Surface finish

Question 1

why is surface finish important

Answer 1

• aesthetic reasons
• safety
• to reduce friction and wear
• to improve mechanical and physical properties
• to assemble

Question 2

components of the section of a machined metallic part

Answer 2

• substrate, not influenced by the cut
• altered layer: modified by temperature and stresses during the cut
• surface texture: profile of the real surface

Question 3

Elements of the surface texture

Answer 3

1. roughness
   - small wavelength deviations from the ideal surface
   - determined mostly by the process
2. waviness
   - larger wavelength deviations
   - due to deflection, vibration
   - roughness is superimposed to waviness
3. lay
   - predominant pattern of the surface texture
4. flaws
   - occasional irregularities on the surface

Question 4

different surfaces definitions

Answer 4

• ideal surface: surface as designed by the designer
• real surface: surface limiting the body, generated by the manufacturing process
• primary surface: surface probed by measuring systems, that “filter” the real surface
• reference surface: surface which the the primary surface is referred to
• mean surface: surface obtained from a least squares fitting

Question 5

definitions of surface roughness

Answer 5

• vertical deviations yi from the mean surface
• average roughness:
  Ra = 1/n * sum(i=1,n) |yi|
• squared roughness
  Rq = sqrt(1/N * sum(i=1,N) yi^2)
• peak to peak roughness
  Rt = Rp + Rv

Ra approximated by Rt / 4

Question 6

methods to measure surface roughness

Answer 6

1. comparison with standard test surfaces
2. electronic measurement
3. optical techniques

Question 7

electronic measurement

Answer 7

• stylus head moves above the surface
• vertical stylus measure the surface profile
• at first, a low pass filtering of the surface is obtained due to the tip of the stylus
• them, a high pass filtering of the obtained signal should be done to remove waviness component.
• to this end, the cutoff length parameter has to be chosen, and influnces the result
• a high pass filtering with cutoff smaller than waviness eliminates the waviness