Page 12 - Fall 2024 EDM Today

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One reason Ra is the most commonly used parameter to defi ne roughness is its robustness.
2) Optical methods

• Focus variation: By varying the vertical position of the optical system and focussing above the part to measure, it
creates a 3D map. It measures typically an Ra of 30 nm.

• White light interferometry: Measures down to sub-nanometer accuracy and can measure Ra down to 40 nm.
• Scanning electron microscopy: Useful for high precision measurements and for the detection of microcracks and
surface defects, it achieves a resolution better than 1 nm.
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EDM Today Magazine EDM Today Magazine VDI3400 Ra [um]
Optical methods are well suited for 2D or 3D measurements of the surface and can provide area measurements
such as Sa, St, Sz, etc., which are extensions of Ra, Rt, and other linear measurements.

Ra[um]
VDI 3400
A MANUAL APPROACH
0
0.10
1.4
23
Another parameter derived from Ra is the VDI class, which is
0.11
24
1.6
1
particularly popular in Europe. It is defi ned by the formula
K = 20*log(10*Ra). This parameter originates from the EDM
26
2.0
0.14
guidelines VDI 3400 and 3402, provided by the Verein Deutscher
2.2
0.16
27
Ingenieure. (To fi nd the Aa in µinch, one has to take the Ra in um, 2 3 4 5 6 0.13 25 1.8
divide by 25.4 and multiply by 1000.)
2.5
0.18
28
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0.20
29
2.8
The VDI 3400 guideline is quite outdated and is currently being
7
30
3.2
0.22
updated, with the new version expected to be available in 2025.
0.25
8
31
3.5
Rulers and texture references, such as those shown in Fig. 4,
enable visual comparison of a surface with the reference without
4.0
0.28
9
32
requiring any instruments. We will discuss the benefi ts of this
4.5
0.32
33
10
approach later.
0.35
34
5.0
11
35
12
0.40
5.6
6.3
13
14
0.50
37
7.1
7.9
0.56
38
15 0.45 36 10.0
39
8.9
16
0.63
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40
0.71
17
Fig. 4: Gaussian fi lter = 2.5 mm, Length = 1.6 mm
11.2
0.79
41
18
42
0.89
19
12.6
14.1
20
43
1.00
1.12
21
15.8
44
17.8
45
1.26
22
Table 1: VDI roughness according
to the VDI 3400 guideline
LIMITATIONS OF THE SYSTEM
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So far, we’ve discussed measurements along
a line. This method has a limitation: it provides
only a partial measurement and may miss any
marks close to the measured line. Therefore,
it’s common practice to measure a surface at
multiple positions (e.g., top, middle, and bottom
for WEDM parts).
Another limitation is that the traditional param-
eters Ra, Rz, Rt do not defi ne the crater size,
look for example at Fig. 5. The two halves of it
represent exactly the same Ra of 0.8 µm. What
is the difference? The difference is the distance
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EDM Today Magazine EDM Today Magazine EDM Today, Fall 2024 Issue
between the peaks, Rsm.
Fig. 5: Same Ra 0.8 with different Rsm
10