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. The unit conversion is straightforward: 1 µm equals approximately 40 µin.

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.

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.

Finish Requirements by Application

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

This range serves as the default for surfaces without specific requirements. Structural components, non-precision mounting surfaces, cosmetically unimportant areas, rough bores and pockets, and most cast surfaces fall into this category. Unless a surface has a specific functional need, there’s no reason to call out anything finer.

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

Standard CNC capability achieves this range without special attention. It suits mating surfaces that don’t require sealing, clearance fits, general mechanical components, most external surfaces, and standard bearing housings. This is what you get from a competent shop running normal programs with appropriate tooling.

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, and transition fits require this level of finish. Achieving it demands careful machining with appropriate tooling, but it remains within the capability of standard CNC equipment when properly programmed.

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

Bearing races, metal-to-metal sealing faces, precision instrument components, high-speed rotating surfaces, and close-tolerance fits operate at this level. Production may require grinding or precision boring rather than conventional machining. See our CNC machining tolerances guide for related information.

Critical and Specialized: Below 16 µin (0.4 µm)

Optical mounting surfaces, semiconductor equipment, gauge blocks and masters, medical implant surfaces, and hydraulic valve spools demand finishes in this range. Production requires grinding, lapping, honing, or specialized processes that go well beyond conventional machining.

Industry-Specific Standards

Different industries have established finish requirements based on decades of 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. The smooth surfaces facilitate cleaning and eliminate crevices where contaminants could accumulate. See our FDA compliance guide for material requirements in food processing applications.

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

Hydraulic systems demand fine finishes to ensure seal life and prevent leakage. Cylinder bores typically require 8-16 µin (0.2-0.4 µm) to support piston seal operation. Piston rods need 8-12 µin (0.2-0.3 µm) where rod seals contact the surface. Valve spools, which must move precisely within close-tolerance bores, often require 4-8 µin (0.1-0.2 µm). Sealing faces can function at 16-32 µin (0.4-0.8 µm) in many applications.

Bearing Surfaces

Bearing finish requirements depend on the bearing type and operating conditions. Ball bearing raceways require 4-8 µin (0.1-0.2 µm) to achieve rated life and low noise. Journal bearing surfaces operate well at 16-32 µin (0.4-0.8 µm), where the slightly rougher finish helps retain lubricant. Thrust surfaces typically need 8-16 µin (0.2-0.4 µm) to handle axial loading without excessive friction.

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. It’s more sensitive to occasional deep scratches or high peaks that Ra might average out, making it valuable for sealing applications where isolated defects matter.

Rt (Total Height) captures the total distance from lowest valley to highest peak. This parameter is critical for sealing applications where a single deep scratch can cause leakage, since Ra would mask such isolated defects.

Rsk (Skewness) indicates whether a surface has more peaks or valleys. Negative skewness, meaning more valleys than peaks, is often preferred for bearing surfaces because the valleys hold lubricant.

For sealing surfaces, consider specifying Rz or Rt limits in addition to Ra. For bearing surfaces, Rsk may be relevant. Highly critical applications often specify multiple parameters to fully characterize the required surface.

Common Mistakes to Avoid

Over-specifying “to be safe” is the most expensive mistake. If a surface doesn’t have a functional requirement for a specific finish, don’t specify one. Let the machinist use standard processes that naturally produce acceptable results.

Specifying finish on non-functional surfaces wastes money on cosmetics. If appearance matters, specify “machine marks acceptable” or “good commercial finish” rather than a tight Ra value. See our DFM guidelines for more cost-saving strategies.

Ignoring process limitations leads to impossible callouts. Some geometries can’t achieve fine finishes regardless of effort. Deep holes, internal corners, and interrupted cuts have practical limits that no amount of money can overcome.

Calling out finish without measurement method creates ambiguity on critical applications. For important surfaces, specify how the measurement should be taken—probe direction, evaluation length, and filter cutoff all affect the measured value.

How to Specify on Drawings

Standard surface finish callouts use the check mark symbol with the Ra value. A basic callout like “63” indicates 63 µin Ra maximum. You can add process requirements such as “32 (ground)” or specify lay direction with symbols like “32 perpendicular.”

For critical surfaces, additional notes should specify measurement direction (perpendicular to lay unless noted otherwise), evaluation length, whether the callout represents a maximum, range, or nominal value, and required measurement equipment or method.

What to Tell Your Supplier

When requesting quotes, clear communication helps suppliers price accurately and identify cost-saving opportunities. Highlight which surfaces are critical on your drawings. Explain why the finish matters—whether for sealing, bearing function, or cosmetic appearance. Indicate whether you have flexibility on non-critical surfaces. Clarify inspection requirements: self-inspect, report data, or certify.

This information helps suppliers understand your actual needs rather than guessing. Often, they can suggest alternatives that meet functional requirements at lower cost.

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.