INTRODUCTION-Factors that determine the performance of a manufactured product, other than mechanical and physical properties, include

Dimensions - linear or angular sizes of a component specified on the part drawing

Tolerances - allowable variations from the specified part dimensions that are permitted in manufacturing.

•A 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" Dimensions on part drawings represent nominal or basic sizes of the part and its features.

A 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.

Dimensioning can be divided into three categories: general dimensioning,geometric dimensioning, andsurface texture.

TYPES OF TOLERANCES

A Dimensional tolerance is the total amount a specific dimension is permitted to vary, which is the difference between maximum and minimum permitted limits of size.

A Geometric tolerance is the maximum or minimum variation from true geometric form or position that may be permitted in manufacture.

Geometric tolerance should be employed only for those requirements of a part critical to its functioning or interchangeability.

Variation is permitted in both positive and negative directions from the nominal dimension Possible for a bilateral tolerance to be unbalanced; for example, 2.500 +0.010, -0.005

Fig: Ways to specify tolerance limits for a nominal dimension of 2.500

Bilateral Tolerance

Unilateral ToleranceVariation from the specified dimension is permitted in only one direction Either positive or negative, but not both Fig: Ways to specify

tolerance limits for a nominal dimension of 2.500

TOLERANCE-

Dimensional tolerance - conventional

Geometric tolerance - modern

unilateral

bilateral

1.00nominal dimensiontolerance

0.95 + 0.10- 0.00 1.05 + 0.00

- 0.101.00 0.05

0.95 - 1.05means a range0.05

REASON OF HAVING TOLERANCE

The raw material may be defect. No manufacturing process is perfect. Nominal dimension (the "d" value) can not be achieved

exactly. Without tolerance we lose the control and as a

consequence cause functional or assembly failure. The work piece may be too slender to take the load.

Tolerances and Manufacturing Processes

Some manufacturing processes are inherently more accurate than others Examples:

Most machining processes are quite accurate, capable of tolerances = 0.05 mm ( 0.002 in.) or better

Sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts must be specified

DIMENSIONAL TOLERANCES (SIZE)Angular size dimension tolerance

It specifies the allowable variation on the size or gap formed by two angular elements of the shape.

Curved dimension tolerance

It is a tolerance on a dimension for a curved feature element measured along the entire path of the curve

Diameter dimension tolerance

It is the allowable variation of the size of a hole in a feature.

Radial Dimension Tolerance

It is the allowable variation for the radial distance from the center of a feature circular curve to a point on the curve.

Location Dimension Tolerance

It is the allowable variation in locating one feature of a point with respect to another.

Angular Dimension Tolerance

It defines the allowable variation in the angle between two elements of a feature.

The tolerance of size is normally defined as the difference between th upper and lower dimensions.

ISO 286 implements 20 grades of accuracy to satisfy the requirements of different industries.

Production of gauges and instruments.IT01, IT0, IT1, IT2, IT3, IT4, IT5, IT6.

Precision and general Industry.IT 5, IT6, IT7, IT8, I9, IT10, IT11, IT12.

Semi finished productsIT11, IT14, IT15, IT16.

Structural EngineeringIT16, IT17, IT18 .

Tolerance Grades

Preferred fits: A specified system of fits for holes and shafts for SI units

Tolerancing Holes and Shafts

- Hole basis •The minimum hole size equals the basic hole size• Uses the symbol “H” in the tolerance specification

- Shaft basis•The maximum shaft size equals the basic shaft size•Uses the symbol “h” in the tolerance specificationEx.-20H7f8 40H8f6

Geometrical Tolerances (Form)

Geometrical Tolerances (Orientation)

Geometrical Tolerances (Orientation)

INDIVIDUAL (No Datum

Reference)

INDIVIDUAL or RELATED

FEATURES

RELATED FEATURES

(Datum Reference Required)

GEOMETRIC CHARACTERISTIC CONTROLS

TYPE OFFEATURE

TYPE OFTOLERANCE CHARACTERISTIC SYMBOL

SYMMETRY

FLATNESS

STRAIGHTNESS

CIRCULARITY

CYLINDRICITY

LINE PROFILE

SURFACE PROFILE

PERPENDICULARITY

ANGULARITY

PARALLELISM

CIRCULAR RUNOUT

TOTAL RUNOUT

CONCENTRICITY

POSITION

FORM

PROFILE

ORIENTATION

RUNOUT

LOCATION

14 characteristics that may be controlled

FIT-

When two parts are to be assembled, the relation resulting from their sixes before assembly is called FIT.

TYPES OF FIT:-1.Clearance fit2. Transition fit3. Interferance fit

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Clearance and Interference fits between two Shafts and a Hole

Surface Texture

• Repetitive or random deviations from the nominal surface which form the pattern of the surface

The topography and geometric features of the surfaceWhen highly magnified, the surface is anything but straight and smooth. It has roughness, waviness, and Flaws

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

Surface RoughnessAverage of vertical deviations from nominal surface over a specified surface length

Figure - Deviations from nominal surface used in the two definitions of surface roughness.

Surface Roughness Specification

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

Elements of surface texture

CONCLUSIONIt is impossible to make a perfect component so when we design a part we specify the acceptable range of features that make-up the part.Traditionally, the tolerance allocation is done based on the hypothesis that the assembly process deals with infinitely rigid bodies. The resultant tolerance of individual components obtained based on this hypothesis will be on the tighter side, thereby increasing the manufacturing cost.In reality all the components of the assembly are deformable bodies and they undergo deformation due to inertia effects. Through finite element simulation, thevalues of deformation due to inertia effects like gravity, angular velocity and temperature effect have been determined in the design and process planning stage itself. Due to this, the tolerance requirements of the given assembly are relaxed to certain extent for critical components, resulting in reduced manufacturing cost and high product reliability.With this approach, the component tolerance values found are the most robust to variation during the product’s application.These benefits make it possible to create a high-quality and cost-effective tolerance design.