Nitinol tolerances cause more scrap than any other line on a drawing. The reason is simple. Engineers borrow tolerance blocks from stainless steel parts, and nitinol does not behave like stainless steel. It moves with temperature, it springs back after forming, and its useful properties are functional rather than dimensional.

This guide explains what to control, how to word it, and where OEMs commonly over-specify.

Why nitinol tolerances behave differently

Three effects make the difference.

Temperature sensitivity. The alloy changes phase near room and body temperature. A part measured at 20 degrees and the same part measured at 37 degrees can give different readings.

Springback. Superelastic material recovers almost fully after bending. Consequently the free-state geometry depends on the shape set, not on the forming tool.

Process coupling. Heat treatment sets both the geometry and the transformation temperature at the same time. Tighten one and you move the other.

Dimensional tolerances on wire and tube

Incoming stock is the easy part. Suppliers of drawn nitinol wire and tube hold diameter closely, and the tolerance is usually driven by cost rather than capability.

Still, state these clearly:

Diameter and its tolerance, plus ovality if it matters.
Surface condition as supplied, since oxide and drawing lubricant affect later steps.
Cold work level or temper, because it changes everything downstream.
Straightness over a defined length.

Material requirements themselves start with ASTM F2063. Reference it rather than rewriting it.

Tolerances on formed geometry

Once a part has been through shape setting, the rules change. Free-state geometry is a process outcome, not a machining result.

Therefore describe formed features in a way that can actually be measured:

Give a nominal and a band, never a bare nominal.
Say whether the part is measured free or constrained.
Define the datum on a feature that exists in the free state.
Use envelope or gauge acceptance for complex curves.

Optical comparators and vision systems suit these parts well. A go/no-go optical envelope is often faster and more repeatable than chasing coordinates.

Functional tolerances matter more than dimensional ones

This is the part most drawings miss. A localization wire or an anchor is bought for how it behaves, not for how it measures.

Control the behaviour directly:

Active Af, with a band, measured by bend and free recovery to ASTM F2082.
Plateau stress on loading and unloading, from a tensile test.
Deployment or recovery force, measured in a fixture that mimics use.
Permanent set after a defined strain.
Fatigue life where the device flexes repeatedly.

Ingot data measured by ASTM F2004 is useful for material release. It is not a substitute for testing the finished component.

State the measurement conditions

A tolerance without conditions is not a tolerance. Every nitinol drawing should say:

The inspection temperature, and the allowed variation around it.
Whether the part is free, constrained or loaded during measurement.
The soak time before measurement.
The measurement method, or an equivalent one.

Add those four lines and most tolerance disputes disappear before they start.

Where OEMs over-specify

Tight dimensions on non-functional features. Every tight band costs inspection time. Ask which features the device actually depends on, then loosen the rest.

Symmetric bands by habit. Processes rarely centre neatly. An asymmetric band that matches the real distribution yields far better.

Hundred percent inspection everywhere. Sampling with capability data is usually stronger evidence than inspecting every part badly.

Copying a stainless drawing. Nitinol tolerances need the temperature and free-state clauses that a stainless drawing never had.

How we inspect

We inspect in a temperature controlled area, using vision systems and optical comparators for geometry and dedicated fixtures for functional tests. Results feed the device history record, so any lot can be traced back to its material and process records.

Finishing happens before final inspection, because electropolishing removes material and shifts dimensions. Measuring before that step measures the wrong part.

Send us the drawing before you release it

The most useful review we do costs nothing. Send us a drawing and we will mark up which nitinol tolerances we can hold comfortably, which will drive scrap, and which should be functional rather than dimensional. That review is part of our normal nitinol processing quoting process under ISO 13485.

Contact our engineers and ask for a drawing review.