1. Re-reamed the holes to H7. A proper chucking reamer restored the holes to Ø16 +0.018/0. The repeatability returned to ±0.01 mm.
2. Added a pin with a shoulder. The new pins have a shoulder (flange) that sits on the workpiece surface. The shoulder prevents the workpiece from lifting. The round pin also has a threaded extraction hole so it can be removed without damaging the fixture.
3. Specified the diamond pin orientation. The diamond pin’s flat sides align with the hole-spacing direction (X). If installed 90° off, the diamond pin locates in X and allows Y movement — the workpiece can shift sideways. The diamond pin orientation is a common setup error. I mark the orientation on the fixture with an arrow.
Taper pin vs dowel pin
| Pin type | Fit | Repeatability | Reusable |
|---|---|---|---|
| Cylindrical dowel (H7/h6) | Transition/light interference | ±0.005 mm | Limited (press fit wears) |
| Taper pin (1:50) | Self-locking, removable | ±0.02 mm | Yes |
| Quick-change ball lock | Spring-loaded ball in hole | ±0.05 mm | Yes |
| Magnetic locator | Non-contact | ±0.1 mm | Yes |
For production fixtures (hundreds of setups), I use a pull-out dowel (with internal thread) that’s pressed in and extracted with a puller. The pin and hole wear together over thousands of cycles. When the fit loosens, the hole is reamed oversize and a new oversized pin is installed. For occasional setups, a taper pin (1:50) is removable and re-usable.
The locator rule: H7 reamed holes, h6 ground pins, diamond pin oriented along the hole-spacing direction. The shifting fixture wasn’t worn pins — an operator drilled the locating holes oversize with a twist drill. Mark pin orientation on the fixture. For high-volume fixtures, use pull-out dowels with a ream-and-pin procedure when the fit loosens.
