What is "VDI" surface finish? Comparison to Ra?

A. Hi. VDI = Verein Deutscher Ingenieure, the Society of German Engineers. There are VDI3400 scales for surface finishes, which are widely used in molds, where tactile & visual texture is to be specified. Yes these textures are often achieved with EDM processes.

Ra is concerned with smoothness; it would be impractical to try to specify a textured finish with the Ra scale. While texturing can possibly offer some limited concealment of surface flaws, a good high quality mold surface is required before texturing, so there may be a relationship between a VDI340 finish and the Ra finish which is needed along the way. See our on-line library article "Textured Molds: High Quality Surface Finishes Make the Difference".

Regards,

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

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 ⇩

Update: The above link is broken. Thankfully, the Wayback Machine saved a copy here.

Please consider a donation to the Internet Archive; it's priceless in this age of temporary, changeable digital records, but is under constant costly legal attacks.

A. I had 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. Hi Eric. We moved your inquiry to an existing thread which explains that VDI3400 is a standard for visual & tactile surface texture rather than smoothness -- often used in the molding field, and the EDM field which is used to texture the molds. VDI33 is one of the classes of surface texture within the VDI3400 standard

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.

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

Profilometers

on Amazon or eBay
(affil link)

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

Profilometer: Mahr Pocket-Surf

on eBay or Amazon
(affil link)

Hi, Simon. Thanks!     🙂

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.

A. VDI = 20log(10Ra)

A. Nr is the number of roughness or the class, using VDI d

Nr=20*log(10*Ra)

Try with the equivalence table.

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, 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.

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

This page has the complete definitions for SPI finishes.

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

https://flic.kr/p/rnv6qr

Ed. update May 2026: This link still works, thanks Andrew  🙂
... but boy do we hate this trend of "short URLs"! The Internet Archive doesn't archive such links, so there's no permanent record.
And hackers try to buy expired URLs posted on sites like ours because people are less wary of following links to malware if they come from trusted sites; it's harder for us to whack-a-mole, and more dangerous with "short URLs" which don't even hint at where they lead, so they are especially attractive to hackers.

A. Check out Xometry - "Injection Moulding Surface Finishes : SPI and VDI" at
https://xometry.pro/en-eu/articles/injection-molding-finishes/
Gives handy SPI and VDI conversion charts.

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

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