Plus/Minus vs Position Tolerance for Hole Patterns

Plus/minus vs position tolerance shows up on almost every bracket with a bolt pattern. Both methods locate holes. They do not give the shop the same zone to hit.

Should a Hole Pattern Use Plus/Minus Dimensions or a Position Tolerance?

Use a position tolerance with basic dimensions when the holes have to locate to datums or to each other for assembly. Plus/minus on each coordinate creates a square zone that rejects good parts and accepts some that will not assemble.

R. Dean Odell walks through that trade in Plus/Minus vs Position Tolerance with GD&T. The description says he shows "some issues with plus/minus dimensioning, and compare that method to GD&T position."

What Plus/Minus vs Position Tolerance Does to the Zone

With ±0.1 mm on X and ±0.1 mm on Y, the allowed center sits in a square. A hole that is 0.12 mm off on a diagonal can fail even though it is closer to true location than a hole that is 0.1 mm off on one axis alone. A position callout with a diameter zone treats every direction the same.

GD&T Basics defines true position / position tolerance as a 3D zone around the theoretically exact location, with datums referenced in the feature control frame. For a hole, the size value is preceded by a diameter symbol so the zone is circular (or cylindrical through thickness).

Basic dimensions (boxed) set the exact pattern. The feature control frame sets how far each hole may wander. That is different from a title-block general tolerance like ISO 2768, which is fine for non-critical sizes but weak for a mating pattern.

How Tight Is Enough?

Only as tight as the assembly needs. A clearance hole pattern that still bolts up with floating fasteners does not need a microscopic zone. Threaded holes that locate another part are a different story; see tapped hole position tolerance.

Our CNC work holds "ISO 2768-m as standard, down to ±0.01 mm where the drawing calls for it." Put the position callout on the drawing when the pattern matters, and keep the datums the same ones the assembly uses. A part can still pass CMM and fail to assemble if the datums or the mating part are wrong; that trap is covered in why a part can pass positional tolerance and still not assemble.