KEVOS® MAKEOS · Quality
Surface Roughness Acceptance (ISO 16 % Rule and Max Rule, Evaluation Length)
Does a measured surface pass its roughness requirement: under the default 16 % rule, are no more than 16 % of the measured values above the upper limit (or below a lower limit), or under the max rule, is no value above the limit — and what evaluation length should each measurement cover?
Acceptance of measured surface roughness values (Ra, Rz and other parameters) against the drawing limits by the comparison rules described in the handbook: with the default 16 % rule no more than 16 % of the measured values may exceed the upper limit and no more than 16 % may fall below a lower limit, and the handbook notes that more than 16 % above the limit may still be accepted when the values are normally distributed and the mean plus one standard deviation lies within the upper limit; with the max rule (the parameter marked max, for example Rz1.5max) no value may exceed the limit anywhere on the surface. The default evaluation length is five sampling lengths, ln = 5 lr.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Rule on the drawing | choice | The comparison rule: the 16 % rule applies unless the parameter on the drawing is marked max. |
| Upper limit of the parameter | µm | Upper limit of the roughness parameter from the drawing, for example Ra 0.8. |
| Lower limit | choice | Whether the drawing also gives a lower limit for the parameter. |
| Lower limit of the parameter (when specified) | µm | Lower limit of the parameter when the drawing specifies one. |
| Measured values | ||
| Number of values measured | values | Total number of values measured on the surface. |
| Values above the upper limit | values | How many of the measured values are above the upper limit. |
| Values below the lower limit | values | How many of the measured values are below the lower limit. |
| Largest value measured | µm | Largest single value measured anywhere on the surface. |
| Smallest value measured | µm | Smallest single value measured. |
| Mean of the measured values | µm | Arithmetic mean of all the measured values. |
| Standard deviation of the measured values | µm | Standard deviation of the measured values. |
| Measuring length | ||
| Sampling length (cut-off) | mm | Sampling length (filter cut-off) used for each value, from the drawing or the default for the parameter. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Rule on the drawing
- What it is
- Comparison rule for the roughness callout: the default 16 % rule, or the max rule when the parameter is marked max.
- Units and format
- Choose one of 2 options from the list: 16 % rule (default, no max marked); Max rule (parameter marked max).
- Example
- The form opens with “16 % rule (default, no max marked)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Changes which measured values decide acceptance.
- Get it right
- Rz1.5max on a drawing means the max rule; Ra 0.8 without max means the 16 % rule.
What this means: Upper limit of the parameter
- What it is
- Upper limit of the roughness parameter from the drawing callout.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0.000001 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Values above it count against the surface.
- Get it right
- Check the parameter (Ra, Rz) matches what the instrument reports.
What this means: Lower limit
- What it is
- Whether the drawing also sets a lower limit for the roughness parameter.
- Units and format
- Choose one of 2 options from the list: None specified; Specified.
- Example
- The form opens with “None specified” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Adds the lower-limit check.
- Get it right
- Lower limits are used on bearing and sealing surfaces that must hold oil.
What this means: Lower limit of the parameter (when specified)
- What it is
- Lower limit of the roughness parameter when the drawing specifies one.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Values below it count against the surface.
- Get it right
- Must be below the upper limit.
Measured values
What this means: Number of values measured
- What it is
- Total number of roughness values measured on the surface.
- Units and format
- Enter a number in values: values. The form accepts 1 values to 100000 values. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 values as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the share that each value over the limit represents.
- Get it right
- More readings make the 16 % share meaningful; one in six is already 16.7 %.
What this means: Values above the upper limit
- What it is
- How many of the measured values are above the upper limit.
- Units and format
- Enter a number in values: values. The form accepts 0 values to 100000 values. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 values as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rejects the surface when more than 16 % are above.
- Get it right
- Count readings, not places; repeat a doubtful reading rather than discard it.
What this means: Values below the lower limit
- What it is
- How many of the measured values are below the lower limit.
- Units and format
- Enter a number in values: values. The form accepts 0 values to 100000 values. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 values as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rejects the surface when more than 16 % are below.
- Get it right
- Only used when a lower limit is specified.
What this means: Largest value measured
- What it is
- Largest single roughness value measured anywhere on the surface.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.85 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Decides the max rule.
- Get it right
- Under the max rule search the whole surface for the worst area.
What this means: Smallest value measured
- What it is
- Smallest single roughness value measured on the surface.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Decides the lower limit under the max rule.
- Get it right
- Used against a lower limit under the max rule.
What this means: Mean of the measured values
- What it is
- Arithmetic mean of all the roughness values measured.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.65 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- With the standard deviation, decides the normal-distribution exception.
- Get it right
- Most roughness testers report it over a set of readings.
What this means: Standard deviation of the measured values
- What it is
- Standard deviation of the roughness values measured.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.08 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger spread makes the exception harder to meet.
- Get it right
- Use the sample standard deviation of the same readings as the mean.
Measuring length
What this means: Sampling length (cut-off)
- What it is
- Sampling length (filter cut-off) used for each roughness value.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the evaluation length of five sampling lengths.
- Get it right
- 0.8 mm is common for turned and ground surfaces; use the drawing value if one is given.
Method and formulas
- 16 % rule: accept when no more than 16 % of the values exceed the upper limit and no more than 16 % fall below the lower limit.
- Exception noted in the handbook: when more than 16 % exceed the upper limit, the surface may still be accepted if the values follow a normal distribution and mean + one standard deviation is within the upper limit.
- Max rule: accept only when no value measured anywhere on the surface exceeds the upper limit (and none falls below a lower limit).
- Default evaluation length ln = 5 × sampling length lr (ISO 4287 R-profile).
- Example: 1 of 12 values above Ra 0.8 µm is 8.3333 % under the 16 % rule; each value covers 4 mm.
Limitations
- Use the rule the drawing calls for; ISO 4288 and ASME B46.1 give further detail on the number of readings and where to take them.
- The normal-distribution exception needs enough readings to judge the distribution; record it on the inspection report.
- Values must be measured with the cut-off and filter the drawing specifies.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Decide whether a part meets its surface roughness callout
- Plan the length each roughness reading must cover
- Settle whether the 16 % or the max rule applies to a drawing callout
Who it is for
- Quality inspectors
- Machinists
- Design engineers
What you get
- Share of values above the upper limit and below the lower limit
- Decision under the rule on the drawing
- Mean plus one standard deviation against the upper limit
- Evaluation length ln = 5 lr
Interpreting the result
- With few readings, one value over the limit is already more than 16 % (1 in 6 is 16.7 %); take more readings before rejecting a marginal surface.
- A max requirement is stricter: a single reading over the limit rejects the surface.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 750. Topic reference; no source tables or recommended settings are reproduced.
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