How much more does Ra 0.8 cost compared to Ra 3.2?

Expect roughly 30–60% more on the affected features, because achieving Ra 0.8 requires finer feeds, additional tool passes, and sometimes grinding or honing operations.

If the drawing does not specify Ra, what is the default?

Most CNC shops default to Ra 3.2 µm on machined surfaces, which is the finish left by sharp carbide tooling at standard feed rates.

Does anodizing change surface roughness?

Yes. Type II anodizing typically adds 5–15 µm of oxide layer, which can increase Ra by roughly 0.2–0.8 µm depending on alloy and anodize thickness.

Can visual comparators replace a profilometer?

For Ra 1.6 and above, visual comparator pads are adequate for production checks; for Ra 0.8 and below, a stylus profilometer gives objective numerical readings.

Surface Finish Ra Values for Machined Parts: A Practical Buyer's Chart

Ra 3.2 Is the Default and Usually Enough

Ra 3.2 µm is the standard as-machined finish on CNC mills and lathes using sharp tooling. Ra 1.6 is achievable with an extra finish pass and covers the majority of functional sealing and bearing surfaces. Below Ra 0.8, costs rise significantly because grinding, honing, or polishing operations are needed.

Ra Value Reference Chart

Ra (µm)	Surface Appearance	Typical Process	Cost Tier	Common Applications
12.5	Visible tool marks, rough	Heavy roughing cuts	Lowest	Non-contact surfaces, weld prep areas
6.3	Obvious tool marks, matte	Standard milling/turning	Low	Internal structure, hidden brackets
3.2	Slight tool marks, smooth matte	Standard CNC with sharp tools	Baseline	General machined surfaces, default finish
1.6	Minimal marks, satin feel	Finish pass at high feed/speed	+10–20%	Sealing surfaces, bearing bores, shafts
0.8	Very smooth, faint marks only	Grinding or fine finish pass	+30–60%	Dynamic seal seats, precision journals
0.4	Nearly polish, directional grain	Honing / lapping	+80–150%	Hydraulic spool bores, medical contact surfaces

Ra vs. Rz: Which One to Call Out

Ra is the arithmetic mean deviation of the surface profile; Rz is the average maximum peak-to-valley height over a sampling length. Ra is the standard callout on most drawings and is what surface roughness comparators and profilometers are calibrated to. Use Rz only if your sealing or fatigue application specifically requires peak-to-valley control.

Three Decisions Before You Specify Ra

Call out Ra on functional surfaces only — applying Ra 1.6 to every face of a part adds cost with no benefit
Do not default to Ra 0.8 across the drawing; start from Ra 3.2 and tighten only the faces that need it
Remember that anodizing, plating, and painting change the final Ra; specify pre-coat and post-coat roughness separately if it matters

How Ra Is Measured

Portable surface roughness testers (profilometers) drag a diamond stylus across the surface and compute Ra electronically. For Ra 3.2 and 1.6, visual comparator pads are sufficient for shop-floor checks. If your drawing specifies Ra below 0.8, request a profilometer trace with the inspection report.

TruPart Precision is a Dongguan-based ISO 9001 CNC shop that measures and documents surface roughness when specified on the drawing. Picking the right Ra value is one of the highest-leverage choices a buyer can make for both cost and part performance.

Frequently Asked Questions

How much more does Ra 0.8 cost compared to Ra 3.2?
Expect roughly 30–60% more on the affected features, because achieving Ra 0.8 requires finer feeds, additional tool passes, and sometimes grinding or honing operations.
If the drawing does not specify Ra, what is the default?
Most CNC shops default to Ra 3.2 µm on machined surfaces, which is the finish left by sharp carbide tooling at standard feed rates.
Does anodizing change surface roughness?
Yes. Type II anodizing typically adds 5–15 µm of oxide layer, which can increase Ra by roughly 0.2–0.8 µm depending on alloy and anodize thickness.
Can visual comparators replace a profilometer?
For Ra 1.6 and above, visual comparator pads are adequate for production checks; for Ra 0.8 and below, a stylus profilometer gives objective numerical readings.