CNC Buyer's Guide

CNC Machining Buyer's Guide

Everything a mechanical engineer or procurement manager needs to know before sending a CNC RFQ — from drawing preparation to supplier selection and first-article approval.

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TL;DR

Always provide a 2D drawing with tolerances — 3D model alone leads to cost blow-ups on tight-tolerance features.
Default to ±0.1 mm unless the function requires tighter — every step tighter costs 2–5× more to achieve and inspect.
3-axis is 40–80% cheaper than 5-axis; redesign for 3-axis access wherever feasible.
EU workshop for prototypes (<5 pcs), Asia for production runs (>50 pcs) — the breakeven is around 20–30 pieces at typical values.

1. Pre-RFQ drawing checklist

Incomplete drawings are the #1 cause of requoting, late deliveries, and surprise costs in CNC machining. Complete this list before sending:

All critical tolerances callout out explicitly — "standard machining tolerance" is ambiguous. Use ISO 2768-m (medium) or ISO 2768-f (fine) as default, then add specific callouts on functional features.
Surface finish specified (Ra in µm) — Ra 3.2 is as-machined, Ra 1.6 is semi-finish, Ra 0.8 requires grinding or lapping. Each step roughly doubles the cost.
Material grade and standard — "aluminium 6061" is insufficient. Specify "Aluminium EN AW-6061-T6" or ASTM B209. Supplier needs to order the right alloy, temper, and form.
Thread callouts complete — include standard (M, G, UNC, UNF), class of fit (6H/6g for metric), and engagement depth. Missing any of these causes errors.
GD&T symbols defined — flatness, parallelism, perpendicularity, and position symbols must have datum references and tolerance values. Ambiguous GD&T is worse than no GD&T.
Post-process requirements — anodising, passivation, zinc plating, heat treatment. These often add 30–80% to part cost and require separate specialist quotes.
Inspection requirements — do you need a CMM report, material cert, or PPAP? Specify inspection level (per PPAP level 2 or 3, or custom).
Drawing revision locked — freeze the drawing before RFQ. Design changes after quotes are in typically require full re-quote.

2. Material → cost decision tree

Material	Relative cost (vs 6061-T6 = 1.0)	Best for	Watch out for
Aluminium 6061-T6	1.0×	Structural, lightweight, anodisable	Not weldable in T6 state
Aluminium 7075-T6	1.4×	High-strength aerospace, cycling	Stress-corrosion risk in marine
Stainless 304	2.2×	Food-grade, corrosion resistance	Work-hardens; slows cutting
Stainless 316	2.8×	Marine, pharmaceutical	Higher raw material cost
Steel 1045	0.8×	Shafts, gears, heat treatable	Rusts without coating
Brass CW614N	1.6×	Connectors, valves, threads	Heavy; poor strength-to-weight
Titanium Grade 5	6–9×	Medical implants, aerospace	Requires specialist tooling

3. 3-axis vs 5-axis: when to upgrade

Most CNC parts can be made on a 3-axis machine with the right feature orientation. Use 5-axis when:

The part requires machining features at multiple compound angles (e.g., turbine blades, complex aerospace brackets).
There are undercuts or deep pockets not accessible from the top face.
You need to machine 5+ faces in one setup to maintain positional accuracy (±0.02 mm GD&T position).
The part geometry requires sculpted surfaces (free-form milling).

If you're paying for 5-axis but your part could be re-designed for 3-axis with a simple feature reorganisation, you're leaving 40–60% cost savings on the table. Ask your supplier to review DFM before quoting.

4. EU vs Asian suppliers: the honest comparison

Dimension	EU workshop (Germany, Poland, Czech)	Asian workshop (China, Vietnam)
Prototype (1–5 pcs)	✓ Preferred — fast iteration, easy communication	Acceptable if lead time allows
Production (50+ pcs)	Higher cost but strong IP protection	✓ Preferred — 30–60% lower unit cost
Lead time	5–12 business days	7–15 business days + 18–35 days shipping
Tolerances	±0.01 mm routinely achievable	±0.01 mm achievable with right supplier
Communication	European business culture, GDPR-native	Time zone gap; use video + written specs
Customs/duty	No customs, no duty, SEPA payment	Import duty 3–12% depending on HS code

Frequently asked questions

What files do I need to get a CNC quote?

A 2D drawing (PDF or DXF) with all critical tolerances, surface finishes, and material callout is mandatory. A 3D model (STEP or IGES) speeds up quoting and reduces misinterpretation. For thread callouts, include thread standard (ISO, UNC, UNF) and engagement depth.

What is the difference between 3-axis and 5-axis CNC machining?

3-axis machines cut along X, Y, Z axes — suitable for prismatic parts with features accessible from one side. 5-axis adds rotation around two axes, enabling complex contours, undercuts, and compound angles in a single setup. 5-axis is 40–80% more expensive per hour but eliminates multiple setups and improves accuracy on complex geometry.

How tight a tolerance can CNC achieve?

Standard CNC tolerances are ±0.1 mm on a 3-axis mill. Precision work achieves ±0.01 mm with appropriate tooling and inspection. Calling unnecessary tight tolerances (e.g., ±0.01 mm on a non-functional face) can increase costs by 3–5× vs ±0.1 mm on the same feature.

What is the minimum order for CNC machining?

Most EU workshops have a €80–€200 minimum per order to cover setup time. Asian workshops typically accept 1-piece orders from €50–€120. For volume orders (50+ pieces), per-part cost drops significantly as setup is amortised.

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