True Position

The position and location tolerances of GD&T are the controls that place a feature where it must be — and of the family, true position is the one that carries the machining world. Where a form tolerance judges a feature’s shape alone and an orientation tolerance holds a feature’s angle to a datum, a location tolerance fixes a feature’s place: where its axis, its centre or its median points must sit relative to the datum reference frame. Position, concentricity and symmetry form the location family, and they differ from every other family in a way that matters on the machine: their tolerance zones are not free to float but are anchored, locked in space by the drawing’s basic dimensions, so that reading a position call-out is reading an exact target that the feature must hit. This entry sets out the location tolerances, with true position — the control behind every bolt pattern and precision bore — at their centre.

Location is a fixed target

The family’s defining idea is that the zone is fixed in space, and the difference from orientation is the whole of it. An orientation zone floats — the two parallel planes hold the feature at the right angle to its datum wherever the feature happens to be — because orientation controls only tilt. A location zone cannot float: true position locks the zone at the exact place the feature is meant to be, measured from the datum frame by dimensions that carry the basic-dimension box, a rectangle drawn around the number. A boxed dimension has no tolerance of its own; its tolerance is the width of the position zone. So where the ordinary dimensioned drawing allows a feature to drift within its plus-and-minus square, and the orientation drawing allows it to tilt, the location drawing says: this feature’s true position is here, at exactly these distances from these datums, and its axis or centre must fall within the zone drawn around that point.

The true position call-out

A true position call-out is read from its feature control frame, the package this wiki’s GD&T entry describes. The first compartment carries the position symbol, a circle crossed by two diameters. The second carries the tolerance — almost always with a diameter symbol in front of it, because the zone for an axis is cylindrical, a round zone around the true position, and the tolerance is its diameter; the compartment may also carry a material-condition modifier, the most important being the maximum-material condition, which grants a bonus of tolerance as a hole is made larger than its least-material limit, since a clearance hole that is bigger genuinely needs less accuracy. The later compartments name the datums the true position is measured from. A call-out of position, a diameter, a tolerance and datums A, B, C reads plainly: the axis must lie within a cylindrical zone of that diameter, centred on the true position located by the boxed dimensions from the datum frame that A, B and C establish.

Why the round zone

True position uses a round zone because that is what a feature actually works against, and the round zone is what makes the control generous rather than harsh. A conventional plus-and-minus location draws a square zone around the true point: the hole may drift by the tolerance in X and the same in Y, but a feature off in both directions at once sits at the zone’s corner, further from the true point than the tolerance suggests, and is rejected even where it would fit its mating pin. The cylindrical zone of true position grants the full tolerance in every direction, so a hole on the diagonal is as acceptable as one straight out — a zone that matches how a round pin fits a round hole. The designer chooses the tolerance the assembly needs and lets the round zone spend it fully.

Position in the making

On the machine, a position tolerance is a demand about relationship: the feature must be cut where the drawing says it is, measured from the datum frame the drawing names. A bolt pattern called out to true position is made by machining all its holes in one setup, located from the same datums the drawing uses, so the pattern holds together and the part sits true to its datum — features cut in separate setups, each re-located from scratch, drift, and the position zone has no room for drift. The holes themselves, whether drilled, reamed or bored, are the work of this wiki’s drilling and holemaking; their position is the work of the setup. That is why the location discipline this wiki treats under datums in setup is inseparable from true position: the drawing’s datum reference frame and the fixture’s locators must be the same three surfaces, or the machine faithfully cuts the feature at the wrong true position.

Concentricity and symmetry

The family’s other two members, concentricity and symmetry, share position’s concern with location but judge it through the feature’s median points rather than its axis or centre, which makes them demanding and, in modern practice, rare. Concentricity keeps the median points of all the opposed elements of one feature — the midpoints of a rotating surface’s opposed sides — within a cylindrical zone about the datum axis, so that the feature is balanced about that axis in a stricter sense than runout checks; symmetry does the same for opposed points about a datum plane, holding two features or the two sides of one feature equally disposed about it. Both are measured by deriving median points from measurements all around the feature, on a roundness machine or a CMM, and both are expensive to satisfy and verify, which is why designers reach for them only where balance or equal disposition genuinely governs function, and why this wiki’s entry on runout, the everyday control of rotating truth, treats the control that most parts actually use.

The location of the feature is the plan

The location tolerances are read by the machinist as the drawing’s most binding instruction, because a located feature has no slack in space. Orientation can be met wherever the feature sits; form is met by shape alone; but position names a target and a round zone, and the whole job of setup and cutting is to put the axis or the centre inside that zone, measured from the datums. Reading the call-out is reading the plan: the boxed dimensions name the target, the datum letters name the surfaces to locate on, the diameter names the room the feature has, and the bonus, if any, grants more as the hole grows. The measurement then checks the plan — the inspector finds the feature’s actual centre relative to the datums and asks whether it fell in the zone, in the GD&T language the drawing speaks. Made to true position and proved against it, a feature is not merely near where it should be: it is where the drawing says it is, within the zone that honest assembly demands.