Introduction to CNC Processing

• 1
  What matters most in an introduction to cnc processing
• 2
  Choosing the right CNC process for the part
• 3
  What happens between your CAD file and a finished part
• 4
  Introduction to cnc processing: which operations fall under it
• 5
  How tolerance and surface finish change the price
• 6
  Documentation and process control to ask for
• 7
  Reading a CNC quote so quotes are comparable
• 8
  How to run a first CNC processing order

CNC processing starts with a 3D model. A CAM programmer turns that model into toolpaths, picks the cutter, sets the feeds and speeds, and defines the workholding. The machine then follows the program to remove material. Nothing about the shape comes from a mold or a pattern, so the first part and the thousandth part are cut from the same instructions.

That is the core advantage over casting or stamping. You can change a dimension in the model and re-cut the part without new tooling. For low and mid volumes, that removes both the tooling cost and the weeks it takes to make it.

Subtractive work also has limits. A cutter is a spinning cylinder. It cannot reach a square internal corner, and it cannot cut a deep narrow slot without deflecting. If a feature is smaller than the smallest cutter available, or deeper than about four times the cutter diameter, the price climbs fast. Sometimes the right answer is to split the part or change the corner radius.

The practical takeaway: design review matters more than machine count. A supplier who reads your model and flags a 0.5 mm internal corner before cutting saves more money than one who quotes the lowest hourly rate.

• 1
  CAD to CAMSTEP and IGES are safest. Send the native solid if the supplier asks for it.
• 2
  Workholding planVises, soft jaws, fixtures or vacuum. It decides how many setups the part needs.
• 3
  First articleMeasure the critical callouts before the run continues.
• 4
  InspectionCMM, micrometers, bore gauges and surface roughness checks, with reports on request.

The term covers more than milling. A full service shop runs milling, turning, mill-turn, drilling, tapping, reaming and boring, plus the secondary steps that make a part usable. There are 127 high-precision CNC machines across our three plants, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers.

Size range matters when you compare suppliers. Maximum processing size is 4,000 mm. Large travels run 4,000 × 400 × 150 mm, medium travels 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact travels 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table handles round work that needs indexing.

Material choice drives the cutting data. Aluminum 6061 and 7075 cut fast and hold tight tolerances. Stainless 304 and 316L work-harden if the feed is too light, so a heavier chip load is safer. Titanium TC4 and Inconel need low surface speed and rigid setups. Plastics like POM, PEEK and PC cut easily but move with temperature, so leave stock and take a light final pass.

Finishing is usually part of the same order. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting and polishing are all available. Laser marking works down to 1.5 mm character height, which matters for traceability marks.

• 1
  Milling3-axis, 4-axis and simultaneous 5-axis for pockets, contours and complex faces.
• 2
  Turning and mill-turnShafts, bushings, connectors and parts that need both operations in one cycle.
• 3
  Secondary operationsDeburring, tapping, heat treatment coordination, plating and marking.
• 4
  Inspection100% inspection before shipment, with reports on request.

Tolerance is not a single number for the whole part. It applies feature by feature. A general block tolerance of ±0.1 mm covers most non-critical dimensions. Tightening one bore to ±0.005 mm may add a reaming step, an in-process check and a slower feed. Tightening every dimension on the drawing the same way multiplies that cost across the part.

Surface finish follows the same logic. As-machined surfaces run Ra 1.6–3.2 μm, which suits brackets, covers and most structural parts. A high finish of Ra 0.8–1.6 μm covers sealing faces and sliding surfaces. Fine finish of Ra 0.2–0.8 μm is for optical seats, bearing bores and hydraulic sealing, and it usually needs a separate finishing pass with a small stepover.

The common mistake is calling out tight tolerance and fine finish everywhere. An engineer who marks only the five or six features that actually function will get a lower price and a shorter lead time, with no loss in part performance.

• 1
  General tolerance±0.1 mm is enough for most non-mating dimensions.
• 2
  Critical featuresHold ±0.005 mm only on bores, spigots and datum faces that mate.
• 3
  As-machinedRa 1.6–3.2 μm, no extra pass, lowest cost.
• 4
  Fine finishRa 0.2–0.8 μm, add a finishing pass and a roughness check.

Certificates tell you what a supplier is allowed to run, not how well they run it. ISO 9001:2015 covers general quality management. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical device work. ISO 27001:2022 covers information security, which matters when your drawings are confidential.

Ask what happens between operations. Raw material should be checked against the mill certificate before cutting. In-process monitoring catches a drift before a whole batch is scrap. Final inspection happens before shipment, and reports are available on request. A supplier with a 99.99% qualification rate is doing all three, not just the last one.

Confidentiality is a separate track. Uploads should be secure, and an NDA should be available on request before you send sensitive geometry. If a supplier hesitates on that, treat it as a signal.

For regulated industries, ask for the inspection plan on paper. Which features are measured, with what instrument, and how often. A supplier who can answer that in detail has run the process before.

• 1
  Material checkMill certificate matched to the alloy and heat lot before cutting.
• 2
  In-processDimensional checks during the run, not only at the end.
• 3
  Final inspection100% before shipment, CMM and hand tools, reports on request.
• 4
  ConfidentialitySecure uploads; NDA available on request.

Two quotes are only comparable when they cover the same scope. Check whether the price includes material, programming, fixturing, finishing and inspection. A low number that excludes anodizing is not a low number.

Ask for the unit price at your volume and the setup or non-recurring cost separately. Setup is charged once, so it matters a lot at quantity 5 and almost nothing at quantity 5,000. If a quote only gives one blended figure, you cannot tell which lever to pull.

Lead time should be stated as a range with a start condition. We quote and return a free DFM analysis within 12 hours, production can start within 24 hours of approval, and parts typically ship in 3–5 days. Historical late-delivery probability is below 2%. Treat any supplier that gives a single hard date without conditions as a risk.

Finally, check the minimum order quantity. There is no minimum order quantity here, so a single prototype and a 10,000+ part run both go through the same process. If a supplier has a high MOQ, that is a sign their setup is built for volume only.

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  ScopeMaterial, programming, fixturing, finishing and inspection all listed.
• 2
  Setup splitNon-recurring cost shown separately from unit price.
• 3
  Lead time basisRange plus the condition that starts the clock.
• 4
  MOQNo minimum order quantity means prototypes and production share one route.