1. Marlborough Boys College
Surface Texture
G Lancaster
Marlborough Boys College

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

5. Surface Texture Terms

6. Roughness
The closely spaced peaks and valleys produced by the machining process

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 Plane of Projection
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
Plane of Projection
Parallel
Perpendicular
Diagonal
Multi
Circular
Radial

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 (Micrometers) = A¹+A²+A³+….+An Cut off length
Ra or CLA
Sample Length
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 = meters = 1 x meters

13. Roughness Grade Numbers
Roughness Average in Micro meters (Ra)
can also be expressed as a
Roughness Grade Number (N) 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 Um
N 1

14. Drawing Symbols
Symbol has an angle of 60 degrees and an open top with two uneven sides. Basic shaped symbol.
Machining optional
Machine finish required
This symbol has a bar added to the top of the standard shaped symbol.
Removal of material not permitted
This symbol is a basic shaped symbol with a circle added in the groove.
Basic symbol with a bar added to the top of the right hand line.
Special surface feature required

15. Drawing Symbol Notation
= Production Method
Roughness Grade Number
or Roughness Average =
C= Sampling Length
(f)= Other Roughness Values
Machining Allowance =
= Direction of Lay using
Lay Symbols
Click on labels for explanations of the notation

16. Lay Symbols

17. Average Values For Processesin 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. Machining Allowance
This is the amount of material that can be removed to achieve desired surface texture

19. 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

20. Methods of Testing Scratch comparison plates.
Each machining process requires it own set.
Optical and mechanical

21. Further Information
Sites
Virtual Machine Shop
Cyberman

22. References