2. Contents
• Definition
• Surface texture effects
• Terms
• Ra or CLA
• Roughness Grade Numbers
• Basic symbol for use on drawings
• Drawing Symbol Notation
• Lay symbols
• Average values for various processes
• Other roughness values
• Methods of testing
• W W W References
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3. Definitions
• Surface texture describes the variation of the
surface from a perfect surface
• Surface texture is made up of
Roughness
Waviness.
Lay
Flaws
It is the topography of the surface
The term surface texture is preferable to surface finish as surface finish
implies coatings and finishing techniques
(Painting, chrome plating etc)
4. Surface Texture Effects
• The appearance
– The finer the texture the higher the aesthetic value
• The function
– Some functions require a fine texture
• The dimensional tolerances possible
– The finer the texture the greater the degree of
dimensional accuracy possible
• The cost
– The finer the texture the greater the cost to produce
7. Waviness
• Repeated widely spaced irregularities
generally resulting from machine
vibrations of chatter in th emachine
8. Profile
Profile is the shape of the surface
It is made up of roughness and weariness.
Profile is influenced by the tool type and shape
9. Lay
Lay is the dominate pattern of marks left on the surface arising from
machining processes (tool type, feed rate, and machine type) Indicated
by symbol
Parallel Perpendicular Diagonal
Multi Circular Radial
Plane of Projection
10. Flaws
• Discrete or irregular infrequent
irregularities these could be scratches, pits
or cracks
11. Errors of Form
• Long form or non cyclic deviations in
surface profile produced by errors in
machine ways, slides or alignments
12. Ra or CLA
The Ra (Roughness Average) or CLA (Centre Line Average) is the
average distance from the centre line to the highest or lowest point. The
Ra or CLA can be found by using the formula
Ra (Micrometers) = A¹+A²+A³+….+An
Cut off length
Micrometers = 0.000001 meters = 1 x 10-6 meters
Sample Length
13. Roughness Average in Micro meters (Ra)
can also be expressed as a
Roughness Grade Number (N)
Roughness Grade Numbers
Ra N
50 Um N 12
25 Um N 11
12.5 Um N 10
6.3 Um N 9
3.2 Um N 8
1.6 Um N 7
Ra N
0.8 Um N 6
0.4 Um N 5
0.2 Um N 4
0.1 Um N 3
0.05 Um N 2
0.025 Um N 1
14. Drawing Symbols
This symbol has a bar added to the
top of the standard shaped symbol.
Basic symbol with a bar added to
the top of the right hand line.
This symbol is a basic shaped symbol
with a circle added in the groove.
Symbol has an angle of 60 degrees and
an open top with two uneven sides. Basic
shaped symbol.
Machining
optional
Machine finish
required
Removal of
material not
permitted
Special surface
feature required
15. Drawing Symbol Notation
= Production Method
= Direction of Lay using
Lay Symbols
C= Sampling Length
(f)= Other Roughness Values
Machining Allowance =
Click on labels for explanations of the notation
Roughness Grade Number
or Roughness Average =
17. Average Values For Processes
Process Ra in µm
50 25 12.5 6.3 3.2 1.6 0.8 0.4 0.2 0.1 0.05 0.025
Gas Cutting
Shaping
Drilling
Milling
Reaming
Turning
Grinding
Lapping
Average
Possible
For More Detailed Information
18. Valores promedio para cada proceso
Process Ra in µm
50 25 12.5 6.3 3.2 1.6 0.8 0.4 0.2 0.1 0.05 0.025
Corte de gas
conformado
barrenado
fresado
rimado
torneado
rectificado
lapeado
Promedio
posible
For More Detailed Information
19. Machining Allowance
• This is the amount of material that can be
removed to achieve desired surface
texture
20. Other Roughness Values
•Rp : is the height of the highest peak in the roughness profile over the
evaluation length
•Rv is the depth of the deepest valley in the roughness profile over the
evaluation length .
•Rt, is the sum of these two, or the vertical distance from the
deepest valley to the highest peak.
•Rz is the sum of the height of the highest peak plus the lowest
valley depth within a sampling length
21. Methods of Testing
• Scratch comparison plates.
• Each machining process requires it
own set.
• Optical and mechanical
