# Dimensions, Tolerances, And Surfaces

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Dimensions, Tolerances, And Surfaces
CHAPTER (2) Dimensions, Tolerances, And Surfaces Dr. Ahmed Abou El-Wafa

Dimensions (ANSI Y14.5M‑1982):
Dimension is “a numerical value expressed in appropriate units of measure and indicated on a drawing and in other documents along with lines, symbols, and notes to define the size or geometric characteristic, or both, of a part or part feature”. 2

Tolerances (ANSI Y14.5M‑1982):
Tolerance is "the total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits". Variations occur in any manufacturing process, which are manifested as variations in part size. Tolerances are used to define the limits of the allowed variation. 3

Bilateral Tolerance Probably the most common Type.
Variation is permitted in both positive and negative directions from the nominal dimension. Two types are found; balanced (a) and unbalanced (b); Balanced Unbalanced (b) 4

Unilateral Tolerance (a)
- Variation from the specified dimension is permitted in only one direction Either positive or negative, but not both. (a) +0.000 -0.007 5

Limit Dimensions - Permissible variation in a part feature size consists of the maximum and minimum dimensions allowed. 6

Typical tolerance limits for various manufacturing processes
7

Surfaces Nominal surface – designer’s intended surface contour of part, defined by lines in the engineering drawing. The nominal surfaces appear as absolutely straight lines, ideal circles, round holes, and other edges and surfaces that are geometrically perfect Actual surfaces of a part are determined by the manufacturing processes used to make it. Variety of processes result in wide variations in surface characteristics. 8

Why surfaces are commercially and technologically important
Aesthetic reasons Surfaces affect safety Friction and wear depend on surface characteristics Surfaces affect mechanical and physical properties Assembly of parts is affected by their surfaces Smooth surfaces make better electrical contacts. 9

Surface Technology Concerned with:
Defining the characteristics of a surface Surface texture Surface integrity Relationship between manufacturing processes and characteristics of resulting surface. 10

A magnified cross‑section of a typical metallic part surface.
11

Surface Texture The topography and geometric features of the surface
When highly magnified, the surface is anything but straight and smooth It has roughness, waviness, and flaws, It also possesses a pattern and/or direction resulting from the mechanical process that produced it (lay). 12

Surface Texture Repetitive and/or random deviations from the nominal surface of an object. Surface texture features. 13

Four Elements of Surface Texture
Roughness - small, finely‑spaced deviations from nominal surface Determined by material characteristics and processes that formed the surface. Waviness - deviations of much larger spacing Waviness deviations occur due to work deflection, vibration, heat treatment, and similar factors. Roughness is superimposed on waviness 14

Four Elements of Surface Texture
Lay - predominant direction or pattern of the surface texture. Possible lays of a surface. 15

Four Elements of Surface Texture
Flaws - irregularities that occur occasionally on the surface Includes cracks, scratches, inclusions, and similar defects in the surface Although some flaws relate to surface texture, they also affect surface integrity. 16

Surface Roughness and Surface Finish
Surface roughness - a measurable characteristic based on roughness deviations. Surface finish - a more subjective term denoting smoothness and general quality of a surface. In popular usage, surface finish is often used as a synonym for surface roughness Both terms are within the scope of surface texture. 17

Surface Roughness Average of vertical deviations from nominal surface over a specified surface length Deviations from nominal surface used in the two definitions of surface roughness. 18

Surface Roughness Parameters
Arithmetic average (AA) based on absolute values of deviations, and is referred to as average roughness where Ra = average roughness; y = vertical deviation from nominal surface (absolute value); and Lm = specified distance over which the surface deviations are measured. 19

Alternative Surface Roughness Equation
Approximation of previous equation is perhaps easier to comprehend where Ra has the same meaning as above; yi = vertical deviations (absolute value) identified by subscript i; and N = number of deviations included in Lm. 20

Cutoff Length A problem with the Ra computation is that waviness may get included To deal with this problem, a parameter called the cutoff length is used as a filter to separate waviness from roughness deviations Cutoff length is a sampling distance along the surface A sampling distance shorter than the waviness eliminates waviness deviations and only includes roughness deviations. 21

Roughness parameter Rq (Rs)
Rq or Rs ………Root Mean Square Roughness Older terms: RMS ...Root Mean Square, Still widely used in some industries (i.e. optical) Rq is the Root Mean Square of the distance of the filtered or unfiltered Roughness Profile from its mean line. Where: N is the number of Data points 22

Roughness parameter Rt (Rmax)
Rt or Rmax ….. Maximum Peak to Valley Height Rt is the distance between the highest Peak to the lowest Valley in the assessment length. Peak Valley 23

Roughness parameter Rz
Rz ….. Average Peak to Valley Height is also known as the ISO 10 point height parameter. It is numerically the average height difference between the five highest peaks and the five lowest valleys within the assessment length. Rz 24

Surface Roughness Specification
Surface texture symbols in engineering drawings: (a) the symbol, and (b) symbol with identification labels. 25

Measurement of Surface Roughness
1- Standard Test Surfaces The surface is compared to the standard surface finish blocks both visually and by the finger nail test. The user judge which standard is closest to the specimen. 2- Stylus Instruments A cone-shaped diamond (or hardened steel) stylus with point radius of about mm and 90º tip angle is traversed across the test surface at a constant slow speed. The vertical movement is converted into an electronic signal that represents the topography of the surface. 26

Measurement of Surface Roughness
3- Optical Techniques These techniques are based on light reflectance from the surface, light scatter or diffusion, and laser technology. They are useful in applications where stylus contact with the surface is undesirable. Some of the techniques permit very high speed operation, thus making 100% inspection feasible. 27

Some processes are inherently capable of producing better surfaces than others;
Typical range of surface roughness values are given in μm (μin.). 28

Surface Integrity Surface texture alone does not completely describe a surface There may be metallurgical changes in the altered layer beneath the surface that can have a significant effect on a material's mechanical properties Surface integrity is the study and control of this subsurface layer and the changes in it that occur during processing which may influence the performance of the finished part or product. 29

Surface Integrity - Evaluation of Surface Integrity
Surface integrity is more difficult to assess than surface roughness. Some techniques to inspect for subsurface changes are destructive to the material specimen. Evaluation techniques include the following: 1- Surface texture: Surface roughness, designation of lay, and other measures provide superficial data on surface integrity. 2- Visual examination: Can reveal various surface flaws such as cracks, craters, laps, and seams. It is augmented by fluorescent and photographic techniques. 3- Microstructural examination: This involves preparing cross sections and obtaining photomicrographs for examination of microstructure in the surface layers compared with the substrate. 30

Surface Integrity 4- Microhardness profile: The part is sectioned, and hardness is plotted against distance below the surface to obtain a hardness profile of the cross section. 5- Residual stress profile: X-ray diffraction techniques can be employed to measure residual stresses in the surface layers of a part. 31

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