Guides / Process

Surface Finish in 5-Axis Machining: What Ra Numbers Mean for Your Part

September 20, 2026

Ra is the number engineers write on a drawing to describe how smooth a surface needs to be, and it is one of the easiest callouts to overspecify by habit. Knowing what a 5-axis machine actually achieves without extra steps helps you write a realistic number instead of guessing.

What Ra actually measures

Ra, arithmetic average roughness, describes the average height of the microscopic peaks and valleys left on a machined surface, usually in micrometers or microinches. It is not the same as flatness or waviness, which describe larger scale deviation across the surface. A part can have excellent flatness and still carry visible tool marks at the Ra scale, and the reverse is also possible.

What a 5-axis machine achieves as-machined

Operation	Typical Ra (as machined)
Roughing pass	6.3 to 12.5 micrometers
Standard finishing pass	1.6 to 3.2 micrometers
Fine finishing pass (light stepover, sharp tool)	0.8 to 1.6 micrometers
Ground or polished surface (secondary operation)	0.1 to 0.8 micrometers

For most functional metal parts, a standard finishing pass at Ra 1.6 to 3.2 is enough. Anything finer usually means a secondary process outside the machining step itself.

Simultaneous 5-axis and surface finish on curved parts

On a flat face, 3+2 positioning and simultaneous 5-axis produce the same finish, since the tool sits at a fixed, favorable angle either way. On a curved surface, simultaneous motion keeps the tool at a consistent angle to the surface as it moves, which reduces the stepover marks that a fixed-angle toolpath leaves on a changing curvature. This is why blades, impellers and blended ports benefit from simultaneous machining for finish, not just for reaching the geometry.

When you need a secondary finishing step

If a drawing calls for Ra finer than about 0.8 micrometers, or a fully polished cosmetic or optical surface, plan on a step after machining. Common options:

• Manual or bead polishing for a mirror or near-mirror finish on a specific surface
• Bead or sand blasting for a uniform matte texture, often before anodizing
• Vibratory tumbling for deburring and a light overall finish on smaller parts
• Grinding or lapping when a flat surface needs both tight flatness and a fine Ra together

Each of these adds a step outside the machining cycle, with its own lead time and cost.

Common finish callouts and what they mean

Drawings sometimes carry Ra in micrometers (µm) and sometimes in microinches (µin), and the two are easy to confuse. Ra 32 microinches is roughly Ra 0.8 micrometers, and Ra 125 microinches is roughly Ra 3.2 micrometers. Always state the unit on the drawing rather than a bare number, since a shop guessing wrong between µm and µin will either overbuild or underbuild the finish.

Where finish actually matters, and where it doesn’t

Surface finish earns its cost on sealing faces, bearing surfaces, surfaces in sliding contact, and cosmetic or optical parts where appearance is functional. It rarely matters on internal, non-mating surfaces, mounting faces hidden inside an assembly, or features that will be painted or coated afterward regardless of the as-machined texture. Calling out a tight Ra everywhere on a drawing, rather than only where it matters, is one of the more common ways buyers accidentally raise their own quote.

What to put on your RFQ

State the Ra value and unit only on the surfaces where it matters, and leave the rest to the shop’s standard finish. If you need a secondary process like polishing or bead blasting, say so explicitly rather than assuming a tight Ra callout will get you there, since some finishes require a distinct operation regardless of how the machining pass is specified.

The short version

A 5-axis machine reaches Ra 1.6 to 3.2 micrometers as standard finishing work, and simultaneous motion helps mainly on curved surfaces, not flat ones. Anything finer needs a secondary step. Specify Ra only where the surface’s function needs it, and the quote will reflect the part rather than a blanket assumption.

For visible marks that Ra does not describe, see witness lines and mismatch on 5-axis parts and our guide to 5 axis machining quality control.

Frequently asked questions

Keep reading

• Thermal Growth on 5-Axis Machines and How Shops Control It
• Collision Checking and Machine Simulation Before a 5-Axis Run