Page 11 - Fall 2024 EDM Today

EDM Today Magazine EDM Today Magazine EDM Today Magazine

PROFILE MEASURE PARAMETERS (SEE FIG. 1)
There are multiple techniques for measuring surface
roughness:
The most used line measures for the roughness are:
1) Stylus profi lometry - This is the most used method:
Ra (Arithmetic Average Roughness): This is the average
a stylus moves across the surface, creating a profi le.
of the absolute values of the surface height deviations
The vertical resolution is in the nanometer range.
measured from the mean plane. It is one of the most
However, the stylus tip limits it to 2 to 10 um; it
widely used roughness parameters.
Rq (Root Mean Square Roughness): This is the root
Two important settings, besides the stylus tip diameter:
mean square average of the surface height deviations measures Ra down to approximately 20 nm.
EDM Today Magazine EDM Today Magazine EDM Today Magazine
from the mean plane. It is typically about 1.1 times larger
- Gaussian fi lter: Used to remove high-frequency noise
and isolating the roughness component. It smooths
than Ra.
the profi le. Typical value: 0.8 mm.
Rz (Mean Roughness Depth): This parameter measures
the average of the absolute values of the fi ve highest
- Cutoff: The cutoff value is the wavelength that
separates the waviness from the roughness. Wave-
peaks and the fi ve deepest valleys over the evaluation
lengths smaller than this value are the roughness,
length.
bigger ones the waviness. Typical value: 2.5 um.
Rt (Total Roughness): This is the vertical distance be-
Measuring length: Typically 2.5 mm.
tween the highest peak and the lowest valley within the
evaluation length.
ent values for the same roughness.
Rmax (Maximum Roughness Depth): This is the maxi- Caution: By varying these parameters, one can get differ-
mum vertical distance between the highest peak and the
See Fig. 2 and 3, the measuring results of the same
EDM Today Magazine EDM Today Magazine EDM Today Magazine
deepest valley in a given evaluation length.
surface.
It becomes immediately clear that with so many parame-
ters involved in defi ning a measurement, the task of mea-
suring a surface is quite complex. And this complexity
arises even when considering just a line measurement,
whereas the roughness is three dimensional.

EDM Today Magazine EDM Today Magazine EDM Today Magazine

Fig. 2: Gaussian fi lter = 0.8 mm, Length = 2.5 mm

Fig. 1: Rz and Ra

EDM Today Magazine EDM Today Magazine EDM Today Magazine
MEASURING STANDARDS AND METHODS

ISO 21920: This is one of the most recent and detailed
standards for measuring roughness. It deals with the
indication of roughness in technical drawings. The stan-
dard is organized into three parts:
• ISO 25178: This standard addresses three-
Fig. 3: Gaussian fi lter = 2.5 mm, Length = 1.6 mm
dimensional surface texture and introduces new
symbols for indicating surface fi nish.
have Ra of 1.979 and 1.864 um, whereas Rz changes
• ISO 4287 and ISO 4288: These standards defi ne One sees that Ra is more robust than Rz, Fig. 2 and 3
EDM Today Magazine EDM Today Magazine EDM Today Magazine
more: 2.28 to 1.5 um.
roughness parameters and measurement
methodologies.
9
EDM Today, Fall 2024 Issue