Locate on the datums the part is toleranced to
A part drawing defines a datum reference frame — the features from which every other dimension is controlled. A fixture that locates on different features than the drawing does inherits the tolerance between them, and that inheritance is invisible until parts start failing at a downstream station.
The rule is simple and frequently violated: locate on the drawing datums wherever physically possible. Where it is not possible, calculate what the substitution costs and record it in the design file. That number becomes part of the fixture's specification rather than a surprise during production.
Both stacks, always
We run every critical dimension twice.
- Worst case. The arithmetic sum of every contributing tolerance at its extreme. It is conservative and often uncomfortable, but it tells you what is physically possible. If a safety-critical or interference condition fails worst case, the design is not acceptable regardless of statistics.
- Root sum square. The statistical combination, valid where contributors are independent and roughly normal. It reflects what you will realistically see in production, and it is what capability studies will measure against.
The gap between the two numbers is the design conversation. A large gap means the design depends on statistical luck; a small gap means the stack is dominated by one or two contributors, which tells you exactly where to spend money on tighter tolerances.
What gets left out
Stacks that only include component tolerances are incomplete. In practice the contributors that get missed are the ones that dominate:
- Clamping deflection — the part moves when you clamp it, especially thin-walled or compliant components.
- Thermal growth — an aluminium fixture in a shop that swings fifteen degrees between night and day shift is not dimensionally constant.
- Locator wear — pins wear, and the stack at month twenty-four is not the stack at commissioning.
- Chip and debris accumulation — the most common cause of a fixture that was capable at FAT and is not capable in production.
- Robot repeatability, where the fixture is loaded automatically — the arm's specification is repeatability at the flange under rated conditions, which is not the same as accuracy at the workpiece.
Design in the recovery
Locators should be replaceable without re-machining the fixture body, and their wear should be measurable with a gauge rather than inferred from part failures. Include a datum check feature that lets an operator verify the fixture in under a minute at shift start. That single feature converts a class of intermittent quality problems into a scheduled check.
Close the loop with measurement
A tolerance stack is a prediction. Validate it with a repeatability study on the actual fixture and actual parts before it goes into production. Where the measurement disagrees with the calculation, the calculation missed a contributor — and finding out which one is worth considerably more than the study cost.
Talk to us about this
If this is relevant to a line you are working on, our engineers are happy to look at the specifics. Related capability: Mechanical Engineering. You can also browse delivered projects, the technology stack or contact the engineering team.