Product Specification Limits & Process Capability Builder

The formula

This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.

How the result is derived

Step by step, from the values you type to the figure on screen.

1. The 7 inputs are read from the form on every keystroke: Nominal GSM, GSM Tolerance, GSM Process SD, Nominal Tensile Strength, Tensile Tolerance, Tensile Process SD and Target Capability.
2. Each value is checked against the accepted range in the input table below. A value outside its range stops the calculation rather than producing a misleading figure — the results blank out and a message appears.
3. The validated values are substituted into the expression above, which resolves Limiting Capability together with every supporting figure in one pass — no value is carried over from a previous entry.
4. The supporting outputs — GSM Capability, GSM Upper Limit, GSM Lower Limit, Tensile Capability, GSM SD for Target and Tensile SD for Target — come from the same pass, so they always describe the same case as the headline figure.
5. Results are rounded for display only. The full-precision value is used throughout the chain, so reading a rounded intermediate figure back into the tool by hand can shift the last digit.

What each input means

Where to read each value on the floor, the unit it must be in, and the range the tool accepts.

What the tool returns

The headline figure and every supporting value it is built from.

Worked example

The tool loads with this case already solved — the Limiting Capability shown above is its answer. Change one value and the difference from this baseline is the sensitivity of the result to that variable.

How to use it

1. Work through the input groups in order — Mass Specification and Strength Specification. The defaults are a realistic case, so you can change one value at a time and watch what moves.
2. There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
3. Read Limiting Capability in the dark results panel — that is the headline figure, expressed in Cp.
4. Check the supporting rows underneath (GSM Capability, GSM Upper Limit, GSM Lower Limit, Tensile Capability, GSM SD for Target and Tensile SD for Target) before acting on the headline — they are where an implausible input usually shows itself first.
5. Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.

Where this is used

• Process planning — establishing Limiting Capability before a trial is booked, so machine time and material in Product Engineering, Specifications & Feasibility are committed against a calculated figure rather than an estimate.
• Costing and quotation — Limiting Capability is an input to the cost sheet, and quoting from a worked number rather than a remembered one is what keeps a margin intact.
• Troubleshooting — when the floor result drifts from plan, entering the measured values (starting with Nominal GSM) shows how much of the gap in Limiting Capability each variable explains.
• Teaching and study — the accepted ranges bracket normal Product Engineering, Specifications & Feasibility practice, so moving one variable at a time shows the shape of the relationship rather than a single answer.

Assumptions and limits

• Cp assumes the process is centred on nominal, which is the optimistic case — a process running off-centre has a lower Cpk than the Cp shown here, and correcting the centring is usually cheaper than reducing the spread. The process standard deviation must come from a stable, in-control process; computing capability from data that contains a special cause produces a number that describes nothing. Capability below about 1.0 means routine rejects are arithmetically guaranteed, and the honest responses are to widen the tolerance, improve the process or decline the specification — not to inspect harder.
• Every input is bounded to the range normal practice occupies (Nominal GSM 20 to 1500 g/m², GSM Tolerance 0.5 to 25 ± % and GSM Process SD 0.1 to 50 g/m², and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
• The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
• Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.

Questions people ask