Machining Factory's Quality Control

Cpk is the measure of whether a process holds a tolerance by margin or by luck. On a representative length characteristic (76.1–76.5 mm), the SPC record shows Cpk = 1.851 (one characteristic on one part, not a blanket capability claim), above the key-characteristic standard of Ppk ≥ 1.67 initial. A stable, centered process holds the tolerance with margin, run after run.

For you, a documented Cpk is the statistical proof behind the tolerance. It is the difference between a supplier who occasionally hits the number and one who lives comfortably inside it.

Two things make a Cpk convincing: the process must be centered, and it must be stable. A process that drifts toward one edge of the band can still show a healthy average while its tail clips the spec limit; a Cpk below the 1.67 key-characteristic standard is exactly that warning. Centered and stable is what turns ±0.003 mm (milling) / ±0.002 mm (turning) from an average into a guarantee.

How Is Material Verified?

Every incoming lot is XRF-checked against its mill certificate before machining. This confirms the alloy is what the drawing specifies, so the tolerance is held in the correct material — a tolerance held in the wrong alloy is a tolerance wasted.

The material check is the first gate of the quality system, and it is what makes the tolerance measurement meaningful.

The XRF check catches the failure a tolerance cannot see: a part machined to ±0.002 mm in the wrong alloy is scrap the moment it goes into service. That is why material is gated before the first cut, not inspected after the last.

What Documents Ship With the Parts?

The measurement travels with the part: CMM inspection reports, SPC control charts, and material certificates ship with the parts. For automotive programs, the APQP/PPAP set — process flow, control plan, PFMEA, and SPC — is available.

These documents are your own evidence of the tolerance. A part with its inspection record is a part whose tolerance can be verified independently, not taken on faith.

The document you ask for depends on what you are trying to prove:

Claim to verify	Document that proves it	What it shows
Size tolerance	CMM inspection report	Each dimension, measured against its band
Geometric tolerance	CMM report	Form and position from coordinate data
Material alloy	XRF result plus mill certificate	The lot matches the drawing's alloy
Process stability	SPC control chart	Cpk on the critical characteristic

What Questions Expose a Tolerance That Is Only Claimed?

Four questions, asked before the order, separate a measurement from a slogan.

• Will the CMM report and SPC chart ship with every batch, or only on request? If it is only on request, the record is not part of the process.
• What is the documented Cpk on the tightest characteristic? A Cpk at or above the 1.67 key-characteristic standard is held by margin; a supplier who cannot produce the number is holding it by luck.
• How is incoming material checked? An XRF check against the mill certificate is the gate that keeps the tolerance held in the right alloy.
• Which certificate numbers back the system? ISO 9001:2015, IATF 16949:2016, and ISO 14001:2015, each with a verifiable identifier.

How Does SPC Turn a Tolerance Into a Guarantee?

Statistical process control is the discipline of watching the process while it runs, not just inspecting the parts after it stops. A control chart plots the dimension against time, and the chart shows the moment the process begins to drift, long before a part goes out of band.

The chart is read by two rules. The points must stay centered on the nominal, and the spread must stay narrow enough that the band has margin on both sides. A process that is centered but too wide clips the spec limit; a process that is narrow but drifting will clip it soon. SPC is what keeps both — center and spread — under control at the same time.

The value to you is that the guarantee is continuous. An end-of-run inspection only proves the parts that were checked; a control chart proves the process stayed in control the whole run. That is the difference between a batch that happened to pass and a process that is known to be stable.

What Is the Difference Between Cpk, Cp, and Ppk?

Three capability indices appear in machining reports, and they answer different questions. Knowing which is which keeps a buyer from reading the wrong number.

Index	What it measures	What it tells you
Cp	The spread of the process against the band	Whether the process is narrow enough, ignoring where it is centered
Cpk	The spread and the centering together	Whether the process holds the band with margin on both sides
Ppk	Long-term performance across many runs	How the process behaves over time, with all real-world drift included

Cp alone is the dangerous one, because it ignores centering. A process can be narrow but sitting against one edge of the band, so Cp looks healthy while parts are still being made out of spec. Cpk fixes that by measuring the distance to the nearest limit, which is why it is the number that matters for a tolerance claim.

The floor's record is Cpk = 1.851 (one characteristic on one part, not a blanket capability claim) on a representative length characteristic, above the key-characteristic standard of Ppk ≥ 1.67 initial. That number is Cpk, not Cp, which is the honest one: it says the process is both narrow and centered, with margin on both sides, run after run.

How Is a CMM Report Read?

A CMM report is a table of every measured feature against its tolerance, and reading it is a skill worth having. Each row states what was measured and whether it passed.

Report field	What it means	What to check
Nominal	The value on the drawing	Matches the print
Actual	The value the CMM measured	Sits inside the band
Deviation	Actual minus nominal	Small relative to the band
Tolerance	The band allowed	Matches the callout
Result	Pass or fail	Every feature passes

The report is your own evidence, and it is readable without a metrology lab. A feature that passes shows a deviation smaller than its tolerance; a feature that is drifting shows a deviation that is growing toward one edge, and that is the warning to look for, because a part that just barely passes is a part on its way to failing.

For geometric callouts, the report lists the form or position result against the geometric band, not just the size. That is the difference between a report that proves the diameter and one that proves the position. A complete report covers both, because a part can be the right size and still be out of position.

What Does a Complete Quality Record Contain?

A single inspection result is a snapshot; a quality record is the whole story. The two are not the same, and if you want proof, you ask for the record, not the snapshot.

Document	What it proves	When it matters
CMM inspection report	Size and geometry, feature by feature	Every batch, to confirm the band
SPC control chart	Process stability across the run	To confirm the band held the whole time
Material certificate	The alloy is what the drawing specified	To confirm the part was made in the right material
XRF result	The incoming lot matched the certificate	To close the material gate before cutting

For automotive programs, the record extends into the APQP and PPAP set — process flow, control plan, PFMEA, and the SPC package. That documentation is the formal proof that the process was planned, controlled, and capable before production ran, and it is what an automotive customer audits against.

The principle is the same at every level: the document that ships with the part is your independent evidence. A part with its record is a part whose tolerance can be checked without asking; a part without one is a part whose tolerance has to be taken on faith. The record is what separates the two.

What this shop actually runs

Capability evidence should come as specifics, not slogans. Behind this guide is a 6,800 m2 plant in Dongguan, China running 100+ CNC machines with 100+ staff. The machine list by category:

• 5-axis machining centres (10+) - Mazak VARIAXIS I-600 (510/910/510 mm travel, 12,000 rpm, 22 kW, 71.6 N·m, 30-tool BT40) and Mazak INTEGREX I-250H S (B-axis 0.0001°, 24 kW, 124 N·m, 38-tool HSK-A63). Run on compound-angle parts, single-setup multi-face prismatic work, and aerospace fittings where datum transfer loss is unacceptable.
• 3-axis / 4-axis machining centres (50+) - Brother SPEEDIO S500Z2N / S700Z2N (10,000 rpm, 14/21-tool ATC, 0.7 s tool-to-tool, Z-axis 2.2 G) and Mazak VCE-430A L / VCE-570C L (10,000 rpm, 118 N·m, 24-tool ATC). Run on prismatic parts, plates, brackets, housings.
• Swiss-type turning centres (10+) - Citizen Cincom A20-3F7 (Φ20 mm, 10,000 rpm), TSUGAMI B0206-III / B0205-III (Φ20 mm, 0.1 µm), STAR SR-32JII / SR-32J Type N (Φ32 mm, 8,000 rpm). Run on long, slender rotational parts at ±0.002 mm.
• CNC turning centres (20+) - HEADMAN T70M/1250 (720 mm swing, 117 mm bore, 3,000 rpm, BMT75 12-station turret). Run on shafts, bushings, flanges up to ±0.002 mm.
• Wire EDM (5+) - Sodick AQ535L (550/350/300 mm travel, ±20° taper, 800 kg workpiece). Run on hardened tool steel, internal splines, sharp internal corners that milling cannot reach.
• Inspection: Hexagon CMM + 20+ gauges - Hexagon CMM for coordinate measurement, plus vision system, profilometer, hardness tester, coating thickness gauge, XRF for material verification. The 10:1 rule (gauge 10x more accurate than the band) is applied to every feature.
• Machine highlights - VARIAXIS I-600: one-setup multi-face 5-axis + MAZA-CHECK auto-calibration; INTEGREX I-250H S: DONE-IN-ONE mill-turn, B-axis 0.0001° + Ai thermal shield; SPEEDIO S500Z2N / S700Z2N: Z-axis 2.2 G + 0.7 s ATC; Swiss-type: guide-bush switchable + 0.1 µm resolution.

Tolerance: ±0.003 mm CNC milling / ±0.002 mm CNC turning / ±0.002 mm grinding and wire EDM. Capability: Ppk >= 1.67 initial on key characteristics, Cpk >= 1.33 ongoing.

Certificates: ISO 9001:2015 (116024/A/0001/UK/En), IATF 16949:2016 (131941/A/0001/SM/En, with design clause 8.3 excluded, stated openly) and ISO 14001:2015 (116024/B/0001/UK/En), issued to Dongguan Zhuohang Ltd, registration 91441900MA4UWT6X39, founded 2005.

Where we ship: Austria, Belgium, the Czech Republic, Denmark, France, Germany, Greece, Hungary, Ireland, Italy, the Netherlands, Spain, Sweden and the United Kingdom in Europe; Brazil, Canada, Mexico and the United States in the Americas; China, Israel, Japan, Singapore and Sri Lanka in Asia; Australia and New Zealand in Oceania; and South Africa in Africa — plus other countries and regions worldwide, shipped from our base in Dongguan, China.

Contact: Felipe Iglesias, [email protected], +86 13126536955.

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