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Precision forming and statistical inspection · Lesson 3 of 4

Precision forming and statistical inspection: methods and worked application

The first-piece loop, what to measure on a Fascia Cover, how an angle error becomes a position error, and worked control charts and capability indices on hypothetical readings, with the assumptions each one needs.

You will be able to: Run a first-piece check, build an individuals and moving-range chart for a short run, compute X-bar and R limits and Cp and Cpk with their assumptions, and choose between 100% and sampled inspection.

14 min · 10 slides · 11 questions

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Slide 1 of 10

The first-piece loop

Form the first piece, measure every angle and leg off the machine, correct and re-form until it matches the order, then record the approval. An Amada punch-and-die (die-bending) patent describes the same: measure, then a corrective fold.

Then chart the run. NIST: an out-of-control action plan (OCAP) details the action to be taken once an out-of-control situation is detected.

From the first piece to a charted runA flow chart. Program from FlashDraft, form the first piece, measure it off the machine. If it does not match the order, correct and re-form. When it matches, record the approval, run with periodic checks and plot them on a chart. If the chart signals, follow the action plan: stop, check the material and the gauge, then go back to the first piece. With no signal, keep running and checking periodically until the run is complete, then go on to the quality check.Program from FlashDraftForm the first pieceMeasure off the machineMatches order?Correct andre-formRecord the approvalRun, check periodicallyPlot on the chartSignal?Action plan:stop, checkmaterial andgaugeRun complete?Quality checkNoYesYesNoNoYesback to first piece

A flow chart. FlashDraft sends legs and signed angles; the first piece is formed and every angle and leg measured off the machine; if it does not match the order it is corrected and re-formed; when it matches, the approval is recorded and the run continues with periodic checks plotted on a chart. A chart signal leads to the action plan (stop, check the material and the gauge) and a dashed line back to forming the first piece; with no signal the run loops back to periodic checks until it is complete, then goes to the quality check.

The loop in words; frequencies are set by the order and the shop, not by this module.

Sources

Videos for this lesson

What this video shows: A tool maker's overview of protractors for measuring angles in setup, layout and inspection.

Video: How to Use Metalworking Protractors with the Starrett C183 Steel Protractor by The L.S. Starrett Company, on YouTube. AFS checked its description against this lesson.

What this video shows: A press-brake example of the measure-then-correct loop: the operator measures the angle and the control calculates the correction. AFS forms on a folder; this is not AFS's machine.

Video: Mitutoyo® wireless digital protractor for press brake by Gasparini Industries, on YouTube. AFS checked its description against this lesson.

Glossary

Moving range
The absolute difference between two successive individual readings; its average, divided by 1.128, estimates the process standard deviation on an individuals chart.
OCAP (out-of-control action plan)
The written steps taken once a control chart signals that a process is out of control, so the response does not depend on who is on shift.
Cp and Cpk
Capability indices: Cp compares the specification width with six process standard deviations; Cpk also accounts for how far the average sits from the nearer limit, so Cpk is never larger than Cp.
Acceptance sampling
Deciding whether to accept or reject a lot from a random sample of it; its purpose is the decision, not an estimate of the lot's quality.