A CNC machining surface finish should be selected around what each surface must do. A bearing seat, an exposed cover, and an electrical contact can need different treatment even when they belong to the same part. Start with the functional requirement, then specify the texture, treatment, protected areas, and acceptance condition.
This guide focuses on machined metal parts. It explains how to compare common finishing routes and turn a finish request into drawing and RFQ information. Metal finishing options should not be transferred to machined plastics without a separate material and process review.
Separate roughness, appearance and treatment
Surface finish can describe several different things in a quotation. Surface roughness is a measured characteristic of the surface profile. Appearance concerns what the finished part looks like under agreed viewing conditions. A surface treatment changes the surface through a process such as anodizing, plating, or passivation.
These requirements can interact, but one does not fully define the others. A part can meet a roughness limit while retaining visible tool marks. A visually uniform surface can still fail a functional requirement. Naming a coating does not establish the dimensional or cosmetic acceptance of the coated part.
| Requirement | What to specify | What it does not establish |
|---|---|---|
| Surface texture | Parameter, units, area and measurement conditions | Color match or freedom from visible scratches |
| Appearance | Visible surfaces, approved reference and inspection conditions | Roughness or dimensional compliance |
| Surface treatment | Material-compatible process and acceptance specification | Automatic suitability for every fit or contact |
| Finished geometry | Critical dimensions and the stage at which they apply | A complete surface texture requirement |
For your drawing, keep these as separate requirements wherever they serve different functions. This makes it easier for the machining and finishing suppliers to identify conflicts before production.
Read roughness requirements in context
Ra is the arithmetic average of the absolute deviations of a roughness profile from its mean line over the specified evaluation length. It is widely used, but it does not describe every feature of a surface. An average alone does not fully represent isolated extremes or the arrangement of peaks and valleys.
State the parameter and units rather than writing a number followed only by the word finish. Ra and Rz describe different aspects of a profile; do not treat them as interchangeable labels or use a universal conversion factor. For a sealing or sliding surface, select the required parameters around the function and the applicable engineering specification.
Also agree where and how the measurement will be made. The inspected area, direction relative to the machining texture, filtering, and evaluation length are part of an interpretable result. A measurement plan should address access to small faces or bores before the drawing is released.
A roughness requirement is not a general flatness, size, or appearance requirement. Record these separately when they matter. Avoid assigning a demanding texture limit to every surface simply because one contact face needs it. Identify the functional region and let the remaining surfaces follow an explicitly agreed requirement appropriate to their use.
Compare finishing routes for metal parts
The right route depends on the material and the required final condition. Use the following comparison to organize a discussion, not as a promise that every supplier provides every process.
| Route | Main reason to consider it | Review before selection |
|---|---|---|
| As machined with defined edge treatment | Retain the machined surface when it meets the requirement | Tool marks, burr removal and critical edge geometry |
| Bead blasting | Create a more uniform matte texture | Protected faces, texture reference and subsequent treatment |
| Brushing or mechanical polishing | Control directional appearance or refine the surface | Direction, geometry, access and material removal |
| Aluminum anodizing | Form an oxide surface for the specified protective or decorative purpose | Alloy, process, dimensional allowance and electrical contacts |
| Stainless steel passivation | Remove free iron contamination through a suitable chemical treatment | Alloy compatibility, cleanliness and acceptance testing |
| Electropolishing | Remove surface metal electrochemically to refine the surface | Removal allowance, access and finished dimensions |
| Plating or powder coating | Apply a selected layer for the specified service or appearance need | Substrate preparation, thickness, coverage and protected features |
Machined texture and mechanical finishing
As machined does not mean mirror polished, burr free, or cosmetically uniform unless those conditions are separately agreed. Define edge treatment as well as the surface condition. An edge needed for assembly or another function should not be altered by an unspecified general deburring instruction.
Bead blasting produces a texture; it should not be used as shorthand for a particular Ra value or guaranteed removal of every machining mark. Brushing introduces a directional appearance. Polishing is a material-removal route whose result depends on the surface and geometry being treated. Ask for the proposed preparation sequence and identify features that must be protected.
Aluminum anodizing
Anodizing converts the aluminum surface to an oxide layer. The process needs to be selected with the alloy, appearance, service conditions, and finished dimensions in mind. Do not assume that a color name also defines coating thickness, preparation, or acceptance requirements.
Anodic oxide is electrically resistive, so an anodized face should not automatically be treated as an electrical bonding surface. Identify required contact areas and have the proposed treatment reviewed for the actual electrical function. This is separate from claiming that the coating provides a rated insulation system.
Alloy selection also belongs in the review. If you are still choosing aluminum stock, the 6061 vs 7075 aluminum comparison explains why the material condition matters alongside machining and finishing requirements.
Stainless steel passivation and electropolishing
Chemical passivation and electropolishing are different operations. Passivation is intended to remove free iron contamination and support the stainless surface’s corrosion resistance. It is not a substitute for polishing away machining marks or removing burrs.
Electropolishing removes surface metal electrochemically. That can be useful for surface refinement, but the removal must be considered against critical dimensions and edges. Choose the process around the alloy and final acceptance requirements rather than assuming either operation guarantees a particular roughness or service life.
Applied coatings and plating
Plating and powder coating add a layer to the part. Their effects on size and coverage need to be considered at holes, threads, seating faces, and other interfaces. A coating specification should identify the selected system and acceptance conditions, not simply a desired color.
For a thermally cured powder coating, review the material and part against the selected product’s curing requirements. The required schedule is specific to the coating system; a generic oven setting is not an adequate process specification. Confirm preparation and curing with the processor rather than carrying assumptions over from another coating.
Protect fits and contact surfaces
Mark the surfaces where finishing could interfere with assembly or function. Common review locations include bearing seats, locating diameters, threaded holes, sealing faces, electrical contacts, and datum features used for final inspection.
For each location, decide whether the requirement applies before or after treatment. If a bore must meet a fit after coating, a satisfactory pre-coating diameter does not by itself demonstrate compliance. The proposed route needs to account for the treatment and include an appropriate final check.
Masking can protect selected regions, but it needs a defined boundary and an agreed result. A broad note such as mask critical areas leaves the processor to decide which areas are critical. Identify the region on the drawing and state whether partial coverage or a transition edge is acceptable.
Do not calculate an anodizing allowance from a universal percentage and assume it applies to every part. Discuss the specified process and preparation with the finisher. Likewise, material-removal operations need an allowance that suits the actual geometry and acceptance limits.
A local finish map is often more useful than a single finish note for the entire component. It can show which faces are cosmetic, which must remain untreated, and which require inspection in the delivered condition.
Set appearance acceptance before production
Terms such as smooth, satin, and black leave room for different interpretations. If appearance affects acceptance, identify the visible surfaces and agree a representative finished reference. Record which differences are acceptable instead of relying only on a photograph viewed on different screens.
Define the inspection conditions appropriate to the product: viewing distance, lighting, viewing angle, and the surfaces being judged. These are project agreements, not universal rules. Distinguish an exposed face from a hidden internal pocket so the quotation reflects the actual cosmetic requirement.
Include permitted contact or handling marks where relevant. Ask the processor where holding or electrical contact is needed and agree acceptable locations. A concealed area may be suitable, but that decision belongs in the review rather than being left until the completed parts arrive.
A cosmetic reference does not replace roughness, thickness, or dimensional inspection. Use it for the appearance question it was approved to answer.
Plan machining, finishing and inspection together
Review the route before releasing the drawing. The machining supplier, finisher, and inspection team need the same revision and the same understanding of the delivered condition. Confirm responsibility for preparation, masking, final inspection, and any required reports.
Consider a hypothetical aluminum housing with a visible outer face, a locating bore, and a grounding pad. The outer face needs an appearance agreement. The bore needs a defined final fit. The grounding pad needs an electrical contact requirement and a reviewed treatment boundary. A single instruction to anodize the housing would leave important decisions unresolved.
For this example, a useful review would identify the three regions on the drawing, agree the proposed treatment, and assign a check to each requirement. It would not infer that the same surface preparation is suitable everywhere. The example illustrates specification planning; it is not a completed customer project or a qualified design.
If a later operation changes an accepted surface, review whether the earlier inspection still establishes the final requirement. Local rework, additional polishing, or post-treatment machining should be handled through an agreed disposition rather than an undocumented shop-floor change.
The CNC machining service page provides a route for discussing the drawing and proposed manufacturing sequence.
Compare quotations on the same scope
Finishing cost depends on more than the process name. Preparation, selective masking, access to features, handling, appearance requirements, inspection, and documentation can change the work involved. Compare quotations against the same drawing revision and finished-part acceptance criteria.
Ask each supplier to identify exclusions and assumptions. One quotation may include only a treatment while another includes preparation, cosmetic approval, and final measurement. Those offers are not directly comparable until the scopes are aligned.
If cost reduction is needed, review requirements surface by surface. Relax a cosmetic requirement only where appearance is not important, or narrow a special texture requirement to its functional area. Do not remove a requirement that supports fit or service performance simply to obtain a lower finishing price.
Prepare finish requirements for an RFQ
Include the following information with your request:
- A dimensioned drawing and 3D model with matching revisions.
- Material grade and condition, quantity, and any permitted substitutions.
- The proposed finish process and applicable specification where required.
- Surface texture parameters, units, locations, and inspection conditions.
- Cosmetic zones, reference samples, and agreed visual acceptance conditions.
- Masking boundaries and features that must remain untreated.
- Dimensions that apply after finishing, plus required inspection records.
- Service exposure, contact requirements, and any later manufacturing steps.
Separate mandatory requirements from preferences so alternatives can be evaluated without changing the intended function. Where a finish is only a starting proposal, explain the problem it is meant to solve and ask the supplier to identify any unresolved process assumptions.
Send your drawing and finish requirements for a project discussion. Include the critical surfaces and the condition in which they must be inspected, so the review can address the finished part rather than an unfinished blank.
Frequently asked questions
Does a lower Ra always make a better part?
No. A lower average roughness value is useful only when it supports the surface’s function and acceptance criteria. It does not prove that size, flatness, appearance, or treatment requirements have been met. Specify the necessary condition for the actual interface.
Will anodizing remove visible tool marks?
Do not rely on anodizing alone to remove them. Agree the preparation and appearance requirements before processing, and use a representative finished reference when cosmetic consistency matters. A coating name is not a replacement for an appearance specification.
Is passivation the same as polishing stainless steel?
No. Chemical passivation addresses surface contamination, while polishing changes surface texture through material removal. Electropolishing is also a material-removal process. Specify the operation and acceptance result needed for the part.
Should critical dimensions be checked after finishing?
When the requirement applies to the delivered, treated part, inspection must establish compliance in that condition. Intermediate checks may also be needed, but they cannot automatically stand in for final acceptance when the treatment affects the feature.
Release a measurable finish requirement
A useful finish specification connects each surface to its function. Define texture, appearance, treatment, and final dimensions separately, then review how the proposed manufacturing route will satisfy them together. Identify masking boundaries and inspection stages before asking suppliers to commit to a quotation.
That gives you a clearer basis for comparing offers, approving samples, and accepting delivered parts. The goal is a surface condition that meets the drawing and service requirements, with evidence appropriate to each requirement.
