ST Statistical tolerance
Statistical tolerance · Modificadores de zona · ASME Y14.5
Example in plan
How to interpret it
On the drawing, “⌖ ⌀ 0.15 ST |A|B|C|” states a statistical tolerance requirement with a cylindrical tolerance zone of ⌀ 0.15 mm, measured from datums A, B, C in that order of precedence.
What controls
An indication that this tolerance has been calculated on the assumption that the process is stable and in statistical control, and is therefore only valid if the part is produced that way. It is not a free, looser tolerance: it is a tolerance that rests on demonstrated process capability, not on inspecting every part. On drawings it is usually accompanied by a second, tighter value, which is the one that applies if the supplier does not maintain statistical control; in that case the choice is not free — it depends on whether or not there is a control plan in force. What it does NOT mean is that out-of-tolerance parts can be shipped because the lot average comes out fine: every part still has to conform; what is added is a requirement on how the process behaves.
How to verify
Each part is measured as usual (CMM, functional gauge or whatever instrument suits the symbol); what changes is the acceptance criterion, which is tracked on a control chart with subgroups and frequency defined in the plan. You need a gauge R&R study of the measurement system beforehand: with an instrument that eats up a large share of the tolerance, the capability indices mean nothing. Results are analyzed by capability indices and for stability (trends, shifts, changes of shift or material lot), and kept as evidence for the customer.
Workshop guideline values
- General
- Capability on the order of Cpk ≥ 1.33 in ordinary series production.
- Accuracy
- Cpk ≥ 1.67 on key characteristics in automotive and safety components.
- High precision
- Cpk ≥ 2.0 on critical characteristics backed by 100 % inspection.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Taking it as permission to accept out-of-tolerance parts if the average comes out fine: every part must still conform.
- Using the statistical value without a control plan, control charts or a measurement system study, when those are exactly what enable it.
- Applying the criterion to a one-off part or a prototype: without a stable process, statistics are meaningless.
How to get it in the workshop / if it comes out of tolerance
It implies an SPC sampling plan and process capability. Do not accept or reject a single part on a statistical basis without the specified plan. On the shop floor it translates into control charts at the machine, an agreed sampling frequency and a defined reaction to a trend: if the characteristic is drifting towards a limit, the machine has to be corrected even though the parts still pass. Without that system set up and documented, this indication cannot be used and you have to go by the arithmetic value.
If capability drops, separate the two possible causes before touching anything: spread (the process varies too much) or off-centering (the process is shifted relative to the center of the tolerance). Off-centering is corrected with an offset adjustment and is cheap; spread forces you to look at tooling, workholding, temperature and material. If an out-of-control point appears, don't adjust the machine by reflex: first look for the cause, because reacting to random noise increases variation. And if the process cannot reach the required capability, apply the tighter arithmetic value and raise the issue: continuing to use the statistical tolerance without control is a nonconformance to the drawing.
Related symbols
Other symbols in Zone modifiers
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.