CNC Machining Materials: Aluminium/Steel/Stainless/Plastics

Choose the material for the finished part

Start with what the part must do: carry a load, resist corrosion, slide against another surface, insulate electrically or retain its shape in service. Give the supplier the operating environment, assembly requirements and any regulated use before requesting material alternatives.

Specify more than a grade name. Include the product form, material condition, required properties, polymer formulation where applicable and surface treatment. Compare the finished-part quotation, including machining, finishing and inspection.

Use the same specification process for replacement components, prototypes and repeat orders. Where failure could cause injury or substantial damage, have the responsible designer approve the material.

Common CNC materials: a practical comparison

Use this table to prepare questions for your designer and supplier. It does not establish suitability for a particular application or rank current material prices. Compare alternatives using the same drawing, quantity, finish and inspection requirements.

Chinese versus ASTM or EN grades: approve evidence, not a conversion chart

Do not approve a substitution between Chinese GB or GB/T, ASTM and EN specifications solely because a supplier calls the materials equivalent. This guide asserts no cross-standard grade equivalence. Ask for the proposed standard and material documents before approving a change.

The official Chinese standards record identifies GB/T 3190-2020 as a chemical-composition standard for wrought aluminium and aluminium alloys. ISO 209:2024 specifies chemical-composition limits for wrought aluminium products and ingots intended for working. These scopes do not demonstrate that a supplied batch meets your product specification.

Ask the supplier to identify the exact grade, standard edition, applicable amendments, product form and delivery condition on both sides of the proposed substitution. Have the responsible designer compare the applicable requirements with the certificate, including composition and relevant mechanical properties.

Check temper, hardness, dimensions and service suitability separately from composition. If the evidence is incomplete, retain the original specification or agree appropriate testing before accepting a change.

For plastics, specify more than POM, ABS or PEEK. Request the actual manufacturer and formulation, including fillers and additives where relevant. Use the applicable material datasheet for selection and batch documentation to identify the supplied stock.

Compare finished-part cost, not material price alone

Request separate quotation items for stock, machining, heat treatment, surface treatment and inspection. Ask suppliers to identify assumptions about stock availability and substitutions.

Keep the drawing and acceptance requirements fixed when comparing materials. Ask each supplier to explain its proposed machining route, workholding, heat treatment and distortion controls. Assess a proposed alternative against the specification as well as the price.

For a detailed quotation breakdown, read CNC machining cost drivers in China. For plate used in fixtures or tooling, see sourcing aluminium tooling plate from China.

Material certificates and traceability: agree the chain before cutting

Specify the documents required in the purchase order. ISO 10474:2013 defines types of inspection documents supplied for steel products according to order requirements. Identify the document type, tests and verification you expect rather than requesting only a 'material certificate'.

For metals, request the original mill certificate showing the manufacturer, material designation, applicable standard, heat or batch identity, and the test results required by your order. Where material comes through a stockholder, request records linking the supplied stock to that certificate.

For plastics, request the stock manufacturer's identity, formulation, production batch and the agreed certificate of conformity or analysis. Request batch identification alongside the material datasheet.

Agree how identification will be preserved when stock is cut and parts are machined, heat-treated or finished elsewhere. Require records connecting incoming material, production lots, subcontracted operations and delivered parts.

Review the evidence before stock is consumed. Agree any independent testing, sampling and acceptance criteria in advance. Review material verification separately from visual and dimensional inspection, and check that the documents identify the material used for the delivered parts.

What to put in your enquiry and purchase order

Send the part drawing and revision, intended use, destination market, quantity and required delivery date. State the material designation, governing standard and edition, applicable amendments, stock form, condition or formulation, finish and required documentation.

Require written approval before material substitution. Ask the quotation to disclose any alternative grade, unavailable stock or certificate limitation.

Agree a sample or first-part review before releasing the remaining production where appropriate. Include material documents in that review, alongside the finished part.

Use drawings and tolerances to send to China for the dimensional specification. Review material and drawing changes together and record both approvals.

If you sell in the US or EU

US entry — checked 18 October 2026: CBP states that the importer of record remains responsible for correct entry documentation and applicable duties, taxes and fees even when using a broker. Give the broker an accurate description of the part, material and use.

US tariffs — checked 18 October 2026: USTR publishes China Section 301 tariff actions and exclusion information. Have the applicable classification and current import measures checked for the actual goods; do not determine tariff treatment from a material name alone.

US food-contact uses — checked 18 October 2026: FDA guidance bases a food-contact material's regulatory status on its constituent substances and applicable regulatory provisions, including specifications and conditions of use. Request evidence for the supplied formulation and intended use rather than accepting a general 'FDA-approved material' claim.

EU CE marking — checked 18 October 2026: official EU guidance says CE marking is required only for products covered by harmonised EU rules that specifically mandate it, and must not be used where no such requirement exists. Assess the actual product and applicable requirements separately from material certification.

UK example: customs value and import VAT

For a shipment to Great Britain, keep the part description, material specification, invoice and transport breakdown consistent. HMRC guidance checked on 18 October 2026 includes relevant delivery and insurance costs up to the applicable place of introduction in customs value. Ask the declarant to confirm that place and any required transport-cost apportionment for the shipping method.

HMRC's import VAT guidance, checked on 18 October 2026, starts with customs value and adds applicable import duties and other import charges excluding VAT itself, plus relevant incidental expenses to the first UK destination and a further UK destination known at importation. Ask the declarant to identify the additions and avoid counting costs already included.

This is a destination-specific example. For shipments elsewhere, obtain the destination authority's valuation and tax rules rather than applying the UK treatment.

Getting help with a material-specific sourcing brief

When commissioning sourcing help, provide the drawing, material requirements, supplier quotation, required certificates, proposed substitutions, quantity and destination.

Agree the scope and deliverables of any quotation review or small-order sourcing work before commissioning it.

Ask the written quotation to state whether factory checks, goods inspections, laboratory testing and third-party services are included, and agree any additional charges in advance.

Sources

The standards and authority sources below support the standards scopes and destination-specific guidance. Manufacturer and industry sources support the limited technical background; their product-specific information is not a guarantee for another supplier's stock. No cross-standard grade equivalence or current material-price ranking is asserted. The two retained NIST links could not be retrieved during this audit and were not relied on.