Ⓣ Tangent plane
Tangent plane · Modificadores de zona · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “∥ 0.1 Ⓣ |A|” states a tangent plane requirement with a tolerance of 0.1 mm, measured from datum A.
What controls
With Ⓣ, the tolerance is not evaluated point by point on the actual surface, but on the theoretical plane resting on its peaks: the contact plane that a rigid plate placed on top would touch. What must lie within the tolerance zone is that tangent plane, not every valley of the surface. The practical consequence is that recesses, pores and marks below the peaks no longer count, because they do not count in the real assembly either: what seats are the high points. What it does NOT do is control the form of the face: waviness or crowning of the surface is not governed by that frame and must be limited separately with flatness or profile if it matters. It is a modifier intended for orientation controls (parallelism, perpendicularity, angularity), not a substitute for a form tolerance.
How to verify
On a CMM, tangent-plane evaluation (an external contact fit, or a least-squares fit constrained to the material side, depending on the software) must be explicitly enabled instead of the default least-squares fit: with the same point cloud the two methods give different results, and the normal fit is always more severe. The equivalent physical check is to place a small granite plate, a surface plate or a ground gauge block on the face and measure the orientation of that plate relative to the datum with a dial indicator: the plate simulates the tangent plane. Probe with enough density to capture the peaks; if probing is sparse, the tangent plane rests on points that are not the highest.
Common mistakes when interpreting it
- Measuring with the software's least-squares fit and rejecting the part: Ⓣ requires evaluating the contact plane on the peaks, which is more lenient.
- Believing Ⓣ controls the form of the surface: it does not limit waviness or crowning, only the orientation of the plane resting on top.
- Applying it out of habit to any frame: the modifier makes sense on orientation controls referenced to a datum, not on a form tolerance with no reference.
How to get it in the workshop / if it comes out of tolerance
Used when the face will be bolted against another rigid part and what matters is how the assembly will seat, not the micro-geometry. It is common on cast faces, textured surfaces or surfaces milled with heavy tool marks, where without Ⓣ a single tool-mark valley could scrap a part that assembles perfectly well. On the shop floor it changes little about how you machine, but a lot about how you accept: do not argue rejections of a face with Ⓣ by looking at the lowest point.
If the face fails even when evaluated as a tangent plane, the problem is real orientation and is corrected like any other parallelism or perpendicularity failure: squareness of the setup, repeatability of the work zero and condition of the cutter. If it fails only with least-squares evaluation and passes as a tangent plane, it is not a process problem but an evaluation-method problem: correct the report. And if the tangent plane rests on three isolated high points (a burr, a ding, leftover scale), remove those defects before measuring again, because they alone are deciding the result.
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