Guides / Quality

Tolerance Stack Up on Assembled Jigs and Fixtures

September 24, 2026

Every part measures in tolerance. The assembly is still out of position. This is not a supplier failure, it is arithmetic that nobody did.

What stacks

Take a simple test jig: a base plate, a spacer, a nest and two dowels. The position of the device seen by the contactor depends on the base plate thickness, the spacer thickness, the nest pocket position, both dowel hole positions on two parts, the clearance in each dowel fit and the flatness of every joint face.

Give each of those a comfortable tolerance and the total can exceed what the function allows, even though every individual part passes.

Where to spend tolerance

Three rules cover most fixtures.

Spend it on what locates, not on what covers. Dowel positions, nest pockets and mounting faces earn tight tolerance. Clearance holes, outlines and cosmetic faces do not.

Reduce the number of joints in the chain. Every interface adds its own error. A nest machined into the plate rather than bolted onto it removes two contributors at once, and often costs less to make.

Use one datum scheme across the family. When every part in the stack references the same scheme, errors add predictably. When each drawing invents its own, they add in ways nobody can compute.

Say which requirement governs

The assembly drawing should carry the functional number: the position of the device relative to the contactor, the parallelism of the seating surface to the base, whatever the fixture exists to hold. Part tolerances are then derived from it and should be written as derived, so a shop that suggests an easier route has something to check its idea against.

Without that, part drawings are tightened defensively. Defensive tolerancing is the single most common avoidable cost on fixture work, and it is covered from the price side in what drives the cost of a semiconductor test jig.

Flatness is part of the stack

Two flat faces bolted together transfer their error into the joint. A plate that is flat free state and bowed when bolted has moved the whole stack, which is why the measurement condition belongs on the drawing. That argument is in flatness, parallelism and datums.

Bolt torque matters for the same reason. If the assembly is measured torqued, say so and give the value.

A practical method

1. Write the functional requirement on the assembly drawing
2. List every contributor in the chain, including fits and joint faces
3. Add them worst case for a fixture programme
4. If the total exceeds the requirement, remove a joint before tightening a tolerance
5. Only then tighten, and only the contributors that actually dominate
6. Decide what is measured on the assembly rather than only on the parts

Step four is the one buyers skip. Removing a joint is usually cheaper than tightening two tolerances, and it makes the fixture easier to rebuild after wear.

Specify the assembly inspection

Say what is checked with the fixture assembled: the functional position, the seating flatness, or a fit check with a gauge or a real device. Parts inspected individually and shipped in a box prove nothing about the stack. Where an assembly check is needed, say who does it and what is recorded, following what a useful first article inspection report contains.

If you are quoting a fixture family, send the RFQ with the assembly drawing included, not only the part drawings. Shops quote the stack far better when they can see it.

Frequently asked questions

More on quality

• Drawing Mistakes That Make a Part Unquotable
• What a Useful First Article Inspection Report Contains
• Flatness, Parallelism and Datums on Semiconductor Plates
• Outgassing, RGA Testing and What a Buyer Can Actually Specify

All guides and the sourcing glossary.