Material & finish for CNC parts

Set each finish requirement by function: appearance, wear, corrosion protection or another specified duty. Define color, coating thickness and sealing-face acceptance separately.

Deburring, polishing, blasting, anodizing, passivation or plating depend on material and order requirements. Confirm the specification before production and inspect at the agreed stage. State any sequence requirements where one treatment follows another.

Process and specification

State treatment, applicable standard and revision, coating thickness and measurement method. For anodizing, specify alloy and color; for passivation or plating, identify the process specification.

Appearance

Identify cosmetic faces, color, gloss, texture and agreed visual acceptance references, including blast or polish references where needed.

Define the surface where the requirement applies

A part can contain a visible outer face, a hidden mounting face and a precision bore. Giving all three the same blanket note may create work or ambiguity without helping the assembly. Mark the surfaces that need the treatment and the exceptions that matter.

If a general finish applies to the whole part, make the local exceptions easy to identify. The drawing should let machining, finishing and inspection find the same face without relying on an informal email description.

Masking is a design choice for a particular surface. It may preserve a fit or contact area, but it also changes where the specified treatment is present. The decision needs the fit, environment and protection requirements together.

Mark the actual boundaries. “Mask critical areas” leaves the finishing supplier to decide which faces are critical, how far the mask extends and what is acceptable at its edge. A section view or a clearly identified surface can resolve a small internal feature more effectively than a general note.

Threads need a finished acceptance condition

Specify the thread designation, class, depth and functional requirements. State whether the thread is treated, masked or completed after treatment, and set the final gauging condition.

Chasing or cutting a thread after treatment can expose base material. Where that affects corrosion protection or cleanliness, the drawing and process review need to resolve it. A repair or rework should not become an undocumented shop decision.

Electrical contacts are local working surfaces

Identify current-carrying or grounding faces and their contact-area requirements. State the mating material, permitted protective treatment and assembled condition used for acceptance.

Keep permitted rack or contact-mark locations separate from the electrically functional area. Where marks would affect appearance or fit, agree an acceptable location and inspection reference.

On an aluminum locating bore, the masking boundary and final anodized condition should include the entry chamfer and adjacent shoulder where they affect the fit. This gives machining, finishing and receiving inspection the same surface definition.

Functional roughness needs a parameter and measurement conditions. Form and waviness have their own controls; color, gloss and permitted defects need separate limits where they affect working or visible faces.

For roughness, name the parameter, units, limit and applicable standard. Include the measurement area and conditions where they affect acceptance. The drawing guide’s roughness section explains why Ra, Rz and the measurement method need to stay connected.

Surface preparation remains part of the result

Surface preparation affects the texture after anodizing, as described in NASA/JSC's process specification. Where a visible face requires blasting or polishing, define the preparation sequence and appearance acceptance before anodizing.

Identify the material, preparation and treatment represented by the appearance sample. Agree viewing conditions for color, gloss and permitted marks, and retain the approved sample with the order records for repeat batches.

Stainless steel: passivation is not decorative plating

Passivation treats the stainless surface and removes contamination such as free iron. Carpenter Technology’s passivation guidance explains the grade-specific process and testing requirements. Specify any required corrosion verification separately from surface appearance.

Specify the stainless grade, condition, treatment standard and required verification. State the sequence where heat treatment or another operation precedes passivation.

Copper and brass: define the working contact

For electrical or thermal components, identify the contact faces and protected surfaces. Specify the substrate and plating separately, with distinct acceptance requirements for conduction and appearance.

Plastics: include the condition in which size matters

Polymer grade, temperature and moisture condition can influence the measured part. Ensinger’s dimensional-stability guidance distinguishes thermal expansion and moisture uptake, which vary between materials.

For a plastic component fitting into a metal assembly, state the service environment and inspection condition. A dimension checked just after machining may not describe every later condition. Review the exact polymer and any conditioning requirement instead of assigning the same behaviour to all engineering plastics.

The machined plastic measurement guide explains how to specify temperature, conditioning and support so the supplier and receiving team assess the fit in the same material state.

A solid plastic part uses the specified polymer throughout. A coated metal part has a separate surface treatment; a covered wheel or roller has a separate covering as part of the component. Those constructions need different material and dimensional definitions.

The Teflon surface-coating collection shows coated round components and long pieces with repeated holes. These use a coated substrate; parts machined from solid PTFE have a different construction.

Coated surfaces, holes and mounting faces

On a round coated part, distinguish the outside working surface from the bore and mounting faces. For a long component with repeated holes, specify whether coating enters the holes and identify the mating contact areas. Keep those requirements in the controlled drawing and finishing specification.

Specify the substrate, coating and surfaces to treat or mask. State whether critical dimensions apply before or after coating, with thickness, adhesion and service requirements. For a separate covering component, include its material and retention method.

The pocketed enclosure study shows recessed parts and an associated machining setup. For a new design of this form, follow the assembly surfaces through the planned finish.

A mounting face can locate the housing, while a bore may locate another component. A cover interface can need flatness and a defined texture, whereas the exterior may mainly need an appearance requirement. Specify each face separately so its finished condition can be checked.

Follow one interface through the process

For a locating bore, the finished fit and datum relationship come first. The treated or masked condition determines the machining allowance, final measurement and records. If cutting after finishing is proposed, check the resulting exposed material and protection before accepting the route.

Use the machining study to review geometry and setup access. The process-selection guide explains how the required faces determine the machining sequence; specify the finish and acceptance criteria for your part.

Separate the lid rim from the clearance cavity

On a pocketed aluminum housing, the closure rim, locating openings and cavity walls can have different jobs. Put the selected treatment and final dimensional requirement on the actual working region. A color requirement on the outside does not identify how a fitted bore or sealing rim will be accepted.

When comparing a finish proposal, ask which surfaces receive preparation, which remain masked and which results accompany the finished part. An appearance sample for one alloy and preparation cannot establish every dimensional or visual requirement for a changed material and route.

A general deburring note can leave important details unresolved. Identify edges that need a defined break, edges that must retain their shape and internal intersections where loose burrs would affect function. Where the drawing requires a radius or chamfer, name it explicitly.

Surface acceptance should also address handling. A sliding face or cosmetic panel can meet the requirement after finishing and still be damaged before delivery. Define separation, protection of threads and seals, and any cleanliness requirement with the packing scope.

Keep rework and touch-up visible

Agree the disposition of damaged surfaces or out-of-limit fits against the drawing and order. Record any permitted polishing, stripping, re-coating or touch-up work, including its effect on the finished requirements.

The packing and shipping guide covers protection and delivery responsibilities. Use it alongside the finish definition when the surface is part of acceptance.