What is parallelism?

Parallelism is an orientation tolerance, shown as ∥ in the feature control frame, that refers to at least one datum. For a surface, the tolerance zone lies between two parallel planes the tolerance value apart, parallel to the datum plane. For an axis, the shape of the zone depends on the indication: with ⌀ in front of the value it is a cylinder parallel to the datum, otherwise two parallel planes. The requirement limits the orientation of the feature, not its distance from the datum; that distance is set by a size or position tolerance.

What does the standard say about parallelism?

The symbol, the zones and the datum reference are defined in ISO 1101; how a datum is derived from a real surface is set out in ISO 5459. Under those rules a parallelism requirement without a datum letter is incomplete. Because the zone covers the whole surface, it also limits that surface's flatness to the same value; a separate flatness requirement only adds something when it is smaller. Where there is no individual requirement but the drawing invokes ISO 2768-2, general parallelism follows from the size tolerance or the general flatness or straightness tolerance, whichever is greater. ASME Y14.5 treats parallelism the same way, as an orientation tolerance with a datum.

How does parallelism work in practice?

Two parallel faces on opposite sides of a plate are usually made in two setups. After turning the part over, it sits on the first face, and the flatness and seating of that face help decide how parallel the second side comes out. Thin or long parts can bow after unclamping as residual stress is released. Whether a tight value is reached by milling, by a light skim after turning over, by stress relieving in between or only by surface grinding depends on material, wall thickness, workholding and the value itself. Features on the same side that must be parallel to each other, such as two bores in one block, are usually made in one setup. Measurement is with a dial indicator on a surface plate or on a coordinate measuring machine, with the datum plane as reference.

What is parallelism often confused with?

Parallelism is not flatness: flatness is a form tolerance without a datum, parallelism an orientation tolerance with a datum that also covers flatness. Nor is it a size tolerance on the thickness. With 20 ±0.1 under the ISO 8015 independency principle, every local thickness may lie between 19.9 and 20.1 mm, even if the plate is 19.9 mm at one end and 20.1 mm at the other. A parallelism of 0.02 to the underside rules that out.

How do you see parallelism in Blake?

In Blake you enter the setups per route and accept or undo them; setup is its own line in the estimate. If you subcontract surface grinding, it comes from your subcontract library as a separate line; if its price is missing, that line stays open. The result is labelled as an estimate. More on estimating.