Table of Contents

What is Surface Finish- Surface Finish Symbol

Introduction to Surface Roughness, Surface Finish:

Whatever may be the manufacturing process used, it is not possible to produce perfectly smooth surface. The imperfections and irregularities are bound to occur. The manufactured surface always departs from the absolute perfection to some extent. The irregularities on the surface are in the form of succession of hills and valleys varying in height and spacing. These irregularities are usually termed as surface roughness, surface finish, surface texture or surface quality. These irregularities are responsible to a great extent for the appearance of a surface of a component and its suitability for an intended application.

Factors Affecting Surface Roughness:-

The following factors affect the surface roughness:
(1) Vibrations
(2) Material of the workpiece
(3) Type of machining.
(4) Rigidity of the system consisting of machine tool, fixture cutting tool and work
(5) Type, form, material and sharpness of cutting tool
(6) Cutting conditions i.e., feed, speed and depth of cut
(7) Type of coolant used

Reasons for Controlling Surface Texture:-

(1) To improve the service life of the components
(2) To improve the fatigue resistance
(3) To reduce initial wear of parts
(4) To have a close dimensional tolerance on the parts
(5) To reduce frictional wear
(6) To reduce corrosion by minimizing depth of irregularities
(7) For good appearance

Element Of Surface Texture:

The various elements of surface texture can be defined and explained with the help of fig which shows a typical surface highly magnified.

element of surface roughness , direction of lay

Roughness Height: This is rated as the arithmetical average deviation expressed in micro-meters normal to an imaginary centre line, running through the roughness profile.
Roughness Width: Roughness width is the distance parallel , to the normal surface between successive peaks or ridges that constitutes the predominant pattern of the roughness.

Waviness: Waviness consists of those surface irregularities which are of greater spacing than roughness and it occurs in the form of waves. These are also termed as moon geometrical errors and constitute irregularities of first and second order. These are caused `due to misalignment of centres, vibrations, machine or work deflections, warping etc.

Lay:

It is the direction of predominant surface pattern produced by tool marks or scratches. It is determined by the method of production used. Symbols used to indicate the direction of lay are given below:
| | = Lay parallel to the boundary line of the nominal surface that is, lay parallel to the line representing surface to which the symbol is applied e.g., parallel shaping, end view of turning and O.D grinding.
⊥= Lay perpendicular to the boundary line .of the nominal surface, that is lay perpendicular to the line representing surface to which the symbol is applied, e.g. , end view of shaping, longitudinal view of turning and O.D. grinding.
X = Lay angular in both directions to the line representing the surface to which symbol is applied, e.g. traversed end mill, side wheel grinding.
M= Lay multidirectional e.g. lapping super finishing, honing.

C= Lay approximately circular relative to the centre of the surface to which the symbol is applied e.g., facing on a lathe.
R= Lay approximately radial relative to the centre of the surface to which the symbol is applied, e.g., surface ground on a turntable, fly cut and indexed on end mill.

Sampling length: It is the length of the profile necessary for the evaluation of the irregularities to be taken into account. It is also known as cut-off length. It is measured in a direction parallelogram general direction of the profile. The sampling length should bear some relation to the type of profile.

Conventional Method for Designing Surface finish:

As per IS: 696 surface texture specified by indicating the following
(a) Roughness value i.e., Ra value in mm
(b) Machining allowance in mm.
(c) Sampling length or instrument cut-off length in mm.
(d) Machining production method, and
(e) Direction of lay in the symbol form as = ⊥, X, M, C, R

ISI Symbols for Indication of surface Finish

The surface roughness is represented in figure. If the matching method is milling, .sampling length is 2.5 mm, direction of lay is parallel to the surface, machining allowance is 3 mm and the representative will be as shown in figure,