Chapter vii · Material Condition & Location

For fifty years we drew a hole's tolerance as a tiny square — rejecting good parts in the corners and passing bad ones on the edges. One man drew a circle.

During WWII the British engineer Stanley Parker kept finding torpedo parts that gauged as scrap yet assembled perfectly; the square tolerance zone was lying, so he redrew it as a circle and saved the batch. Parker's circle has a name now: position. In Y14.5-2018 it is the only location characteristic left standing — concentricity and symmetry were retired, their jobs handed to position, profile, and runout. When you need to say where a feature of size lives, this is the symbol you reach for.

Position locates the axis (or centre-plane) of a feature of size within a tolerance zone — usually a cylinder, marked with the ⌀ — centred on true position. True position is not a measurement; it is set by basic dimensions, the boxed, exact numbers the zone is built around. The feature's job is to put its axis somewhere inside that zone.

Here is the whole contract for one hole: the ⌀ tells you the zone is round, the basics fix its centre, and the datums A·B·C say which way the part is held while you check it.

Position callout

Position

Worst-case boundary

Left, the callout: position, ⌀0.4 at MMC, to A·B·C. Right, what it guarantees — a constant virtual condition boundary (inner dashed circle) that always stays clear, no matter where inside its zone the axis lands. That boundary is exactly the size of the gauge.

Virtual condition is the heart of it. It is the single worst-case boundary a mating part can ever encounter — the size you build the go-gauge to. For an internal feature (a hole) at MMC, VC = MMC − geoTol; for an external feature (a pin) at MMC, VC = MMC + geoTol. Two parts assemble for certain when VC(pin) ≤ VC(hole). Get the virtual condition wrong by a hair and ten thousand brackets ship that won't bolt to their mating plate on the line — the recall is measured in trucks, not parts.

That turns inspection into something physical. Build a pin to the hole's virtual condition and try to drop it through. It either goes or it doesn't — pass or fail you can feel.

The go-gauge is a single steel pin ground to the hole's virtual condition — the constant worst-case boundary. Move the hole off true position, or let it grow past MMC to earn bonus. If the pin drops through, the part assembles. Pass/fail you can feel.

Hole offset from true position +0.10 mm Actual hole size 10.00 mm

position error (⌀): mm
bonus earned: mm
tolerance available: mm
virtual condition (gauge ⌀): mm

Notice the gift hidden in the size slider. As the hole grows past MMC it has more material to spare, so the position requirement loosens by exactly that much — the bonus tolerance from the previous chapter. The gauge encodes it automatically: a bigger hole simply has more room to slip over the same fixed pin. The drawing never gets looser; the part earns it.

Composite position — the two-segment trap

Stack two position rows under one symbol and you get a composite control. The upper segment — the PLTZF (pattern-locating tolerance zone framework) — locates the whole pattern of features to the datum reference frame with a relatively large zone. The lower segment — the FRTZF (feature-relating tolerance zone framework) — controls the features' spacing and orientation to each other with a tighter zone.

The classic error is to read the tight lower segment as relocating the pattern more precisely to the datums. It does not. The FRTZF refines feature-to-feature relationship and orientation only; it never tightens the pattern's location to the DRF. Location to the datums lives entirely in the upper segment.

For the advanced reader → Fastener formulas and the projected zone

Sizing the position tolerance for an assembly comes down to two cases, where H is the MMC of the clearance hole and F is the MMC of the fastener:

Floating fastener — a bolt passing through clearance holes in both parts, nut on the far side: each part may spend the full clearance, so T = H − F. Fixed fastener — one part tapped (or with a pressed pin) so the fastener has no clearance to spend on its own end: the budget is split, T = (H − F) / 2. The ÷2 is not a fudge factor; it is because the fixed feature contributes zero clearance, so all of the misalignment must fit in the clearance part alone.

And where a bolt or pin projects out of a tapped hole, a tilt at the surface becomes a much larger swing at the top of the fastener. The projected tolerance zone Ⓟ moves the cylindrical zone out of the part to the height the mating feature actually reaches — so the threaded hole's tilt is controlled where it can cause interference, not where it's drilled.

Try it

Will the Gauge Drop?

A clearance hole is position ∅0.4 Ⓜ, hole MMC = ∅10.0. What virtual-condition boundary is the functional gauge pin built to?

The hole is produced at ∅10.3 — bigger than its ∅10.0 MMC. What happens to the position requirement?

Two plates are joined by a bolt through a clearance hole in each, with a nut on the far side. Which fastener formula sizes the position tolerance?

Position is the workhorse because it is honest about what a hole is for. It does not ask the machinist to chase a perfect centre; it asks only that the part will assemble — and then hands you a gauge that settles the question without a single number. No argument, no spreadsheet, no inspector's signature — just a pin that drops with a soft clack, or one that stops dead. The drawing already decided; the gauge only says it out loud.

A drawing that ends in "the gauge drops" has done its whole job. The promise made to a stranger becomes a thing you can pick up and feel go.

Position locates a hole's axis. But what locates a curved surface that has no axis at all? One symbol does almost everything. Chapter viii — profile.