Surface roughness of investment castings should be specified where it affects function. A sealing face, a visible contour and an internal flow passage do not necessarily need the same finish. Applying one demanding value to the entire component can increase work while leaving the important surface requirement poorly defined.

Identify the job performed by each surface

A sealing interface may require machining to establish both geometry and finish. An outside contour may only need an agreed visual condition. An internal passage can introduce separate concerns about cleanliness and access. These requirements should be described independently rather than merged into a general smooth finish note.

Consider the delivered component after all specified operations. The condition of a rough casting is not the same as the condition after machining, blasting or polishing, so the drawing should make clear which stage the requirement governs.

Name the roughness parameter and measurement basis

Different roughness parameters are not interchangeable labels. The requirement should identify the intended parameter and the applicable measurement approach. A number without units or a defined surface can be interpreted in several ways.

Where access prevents straightforward measurement, agree an appropriate verification method before production. Comparing an internal casting surface with an unrelated external sample may not provide meaningful evidence for the actual requirement.

Do not confuse roughness with every surface defect

Roughness describes a particular aspect of surface texture. Cracks, pits, inclusions and local damage create separate acceptance questions. A favorable roughness measurement does not establish that the surface is free of unacceptable discontinuities.

Use separate criteria where needed. This allows the foundry and inspector to distinguish ordinary texture variation from a condition that requires disposition under the component specification.

Let finishing serve a defined purpose

Additional finishing can change cost, geometry and accessibility requirements. Ask which operation is proposed and how it will be verified. A brighter appearance alone does not demonstrate that a sealing, hygienic or corrosion-related requirement has been met.

Tousin Metal can discuss surface requirements for a casting with marked drawing regions and the intended service. A targeted specification helps deliver the surface that performs the required job, while avoiding unnecessary finishing on areas that do not benefit from it.