What Is Surface Roughness Ra and How Should You Specify It for CNC Machined Parts?

Surface roughness Ra appears on many CNC machining drawings. It is also one of the finish notes that gets copied too easily from old prints. If you buy milled housings, turned shafts, valve blocks, brackets, or custom metal parts, the Ra number can affect machining time, inspection work, and how the part performs after assembly. For a wider view of machining routes, you can also review related manufacturing processes.

The notes below refer to public information from ISO 21920-2:2021, ASME B46.1-2019, NIST surface metrology materials, Sandvik Coromant technical guidance, and Kennametal machining calculators. These references do not give one fixed Ra value for every CNC part. They point to a more practical way of working: call out the finish where the surface has a real function, and leave non-critical faces with a reasonable general finish.

What Is Surface Roughness Ra and Why Does It Matter?

Ra is one number used to describe the average height change in a measured surface profile. It helps designers, machinists, inspectors, and buyers talk about surface finish with the same basic term. It is not a full description of the surface, though. Two parts can have the same Ra value and still act differently in sealing, oil holding, wear, or visual appearance.

Ra as an Average Height Value

ISO 21920-2:2021 defines profile surface texture terms and parameters, including the arithmetic mean deviation of a profile. In normal shop language, Ra is the average of the peaks and valleys measured from a mean line over a set evaluation length.

A lower Ra number means the measured profile is smoother. For example, Ra 3.2 µm is rougher than Ra 0.8 µm, even when both surfaces may look acceptable during a quick visual check.

Roughness, Waviness, and Lay Are Different

ASME B46.1-2019 covers surface roughness, waviness, and lay. NIST explains the difference in a practical way: roughness is the close surface pattern usually left by cutting tools or grinding grit, while waviness has wider spacing and may come from vibration, chatter, or machine movement.

Lay is the main direction of the surface marks. A turned shaft, a face-milled plate, and a ground rail may all meet a similar Ra value, but the tool marks can run in very different directions.

Ra Cannot Tell the Whole Surface Story

Ra can hide sharp peaks because it averages the profile. That is acceptable for many general machined parts, but it can miss trouble on sealing faces, sliding guides, and fatigue-sensitive edges.

If a surface needs to hold oil, seal gas, or resist fretting, Ra alone may not be enough. In that case, Rz, Rq, bearing ratio, lay direction, or a process note may be needed. It may look like a small drawing detail, but one leaking seal can cost much more than the extra note.

How Is Ra Measured on Machined Parts?

Ra measurement depends on the instrument, cutoff, filter, inspection direction, and measurement position. If a drawing only says Ra 0.8 with no method or location, the buyer and supplier may not measure the same thing. The part is not always wrong; the inspection setup may simply be different.

Stylus Profilometers for Shop Inspection

A contact stylus profilometer is still widely used in machining inspection. The stylus moves across the surface and records vertical movement along a short trace.

The instrument then filters the profile and calculates Ra. For a flat milled face, this is usually straightforward. For a small bore, narrow groove, curved blade, or delicate coating, access, stylus tip size, and measurement direction can change the result.

Cutoff Length and Filter Settings

Cutoff settings separate roughness from waviness. If the cutoff is too short, the reading may miss real tool marks. If it is too long, waviness may be included in the roughness result.

ISO and ASME standards give rules for filtering and evaluation, but many drawings leave these details out. For a critical surface, write the required standard and inspection direction instead of only writing Ra 0.8.

Units in Micrometers and Microinches

Most metric drawings use micrometers, shown as µm. Many U.S. drawings use microinches, shown as µin. The conversion is simple: 1 µm equals about 39.37 µin.

Common equivalents are useful during drawing review and quoting:

• Ra 3.2 µm is about 126 µin, often rounded to 125 µin.
• Ra 1.6 µm is about 63 µin.
• Ra 0.8 µm is about 32 µin.
• Ra 0.4 µm is about 16 µin.

Always check the unit before release. Confusing 32 µin with 32 µm is not a minor typing error; it changes the finish class completely.

Which Ra Values Fit Common CNC Processes?

No public ISO, ASME, or NIST document gives a universal CNC price table by Ra value. Reliable public cost multipliers are also not available for every material, geometry, and machine shop. That needs to be said plainly. Even so, process capability follows patterns seen in normal production. A standard machined finish is not the same as a fine turned finish, and both are different from grinding, honing, or polishing.

Ra 3.2 µm for General Machined Faces

Ra 3.2 µm is a common callout for general as-machined surfaces, such as mounting faces, non-sealing covers, brackets, and structural blocks. It is usually achievable on many milled and turned parts when the machine is stable and the cutter is sharp.

It should not be used as a blanket rule without looking at the function. A visible chatter mark can still cause a problem, even if one average Ra reading passes in one measured area.

Ra 1.6 µm for Better Bearing or Mounting Contact

Ra 1.6 µm is often used for better contact faces, bearing shoulders, improved turned diameters, and cleaner milled surfaces. Sandvik Coromant public guidance notes that surface finish is directly related to feed rate and nose radius in turning.

In one published table for achieving about Ra 1.6 µm with Rmax 8.0 and N7 finish, larger nose radii allow higher maximum feed. That is a simple reminder that tool geometry, cycle time, and finish are tied together.

Ra 0.8 µm and Finer for Sealing or Sliding Areas

Ra 0.8 µm, Ra 0.4 µm, or lower may be needed for hydraulic sealing lands, valve seats, sliding contact, precision shafts, and cosmetic stainless parts. These finishes often need a slow finishing pass, sharp inserts, stable clamping, grinding, honing, lapping, or polishing.

If only one narrow seal band needs Ra 0.8 µm, call out that band only. Do not make the whole block meet the same finish unless every face really needs it.

How Does Ra Affect Cost, Function, and Lead Time?

A smoother surface is not automatically better. It can reduce friction, help sealing, and improve appearance, but it can also remove useful oil pockets or add work that brings no benefit. The right Ra value is the one that matches the surface function, material, and inspection plan.

Lower Feed Rates and Extra Finish Passes

Turning theory shows why smoother finishes often take more time. Kennametal provides a public surface finish calculator based on theoretical planning values and notes that actual results can vary.

In real production, reducing feed, changing the insert radius, adding a spring pass, or replacing a worn tool can improve Ra. Each choice can also slow the cycle, add inspection steps, or increase operator attention. See also: Machines.

Process Choice from Milling to Grinding

Milling and turning can cover many standard surface finish requirements. Grinding is often used when tighter size control and a more consistent texture are needed.

Honing may be a better fit for bores. Lapping or polishing can reach very fine surfaces, but they add handling and may affect flatness or edge condition if the process is not controlled. A small stainless valve stem and a large aluminum housing should not be judged by the same process expectation.

Inspection Time and Rework Risk

Tight Ra callouts need inspection access and repeatable measurement. A deep groove may need a special stylus, and a large plate may need readings at several locations.

If the drawing does not say where to measure, the supplier may check the easiest area while the buyer checks the hardest area. This is how a finished order turns into a long email chain. Clear notes usually cost less than sorting and rework.

How Should You Specify Ra on a Drawing?

A useful Ra callout is specific, measurable, and linked to function. It tells the shop which surface matters and lets the rest of the part stay economical. This matters even more in export manufacturing, where the drawing may pass through purchasing, engineering, machining, finishing, and inspection teams in different countries.

Call Out Only Functional Surfaces

Start by asking what the surface does. Does it seal, slide, locate another part, carry a bearing, or need paint or anodizing? If a side wall is only for clearance, a general machined finish may be enough.

If a groove supports an O-ring, Ra and lay direction may matter. A practical drawing might use Ra 0.8 µm on the seal land and Ra 3.2 µm on non-contact outer faces.

Add Direction, Area, and Acceptance Rules

Mark the exact area that needs the finish. Add lay direction if sliding or sealing depends on the tool marks, and state whether the Ra value is a maximum value.

If several readings are required, say how many and where. For a round shaft, inspection along the axis can read differently from inspection around the circumference. This small point is easy to miss until the parts are already delivered.

Match ISO or ASME Language

Use one drawing language instead of mixing old and new symbols copied from different files. ISO 21920-2:2021 replaced older ISO 4287 terminology for profile surface texture.

ASME B46.1-2019 remains a key U.S. reference for roughness, waviness, and lay. If the customer drawing follows ASME, keep that system. If it follows ISO, keep ISO notation and units consistent.

What Mistakes Should You Avoid When Buying CNC Parts?

Most Ra problems come from over-specifying, under-specifying, or using Ra as if it were only a cosmetic grade. A machined surface is not just a number on a print. It is the result of material, tool, machine rigidity, coolant, feed, speed, and finishing method.

Using One Ra for Every Face

A single all-over Ra 0.8 µm callout can make a simple part harder to quote and slower to build. It may force the supplier to finish faces that nobody touches after assembly.

Use a general finish for the part, then add tighter Ra only where needed. This keeps the drawing cleaner and makes the quote closer to the real work.

Mixing Ra, Rz, and Polished Appearance

Ra is not the same as Rz, and neither one guarantees a mirror look. A polished part may look bright but still fail a sealing roughness requirement if scratches remain.

A ground surface may look matte but seal well. If appearance matters, add a visual requirement or sample approval. If peak height matters, consider Rz together with Ra.

Ignoring Material and Coating Effects

Aluminum, stainless steel, brass, titanium, and hardened steel do not cut the same way. Coatings can also change the final surface after machining.

Anodizing, plating, bead blasting, and passivation may affect texture or appearance. If the final surface is measured after finishing, state that on the drawing. Otherwise, the supplier may measure before coating while the buyer checks after coating.

FAQ

Q1: What Does Surface Roughness Ra Mean? A: Ra means the arithmetic average deviation of a measured surface profile from its mean line. It gives one roughness value, usually in µm or µin, but it does not describe every peak, valley, or lay direction.

Q2: Is a Lower Ra Always Better? A: No. A lower Ra can help sealing, sliding, or appearance, but it can also raise cost and remove useful surface texture. Choose a lower Ra only when the surface function needs it.

Q3: What Is a Common Ra for CNC Machined Parts? A: Ra 3.2 µm is often used for general machined surfaces. Ra 1.6 µm or Ra 0.8 µm may be used for more demanding contact, sealing, or sliding surfaces. Actual capability depends on material, geometry, tooling, and machine condition.

Q4: Should You Use Ra or Rz on a Drawing? A: Use Ra for common average roughness control. Add Rz when peak-to-valley height matters, such as sealing faces, fatigue-sensitive surfaces, or sliding contact. Critical parts may need both.

Q5: How Can You Avoid Paying Too Much for Surface Finish? A: Do not apply tight Ra values to every surface. Mark only the functional areas, define units and inspection direction, and match the callout to ISO or ASME rules. Clear notes help the supplier quote the real work.