Troubleshooting: Supplier Inability to Hold Tolerances

A stamping part that misses spec does not fail because the machine is broken. It fails because something in the process chain drifted, or the specification was too tight for the process as specified. This guide walks through the common symptoms, the likely causes, and the fixes engineers use when a stamping supplier cannot hold tolerance.

What does a tolerance failure actually look like

Before opening a CAPA or a supplier scorecard, define the failure mode. A part is not “out of tolerance” in one way. It is too long, too short, too thick, too thin, off center, or warped. The symptom tells you where to look.

• Dimensional drift over a production run
• Repeated out-of-spec measurements on the same feature
• First piece in spec, later pieces out
• Pass on one shift, fail on the next
• Pass on one press, fail on another
• Pass on one material lot, fail on the next
• Pass at room temperature, fail after heat exposure

Each symptom points to a different layer of the process. The table below maps the most common symptoms to the causes engineers typically find and the first fixes to apply.

Symptom	Likely cause	What to do
Gradual drift over a production run	Tool wear, die set misalignment, or lubricant change	Inspect die edges, check punch land, review lubrication records
First piece good, later pieces off	Press tonnage, back pressure, or material springback variation	Verify press settings, pull material certs, check springback compensation
Pass on one shift, fail on the next	Operator setup change, unrecorded tool adjustment	Freeze setup parameters, require signed setup sheets
Pass on one press, fail on another	Press accuracy, bed flatness, or tooling fit	Run gauge checks on each press, verify tooling seating
Pass on one material lot, fail on the next	Material hardness, coating, or gauge variation	Require material certs, check lot traceability, review coating spec
Pass at room temperature, fail after heat	Thermal expansion or coating stress	Specify operating temperature range, review coating and heat treatment specs

The table is a starting point, not a verdict. The next step is to isolate the variable.

How to separate tooling from material from setup

The fastest way to diagnose a tolerance issue is to control variables. Change one thing at a time and measure the effect.

1. Run the same tooling, the same material lot, and the same press settings on the same press. Measure the first ten and the last ten. If the drift is inside the tooling or setup window, the issue is not material.
2. Swap the material lot while keeping the tooling and press identical. If the part moves, the material or coating is the driver.
3. Move the same tooling and material to a different press. If the part moves, the press or tooling fit is the driver.

This isolates the problem without guessing. It also creates a record that is useful in vendor qualification. If you are evaluating a new supplier, these tests belong in the qualification package. They show whether the supplier can hold tolerance under controlled conditions.

What to check in the tooling itself

Tooling is the most common root cause of dimensional drift. A die that is worn, a punch that is chipped, or a set that is misaligned will produce a part that looks fine for a while and then drifts.

Check these items:

• Punch land condition. A worn punch land shortens the shear and changes the burr, which changes the dimensional stability.
• Die edge wear. A worn die edge produces a different fracture surface and can shift the part center.
• Die set alignment. A set that is not square or not level will produce a part that is off center or twisted.
• Die spring preload. If the die spring is fatigued, the set will not return to the same position after each stroke.
• Tooling mounting. A loose or shifted tooling plate changes the part position relative to the press.

If the tooling is worn beyond the supplier’s stated life, the fix is a tooling overhaul or replacement. If the tooling is within life but the part is off, the cause is usually setup, material, or press accuracy.

How material and coating affect tolerance

Material variation is the second most common cause. A part that holds spec on one lot can miss on the next because the hardness, the gauge, or the coating changed.

Material checks:

• Hardness. Softer material forms differently and springs back more.
• Gauge. A thicker or thinner sheet changes the bend and the flat dimension.
• Coating. A coating adds a layer that affects friction, formability, and the final surface dimension.
• Lot traceability. If the supplier cannot tie the part to a material lot, you cannot diagnose a failure.

Coating changes can be subtle. A different coating or a different coating thickness can alter the friction in the die and the final part dimension. If the specification includes a coating, the tolerance must be written with the coating in mind. A part that is in spec before coating can be out of spec after coating if the coating adds material to a critical surface.

Press setup and operator discipline

A press that is accurate in the shop can produce bad parts if the setup is not controlled. The operator changes the die height, the back pressure, or the lubricant, and the part moves.

Setup controls:

• Die height set and recorded.
• Back pressure set and recorded.
• Lubricant type and amount recorded.
• Press tonnage verified.
• Setup sheet signed by the operator and reviewed by the engineer.

If the supplier cannot produce a signed setup sheet for the last run, the tolerance problem is not just mechanical. It is a process control problem. The supplier selection process should require this level of documentation before the part goes into production.

How to structure a vendor qualification test

Vendor qualification is not a one-time inspection. It is a series of tests that prove the supplier can hold tolerance under real conditions.

A basic qualification package includes:

• Material certs for at least two lots.
• Tooling life records and any known wear.
• Press setup sheets for at least two shifts.
• Dimensional reports for at least two batches.
• A comparison of the first piece and the last piece in each batch.
• A review of the coating specification and any changes.

Run the qualification test on a small sample. If the supplier passes, move to a trial production run. If the supplier fails, document the failure mode and require a corrective action plan before the next run.

When to escalate to a root cause analysis

If the same part fails twice, or if the failure moves from one feature to another, the issue is not a one-off. It is a system problem.

Escalate when:

• The same part fails on two consecutive batches.
• The failure moves from one feature to another.
• The supplier cannot produce the setup sheets or material certs.
• The tooling is within life but the part still drifts.
• The press is accurate but the part still misses.

A root cause analysis should include a fishbone diagram, a 5 why analysis, or an equivalent method. The goal is to find the single cause that explains the failure, not to list every possible cause.

Prevention: how to keep the supplier in tolerance

Prevention is cheaper than rework. The supplier selection process should build in controls that catch drift before it becomes a nonconformance.

• Require material certs with lot traceability on every delivery.
• Require tooling life records and a plan for tooling overhaul.
• Require signed setup sheets for every run.
• Require dimensional reports for the first and last piece of each batch.
• Require a coating specification and a review of any coating changes.
• Require a corrective action plan for every nonconformance.

These controls are not optional. They are the minimum for a supplier that will hold tolerance on a production part.

How to write the specification so the supplier can hit it

A tolerance that is impossible to hit is not a supplier problem. It is a specification problem.

• Write tolerances that match the process capability.
• State the material and coating with enough detail to control variation.
• State the temperature range for the part in use.
• State the inspection method and the acceptance criteria.
• State the tooling life expectation.

If the specification is written for a process that does not exist, the supplier will not be able to hold the tolerance. The specification must be realistic for the process.

Final note on supplier selection

Vendor qualification is the bridge between a good quote and a reliable part. A supplier that can hold tolerance under controlled conditions is a supplier that can hold tolerance in production. The tests in this guide are not a formality. They are the minimum evidence that the supplier can deliver the part you need.