Sample CNC Machining Services: How to Pick a Supplier

• 1
  Key takeaways
• 2
  Which process fits your sample part
• 3
  What sample CNC machining services can actually hold
• 4
  Match the sample to the right machine
• 5
  How to read a sample machining quote
• 6
  Inspection, certification and material traceability
• 7
  Materials and finishes available for samples
• 8
  Seven steps to place a sample order

A sample part is where the supplier shows what the process can do. Tolerance is the first number to agree on. GreatLight works to ±0.005 mm (±0.0002 in) on qualified features. That number applies to a defined geometry, a defined material and a defined measuring method. It does not apply to every dimension on every drawing.

Surface finish is the second number. As-machined finish sits at Ra 1.6–3.2 μm. A high-quality finish is Ra 0.8–1.6 μm. A fine finish is Ra 0.2–0.8 μm, and that usually means extra finishing passes and longer cycle time. Put the finish callout only where the part needs it. A sealing face needs Ra 0.8. A bracket does not.

Tight tolerance and fine finish both add cost, and neither fixes a bad design. If a 0.005 mm tolerance sits on a feature that floats in the stack-up, the sample will pass inspection and the assembly will still fail. Review the functional dimensions before you send the RFQ.

• 1
  State the datumEvery tight tolerance needs a datum that a machinist can indicate on.
• 2
  Keep tight tolerances localApply ±0.005 mm to mating features, not to the whole drawing.
• 3
  Separate cosmetic from functional finishRa 0.2–0.8 μm on a sealing face; Ra 1.6–3.2 μm elsewhere.

Sample CNC machining services are not one process. They are a family of setups, and the setup decides the accuracy. A 3-axis mill cuts from one direction. A 4-axis mill adds a rotary table so the part can be indexed to several faces. A 5-axis machine tilts the tool or the table so the cutter approaches the work from almost any angle.

The practical difference shows up in the number of setups. Every re-fixture adds a datum shift. On a part with features on four sides, 3-axis machining may need three or four setups, and each one can move the part by 0.01 mm or more. A 4-axis or 5-axis machine does the same work in one or two setups.

Consider 5-axis when the part has deep pockets, undercuts, contoured surfaces or a shape you cannot reach with a straight tool. Consider 3-axis when the part is prismatic and the volume matters more than the geometry. For round parts with cross features, a mill-turn center keeps everything concentric in one chuck.

Machine capacity matters too. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm. Common travels are 4,000 × 400 × 150 mm, 750 × 1,150 × 550 mm, 600 × 600 × 600 mm, 500 × 500 × 450 mm and 500 × 310 × 200 mm. Rotary tables go up to Ø400 mm.

• 1
  3-axisFlat plates, covers, brackets, simple pockets. Lowest cost per part.
• 2
  4-axisParts with four-sided features. One rotary setup replaces several re-fixtures.
• 3
  5-axisImpellers, blades, organic surfaces, deep cavities. Shorter tools, better rigidity.
• 4
  Mill-turnShafts and fittings with milled flats or cross holes. Concentricity held in one chuck.

A useful quote breaks the price into lines: material, machining, finishing, inspection and freight. When those lines are bundled into one number, you cannot tell whether the supplier is cheap on machining or expensive on finishing. Ask for the split before you compare two suppliers.

Lead time has two clocks. The first is the quote and DFM review. GreatLight returns a quotation and a free DFM analysis within 12 hours. The second is production. Production can start within 24 hours of a released order, and parts ship in 3–5 days. Historical late-delivery probability is below 2%. Those numbers assume a clean drawing and a material that is in stock.

MOQ is the next filter. Some shops will not quote a single piece, or they quote a setup price that makes one part uneconomic. GreatLight has no minimum order quantity: one prototype or a run of 10,000+ parts. That matters when you only need a fit-check sample before a design review.

Watch for the hidden step. A quote that ignores deburring, anodizing or a first-article report will grow after the PO. List every secondary operation in the RFQ so the comparison stays honest.

• 1
  Split the priceMaterial, machining, finishing, inspection, freight on separate lines.
• 2
  Ask for the DFM noteA supplier who flags thin walls and deep pockets is reading the model.
• 3
  Name the secondary operationsDeburr, anodize, laser mark, plating. Unlisted work becomes a change order.

A sample part is only useful if you can trust the numbers printed next to it. GreatLight inspects 100% of parts before shipment. The flow is raw material check, in-process monitoring and final inspection. Inspection reports are available on request. Ask for the report with the sample so you can compare the measured value to the drawing callout.

Material traceability is the second half. A 6061-T6 part and a 6061-T651 part machine differently and behave differently in service. Ask for the mill certificate when the part carries load or sees temperature. The same applies to 17-4PH, Ti-6Al-4V and Inconel samples.

Certifications tell you which industries the quality system was built for. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The first covers general quality. IATF 16949 supports automotive and EV work. ISO 13485 supports medical devices. ISO 27001 covers information security, which is the one to look for when your files are confidential.

Qualification rate across production is 99.99%. Treat that as a system number, not a promise about your specific part. The sample is what proves your geometry.

• 1
  Ask for the FAI reportFirst-article inspection ties measured values to drawing callouts.
• 2
  Request the mill certLoad-bearing and high-temperature parts need documented heat lots.
• 3
  Check the cert scopeIATF 16949 for automotive, ISO 13485 for medical, ISO 27001 for data.

Material choice drives both the machining plan and the finish. Aluminum 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 cover most brackets, housings and heat sinks. Stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH cover food-contact, marine and high-strength parts. Steel grades include 1018, 1045, 4130, 4140, 4340, A36 and tool steel.

Copper and brass options include C101, C103, C110, beryllium copper, C27400, C28000 and C36000. Titanium and specialty work covers TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B / AZ91D. Plastics for samples include ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre.

Finishing changes dimensions, so it belongs in the sample plan. Anodizing, hardcoat and plating add or remove a few micrometres. If a plated hole must hold a press fit, tell the machinist before the part is cut, not after. Bead blasting, tumbling, brushing and polishing change the surface but not the tolerance. Laser marking and engraving need a minimum character height of 1.5 mm.

Pick the finish that the production part will actually use. A sample with a hand-polished surface does not predict how a production anodize line will look.

• 1
  AnodizingClear, colour, hardcoat and conductive. Hardcoat builds thickness on edges.
• 2
  PlatingElectroless nickel, zinc, silver and gold. Masking affects cost.
• 3
  Coating and oxidePowder coating and black oxide. Black oxide holds tight dimensions.
• 4
  Mechanical finishBead blasting, tumbling, brushing and polishing.