Surface Roughness In Machining: The Ultimate Guide

Definition of surface roughness

It is quantified by the deviation between the direction of the normal vector of the real surface and its ideal form.

It is understood that the machined surface has small spacing and small unevenness of peaks and valleys.

The distance (wave distance) between two wave peaks or two wave valleys is very small (below 1mm), which belongs to micro geometric error.

If these deviations are large, the surface is rough;

If they are small, the surface is smooth.

Theoretical (geometric) surface roughness.

How to obtain the surface roughness value?

The theoretical surface roughness during turning represents the roughness value under the minimum cutting condition.

The formula is as follows:

surface roughness formula

In the above formula:

Rzh: theoretical surface roughness, unit: µ m.

f: Feed per revolution, in mm / rev.

The feed rate is calculated as follows:

feed rate

In the above formula

L is the cutting length per minute, in mm / min.

N is the main axis spindle speed, in min-1.

For example, when the spindle speed is 500min-1 and the cutting length per minute is 120mm / min, what is the feed rate per revolution?

Answer: substitute n = 500 and I = 120 into the formula.

That is 0.24 mm / rev.

Rrε: Also recorded as re, corner radius of insert, unit: mm.

To understand the above formula, refer to the figure below.

Ra, Rz and Ry definitions

Ra: arithmetic mean deviation of contour, take the arithmetic mean of the absolute value of contour offset in degree L. Main evaluation parameters.

The average height between the peak and valley is R5 degrees, and the average height between the peak and valley is 5 degrees.

Generally, it is only used to represent relatively short and small surfaces.

Ry: the maximum height of the contour, taking the distance between the contour peak line and the contour bottom line in degree L.

It is basically not used alone. It can be regarded as a limit deviation value.

Arithmetic mean deviation Ra

Ra is calculated as follows:

Ra calculation formula

When the roughness curve is represented by y = f (x), take the x-axis as the average line direction and the y-axis as the vertical magnification of the roughness curve.

When the magnification is within the range of the sampling reference length “ℓ”, it is represented by microns.

Ra can objectively reflect the geometric characteristics of the measured contour.

Ra value can be measured directly by electric profiler, but it is not intuitive.

Ra value

Micro unevenness ten point height Rz

Rz is calculated as follows:

Rz can be obtained from the average distance between the average line and the five peaks (Yp) and the average value of each distance (Yv) between the average line and the five valleys (in microns).

Rz is used to evaluate the height parameters of surface roughness, which is intuitive and easy to measure on optical instruments, but it is limited to reflect the geometric characteristics of the measured contour.

Yp1, Yp2, YP3, yp4 and yp5 in the above figure are the distance from the average line to the highest five peaks within the sampling reference length “ℓ”.

Yv1, yv2, yv3, yv4 and yv5 are the distance from the average line to the lowest 5m valley within the sampling reference length “ℓ”.

Maximum height of contour Ry

Ry is calculated as follows:

Ry is derived from the distance (in microns) between the highest peak and the lowest valley within the sampling reference length (ℓ) to the mean line direction of the roughness curve.

Parameter Ry is easy to measure.

When the measured surface is very small and RZ is not suitable for evaluation, Ry can be used.

Maximum height of contour Ry

Root mean square RMS

The RMS is calculated as follows:

RMS Calculation Formula

In the above formula:

L, the evaluation length

Z (x), as a function of contour height

The formula is approximately equivalent to:

RMS is the root mean square of the deviation of the contour length from the mean line and is recorded in the evaluation length.

Ra and RMS are both expressions of surface roughness, but their calculation methods are different.

Ra is calculated as the average peak and valley of the micro roughness of the surface.

RMS is calculated as the root mean square of the microscopic root mean square of the surface.

Each value is measured using the same surface peak and valley height measurements, but using different formulas.

Symbols and changes of Ra, Rz and Ry in JIS standard

TypeAccording to JIS B 0601-1994 standardAccording to JIS B 0601-2001 standard
Maximum height of the profilesRyRz
Ten point height of irregularities  RzRzJIS
Arithmetic mean deviation of contourRaRa

National definitions of surface accuracy and industrial standards (part)

China:

GB / T 3505 2000 surface roughness terms and parameters;

GB / T 1031-1995 surface roughness parameters and their values;

GB / T 131-1993 mechanical drawing surface roughness symbols, codes and notes.

Japan:

JIS B 0601-2001, definition and name, jisb diagram. Surface roughness.

JIS B 0601-1982, definition and name, jisb diagram. Surface roughness, old version.

JIS B 0031-1982, definition and name, jisb diagram. Surface roughness, old version.

JIS B 0031-1994, definition and name, jisb diagram. Surface roughness, old version.

JIS B 0601-1994, definition and name, jisb diagram. Surface roughness, old version.

Britain:

BS EN ISO 4287:2000, definition of common parameters.

Germany:

Din 4768 (1990) standard for surface roughness in various fields (including metal castings).

Din 4771 (1977) measures the surface contour height P ₜ.

Din 4775 (1982) measures the surface roughness of the workpiece.

Din 4776 (1990) measuring surface roughness; Parameters R 1, R 4, RVK, MR1, MR2 are used to describe the material part in the roughness profile (profile bearing length ratio); Measurement conditions and evaluation procedures.

Din 4777 (1982) surface measurement; Contour filter of electric contact needle measuring instrument; Phase correction filter.

DIN ISO 1302 standard for surface roughness in various fields, including metal castings.

France:

NF e05-015 (1984), geometrical product specifications (GPS) – surface texture: contour method – terms, definitions and surface texture parameters

NF e05-016 (1978), geometric product specification (GPS) – surface texture indication in technical product documentation

NF e05-017 (1972), surface condition of products and surface measurement method.

Europe:

EN 10049 2005, the European standard defines the measurement conditions of surface roughness parameters of metal flat products not coated (cold rolled and hot rolled pickled steel) and coated with metal coatings (e.g. zinc, aluminum, tin, chromium).

American national standard:

ASME B46. 1-2009, surface texture (surface roughness, waviness and position).

ASME B46. 1-1995, surface texture (surface roughness, waviness and position), old edition.

ASME B46. 1-1985, surface texture (surface roughness, waviness and position), old edition.

ANSI B4. 2-1978, surface texture. Surface roughness, undulation and undulation.

International standards:

ISO 1320-2002, the main standard, provides the definition of common parameters.

ISO 25178-71:2012, geometrical specifications (GPS), surface texture

ISO 468-1982, surface roughness – parameters, their values and general rules for specifying requirements

ISO 4287-1997, definitions of common parameters, such as those given by Rx.

ISO 4287/1-1984,

ISO 4288-1996, geometrical product specifications, contour method – rules and procedures for surface texture evaluation

ISO 4288-1985,

ISO 1320-1978,

ISO 12085-1996, contour method – pattern parameters.

Symbols and codes of surface roughness

Basic concepts

As shown in the figure below, the surface roughness value, cut-off value or reference length, processing method, grain direction, surface fluctuation, etc. are represented around the surface symbol.

Basic concepts of surface roughness

a. Ra value

b. Processing mode

c. Critical value, evaluation length

C ´, reference length, evaluation length

d. Texture direction

f. Parameters other than Ra (e.g. parameters / cut-off level)

g. Surface fluctuation

Basic symbol style (JIS B 0610 and DIN ISO 1302, DIN 4768)

Basic symbol style (JIS B 0610 and DIN ISO 1302, DIN 4768)

Materials to be removed during processing (JIS B 0610 and DIN ISO 1302, DIN 4768)

Materials to be removed during processing (JIS B 0610 and DIN ISO 1302, DIN 4768)

It is forbidden to remove materials during processing (JIS B 0610 and DIN ISO 1302, DIN 4768).

Preset the upper limit of RA (JIS B 0610 and DIN ISO 1302, DIN 4768)

Calibrate texture direction (JIS B 0610 and DIN ISO 1302, DIN 4768)

Preset the upper and lower limits of Ra (JIS B 0610 and DIN ISO 1302, DIN 4768)

Calibration processing method (JIS B 0610 and DIN ISO 1302, DIN 4768)

Comparison table of triangular symbols of surface roughness with Ra, Rz and Ry

Triangle symbol is a simple way to mark the accuracy grade of machined surface in machining design.

It should be noted that the finishing symbols (triangle ▽ and wave ~) are cancelled from the JIS standard revised in 1994.

Here is a brief introduction to the triangle symbol.

Ra unit µmRy unit µmRzJIS unit µmTriangle symbol
0.0025
0.05
0.1
0.2
0.1
0.2
0.4
0.8
0.1
0.2
0.4
0.8
∇∇∇∇
0.4
0.8
1.6
1.6
3.2
6.3
1.6
3.2
6.3
∇∇∇
3.2
6.3
12.5
25
12.5
25
∇∇
12.5
25
50
100
50
100

Comparison table of surface roughness

Rz (µm)VDI-3400Ra (µm)RMS (µinch)Class ISO 1302
0.080.01250.5 
0.160.0251 
0.320.052 
0.6400.104.1N3
0.7010.114.5 
0.7620.124.9 
0.9030.145.7 
1.0240.156.6 
1.1550.177.4 
1.2860.208.2N4
1.4170.239.0 
1.6080.2510.2 
1.7990.2811.5 
2.05100.3213.1 
2.24110.3514.3 
2.56120.4016.4N5
2.88130.4518.4 
3.20140.5020.5 
3.58150.5622.9 
4.03160.6325.8 
4.48170.7028.7 
5.12180.8032.8 
5.76190.9036.9N6
6.40201.0041.0 
7.17211.1245.8 
8.06221.2651.6 
8.96231.4057.3 
10.24241.6065.5 
11.52251.8073.7N7
12.80262.0081.9 
14.08272.2090.1 
16.00282.50102.4 
17.92292.80114.6 
20.48303.20131.1N8
22.40313.50143.4 
25.60324.00163.8 
334.50 
32.00345.00204.8 
355.60 
40.32366.30258.0N9
377.00 
51.20388.00327.7 
399.00 
64.004010.00409.6 
4111.20 
4212.60N10
4314.00 
4416.00 
4518.00 

*VDI:Verein Deutscher Ingenieure 

*RMS:Root mean square 

Comparison table of processing methods and processing accuracy

Symbol

  • ○ Default machining (standard)
  • ● Precision
  • ◇ Rough machining
  • ◈ Medium
  • ◆ Fine

Ra arithmetic mean

0.025

0.05

0.1

0.2

0.4

0.8

1.6

3.2

6.3

12.5

25

50

100

Ry maximum peak

0.1

0.2

0.4

0.8

1.6

3.2

6.3

3.2

12.5

25

100

200

400

Evaluation length mm

0.25

0.8

2.5

8

25

Triangle indication

∇∇∇∇

∇∇∇

∇∇

Processing technology

 

 

Forging

 

 

 

 

 

 

 

Casting

 

 

 

 

 

 

 

Die casting

 

 

 

 

 

 

 

 

 

Cold rolling

 

 

 

 

 

 

 

Drawing

 

 

 

 

 

 

 

 

 

Extruding

 

 

 

 

 

 

 

 

 

Tumbling 

 

 

 

 

 

 

 

 

 

Sandblasting

 

 

 

 

 

 

 

 

 

 

Rolling

 

 

 

 

 

 

 

 

 

 

Face cutter grinding

 

 

 

 

 

 

 

Planing

 

 

 

 

 

 

 

 

 

Carving Slotting

 

 

 

 

 

 

 

 

 

Cutter grinding

 

 

 

 

 

 

 

Precision boring

 

 

 

 

 

 

 

 

 

Filing

 

 

 

 

 

 

 

 

Round grinding

 

 

Boring

 

 

 

 

 

 

 

 

Drilling

 

 

 

 

 

 

 

 

 

 

Reaming

 

 

 

 

 

 

 

 

Broach grinding

 

 

 

 

 

 

 

 

Shaving

 

 

 

 

 

 

 

 

 

Grinding

 

 

Hone finishing

 

 

 

 

 

 

 

 

Super finishing

 

 

 

 

 

 

 

 

 

Buffing

 

 

 

 

 

 

 

Paper finishing

 

 

 

 

 

 

 

 

 

Lapping

 

 

 

 

 

 

 

 

 

Liquid honing

 

 

 

 

 

 

 

 

Burnishing

 

 

 

 

 

 

 

 

 

 

Surface rolling

 

 

 

 

 

 

 

 

 

 

Electric discharge carving

 

 

 

 

 

 

 

 

Wire cut electric spark

 

 

 

 

 

 

 

 

 

 

Chemical polishing

 

 

 

 

 

 

 

 

Electrolytic abrasion

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
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