SURFACE ROUGHNESS CONVERSIONS

Charts relating toSurface Roughness Values ,Process Texture,Roughness Drawing SymbolsandDirection 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

RtRz*Ra / CLA / AARq / RMS / Rs
µmµmµm µ"µmµ"
0.060.030.0060.20.0070.2
0.080.040.0080.30.009 0.3
0.10.050.010.40.0110.4
0.120.060.0120.50.0130.5
0.150.080.0150.6 0.0180.7
0.20.10.020.080.0220.9
0.250.120.0251.00.0271.1
0.30.15 0.031.20.0331.3
0.40.20.041.60.0441.8
0.50.250.052.00.0552.2
0.6 0.30.062.40.0662.6
0.80.40.083.20.0883.5
0.90.50.104.00.114.4
1.00.60.124.80.135.2
1.20.80.156.00.187.2
1.61.00.208.00.22 8.8
2.01.20.2510.00.2710.8
2.51.60.312.00.3313.2
3.02.00.416.0 0.4417.6
4.02.50.520.00.5522.0
5.03.00.624.00.6626.0
10.06.01.2 48.01.352.0
15.010.02.5100.02.7108.0
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

RaRaRMSCLA RtNCUT-OFF LENGTH
micro-meters µmmicro-inches µ"(N)Inchesmm
0.025 11.110.310.0030.08
0.0522.220.520.010.25
0.144.440.830.010.25
0.288.881.240.010.25
0.41617.616250.010.25
0.83232.532460.03 0.8
1.66364.363870.030.8
3.2125137.5125138 0.12.5
6.32502752502590.12.5
12.5500550500 50100.12.5
25100011001000100110.38
5020002200 2000200120.38

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.)

Process50
(2000)
25
(1000)
12.5
(500)
6.3
(250)
3.2
(125)
1.6
(63)
0.80
(32)
0.40
(16)
0.20
(8)
0.10
(4)
0.05
(2)
0.025
(1)
Flame Cutting
Snagging
Sawing      
Planing, Shaping      
Drilling    
Chemical Milling     
Elect. Discharge Mach.     
Milling       
Broaching    
Reaming    
Electron Beam     
Laser     
Electro-Chemical         
Boring, Turning          
Barrel Finishing      
Electrolytic grinding     
Process50
(2000)
25
(1000)
12.5
(500)
6.3
(250)
3.2
(125)
1.6
(63)
0.80
(32)
0.40
(16)
0.20
(8)
0.10
(4)
0.05
(2)
0.025
(1)
Roller Burnishing    
Grinding        
Honing       
Electro-Polish        
Polishing       
Lapping      
Superfinishing      
Sand Casting    
Hot Rolling   
Forging     
Perm Mold Casting   
Investment Casting     
Extruding      
Cold Rolling, Drawing     
Die Casting   

KEY:-

 Average Application
 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.

SYMBOLINTERPRETATION
surface finish symbol - basicSurface may be produced by any method
surface finish symbol - permittedRemoval of material is not permitted
surface finish symbol - not permitted Removal of material by machining
surface finish symbol - specialIf special surface characteristic is to be indicated

~

Roughness Grade N12
surface finish symbols one triangleRoughness Grade N11 & N10
surface finish symbols two trianglesRoughness Grade N9, N8 & N7
surface finish symbols three trianglesRoughness Grade N6, N5 & N4
surface finish symbols four triangles Roughness Grade N3, N2 & N1
surface finish symbolA:- 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 usedParallel Direction of Lay
Perpendicular to the plane of projection of the view in which the symbol is usedPerpendicular Direction of Lay
XCrossed in two angled directions relative to the plane of projection of the view in which the symbol is usedCrossed Direction of Lay
MMulti directionalMulti Direction of Lay
CApproximately circular, relative to the centre of the surface to which the symbol is appliedCircular Direction of Lay
RApproximately radial, relative to the centre of the surface to which the symbol is appliedRadial Direction of Lay