Straightness GD&T on long turned parts: surface element or derived median line?

What is the difference between surface straightness and median line straightness?

Surface element straightness limits each straight line you could draw along the outside of the cylinder, one line at a time, while derived median line straightness limits the bend of the axis made by joining the centers of every cross-section. The first is applied to a surface; the second is applied to a feature of size.

On a drawing, surface element straightness is shown by pointing the leader of the feature control frame at the surface, or at an extension line of the surface in the profile view, with no diameter symbol in front of the tolerance. Its tolerance zone is two parallel lines, separated by the tolerance value, in each plane that contains the axis. Every longitudinal element is checked on its own, and the elements do not have to be parallel to each other, so it catches a bow, a waisted or barrel-shaped diameter, and chatter along the length.

Derived median line straightness is shown by placing the frame under the size dimension or attaching it to the dimension line, with a diameter symbol in front of the tolerance. Its zone is a cylinder whose diameter equals the tolerance, and the thing that has to fit inside it is the derived median line: the line through the centers of the actual local cross-sections. Because a barrel or waist shape leaves the centers in a straight row, this control does not see those shapes; it sees bend.

ISO 1101:2017 draws the same distinction under different names, straightness of a line on a surface and straightness of an extracted median line, and uses similar cues: an arrow in line with the size dimension points to the median line, and the diameter symbol in front of the value makes the zone a cylinder. A frame that carries the diameter symbol but has its leader pointing at the surface is internally contradictory, and a supplier should query it before quoting rather than guess which one was meant.

Why does Rule #1 decide which straightness GD&T callout a long pin needs?

Under ASME Y14.5, a diameter that carries only a size tolerance must fit inside a perfect cylinder at its maximum material size, so the size tolerance already limits bow. On a long pin with a tight diameter that limit can be stricter than the function needs, and median line straightness is the tool that relaxes it.

Rule #1, the envelope principle, says that when a feature of size is produced at its maximum material condition it must have perfect form. A pin turned exactly to its upper limit therefore cannot be bent at all, and a pin at its lower limit may bend only by the difference between its actual size and the upper limit. Adding surface element straightness tightens this further but can never loosen it, which is why a surface straightness tolerance larger than the size tolerance adds nothing on an ASME drawing.

That is the trap on long, slender parts. A designer calls a tight diameter for a sliding fit, and does not realize that the same tolerance has quietly become the straightness limit over the whole length. Free-machining bar, released from its residual stress as material comes off, and a slender workpiece pushed away by the cutting force both tend to leave a small bow that a short part would never show. Under Rule #1 every one of those pins is a reject, even if it would assemble and work.

Derived median line straightness with a diameter symbol is the exception the standard provides. It turns off the envelope for that feature and replaces it with a virtual condition: for a pin at MMC, the maximum size plus the straightness tolerance. The mating hole is then designed around that virtual condition, and the pin may be as bowed as the tolerance allows while still meeting its tight local size at every cross-section.

ISO drawings work the other way round. ISO 8015:2011 makes independency the default, so size and form are separate requirements unless the size carries the envelope symbol, a circled E. An ISO drawing of a long pin with only a size tolerance and no straightness callout therefore controls bow only through whatever general tolerance note it cites, which is why the ISO 2768-2 classes below matter more than many designers expect. The guide to ASME Y14.5 vs ISO GPS covers the default rules in full.

What straightness applies when an ISO drawing gives none?

If the title block cites ISO 2768-2, its general geometrical tolerance class sets the straightness of every feature without an individual callout. The allowance grows with length, so a long pin on a class L drawing may bow far more than its designer assumed.

ISO 2768-2:1989 has been withdrawn and replaced by ISO 22081:2021, which handles general geometrical specifications differently, but drawings that cite classes H, K or L are still common and are read the way they were written. On a new drawing, a long pin whose straightness matters should get its own straightness frame rather than rely on either general note.

The table reproduces the straightness and flatness column of ISO 2768-2. The length is the length of the line being assessed, not the overall part length, though on a plain pin they are the same. Read it against your own part: a pin in the 100 to 300 mm range on a class K drawing is allowed considerably more bow than its diameter tolerance suggests, per ISO 2768-2:1989. The broader use of the general tolerance note is covered in the ISO 2768 and NADCA tolerance guide.

Why do long pins pass straightness after turning and fail after plating?

Plating itself rarely bends a steel pin, but the steps around it can cause rejects: handling and tumbling of slender parts, and plating build-up that shrinks the room left inside the size envelope. The drawing has to say at which stage straightness is measured, including after any post-plate bake.

Thin electroplated coatings such as zinc to ASTM B633 add thickness on every side of the diameter, so a pin turned near its upper limit can end up at or over the maximum material size after plating. Under Rule #1, that leaves almost no room for bow, and a part that was legitimately bent within the envelope before plating fails after it with no change in shape. Derived median line straightness at MMC is far less sensitive to this, because the gauge works at virtual condition rather than at the bare maximum size.

The other causes are mechanical. Slender pins processed in bulk can bend each other, so a part that cannot tolerate that needs a handling note on the drawing or purchase order. Hardened steel parts that need a hydrogen embrittlement bake after zinc plating go through one more heat cycle after the last measurement point, so if straightness is critical, state that it applies after the bake; the hydrogen embrittlement baking guide covers when that bake is required. Start from bar bought to a known straightness: cold-finished steel bar to ASTM A108 carries its own permissible straightness variation, and that is the starting point for any part turned from it.

Write the stage into the note, for example “dimensions and straightness apply after plating” or “straightness before plating; size after plating”, so turning and finishing are held to the same target.