10.5 Profile Applications: Conicity, Extruded and Irregular Shapes, and Profile-Controlled Datum Features

10.5 Profile Applications: Conicity, Extruded and Irregular Shapes, and Profile-Controlled Datum Features

Quick Answer: Para. 8.4 covers what profile is actually applied to. Para. 8.4.1.1 treats coplanarity — two or more surfaces with all elements in one plane — as a single interrupted surface. Para. 8.4.2, Conicity, states that a profile tolerance may control the conicity of a surface in two ways: as an independent control of form, or as a combination of size, form, orientation, and location — and in each case the feature must still be within its size limits. Profile is also the standard means of defining an extruded (linear-swept) outline and any irregular shape that cannot be described by analytic geometry, and it is the normal way to satisfy the para. 4.9 requirement to control a complex datum feature directly.

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Conicity (Para. 8.4.2)

A cone is the most instructive profile application because it exposes the full range of what profile can control.

The standard's own two illustrations map onto the first and second rows: a conical feature controlled by a profile of a surface tolerance where conicity of the surface is a refinement of size, and the same control oriented to a datum axis. The recurring sentence is the one candidates skip: in each case, the feature must be within size limits.

Why this matters. Profile applied without datums to a cone does not replace the size dimension. A cone can be perfectly conical, perfectly formed, and still be rejected because its diameter at the gage line is out of tolerance. Profile refines; the size dimension still governs.

Recall the taper specification options of para. 2.13, which cross-reference this paragraph directly. Method (b) — a size tolerance combined with a profile of a surface tolerance applied to the taper — is method 8.4.2 seen from the General Tolerancing side, and method (d) is a composite profile tolerance.

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Extruded and Irregular Outlines

Where a part cross-section is swept along a straight line — an extrusion, a machined pocket of constant section, a stamped bracket outline — the true profile is defined by basic dimensions in the section view, and a profile of a surface tolerance controls the swept surface over the stated extent. Two mechanisms from para. 8.3.1 do most of the work:

• All-around (8.3.1.4): where a profile tolerance applies all around the true profile of the designated features of the part, in the view where it is specified. The symbol is a circle at the bend of the leader. It follows the 2D outline in that view — it does not wrap onto surfaces in other views.
• All-over (8.3.1.6): a profile tolerance may be applied all over the 3-dimensional profile of a part unless otherwise specified. The symbol is two concentric circles at the leader bend.
• Limited segment (8.3.1.5): where segments of a profile have different tolerances, the extent of each may be indicated by reference letters and the between symbol — for example A ↔ B.

For genuinely irregular shapes the boundary concept applies, and para. 8.8 combines profile with position so that the profile tolerance controls the shape and size of the feature while the positional tolerance establishes a theoretical boundary shaped identically to the true profile.

         ALL-AROUND (8.3.1.4)        ALL-OVER (8.3.1.6)       BETWEEN (8.3.1.5)
              ○                           ◎                       A ↔ B
        ┌────────────┐              ┌────────────┐          ┌──●──────●──┐
        │            │              │▓▓▓▓▓▓▓▓▓▓▓▓│          │  A      B  │
        │  2D outline│              │▓ every 3D ▓│          │ only this  │
        │  in THIS   │              │▓ surface  ▓│          │  segment   │
        │    view    │              │▓▓▓▓▓▓▓▓▓▓▓▓│          │            │
        └────────────┘              └────────────┘          └────────────┘

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Coplanarity as a Profile Application (Para. 8.4.1.1)

Coplanarity is the condition of two or more surfaces having all elements in one plane. A profile of a surface tolerance may be used where it is desired to treat two or more surfaces as a single interrupted or noncontinuous surface. In this case, a control is provided similar to that achieved by a flatness tolerance applied to a single plane surface.

The specification pattern is a profile feature control frame with no datum reference (when the requirement is purely that the pads be co-surfaced) or with datums (when the interrupted surface must also be located), accompanied by a note such as 2 SURFACES or a 2X designation. Flatness cannot do this job — flatness applies to one surface — which is why coplanarity is a profile topic rather than a form topic.

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Profile-Controlled Datum Features

Para. 4.9 requires datum features to be controlled directly by geometric tolerances or indirectly by dimensions such as size. For a complex or mathematically defined datum feature (para. 4.13), there is no size dimension to lean on and no simple form control that fits — so profile of a surface applied to the datum feature itself is the standard answer.

Two patterns recur:

1. Profile without datums on the datum feature: controls the datum feature's own form, so that the theoretical datum feature simulator has a well-formed surface to contact.
2. Profile referencing higher-precedence datums on the datum feature: controls the datum feature's form and its orientation or location relative to datums earlier in the precedence order. Datum feature B may be profiled to A; it may never be profiled to C.

Para. 8.4.1.1 supplies a third pattern that is easy to miss: interrupted coplanar surfaces may themselves serve as a datum feature, with the profile-controlled composite surface establishing a single datum plane from all of the pads together.