Chapter viii · The Universal Control & Rotation
There is one symbol in the whole language that can replace almost all the others. Most engineers are a little afraid of it — and they should be.
Every control so far has been a specialist. Flatness watches a plane. Perpendicularity watches an angle. Position watches the axis of a hole. Profile watches the shape itself — the whole surface, point for point, against the ideal.
The ideal is the true profile: a contour defined entirely by basic dimensions, the boxed, exact numbers that carry no tolerance of their own. Profile then wraps that true profile in a uniform band of allowed deviation. Profile of a surface ( ) does this in three dimensions, over the entire face. Profile of a line ( ) does it in two, cross-section by cross-section.
That band is why profile is the universal control. Give it datums and a single callout governs the surface's form, orientation, and location at once — the synthesis of every family you have met so far. The one thing it still can't do is locate a feature-of-size axis or centre-plane; that remains position's job.
A turbine-blade batch once shipped whose airfoils each passed every flatness, angle, and location check in isolation — but the blended shape between them was never bounded as a whole, and the assembled rotor failed its flow test. One profile-of-a-surface callout to the right datums would have caught it.
No datum
Form + orientation of the surface, relative to nothing. The band floats with the part.
Referenced to A·B
The same 0.8 band — now locked in space. One callout: form, orientation, and location.
Drag the surface below. The dashed line is the perfect basic profile; the two rails are the band. Notice profile has no axis and no feature of size — it judges the surface, and it doesn't care which direction an error runs.
- Total zone
- 0.80 mm
- Outboard side
- 0.40 mm
- Inboard side
- 0.40 mm
- Controls
- form + orientation
By default the band is equally disposed — bilateral, half outboard and half inboard of the true profile. Flip the toggle to Ⓤ unequal and watch the whole band slide off the surface to one side: that is the unequally-disposed modifier doing its work, and it's the single most misread feature of profile.
The Ⓤ modifier — and the second number that comes with it
The default profile zone straddles the true profile evenly. The unequally-disposed-profile modifier Ⓤ changes that. It is written after the tolerance and is followed by a second value giving how much of the total zone lies on the add-material (outboard) side.
So profile 0.4 Ⓤ 0.1 means: total band 0.4 mm, of which 0.1 mm sits outboard and the remaining 0.3 mm inboard of the true profile. The basic profile no longer runs down the middle of the zone — it sits 0.1 from one rail and 0.3 from the other.
Two glyphs people confuse with Ⓤ are worth nailing down: the dynamic-profile modifier is a triangle, △ — not a circled-D — and the continuous-feature modifier is a circled "CF", not a circled-C. And note the edition trap: the Ⓤ modifier itself only arrived with Y14.5-2009 — pre-2009 prints expressed unequal disposition with phantom lines, a different convention entirely, so an old drawing and a new one can mean different things by "unequal."
For the advanced reader → Why profile can replace almost everything — but isn't position
Because the true profile is built from basics and the band controls every point of the surface against it, a datum-referenced profile callout simultaneously bounds form (the surface can't wander within the band), orientation (the band is fixed in attitude to the datums), and location (the band is fixed in position too). One symbol, three families. That is the synthesis the chapter promised.
But profile and position are not interchangeable. Profile controls surfaces; position controls features of size — the axis or centre-plane of a hole, a pin, a slot. A bolt cares about a hole's axis, which is position's job; a mating face cares about the surface, which is profile's. Our bracket's mounting face is exactly this kind of surface: profile, referenced to A and B, locks the whole face flat, square, and in the right place against the mating plate in one stroke — where it would otherwise take flatness plus perpendicularity plus position. The myth that "profile only controls form" is half the error here; the other half is "profile and position do the same thing." They cover different geometry, and the best drawings use each where it belongs.
Read the Band
A surface is profile-of-a-surface 0.4 with NO datum references. What does that 0.4 band control?
A frame reads profile 0.4 Ⓤ 0.1. How is the 0.4 band split around the true profile?
On a 2024 drawing a triangle (△) is attached to a profile callout. What is it — and what is it NOT?
Profile is the language folding back on itself: one band, defined by exact numbers, that can say what an entire toolbox of other symbols would otherwise have to say in pieces.
It is feared because it is powerful — and power, on a drawing, is just responsibility you can't hand off. Spend it well.
Everything so far held still. But a driveshaft doesn't. What happens when the part spins? Chapter ix — runout.