VDI What does this mean in surface finish terms?

A. Hi. VDI = Verein Deutscher Ingenieure, the Society of German Engineers. There is a VDI scale for surface finishes, which is widely but not exclusively used in the EDM field. My lack of German has hampered my search for a cross reference chart or a description of what the VDI scale is about, but hopefully a reader will help.

Regards,

A. I had the same problem with VDI grades. The solution came from Rubert & co. England. They offer EDM finish scale to VDI 3400 standard. It comes with comparison table Ra values. For more information: www.rubert.co.uk/comparison.php ⇩

[Ed. update: The above link has broken. Thankfully, The Internet Archive preserved a copy
These days information often appears on the internet, then disappears forever. If that bothers or frightens you please consider a donation to The Internet Archive.

A. I have the same problem to find a correspondence between VDI and Ra, and after some search I made this comparison table :

VDI   RA (in micrometer)
10    0.32
12    0.4
15    0.56
18    0.8
21    1.12
22    1.26
24    1.6
27    2.24
30    3.15
33    4.5
36    6.3
39    9
42   12.5
45   18

Best Regards.

A. Bayer Plastics has some good Information about surfaces and injection molding available on their web- Site.

According to a (German) Document the VDI surfaces correspond to the following Ra / Rz Values and require the following drafting angles:

VDI;   Ra;     Rz;    DA-PA;   DA-PC;   DA-ABS
12;    0,40;   1,5;   0;      1,0;     0,5
15;    0,56;   2,4;   0,5;     1,0;     0,5
18;    0,80;   3,3;   0,5;     1,0;     0,5
21;    1,12;   4,7;   0,5;     1,0;     0,5
24;    1,60;   6,5;   0,5;     1,5;     1,0
27;    2,24;  10,5;   1,0;     2,0;     1,5
30;    3,15;  12,5;   1,5;     2,0;     2,0
33;    4,50;  17,5;   2,0;     3,0;     2,5
36;    6,30;  24,0;   2,5;     4,0;     3,0
39;    9,00;  34,0;   3,0;     5,0;     4,0
42;   12,50;  48,0;   4,0;     6,0;     5,0
45;   18,00;  69,0;   5,0;     7,0

DA-PA = Drafting Angle for Polyamide
DA-PC = Drafting Angle for Polycarbonate
DA-ABS = Drafting Angle for Acrilnitrile-Butadiene-Styrol

glass reinforced materials require more drafting

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But be careful: The table with the VDI related draft angles Mr. Lorenz Wiedemar provided (he says it's from BAYER plastics) is possibly not right!

For example:

STANDEX International communicates that their VDI 36 equivalent (MT 11540) requires minimum 3,5 even for PA !!
So just ask your surface supplier BEFORE being fooled by this table (like me).

Simon Mueller
- Fuerth, Bavaria, Germany
February 22, 2012

Hi, Simon. Thanks!

I've never found any surface standards for which "conversion" or "substitution" is actually possible. Rather, it's like comparing people's height vs. weight, or an engines torque vs. horsepower -- one parameter tends to "track" the other, but they cannot actually be converted. We see this in Ra vs Rz where the "conversion factor" must be low in one direction to be relatively safe and much higher in the other direction to be relatively safe because they are simply not measuring the same thing.

My unfortunate lack of German prevents me from understanding VDI standards and how they translate to other systems, but it would greatly surprise me if there is an actual "conversion"; I'll bet it's "usually roughly equivalent to somewhere around . . ."   :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. VDI = 20log(10Ra)

A. VDI Table
In the following, you can determine the roughness for your application according to VDI table.

VDI3400 Ra = AA = CLA              ISO1302 Rt
0-45    µm       µinch    N3-N10   µm
-----  -----    -----    -----    -----
0    |  0.1   |   4    |   N3   |
1    |  0.11  |   4.4  |        |
2    |  0.12  |   4.8  |        |
3    |  0.14  |   5.6  |        |
4    |  0.16  |   6.4  |        |
5    |  0.18  |   7.2  |   N4   |
6    |  0.2   |   8    |        |
7    |  0.22  |   8.8  |        |  
8    |  0.25  |  10    |        |
9    |  0.28  |  11.2  |        | 
10   |  0.32  |  12.8  |        |  
11   |  0.35  |  14    |   N5   |
12   |  0.4   |  16    |        |  1.6
13   |  0.45  |  18    |        |
14   |  0.5   |  20    |        |
15   |  0.56  |  22.4  |        |  3.2
16   |  0.63  |  25.2  |        | 
17   |  0.7   |  28    |        |
18   |  0.8   |  32    |   N6   |  5
19   |  0.9   |  36    |        |
20   |  1     |  40    |        |
21   |  1.12  |  44.8  |        |  7.5
22   |  1.26  |  50.4  |        | 
23   |  1.4   |  56    |        |
24   |  1.62  |  63    |        |  12
25   |  1.8   |  72    |   N7   |
26   |  2     |  80    |        |
27   |  2.2   |  88    |        |  16
28   |  2.5   | 100    |        |
29   |  2.8   | 112    |        |
30   |  3.2   | 125    |   N8   |  20
31   |  3.5   | 140    |        |
32   |  4     | 160    |        |
33   |  4.5   | 180    |        |  25
34   |  5     | 200    |        |
35   |  5.6   | 224    |        |
36   |  6.3   | 250    |   N9   |  37
37   |  7     | 280    |        |
38   |  8     | 320    |        |
39   |  9     | 360    |        |  46
40   | 10     | 100    |        |
41   | 11.2   | 448    |        |
42   | 12.6   | 500    |   N10  |  60
43   | 14     | 560    |        |
44   | 16     | 640    |        |
45   | 18     | 760    |        |  85

A. VDI is a textured finish usually from spark eroding, for polished finish you should work in microns, lower the number higher the polish.

A. VDI, while I am less aware of the call-out, is an EDM finish loosely related to the roughness of the resultant finish. SPE/SPI is a modified surface finish ranging from D-3 to A-1. D surfaces are media blasted. C surfaces are stone polished, B surfaces are paper abraded. A surfaces are diamond polished.

http://sftusa.com/polishing/spi-guidelines-chart.html ⇩

This page has the complete definitions for SPI finishes.

Ed. update 1/20/24: That site is not loading. Thankfully, The Internet Archive preserved a copy of the chart
These days information often appears on the internet, then disappears forever. If that bothers or frightens you please consider a donation to The Internet Archive.

A. See chart I obtained, with SPI, VDI and Ra Rz, in my flicker:

https://flic.kr/p/rnv6qr

A.
VDI=20*LOG(10*Ra)
Ra=10^((VDI-20)/20)
Rmax=4*Ra

A. Check out Xometry - Injection Moulding Surface Finishes : SPI and VDI

Gives handy SPI and VDI conversion charts.