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.
SURFACE ROUGHNESS VALUE CONVERSIONS
|Rt||Rz*||Ra / CLA / AA||Rq / RMS / Rs|
|Comparison values may vary by up to 25% .1µm=.001 mm=40. µ" or|
|1 Micron=.00004 . 1=.0254µm or .001=.0254mm|
|* Same as Rtm is U.S.|
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.
SURFACE ROUGHNESS CONVERSION CHART
|micro-meters µm||micro-inches µ"||(N)||Inches||mm|
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
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.)
|Elect. Discharge Mach.|
|Perm Mold Casting|
|Cold Rolling, Drawing|
|Less Frequent Application|
|Higher or lower values may be obtained under special conditions|
SURFACE ROUGHNESS SYMBOLS AND DEFINITIONS
The quality of a surface texture is defined by certain symbols on technical drawings, these are explained below.
|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.
|=||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|
|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|