The critical dimension that drifts before the first scrap
Measurements of critical characteristics become a control chart with the Western Electric rules and Cpk per die and per week. Die wear shows when Cpk falls below the target, while no part is out of tolerance yet.
Illustrative scenario: it describes a typical case, not a customer project.
The first scrap comes back from the customer. The dimension had been moving for weeks.
On a front cross member the reference hole has a half-millimetre tolerance, and the car maker asks for a Cpk of at least 1.33 on critical characteristics. The measurements exist, five parts every five hundred strokes, but they end up in a report read when a part is already out.
A wearing die does not produce scrap at once: it moves the dimension slowly, cycle after cycle, and each re-sharpening lasts a little less than the one before. Seeing it means watching Cpk over time and comparing cycles, every week and for every die.
- Who it's for
- Quality managers, stamping engineers, die maintenance and whoever follows PPAP and process capability towards the customer.
In Muvia, step by step
Real product screens, recorded on a project with sample data.
The video · Measurements of critical characteristics become a control chart with the Western Electric rules and Cpk per die and per week. Die wear shows when Cpk falls below the target, while no part is out of tolerance yet.
What you get
- Act before the first scrap
- Cpk falling below target is the signal: the die is restored while parts are still good.
- The real life of a re-sharpening
- Overlaid cycles say how many strokes the die holds, and maintenance is planned on data.
- Capability ready for the customer
- Cpk per characteristic and per week, with the actions written beside it, in a document that is always current.
- Measurements you can trust
- Impossible readings are flagged before they enter the chart, not discovered afterwards.
For the technical team
How it is built in Muvia
- 1
Connect measurements and strokes
Measurements arrive as files from the measuring machine, the stroke counter from the edge node next to the press. A data check flags impossible readings, such as a gauge writing 9.999 mm.
- 2
Compute chart and Cpk in Python
A function computes mean and range per subgroup, 3σ limits, the four Western Electric rules and Cpk per die and week. A flow runs it and writes three tables.
- 3
Overlay the re-sharpening cycles
In Analysis the Layers view aligns every cycle on its re-sharpening: you see how many strokes the die holds before it starts drifting, and whether each cycle lasts less than the last.
- 4
Decide with the weekly notebook
A notebook brings together Cpk per week, parts out of tolerance and the decision taken, and prints to PDF for the quality meeting or the customer.
Python function
import pandas as pd
def transform(inputs, params, ctx):
m = inputs["rows"] # one row per measured part
lsl, usl = params["lower_limit_mm"], params["upper_limit_mm"]
m["week"] = m["ts"].dt.to_period("W").dt.start_time
g = m.groupby(["die", "week"])["deviation_mm"]
out = g.agg(mean="mean", sigma="std", parts="count",
out_of_tol=lambda s: ((s < lsl) | (s > usl)).sum())
out["cpk"] = pd.concat([
(usl - out["mean"]) / (3 * out["sigma"]),
(out["mean"] - lsl) / (3 * out["sigma"]),
], axis=1).min(axis=1)
out["below_target"] = out["cpk"] < params["cpk_target"]
return {"rows": out.reset_index()}- The data it needs
- Measurements of critical dimensions from the coordinate measuring machine, part by part
- The press stroke counter from the Siemens S7 PLC
- Re-sharpenings, die changes and interventions from the maintenance log
- Tolerances and Cpk target per characteristic
More cases in this line
All use cases- OEE and losses on stamping and welding linesThe counters of presses and welding cells become OEE shift by shift and the six big losses, computed every morning by a function your team writes. When a press slows down after a die change, the dashboard colours it before the week is lost.
- Process quality and traceabilityProcess parameters from the PLC, lots from the line database, orders and scrap from the ERP, all in the same engine. Every lot has its process history, and an alarm goes off when a parameter leaves its window while the lot is being made.
- The control chart that warns you firstWeighings from the checkweigher become an X̄-R chart with 3σ limits and the Western Electric rules, recalculated at every shift change. A worn nozzle shows up hours before the scrap does, and Cpk shift by shift tells you where to act.
Tell us which machines you run. We'll tell you how to read them.
PLC brand, protocol, how the floor is connected: with those three answers, in a demo with one of our engineers, we show you how Muvia Industrial puts line data next to the rest of the business.