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Surface Finish Basics

Guide · Updated October 28, 2026

Surface finish describes the fine peaks and valleys left by machining. Drawings call it out as Ra (or Rz) in microinches or micrometers, and the biggest levers you control are feed, nose radius, tool sharpness and rigidity.

Check the size too

Finish is only half of a good part. Build exact gage stacks to verify size with the free Gage Block Calculator.

Step by step

Read the callout: Find the finish symbol on the drawing: a value like 63 means Ra 63 µin (1.6 µm) maximum unless noted otherwise.
Pick the method: Use a stylus profilometer for numbers, or a finish comparator plate (feel and look) for a quick shop check of the same process.
Measure across the lay: Run the stylus perpendicular to the tool marks, at the cutoff length the drawing or standard calls for (often 0.030 in / 0.8 mm).
Compare to spec: Take a few readings in different spots and compare to the maximum allowed.
Adjust the process: If rough, reduce feed, use a larger nose radius or wiper insert, raise speed into the tool’s sweet spot, and fix chatter or a dull edge.

Worked example

Turning with a 1/32 in (0.03125 in) nose radius at 0.008 IPR. Theoretical peak-to-valley ≈ f² ÷ (8 r) = 0.000064 ÷ 0.25 = 0.000256 in (256 µin). Ra is roughly a quarter of that: f² ÷ (32 r) ≈ 64 µin Ra.

The drawing calls for 32 Ra. Halving the feed to 0.004 IPR cuts the theoretical value by four: about 16 µin Ra, which leaves room for real-world wear and vibration. A 1/64 in radius at 0.008 IPR would be about 128 µin, so a bigger radius helps too.

Ra vs Rz

Ra is the average deviation from the mean line, so one deep scratch barely moves it. Rz averages the highest peak to lowest valley over several sample lengths and is usually 4–7 times Ra on turned surfaces. Common callouts: 125 Ra general machining, 63 good machined, 32 fine turned or ground-like, 16 and below ground, honed or lapped. 1 µm ≈ 40 µin.

Tips

Feed and nose radius matter most on a lathe: turning speeds and feeds.
On a mill, chip load and step-over control the finish: milling speeds and feeds.
Profile and small details can be checked on an optical comparator.
Coolant and a sharp edge reduce built-up edge, which tears the surface: cutting fluid basics.
Chatter marks usually mean a rigidity problem, not a speed problem: shorten stickout first.

FAQ

What does Ra 32 mean?

An average roughness of 32 microinches (about 0.8 µm). Lower numbers are smoother.

How do you measure surface finish in a machine shop?

With a stylus profilometer for numbers, or by comparing to a finish comparator plate by sight and fingernail for a quick check.

How do I get a better finish on a lathe?

Lower the feed, use a larger nose radius or wiper insert, keep the edge sharp, run the speed the insert likes and remove chatter by reducing stickout.

What is the difference between Ra and Rz?

Ra is an average deviation; Rz is an average of peak-to-valley heights, so it reacts more to scratches and is a larger number.

Is there a free calculator for checking part size?

Yes. The Gage Block Calculator at gageblockcalc.com builds any inch or metric stack, free with no login.

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General shop information only — follow your tooling, machine and gage manufacturers' instructions and your quality system.