± Plus/minus tolerance
Plus/minus tolerance · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “50 ±0.1” states a plus/minus tolerance requirement.
What controls
The classic way of expressing the permissible variation of a linear or angular dimension by stating the nominal value and the permitted deviations. It defines an acceptance zone in a single measurement direction, that of the dimension itself, and therefore does not replace geometric tolerances: it says nothing about form, orientation or combined location. When it is used in two directions to locate a hole, the resulting zone is a rectangle, with the consequence that the corners allow larger deviations than the sides, which rarely makes functional sense.
How to verify
An instrument suited to the size of the tolerance: a caliper for wide tolerances, a micrometer or dial indicator on a stand for tight ones, and limit gauges in series production. The practical rule is that the resolution and uncertainty of the instrument must be a small fraction of the tolerance; measuring a tolerance in hundredths of a millimeter with a caliper is not a valid measurement. For temperature-sensitive dimensions, the part must be thermally stabilized.
Workshop guideline values
- General
- Tolerances of ±0.1 to ±0.3 mm on ordinary machined dimensions with no special requirements.
- Accuracy
- Tolerances of ±0.02 to ±0.05 mm on carefully machined fits and mating features.
- High precision
- Tolerances of ±0.005 to ±0.01 mm, which require grinding, temperature control and suitable instruments.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Locating a hole with tolerances in two directions and believing the acceptance zone is circular: it is a rectangle.
- Measuring a tight tolerance with an instrument whose uncertainty is of the same order as the tolerance.
- Considering the part's function controlled by dimensional tolerances when what is failing is the geometry.
How to get it in the workshop / if it comes out of tolerance
It coexists with geometric language on almost every drawing: dimensional tolerancing controls size and geometric tolerancing controls where and how. If a dimension has no explicit tolerance, the drawing's general tolerance applies, and that must be checked before manufacturing. Bear in mind that aiming for the middle of the tolerance is not always best: on clearance holes it pays to work toward the side that gives more assembly margin.
Parts drifting progressively toward one end of the tolerance: that is tool wear, and it is corrected with scheduled offset compensation, not by waiting for a reject. Wide scatter with no trend: look at workholding, coolant and rigidity. And if the dimension is met but the assembly does not go together, the problem is not dimensional but geometric: position or orientation needs to be controlled.
Related symbols
Other symbols in Dimension indicators
Related tools
ISO fits
Calculates indicative ISO 286 tolerances for shaft and hole, H7/g6, H7/h6, H7/k6 and H7/p6 fits with clearance, transition and interference graph.
Inspection Plans
Upload a plan, extract dimensions and GD&T with AI and export your inspection guideline in PDF or CSV.
Shop trigonometry
Workshop trigonometry: right triangle, C chamfer, Z countersink, sine bar, taper and polar→X/Y for CNC operators.