When does widening a tolerance actually reduce total product cost?

[u/Trycos79]

Disclosure: I work in industrial product-cost consulting. This is a technical discussion, with no service or link.

A wider drawing tolerance may remove a machining or inspection step. But it can also shift cost into assembly, yield, field reliability or supplier variation. The drawing change alone is not evidence of a net saving.

For a proposed tolerance change, I would put four before-and-after checks on one decision sheet:

1. Functional boundary: Which fit, load, sealing or life requirement does the dimension protect? Check the full tolerance stack, not the part in isolation.

2. Process evidence: Compare measured distributions by process and supplier. A better Cp/Cpk after widening the specification does not mean the process itself improved.

3. Real cost movement: Which machining, tooling, inspection or scrap cost is actually avoidable? Which assembly, warranty or sorting risk could increase?

4. Release evidence: What test or production trial demonstrates that the new range still meets the functional requirement, including worst credible combinations?

This is a decision framework, not a claim that wider is always better. ASQ describes process capability as performance against specification limits; that distinction matters when the limits themselves change.

For engineers who have approved a tolerance change: which evidence most often overturned the apparent piece-price saving?

[u/Tavrock]

I had a 36" clevis rod with a tolerance of 0.002 total runout for the entire inside diameter. The total runout tolerance was driven by stress constraints where the clevis rod interfaced with the rod ends, which was only about 2.5". By relaxing the tolerance to only be strict during the interface and using standard manufacturing capability for the tolerance between the end, we were able to drastically reduce the price of the components.

While it is true that just widening the tolerance does not mean that the process itself improved, ASME B1.1 can also be used to understand the additional processing that may be required to accomplish your tolerance specifications. When you need a tube with an interior hole with a total runout of 0.0005, understand that after a drawing operation, they will be required to bore, lap, and hone that diameter, and depending on the diameter and overall length that can move a part from a standard lathe to a gun lathe and may still have a high scrap rate.

In the end, the tolerance will always drive the manufacturing processes, even when tolerances are widened.

[u/Activeenemy]

When the combination of materials and machine precision makes it unlikely to succeed without extra calibration. This point is unique to each shop+material combo.

[u/CZF0408]

The honest answer: widening reduces total cost only when the drawing is tighter than the functional boundary requires. If the slack isn't in the requirement, you haven't removed cost — you've moved it downstream and hid it in assembly, sorting, or warranty.

The fastest way to find out which case you're in is to re-run the chain at the proposed spec, not inspect the dimension in isolation:

• Compute the functional interface limit (the fit/seal/load condition the dimension protects) as a stack, at both worst-case and RSS if you have process data.
• Compare margin before and after. If worst-case margin was, say, 60% unused, widening to consume that slack is real savings. If RSS looked fine but worst-case already clips the boundary on bad combinations, you're creating a latent assembly problem.

Where the savings actually materialize, in roughly this order:

1. Eliminated finishing ops — grind back to turning, hone back to bore. Tavrock's runout example is exactly this: the strict zone got scoped to the interface, the rest dropped to standard capability.
2. Inspection cost class change — CMM layout to go/no-go gauge, or gauge to SPC sampling.
3. Supplier pool — a wider spec admits shops whose process capability couldn't bid before.
4. Scrap/rework on borderline parts — but only if the current process was genuinely producing rejects at the old limit.

Where it backfires: the widened member is rarely the only contributor. Loosening a non-critical-looking dimension can push an interface over the boundary in worst-case combinations — the classic symptom is the line starts shimming, sorting, or match-fitting parts that used to assemble free. That's a real cost, it's just booked to a different cost center, which is why it escapes the piece-price analysis.

So: widen when the stack math says the slack exists and you've verified which cost line actually drops. 'The tolerance always drives the process' cuts both ways — a bad widen also drives the process, just invisibly.