Inside a CNC Machined Part: Process Route and the Three-Stage CMM Check Before Shipment

The parts in the photo above are 6061-T6 aluminum bodies machined for an industrial customer: stepped pockets on the top face, bored holes through the body, a machined flange and a set of threaded mounting points. Nothing about the shape is unusual. What makes them shippable is the route they took and the measurements taken at the end of it.

This article walks through that route step by step, and then through the three-stage coordinate measuring machine check that decides whether a batch leaves the factory or goes back to the machine.

1. Drawing and DFM review. Before quoting, we check whether the tolerance on the drawing is achievable with the geometry as drawn. A 0.01 mm position callout on a hole 4 mm from a thin wall is not the same problem as the same callout in solid material. Anything that will cost the customer money for no functional gain is flagged, in writing, before the order starts.
2. Material and blank preparation. 6061-T6 plate is verified against the mill certificate for grade and temper. The blank is cut oversize so the part can be faced square on all sides before any feature is machined.
3. Stress relief. Aluminum plate carries residual stress from rolling. Removing 60 percent of the material from one side releases it and the part bows. Rough machining, then stress relief, then finishing is what keeps a 200 mm plate flat to 0.02 mm.
4. Roughing. Every pocket and bore is roughed with 0.4 to 0.5 mm of stock left on all surfaces. Roughing is where the heat goes; leaving stock means the finishing pass cuts cool material and holds size.
5. Finishing in one clamping. Pockets, bores and the flange are finished without re-fixturing wherever the geometry allows, because every re-clamp adds a new datum error. Related features that must align are finished in the same setup.
6. Holes and threads. Critical holes are drilled undersize, bored to position, then reamed. Threads are cut or thread-milled to a controlled depth so a fastener seats on the face and not on the last thread.
7. Deburr and surface finish. Hand deburring on edges, cross-hole breakouts and thread starts, then cleaning. Anodizing follows with sealing faces, bores and threads masked so the coating does not change a fit.
8. Final wash and inspection. Parts are washed, dried, and released to the CMM queue. Nothing is packed before it has a report.

New buyers often assume accuracy is a function of the machine's specification. In practice a modern five-axis centre is the least variable part of the system. What moves a dimension is everything around it:

• Fixture rigidity. A part that vibrates during finishing leaves chatter marks and a wall that is 0.02 mm off.
• Thermal drift. A spindle running for six hours grows; we finish critical features early in the run and check the first part against the last.
• Tool wear. A worn end mill cuts undersize and pushes material instead of shearing it. Tool life is logged per programme, not judged by sound.
• Order of operations. Boring a hole after the surrounding pocket is finished, rather than before, is often the difference between 0.01 mm and 0.03 mm of position error.

Every batch on our floor passes three coordinate measuring machine checks, not one. They answer different questions, which is why one is not enough.

The first part off the machine is programmed feature by feature against the drawing: datums aligned, then position, flatness, perpendicularity, bore size and depth. If a dimension is out, the programme is corrected before the second part is cut. The report travels with the batch.

During the run, the operator measures key features at a defined interval. CMM sampling catches drift that a single first article cannot: tool wear, thermal growth, a fixture that has loosened. If a trend appears, the offset is corrected while the batch is still running.

Before packing, every part is checked on the features that decide whether it assembles: bore position, pocket depth, flange flatness, hole pattern. Small and medium batches are inspected one by one; for high-volume runs we use a sampling plan agreed with the customer and a dedicated checking fixture.

The machine itself is calibrated and its probing verified against a certified sphere before a batch is released. Measurement is done in a temperature-controlled room, because an aluminum part measured at 28 degrees reads several microns larger than the same part at 20 degrees.

A CMM measures geometry. It does not see a burr inside a cross-drilled passage, a thread that is one turn shallow, a tool mark across a seal path or a part that leaks. That is why the inspection plan is layered:

• Visual and borescope: burrs, tool marks, chips left in cavities
• Thread and pin gauges: fit and effective depth
• Roughness tester: sealing and sliding surfaces
• Leak or pressure test: wherever the part carries gas or fluid

A report full of green numbers and a part that leaks is a failure of the inspection plan, not of the machine. The layers exist to prevent exactly that.

For a one-off prototype, the CMM first article is the only thing standing between a good drawing and a part that does not fit. For a production order, the value is in the trend: a supplier who measures every batch knows when a tool is wearing out before the customer finds out at assembly.

Custom work is our normal business. One piece is a valid order with no tooling charge and no setup fee, prototypes ship in five to eight working days, and batch orders in 15 to 30 working days after drawing approval. Drawings and models are handled under NDA on request, and no customer part appears in samples, catalogues or website photography without written release.

Founded in 2008, Changxinda operates four-axis and five-axis machining centres, turn-mill lathes, CNC lathes and milling machines with 90 to 102 staff under an ISO 9001:2015 system. More than 200 customers are served, 50 to 60 percent of output is exported, and annual sales sit between USD 10 million and 20 million. Our ten customer industries are consumer electronics, automation and industrial equipment, semiconductor and vacuum, medical devices, automotive, aerospace and UAV, optics and photonics, communications, robotics, and instrumentation.

Send a drawing or STEP model and we will return a DFM review, a quotation and a confirmed delivery date within one working day.