Sourcing CNC Machined Parts from China: Drawings, Tolerances, and Quotes

Getting accurate CNC machining quotes from Chinese suppliers depends almost entirely on the quality of your request-for-quote package. A complete drawing set with clearly marked tolerances, correct material designations, and defined inspection requirements will produce comparable quotes and fewer disputes at delivery. This guide covers what to include, how Chinese shops interpret tolerances and material grades, and how to structure first article inspection so problems surface before mass production.

What a Complete RFQ Package Contains

Most quoting delays and pricing errors trace back to incomplete packages. A shop that receives only a 3D model will either guess at your requirements or quote conservatively to cover unknowns. Either way, the number you receive is not the number you will pay. A complete package for a machined part includes five elements.

• A 3D model in STEP format (.stp or .step). STEP is the neutral exchange format every Chinese shop can open. Native SolidWorks or NX files are usually fine as a supplement, but STEP should be the reference. Avoid sending only STL meshes; they are approximations and cannot drive precision machining.
• A 2D drawing in PDF. The 3D model defines geometry; the 2D drawing defines intent. It carries tolerances, datums, surface finish callouts, thread specifications, and notes. A shop cannot infer from a solid model which of your two hundred dimensions actually matter.
• Material designation with a specific grade. Write "AL 6061-T6," not "aluminum." Write "AISI 304 stainless," not "stainless steel." If you accept equivalents, say so explicitly on the drawing, for example "6061-T6 or GB equivalent."
• Surface treatment and finish requirements. Anodizing type and color, plating thickness, passivation, bead blasting, or "as machined" — state it. Include the surface roughness target (for example Ra 1.6 or Ra 3.2) on functional surfaces.
• Quantity tiers. Ask for pricing at several volumes, such as 10, 100, 500, and 2,000 pieces. Setup and programming costs amortize across the batch, so per-piece prices drop sharply with volume, and the tier structure tells you how a shop's cost model works.

Tolerances: ISO 2768 for General Dimensions, Explicit Callouts for Critical Ones

The most common and effective tolerancing strategy is a two-layer approach. In the drawing's title block, state a general tolerance standard such as "ISO 2768-mK" (medium class for linear dimensions, class K for geometric tolerances). Chinese shops know ISO 2768 well; the domestic equivalent GB/T 1804 is modeled on the same system. This covers every dimension you have not individually toleranced.

Then, for the dimensions that genuinely control fit or function — bearing bores, mating faces, thread depths, sealing surfaces — apply explicit tolerances directly on those dimensions, along with geometric controls (flatness, perpendicularity, position) where needed. Two rules make this work in practice.

• Do not blanket-tighten. A drawing that puts ±0.01 mm on every dimension will be quoted as if every dimension needs precision grinding, and the price reflects that. Tight tolerances should be the exception, applied only where the design requires them.
• Do not leave critical features to the general tolerance. ISO 2768-m allows roughly ±0.1 to ±0.3 mm on typical mid-size dimensions. If a press-fit bore is covered only by the general tolerance, the shop can deliver a part that is dimensionally "correct" and functionally useless.

If your drawing uses GD&T per ASME Y14.5, confirm during quoting that the shop's inspection department can measure it. Capable suppliers in the machinery and equipment manufacturing sector routinely work to GD&T, but it is worth verifying that they own a CMM rather than only calipers and micrometers.

Material Grades: GB Designations and Their International Equivalents

Chinese mills certify material to GB (national standard) grades. When your drawing specifies an AISI, ASTM, or EN grade, the shop will normally purchase the closest GB equivalent. For common engineering materials the equivalence is close and well established, but you should know what substitution is happening and ask for the mill certificate so you can verify the chemistry yourself.

When a Chinese supplier's own drawing or quote lists a GB grade you do not recognize, ask for the chemistry range and cross-check it against the grade you designed for. Equivalence tables are a starting point, not a certification.

Six Reasons Quotes for the Same Part Vary Widely

It is normal to receive quotes that differ by a factor of three or more for an identical drawing. Before assuming the low bidder is cutting corners or the high bidder is padding, understand where the spread comes from.

1. Machine class and process routing. One shop runs the part in two operations on a 3-axis mill with a refixture; another does it in one setup on a 5-axis machine. Cycle times, labor, and risk differ accordingly.
2. Tolerance interpretation. Some shops quote your drawing as written; others silently assume they can hit "close enough." A very low quote sometimes means the shop did not read the tolerances at all.
3. Material sourcing. Certified imported bar stock, domestic certified GB material, and uncertified market-grade material carry very different costs. If your RFQ does not demand mill certificates, quotes will not be comparing the same input.
4. Outsourced finishing. Anodizing, plating, and heat treatment are usually subcontracted. Shops with established finishing partners quote these tightly; others add generous margins to cover uncertainty.
5. Overhead structure and order-size fit. A 20-machine job shop and a 300-machine production plant have different break-even batch sizes. A quote that seems high may simply mean your volume is wrong for that factory.
6. Inspection and documentation burden. Full dimensional reports, material certs, and FAI documentation take real hours. Shops that include them quote higher than shops that plan to ship parts in a bag with no paperwork.

The practical countermeasure is to make quotes comparable: send the same complete package to every candidate, state inspection and documentation requirements up front, and ask each shop to note any dimension it cannot hold. Browsing suppliers by capability in a structured manufacturer directory helps you shortlist shops whose equipment actually matches your part before you spend time quoting.

How to Require First Article Inspection (FAI)

First article inspection is your main defense against discovering problems after 1,000 pieces are on a boat. Write it into the purchase order, not an email thread. A workable FAI clause covers four points.

• Scope. The supplier measures every dimension and note on the drawing — not a sample of dimensions — on one to three parts produced with the actual production process, and records actual measured values against nominals and tolerances. If you work in an AS9102-style format, provide the report template.
• Evidence. The report ships with supporting documents: material mill certificate, finishing certificates (anodize thickness, plating composition), and photos of the measured parts with serial markings.
• Approval gate. Production does not proceed until you approve the FAI in writing. Build the review time into your schedule; a rushed FAI approved under delivery pressure protects nobody.
• Re-triggering. A new FAI is required after any change in process, machine, material source, or subcontractor, and after any drawing revision. Say this explicitly, because suppliers will otherwise treat FAI as a one-time formality.

For critical parts, pair the FAI with an independent check: have a third-party inspector or your own engineer re-measure a handful of key dimensions. Agreement between the two reports tells you the supplier's metrology can be trusted for ongoing production. Suppliers in the fabricated metal products sector that regularly serve automotive or aerospace customers will find none of this unusual; resistance to basic FAI requirements is itself useful information about a shop.

Key Takeaways

• Quote from a complete package: STEP model, 2D PDF drawing, specific material grade, surface finish, and quantity tiers.
• Use ISO 2768 general tolerances in the title block and explicit callouts only on dimensions that control fit and function.
• Know the GB equivalents of your materials (304 = 06Cr19Ni10, 1045 = 45 steel) and always request mill certificates.
• Wide quote spreads usually reflect different assumptions about process, material, finishing, and documentation — make those assumptions explicit to compare fairly.
• Put FAI requirements in the purchase order with a written approval gate before production, and re-trigger FAI on any process or drawing change.

This guide is editorial reference material, not legal or transactional advice. Verify supplier claims and regulatory requirements independently. See our data sources and editorial policy.