Surface Finishes Explained: Ra, Coatings & Specification Guide (2026)

Q: What does Ra mean and how is it measured?

Ra (arithmetic average roughness) is the average deviation of the surface profile from the mean line, measured over a sampling length per ISO 4287. It is the most commonly specified surface roughness parameter. Ra is measured with a contact profilometer (a diamond stylus drags across the surface) or a non-contact optical profilometer. Values are expressed in microinches (μin.) in the US or micrometers (μm) internationally. Lower Ra = smoother surface. Typical as-machined CNC milling: Ra 63–125 μin. (1.6–3.2 μm).

Q: What is the difference between Type II and Type III anodizing?

Type II (standard/decorative) anodizing grows a 0.3–1.0 mil (8–25 μm) oxide layer on aluminum. It provides moderate corrosion resistance, accepts dye colors (black, blue, red, gold), and has minimal dimensional impact. Type III (hardcoat) anodizing grows a 1.0–3.0 mil (25–75 μm) oxide layer that is significantly harder (60–70 HRC equivalent), more wear-resistant, and more corrosion-resistant. Type III is typically dark gray or black and is harder to dye in bright colors. Type III adds roughly half the coating thickness to each surface (growth is ~50% into the base metal, ~50% outward), so account for this in your tolerances.

Q: When should I specify passivation vs. electropolishing?

Passivation (per ASTM A967 or AMS 2700) is a chemical treatment that removes free iron from stainless steel surfaces and restores the chromium oxide passive layer. It does not change the surface finish. Use passivation when you need corrosion resistance on stainless steel without changing the appearance or roughness. Electropolishing removes 0.0002–0.001 in. (5–25 μm) of material through an electrochemical process, smoothing the surface to Ra 4–16 μin. (0.1–0.4 μm) and improving corrosion resistance beyond passivation alone. Use electropolishing when you need both a mirror-like finish and enhanced corrosion performance — common for medical devices, pharmaceutical equipment, and food-contact surfaces.

Q: How does surface finish affect part cost?

Each step of surface finish improvement adds cost in two ways: (1) machining time — achieving Ra 32 μin. requires slower feed rates and finer tools than Ra 125 μin., roughly doubling cycle time per surface, and (2) secondary finishing — anodizing adds $2–$8/part for small brackets, powder coating adds $3–$12/part, electropolishing adds $5–$25/part depending on geometry and batch size. As a guideline: leaving a part as-machined (Ra 63–125 μin.) is the baseline cost. Specifying Ra 32 μin. adds 20–40% to machining cost on affected surfaces. Adding anodize or powder coat adds $2–$12/part. Specifying Ra 8–16 μin. (grinding/polishing) can double the total part cost.

Q: How do I call out a surface finish on my engineering drawing?

Use the surface finish symbol (a checkmark-like mark per ASME Y14.36 or ISO 1302) placed on the surface with a leader line. Write the maximum Ra value in the symbol: e.g., "32" for Ra 32 μin. or "0.8" for Ra 0.8 μm. For secondary finishes, call them out in the general notes: "FINISH: TYPE II ANODIZE, BLACK, PER MIL-A-8625 TYPE II CLASS 2" or "POWDER COAT: RAL 7016 ANTHRACITE GRAY, 2–4 MIL." Specify the standard (MIL-A-8625 for anodize, ASTM B633 for zinc plating, ASTM A967 for passivation). Missing the standard creates ambiguity — "anodize" alone does not tell the finisher whether you want Type II or Type III, what thickness, or what color.

Q: Can I anodize all aluminum alloys?

All aluminum alloys can be anodized, but the results vary significantly. 6061-T6 and 5052-H32 produce consistent, uniform anodize layers with good dye uptake — these are the preferred alloys for anodized parts. 7075-T6 anodizes well but may show slight color variation. 2024-T3 is difficult to anodize uniformly due to its high copper content (3.8–4.9%) — expect mottled or inconsistent coloring. Cast aluminum alloys (A356, A380) produce a porous, dull anodize with limited dye absorption. If your part requires cosmetic anodizing, specify 6061-T6 or 5052-H32 and note the finish in the material callout.

Why surface finish is referenced on every other MakerStage guide

Surface finish appears in the CNC machining guide (as-machined Ra), the sheet metal guide (powder coat, anodize), the GD&T guide (surface finish symbols), and the tolerances guide (finish vs. cost). This page is the central reference that ties them all together. Bookmark it — you will come back to it every time you specify a finish on a drawing.

Section 1 of 9

What Is Surface Finish (Ra)?

Surface finish quantifies the roughness of a machined or finished surface. The most common parameter is Ra (arithmetic average roughness) per ISO 4287 — the mean deviation of the surface profile from the centerline, measured in microinches (μin.) or micrometers (μm). Lower Ra = smoother surface. Ra is measured with a profilometer (contact or optical) and is the number you write in the surface finish symbol on your drawing.

Ra Reference Table — Process vs. Achievable Finish

Ra (μin.)	Ra (μm)	Typical Process	Application	Relative Cost
250	6.3	Rough machining, sawing	Non-functional hidden surfaces	1×
125	3.2	Standard CNC milling	General as-machined default	1×
63	1.6	Fine milling, standard turning	Mating surfaces, bearing housings	1.2–1.5×
32	0.8	Fine turning, finish milling	O-ring grooves, sealing faces	1.5–2×
16	0.4	Grinding, fine boring	Precision bearing journals	2–3×
8	0.2	Lapping, honing, polishing	Optical, gauge faces	3–5×
4	0.1	Superfinishing, electropolishing	Medical implants, mirror finish	5–10×

Section 2 of 9

As-Machined Finishes

The finish left by the cutting tool with no secondary processing. This is the baseline — every other finish adds cost and lead time on top of this.

CNC milling: Ra 63–125 μin. (1.6–3.2 μm)

The standard as-machined finish from a carbide end mill at normal feeds/speeds. Tool marks (cusps) are visible under magnification. Acceptable for most non-cosmetic, non-sealing surfaces. This is the finish you get if your drawing does not call out a specific Ra value.

CNC turning: Ra 32–63 μin. (0.8–1.6 μm)

Turning naturally produces a finer finish than milling because the tool is in continuous contact with the rotating workpiece. Standard turned finishes are suitable for bearing housings, O-ring grooves, and mating bores without additional finishing.

Grinding: Ra 4–16 μin. (0.1–0.4 μm)

Surface grinding, cylindrical grinding, or centerless grinding produces the finest as-machined finishes. Used for bearing journals, precision bores, gauge surfaces, and any feature requiring Ra ≤16 μin. Adds a secondary operation after rough CNC machining.

Wire EDM: Ra 8–32 μin. (0.2–0.8 μm)

Wire EDM achieves fine finishes through multiple skim passes (rough cut + 2–3 finish passes). The finish improves with each pass but at the cost of additional machine time. A single rough cut yields Ra ~125 μin.; three skim passes bring it to Ra 8–16 μin.

Section 3 of 9

Mechanical Finishes

Mechanical finishes use physical media (beads, grit, abrasives) to alter the surface texture without chemical processes.

Bead Blasting

Glass bead or aluminum oxide media propelled at the surface to create a uniform matte texture. Hides tool marks and fingerprints. Specify media type and mesh: "glass bead, 80–120 mesh, uniform matte." Often used as a pre-treatment before anodizing. Ra typically 63–250 μin. depending on media and pressure.

Typical cost: $1–$4/part (small brackets)

Tumble / Vibratory Deburring

Parts are placed in a drum or vibratory bowl with ceramic or plastic media. The media abrades edges and surfaces, removing burrs, rounding corners, and producing a uniform satin finish. Economical for high volumes. Specify "tumble deburr, break all edges 0.005–0.015 in."

Typical cost: $0.50–$2/part in batch

Brushing (Scotch-Brite)

Abrasive pad or belt produces a directional linear grain pattern on flat surfaces. Common for stainless steel panels, appliance fronts, and architectural metalwork. Specify grain direction relative to a datum: "#4 satin finish, grain parallel to long edge." Typical Ra 16–63 μin.

Typical cost: $2–$6/part

Section 4 of 9

Anodizing (Aluminum Only)

Anodizing is an electrochemical process that grows a hard aluminum oxide layer on the surface of aluminum parts. It is the most commonly specified secondary finish for CNC-machined and sheet metal aluminum components.

Type	Thickness	Hardness	Colors	Standard	Typical Use
Type II (Decorative)	0.3–1.0 mil (8–25 μm)	~40 HRC equivalent	Clear, black, blue, red, gold, custom	MIL-A-8625 Type II	Consumer electronics, enclosures, panels
Type III (Hardcoat)	1.0–3.0 mil (25–75 μm)	60–70 HRC equivalent	Natural dark gray/black (limited dye options)	MIL-A-8625 Type III	Wear surfaces, slides, rails, actuator bodies

Dimensional impact

Type III hardcoat grows approximately 50% into the base metal and 50% outward. A 2.0-mil hardcoat adds ~1.0 mil (0.001 in.) to each surface. On a bore with ±0.001 in. tolerance, this matters — either machine the bore 0.002 in. oversize before anodizing, or mask the bore to prevent coating. Call this out on your drawing: “MASK BORE B PER SPEC” or “MACHINE TO FINISH SIZE AFTER ANODIZE.”

Section 5 of 9

Chemical & Electrochemical Finishes

Passivation

Acid bath (citric or nitric per ASTM A967 or AMS 2700) removes free iron from stainless steel surfaces and restores the chromium oxide passive layer. Does not change dimensions or surface finish. Required for medical, food-contact, and marine stainless parts. Specify: "PASSIVATE PER ASTM A967, CITRIC ACID METHOD."

Compatible: Stainless steel (304, 316L, 17-4 PH)

Typical cost: $1–$5/part

Electropolishing

Electrochemical process that removes 0.0002–0.001 in. (5–25 μm) from the surface, preferentially dissolving peaks and smoothing the micro-profile to Ra 4–16 μin. (0.1–0.4 μm). Improves corrosion resistance beyond passivation and produces a bright, reflective finish. Specify: "ELECTROPOLISH PER ASTM B912, REMOVE 0.0005 in. MINIMUM."

Compatible: Stainless steel, some nickel alloys

Typical cost: $5–$25/part

Chromate Conversion (Alodine / Chem Film)

Chemical coating on aluminum that provides corrosion resistance and serves as a primer for paint adhesion. Adds negligible thickness (<0.0001 in.). Available as clear (Class 3) or gold/iridescent (Class 1A per MIL-DTL-5541). Does not affect dimensions. Often used on aluminum parts that will be painted or powder coated.

Compatible: Aluminum alloys

Typical cost: $1–$3/part

Black Oxide

Chemical conversion coating on carbon steel and stainless steel that produces a dark black appearance. Adds negligible thickness (<0.0001 in.) and provides mild corrosion resistance (typically supplemented with oil or wax). Specify: "BLACK OXIDE PER MIL-DTL-13924, CLASS 1." Commonly used on fasteners, tooling, and firearms components.

Compatible: Carbon steel, stainless steel

Typical cost: $0.50–$3/part

Section 6 of 9

Plating

Plating deposits a thin metal layer onto the part surface through electrochemical (electroplating) or chemical (electroless) processes.

Zinc Plating

Sacrificial corrosion protection for carbon steel. 0.2–0.5 mil (5–12 μm) zinc layer with optional chromate conversion (clear, yellow, or black). Per ASTM B633. The most economical plating for steel parts.

Typical cost: $0.50–$3/part

Nickel Plating (Electroless)

Uniform nickel-phosphorus coating regardless of part geometry (no "dog bones" at edges). Adds corrosion and wear resistance. Typical thickness: 0.3–1.0 mil. Per MIL-C-26074 or ASTM B733. Used on complex geometries where electroplating would be non-uniform.

Typical cost: $3–$10/part

Chrome Plating (Hard Chrome)

Hard chromium electroplate (1–10 mil) for extreme wear resistance. Used on hydraulic cylinder rods, piston rings, and bearing surfaces. Per SAE-AMS-QQ-C-320 (formerly QQ-C-320). Note: hexavalent chrome is increasingly restricted (REACH, RoHS) — specify trivalent chrome alternatives where regulations apply.

Typical cost: $5–$20/part

Section 7 of 9

Organic Coatings

Powder Coating

Electrostatically applied dry powder cured at 350–400°F (175–200°C). Produces a durable, uniform, scratch-resistant coat in any RAL color. Thickness: 2–4 mil (50–100 μm). Widely used for enclosures, brackets, and outdoor hardware. Cannot be applied to plastics or materials that cannot withstand the cure temperature.

Typical cost: $3–$12/part

Wet Paint (Spray)

Liquid paint applied by spray gun. Available in virtually any color, gloss level, and specialty formulation (high-temp, anti-static, conductive). Thinner than powder coat (0.5–2 mil typical). Used when powder coat cure temperature is too high, when very thin coats are needed, or for touch-up and small quantities.

Typical cost: $2–$8/part

E-Coat (Electrocoat)

The part is submerged in a paint bath and an electrical current deposits a uniform coating into every recess and cavity. Typical thickness: 0.6–1.2 mil. Used for high-volume automotive and appliance parts where uniform coverage of complex geometry is required. Primer coat only — typically followed by a topcoat for cosmetic parts.

Typical cost: $1–$4/part (high volume)

Section 8 of 9

Finish Comparison Table

Finish	Compatible Metals	Thickness Added	Corrosion Protection	Wear Resistance	Colors	Cost/Part
Type II Anodize	Aluminum	0.3–1.0 mil	Moderate	Low–moderate	Many (dye)	$2–$8
Type III Hardcoat	Aluminum	1.0–3.0 mil	High	High (60–70 HRC)	Dark gray/black	$5–$15
Passivation	Stainless steel	None	Moderate (restores passive layer)	None	None (no change)	$1–$5
Electropolish	Stainless steel	Removes 0.0002–0.001 in.	High	None	Mirror bright	$5–$25
Powder Coat	Steel, aluminum	2–4 mil	High	Moderate	Any RAL color	$3–$12
Zinc Plating	Carbon steel	0.2–0.5 mil	Moderate (sacrificial)	Low	Clear/yellow/black	$0.50–$3
Electroless Nickel	Most metals	0.3–1.0 mil	High	Moderate–high	Silver/gray	$3–$10
Hard Chrome	Most metals	1–10 mil	High	Very high	Chrome silver	$5–$20
Chromate Conv.	Aluminum	<0.0001 in.	Low–moderate	None	Clear/gold	$1–$3
Black Oxide	Steel	<0.0001 in.	Low (needs oil)	None	Black	$0.50–$3

Section 9 of 9

How to Specify Surface Finish on Your Drawing

A properly specified finish eliminates RFIs and ensures the finisher delivers what you expect. Here is exactly what to include.

Ra callout on the drawing surface

Place the surface finish symbol (per ASME Y14.36 or ISO 1302) on surfaces where roughness matters functionally. Write the maximum Ra value: e.g., "32" for 32 μin. Ra. Surfaces without a callout default to the general note (typically "125 μin. Ra UOS").

Finish callout in general notes

Specify the secondary finish with full detail: "FINISH: TYPE II ANODIZE, BLACK, PER MIL-A-8625 TYPE II CLASS 2, 0.7 MIL NOM." Include: finish type, color, specification standard, and thickness. Missing any element creates ambiguity.

Masking requirements

If certain features must remain uncoated (threads, press-fit bores, mating surfaces), call out masking: "MASK THREADS AND BORE B PRIOR TO ANODIZE." Without this note, the finisher coats everything — and you get anodize build-up in your press-fit bore.

Pre-finish and post-finish dimensions

For coatings that add meaningful thickness (Type III anodize, hard chrome, powder coat), specify whether dimensions on the drawing are before or after coating. Standard practice: dimensions are after coating unless noted. If machining to a tight tolerance, note: "DIMENSION APPLIES AFTER COATING" or "MACHINE BORE TO 0.502 in. BEFORE ANODIZE."

Common Questions

Frequently Asked Questions

What does Ra mean and how is it measured?

Ra (arithmetic average roughness) is the average deviation of the surface profile from the mean line, measured over a sampling length per ISO 4287. It is the most commonly specified surface roughness parameter. Ra is measured with a contact profilometer (a diamond stylus drags across the surface) or a non-contact optical profilometer. Values are expressed in microinches (μin.) in the US or micrometers (μm) internationally. Lower Ra = smoother surface. Typical as-machined CNC milling: Ra 63–125 μin. (1.6–3.2 μm).

What is the difference between Type II and Type III anodizing?

Type II (standard/decorative) anodizing grows a 0.3–1.0 mil (8–25 μm) oxide layer on aluminum. It provides moderate corrosion resistance, accepts dye colors (black, blue, red, gold), and has minimal dimensional impact. Type III (hardcoat) anodizing grows a 1.0–3.0 mil (25–75 μm) oxide layer that is significantly harder (60–70 HRC equivalent), more wear-resistant, and more corrosion-resistant. Type III is typically dark gray or black and is harder to dye in bright colors. Type III adds roughly half the coating thickness to each surface (growth is ~50% into the base metal, ~50% outward), so account for this in your tolerances.

When should I specify passivation vs. electropolishing?

Passivation (per ASTM A967 or AMS 2700) is a chemical treatment that removes free iron from stainless steel surfaces and restores the chromium oxide passive layer. It does not change the surface finish. Use passivation when you need corrosion resistance on stainless steel without changing the appearance or roughness. Electropolishing removes 0.0002–0.001 in. (5–25 μm) of material through an electrochemical process, smoothing the surface to Ra 4–16 μin. (0.1–0.4 μm) and improving corrosion resistance beyond passivation alone. Use electropolishing when you need both a mirror-like finish and enhanced corrosion performance — common for medical devices, pharmaceutical equipment, and food-contact surfaces.

How does surface finish affect part cost?

Each step of surface finish improvement adds cost in two ways: (1) machining time — achieving Ra 32 μin. requires slower feed rates and finer tools than Ra 125 μin., roughly doubling cycle time per surface, and (2) secondary finishing — anodizing adds $2–$8/part for small brackets, powder coating adds $3–$12/part, electropolishing adds $5–$25/part depending on geometry and batch size. As a guideline: leaving a part as-machined (Ra 63–125 μin.) is the baseline cost. Specifying Ra 32 μin. adds 20–40% to machining cost on affected surfaces. Adding anodize or powder coat adds $2–$12/part. Specifying Ra 8–16 μin. (grinding/polishing) can double the total part cost.

How do I call out a surface finish on my engineering drawing?

Use the surface finish symbol (a checkmark-like mark per ASME Y14.36 or ISO 1302) placed on the surface with a leader line. Write the maximum Ra value in the symbol: e.g., "32" for Ra 32 μin. or "0.8" for Ra 0.8 μm. For secondary finishes, call them out in the general notes: "FINISH: TYPE II ANODIZE, BLACK, PER MIL-A-8625 TYPE II CLASS 2" or "POWDER COAT: RAL 7016 ANTHRACITE GRAY, 2–4 MIL." Specify the standard (MIL-A-8625 for anodize, ASTM B633 for zinc plating, ASTM A967 for passivation). Missing the standard creates ambiguity — "anodize" alone does not tell the finisher whether you want Type II or Type III, what thickness, or what color.

Can I anodize all aluminum alloys?

All aluminum alloys can be anodized, but the results vary significantly. 6061-T6 and 5052-H32 produce consistent, uniform anodize layers with good dye uptake — these are the preferred alloys for anodized parts. 7075-T6 anodizes well but may show slight color variation. 2024-T3 is difficult to anodize uniformly due to its high copper content (3.8–4.9%) — expect mottled or inconsistent coloring. Cast aluminum alloys (A356, A380) produce a porous, dull anodize with limited dye absorption. If your part requires cosmetic anodizing, specify 6061-T6 or 5052-H32 and note the finish in the material callout.