Often engineering drawings call for a range of surface roughness.  Traditionally this is a roughness value expressed in Ra although other measurement types such as Rz are becoming more common.  But what do they mean?

Here is an interesting short article, courtesy of Misumi that explains the different types:

TECHNICAL DATA – SURFACE ROUGHNESS

EXCERPT FROM JIS B 0601(1994) AND JIS B 0031(1994)

Categories of Surface Roughness

Definitions and indications for surface roughness parameters (for industrial products) are specified.

They are arithmetical mean roughness(Ra), maximum height(Ry), ten-point mean roughness (Rz), mean spacing of profile irregularities (Sm), mean spacing of local peaks of the profile(S)and profile bearing length ratio (tp). Surface roughness is given as the arithmetical mean value for a randomly sampled area.

Mean centre line roughness (Ra) is defined in the annexes of JIS B 0031 and JIS B 0601.

Typical ways for obtaining surface roughness:

1. Arithmetical mean roughness (Ra)

A section of standard length is sampled from the mean line on the roughness chart. The mean line is laid on a Cartesian coordinate system where in the mean line runs in the direction of the x-axis and magnification is the y-axis. The value obtained with the formula on the right is expressed in micrometres (µm) when y=f(◊).

2. Maximum peak (Ry)

A section of standard length is sampled from the mean line on the roughness chart. The distance between the peaks and valleys of the sampled line is measured in the y direction. The value is expressed in micrometres (µm).

Note: To obtain Ry, sample only the standard length. The part, where peaks and valleys are wide enough to be interpreted as scratches, should be avoided.

3. Ten-point mean roughness (Rz)

A section of standard length is sampled from the mean line on the roughness chart. The distance between the peaks and valleys of the sampled line is measured in the y direction. Then, the average peak is obtained among 5 tallest peaks (Yp), as is the average valley between 5 lowest valleys(Yv).  The sum of these two values is expressed in micrometres (µm).

Reference:Relationship Between Arithmetical Mean Roughness (Ra) and Conventional Symbols

Note: The interdependence for 3 classes is not strictly enforced.

Note: The evaluation lengths of Ra: Ry and Rz: Five times the cut-off value and standard length respectively.

TECHNICAL DRAWINGS –  METHOD OF INDICATING SURFACE TEXTURE ON DRAWING

EXCERPT FROM JIS Z B0031(1994)

Positions of respective indicating symbols relative to indicating symbol of surface. Each grain surface position is indicated as shown in Drawing 7. This includes surface roughness, cut-off value or reference length, processing method, symbol of direction of lay, surface waviness, etc…

Drawing 7 Entry position of each indication

a: Value of Ra

b: Processing method

c: Cut-off value // valuation length

c: Reference length // valuation length

d: Symbol of direction of lay

f: Parameter other than Ra (With tp, parameter/cut-off level)

g: Surface waviness (according to JIS B 0610)

Note: Items other than a and f are added as necessary.

Reference: The location of lay of e in Drawing 7 is given as the finish allowance in ISO 1302.

Original article posted by Misumi at: de.misumi-ec.com/pdf/tech/mold/09_mo1517.pdf