Machining Surface Finish Chart: Ra by Process

A machining surface finish chart maps each cutting process to the surface roughness, or Ra value, it can realistically hold. Standard CNC turning and milling land between 1.6 and 6.3 micrometers (63 to 250 microinches). Grinding reaches 0.1 to 1.6 micrometers, and lapping goes as fine as 0.025 micrometers (1 microinch).

This guide is built for choosing a process, not for converting units. You get a capability table that names the physical limit behind each number, a turning versus milling comparison, how material changes the result, a feature by feature selection matrix, and the three levers our engineers pull before adding an operation. If you need to convert between Ra, RMS, Rz, and N grades, use our surface finish chart instead.

One note on sources. The roughness parameters here follow ASME B46.1 and ISO 21920-1, which govern how Ra is measured and called out. The process ranges come from published machining references and from our own 2026 production data, because no standard promises a finish for a given process.

What Surface Finish Can Each Machining Process Achieve?

Machining spans a huge range, from a coarse 12.5 micrometers off a roughing cutter down to 0.025 micrometers off a lapping plate. The table below is the quick reference. The last column is the part most charts leave out: what actually stops each process from going finer.

Which Process Gives the Smoothest Finish?

Lapping and superfinishing give the smoothest finish, down to 0.025 micrometers. Honing and fine grinding sit just above them near 0.1 micrometers. All three are slow and skill heavy, so they are reserved for gauge blocks, bearing races, and seals that leak at anything rougher.

What Is the Roughest Machining Finish?

Rough milling is the coarsest common machined finish at 6.3 to 12.5 micrometers. The cutter moves fast and takes deep cuts, so it leaves obvious marks. It is also the cheapest metal removal you can buy, which is exactly why it belongs on hidden faces.

Surface Finish by Machining Process

Here is what each process is good at and how far you can push it before the cost curve turns.

CNC Turning

Turning holds 1.6 to 6.3 micrometers on shafts, pins, and bushings. A larger nose radius paired with a slower feed is the fastest way to improve it, because feed marks are geometric and predictable. A dedicated fine pass reaches 0.4 micrometers. Below that, plan on grinding. See our CNC turning services for round part work.

CNC Milling

Milling covers the same 1.6 to 6.3 micrometer band, but it gets there less evenly. The cutter leaves scallops, so the step-over drives the result more than the feed does. Cut the step-over, raise the RPM, and use a sharp finisher, and 0.8 micrometers is reachable on aluminium and steel.

Drilling and Reaming

A drilled wall runs 1.6 to 6.3 micrometers because chips scrape the bore on their way out. Reaming after drilling brings it to 0.4 to 1.6 micrometers and tightens the hole at the same time. Specify reaming when the hole takes a press fit, a seal, or a sliding pin.

Grinding

Grinding reaches 0.1 to 1.6 micrometers with an abrasive wheel. It is the default for bearing seats and for hardened parts above roughly 45 HRC, where carbide tooling struggles. Wheel grit and how recently the wheel was dressed matter more than machine speed.

Honing

Honing uses abrasive stones to reach 0.1 to 0.8 micrometers and leaves a crosshatch pattern that holds an oil film. That pattern is the point. Cylinder bores and hydraulic components need it to seal and stay lubricated under pressure.

Lapping and Superfinishing

Lapping rubs the part against a charged plate to reach 0.025 to 0.2 micrometers. The result is mirror like and slow to produce. It shows up on optical mounts, gauge blocks, and the small number of seals where nothing rougher survives.

Turning vs Milling Surface Finish

Both land in the same numeric range, so buyers often treat them as equal. They are not. Turning produces a continuous spiral, which means the roughness is repeatable and easy to predict from feed and nose radius. Milling produces overlapping scallops, so the finish varies with toolpath and part geometry.

Bottom line: getting a round feature to a fine finish is usually cheaper than getting a flat one there. If a design allows either, we recommend putting the tight callout on the turned feature.

Does Material Change the Achievable Surface Finish?

Yes, and this is where most finish problems start. Two shops running the same program on the same machine will get different Ra values from 6061 aluminium and 316 stainless. The table below reflects typical results across our shops in 2026, using standard production tooling rather than best case lab conditions.

Which Process Should You Use for Each Feature?

Process choice is a feature level decision, not a part level one. Use this matrix to route each surface before you finalise the drawing.

How to Improve Surface Finish in Machining: The Three Lever Ladder

When a part comes off too rough, the reflex is to add grinding. That is the most expensive answer and usually the third best one. Our engineers climb three rungs in order, and each rung costs more than the one below it.

• Rung one, Tool. Fit a sharp, undamaged insert with the right nose radius and coating for the material. A worn tool is the single most common cause of a rough surface, and swapping it costs minutes.
• Rung two, Path. Slow the finishing feed, lighten the final depth of cut, and shrink the step-over. This adds cycle time on the same machine and the same setup, which is the cheapest real improvement available.
• Rung three, Process. Only when rungs one and two are exhausted, add grinding, honing, or lapping. This means a new operation, a new setup, and often a new fixture.

Here is how that plays out. A customer needed 0.4 micrometers on one sealing face of a steel housing and asked us to fine mill the whole part. We milled everything to 3.2 micrometers, then added a short targeted grind on the sealing face alone. The part passed first article inspection on a calibrated profilometer and cost less than fine milling six faces. That kind of routing sits inside our CNC machining and surface finishing services.

For how finish sits alongside tolerance, material, and volume in the total price, read our guide on what it costs to get a metal part made. For what each number means once it reaches a drawing, see our breakdown of surface finish Ra values.