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

CHAPTER (2)

Dimensions, Tolerances, And Dimensions, Tolerances, And SurfacesSurfaces

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”.

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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.

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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);

BalancedUnbalanced

(b)

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Unilateral Tolerance

- Variation from the specified dimension is permitted in only one direction Either positive or negative, but not both.

(a)

+0.000-0.007

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Limit Dimensions

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

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Typical tolerance limits for various manufacturing processes

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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.

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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.

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Surface Technology

Concerned with:

Defining the characteristics of a surface

Surface texture

Surface integrity

Relationship between manufacturing processes

and characteristics of resulting surface.

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Metallic Part Surface

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

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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).

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Surface Texture

Repetitive and/or random deviations from the nominal surface of an object.

Surface texture features.

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Four Elements of Surface Texture

1. Roughness - small, finely‑spaced deviations from nominal surface Determined by material characteristics and

processes that formed the surface.

2. Waviness - deviations of much larger spacing Waviness deviations occur due to work

deflection, vibration, heat treatment, and similar factors.

Roughness is superimposed on waviness

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Four Elements of Surface Texture

3. Lay - predominant

direction or pattern

of the surface

texture.

Possible lays of a surface.15

Four Elements of Surface Texture

4. 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.

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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.

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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.

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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.

dxL

yR

mL

0 ma ∫

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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.

N

1i

ia N

yR

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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.

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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

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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

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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

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Surface Roughness Specification

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

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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 0.005 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.

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3- Optical Techniques

These techniques are based on light reflectance from the surface, light scatter or diffusion, and laser technology.

Measurement of Surface Roughness

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.

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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.

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- 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.

Surface Integrity

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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.

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Web site:

http://www.staff.zu.edu.eg/awafa/http://www.staff.zu.edu.eg/awafa/

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