Requirement Traceability: Where Did This Tolerance Come From?

The question nobody can answer

A cordless drill is in production. A new process engineer is reviewing the spindle line and notices that the bearing seat gets an extra finish grinding pass to hold a tight tolerance. It’s slow and expensive. Dropping it would save real money on every unit. So she asks: why is this tolerance so tight?

The drawing says “per specification”. The specification says “per design”. The engineer who designed the spindle moved to another team two years ago. Everyone suspects it matters; nobody can say how much, or to whom. So either the pass stays forever “just in case”, or it gets cut and the warranty returns tell you six months later that it mattered after all.

Requirement traceability is the ability to walk from that tolerance back to the customer need it serves — and, in QFD, to see how much of its importance each need accounts for.

Linked is not the same as weighted

Requirements management tools are good at links: this parameter is connected to that requirement, which is connected to that customer need. That tells you a relationship exists. It doesn’t tell you whether the tolerance is 80% about drilling accuracy and 20% about durability, or the other way round.

A multi-stage QFD already contains that information. Every link has a strength (9, 3 or 1), every requirement has a weight, and the weights flow from stage to stage. Traceability in QFD Tool simply reads that chain backwards.

The drill, from customer to grinding machine

Here’s a simplified version of the drill’s four-stage chain. The highlighted items are the thread behind our tolerance.

Customers want a drill that doesn’t wobble. Wobble is mostly chuck runout. Runout depends on how precisely the spindle sits in its bearings and how straight it is. And the bearing seat tolerance is held by the finish grinding pass and checked by in-process gauging.

How QFD Tool calculates each need’s contribution

The calculation starts at the last stage, where every process parameter has a priority. Each priority is then split among the requirements that feed it, in proportion to the relationship strengths, and the split is repeated stage by stage until it reaches the customer needs. The result: every item in the chain gets a contribution in percent, and the customer needs at the top add up to 100% of what happens at the bottom.

You see it in the traceability graph: all stages laid out left to right, every relationship drawn as a line. Hover over a customer need and everything it influences downstream lights up, with its contribution in the tooltip. To answer a bottom-up question, start at the parameter and follow the lines back to the left.

The answer for the drill

The finish grinding pass sits on the path from “drills straight, no wobble” — the need customers rated highest and the one with the largest contribution to the final output. Drop the pass, and you save money on exactly the thing buyers care about most. The pass stays, and now there’s a reason written down.

The same graph points somewhere else for savings. The paint gloss inspection traces back only to “looks professional”, a low-weight need with a small contribution. That’s where a cheaper control plan won’t hurt.

Dominance: when one need runs the show

Sometimes the graph shows one customer need responsible for most of the final output. That’s not necessarily wrong, but it’s fragile: the whole design rests on one assumption about what customers want. If that weight was off, everything downstream is off with it.

QFD Tool’s dominance analysis lists the driving requirements with their share and a risk level: above 30% is flagged as medium, above 50% as high. Dominant needs are also highlighted in the graph. A high flag is a prompt to double-check the survey data — or to split a vague need like “good quality” into the specific things customers actually mean.

What teams use traceability for

• Cost reduction without guesswork. Parameters with a small contribution are the first candidates for looser tolerances and lighter controls.
• Change impact. Before you change a customer requirement’s weight, hover over it and see which parts and processes will move.
• Design reviews. Export the stages and the graph to PDF or SVG and attach them to the review package, so reviewers see the reasoning, not just the numbers.
• Onboarding. A new engineer can see why the spec is the way it is without tracking down the people who wrote it.

In regulated fields such as medical devices or automotive, design inputs are expected to trace back to user needs. QFD Tool doesn’t replace your requirements management or quality system, but it adds what a link table can’t: how much each need matters to each decision.

Traceability is only as good as the chain behind it. If you haven’t built one yet, start withthe first House of Quality and thenconnect the stages.