Surface finish table for Ra, Rz and machining processes
What a surface finish table is really telling you
A surface finish table is a quick reference for comparing roughness values, units and likely manufacturing processes. It is useful when a drawing calls out Ra 1.6 μm, 63 μin, N7 or a similar requirement, and the designer, buyer or machinist needs to understand what that means on the shop floor. The table should not be treated as a universal conversion chart for every roughness parameter. Ra, Rz and Rq describe different features of a measured profile, and the reported result depends on the measurement method, cutoff, filter, direction and actual process. Used correctly, the table helps teams choose a finish that is functional, measurable and economically realistic for surface finishing work.
The common error is to read a finish table as a guarantee. A turned, milled, ground or honed surface may fall within a typical range, but material, tool condition, machine rigidity, coolant, feed, speed and inspection setup can all move the result. For controlled parts, the drawing and inspection plan need to define the parameter and acceptance method, not only a roughness number.
Key roughness terms before using the table
Surface texture is usually separated into roughness, waviness and lay. ASME B46.1 describes surface texture as including these constituents and provides terminology for specifying surface parameters. ISO 21920-1:2021 covers the indication of profile-based surface texture in technical product documentation. These standards matter because shop shorthand can be ambiguous across regions, industries and drawing systems.
Ra
Ra is the arithmetic mean roughness of a profile after the specified filtering and evaluation rules are applied. It is widely used because it gives one stable average value, but it does not show whether the surface has sharp peaks, deep valleys or a directional pattern. Two surfaces can have the same Ra and behave differently in sealing, sliding, fatigue or coating adhesion.
Rz
Rz is a height-based profile parameter related to peak-to-valley behavior. It is often more sensitive than Ra to isolated peaks and valleys. Because Rz is not simply another unit for Ra, a drawing should not replace one with the other unless the customer, supplier and inspection method have agreed on the relationship for that surface and process.
Rq or RMS
Rq, often called RMS roughness, is the root mean square value of profile deviations from the mean line. NIST descriptions of profile roughness define Rq from the squared deviations of the measured profile. Rq gives greater weight to larger deviations than Ra, so it is not identical to Ra even when both are measured on the same trace.
N grades
N grades are older roughness grade numbers that still appear in shop charts and legacy drawings. They are convenient shorthand, but modern drawings are clearer when they state the actual parameter, value, units and standard reference.
Surface finish table for common Ra values
The table below gives common Ra values in micrometres and microinches, with the roughness grade commonly associated with each value. The Rz column is a planning band only, based on the often-used assumption that Rz may be several times Ra on many machined surfaces. It is not a standard conversion and should not be used for final acceptance.
| Ra μm | Ra μin | Common N grade | Planning-level Rz band μm | Typical interpretation |
|---|---|---|---|---|
| 50 | 2000 | N12 | 200-350 | Very rough primary surface or heavy stock condition |
| 25 | 1000 | N11 | 100-175 | Rough casting, forging or coarse cut surface |
| 12.5 | 500 | N10 | 50-88 | Coarse machined or saw-cut surface |
| 6.3 | 250 | N9 | 25-44 | Rough turning or rough milling range |
| 3.2 | 125 | N8 | 13-22 | General machined finish for non-critical faces |
| 1.6 | 63 | N7 | 6.4-11 | Good machined finish for many mating surfaces |
| 0.8 | 32 | N6 | 3.2-5.6 | Fine machining or light finishing operation |
| 0.4 | 16 | N5 | 1.6-2.8 | Fine ground, honed or carefully finished surface |
| 0.2 | 8 | N4 | 0.8-1.4 | Precision ground, honed or polished surface |
| 0.1 | 4 | N3 | 0.4-0.7 | Very fine lapped or superfinished surface |
| 0.05 | 2 | N2 | 0.2-0.35 | Special precision finish |
| 0.025 | 1 | N1 | 0.1-0.18 | Extremely fine finish for specialized uses |
The unit conversion is straightforward: 1 μm is approximately 39.37 μin. The harder decision is whether the number fits the function. A clearance surface usually does not need the same finish as a dynamic seal, bearing journal, gauge face or optical component.
Typical surface finish by manufacturing process
Process-based ranges are most useful during estimating and design-for-manufacturing review. They help show when a requirement may push the part into a secondary operation. The ranges below are intentionally broad because actual results depend on material, tool geometry, feed, machine condition, workholding and inspection setup.
| Process or operation | Typical Ra range μm | Typical Ra range μin | Design note |
|---|---|---|---|
| Sand casting or rough forging | 12.5-50 | 500-2000 | Functional surfaces usually need machining if tighter texture is required |
| Sawn, flame-cut or rough cut edge | 6.3-25 | 250-1000 | Often visible and irregular; not ideal for sealing or precision contact |
| Rough turning or rough milling | 3.2-12.5 | 125-500 | Suitable for stock removal and many non-critical faces |
| Finish turning or finish milling | 0.8-3.2 | 32-125 | Common economical range for many CNC machined surfaces |
| Reaming or broaching | 0.4-1.6 | 16-63 | Useful for controlled holes and repeated features when tooling is stable |
| Grinding | 0.1-1.6 | 4-63 | Often selected for tighter texture, geometry control or hardened materials |
| Honing | 0.05-0.4 | 2-16 | Common for bores, crosshatch patterns and controlled bearing or sealing surfaces |
| Lapping or superfinishing | 0.012-0.2 | 0.5-8 | Used where very fine texture and form control justify added cost |
| Polishing or electropolishing | 0.05-0.8 | 2-32 | Result depends strongly on starting surface and process control |
A practical way to use these ranges is to watch the process boundaries. Moving from Ra 3.2 μm to Ra 1.6 μm may be possible with improved cutting conditions. Moving from Ra 1.6 μm to Ra 0.4 μm may require grinding, honing or another secondary operation. Moving below Ra 0.1 μm is a specialized finishing problem, not simply a tighter note on the drawing.
How to choose the right finish for a drawing
Start with function, not appearance. If the surface is a non-contact exterior face, a coarse machined finish may be enough. If it locates another component, a more controlled machined surface may be needed. If it slides, seals, carries lubricant or sees cyclic stress, the correct value depends on contact pressure, material pair, lubricant, coating, heat treatment and operating environment.
- Non-critical clearance faces: Ra 3.2-12.5 μm is often adequate, provided burrs and sharp edges are controlled separately.
- General machined mating faces: Ra 1.6-3.2 μm is a common design range, but flatness and dimensional tolerance may matter more than roughness alone.
- Sliding or sealing surfaces: Ra 0.2-0.8 μm may be considered, but valley structure, lay direction and lubrication behavior can be as important as the average value.
- Precision gauge, hydraulic or bearing-related surfaces: Ra 0.1-0.4 μm may be appropriate only when the process and inspection plan can reliably support it.
- Decorative or corrosion-related surfaces: appearance and cleanability may require polishing or chemical finishing, but Ra alone does not fully describe gloss, color or passivation quality.
Do not use the table to hide uncertainty. If the finish is critical, state the actual requirement and review it with manufacturing and quality teams before release. A slightly looser but measurable requirement is often better than a fine finish that adds cost without improving performance.
How to write surface finish callouts clearly
A clear callout names the parameter, value, limit condition and units. For example, Ra 1.6 μm maximum is clearer than a note that only says 63 finish, because 63 can be read differently if the unit system or parameter is not understood. If the drawing uses ISO symbols, keep the notation consistent with ISO 21920 practice. If it uses an ASME system, reference the applicable ASME standard and company drafting rules. See also: CNC Machining.
For critical surfaces, consider adding the following information:
- Parameter, such as Ra, Rz or Rq.
- Units, such as μm or μin.
- Maximum, minimum or range requirement.
- Measurement direction, especially when lay affects function.
- Evaluation length, cutoff or filter when required by the specification.
- Area to be measured and any excluded edges, holes, blends or defects.
- Required lay direction when sealing, sliding or visual grain matters.
- Inspection method if contact stylus, optical measurement or a customer-specific method is required.
It is also useful to avoid mixing unrelated goals in one surface note. Roughness does not control flatness, waviness, burr condition, dents, porosity, cracks, coating thickness or cleanliness unless those requirements are separately specified.
Measurement limits that can change the result
Surface finish values are measurement results, not properties that exist independently of the measurement method. A contact stylus traces a line across the surface. Optical instruments can capture areal data, but they may respond differently to reflectivity, steep slopes, transparent coatings or contamination. Profile parameters such as Ra and Rz should not be casually compared with areal parameters such as Sa or Sz.
Direction also matters. Measuring across turning marks can give a different result than measuring along them. Filters and cutoff lengths separate roughness from waviness, so changing the cutoff can change the reported value. Reference specimens and calibrated instruments help, but they do not replace a well-defined inspection plan.
For production control, measure representative areas, avoid burrs unless burr condition is part of the requirement, and record the setup used. If a supplier and customer disagree over a reading, the cause is often not the arithmetic but a difference in instrument, cutoff, trace direction or surface location.
Frequently asked questions
Is Ra the same as RMS roughness?
No. Ra is an arithmetic average of absolute profile deviations, while RMS roughness usually refers to Rq, the root mean square of profile deviations. Rq gives more weight to larger deviations, so the values are related but not identical.
Can I convert Ra to Rz?
Only as a rough planning estimate. Many shop charts use approximate ratios, but the actual relationship depends on the surface profile and manufacturing process. For acceptance inspection, specify and measure the required parameter directly.
What does N6 surface finish mean?
N6 is commonly associated with Ra 0.8 μm, or about 32 μin. Because N grades are shorthand, a modern drawing is clearer when it states Ra 0.8 μm maximum, or the exact parameter and limit required.
What surface finish can CNC milling achieve?
Many finish-milled surfaces fall around Ra 0.8-3.2 μm under suitable conditions, but this is not guaranteed. Tool geometry, feed per tooth, machine rigidity, material, coolant and tool wear can shift the actual result.
Why does a tighter finish increase cost?
A tighter finish can reduce feed rate, require sharper tooling, add inspection time or force a secondary process such as grinding, honing, lapping or polishing. The cost increase is largest when the requirement crosses from normal machining into a separate finishing operation.