CNC Machining Surface Finishes Guide

CNC machining surface finishes affect more than appearance. They influence friction, wear, corrosion behavior, coating adhesion, inspection results, and whether a part still meets tolerance after post-processing. For engineering buyers, the finish decision should be made before drawings are released, not after parts come off the machine.

The key issue is that surface finish is both a machining outcome and a post-processing choice. A part can be left as-machined, refined by changing machining parameters, textured by bead blasting, coated by anodizing or powder coating, plated for corrosion or hardness, or treated by passivation or electropolishing. Each option changes risk in a different way.

This guide focuses on feasibility. It explains what CNC machining surface finishes can do, where they fail, and what should be checked before selecting a finish for aluminum, stainless steel, steel, titanium, and coated metal parts.

What CNC Machining Surface Finishes Are and Why They Matter

A CNC surface finish is the condition of the part surface after machining or after a secondary finishing process. It includes roughness, tool marks, texture, lay direction, coating thickness, color, edge condition, and visual uniformity.

For many parts, the finish is a functional requirement. A sliding surface may need lower roughness to reduce friction. A visible enclosure may need a uniform matte or brushed texture. A stainless steel medical component may need a smooth and cleanable surface. A gear may need precision grinding and plating rather than a decorative finish.

Surface finish choices also affect procurement. Extra finishing can add process steps, masking, inspection work, and rework risk. Some finishes add material. Others remove material. Some change edges. Some hide tool marks. Others make tool marks more visible.

What is the standard surface finish for CNC machining?

The common standard CNC machining surface finish is Ra 3.2 μm, also expressed as 125 μin. This is typically achieved by standard milling or turning without extra grinding, polishing, or special surface preparation.

Ra means arithmetic average roughness. It is a common way to describe the average height variation of microscopic peaks and valleys on a surface. A lower Ra value means a smoother measured surface, though it does not describe everything about texture, direction, or visual appearance.

Finer finishes such as Ra 1.6 μm, 0.8 μm, or 0.4 μm can be specified when the part needs better sealing, smoother sliding, lower friction, or improved visual quality before coating. These finishes usually require more control over machining parameters, tooling, and inspection. They may also limit how fast the part can be machined.

The standard as-machined finish is often acceptable for brackets, fixtures, prototype housings, and non-cosmetic structural parts. It may not be acceptable for visible product surfaces, sealing faces, moving contact areas, sterile components, or parts that will receive a finish that highlights tool marks.

How Ra roughness affects CNC part performance

Ra roughness affects CNC part performance because the real contact area between surfaces depends on microscopic peaks and valleys. A rougher surface may increase friction, retain debris, accelerate wear, or make sealing less reliable. A lower Ra value can improve cleaning, appearance, or some contact conditions, but it is not universally better for every functional surface. Sliding and sealing performance also depend on material pair, lubrication regime, contact pressure, and surface lay, so roughness should be specified by function rather than by preference alone.

Ra is not the only factor. Two parts with the same Ra can look and behave differently if the tool path, lay direction, or process changes. A milled surface with directional tool marks can have the same Ra as a blasted surface, but the blasted surface will look more uniform and matte. A polished surface may be smoother and brighter, but it is not always the best substrate for coatings that depend on surface preparation for adhesion. Coating suitability depends on the coating system, pretreatment method, cleanliness, and required anchor profile rather than smoothness alone.

For functional parts, the finish should be tied to the surface’s job. A cosmetic external face may need uniform texture. A bearing or sliding contact face may need lower roughness. A coated aluminum part may need tool marks controlled before anodizing. A stainless steel component in a clean environment may need electropolishing and passivation rather than only mechanical polishing.

As-machined finish vs post-processed finish: tool marks, texture, and function

An as-machined finish keeps the surface left by the cutting tool. Minor tool marks are expected at the common Ra 3.2 μm level. This finish has the lowest process complexity because it avoids secondary finishing, but it also leaves the machining pattern visible.

A post-processed finish changes the machined surface after cutting. Deburring removes sharp edges and burrs. Bead blasting produces a uniform matte texture. Brushing creates a linear satin pattern. Polishing smooths and brightens the surface. Anodizing adds a controlled oxide layer on aluminum or titanium. Powder coating adds a baked coating layer. Plating adds a metallic layer. Passivation improves stainless steel corrosion resistance without changing the appearance much. Electropolishing removes a thin surface layer from stainless steel, leaving a smoother and brighter surface.

The main decision is whether the surface requirement is dimensional, functional, cosmetic, or corrosion-related. A finish that improves appearance can harm precision. A finish that improves wear resistance can change dimensions. A finish that hides tool marks may round edges. A finish that adds corrosion protection may require masking on threaded holes, sealing faces, or tight-fit features.

Table: Common Ra values for CNC machining surface finishes [References: standards bodies, industry sources]

Ra values should be called out only where they matter. Applying a tight Ra requirement to every surface can increase cost and lead time without improving the part.

Feasibility: Can the Finish Be Applied to the Part?

Finish selection starts with feasibility. A surface finish must be compatible with the material, geometry, tolerances, edge requirements, and inspection plan. A finish that works well on a simple cover plate may create problems on a small precision machined component with threaded holes and close-fitting features.

The feasibility review should happen before release. Once a part is machined, changing the finish can require redesign, masking changes, or remanufacture.

Material compatibility: aluminum, stainless steel, steel, titanium, and coated metals

Aluminum is commonly finished by anodizing, bead blasting, brushing, polishing, powder coating, or plating. Type II anodizing is commonly used for corrosion resistance and decorative color on aluminum, while Type III hardcoat anodizing is used when higher wear resistance is needed and dimensional change has been planned. Appearance can vary by aluminum alloy and surface preparation, and hardcoat is not automatically suitable for close fits, threads, or sealing features without explicit dimensional allowance.

Stainless steel is commonly finished by passivation, electropolishing, mechanical polishing, brushing, or bead blasting. Passivation improves corrosion resistance without a major visual change. Electropolishing is often used on stainless steel when a smoother, brighter, and more cleanable surface is required, especially for cleanability-critical or hygienic applications. Its suitability still depends on geometry, edge effects, stock removal, and whether critical dimensions are controlled before or after finishing.

Steel parts often use plating, coating, painting, powder coating, or precision grinding, depending on the application.

Gears may need precision grinding or honing for profile accuracy, while any plating or coating should be treated as application-specific rather than a default hardness solution. Surface treatment on gear teeth must be evaluated for dimensional effect, contact pattern, and lubrication conditions.

Titanium can be anodized, and it may also receive other surface treatments depending on the application. The finish decision should consider wear behavior, corrosion needs, and whether dimensional change is acceptable.

Coated metals need extra caution. If the base material already has a coating, machining may expose substrate material at cut faces. A new finish may not behave uniformly across coated and exposed areas. The drawing should define whether coating is required only after machining, before machining, or both.

How tight tolerances limit post-processing options

Tight tolerances limit post-processing because many finishes add material, remove material, or change edges. Anodizing adds an oxide layer. Type II anodizing is commonly 4–12 μm thick. Clear Type II anodizing is commonly 4–8 μm, while black-dyed Type II anodizing is commonly 8–12 μm. Type III hardcoat anodizing is much thicker, commonly 25–100 μm, or 0.001–0.004 inches.

Powder coating adds a baked coating layer. Plating adds a metallic layer. Electropolishing removes material from the surface. Mechanical polishing removes and smooths material in a less uniform way than a controlled machining pass. Bead blasting changes texture and can round edges.

For precision features, the drawing should define which surfaces are finished and which are masked. Threads, bearing seats, sealing faces, datum surfaces, and close-fit bores often need special attention. If the finish changes the feature enough to affect assembly, the tolerance may need to apply after finishing rather than before finishing.

When bead blasting is not suitable for tight tolerance parts

Bead blasting is not suitable for tight tolerance parts when edge definition, small features, or controlled fit dimensions must be preserved. The process creates a matte texture by impacting the surface with media. This can soften sharp transitions, round edges, and change surface condition in ways that are hard to control on small or delicate features.

Bead blasting is often used for visual uniformity on non-precision surfaces. It can reduce the visual contrast of tool marks and create a consistent matte appearance. But it should be avoided or tightly controlled on sealing surfaces, precision bores, bearing seats, thin edges, threads, and mating features.

The difference between bead blasting and sand blasting is mainly the media and aggressiveness. Glass bead blasting is commonly used when a more uniform, less aggressive matte texture is desired. Sand blasting can be more aggressive. The terms are sometimes used loosely, so the drawing or purchase specification should define the required texture and any surfaces that must be protected.

Checklist: Finish feasibility review before releasing drawings

Before drawings are released, review these points:

How CNC Surface Finishing Works

CNC finishing starts before any secondary process. The surface left by machining sets the baseline. If the machined surface has chatter, heavy tool marks, burrs, or inconsistent texture, post-processing may not fully correct it. In some cases, post-processing makes the issue more visible.

A good finish strategy therefore combines machining control, deburring, finishing, and inspection.

Machining parameters that improve surface finish quality

Machining parameters that improve surface finish quality include spindle speed, feed rate, depth of cut, tool path strategy, and cut stability. High-speed machining with optimized parameters can reduce tool marks by using high spindle speeds and light cuts. The result is a better inherent surface before any coating, blasting, or polishing.

Light finishing passes are often used after roughing passes. Roughing removes material quickly, while finishing focuses on geometry and surface quality. A stable setup also matters because vibration can leave chatter marks that are hard to remove without extra work.

Coolant, chip evacuation, and cutting conditions also affect finish. If chips are recut or trapped between the tool and workpiece, they can scratch the surface. If heat builds up, the surface condition can become less consistent.

Factors affecting surface roughness in CNC milling

Several factors affecting surface roughness in CNC milling are tied to the tool, machine, workholding, material, and geometry. Thin walls can vibrate. Deep pockets may require longer tools, which are less rigid. Small internal radii may force smaller tools and lighter cuts. Harder materials may require slower, more controlled machining to avoid poor surface quality.

Part geometry is a major constraint. Large flat faces, deep slots, internal corners, and interrupted cuts can all show different tool patterns. A drawing that calls for the same Ra on every surface may not reflect real machining conditions.

For milled parts, surface roughness should be specified with the surface function in mind. A cosmetic face, a gasket land, and an internal clearance pocket do not need the same finish unless the application requires it.

How cutting tools influence machined surface quality

Cutting tools influence machined surface quality through sharpness, geometry, coating condition, tool wear, and rigidity. A worn tool can leave visible lines, tearing, burrs, or poor edge quality. A tool with the wrong geometry for the material can rub instead of cut cleanly.

Tool deflection is another issue. Long, small-diameter tools are less stiff. They can leave inconsistent surfaces, especially in deep pockets or narrow features. This can also affect dimensional accuracy.

Tool choice should match the material and surface requirement. If the part will be anodized, visible tool marks should be controlled before anodizing because anodizing may not hide them. If the part will be polished, the machined surface still matters because polishing time and consistency depend on the starting condition.

Process diagram: machining → deburring → finishing → inspection

A typical finishing route follows this sequence:

CNC machining

↓

Deburring and edge cleanup

↓

Surface finishing or coating

↓

Inspection of dimensions, roughness, appearance, and protected areas

This sequence matters because each step changes the part. Deburring can remove sharp edges. Finishing can add or remove material. Inspection must confirm the final state, not only the machined state. For critical parts, dimensions should be checked after finishing when the finish can affect fit or function.

Advantages and Limitations of Common Finish Options

No finish is best for every CNC machined part. The right option depends on material, function, tolerance sensitivity, corrosion needs, wear needs, appearance, and whether the part has features that must be masked.

Table: As-machined, bead blasted, brushed, polished, anodized, powder coated, plated, passivated, and electropolished finishes

As a quick decision rule, finishes that add or remove material, require masking, or depend on line-of-sight access create the most risk for close-fit bores, threads, datum surfaces, and small internal features. Coated and plated finishes also require agreement on contact points, edge buildup, and which dimensions apply after finishing.

Anodizing vs powder coating for CNC aluminum parts

Anodizing vs powder coating for CNC aluminum parts should be decided by function first. Type II anodizing is often used when corrosion resistance, color, and a relatively thin surface layer are needed. Type III hardcoat anodizing is used when wear resistance is more important and the added thickness can be managed.

Powder coating is used when a thicker protective and decorative coating is acceptable. It can work well for frames, handles, covers, and other parts where visual durability and corrosion resistance matter more than very tight machined fits.

For precision aluminum parts, anodizing often gives better dimensional control than a heavy coating, but Type III hardcoat still has significant thickness. Powder coating can create higher risk on close-fitting features, threads, datum surfaces, and sliding interfaces unless those areas are masked or designed with coating buildup in mind.

Electropolishing vs mechanical polishing for stainless steel parts

Electropolishing vs mechanical polishing for stainless steel parts is a functional decision. Mechanical polishing uses abrasive contact to smooth or brighten a surface. It can create a good visual result on accessible surfaces, but access, operator technique, and geometry can affect consistency.

Electropolishing removes a surface layer electrochemically. It smooths and brightens stainless steel and is preferred when the part needs a cleanable, corrosion-resistant surface, such as sterile components. It is often paired with passivation when corrosion behavior is critical.

Mechanical polishing may be enough for visible stainless parts where appearance is the main goal. Electropolishing is more appropriate when cleanability, corrosion resistance, and surface uniformity are more important than only appearance.

Tradeoffs between matte finish and smooth finish on CNC parts

The tradeoffs between matte finish and smooth finish on CNC parts are often misunderstood. A matte bead blasted part can look uniform but may not be smoother in a functional sense. A smooth polished part may have lower roughness but may show fingerprints, handling marks, or small defects more easily.

Matte finishes are useful when the goal is visual consistency and reduced glare. They can hide small tool marks, but blasting can alter edges and small features. Smooth finishes are useful for sliding, sealing, cleaning, and bright appearance, but they can increase processing time and inspection needs.

The key point is that matte does not automatically mean low Ra, and low Ra does not automatically mean good cosmetic appearance. The drawing should separate roughness requirements from visual texture requirements.

Common Problems, Risks, and Failure Scenarios

Surface finish problems often come from unclear specifications. A drawing may say “anodize black” without saying which surfaces are cosmetic, which holes must be masked, or whether tool marks are acceptable. Another drawing may call out a fine Ra everywhere, even on hidden relief pockets where it adds no function.

Risks increase when finishing is treated as an afterthought.

Causes of poor surface finish in CNC machining

Common causes of poor surface finish in CNC machining include worn tools, unstable cutting, vibration, poor chip evacuation, unsuitable feed and speed, and geometry that forces long or small tools. Material behavior also matters. Some materials cut cleanly under the right conditions, while others can tear, smear, or burr.

Chatter is a frequent surface problem. It leaves repeating marks that may remain visible after coating or anodizing. Burrs are another problem because they can break loose, interfere with assembly, or create sharp edges that finish unevenly.

Poor surface finish can also come from overspecification. If a part requires a very fine Ra on surfaces that are hard to access, the process may need extra setups, special tools, or polishing. That adds risk and may still produce variation across the part.

Impact of tool marks on anodized aluminum components

The impact of tool marks on anodized aluminum components is important because anodizing does not erase the machined surface. It grows a surface oxide layer that follows the existing texture. If tool marks, chatter, or inconsistent cutter paths are present before anodizing, they can remain visible after the finish.

This is a common issue on customer-facing aluminum parts. A standard as-machined Ra 3.2 μm surface may be acceptable structurally but may look uneven after clear or dyed anodizing. Bead blasting or brushing before anodizing can improve visual uniformity, but those steps may affect edges and dimensions.

For cosmetic anodized parts, the drawing should define cosmetic zones and acceptable tool mark direction or texture. Critical dimensions should be reviewed with anodizing thickness in mind.

Risks of powder coating on precision machined surfaces

Risks of powder coating on precision machined surfaces come from coating buildup and coverage variation. Powder coating adds a baked layer to the part. This can interfere with threads, close-fit bores, slots, bearing seats, datum surfaces, and mating faces.

Powder coating can also soften sharp visual details and reduce the clarity of machined edges. This may be acceptable on a handle, cover, or frame. It may not be acceptable on precision mechanical interfaces.

If powder coating is needed on a precision part, masking should be defined clearly. The drawing should state which features must remain uncoated and which dimensions apply after coating.

Common problems with anodized CNC aluminum surface quality

Common problems with anodized CNC aluminum surface quality include visible tool marks, uneven appearance, color variation, and edge effects. Some of these issues start in machining. Others come from surface preparation or differences in how features receive the finish.

Black-dyed Type II anodizing commonly has a thicker range than clear Type II anodizing, so dimensional effects may differ. Type III hardcoat anodizing is thicker again and should be treated as a functional coating, not only a color choice.

For aluminum parts that must look uniform, the pre-anodize surface preparation matters. Bead blasting, brushing, or polishing may be needed, but each one changes texture and may affect precision features.

Cost, Tolerance, and Lead Time Decision Factors

Surface finish affects cost and lead time through added operations, handling, masking, inspection, and rework risk. The part may also need a different machining strategy if the finish callout requires lower Ra before post-processing.

In relative terms, as-machined, passivated, and simple blasted finishes are usually lower process burden; anodizing, brushing, and standard plating are often medium; and electropolishing, hardcoat anodizing, complex masking, and appearance-critical coated finishes are often higher burden. Quote variability usually increases when the part has tight post-finish dimensions, difficult masking, strict cosmetic standards, or outsourced finishing steps.

No reliable cost decision can be made from finish name alone. The same finish can be simple on a large open face and difficult on a small part with internal threads, thin walls, and multiple masked zones.

Industry-level cost drivers: finish type, masking, batch size, inspection, and rework risk

Finish type is the first cost driver. As-machined parts usually require fewer steps than plated, anodized, polished, or electropolished parts. Coatings and chemical processes add handling, cleaning, and process control.

Masking adds cost because selected areas must be protected from coating or finishing. Threads, bores, sealing faces, electrical contact areas, and datums often need masking. Complex masking increases risk because a missed or poorly protected area can make the part unusable.

Batch size also matters. Finishing often has setup and handling effort. Very small batches can carry more cost per part because the setup burden is spread over fewer parts. Inspection adds cost when Ra, coating coverage, appearance, or post-finish dimensions must be verified.

Rework risk is often hidden. If a cosmetic finish exposes tool marks or coating enters a precision bore, the part may need rework or remake. Clear drawings reduce this risk.

How bead blasting changes dimensional accuracy of machined parts

Bead blasting changes dimensional accuracy of machined parts mainly by altering the surface and edge condition. It impacts the surface with media to create a matte texture. This can round edges and slightly change the effective size of delicate features.

The risk is higher on small parts, thin edges, fine threads, sharp corners, and tight-tolerance mating surfaces. A large nonfunctional outside face may tolerate blasting well. A precision bore or bearing seat may not.

When bead blasting is specified, identify areas to avoid blasting. If the whole part must be blasted for appearance, the design should allow for edge softening and texture change.

Surface finish limitations for precision CNC machined parts

Surface finish limitations for precision CNC machined parts are tied to final dimensions. If a finish adds thickness, the machined dimensions may need to be offset before finishing. If a finish removes material, the part may need stock allowance. If a finish rounds edges, sharpness requirements may need to be relaxed or protected.

Precision parts should define whether tolerance applies before or after finishing. In most functional cases, the final inspected condition is what matters for assembly. This is especially important for hardcoat anodizing, plating, powder coating, and electropolishing.

Small parts create extra limits. Handling, masking, and inspection become harder as features shrink. A finish that is easy on a large housing may be risky on a small machined insert.

Table: Finish thickness, dimensional impact, tolerance risk, and added process steps [References: industry sources]

Applications and Use Cases by Performance Requirement

The best surface finish depends on the part’s job. A prototype bracket, a medical stainless steel component, an aluminum enclosure, and a steel gear should not use the same selection logic.

Surface finish requirements for functional prototype parts

Functional prototype parts often use as-machined finish when the goal is fit testing, mechanical validation, or early assembly checks. Ra 3.2 μm is often enough for non-cosmetic prototype surfaces. This keeps the process simpler and avoids finish-related dimensional changes.

If the prototype must represent production appearance, bead blasting, brushing, anodizing, or powder coating may be needed. But that can change how the part fits. For prototypes with tight interfaces, it is safer to finish only cosmetic surfaces or to inspect after finishing.

Prototype drawings should separate “test-critical” surfaces from “appearance-only” surfaces. This prevents a cosmetic finish from damaging the data needed from the prototype.

Choosing a surface finish for wear-resistant metal parts

Choosing a surface finish for wear-resistant metal parts starts with the contact condition. Aluminum parts may use Type III hardcoat anodizing when wear resistance is needed and the thickness can be managed. Steel gears or similar mechanical parts may rely on precision grinding for accurate profiles and nickel plating for hardness or wear resistance.

A smooth finish can reduce friction, but wear resistance may also require a hard surface layer. The designer should avoid choosing a finish only by appearance. A polished aluminum part may look smooth, but it does not provide the same wear behavior as hardcoat anodizing.

For wear surfaces, define the functional face, required roughness if needed, coating type, and final inspection condition.

When electropolishing is preferred for stainless steel components

Electropolishing is preferred for stainless steel components when cleanability, corrosion resistance, and a bright smooth finish are required. It is often used for sterile or medical-style parts because it smooths and brightens the surface by removing a surface layer.

Passivation can also improve stainless steel corrosion resistance, but it does not create the same bright, smoothed surface. Mechanical polishing can improve appearance, but it may not be as consistent on complex shapes or internal features.

Electropolishing should be reviewed for dimensional impact. If the part has tight tolerance features, those surfaces may need masking or post-process inspection.

Best surface finishing methods for small CNC machined parts

The best surface finishing methods for small CNC machined parts are usually the ones that control handling, edge change, and masking risk. As-machined, passivated stainless steel, carefully controlled anodizing, or selective finishing may be more reliable than aggressive blasting or thick coatings.

Small parts are sensitive to edge rounding and coating buildup. Bead blasting can change small edges. Powder coating can fill fine details. Thick anodizing or plating can affect small bores and slots.

For small components, avoid broad finish notes such as “finish all over” unless that is truly required. Define protected features and inspect final dimensions where assembly depends on them.

Decision Guide for Selecting the Right Surface Finish

A good finish decision follows the part function. Start with the material, then check tolerance sensitivity, corrosion or wear needs, appearance, and inspection requirements. Do not start with color or texture unless the part is purely cosmetic.

Flowchart: choose by material → tolerance → corrosion/wear need → appearance → inspection requirement

Start

↓

Identify base material

↓

Aluminum? → Consider as-machined, bead blast, brushed finish, polishing, Type II anodizing, Type III hardcoat anodizing, or powder coating

Stainless steel? → Consider as-machined, brushed finish, polishing, passivation, or electropolishing

Steel? → Consider as-machined, grinding, plating, powder coating, painting, or protective coating

Titanium? → Consider as-machined, polishing, or anodizing where specified

↓

Check tolerance sensitivity

↓

Tight fits, threads, bores, datums, or sealing faces?

↓

Yes → Avoid uncontrolled blasting or thick coatings; define masking areas and inspect dimensions after finishing

No → Broader surface finish options can be used

↓

Need corrosion or wear resistance?

↓

Corrosion resistance → Consider anodizing, passivation, plating, powder coating, or other protective coatings

Wear resistance → Consider hardcoat anodizing, plating, grinding, or polishing depending on material type

↓

Need specific appearance?

↓

Matte finish → Bead blasting or similar textured finish, if tolerances allow

Satin linear finish → Brushed finish

Bright or smooth finish → Polishing or electropolishing, depending on material

Colored finish → Anodizing or powder coating where compatible

↓

Define inspection requirements

↓

Ra surface roughness callouts, cosmetic zones, masking notes, final dimensions, and coating acceptance criteria

Is anodizing or powder coating better for CNC aluminum parts?

For CNC aluminum parts, anodizing is often better when the part needs corrosion resistance, color, and a thinner controlled surface layer. Type II anodizing is common for decorative and protective use. Type III hardcoat anodizing is better for wear resistance when the design can handle 25–100 μm thickness.

Powder coating is better when a thicker protective color coating is acceptable and the part does not depend on tight exposed machined features. It is common for frames, handles, and covers. It is higher risk for precision surfaces unless masking is defined.

The decision should be based on final function. For close-fit aluminum parts, check anodizing thickness and masking. For visible structural parts with loose fit areas, powder coating may be acceptable.

Surface treatment options for electropolished stainless steel parts

Surface treatment options for electropolished stainless steel parts often include electropolishing alone or electropolishing followed by passivation. Electropolishing smooths and brightens the surface by removing a surface layer. Passivation improves corrosion resistance with little visual change.

Mechanical polishing can be used before electropolishing when the starting surface needs improvement, but the design must allow access. Brushing may be used where a linear satin appearance is desired instead of a bright electropolished look.

For stainless steel parts with sterile or cleanable requirements, define the surface zones that require electropolishing. Do not assume internal features will receive the same finish as open external faces unless the process plan supports it.

Drawing checklist: Ra callouts, masking notes, cosmetic zones, and inspection criteria [References: standards bodies]

State where roughness will be measured, because stylus access, sampling length, and feature size can limit whether a requirement is verifiable on the actual surface. Also define cosmetic acceptance, coating-thickness measurement method, masking needs, rack or contact-mark allowances, and whether final dimensions are inspected before or after finishing.

ASME Y14.36 surface texture symbol standards recommend clearly defining roughness symbols, lay direction, machining allowance, and surface treatment requirements on engineering drawings to reduce manufacturing and inspection risk. Include:

In short, choose CNC machining surface finishes by engineering risk. Use as-machined finish when function allows visible tool marks and standard Ra is acceptable. Use bead blasting or brushing for visual uniformity when tolerances are not highly sensitive. Use anodizing, plating, powder coating, passivation, or electropolishing when corrosion, wear, cleanability, or appearance require it. Avoid finishes that add, remove, or distort material on precision features unless the drawing accounts for the change.

FAQs

References

https://www.iso.org/standard/10132.html

https://www.iso.org/standard/4287.html

https://www.asme.org/codes-standards/find-codes-standards/y14-36-surface-texture-symbols

https://www.astm.org/b0633-23.html