A surface roughness chart compares roughness values in microinches (µin) and micrometers (µm). For the same Ra value, multiply µin by 0.0254 to obtain µm. Thus, 32 µin equals 0.8128 µm, and 125 µin equals 3.175 µm. Ra, Rq and Rz are different parameters, not interchangeable units.
Use the surface finish chart below to check units, then confirm the parameter, drawing standard and inspection conditions before specifying a CNC machined surface.
Surface Roughness Chart: Ra in Microinches and Micrometers
This table converts Ra between units. It does not convert Ra into Rq, Rz or another roughness parameter. Lower Ra means smaller average profile height deviations under the same evaluation conditions.
| Ra (µin) | Ra (µm), exact conversion |
|---|---|
| 1 | 0.0254 |
| 2 | 0.0508 |
| 4 | 0.1016 |
| 8 | 0.2032 |
| 16 | 0.4064 |
| 32 | 0.8128 |
| 63 | 1.6002 |
| 125 | 3.175 |
| 250 | 6.35 |
| 500 | 12.7 |
| 1,000 | 25.4 |
| 2,000 | 50.8 |
The conversion works in either direction:
- Ra in µm = Ra in µin × 0.0254
- Ra in µin = Ra in µm ÷ 0.0254
These are numerical conversions, not statements about instrument resolution or measurement uncertainty. The table also does not assign a guaranteed finish to milling, turning, grinding or polishing.
What Do 32 and 125 Finishes Mean?
What does “surface finish 32” mean?
Under a drawing convention that specifies Ra in microinches, surface finish 32 means Ra 32 µin, equivalent to 0.8128 µm. It is often paired with the rounded metric value Ra 0.8 µm. Confirm the drawing convention before interpreting a bare “32.”
The number alone does not identify whether the requirement is an upper limit, a range or another specified condition. Check the title block, general notes and local symbol together.
What does “125 machine finish” mean?
The phrase 125 machine finish commonly refers to Ra 125 µin, equivalent to 3.175 µm and often shown as approximately Ra 3.2 µm. It describes a roughness level under that convention; it does not specify one machining process or guarantee a scratch-free appearance.
Before comparing two quotes, check that both suppliers have interpreted the parameter, units and controlling limit in the same way.
Exact conversions and rounded values can set different limits
Rounded pairs are useful for comparison, but they can change an acceptance boundary. Ra 32 µin maximum and Ra 0.8 µm maximum are not numerically identical requirements.
For a calculated illustration, 0.810 µm is below 0.8128 µm but above 0.8000 µm. Under a specified maximum-limit rule, that difference can change the numerical comparison. This illustration excludes measurement uncertainty and decision-rule effects; actual acceptance must follow the agreed inspection requirements.
Keep the controlling unit and limit on the drawing. If a secondary-unit value is supplied for reference, identify it accordingly rather than creating two conflicting limits.
What Do Ra, Rq and Rz Describe?
Ra is the arithmetic mean of the absolute profile height deviations from the mean line over the evaluation length. It provides an average roughness value, so surfaces with different peak and valley shapes can share the same Ra.
Other parameters describe different aspects of machined surface roughness:
| Parameter | What it describes |
|---|---|
| Ra | Arithmetic mean of absolute profile height deviations. |
| Rq, or RMS roughness | Square root of the mean of squared profile height deviations. |
| Rz | A peak-to-valley height parameter whose precise definition depends on the governing standard. |
| Rt | Total height from the highest peak to the deepest valley over the evaluation length. |
There is no universal exact multiplier that converts Ra into Rq or Rz. A fixed-ratio comparison may be an estimate for particular surface profiles, but it cannot establish drawing compliance. Measure or calculate the specified parameter from suitable profile data.
Roughness also differs from waviness and lay. Roughness concerns finer profile variations; waviness describes broader variations separated through the evaluation method. Lay is the predominant direction of the surface pattern. A Ra value alone does not specify flatness, waviness, machining-mark direction or individual defect limits.
For sealing interfaces, select texture requirements alongside the seal design and geometry. A general chart cannot establish a universal seal finish. See the related discussion of CNC machined valve sealing surfaces for application context.
How to Read Surface Texture Symbols
A surface roughness chart provides numerical comparisons; surface finish symbols connect a texture requirement to a particular surface on the drawing. Read the graphic together with its parameter, value, units, referenced standard and any supplementary notes.
Start by identifying the standard and revision. ISO 21920-1:2021 replaced ISO 1302:2002 for ISO profile texture indications. ASME Y14.36-2018 (R2024) addresses surface texture symbols within the ASME system. Existing drawings may retain requirements issued under earlier standards.
Recognizing legacy ISO 1302 forms
The following descriptions apply to familiar legacy ISO 1302 forms. They are a recognition aid, not a complete symbol-placement guide for every standard.
| Legacy symbol feature | Meaning |
|---|---|
| Basic two-leg symbol | Does not, by itself, require or prohibit material removal. |
| Bar added across the basic symbol | Material removal by machining is required. |
| Circle added within the basic symbol | Material removal is not permitted. |
The circle in the last row must not be confused with a separate all-around indication. Its position within the symbol matters.
When reading symbols for surface roughness, avoid assigning meaning from a detached icon. The local requirement may override a general drawing note, and supplementary indications can control lay or other conditions.
For new drawings, create roughness symbols using the applicable standard and a verified CAD symbol library. Do not combine legacy ISO symbol placement with current ISO or ASME defaults without checking the complete notation.
How to Write a Useful Surface Texture Requirement
A surface finish callout should identify the required texture and where it applies. Start with the surface function, choose the relevant parameter and limit, and state the governing standard. Add specific lay, process or inspection requirements when the application needs them.
A practical order is:
- Identify the controlled area. Distinguish a sealing land, bore or sliding face from nearby nonfunctional surfaces.
- Specify the parameter and limit. Preserve units and distinguish an upper limit from a permitted range.
- Reference the standard and revision. Use its notation and applicable defaults consistently.
- Define additional functional controls. Address lay, waviness or defects separately when Ra cannot describe the requirement.
- Identify the surface condition for acceptance. State whether inspection applies before or after a specified treatment.
For example, an engineering handoff might record: “Surface A; Ra maximum 0.8 µm; inspect before anodizing; governing standard and inspection plan as identified on the controlled drawing.” This is an illustrative requirement record, not a complete graphical callout or a recommended finish for every part.
For anodized components, the acceptance stage belongs in the surface texture callout or associated specification. Review how anodizing affects 6061 aluminum part fits when coordinating texture requirements with dimensional tolerances.
Need a quote for parts with controlled surface texture?
Submit your CNC machining requirements with the model, controlled drawing, material, quantity and identified critical surfaces. Include the required finish and acceptance stage so they can be considered in the quotation.
How to Measure and Verify Surface Roughness
A stylus profilometer records a profile as its tip traverses the surface. For a directional machining pattern, the trace is generally taken across the lay to capture the height variations, unless the applicable specification requires another direction. The stylus moves along the surface; “perpendicular to the lay” describes the trace direction.
The measured profile must then be evaluated using the required parameter and settings. A displayed Ra number without those conditions is insufficient for a reliable supplier-to-customer comparison.
Agree on the inspection conditions
Use the invoked standard’s defaults where applicable and record the resulting setup. Agree on departures or additional requirements before manufacturing.
| Inspection item | What to establish |
|---|---|
| Requirement | Parameter, controlling units, limit and standard revision. |
| Profile processing | Applicable filters and nesting indices or cutoff settings. |
| Evaluation length | Length evaluated and any required section settings. |
| Trace location and direction | Controlled surface, measurement path and orientation to lay. |
| Instrument setup | Measurement method and relevant stylus geometry or acquisition settings. |
| Part condition | Required treatment stage and a clean measurement area. |
| Acceptance | Sampling, decision rule and treatment of measurement uncertainty. |
ISO 21920 separates drawing indications, parameter definitions and specification operators across its three parts. Its defaults should not be silently substituted for those governing an older drawing. A supplier’s normal instrument preset may therefore need review against the actual requirement.
If a short land or small bore prevents the intended trace, resolve the measurement method and acceptance plan before production. Do not replace the specified evaluation length with a convenient setting and assume the results remain equivalent.
Use the Chart to Specify a Machinable, Verifiable Finish
Select the finish from the functional requirement, then assess the manufacturing route. Material, feature geometry, tool access, machining strategy and any secondary finishing operation affect what is practical. A lower Ra number alone does not select the process or prove that a component will perform better.
For custom CNC machined parts, identify the surfaces that need individual texture control and give the remaining surfaces a suitable general requirement. Let the quotation identify any additional finishing or inspection scope needed for the controlled features.
Use this surface roughness chart for unit lookup, then carry the selected parameter, governing limit and inspection conditions into the drawing. For a project inquiry, prepare the CAD model and drawing for quotation and request a CNC machining quote with material, quantity, critical finishes and required inspection documentation.
