- 6061-T6 bar is verified at four stages: goods-in, first-off after programme setup, in-cycle during the run, and a visual pass before finishing.
- Anodizing, plating and powder coating seal whatever machining left behind, so defects have to be found before a part enters the coating line.
- Our CNC capability runs 3-, 4- and 5-axis machining centres, with CMM, video measuring, roughness and spectrometer equipment on the inspection side.
- General tolerance is DIN/ISO2768-f or better, ±0.01mm available on critical features, and machined roughness from Ra0.2 to Ra3.2.
The process, step by step
The process moves one job through the cell. A 6061-T6 bar arrives with its mill certificate, is loaded onto the machining centre, and leaves as a component that still has to be anodized, plated or powder coated. Those finishing steps seal whatever the machining left behind, so inspection has to come first.
At the machine, the operator works from the drawing rather than from the previous part. That is where the rework comes from: a bar slightly oversize, a fixture that shifted since the last setup, a cutter starting to leave a step on a bore. The checks on the shop floor are ordinary — a calliper on a critical dimension, a gauge on a hole, a hand across a finished face — but they happen at defined stages rather than at the end.
Each bar is checked before it goes into the machine. Diameter and straightness are measured against the drawing, the surface is inspected for damage, and the heat number is matched to the certificate that arrived with it. 6061-T6 is defined by a narrow chemistry — magnesium at roughly 0.8 to 1.2% and silicon at 0.4 to 0.8% — so a mislabelled bar can sit close to another alloy on paper and still cut differently. Incoming verification includes spectrometric analysis.
Where the inspection happens
Four stages cover the route from goods-in to finishing. Each answers a different question, and none replaces the final CMM check.
| Stage | What is checked | Equipment | What it catches |
|---|---|---|---|
| Goods-in | Bar diameter, straightness, surface condition, heat number against mill certificate | Callipers, straightedge, spectrum analyser | Mixed or wrong bar, damaged surface |
| First-off | All critical dimensions on the drawing | CMM, video measuring system | Wrong tool offsets, fixture shift, stale programme |
| In-cycle | Critical dimensions and feature size at set intervals | Hand gauges, video measuring | Tool wear, parameter drift, thermal growth |
| Before finishing | Visual pass for burrs, marks and handling damage, plus roughness on critical faces | Visual, rough meter | Defects that coating would seal in |
Final dimensional verification is done on the CMM for the features your drawing calls critical. Our general tolerance is DIN/ISO2768-f or better, ±0.01mm is available as a minimum on critical features, and roughness runs from Ra0.2 for mirror surfaces through Ra3.2 for general machined faces. Aluminium 6061 bar is supplied to ASTM B211, which sets the composition and mechanical property requirements for rolled or cold finished bar, rod and wire.
What arrives with the parts
Every shipment leaves with the records that describe it: the dimensional report against your drawing, the mill certificate for the bar, the inspection records from the stages above, and coating records where a surface treatment was applied. Our plant operates under ISO 9001:2015 and SA8000, and every process from raw material to finished part carries traceable files.
Surface treatment is what makes a late machining judgement expensive. An anodic layer is only microns thick and follows the substrate profile, so a coarse machining mark or a rough Ra stays visible in the finished part rather than disappearing under the coating. Anodic oxide coatings on aluminium are specified under ASTM B580. Polishing, sand blasting, colourless oxidation, plating and powder coating are all available, and each puts a different constraint on the finish arriving at it.
Five questions to ask before you order
Most of my work sits one step earlier: reviewing drawings and confirming material before a job is scheduled. If you are comparing suppliers on aluminium parts, these are the questions I would ask — including of our own plant.
- Which stage verifies a given dimension. A supplier who inspects only finished parts finds defects after the value has been added.
- What the incoming check covers. Chemistry verification and heat-number matching separate 6061-T6 from a lookalike bar.
- What the tolerance figure means in practice. ±0.01mm as a minimum on critical features is a different promise from ±0.01mm as standard.
- How distortion is handled. 6061-T6 carries residual stress from solution treatment and ageing, and T651 is stress relieved by stretching after quenching, so the temper on the quotation matters.
- What happens to a part that fails. Quarantine, root cause and a recorded disposition tell you whether the quality system is real.
Frequently Asked Questions
What tolerance can 6061-T6 aluminium parts hold?
Our general tolerance standard is DIN/ISO2768-f or better. On critical features we work to ±0.01mm as a minimum capability, and the tightest achievable figure depends on feature size, material condition and setup. Roughness runs from Ra0.2 for mirror surfaces to Ra3.2 for general machined faces.
Why do 6061-T6 parts warp after machining, and how do you prevent it?
6061-T6 is solution heat treated and artificially aged, and that process leaves residual stress in the bar. When machining removes material asymmetrically, the stress releases and the part moves. The countermeasures are roughing and stress relief before finishing, symmetric material removal patterns, and T651 temper where the drawing allows it. Because the answer depends on part geometry, we look at it at drawing review rather than after the first batch.
Which surface treatments are compatible with machined 6061-T6 parts?
6061-T6 accepts anodizing, powder coating, plating, polishing and sand blasting. The constraint is the surface underneath. An anodic layer is only microns thick and follows the substrate profile, so a coarse machining mark or a rough Ra stays visible in the finished part rather than being covered. That is why the finish is inspected before parts enter the coating line, not after.
What is the difference between 6061-T6 and 6061-T651?
Both are solution heat treated and artificially aged to the same nominal strength — around 310 MPa tensile and 276 MPa yield for 6061-T6. The difference comes after the quench: T651 is stretched to relieve residual stress, making it the usual choice for thin sections and tight flatness requirements.
Working on 6061-T6 aluminium parts?
Send the drawing and the tolerance you need, and we will confirm the machining route and inspection stages before scheduling.
Alison coordinates technical drawing review, material selection and order scheduling for OEM and ODM buyers in Europe, North America and Russia. The plant behind cnnbxinye.com was founded in 1996 and runs 130 people across 16,000 m² of production floor in Ningbo Jiangbei, with ISO 9001:2015 and SA8000 certification and core production equipment imported from Japan. Alongside CNC machined parts, the site builds forging rings, lost-wax castings and stamped steel components for the same buyer base.
Inspection stages described here apply to CNC machined 6061-T6 aluminium parts produced to our standard general tolerance. Capability, tolerance and surface treatment ranges are confirmed against each drawing and quotation.
Post time: Nov-05-2026