GlossaryKind of term: Manufacturing termStraightness
What is straightness?
Straightness is a form tolerance that limits how far a line on a surface, or the axis of a shaft or bore, may deviate from a straight line.
Straightness ⏤ is a form tolerance with no datum. For a line on a surface, such as a generator line of a cylinder or a line on a flat face in a stated direction, the tolerance zone lies between two parallel straight lines separated by the tolerance value. With ⌀ in front of the value, the requirement applies to the derived median line of a shaft or bore and the zone is a cylinder of that diameter. On a ⌀20 mm shaft 300 mm long with straightness ⌀0.03 mm, the derived median line has to stay within a ⌀0.03 mm cylinder, even when every measured diameter is within size.
What does the standard say about straightness?
Zone and symbol are defined in ISO 1101. Under the independency principle of ISO 8015 a size tolerance does not limit straightness; only the envelope requirement Ⓔ or a straightness requirement of its own controls the form. Under ASME Y14.5 a feature of size carries an envelope by default, so the standard the drawing refers to decides what a value means. An axis straightness can carry Ⓜ after the value; the permitted deviation then grows as the shaft departs from its maximum material size.
How does straightness work in practice?
On a long, slender shaft straightness is mostly about deflection during turning. Depending on the length-to-diameter ratio, the material and the tolerance, that calls for a tailstock centre, a steady or follow rest, lighter cuts or extra finishing passes. If the requirement is tighter than turning holds reliably, grinding between centres or centreless grinding follows, often subcontracted. After hardening or heavy roughing a shaft can come out bent and need straightening or re-machining.
What is straightness often confused with?
Straightness is not flatness: straightness limits lines, flatness the whole surface. Nor is the straightness of an axis the same as runout: runout is measured while the part turns about a datum axis, and it also picks up roundness and the position of the axis.
How do you see straightness in Blake?
A straightness tolerance from the drawing sits in Blake with the other tolerances as a proposal on the part, and you confirm or correct the value. If you subcontract grinding between centres, it enters the estimate as its own line from your subcontract library; while the price is missing, that line stays open. More on estimating.
Frequently asked questions
What is the difference between straightness with and without ⌀?
Without ⌀ the straightness applies to lines on the surface, with a zone between two parallel lines. With ⌀ it applies to the derived median line of the shaft or bore, and the zone is a cylinder around that line.
How do you measure the straightness of a shaft?
For surface lines, with a dial indicator along a straightedge, between centres or on a coordinate measuring machine. For the axis you need the centres of many cross-sections, so a coordinate measuring machine or form tester. In V-blocks, roundness shows up in the reading.
Does a shaft stay straight through hardening?
Not by itself. Hardening and released residual stress can bend a slender shaft. Straightening, grinding or re-machining follows, and that step belongs in the route and the estimate before you quote.