Surface quality

In metrology, surface quality is a term which includes the geometrical state and the physiquo-chemical state.

Criteria

The physical criterion R is microgeometrical, and it is itaverage amplitude of roughness (roughness).

The statistical criterion R_has is statistics, and it is itarithmetic average deviation compared to the average line.

R = 5 R_has approximately

Measuring apparatus

There are several methods to evaluate a surface quality:

• by comparison (samples viso-tactile);
• by measurement (electronic instruments with sensors or optical apparatuses).

Roughness It is the whole of the irregularities of a surface in micrographic and macrographic matter. The machined surfaces are not perfect, they present irregularities due to the processes of machining, with the tools, with the matter, etc. The functional role of a surface depends on a certain number of factors, in particular of the surface quality (sealing, slip, etc.). More the index of roughness is weak, more it is difficult to obtain, what increases necessarily the manufacturing cost. Geometrical surface Surfaces perfect; On the drawing, it is geometrically defined by the engineering and design department, using nominal sizes. Surface specified Surface resulting from geometrical surface, transform by the office of the studies which prescribes the limits of realization of this surface using symbols and numerical values in complement of the nominal sizes of the drawing. Surface measured Surface determined using the measuring instruments starting from real surface. Measured surface, resulting from the exploration of real surface will have to be the image most brought closer this one. Real surface Surfaces obtained during manufacture. L Basic length of average profile RP average Depth of roughness. Arithmetic mean of the values of the ordinate there of all the points of the profile over the basic length L. Ra Arithmetic average deviation. Arithmetic mean of the absolute values of the ordinate y' (between each point of the curve and the axis Ox').

Values of roughness

Surface qualities and function

Maximum value of R in µm:

• with relative displacements:
  • sliding friction
    • very difficult
      • Steel: maximum value of R = 0,4 and W ? 0,3 R
      • ...
    • ...
  • rolling friction
  • resistance to hammering
  • fluid friction
  • dynamic sealing

• fixed assemblies
  • static sealing
  • assembly fixes without constraint
  • assembly fixes with constraint
  • adherence (joining)

• without external pressure:
  • coat of paint
  • dépot electrolytic
  • measure

• with external pressures:
  • load-carrying capacity alternate
  • cutting tools face of cut

Manufactoring processes and surface qualities

Roughness R_has out of micrometers or roughness R out of micrometers

Roughness of rough surfaces:

• Stamping: R_has / R 12,5/ 40 - 3,2/ 10 - (1,6/ 4)
• Forging: R_has / R 3,2/ 10
• Rolling cold (on copper): R_has 0,6 with 0,08µm/R_max = 1,0 with 1,5µm

Roughness of machined surfaces:

• Boring:
  • tool steel rapid R_has / R (6,3/ 16) - 3,2/ 10 - 1,6/ 4 - (0,8/ 2)
  • tool carburize or diamond R_has / R (3,2/ 10') ' - 1,6/ 4 - 0,8/ 2 - (0,4/ 1)
  • with the borer R_has / R (6,3/ 16) - 3,2/ 10 - 0,8/ 2 - (0,4/ 1)
• ...
• Turning:
  • tool high speed steel R_has / R (12,5/ 40) - 6,3/ 16 - 1,6/ 4 - (0,8/ 2)
  • carbide-tipped tool or diamond R_has / R (3,2/ 10) - 3,2/ 10 - 0,4/ 1 - (0,4/ 1)