Surface Texture PDF

Surface Texture  Surface Texture 1- Introduction Repetitive or random deviations from the normal surface which form the pattern of the surface. Surface texture includes roughness, waviness, lay and flows. Every part’s surface is made up of texture and roughness which varies due...

Surface Texture  Surface Texture 1- Introduction Repetitive or random deviations from the normal surface which form the pattern of the surface. Surface texture includes roughness, waviness, lay and flows. Every part’s surface is made up of texture and roughness which varies due to manufacturing techniques and the part structure itself. There can be many ways; throw which the surface roughness plots can be interpreted. Because of this reason only, there are varieties of methods prevailing in the engineering world for indicating surface roughness values in the drawing. As per the data, worldwide more than 128 conventions are available for interpreting above graph and presenting the results in the drawing! The most common and popular method amongst all is interpreting thro average roughness indication. This is known as Ra – in which, R stands for „Roughness‟ and „a‟ stands for „average‟. The other methods are Rz, Rt, Rmax etc. Below is the list of popular parameters: It is a common joke in surface finish circles that ―RA‖ stands for Regular Army and ―Ra‖ is the chemical symbol for Radium; only ―Ra‖ is the average roughness of a surface. This emphasizes that the ―a‖ is a subscript. 2- Surface roughness indication in drawing The earlier method of indicating surface roughness through triangle symbols is now superseded, however still many companies are following the older practice. Below is the comparison table for all 3 ways of indicating surface roughness symbol in the drawing. 3- Indication of surface texture The basic symbol consists of two legs of unequal length inclined at approximately 60 degrees to the line representing the considered surface The symbol must be represented by thin line If the removal of material by machining is required, a bar is added to the basic symbol, If the removal of material is not permitted, a circle is added to the basic symbol. When special surface characteristics have to be indicated, a line is added to the longer arm of any of the above symbols, The value or values defining the principal criterion of roughness are added to the symbols If it is necessary to impose maximum and minimum limits of the principal criterion of surface roughness, both values shall be shown If it is required that the required surface texture be produced by one particular production method, this method shall be indicated in plain language on an extension of the longer arm of the symbol Complete symbol ` 4- Graphical symbol for “all surfaces around a workpiece outline” When the same surface texture is required on all surfaces around a workpiece outline (integral features), represented on the drawing by a closed outline of the workpiece, a circle shall be added to the complete graphical symbol illustrated in as shown in Figure.1. Fig.1:Surface texture requirement for all six surfaces represented by outline on workpiece. 5-Position on drawings and other technical product documentation 5-1- General Surface texture requirements shall be indicated only once for a given surface and, if possible, on the same view where the size or location, or both, are indicated and toleranced. Position and orientation of graphical symbol and its annotation. 5-2 - Position and orientation of graphical symbol and its annotation a- General The general rule is that the graphical symbol together with the complementary information shall be oriented so that they are readable from the bottom or right-hand side of the drawing, in accordance with ISO 129-1 (see Figure 2). Fig.2: Direction of reading of surface texture requirements b- On outline or by reference line and leader line The surface texture requirement (graphical symbol) shall touch the surface or be connected to it by means of a reference/leader line terminating in an arrowhead. As a general rule, the graphical symbol, or the leader line terminating in an arrowhead (or other relevant terminator), shall point at the surface from outside the material of the work piece — either to the outline (representing the surface) or the extension of it (see Figures 3and 4). Fig.3: Surface texture requirements on contour line representing surface  Fig.4: Alternative use of reference lines and leader lines c- On dimension line in connection with feature-of-size dimension If there is no risk of misinterpretation, the surface texture requirement may be indicated in connection with the dimensions given, as shown in Figure 5. Fig.5: Surface texture requirement — Feature-of-size dimension d- On tolerance frame for geometrical tolerances The surface texture requirement may be placed on top of the tolerance frame for geometrical tolerances (according to ISO 1101), as shown in Figure 6,( a) and (b). Fig.6: Surface texture requirement — Geometrical tolerances indication e- On extension lines The surface texture requirement may be directly placed on extension lines or be connected to it by a reference/leader line terminating in an arrowhead, as shown in Figures 3 and 7. f- Cylindrical and prismatic surfaces Cylindrical as well as prismatic surfaces may be specified only once if indicated by a centreline and if each prismatic surface has the same surface texture requirement (see Figure 7). However, each prismatic surface shall be indicated separately if different surface textures are required on the individual prismatic surfaces (see Figure 8). Fig.7: Surface texture requirement -Extension lines of cylindrical features Fig.8: Surface texture requirements — Cylindrical and prismatic surfaces 6-Indication of two or more manufacturing methods If it is necessary to define surface texture both before and after treatment, this shall be explained in a note or in accordance with Figure 9. Fig.9: Indication of surface texture requirement before and after treatment (in this case, coating)