Form and position controls in GD&T: Are “tolerances of form and position” typically not used to control straightness, or is straightness itself a form tolerance?

Introduction / Context:Geometric dimensioning and tolerancing (GD&T) classifies controls into form, orientation, location, and runout. Understanding which group each control belongs to is fundamental for specifying and inspecting parts effectively.

Given Data / Assumptions:

• Straightness is a form control.
• Form tolerances (straightness, flatness, circularity, cylindricity) do not require datums.
• Position, orientation, and runout controls may reference datums to define relationships among features.

Concept / Approach:The statement claims form/position tolerances are typically not used to control straightness. This is incorrect because straightness is specifically a form tolerance and is directly used to control the shape of an axis, line element, or surface generator without needing datums. Position is a different category, but the question lumps “form and position” and denies their use for straightness, which misstates GD&T fundamentals.

Step-by-Step Solution:Identify straightness as one of the four basic form controls.Recognize that GD&T symbols explicitly include straightness to control form.Compare this reality with the claim that form/position are not used for straightness.Conclude the statement is incorrect.

Verification / Alternative check:Any GD&T symbol chart lists straightness under form, confirming its role as the direct method to control straightness.

Why Other Options Are Wrong:“Correct” contradicts definitions. “Only controlled by size limits,” “perpendicularity,” or “datumed features” misidentify the correct control category.

Common Pitfalls:Confusing straightness (form) with perpendicularity (orientation) or position (location).

Final Answer:Incorrect