CNC Milling Services Quality Control

A certificate is a pre-audit you get for free: it tells you the shop already runs a documented, third-party-surveilled system, so you do not have to build that confidence from scratch. The scope is stated honestly, because an over-claimed scope is a liability, not an asset.

Standard	Certificate number	Scope
ISO 9001:2015	116024/A/0001/UK/En	Machining of metal parts
IATF 16949:2016	131941/A/0001/SM/En	Manufacture of CNC metal machining parts (excludes product design, Clause 8.3)
ISO 14001:2015	116024/B/0001/UK/En	Environmental management related to machining of metal parts

Honest scope note: IATF 16949 excludes product design under Clause 8.3. We are a build-to-print manufacturer.

How are dimensions verified before shipping?

Each instrument verifies a different part of the drawing, and the results are documented rather than assumed. The CMM verifies position and profile against the datums; the laser microscope verifies micro-features and surface form; the XRF verifies material. The instruments below are the ones that turn a tolerance from a claim into a measured value.

• Hexagon GLOBAL S CMMs (2) for dimensional verification, plus vision measuring machines and a gear measuring center.
• KEYENCE VK-X3000 laser microscope and SURFCOM 1800D for surface finish and profile.
• XRF (iCHEQ) spectrometer for incoming material composition checks against the mill certificate.
• HVS-1000A micro-Vickers hardness tester, Fischer coating thickness gauge, salt spray and tensile testers.
• SPC: control charts on key characteristics, with Ppk ≥ 1.67 at the initial study on critical characteristics and Cpk ≥ 1.33 in production on general characteristics.

How is true position and profile measured?

True position and profile are relationships, not single dimensions, and they are measured on a CMM against the drawing's datums. Our Hexagon GLOBAL S CMMs measure the actual feature location and surface form, so a hole that is on size but off position — or a surface that is within size but out of form — is caught before it ships. The CMM builds the datum reference frame from the drawing, then measures each controlled feature against it.

How is process capability measured?

Characteristic class	Cpk target	What it means
Key characteristics	≥ 1.67	The process is stable and centered well inside the tolerance band
General characteristics	≥ 1.33	The process meets standard capability expectations

On a representative length characteristic (76.1–76.5 mm), our SPC shows Cpk = 1.851 (one characteristic on one part, not a blanket capability claim), measured over 25 subgroups. The Cpk is computed from measured values, not stated as a slogan, which means the tolerance is held repeatedly rather than demonstrated once on a lucky part.

How is incoming material verified?

Every incoming lot is checked by XRF spectrometer against its mill certificate before it reaches a machine. A wrong alloy can pass every dimensional check and still fail, which is why material comes first. The XRF reads the actual elemental composition and compares it to the certificate, so a mixed or substituted lot is caught at the door rather than discovered on your dock.

What documentation ships with a measured part?

Documentation is what turns a promise into evidence, and the evidence is what lets you trust a batch without opening every box. A measured part ships with the documents that prove it is what the drawing asked for.

• CMM dimensional reports for critical dimensions, with measured values and deviations against the datum frame.
• SPC control charts for process-critical characteristics, showing the chart behind the Cpk.
• Material certificates, with XRF verification against the mill certificate.

How do you audit a CMM program, not just accept a CMM report?

A CMM report is only as good as the program behind it. A program that references the wrong datum, or samples a surface with a handful of points instead of a grid, returns numbers that look precise and are still wrong. Before you trust a report, ask how the program was built: which datums it references, how many points it samples per feature, and whether the alignment is re-established for each part.

The same question applies to the machine behind the program. A CMM that is not verified against a known standard can drift and still print confident digits. The report should name the machine and the program, not just the values.

• Ask which datums the program references. They must match your drawing's datum frame.
• Ask for the point count per feature. A plane needs enough points to be trusted; a profile needs more.
• Ask how the CMM is calibrated and how often. The answer should be a schedule, not a shrug.
• Ask whether alignment is re-established per part. A one-time alignment will not catch a drifting fixture.

Why a CMM report beats a capability statement

A capability statement says a shop can hold a tolerance. A CMM report from your actual part says it did. For GD&T characteristics, the report should list each characteristic, its datum frame, the measured value, and the deviation — tied to the specific lot. Zhuohang Precision Machining Factory verifies critical dimensions on Hexagon CMMs and form on a KEYENCE laser microscope, and tracks the process at Ppk ≥ 1.67 at the initial study on critical characteristics and Cpk ≥ 1.33 in production on general characteristics. The report, not the brochure, is what you file and what you defend if a question arises later.

What is the difference between Cp and Cpk?

Both numbers describe how a process fits inside its tolerance band, but only one tells you whether the process is centered. Cp measures the width of the process spread against the width of the tolerance and assumes the process sits in the middle of the band. Cpk corrects for position: it measures the distance from the process mean to the nearer limit, so a process that drifts off-center shows a falling Cpk even when its spread has not changed.

The difference is why Cpk is the number you should ask for. A process can show a healthy Cp while quietly drifting toward one limit, producing parts that are still within spec today but are one shift away from out of spec tomorrow. Cpk catches that drift, because it collapses the moment the mean moves. When a supplier reports Cpk rather than Cp, it is reporting a number that accounts for both spread and centering, which is the number that actually predicts how many parts will fall out of tolerance.

How is the CMM itself kept trustworthy?

A CMM is a measurement instrument, and like any instrument it drifts unless it is checked against a known standard. The trust in a CMM report rests on two things: the machine is calibrated on a schedule, and the calibration is traced to a recognized reference. A CMM that is not calibrated can return confident digits for a wrong measurement, and the error is invisible in the report because the report is internally consistent.

The same discipline applies to the probes and the software. A probe that is worn or a temperature compensation that is off will shift every measurement the machine makes. When you audit a supplier's measurement, ask about the calibration schedule and the reference it is traced to. The answer should be a schedule and a standard, not a shrug, and it is the same question you would ask about any gauge you rely on.

Why is a defect cheapest to catch at the machine?

A defect caught at the machine costs the scrap value of the stock. The same defect caught at your incoming inspection costs the part plus freight and the inspection hour. On your assembly line it costs a stoppage; in your customer's hands it costs a return and a reputation. The cost climbs at every step the defect travels, which is why the supplier's internal quality system is your cost control, not just their problem.

Everything a shop does to catch a defect at the machine — SPC charts, in-process checks, CMM verification before boxing — is a discount on the rework bill you would otherwise pay downstream. The cheapest defect is the one that never ships, and a shop that keeps defects at the machine is the shop that keeps them off your ledger.

How does traceability turn a measurement into a record?

A measurement is a single point in time; a record is the chain that makes that point useful later. Traceability links the measured dimension back to the material lot, the machine, the operator, and the inspection result, so a question raised months after shipment can be answered with a document instead of a guess. The MES/ERP system records that chain on every lot.

The difference shows up when something fails. With traceability, a non-conformance can be contained to the affected lot — the parts from one machine, one material batch, one shift — instead of treating an entire order as suspect. Without it, the only safe response is to replace everything, because you cannot prove which parts are good and which are bad. That is the costliest kind of failure, and traceability is the cheapest insurance against it.

For regulated buyers traceability is a requirement; for everyone else it is the difference between isolating a problem and guessing at it. The chain begins with an XRF check of incoming material and runs through every dimension on the CMM report, so the record is complete from the raw stock to the shipped part.

• Material lot is XRF-checked and matched to the mill certificate at receiving.
• Machine and operator are recorded against every production batch.
• Inspection results are stored against the lot, not filed loose.

What this shop actually runs

Ask for capability evidence and the answer should be specific. The facility behind this guide runs 100+ CNC machines on a 6,800 m2 floor in Dongguan, China, 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). Built for 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). Built for 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). Built for 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). Built for shafts, bushings, flanges up to ±0.002 mm.
• Wire EDM (5+) - Sodick AQ535L (550/350/300 mm travel, ±20° taper, 800 kg workpiece). Built for 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.

Tolerances held: milling ±0.003 mm, turning ±0.002 mm, grinding and wire EDM ±0.002 mm. Capability: Ppk >= 1.67 initial and Cpk >= 1.33 in production on key characteristics.

Certifications: ISO 9001:2015 (116024/A/0001/UK/En), IATF 16949:2016 (131941/A/0001/SM/En, clause 8.3 design excluded, disclosed openly) and ISO 14001:2015 (116024/B/0001/UK/En), issued to Dongguan Zhuohang Ltd, USCC 91441900MA4UWT6X39, established 2005.

Service area: 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 — built in China, shipped worldwide.

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

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