Measurement of surface finishes is done with instruments called profilometers. Experience proves that measurement of surface finishes by comparison using a surface finish gauge does not give an error above 2 CH classes. The criteria of roughness are the Ra (Europe) – CLA (UK) – AA (USA) To obtain a grading of roughness, Charmilles has defined a CH scale, which is however cross referenced with the standards that are in common use.
Definition of Roughness criteria
No CH = 20 log (10 Ra) (µm)
Some significant rules and figures
1 µm Ra = CH20
10 µm Ra = CH40 If Ra = Ra x 2 CH = CH+6 ex: 1 µm Ra = CH20
- µm Ra = CH26
- µm Ra = CH32
between 2 successive CH units is 1.12
between 2 successive CH units is 12.2%
The MAX Rt: roughly equal to 8 x Ra
The mean Rt: is the arithmetic mean of the 5 Max Rt recorded on the 5 sample lengths selected.
(For establishing the technology: 0.8mm)
Mean Rt = 5 x the Ra value
In surface finish measurements a tolerance of + 2 CH units is accepted.
The American and English standards admit the following relationship between the values V(RMS) and V(Ra):
V(RMS) = 1.11 V(Ra)
The French standard NF 05051 is tied in with the ISO 1302 standard concerning classes of surface finish.
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