Aluminum Casting Surface Finish: Specify the Requirement

Surface finish can refer to as-cast texture, a machined surface or an applied finish. These are different purchasing requirements. A useful casting RFQ identifies which surface is being controlled, the delivered state and the evidence needed to accept it.

Separate the Three Requirements

A surface that looks smooth may still fail a dimensional, sealing or coating requirement. Conversely, a functional surface need not receive a cosmetic treatment unless the design or purchase specification requires it.

Write Roughness Requirements Unambiguously

State the parameter and unit together. Ra and root-mean-square roughness, commonly designated Rq, are different parameters; do not treat an RMS value as an Ra value without an approved basis. When reviewing older data, verify whether the unit is micrometers, microinches or another explicitly specified unit.

Record the applicable drawing convention and measurement method, including direction and filtering/evaluation conditions where required. If a source shows a descending range or an unclear comparison symbol, check the original controlled information before editing it. Do not guess a missing digit or silently reverse a range.

This guide does not publish a universal roughness range for resin-sand, gravity or die-cast surfaces. Ask for evidence on a comparable feature and distinguish a typical process observation from a contractual limit.

Plan Machining Around Functional Surfaces

For sealing faces, bearing seats, datum pads and mating interfaces, define the final geometry and surface requirement together. Review stock, access, fixturing, tool marks, edge condition and the stage at which acceptance will be checked. A roughness value alone cannot replace flatness, waviness, defect or leak-test requirements when those are needed by the design.

See the CNC machining capability page and the casting tolerance guide when preparing the drawing package. The supplier should identify assumptions that prevent a firm quotation rather than substitute a generic surface range.

Scope Coating, Masking and Dimensional Effects

For anodizing, powder coating, painting or another specified finish, provide the approved finish specification and edition or a buyer-controlled requirement. Agree the substrate condition, preparation, required thickness and measurement method, color or appearance where relevant, and acceptance evidence.

Mark masked bores, threads, seals, datum faces and electrical-contact areas. Identify whether dimensions apply before or after the finish and who accounts for its effect on fits. Specify how handling and packaging protect the accepted surfaces. Do not place a single unverified coating-thickness range on every casting family.

Hardness requirements also need a defined material, location, test method and scale. Do not convert a coating-hardness statement into an HRC value without an applicable, approved basis.

RFQ Surface Schedule

For each controlled area, include:

• •Feature ID and drawing/model reference.
• •As-cast, machined, coated or masked final state.
• •Geometry and texture requirements with parameters and units.
• •Finish system and thickness/appearance criteria where applicable.
• •Inspection method, sampling and record requirements.
• •Repair, touch-up and deviation approval rules.
• •Cleaning, corrosion protection and packaging requirements.

FAQ

Does a coating remove the need for machining?

Not automatically. Required geometry, fits and functional surfaces still need a defined manufacturing and acceptance plan.

Can a supplier choose the roughness unit from context?

It should not. Resolve the drawing or specification ambiguity before quotation and inspection planning.

What is the most useful first step?

Send a marked surface schedule with the drawing through the surface-finish RFQ route. Ask the supplier to return proposed operations, exclusions and any missing acceptance details.