Surface Finish Specifications Explained: Ra, Rz, and What They Mean for Your Parts

Surface finish specifications appear on nearly every precision machined part drawing — and are misunderstood more often than almost any other callout. Engineers specify Ra values out of habit. Buyers accept inspection reports without knowing what the numbers mean. Machinists apply finish requirements without knowing why a particular value was chosen.

Getting surface finish right matters. On sealing surfaces, bearing bores, sliding interfaces, and biomedical implants, the wrong finish creates functional problems that dimensional accuracy alone won’t prevent. Understanding the difference between Ra and Rz — and when each applies — helps you specify finish requirements that match your application and communicate them clearly to your machine shop. FM Machine’s precision CNC machining services include surface finish control and verification as part of standard quality practice.

What Surface Finish Actually Measures

A machined surface is not perfectly smooth. At the microscopic level, it has peaks and valleys left by the cutting tool, grinding wheel, or finishing process. Surface finish measurement quantifies the texture of those peaks and valleys using a contact profilometer — an instrument that drags a fine stylus across the surface and records the vertical displacement.

The recorded profile is used to calculate parameters that describe surface texture. Ra and Rz are the two most commonly specified.

Ra: Average Roughness

Ra is the arithmetic mean of the absolute deviations of the surface profile from the mean line over a sampling length. In plain terms, it’s the average height of the peaks and valleys across the measured surface.

Ra is expressed in microinches (µin) in the inch system and micrometers (µm) in metric. Common Ra values in machining:

• 250 Ra (µin): Rough machined surface — heavy cuts, visible tool marks
• 125 Ra: General machined finish — standard milling or turning, faint tool marks
• 63 Ra: Improved machined finish — most production CNC work
• 32 Ra: Smooth finish — finish milling or turning with controlled parameters
• 16 Ra: Fine finish — requires careful setup, light cuts, sharp tooling
• 8 Ra: Ground or fine-honed surface — typically requires a secondary operation
• 4 Ra and below: Lapped, superfinished, or electropolished — specialized processes

Ra is useful for general surface texture specification and is widely understood across manufacturing. Its limitation is that it averages the profile — two surfaces with identical Ra values can have very different peak heights and valley depths, which affects how they perform in service.

Rz: Average Maximum Height

Rz measures the average of the maximum peak-to-valley heights across multiple sampling lengths. Because it focuses on the extremes of the profile rather than the average, Rz is more sensitive to isolated high peaks or deep valleys that Ra would average away.

For applications where a single high peak could cause contact stress, wear, or leakage, Rz provides better control than Ra. Sealing surfaces, bearing races, and sliding valve interfaces often benefit from Rz specification because a surface with a few isolated peaks — which might show an acceptable Ra — could fail under load or prevent a seal from seating.

The general relationship between Ra and Rz varies by manufacturing process, but Rz is typically 4 to 7 times the Ra value on a well-controlled machined surface.

When to Specify Ra vs. Rz

Ra is appropriate for general surface texture control where average roughness governs the application — cosmetic appearance, general wear resistance, paint adhesion, or assembly fit where no single peak causes a functional problem.

Rz is more appropriate when the application is sensitive to peak heights:

• Static and dynamic sealing surfaces where a peak could bridge a seal
• Sliding bearing surfaces where peak contact governs wear rate
• Fatigue-critical surfaces where high peaks act as stress concentrators
• Surfaces receiving thin coatings where peak breakthrough affects coating performance

In practice, many drawings specify Ra where Rz would be more appropriate. If your application involves sealing or wear-critical surfaces, discuss with your machine shop whether adding an Rz requirement provides more meaningful control.

How Machining Processes Relate to Surface Finish

Different machining processes produce characteristic surface finishes:

• Rough milling/turning: 125–250 Ra
• Finish milling/turning: 32–125 Ra depending on parameters
• Grinding: 8–32 Ra
• Honing: 4–16 Ra with characteristic crosshatch pattern
• Lapping: 1–4 Ra
• Electropolishing: Below 8 Ra with improved corrosion resistance

Specifying 8 Ra on a milled surface implies a secondary grinding operation. Specifying 32 Ra on a bore that requires a honed crosshatch for oil retention misses the functional requirement. The finish callout and the process that achieves it need to match.

Surface Finish Measurement and Verification

Surface finish specified on a drawing should be verified during inspection. Contact profilometry with a calibrated instrument is the standard method. For curved surfaces and complex geometry, non-contact optical profilometry is available where stylus access is limited.

FM Machine’s machined parts inspection capabilities include surface finish measurement to verify Ra requirements alongside dimensional inspection. If your drawing specifies finish, we verify it.

Specify Finish for Function, Not Habit

The most common surface finish mistake is applying 63 Ra or 32 Ra everywhere on a drawing without considering what each surface actually requires. That drives unnecessary secondary operations on non-critical surfaces and — worse — may leave functional surfaces underspecified.

Review your finish callouts against the function of each surface. Where finish affects sealing, wear, or fatigue, specify the right parameter. Where it doesn’t, leave it at the general machining note and avoid unnecessary cost.

Request a quote from FM Machine and include your surface finish requirements — we’ll machine it to spec and verify it before it ships.