How to set critical-to-quality dimensions

What critical to quality means

A critical-to-quality dimension, usually shortened to CTQ, is a characteristic whose variation has a direct effect on whether the part works. It might be a bore diameter that controls a bearing fit, a hole position that decides whether a connector aligns, a plating thickness that controls corrosion or a flatness that decides whether a gasket seals.

The label is not a grade of difficulty. A dimension can be hard to measure and still be irrelevant to the function. Another can be easy to measure and decide the whole assembly. CTQ selection is about consequence, not about tolerance width or machining effort.

A CTQ list also has a commercial purpose. It tells the supplier where to spend inspection effort and where a process change needs approval. It tells the buyer which results to review in the first article and which to watch in production.

The list should be short. If every dimension is critical, the supplier has no priority and the inspection plan becomes a full layout by default. That is expensive and it dilutes attention from the few features that matter.

Start with the function, not the drawing

Write down what the part does in one sentence. A busbar carries current between two joints. A contact spring holds a force over a gap. A housing locates a terminal and keeps a creepage distance. The sentence should name the surfaces, forces and clearances that make the function possible.

Then list the ways the part can fail that function. The joint can overheat, the contact can lose force, the terminal can sit too far in or the housing can track across a surface. Each failure mode points to a characteristic that can detect it.

This order matters. If you start with the drawing, every dimension looks equally important. If you start with the failure, some dimensions become obviously central and others become obviously secondary. The drawing is the output of the analysis, not the input.

The failure list should include the assembly, not only the part. A hole that is in the wrong place may be a perfectly good hole. Its status as a CTQ comes from the mating pin, the joint and the clearance it has to preserve.

Select the characteristic that detects the failure

For each failure mode, choose a characteristic that changes when the failure is present. A joint that can overheat might be detected by hole position, contact area, plating thickness or bolt torque. The right characteristic is the one that is closest to the failure and practical to measure.

Prefer a characteristic that is measurable at the supplier and at goods-in. A dimension that can only be checked after assembly is a poor production control, even if it is functionally important. If the only available check is a functional test, say so and plan the test.

Avoid proxies that do not track the function. A surface finish callout is not a substitute for contact resistance if the joint depends on real contact area. A hardness callout is not a substitute for spring force if the spring geometry also varies. The characteristic should respond to the failure, not merely correlate with it in one batch.

The table below is an illustrative selection for a busbar. The point is the reasoning, not the particular part.

Give each CTQ a zone and a reason

A CTQ needs a tolerance that comes from the function. For a clearance hole, the zone comes from the fastener and the assembly allowance. For a seal face, the flatness comes from the gasket and the pressure. For a current path, the thickness comes from the temperature rise and the material.

Do not simply tighten the general tolerance. A direct tolerance should be calculated or justified, and the reason should be recorded. If the number is inherited from an older drawing, check whether the assembly still needs it. A tight tolerance with no reason is a cost with no benefit.

Where the function is geometric, use a geometrical control instead of a tighter linear tolerance. Position, profile, perpendicularity and runout describe the relationship that matters. A plus or minus value on every coordinate can be tighter and still allow the failure.

State the standard and the modifier. A position callout at MMC has a different acceptance boundary from one regardless of feature size. A profile callout with datums controls location as well as form. The CTQ is not complete until the control is unambiguous.

Choose the method and the sample

Every CTQ needs a method. A caliper is fine for a general length and wrong for a tight bore. A CMM is right for a hole position and unnecessary for a cosmetic edge. The method should have enough resolution and repeatability for the tolerance it checks.

Gauge R&R matters when the tolerance is tight relative to the process. If the measurement system uses a large share of the tolerance, a pass or fail decision is weak. For a CTQ, use a method with a known uncertainty and a fixture that repeats the datum scheme.

The sample plan should match the risk. A first article checks every characteristic. Production may check the CTQ on a defined frequency, with a reaction plan if a result drifts. The frequency should reflect the process stability and the consequence of a failure, not a habit.

Record the method in the inspection plan and the first article report. Name the instrument, the fixture, the datum alignment and the calibration status where relevant. A result without a method cannot be repeated or defended.

Put the CTQ into the control plan

A CTQ that is not in the control plan is a wish. The control plan should list each characteristic, the specification, the method, the sample size and frequency, the person responsible and the reaction plan.

The reaction plan is the part most often missing. If a CTQ result is out of limit, what happens to the batch? Is it quarantined, sorted, reworked or scrapped? Who decides whether a concession is possible? A control plan without a reaction is a measurement without a consequence.

The supplier should see the CTQ list at RFQ stage. It affects the process, the inspection time and the price. A supplier who discovers the critical calls after the first article may need a new fixture or a different machine, and the schedule will move.

Review the list when the design, the process or the supplier changes. A characteristic that was critical on the prototype may become stable in production. A new assembly may make a previously minor dimension critical. The list is a living document, not a one-time label.

• Name the characteristic and the failure it detects.
• State the specification and the reason for the limit.
• Name the method, fixture and datum alignment.
• Set the sample size and frequency.
• Write the reaction plan for an out-of-limit result.
• Review the list at every design or process change.

Common mistakes

The first mistake is marking every dimension as critical. It removes the priority and adds inspection cost without improving the part. The second is choosing a characteristic because it is easy to measure rather than because it detects the failure. The third is copying a CTQ list from a similar part without checking the new function.

The fourth is a tolerance without a method. The drawing says 0.05 mm but the bench has a caliper with 0.02 mm resolution and no fixture. The fifth is a method without a reaction plan. The inspector records a failure and the batch keeps moving.

The sixth is treating a CTQ as a one-time first article check. A characteristic that matters at launch matters in production. The seventh is failing to tell the supplier. A critical call that appears only in the buyer's internal spreadsheet is not a requirement.

Review the list with the supplier

A short review with the supplier is worth more than a long specification. Walk through each CTQ, the failure it prevents and the method you expect. Ask whether the supplier can measure it the same way and whether the process can hold it.

If the supplier proposes a different method, check that it measures the same requirement. A functional gauge and a CMM can both check a position callout, but they may not agree at the edge of the zone. Agree the method before the first article, not after a dispute.

Record the agreed list in the purchase order or the quality agreement. That turns a drawing note into a commercial requirement. It also gives both sides a reference when a batch is questioned.

The final test is whether the list changes a decision. If the supplier would do exactly the same thing with or without the CTQ label, the list is not doing any work. If it changes the process, the inspection or the reaction to a bad result, it is doing its job.