Machining Surface Finish Chart: Ra by Process

Machining Surface Finish Chart: Ra by Process

A machining surface finish chart shows the surface roughness, written as an Ra value, that each machining process can reach. As a quick rule, grinding, honing, and lapping produce the smoothest surfaces, while standard turning and milling land around 3.2 micrometers (125 microinches), the common as-machined finish. Match the process to the finish you need and you avoid both rough parts and wasted cost.

Below you will find a full process-to-Ra table, a plain breakdown of each process, a turning versus milling comparison, and the levers our engineers use to get a finer finish. Every value here matches the ASME B46.1 and ISO 21920 standards.

What Surface Finish Can Machining Achieve?

Machining can reach a wide range of finishes, from a rough 6.3 micrometers down to a mirror-smooth 0.05 micrometers. The exact result depends on the process, the tooling, and the speeds and feeds used. The chart below is the quick reference for the achievable surface finish by process.

Which Process Gives the Smoothest Finish?

Lapping and superfinishing give the smoothest finish, reaching as low as 0.025 micrometers (1 microinch). Honing and fine grinding come next. These processes are slow and skilled, so they are saved for parts that truly need a polished surface, like bearings and gauge blocks. To learn what each value means in use, see our guide to surface finish Ra values.

Surface Finish by Machining Process

Here is how each common process performs, what it is best at, how to push it finer, and what that does to cost. Use this as your surface finish scale for machining.

CNC Turning

Standard CNC turning produces a 1.6 to 6.3 micrometer finish on round parts like shafts and pins. To go finer, use a sharp insert, a smaller nose radius with a slow feed, and a light final pass. A fine turning pass can reach 0.4 micrometers. Pushing below that usually means adding grinding. For round-part work, see our CNC turning services.

CNC Milling

Standard CNC milling lands in the same 1.6 to 6.3 micrometer range as turning, the everyday machining finish. A fine milling pass with a sharp cutter, high RPM, and a small step-over can reach 0.8 micrometers. The milling surface finish you get depends heavily on the final pass, so a dedicated finishing pass is often the cheapest way to improve it.

Drilling and Reaming

A drilled hole wall is fairly rough, around 1.6 to 6.3 micrometers, because the drill cuts fast and the chips drag. To smooth a hole, follow drilling with reaming, which brings the wall down to 0.4 to 1.6 micrometers. Use reaming when a hole needs a press fit, a seal, or a sliding part inside it.

Grinding

Grinding uses an abrasive wheel to reach 0.1 to 1.6 micrometers, far smoother than turning or milling. It is the standard step for bearing seats, seal surfaces, and hardened parts that are too tough to machine cleanly. The grinding surface finish chart values above show why it is the go-to when a part needs to be both hard and smooth.

Honing

Honing uses fine abrasive stones to reach 0.1 to 0.8 micrometers, with a crosshatch pattern that helps hold oil. It is the standard finish for cylinder bores and hydraulic components. The honing surface finish chart range above makes it ideal where parts slide under pressure and need to seal.

Lapping and Superfinishing

Lapping rubs the part against a soft plate loaded with fine abrasive, reaching 0.025 to 0.2 micrometers, the smoothest finish in the shop. The lapping surface finish is mirror-like and used on gauge blocks, optical parts, and the most critical seals. It is slow and costly, so it appears only where nothing rougher will work.

Turning vs Milling Surface Finish

Turning and milling produce a similar standard finish, both around 1.6 to 6.3 micrometers, but they get there differently. Turning spins the part against a fixed tool, so it leaves a smooth, even spiral pattern that often looks finer. Milling spins the tool against a fixed part, so the finish depends more on the step-over and the final pass.

In practice, turning a round part to a fine finish is often easier and cheaper than milling a flat one to the same number. The table below compares the three main metal-removal processes side by side.

How to Improve Surface Finish in Machining: The Three Levers

When a finish is not smooth enough, you do not always need a new process. Our engineers reach for three levers first, in order, before adding a costly grinding or honing step.

• Tool: Use a sharp, undamaged insert with the right nose radius. A worn tool is the most common cause of a rough finish.
• Path: Slow the feed, lighten the final pass, and reduce the step-over so the tool leaves smaller marks.
• Process: Only if Tool and Path are maxed out, add a finer operation like grinding or honing.

Here is a real example. A customer needed a 0.4 micrometer finish on one sealing face of a steel housing. The first instinct was to fine-mill the entire part, which is slow and expensive. Our team instead milled the whole part to a normal finish and added a quick targeted grind on only the sealing face. The part hit the spec, passed first-article inspection on a calibrated profilometer, and cost less than over-machining every surface. That kind of process tuning is built into our CNC machining and surface finishing services.

How Surface Finish Choice Affects Cost

Surface finish is one of the biggest hidden cost drivers in machining. Going from a standard finish to a fine one can add a second operation, which raises both time and price. The smartest move is to specify a tight finish only where the part actually needs it.

The most common mistake we see is asking for a fine finish on every surface, even faces that never touch another part. Our DFM review flags this on every quote and typically saves 15 to 30 percent. To see how finish fits into total part price, read our guide on how much it costs to get a metal part made. For the full roughness reference and unit conversions, see our surface finish chart and our surface finish conversion chart.