How to measure roughness?

Principle of measurement using a roughness meter:

A tip travels over a defined length on the surface to be measured and measures the displacement of the sensor as a function of surface irregularities.

Types of profile curves:

The roughness profile corresponds to the primary profile corrected for ripple effects

CutOff, number of repetitions, evaluation length

During a test, we define a displacement length before and after the evaluation (pre-travel and post-travel)

An evaluation length called cutoff

and a number of repetitions of the evaluation

For small parts:

• Reduce the number of "N"
• Remove or reduce pre / post travel
• Do not reduce the "cutoff"

What should I know before measuring?

Mandatory information

• Which standard: ANSI, ISO, JIS, DIN, etc.
• Which filter to use (refer to the standard)
• What parameters are to be measured: Ra, Rq (Rms), Rt, Rz, Rpk ...
• Cutoff and Multiplier (N)

Complementary

• Measuring force Angle and radius of the stylus to be used
• Additional filter (shape, average line, etc.)

Standards:

Filters:

Which Cutoff to choose?

Definitions of roughness parameters:

Ra: Arithmetic mean deviation of the profile, Ra (JIS1982): Deviation from the arithmetic mean of roughness

(JIS1994, JIS2001, ISO1997, ANSI, VDA ...)

Ra is the arithmetic mean of the absolute values of the deviations of the evaluation profile (Yi) from the mean line.

For ANSI, Ra is defined over the entire evaluation length

Rq Root mean square deviation of the profile

(JIS2001, ISO1997, ANSI, VDA ...)

Rq is the square root of the arithmetic mean of the squares of the deviations (Yi) from the mean line at the evaluation profile.

For ANSI, Rq is defined over the entire evaluation length

Rz, Rmax, Ry: Maximum profile height

Rz (JIS2001, ISO1997, ANSI, VDA), Rmax (JIS1982), Ry (JIS1994)

Divide the assessment profile into components based on the sample length. Then, for each component, obtain the sum (Zi) of the highest point of the mean line (Pi) and the lowest point of the mean line (Vi). The average of these sums is Rz, Rmax (for JIS1982), or Ry (for JIS1994).

Vertices / peaks and valleys / bottoms of the rating profile When the rating profile contains an average line, the parts of the profile that project above the average line are called “peaks”, and the parts of the profile that are above the average line. project below the middle line are called “valleys”. The highest point of each vertex is called the “peak”, and the lowest point of each is called the “bottom”.

Rp: Highest peak

(JIS2001, ISO1997, ANSI, VDA, Free), Rpm (ANSI)

Divide the assessment profile into components based on the sample length. Then, for each component, get the distance from the highest point (Rpi) to the mean line. Rp is the average of the Rpi values that were obtained from the components.

Rp (ANSI) is defined as the maximum peak height over the evaluation length.

Rv: Maximum trough depth

(JIS2001, ISO1997, ANSI, VDA, Free)

Divide the assessment profile into components based on the sample length. Then, for each component, obtain the distance from the lowest point (Rvi) of the mean line. Rv is the average of the Rvi values that were obtained from the components.

Rv (ANSI) is defined as the maximum bottom depth over the evaluation length.

Rt: Maximum roughness

(JIS2001, ISO1997, ANSI, VDA, Free)

Rt is the sum of the distance from the mean line to the highest peak and the distance from the mean line to the deepest bottom, for the entire evaluation length

For more information, do not hesitate to consult the documents attached to this article or contact Labomat Essor