Surface Roughness and Texture

Charts relating to Surface Roughness Values, Process Texture, Roughness Drawing Symbols and Direction of Lay.

SURFACE ROUGHNESS VALUE CONVERSIONS

Surface texture is comprised of three components: roughness, waviness and form.

Roughness is a function of the machining process; waviness is the component that is superimposed by roughness; and form is the overall shape of the surface minus contributions from roughness and waviness.

Roughness Average can be measured and interpreted using various mathematical models, the most common being Ra or Roughness Average (also known as CLA and AA).

Roughness is measured in microns, µm or microinches, µ

Ra measurement for a sample length L is the mean height of the surface profile(peaks and inverted valleys). Ra is the arithmetic average value of the departure from profile from the center line.

So, for four values: Ra / CLA / AA = W + X + Y + Z

Root Mean Square or RMS Roughness (Rq / RMS / Rs), is another averaging measurement well known in statistics as the Root Mean Square value.

For example, the arithmetic average (AA) of four values, W, X, Y, Z is: AA = W + X + Y + Z.

Whereas RMS value = For statistical measurements, RMS values are the preferred measure.

Rt is the maximum peak-to-valley height in the sampling length Rz / Rtm: Rz is the (ISO) ten-point height measurement and, in the USA, is known as Rtm which is (RzDIN) mean peak-to-valley height.

EQUIVALENT Ra VALUES FOR SURFACE FINISH GRADE numberS

Ra = Roughness, average in micro-meters & micro-inches

RMS = Root Mean Square in micro-inches

CLA = Center Line average in micro-inches

Rt = Roughness, total in microns

N = New ISO (Grade) Scale numbers

Cut-Off Length = Length required for sample.

Conversions (math):

CLA (micro inches) = Multiply Ra(µm) x 40

RMS * (acceptable 1.1 - 1.7 factor) = Multiply CLA x 1.1

Parameter Conversion Formula

To convert a known Ra value to a different parameter, use the following formula:

Ra x Parameter Factor = Desired Parameter

Example:-

Ra to Rt:

1.0 micrometer Ra x 8.7 (Rt Factor) = 8.7 micrometer Rt

40 microinch Ra x 8.7 (Rt Factor) = 348 microinch Rt

PROCESSES AND ASSOCIATED SURFACE ROUGHNESS VALUES

Roughness Average Micrometers µm (Microinches µin.)

SURFACE ROUGHNESS SYMBOLS AND DEFINITIONS

The quality of a surface texture is defined by certain symbols on technical drawings, these are explained below.

SYMBOL	INTERPRETATION
	Surface may be produced by any method
	Removal of material is not permitted
	Removal of material by machining
	If special surface characteristic is to be indicated
~	Roughness Grade N12
	Roughness Grade N11 & N10
	Roughness Grade N9, N8 & N7
	Roughness Grade N6, N5 & N4
	Roughness Grade N3, N2 & N1
	A:- Roughness value, Ra in Micrometres, Roughness Limits or N11 & N10 Grade B:- Production Method/ Treatment of Coating C:- Sampling Length D:- Direction of Lay E:- Maching Allowance F:- Other Roughness Values

DIRECTION OF LAY

The direction of lay is the direction of the predominant surface pattern, this is usually defined by the production method.

SYMBOL	INTERPRETATION
=	Parallel to the plane of projection of the view in which the symbol is used
⊥	Perpendicular to the plane of projection of the view in which the symbol is used
X	Crossed in two angled directions relative to the plane of projection of the view in which the symbol is used
M	Multi directional
C	Approximately circular, relative to the centre of the surface to which the symbol is applied
R	Approximately radial, relative to the centre of the surface to which the symbol is applied