3 Axis CNC Machining Quality Control

How do you verify a tolerance claim instead of trusting it?

A tolerance on a sales page means nothing without measurement behind it. The test is simple: ask for a CMM report or SPC chart from a real job. A shop that measures in-house and ships the report with your parts proves the tolerance; a shop that can only recite the number is asking you to take it on faith.

Our tolerances are tied to the process, and every critical dimension is checked on a CMM rather than assumed. The inspection bench is the same class of equipment a buyer would use to audit us:

• Hexagon GLOBAL S CMMs for dimensional verification, plus a gear measuring center and vision measuring machines.
• KEYENCE VK-X3000 laser microscope and SURFCOM 1800D for surface finish and profile.
• XRF (iCHEQ) spectrometer for incoming material composition checks.
• HVS-1000A micro-Vickers, Fischer coating gauge, salt spray and tensile testers.

What does process capability (Cpk) actually tell you?

Cpk is the honest way to describe how consistently a process hits a tolerance, not a best case, but the spread across a real run. A Cpk of 1.67 means the process output sits comfortably inside the tolerance band with margin for drift; a Cpk of 1.33 is the standard bar for general characteristics.

Characteristic type	Cpk target	What it means
Key characteristics	≥ 1.67	Critical-to-function dimensions, held with margin for drift
General characteristics	≥ 1.33	Standard dimensions, held within a normal process window

On a representative length characteristic (76.1–76.5 mm), our SPC control charts show Cpk = 1.851 (one characteristic on one part, not a blanket capability claim) across 25 subgroups — comfortably above the key-characteristic bar. That is the kind of number you can ask any shop for; if they cannot produce it, their tolerance claim is a slogan, not a specification.

The distinction between Cpk and a simple tolerance check matters because one predicts and the other only records. A tolerance check tells you the single part in front of you was inside the band; Cpk tells you how the whole run is distributed against the band, and therefore how likely the next part is to stay inside it. Buyers who only ask "is it in tolerance" are buying a snapshot; buyers who ask "what is the Cpk" are buying a process.

Which certifications should a quality system hold?

Certifications are your cheapest pre-audit — they tell you a shop already runs a documented, third-party-surveilled system. We hold three, with scope stated honestly.

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 — the process control (SPC, control plans, PFMEA, traceability) runs on every job, but design responsibility is yours unless we scope it separately.

How is incoming material verified?

The quality chain starts before cutting. Every incoming lot of raw material is checked by XRF spectrometer against its mill certificate, so the alloy you specify is the alloy you receive, and a mixed or substituted lot is caught at the door rather than discovered after machining.

This is the same discipline IATF 16949 audits look for, and it runs on every job, not just automotive ones. If a non-conformance ever surfaces, we can tell you exactly which lot, which machine and which operator produced it, and isolate the impact instead of guessing.

Material verification is the cheapest insurance a quality system can buy, because a dimensional check on the wrong alloy is a perfect measurement of the wrong part. The XRF check happens at the door, before the stock reaches a machine, which is the only point where a substitution can be caught cheaply. Once a wrong-grade bar is cut into parts, the mistake is either discovered at inspection — after the machine time is spent — or worse, in the field. The discipline of checking at incoming is what keeps the rest of the quality chain honest.

How are parts traced back to the raw material?

For regulated buyers, traceability is not optional. Our MES/ERP system records the full chain on every lot: raw material source and usage, process parameters, machine data, production batch, machining time, operator, and inspection results. That record is what turns a quality system from a policy on the wall into a capability you can audit.

What APQP / PPAP documentation is available?

For automotive and other programs that require it, we produce the full APQP/PPAP package: a process flow chart, control plan, SPC charts, and PFMEA. These are the documents you use to confirm the process is planned and controlled, not improvised. Ask for them on any program where dimensional conformance is safety- or function-critical.

What is the quality policy behind the numbers?

The policy is straightforward and enforced in three gates: no non-qualified material accepted, no defective parts produced, and no defective parts delivered. The point is not the slogan — it is that each gate has a documented check behind it: XRF at incoming, SPC and in-process checks during machining, and CMM verification at final inspection before packing.

What does the inspection bench actually measure?

An inspection bench is only as credible as the instruments on it, because each measurement answers a different question. A caliper answers "is it roughly the right size"; a CMM answers "is every critical dimension where the drawing says, to a few microns"; an XRF answers "is this the alloy I ordered"; a roughness tester answers "is the surface smooth enough to seal or slide." A serious shop runs all of them, not one.

Instrument	What it measures	Why you should care
Hexagon GLOBAL S CMM	Dimensional conformance of critical features	The same class of machine an auditor would use to check the shop
Vision measuring machine	Small, flat features and 2D profiles	Fast verification of holes, slots and edge positions
XRF spectrometer	Alloy composition at incoming inspection	Catches a substituted or mixed lot before it is cut
SURFCOM roughness tester	Surface roughness (Ra) and profile	Confirms a sealing or sliding surface will behave
Micro-Vickers hardness tester	Material hardness	Confirms heat treatment landed where specified
Salt spray and tensile testers	Corrosion and mechanical behavior	Validates finish and material on samples

The point of listing instruments is not the brand names — it is that each critical property on a drawing maps to a specific measurement, and a shop should be able to tell you which instrument checks which dimension. A shop that cannot name the instrument behind a tolerance claim is asking you to take that claim on faith.

How does SPC work in practice, not in theory?

Statistical process control sounds abstract until you see it as a run chart. A critical dimension is measured across a production run, subgroup by subgroup, and plotted against its control limits. What you are looking at is not a single pass or fail — it is the spread and the drift, which is what actually predicts whether the next hundred parts will stay in tolerance.

Cpk is the number that summarizes that chart. A Cpk of 1.67 means the process output sits well inside the tolerance band with room for drift before a part goes out of spec; a Cpk of 1.33 is the standard bar for general characteristics. On a representative length characteristic (76.1–76.5 mm), our control charts run at Cpk = 1.851 (one characteristic on one part, not a blanket capability claim) across 25 subgroups — above the key-characteristic target. Ask any shop for that number on a real feature; if they cannot produce it, their tolerance claim is a slogan, not a specification.

SPC is also what turns "we measured one part" into "we watched the whole run." A single CMM report proves the first part was right; an SPC chart proves the process stayed right. For a production program, the second is the one that matters, because it is the difference between a good sample and a stable process.

How are non-conformances handled and contained?

A quality system is judged not by whether defects happen — every shop occasionally produces one — but by what happens when one is found. The test is containment: can the shop name the lot, the machine, the operator and the material, and isolate the affected parts instead of guessing.

That is exactly what the traceability record enables. When a non-conformance surfaces, the MES/ERP record narrows it to a specific lot and machine, so containment is a data lookup rather than a warehouse-wide recall. The parts are quarantined, the root cause is written down, and a corrective action is put in place — the same discipline an IATF 16949 auditor expects, applied to every job rather than just the automotive ones. You should ask one question of any supplier: "show me your last non-conformance and what you did about it." A shop with a real system answers with a record; a shop without one answers with silence.

How do you read a CMM report like an auditor?

A CMM report is the evidence a tolerance claim lives or dies on, and reading it is a skill worth having. Three things to check before you accept one.

• Which dimensions are reported. The report should list the criticals from your drawing, by number, not a generic "all in tolerance" summary.
• What the deviation is. Each reported dimension should show its measured value and its deviation from nominal, so you can see how much margin is left — a dimension sitting at the edge of tolerance is different from one in the middle.
• Who measured it and when. The report should carry a date, an operator or inspector, and the machine that produced the part, so it traces back to a real event rather than a template.

A report that passes all three is the difference between a claim and a measurement. It is also the document you keep for your own audit trail — the thing that proves, months later, that the batch you received was the batch you ordered.

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). Cut 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). Cut 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). Cut 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). Cut on shafts, bushings, flanges up to ±0.002 mm.
• Wire EDM (5+) - Sodick AQ535L (550/350/300 mm travel, ±20° taper, 800 kg workpiece). Cut 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.

Frequently Asked Questions

Standards referenced on this page

• IATF 16949:2016 — Automotive quality management system standard
• ASTM A582/A582M-12 — Standard Specification for Free-Machining Stainless Steel Bars
• ISO 14001 — Environmental management systems — Requirements with guidance for use