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Surface Roughness

 
Surface finishes recommended for Portable Hardness Testers.
 
Table 1: Maximum surface finish according to the Vickers standard.

 

Rebound D-type probe
(most common)
3 mm ball
11 Nmm

Rebound G-type probe
(for castings & rough surfaces)
6 mm ball,
100 Nmm

Ultrasonic
(Most common)
50 N
5 kp

Ultrasonic
(light load)
10 N
1 kp

Rt value

<=10µm

<=30µm

<=10µm

<=5µm
 
 
 
 
 
 
 
 
 
Common expressions for surface roughness are:

Standard grit reference ( Grit No.)

- refers to the grit of a surface finishing medium or method, which does not provide a consistent measure of roughness, since results depend on a part's material, finishing method, lubricant used (if any), and applied work pressure.

N

- New ISO (Grade) Scale numbers. These are used on manufacturing drawings that specify surface finish in terms of an ISO standard. Each roughness grade number can be correlated to a specific Ra number that is expressed in microns.

Ra

- Roughness average, most commonly expressed in micrometers (microns). This is the most universally recognized and used international standard of roughness measurements. It is the arithmetic mean of the absolute departures of a roughness profile from the mean line of the measurement. Ra may also be expressed in micro inches.

CLA

- Center Line Average in micro-inches. This is a conversion using Ra(μm) x 39.37.

RMS

- Root Mean Square in micro-meters or micro-inches; i.e.., the average of peaks and valleys of a material's surface profile as calculated from a number (n) of measurements (x) along the sampling length:
RMS

Rp

- Maximum profile peak height

RSm

- The mean spacing between profile peaks on the mean line, measured along the sampling length.

Rt

- The total height of a roughness profile, typically expressed in microns, is the maximum peak-to-valley height along the assessment length.
Table 2. Conversion chart for for International Roughness Specification
Grit No. ISO No. Ra (μm) Ra (μin.) CLA (μin.) RMS (μin.)1 Rt (μm)2
----------- N12 50 2000 2000 2200 200
----------- N11 25 1000 1000 1100 100
----------- N10 12.5 500 500 550 50
60 N9 6.30 250 250 275 25
----------- N8 3.20 125 125 137.5 13
80 ----------- 1.80 71 71 78 9.0
----------- N7 1.60 63 63 64.3 8.0
120 ----------- 1.32 52 52 58 6.6
150 ----------- 1.06 42 42 46 5.3
----------- N6 0.80 32 32 32.5 4.0
180 ----------- 0.76 30 30 33 3.8
220 ----------- 0.48 19 19 21 2.4
----------- N5 0.40 16 15 17.6 2.0
240 ----------- 0.38 15 12 17 1.9
320 ----------- 0.30 12 9 14 1.5
400 ----------- 0.23 9 8 10 1.3
----------- N4 0.20 8 4 8.8 1.2
500 N3 0.10 4 2 4.4 0.8
----------- N2 0.05 2 1 2.2 0.5
----------- N1 0.025 1 1 1.1 0.3


Table 3. Manufactured parts, surface finish.

Manaufacturing surface finishes.

Source: Wikipedia: https://en.wikipedia.org/wiki/Surface_finish

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