CNC machining process guide: from CAD file to inspected part
This CNC machining process guide walks through the six steps we run every day in Dongguan: file review, DFM, programming, setup, cutting, and inspection. It is written for design engineers and sourcing engineers who need to know what happens to their model after they upload it, and which decisions they still control.
What matters before you read the steps
File review: what we check before anything is cut
Every job starts with the CAD file. We accept STEP, IGES, X_T, and native SolidWorks or Fusion files. The first question is not how to cut the part but whether the model is complete: are there open surfaces, zero-thickness walls, or threads modeled as cosmetic decals instead of real geometry? A model that looks clean on screen can still hide a 0.3 mm wall behind a fillet.
The second check is datum structure. If the drawing calls out a positional tolerance but names no datum, the inspector has nothing to measure against. We ask for A, B, and C datums on any feature that carries a tolerance tighter than ±0.1 mm. On a typical aluminum housing this takes ten minutes to add and saves a rework loop later.
Third, we look at the tolerance callouts against the geometry they describe. A ±0.005 mm bore in a 6061-T6 plate is achievable; the same callout on a 4,000 mm long extrusion is not, because temperature and clamping force move the part more than the tool does. When a callout is unrealistic we say so in the DFM report rather than quoting a number we cannot hold.
Material selection usually lands here too. Aluminum 6061-T6 covers most brackets and housings. Stainless 303 machines cleanly for shafts, 316L for anything that sees washdown or saline. Titanium TC4 and Inconel need slower speeds and more tool changes, which shows up in price, so we flag that before the quote goes out.
- 1Send a STEP fileIt carries solid geometry without translation loss.
- 2Add datumsAny feature tighter than ±0.1 mm needs an A-B-C reference.
- 3Mark critical featuresWe will tell you what they cost to hold.
DFM analysis and quotation within 12 hours
Our DFM report lists every feature that will slow the job down or raise risk. Thin walls under 0.8 mm in aluminum, deep pockets with an aspect ratio past 4:1, and sharp internal corners that force a 1 mm end mill are the usual entries. Each one comes with a suggested change and the time it would save.
We also check whether the part can be held. A thin plate with a tight flatness callout needs either a vacuum fixture or a support plate, and both add setup time. A long shaft with a concentricity callout between two ends may need to be turned in one operation on a mill-turn center rather than in two separate lathe setups.
The quotation and the DFM notes arrive together, within 12 hours of upload. Production can start within 24 hours after you approve. The report is written by the same programmer who will write the toolpaths, so the numbers are not a sales estimate.
If the part is a prototype, we usually suggest leaving cosmetic surfaces as-machined first and adding the finish after the fit is confirmed. Anodizing a part that later needs a boss moved wastes both the coating and the lead time.
- 1Free DFMDelivered with the quote, not as a separate paid step.
- 2Written by the programmerThe person who cuts the part signs off on the cycle time.
- 3Prototype tipConfirm fit before you pay for cosmetic finishing.
CAM programming and toolpath strategy
The programmer imports the model into CAM and builds the operation list. For a three-axis part, that is usually face, rough, semi-finish, finish, then drill and tap. For a five-axis part, the programmer looks for the orientation that reaches the most features in one setup. On a housing with ports on five sides, one five-axis setup can replace four three-axis setups.
Roughing uses the largest tool the geometry allows, typically a 12 mm or 16 mm end mill at 0.5–1.0 mm radial depth of cut for aluminum. Finishing steps down to a 6 mm or 3 mm tool, and only to 1 mm where a small internal radius demands it. Every tool change costs time, so the programmer groups features by tool rather than by feature type.
Feeds and speeds come from the material, not from habit. Aluminum 6061 runs at 300–500 m/min surface speed with carbide. Stainless 316L drops to 120–180 m/min and needs constant coolant. Titanium runs slower still, around 40–60 m/min, with high-pressure coolant to keep the edge alive.
The output is G-code plus a setup sheet. The setup sheet lists the stock size, the work offset, the tool list, and the inspection points. It travels with the job to the machine, so the operator does not have to guess what the programmer intended.
- 1Group by toolFewer tool changes beat a tidy feature-by-feature order.
- 2Five-axis orientationPick the angle that reaches the most faces at once.
- 3Setup sheet travelsOffset, tools, and inspection points stay with the job.
Steps 4 to 6: setup, cutting, and inspection
- 1Prepare the stock and fixtureCut stock to size with 1–2 mm allowance per face. Mount in a vise, soft jaws, or a custom fixture. For a Ø400 mm rotary table job, indicate the fixture to within 0.01 mm before the first cut. Skip the indication and every later dimension inherits the error.
- 2Set work offsets and tool lengthsTouch off X, Y, and Z against the datum faces named on the drawing, not against a convenient corner. Measure every tool on the presetter or in the spindle. A 0.02 mm error in a tool length shows up as a 0.02 mm error in the floor of every pocket that tool cuts.
- 3Run the first articleCut one part and stop. Measure the critical features before running the rest of the batch. On a ±0.005 mm bore, use a bore gauge or a CMM, not calipers. If the first article is out, correct the offset rather than adjusting the program by hand.
- 4Cut the production runRun the batch with in-process monitoring. Check a feature every few parts to catch tool wear. Carbide inserts in aluminum can hold size for hundreds of parts; the same insert in stainless may drift after 30. Watch the chip color and the sound, not just the numbers.
- 5Deburr and finishBreak all sharp edges by hand or in a tumbler. If the drawing calls for anodizing, bead blasting, or electroless nickel, the parts go to finishing after deburr. Mask threads and mating faces before coating. Laser marking needs a minimum character height of 1.5 mm to stay legible.
- 6Final inspection and packInspect 100% of parts before shipment. That covers raw material check, in-process monitoring, and final inspection. Reports are available on request. Parts are packed to avoid edge damage in transit, and each box carries the job number so a measurement can be traced back.
Which machine type fits which part
Use this to judge what a job needs before you ask for a quote.
| Part feature | Machine type | Typical tolerance | When it is the wrong choice |
|---|---|---|---|
| Flat plate, holes on one face | 3-axis mill | ±0.05 mm | Needs 4+ faces machined |
| Pockets on 3–4 sides | 4-axis mill | ±0.02 mm | Undercuts and compound angles |
| Ports on 5 sides, complex angles | 5-axis center | ±0.005 mm | Simple 2D plate work |
| Turned shaft with cross holes | Mill-turn center | ±0.01 mm | Part is longer than 4,000 mm |
| Prototype, one-off geometry | 3-axis or 5-axis | ±0.05 mm | High volume, lower cost per part needed |
| Large frame, 4,000 mm class | Gantry-style 3-axis | ±0.05 mm | Tight tolerances on small features |
Where this process earns its keep
A CNC machining process guide is only useful if it changes what you do next. Send a STEP file with datums and marked critical features, read the DFM notes before you approve the quote, and confirm fit before you pay for cosmetic finishing. That sequence removes most of the rework we see.
Questions we get during the process
What file format should I send?
STEP is the safest choice because it carries solid geometry without translation loss. IGES works but can break surfaces into patches. Native SolidWorks, Fusion, and X_T files are fine too. If you only have a 2D drawing, we can quote from it, but the DFM notes will be shorter because we cannot measure wall thickness from views alone.
How tight a tolerance can you actually hold?
We hold ±0.005 mm on critical features when the geometry supports it. That means a feature with a defined datum, a rigid setup, and a material that does not move much under clamping. On long parts or thin walls, the practical limit widens. We would rather quote ±0.02 mm and hold it than quote ±0.005 mm and argue about it later.
Do I need to pay for finishing separately?
Finishing is quoted as a line item because it is a separate operation with its own lead time. Anodizing, plating, powder coating, and black oxide all add days. Bead blasting, tumbling, and brushing are faster. If your part is a prototype and the finish is cosmetic, run it as-machined first and coat the version that fits.
What is the minimum order quantity?
There is no minimum order quantity. We run from one prototype to 10,000+ part runs. A single part gets the same DFM review as a production order, though the per-part price is higher because setup is spread over one piece instead of many.
How do you handle confidential designs?
Uploads are secure and confidential, and we sign an NDA on request. We do not share customer files or part photos. If your program has an export-control or ITAR requirement, tell us at the quote stage so we can confirm whether we can take the job before any file moves.
What happens if the first article is out of tolerance?
We stop the run and correct it. Usually the cause is a work offset or a tool length that drifted, not the program. The operator re-measures the datum, adjusts the offset, and cuts another first article. If the geometry itself is the problem, the programmer revises the toolpath and we re-run the DFM check on the affected feature.
Send your model and get a DFM report with the quote
Quotation and free DFM analysis within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days.
12-hour quote100% inspectionNo minimum order quantity
More process notes and machining examples
We publish setup notes, tooling trials and inspection data from the factory floor.
