This article is part of the CNC Surface Finishing Guide: Anodizing, Plating, Coatings & Ra on CNX Precision.
Surface roughness controls how a machined part looks, feels, and functions. For engineers, CNC surface roughness is a specification on almost every drawing. Get it wrong, and parts fail early or cost too much. Get it right, and your parts meet their design intent. This guide explains Ra values in plain language. You will also learn how to specify finishes correctly on your drawings.
CNC Surface Roughness: Ra vs Rz Explained
Every machined surface has microscopic peaks and valleys. Cutting tools leave these marks during machining. Roughness describes the size of those deviations. It is different from waviness and form, which cover larger shapes. In short, roughness is the fine texture of the surface.
Ra is the arithmetic average of the profile deviations. Imagine a center line through the surface profile. Ra is the average distance between the profile and that line. Therefore, Ra gives a simple, repeatable number. Most drawings around the world use Ra.
Rz is another common parameter. It averages the five highest peaks and the five lowest valleys. Rz is more sensitive to single defects. For example, a scratch that barely affects Ra can push Rz higher. Use Rz for sealing surfaces and highly loaded joints. In contrast, Rq gives a root mean square average that stresses outliers even more.
Measurement settings also matter. The cut-off length filters out waviness and defines the sampling window. A longer cut-off averages more surface, which can change the Ra number. Therefore, compare reports only when the settings match. In practice, most shops use a standard cut-off such as 0.8 mm. Ask for the setting when you review a report.
| Parameter | Definition | Typical Use |
|---|---|---|
| Ra | Arithmetic average deviation | General specification |
| Rz | Average of five peak-to-valley heights | Sealing surfaces |
| Rq | Root mean square average | Research and analysis |
| Rmax | Single highest peak-to-valley height | Critical optical parts |
When you specify CNC surface roughness, Ra is the safest starting point. It is well understood by every machine shop. If the function demands more, add Rz as a secondary limit. Most importantly, do not mix units. State micrometers or microinches, but never both without a note.
Common Ra Values and Their Applications
Most CNC surface roughness specs fall between 0.4 and 3.2 µm. Each range suits different functions. The table below shows common CNC surface roughness values and their typical uses.
| Ra Value | Typical Finish | Common Applications |
|---|---|---|
| 0.4 µm | Ground or fine machined | Bearing journals, seals |
| 0.8 µm | Fine machining | Precision shafts, hydraulic parts |
| 1.6 µm | Standard machining | General mechanical parts |
| 3.2 µm | Standard milling and turning | Non-critical surfaces |
Ra 1.6 is the workhorse value. Most standard CNC parts are quoted at this level. Ra 0.8 is common when parts mate or slide together. Ra 0.4 requires grinding or special finishing, which adds cost. In contrast, Ra 3.2 is fine for hidden or cosmetic surfaces. Remember that tighter roughness means longer cycle times. Therefore, specify the loosest value your function allows.
Roughness also affects cost and lead time. A finish of Ra 0.4 may need grinding, lapping, or polishing. Those steps add operations and inspection. For example, an extra finishing operation can add days to delivery. In addition, roughness interacts with coatings. A smooth surface accepts anodizing more evenly. Therefore, think about finishing early in the quoting stage.
The direction of lay is part of the spec as well. Machining marks usually run in one direction. For sealing surfaces, marks should run in a controlled pattern. Your drawing can state the lay direction with the roughness symbol. For example, write circular, perpendicular, or parallel to the surface. A clear callout prevents arguments at inspection.
How to Specify and Measure Surface Roughness
The drawing symbol for CNC surface roughness looks like a check mark. The Ra value sits above the symbol, in micrometers. For example, write Ra 1.6 for a standard finish. You can also add a machining allowance if you plan secondary operations.
Rules for Realistic Roughness Callouts
- Only specify what the function needs.
- Use standard values such as 0.4, 0.8, 1.6, or 3.2 µm.
- Add roughness to critical surfaces only.
- Do not call out roughness on internal threads.
- Note the direction of lay when direction matters.
Many engineers over-specify roughness by habit. A shiny finish on a hidden face adds cost without value. In addition, unrealistic values cause rework loops. For instance, a designer asks for Ra 0.2 on a milled face. The machinist must grind it, which changes the geometry. Therefore, match the finish to the function. Ask your shop for feedback before you finalize the drawing.
How Roughness Is Measured
Measuring CNC surface roughness requires a profilometer. A stylus travels across the surface and records the profile. Optical instruments do the same without contact. In either case, take several measurements at different locations. Roughness varies across a part, especially on curved features. In addition, the direction of tool marks affects the reading. Therefore, measure along the direction that matches your drawing callout. Ask the shop to include the evaluation length in the report.
Process Variables That Control Roughness
Process choices control the final surface. Feed rate, spindle speed, tool nose radius, and material all matter. For example, a larger nose radius generally leaves a smoother surface. A finishing pass with light depth of cut helps as well. A skilled machinist balances these variables to hit your target. Ask your shop how it plans to achieve a specific Ra value. Then, check the inspection report when parts arrive.
Quality control closes the loop. Every batch should come with a roughness report. The report lists measured values, locations, and the instrument used. Check the numbers against your drawing callouts. If a value is out of spec, ask for the corrective action. This habit catches drift before it becomes a recall.
Frequently Asked Questions
What is the difference between Ra and Rz?
Ra is the average deviation of the whole profile. Rz averages the five highest peaks and five lowest valleys. Ra is easier to measure and more common. Rz catches isolated defects better. In practice, use Ra for general parts and Rz for seals. For high-precision work, request both values.
What is a good CNC surface roughness for machined parts?
For general parts, Ra 1.6 µm is standard and cost-effective. Use Ra 0.8 µm for sliding or sealing features. Use Ra 0.4 µm only when the function demands it, because it costs more. When in doubt, choose 1.6 µm and validate with a prototype.
How can I reduce surface roughness on CNC parts?
Increase spindle speed and reduce feed rate. Use a larger tool nose radius. Add a finishing pass with light depth of cut. Also, check tool wear and use proper coolant. Your machinist can tune all these variables. A test coupon can confirm the target before you run a batch.
Surface roughness is a language between you and the machine shop. Learn it, and you will order better parts for less money. Need help specifying CNC surface roughness? Send your drawing to CNX Precision. Our engineers will review the roughness callouts and suggest cost-effective options.
For reference, machining standards such as the ISO 2768 general tolerance standard define the quality and tolerance requirements we follow.
For related information, see our guide to cnc machining service and 5-axis cnc machining and cnc milling vs turning and cnc machining tolerances.