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Surface Finish Degree (Smoothness) Comparison Table
between Rz, Ra and RMS for Iron and Steel Castings

What standard can be used for the surface finish degree, or we called smoothness degree for the iron and steel castings?

The following comparison table was organized by us. You can find the comparison for the new standard in China for the Ra and Rz, and the micron, microinch Ra in USA.  In addtion, we also lised the old grades in China for your reference. The old grades are still widely used in China. We usually called the Downward-pointing Triangle as "hwar". However, we do not suggest the purchasers or buyers to denoted the China old grades on the drawings or inquirements. The comparison is just for your reference, should not be taken as the inspection standard.

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China Old Grade

China New
Ra

China New
Rz

USA micron
Ra

USA microinch
Ra

1

50

200

 

 

2

25

100

 

 

3

12.5

50

 

 

4

6.3

25

8.00

320

6.30

250

5

3.2

12.5

5.00

200

4.00

160

3.20

125

6

1.6

6.3

2.50

100

2.00

80

1.60

63

7

0.8

6.3

1.25

50

1.00

40

0.80

32

8

0.4

3.2

0.63

25

0.50

20

0.40

16

9

0.2

1.6

0.20

12.5

10

8

10

0.1

0.8

0.10

 

  

RMS (microinch)

RMS
(µm)

RA (microinch)

RA
(µm)

Grit Size

80 58
47
34
17
14

2.03 1.47
1.2
0.6
0.43
0.36

71 52
42
30
15
12

1.90 1.32
1.06
0.76
0.38
0.30

80 120
150
180
240
320

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