Ⓟ Projected tolerance zone
Projected tolerance zone · Modificadores de zona · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The tolerance zone is projected in depth beyond the face on the drawing (Ⓟ symbol): typical on threads or pins to guarantee alignment along the whole functional length.
What controls
Moves the position tolerance zone outside the material, into the space where the part fitted into the hole (a stud, a bolt or a pin) will protrude, instead of evaluating it within the hole itself. The value that accompanies Ⓟ is the projection height, which normally matches the thickness of the mating part. It is the only way to control the lean of the fitted part: a tapped hole can be perfectly located at the face and, if it is tilted, make the bolt clash with the part above.
How to verify
On a CMM, the actual axis of the hole is measured by probing several sections at different depths to obtain its tilt, and that axis is extrapolated to the stated height P: the deviation evaluated is the one at that height, not at the hole mouth. In the shop, the direct method is to fit a calibrated pin or stud into the hole and measure its position at the free end, at the projection height. A functional gauge with pins of length equal to P does the check in one go on production parts.
Datums and notes
Datums are referenced just as in any position tolerance and are simulated the same way: what changes is where the zone lies, not what it is measured from.
Workshop guideline values
- General
- Projection heights of 8–20 mm, equal to the thickness of the part mounted on top.
- Accuracy
- Projected zones of 0.05–0.1 mm on bolted joints with little clearance.
- High precision
- 0.01–0.03 mm projected on assemblies with locating dowel pins or guides.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Confusing the value after Ⓟ with the tolerance value: the number following Ⓟ is the projection height in mm, not the size of the zone.
- Measuring the position at the hole mouth and accepting it: the projected zone is evaluated above, where the bolt sits, and there the tilt has already been amplified.
- Applying it to a simple clearance through hole, where it adds nothing: it makes sense on tapped or fitted holes that hold a protruding part.
How to get it in the workshop / if it comes out of tolerance
Mandatory on many drawings for tapped holes that receive a bolt: the P value gives the projection height. Prevents studs that "lean" out of the zone.
The cause is almost always tilt, not offset: the hole starts in the right place but runs off. Spot drill before drilling, use short or rigid drills, avoid entering on an inclined or irregular surface and check spindle perpendicularity to the table. In tapping, a tap that goes in forced or misaligned tilts the effective axis even if the tap drill hole was straight. The larger P is, the more any tilt is penalized: a small angular error is multiplied by the projected height.
Related symbols
Other symbols in Zone modifiers
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