Compare a measured surface roughness value against a specified limit and determine whether the numerical result passes or fails.
| Item | Value | Interpretation |
|---|
A surface roughness acceptance check compares a measured surface-texture value with a specified requirement. The requirement may be expressed as a maximum permitted roughness or, in some applications, a minimum value.
For a typical maximum surface-finish requirement, a lower numerical roughness value is generally better than a higher value. The measurement must also use the same roughness parameter and unit as the specification.
A maximum limit means the measured value must be less than or equal to the specified value.
For example, if a drawing specifies Ra 1.6 µm maximum and the measured Ra is 1.2 µm, the numerical comparison passes because 1.2 is below 1.6.
A minimum requirement means the measured value must be greater than or equal to the specified lower limit.
Minimum roughness requirements are less typical for ordinary surface-finish specifications, but the calculator supports them for situations where a lower numerical boundary is explicitly required.
The acceptance margin shows how far the measurement is from the specified limit.
For a maximum requirement:
A positive margin indicates that the measured value is below the maximum limit. A zero margin means the measurement is exactly at the limit.
For a minimum requirement:
A positive margin indicates that the measurement is above the minimum requirement.
Ra, Rq and Rz are different surface-texture parameters. A measured Ra value should be compared with an Ra specification, not directly with an Rz or Rq specification.
Metric drawings commonly use micrometres (µm), while some specifications use microinches (µin). The physical value can be converted when the same roughness parameter is retained.
This tool expects the measured value and limit to be entered using the same unit, so the comparison does not accidentally mix µm and µin.
| Ra | Approx. µin | General Reference |
|---|---|---|
| 0.025 µm | 1 µin | Very fine finish |
| 0.05 µm | 2 µin | Very fine finish |
| 0.1 µm | 4 µin | Fine finish |
| 0.2 µm | 8 µin | Fine finish |
| 0.4 µm | 16 µin | Fine machined finish |
| 0.8 µm | 32 µin | Machined finish |
| 1.6 µm | 63 µin | Common machined finish |
| 3.2 µm | 126 µin | General machining range |
| 6.3 µm | 248 µin | Rougher machined finish |
| 12.5 µm | 492 µin | Coarse surface |
These are general Ra reference values and are not acceptance limits for a particular component.
A numerical comparison can quickly indicate whether a measurement is above or below a specified limit. It does not replace the actual inspection procedure.
The applicable drawing, specification or standard may define how many measurements are required, where measurements are taken, the measurement direction, cutoff length, filtering method and instrument requirements.
This tool performs a numerical acceptance comparison only. It does not certify a component, determine compliance with a particular manufacturing standard or validate the measurement procedure.
If a drawing, specification, customer requirement or applicable standard defines a specific surface-texture measurement procedure, that requirement takes precedence over this calculator.
How does this tool determine pass or fail? For a maximum limit, it passes when measured roughness is less than or equal to the specified limit. For a minimum limit, it passes when measured roughness is greater than or equal to the specified limit.
What does the acceptance margin mean? It shows the numerical distance between the measurement and the specified boundary. A positive margin indicates the measurement is on the acceptable side of the selected limit.
Can I compare Ra with Rz? No. The measured value and specified limit must use the same roughness parameter.
Can I use µm and µin? Yes, but both values must be entered using the same unit. Convert them first if the measurement and specification use different units.
What happens when the measured value equals the limit? The calculator treats equality as passing for both maximum and minimum criteria because the measurement meets the stated boundary.
Does a pass result certify the surface? No. It only confirms that the entered numbers satisfy the selected mathematical comparison.