Surface Finish Specifications: A Practical Guide to Ra Values

Surface finish specifications cause more confusion—and more unnecessary cost—than almost any other drawing callout. Engineers often specify finishes tighter than needed “just to be safe,” not realizing that each step finer can add 15-50% to machining costs.

This guide explains what surface finish numbers actually mean and provides practical guidance for specifying the right finish for your application.

What Ra Actually Measures

Ra (Roughness Average) is the arithmetic mean of surface deviations from the centerline, measured in either microinches (µin) or micrometers (µm).

Think of it this way: if you could slice through a surface and measure every peak and valley from an imaginary centerline, Ra is the average of all those measurements.

Unit conversion: 1 µm = 40 µin (approximately)

Common specifications you’ll encounter:

Ra (µin)	Ra (µm)	Common Name
250	6.3	Rough machining
125	3.2	Standard machining
63	1.6	Fine machining
32	0.8	Precision machining
16	0.4	Fine grinding
8	0.2	Lapping/Honing
4	0.1	Superfinishing

The Cost of Smooth

Here’s what most engineers don’t learn in school: surface finish cost is not linear.

Going from 125 µin to 63 µin might add 20% to machining time. Going from 32 µin to 16 µin might double it. Achieving single-digit Ra values can require entirely different processes—grinding, lapping, or honing instead of standard machining.

Approximate cost multipliers (vs. 125 µin baseline):

Finish	Cost Multiplier	Process
125 µin (3.2 µm)	1.0x	Standard milling/turning
63 µin (1.6 µm)	1.2x	Fine milling/turning
32 µin (0.8 µm)	1.5x	Precision machining
16 µin (0.4 µm)	2.0-2.5x	Grinding
8 µin (0.2 µm)	3.0-4.0x	Fine grinding/Lapping
4 µin (0.1 µm)	5.0-8.0x	Lapping/Superfinishing

The message: specify the roughest acceptable finish, not the smoothest achievable.

Quick Reference: Finish by Application

Non-Critical Surfaces: 125-250 µin (3.2-6.3 µm)

• Structural components
• Mounting surfaces (non-precision)
• Cosmetically unimportant areas
• Rough bores and pockets
• Most cast surfaces (as-cast or minimal machining)

This is the default for surfaces without specific requirements.

General Machined Surfaces: 63-125 µin (1.6-3.2 µm)

• Mating surfaces (non-sealing)
• Clearance fits
• General mechanical components
• Most external surfaces
• Standard bearing housings

Standard CNC capability without special attention.

Precision Fits and Sealing: 32-63 µin (0.8-1.6 µm)

• O-ring grooves and glands
• Hydraulic cylinder bores
• Press-fit interfaces
• Sliding contact surfaces
• Transition and locational fits

Achievable with careful machining and appropriate tooling.

High-Precision Applications: 16-32 µin (0.4-0.8 µm)

• Bearing races
• Sealing faces (metal-to-metal)
• Precision instrument components
• High-speed rotating surfaces
• Close-tolerance fits

May require grinding or precision boring.

Critical/Specialized: Below 16 µin (0.4 µm)

• Optical mounting surfaces
• Semiconductor equipment
• Gauge blocks and masters
• Medical implant surfaces
• Hydraulic valve spools

Requires grinding, lapping, honing, or specialized processes.

Industry-Specific Standards

Different industries have established finish requirements based on experience:

Food and Beverage

Sanitary surfaces typically require 32 µin (0.8 µm) or better to prevent bacterial harborage. Dairy equipment often specifies 20-32 µin, electropolished.

Pharmaceutical (ASME BPE)

Designation	Ra Max	Application
SF1	40 µin (1.0 µm)	Utility services
SF2	30 µin (0.76 µm)	General process
SF3	25 µin (0.64 µm)	Clean process
SF4	15 µin (0.38 µm)	High-purity/Biotech

Hydraulic Components

• Cylinder bores: 8-16 µin (0.2-0.4 µm)
• Piston rods: 8-12 µin (0.2-0.3 µm)
• Valve spools: 4-8 µin (0.1-0.2 µm)
• Sealing faces: 16-32 µin (0.4-0.8 µm)

Bearing Surfaces

• Ball bearing raceways: 4-8 µin (0.1-0.2 µm)
• Journal bearing surfaces: 16-32 µin (0.4-0.8 µm)
• Thrust surfaces: 8-16 µin (0.2-0.4 µm)

Beyond Ra: Other Roughness Parameters

Ra is the most common specification, but it doesn’t tell the whole story. Two surfaces with identical Ra can have very different characteristics.

Rz (Average Maximum Height) Measures the average of the five highest peaks to five lowest valleys. More sensitive to occasional deep scratches or high peaks that Ra might average out.

Rt (Total Height) The total distance from lowest valley to highest peak. Critical for sealing applications where a single deep scratch can cause leakage.

Rsk (Skewness) Indicates whether a surface has more peaks or valleys. Negative skewness (more valleys) is often preferred for bearing surfaces—the valleys hold lubricant.

When to specify beyond Ra:

• Sealing surfaces: Consider Rz or Rt limits
• Bearing surfaces: Consider Rsk
• Highly critical applications: Specify multiple parameters

Common Mistakes to Avoid

Over-specifying “to be safe” If a surface doesn’t have a functional requirement for a specific finish, don’t specify one. Let the machinist use standard processes.

Specifying finish on non-functional surfaces Cosmetic surfaces rarely need finish callouts. If appearance matters, specify “machine marks acceptable” or “good commercial finish” rather than a tight Ra.

Ignoring process limitations Some geometries can’t achieve fine finishes. Deep holes, internal corners, and interrupted cuts have practical limits.

Calling out finish without measurement method For critical applications, specify how the measurement should be taken—probe direction, evaluation length, and filter cutoff.

How to Specify on Drawings

Standard surface finish callouts use the check mark symbol with Ra value:

• Basic: ✓ 63 (63 µin Ra maximum)
• With process: ✓ 32 (ground)
• Specific direction: ✓ 32 ⊥ (perpendicular lay)

For critical surfaces, add notes specifying:

• Measurement direction (perpendicular to lay unless noted)
• Evaluation length
• Whether the callout is maximum, range, or nominal
• Required measurement equipment or method

What to Tell Your Supplier

When requesting quotes, communicate:

1. Which surfaces are critical (highlight on drawings)
2. Why the finish matters (sealing, bearing, cosmetic)
3. Whether you have flexibility on non-critical surfaces
4. Inspection requirements (self-inspect, report data, certify)

This information helps suppliers quote accurately and identify cost-saving opportunities you might have missed.

Working With NextGen Components

Our machining partners maintain capabilities from standard machined finishes through precision grinding. When you submit drawings, we review finish specifications and flag any callouts that may significantly impact cost—often suggesting alternatives that meet functional requirements at lower cost.

Have questions about finish specifications for your application? Contact our engineering team for guidance.