Understanding Dimensions, Tolerances, and Surfaces in Manufacturing

ENM202 ANADOLU U N I V E R S I T Y Industrial Engineering Department Lecture 3 – Dimensions, Tolerances and Surfaces Spring 2007 Saleh AMAITIK

Manufacturing Processes Affected Areas Tolerance & Surfaces Product Design Quality Control Manufacturing Spring 2005

Manufacturing Processes Dimensions, Tolerances and Surfaces In addition to mechanical and physical properties, other factors that determine the performance of a manufactured product 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. Surfaces - surface finishes obtained by manufacturing process that produce the shape Spring 2005

Manufacturing Processes Dimensions 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. The dimension indicates the part size desired by the designer, if the part could be made with no errors or variations in the manufacturing process Spring 2005

Manufacturing Processes Tolerances 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 Spring 2005

Manufacturing Processes Why is tolerancing necessary? It is impossible to manufacture a part to an exact size or geometry Assemblies: Parts will often not fit together if their dimensions do not fall within a certain range of values Tolerances are needed to control the dimensions of any two mating parts so that any two parts may be interchangeable. Tolerances on parts contribute to the expense of a part, The smaller the tolerance the more expensive the part Spring 2005

Manufacturing Processes 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 toleranceis 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. Spring 2005

Manufacturing Processes How Is Tolerance Specified? Tolerances can be expressed in Several ways: General Tolerances A note may be placed on the drawing which specifies the tolerance for all dimensions except where individually specified ALL DECIMAL DIMENSIONS TO BE HELD TO ±0.020 Specific Tolerances The tolerance for a single dimension may be specified with the dimension based on one of the following methods • Limits • Unilateral tolerance • Bilateral tolerance Spring 2005

Manufacturing Processes Specific Tolerances Limit Dimensions Permissible variation in a part feature size, consisting of the maximum and minimum dimensions allowed Spring 2005

Manufacturing Processes Specific Tolerances Unilateral Tolerance Variation from the specified dimension is permitted in only one direction, either positive or negative, but not both. Spring 2005

Manufacturing Processes Specific Tolerances Bilateral Tolerance Variation is permitted in both positive and negative directions from the nominal dimension. It is possible for a bilateral tolerance to be unbalanced; for example, 2.500 +0.010, -0.005 Spring 2005

Manufacturing Processes 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. Spring 2005

Manufacturing Processes Dimensional Tolerances (Size) 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. Spring 2005

Manufacturing Processes Geometrical Tolerances (Form) Spring 2005

Manufacturing Processes Geometrical Tolerances (Location) Spring 2005

Manufacturing Processes Geometrical Tolerances (Orientation) Spring 2005

Manufacturing Processes Tolerance Grades 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 products IT11, IT14, IT15, IT16. Structural Engineering IT16, IT17, IT18 . Spring 2005

Manufacturing Processes ISO Tolerance Band "T "micrometres = (m-6) based on ISO 286 IT Grades 1 to 14 Spring 2005

Manufacturing Processes Fits between Mating Parts Fit is the general term used to signify the range of Looseness and Tightness of mating parts Spring 2005

Manufacturing Processes Important Terms Nominal size: a dimension used to describe the general size. Basic size: the theoretical size used as a starting point for the application of tolerances. Actual size: the measured size of the finished part after machining. Limits: the maximum and minimum sizes shown by the toleranced dimension Allowances: the minimum clearance or maximum interference between parts, or the tightest fit between two mating parts. Tolerance (Tolerance zone): the total allowable variation in a dimension; the difference between the upper and lower limits Spring 2005

Manufacturing Processes Tolerancing Holes and Shafts Preferred fits: A specified system of fits for holes and shafts for SI units • - 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 specification Spring 2005

Manufacturing Processes Fits Types The most common types of fit found in industry Clearance Fit occurs when two toleranced mating parts will always leave a space or clearance when assembled. Interference Fit occurs when two toleranced mating parts will always interfere when assembled. Transition Fit occurs when two toleranced mating parts are sometimes an interference fit and sometimes a clearance fit when assembled. Spring 2005

Manufacturing Processes Clearance and Interference fits between two Shafts and a Hole Spring 2005

Manufacturing Processes Transition fit between a Shaft and a Hole Spring 2005

Manufacturing Processes Fit Type Determination If feature A of one part is to be inserted into or mated with feature B of another part, the type of fit can be determined by the following: The Loosest Fit is the difference between the smallest feature A and the largest feature B. The Tightest Fit is the difference between the largest feature A and the smallest feature B. Spring 2005

Manufacturing Processes ISO Tolerances for Holes (ISO 286-2) Spring 2005

Manufacturing Processes ISO Tolerances for Shafts (ISO 286-2) Spring 2005

Manufacturing Processes Recommended Fits Spring 2005

Manufacturing Processes Tolerances and Manufacturing Processes Some manufacturing processes are more accurate than others Examples: • Most machining processes are quite accurate, capable of tolerances = 0.05 mm or better • Sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts must be specified Spring 2005

Manufacturing Processes Manufacturing Processes associated with ISO IT Tolerance Grade Spring 2005

Manufacturing Processes Metric Symbols of Fits Spring 2005

Manufacturing Processes Creating a Clearance Fit using The Basic Hole System Given the following fit Φ40 – H11/c11 From table for hole diameter = 40 and H11 we find Upper deviation = +160 μm & Lower deviation = 0 From table for shaft diameter = 40 and c11 we find Upper deviation = -120 μm & Lower deviation = -280 μm Calculations of dimension limits for hole and shaft • Maximum hole diameter = 40 + 0.16 = 40.16 mm • Minimum hole diameter = 40 + 0 = 40 mm • Maximum shaft diameter = 40 +(-120) = 39.88 mm • - Minimum shaft diameter = 40 + (-280) = 39.72 mm Maximum clearance = Maximum hole diameter – Minimum shaft diameter = 40.16 – 39.72 = 0.44 mm Minimum clearance = Minimum hole diameter – Maximum shaft diameter = 40 – 39.88 = 0.12 mm Allowances = minimum clearance = 0.12 mm = 120 μm Spring 2005

Manufacturing Processes Creating an Interference Fit using The Basic Hole System Given the following fit Φ40 – H7/s6 From table for hole diameter = 40 and H7 we find Upper deviation = +25 μm & Lower deviation = 0 From table for shaft diameter = 40 and s6 we find Upper deviation = +59 μm & Lower deviation = +43 μm Calculations of dimension limits for hole and shaft • Maximum hole diameter = 40 + 0.025 = 40.025 mm • Minimum hole diameter = 40 + 0 = 40 mm • Maximum shaft diameter = 40 + 0.059 = 40.059 mm • - Minimum shaft diameter = 40 + 0.043 = 40.043 mm Maximum interference = Maximum shaft diameter – Minimum hole diameter = 40.059 – 40 = 0.059 mm Minimum interference = Minimum shaft diameter – Maximum hole diameter = 40.043– 40.025 = 0.018 mm Allowances = maximum interference = 0.059 mm = 59 μm Spring 2005

Manufacturing Processes Surfaces Nominalsurface - 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 • The variety of manufacturing processes result in wide variations in surface characteristics Spring 2005

Manufacturing Processes Why Surfaces are 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. Spring 2005

Manufacturing Processes Surface Technology Concerned with: • Defining the characteristics of a surface • Surface texture • Surface integrity • Relationship between manufacturing processes and characteristics of resulting surface Spring 2005

Manufacturing Processes A magnified cross‑section of a typical metallic part surface Spring 2005

Manufacturing Processes 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 Spring 2005

Manufacturing Processes Surface Integrity Concerned with the definition, specification, and control of the surface layers of a material (most commonly metals) in manufacturing and subsequent performance in service. Manufacturing processes involve energy which alters the part surface. The altered layer may result from work hardening (mechanical energy), or heating (thermal energy), chemical treatment, or even electrical energy Surface integrity includes surface texture as well as the altered layer beneath Spring 2005

Manufacturing Processes Four elements of surface texture • Roughness - small, finely‑spaced deviations from nominal surface determined by material characteristics and process that formed the surface • Waviness - deviations of much larger spacing; they occur due to work deflection, vibration, heat treatment, and similar factors • Roughness is superimposed on waviness 3. Lay - predominant direction or pattern of the surface texture 4. Flaws - irregularities that occur on the surface • Includes cracks, scratches, inclusions, and similar defects in the surface. Spring 2005

Manufacturing Processes 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 Spring 2005

Manufacturing Processes Surface Roughness Average of vertical deviations from nominal surface over a specified surface length Surface roughness can be approximately calculated using the following equation where Ra = average roughness; yi = vertical deviations (absolute value) identified by subscript i; and n = number of deviations included in Lm Spring 2005

Understanding Dimensions, Tolerances, and Surfaces in Manufacturing

Understanding Dimensions, Tolerances, and Surfaces in Manufacturing

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