Why is Parallelism used?
Parallel mating faces can bind a slide or create an uneven gap even when each face is individually flat.
ORIENTATION CONTROL / GD&T SYMBOL GUIDE
PARALLELISM
Parallelism controls the orientation of a plane or axis relative to a reference datum. Depending on the controlled feature, its zone can be the space between two planes parallel to the datum, the area between two parallel lines, or a cylinder whose axis is parallel to the datum. The example below controls a surface between two planes.
INTERACTIVE APPLICATION
GD&T parallelism example: upper plate face
NO SEPARATE CONTROL
The top face tilts relative to datum A
- No separate orientation zone is stated relative to A.
Limitations
- Parallel mating faces can bind a slide or create an uneven gap even when each face is individually flat.
- A slide may bind or wear unevenly.
PARALLELISM CONTROL
The top face stays parallel to datum A
- The illustrated surface must remain between two planes 0.2 mm apart and parallel to datum A.
- For an axis application, the derived axis must stay inside its specified 0.2 mm zone parallel to datum A; its frame must identify the zone shape.
Advantages
- The mating gap remains more consistent.
- The drawing states the intended requirement for inspection and assembly.
Where is Parallelism used?
Plate faces, slideways, guide rails, spacers and opposed bearing surfaces.
Steps to add parallelism tolerance
Parallelism sets orientation relative to datum A, not the size of the plate. The controlled surface must fit between the two parallel tolerance-zone planes shown below.
Two planes 0.2 mm apart and parallel to datum A enclose the whole controlled face; the zone may translate but cannot tilt relative to A.
On the drawing
- Select the face or feature that will act as the datum and attach a datum feature symbol.
- Determine the acceptable parallelism tolerance for the application.
- Attach a feature control frame to the surface or axis being controlled:
- parallelism symbol in the first box
- tolerance value in the second box
- datum reference in the final box
Parallelism limits orientation relative to a datum; it does not control feature size. Here, the 0.2 mm zone applies to the upper surface, and the boxed A identifies the lower datum feature.
Commonly defined variable values
The controlling value is a tolerance-zone width, not a universal process grade. For this parallelism example, the drawing states the controlled feature and applicable extent.
Scroll the table sideways to see every column.
| Application | Illustrative starting value | Example feature | Notes / caution |
|---|---|---|---|
| Machined plate faces | 0.05–0.20 mm | 200 mm plate | Ensure datum A is stable. |
| Sliding guide face | 0.02–0.10 mm | 300 mm rail | Consider full travel. |
| Precision spacer | 0.01–0.05 mm | 100 mm spacer | Check both faces' form. |
These are planning examples, not universal rules or guaranteed process capabilities. Set the actual tolerance from function, size, length, process capability, inspection method and the standard specified on the drawing.
How is parallelism measured in real life?
Establish datum A on the lower face, then measure the indicated face relative to that plane.
- Surface plate + indicator: Seat datum A on a suitable simulator, then sweep the controlled face without re-levelling it.
- Height gauge: Sample a grid across the face while the datum is established on the inspection plate.
- CMM: Establish datum A first, then evaluate the face against a zone oriented to it.
ASME Y14.5-2018 (R2024)
Under ASME Y14.5, the datum feature simulator establishes A. Surface parallelism constrains orientation and form, but not location.
ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.
LEARN BY DOING
Practise GD&T on engineering drawings
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