CNC Parts Buyers Guide

• 1
  How a CNC part is actually made
• 2
  Reading tolerance and datum callouts on a drawing
• 3
  Material choice and how it changes the quote
• 4
  Surface finish, coatings, and what they hide
• 5
  How to judge a CNC supplier's process control
• 6
  When each choice makes sense

CNC parts start as a solid block, bar, or casting. A CAM programmer turns your 3D model into toolpaths, and the machine removes material along those paths. Nothing here is additive, so the geometry you can reach is limited by tool length, tool diameter, and which direction the spindle can approach the surface.

That constraint is the first thing a buyer should understand. Deep pockets, sharp internal corners, and undercuts all have a price. A Ø6 mm end mill cannot cut a 3 mm internal corner radius. A 100 mm deep pocket needs a long tool, and long tools deflect, so the shop has to slow down and take lighter passes.

Typical workholding adds another layer. The part must be held rigidly enough to survive cutting forces without moving. Thin walls, tall ribs, and unsupported overhangs vibrate. When that happens, surface finish degrades and dimensions drift. On a 4,000 mm part the same rule applies, just on a bigger scale.

Five-axis machining changes what is possible. With 16 simultaneous 5-axis centers, angled faces and compound features can be cut in one setup, which removes the re-fixturing error that stacks up across multiple operations. For parts with features on five or more faces, that is usually the difference between holding ±0.005 mm and chasing it.

• 1
  Reach beats powerIf the tool cannot touch the feature, no spindle speed fixes it.
• 2
  Fewer setups, fewer errorsEach re-fixture adds its own positional deviation.
• 3
  Stiffness sets finishVibration shows up as chatter, not as a dimension error at first.

A tolerance is only meaningful against a datum. If a drawing shows ±0.05 mm on a hole position but does not say which face or bore is the datum, the shop will pick one, and it may not be the one your assembly uses. That mismatch is a common source of parts that measure fine in inspection and still fail to fit.

Tolerance stack-up matters more than any single number. Five features each at ±0.05 mm can put the outermost feature 0.25 mm away from nominal in the worst case. If the mating part has the same allowance, the assembly may not close. Ask for a stack-up check before you release the drawing, not after the parts arrive.

Tighter is not automatically better. Going from ±0.1 mm to ±0.005 mm usually means more setups, slower feeds, temperature-controlled inspection, and a higher scrap rate. On a batch of 10,000 parts that cost is real. Apply tight tolerance only to the features that actually control fit and function.

General tolerances cover everything else. A title-block note like ±0.1 mm on unmarked dimensions keeps the drawing readable and keeps the quote honest. If every dimension carries its own tolerance, the programmer has to treat each one as a critical feature, and the price reflects that.

• 1
  Pick datums the assembly usesA datum that is easy to machine is not always the right one.
• 2
  Check stack-up earlyWorst-case addition is quick and prevents expensive rework.
• 3
  Reserve tight toleranceOnly the fit-critical features need the tight number.

Aluminum 6061-T6 is the default for most machined parts. It cuts fast, holds tolerance well, and takes anodizing cleanly. When you need higher strength, 7075 offers roughly twice the yield strength of 6061, but it machines slower and is more prone to stress corrosion in some environments. Choose it because the load case demands it, not because it sounds stronger.

Stainless behaves differently. Grades 303 and 304 cut reasonably well; 316 and 316L resist corrosion better but work-harden quickly, so the shop has to keep the tool moving and avoid rubbing. Grade 17-4PH gives high strength after heat treatment, which means the part may be machined in one condition and used in another. That sequence has to be planned.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the difficult end. Both generate heat at the cutting edge and wear tools fast. Cycle times can be several times longer than the same part in aluminum, and the material cost is higher. If a titanium part is not required by weight or temperature, aluminum or steel usually delivers the same function for less.

Plastics are not automatically easier. POM and PEEK hold dimensions well, but ABS and PP deflect under clamping pressure. PEEK is expensive and abrasive to tooling. Carbon fiber reinforced plastic wears carbide quickly and produces dust that needs extraction. For prototypes, these differences often decide the process before the geometry does.

• 1
  6061-T6 for general workGood strength, easy machining, clean anodizing.
• 2
  316L when corrosion mattersExpect slower cutting and more attention to work hardening.
• 3
  Titanium and Inconel cost timeHigher material cost plus longer cycle time.

Surface roughness is specified as Ra, and the number drives the machining strategy. As-machined at Ra 1.6-3.2 μm is normal for most functional surfaces. Ra 0.8-1.6 μm requires finer passes and sharper tooling. Ra 0.2-0.8 μm usually means a secondary operation or a dedicated finishing pass, and it costs accordingly.

Coatings change dimensions. Anodizing builds an oxide layer that can add 5-25 μm per surface depending on type. Hardcoat anodizing is thicker than decorative clear anodizing. If a bore is anodized after machining, the hole gets smaller. The shop has to account for that in the pre-plate dimension, and the drawing should say so.

Plating follows the same logic. Electroless nickel, zinc, silver, and gold all add thickness. Silver and gold are used for conductivity, so masking matters: if the contact area is plated, resistance changes. Laser marking needs a minimum character height of 1.5 mm to stay legible, and it should not be placed on a sealing surface.

Cosmetic requirements deserve their own note. Bead blasting, brushing, and polishing produce different appearances, and a scratch that is invisible on a bead-blasted part shows clearly on a polished one. If the part is visible to an end user, say so on the drawing. Otherwise the shop will optimize for function, which is the right default.

• 1
  Ra drives the toolpathFiner finish means slower passes and more inspection time.
• 2
  Coatings add thicknessAllow for it on tight bores and threads.
• 3
  Mask conductive areasSilver and gold plating change contact resistance.

Certifications tell you what system the shop runs, not how well it runs it. ISO 9001:2015 covers general quality management. IATF 16949:2016 adds automotive requirements, including traceability and change control. ISO 13485:2016 is written for medical devices. ISO 27001:2022 covers information security, which matters if your drawings are confidential.

Ask how inspection is done. A shop that inspects 100% of parts before shipment and can produce raw material certificates, in-process records, and a final inspection report is easier to audit than one that samples. Ask whether the CMM is temperature-compensated and how often it is calibrated. Those details separate a real system from a certificate on the wall.

Lead time claims need a basis. Quotation and free DFM analysis within 12 hours is a process claim, and production starting within 24 hours depends on material availability. Parts shipping in 3-5 days applies to standard work, not to a 4,000 mm weldment. Ask which step the clock starts at: PO receipt, material arrival, or first cut.

Machine count and type matter for capability. A shop with 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers can route work to the right machine instead of forcing it onto one. That flexibility is what keeps a schedule stable when one job runs long.

• 1
  Match certificate to industryMedical and automotive have different requirements.
• 2
  Ask for inspection recordsReports on request are a sign the data exists.
• 3
  Define the lead-time clockStart from PO, not from a vague promise.