起源故事 · Cp 怎么进入工业标准
Origin Story · How Cp Became an Industry Standard
1951 年 Juran 在《质量控制手册》第一版里就有"过程能力"概念,但只是描述性的。
真正把 Cp 写成公式并标准化,是 1980 年代美国汽车业的质量危机——日本车涌入美国市场,
福特、克莱斯勒、通用三家联手推出 QS-9000 + AIAG SPC 手册(1986 起草,1995 正式版),
要求所有供应商交件必须报 Cp / Cpk,分母统一为 6σ(呼应 Shewhart 1924 的 ±3σ 控制限)。
从此 Cp 从一个学术术语变成采购合同条款:Cp ≥ 1.33 才能进入合格供应商名录,关键件要 ≥ 1.67,安全件要 ≥ 2.0。 这套标准后来被 ISO/TS 16949、IATF 16949 全盘继承,今天全球汽车业仍在用。 Juran's 1951 Quality Control Handbook already mentioned "process capability," but only in words. What turned Cp into a real formula and an industry standard was the quality crisis in 1980s US auto — Japanese cars were flooding the American market. Ford, Chrysler and GM banded together and shipped QS-9000 + the AIAG SPC manual (drafted 1986, formally released 1995), making every supplier report Cp / Cpk with 6 σ as the denominator (a nod to Shewhart's ±3 σ control limits from 1924).
From that moment Cp stopped being an academic term and became a line in the purchasing contract: Cp ≥ 1.33 to get on the approved-supplier list, ≥ 1.67 for critical parts, ≥ 2.0 for safety parts. ISO/TS 16949 and IATF 16949 inherited the whole package — and global auto still runs on it today.
从此 Cp 从一个学术术语变成采购合同条款:Cp ≥ 1.33 才能进入合格供应商名录,关键件要 ≥ 1.67,安全件要 ≥ 2.0。 这套标准后来被 ISO/TS 16949、IATF 16949 全盘继承,今天全球汽车业仍在用。 Juran's 1951 Quality Control Handbook already mentioned "process capability," but only in words. What turned Cp into a real formula and an industry standard was the quality crisis in 1980s US auto — Japanese cars were flooding the American market. Ford, Chrysler and GM banded together and shipped QS-9000 + the AIAG SPC manual (drafted 1986, formally released 1995), making every supplier report Cp / Cpk with 6 σ as the denominator (a nod to Shewhart's ±3 σ control limits from 1924).
From that moment Cp stopped being an academic term and became a line in the purchasing contract: Cp ≥ 1.33 to get on the approved-supplier list, ≥ 1.67 for critical parts, ≥ 2.0 for safety parts. ISO/TS 16949 and IATF 16949 inherited the whole package — and global auto still runs on it today.
1 三步拆 Cp:门 → 钟 → 比例 Three Steps: Door → Bell → Ratio
第 1 步 · 画门Step 1 · Draw the Door2 Cp 等级条:指针告诉你在哪一档 Cp Grade Bar: The Pointer Tells You Where You Sit
<1 不合格 · 1–1.33 临界 · 1.33–1.67 良好 · ≥1.67 优秀 · ≥2.0 六西格玛级 < 1 not capable · 1–1.33 marginal · 1.33–1.67 good · ≥ 1.67 excellent · ≥ 2.0 Six Sigma grade
3 各行业典型 Cp Typical Cp by Industry
半导体光刻 Cp=2.5:超精密工艺,几 nm 线宽,0 缺陷文化的物理基础。
Semiconductor lithography, Cp = 2.5: ultra-precise process, few-nm linewidths — the physical basis of the zero-defect culture.
注塑常规 Cp=1.5:成熟工艺、模具温控好,行业平均水平。
Standard injection molding, Cp = 1.5: mature process, well-tuned mold thermals — solid industry average.
手工焊 Cp=0.7:依赖个人技能,必须靠 100% 自检 + 全检兜底。
Hand soldering, Cp = 0.7: skill-dependent — only 100% self-inspection plus full inspection keeps it shippable.
食品包装 Cp=1.3:临界值,刚过合格线,σ 稍涨就出问题。
Food packaging, Cp = 1.3: right on the edge — any uptick in σ and you start failing.
一句话In One Line
Cp 是过程的"理论最佳"——只要居中,就能达到的废品率。
它不告诉你今天废品多少,只告诉你"如果你愿意把 μ 调到中心,最少能做到几个 σ 内"。
改进路径完全可以由 Cp 与 Cpk 的关系决定:
· Cp 不够(钟比门胖)→ 必须降 σ,投资设备 / 工艺,贵;
· Cp 够但 Cpk 不够(钟塞得下但偏了)→ 只需调 μ,便宜;
· Cp 和 Cpk 都够 → 进入维持期,重点防漂移(SPC 控制图见 PC04)。 Cp is the "theoretical best" of your process — the defect rate you'd hit if you were centered. It doesn't say how much scrap you make today; it says "if you bothered to recenter μ, here's the smallest σ envelope you'd live inside." That alone fixes the improvement path:
· Cp too low (bell fatter than door) → you must shrink σ — equipment or process investment, expensive;
· Cp fine but Cpk low (bell fits but sits off-center) → just recenter μ, cheap;
· Both fine → maintenance mode, watch for drift (SPC control charts, see PC04).
· Cp 不够(钟比门胖)→ 必须降 σ,投资设备 / 工艺,贵;
· Cp 够但 Cpk 不够(钟塞得下但偏了)→ 只需调 μ,便宜;
· Cp 和 Cpk 都够 → 进入维持期,重点防漂移(SPC 控制图见 PC04)。 Cp is the "theoretical best" of your process — the defect rate you'd hit if you were centered. It doesn't say how much scrap you make today; it says "if you bothered to recenter μ, here's the smallest σ envelope you'd live inside." That alone fixes the improvement path:
· Cp too low (bell fatter than door) → you must shrink σ — equipment or process investment, expensive;
· Cp fine but Cpk low (bell fits but sits off-center) → just recenter μ, cheap;
· Both fine → maintenance mode, watch for drift (SPC control charts, see PC04).
常见误用Common Mistakes
报告只看 Cp。必须 Cp / Cpk 一起看——偏心问题会藏在漂亮的 Cp 数字背后,Cpk 才暴露真相。
Reporting Cp alone. Always pair Cp with Cpk — a pretty Cp can hide a nasty offset, and only Cpk surfaces it.
Cp 用 σ_overall(整体)算。Cp / Cpk 用 σ_within(组内短期),σ_overall 算出来的叫 Pp / Ppk(见 47-cp-cpk-pp-ppk)。
Computing Cp with σ_overall. Cp and Cpk take σ_within (short-term, within-subgroup). σ_overall gives you Pp / Ppk — different beast (see PC08 / 47-cp-cpk-pp-ppk).
"Cp 越大越好,1.0 就够"。行业基准 Cp ≥ 1.33,关键件 ≥ 1.67,安全件 ≥ 2.0;Cp=1.0 = 门刚好等于钟,任何一点漂移就出废品。
"Bigger Cp is better, 1.0 is enough." Industry baseline is Cp ≥ 1.33, critical parts ≥ 1.67, safety parts ≥ 2.0. Cp = 1.0 means door exactly equals bell — any tiny drift and you're making scrap.