GlossaryKind of term: Manufacturing termGeometric tolerance (GD&T)
What is a geometric tolerance (GD&T)?
A geometric tolerance limits the form, orientation, location or run-out of a feature, in addition to its size, and sits in a feature control frame.
A geometric tolerance is a requirement on the geometry of a feature: the width or diameter of a zone within which a surface, an axis or a profile must lie. ISO 1101 groups the characteristics in four families. Form: straightness ⏤, flatness ⏥, roundness ○ and cylindricity ⌭. Orientation: parallelism ∥, perpendicularity ⟂ and angularity ∠. Location: position ⌖, coaxiality and concentricity ◎, and symmetry ⌯. Run-out: circular run-out ↗ and total run-out ⌰. Line and surface profile ⌒ ⌓ count as form without a datum and as orientation or location with one. The requirement sits in a feature control frame with symbol, value, any modifiers and datums.
What does the standard say about a geometric tolerance (GD&T)?
ISO 1101 sets the symbols, tolerances and zones, ISO 5459 the datums and datum systems, and ISO 8015 the fundamental rules, including the independency principle. ASME Y14.5, where the term GD&T comes from, applies different default rules, such as the envelope on features of size. A drawing should therefore state which standard applies. Without that reference a frame cannot be read unambiguously in every case, especially where modifiers and datums are involved.
How does a geometric tolerance (GD&T) work in practice?
Geometric tolerances often shape the route more than the dimensions do. Features with a tight orientation or location tolerance between them are kept in one setup where possible. Where the part has to be reclamped, the repositioning has to stay within the tolerance, through alignment, probing or workholding that repeats closely enough. Whether a 5-axis machine or a second setup is the better choice depends on quantity, access, workholding and rates. Inspection takes place on a coordinate measuring machine or on the surface plate, depending on the requirement and its zone; probing on the machine is an in-process check, not an independent measurement.
What is a geometric tolerance (GD&T) often confused with?
A geometric tolerance is not a tighter dimensional tolerance: one governs how big a feature is, the other where it lies or what shape it has. Under the independency principle they apply separately. And GD&T is strictly the name of the ASME system; ISO speaks of geometrical product specification, with partly different rules.
How do you see a geometric tolerance (GD&T) in Blake?
In Blake the geometric tolerances from the drawing sit with the part as proposals, ready to confirm or correct. The number of setups is your decision: you enter them per route, then accept or undo them. Setup counts as a separate line in the estimate, and the result is labelled as an estimate. More on the factory.
Frequently asked questions
Does every geometric tolerance need a datum?
No. Form tolerances such as flatness and roundness concern the feature itself and take no datum. Orientation and run-out tolerances need one. A position tolerance usually has datums; under ISO it may also apply to a pattern of features without one.
Do features with a position tolerance between them have to stay in one setup?
Not necessarily, but it is often the safest choice. After reclamping, the new position has to fall within the tolerance, through alignment, probing or precise workholding. Whether that works depends on the value, the workholding and the machine.