Engineering Drawing Dimensions and Tolerances PDF

Topic 5: Dimensions Dimensions and general notes describing the size and location of a part or a feature Details related to the construction or manufacture of the part Dimensions is a numerical value used to define the attributes of a part or feature in technical drawing. Features: Size, Geometric...

Topic 5: Dimensions Dimensions and general notes describing the size and location of a part or a feature Details related to the construction or manufacture of the part Dimensions is a numerical value used to define the attributes of a part or feature in technical drawing. Features: Size, Geometric Characteristics, Location, Surface Texture Main Goals of dimensioning 1. Use only the essential dimensions to fully describe a part 2. Select and arrange dimensions to support the function and mating relationship of the part 3. Do not specify the manufacturing methods to be used in building the part 4. Arrange dimensions for optimum readability 5. Unless otherwise stated, assume angles to be 90 degrees Basic Terminology Elements of Dimensioning Angular Dimension - A measurement specifying angles between lines or edges in design Reference Dimension - Dimensions that are only for information or guidance Dimension Value - A numerical value indicating size or distance in technical drawings Arrow - mark dimension line ends to indicate measurements limits Radius Symbol - geometric notation to denote the measurement from the center of a circle or arc to its outer edge. Diameter Symbol - Represents the full distance across a circle or circular feature, passing through its center Dimensioning Principles - Use thin lines with 2 projection lines marking their start and end points - Projection lines are drawn perpendicularly to the dimensioned element but do not touch it Size dimensioning - show the sizes of geometrical shapes Location dimensioning - show the dimensions locating those elements with respect to one another Manufacturing Specifications - Tolerances (+/- types, geometrical control) - Surfaces finishes - Materials used - Number of parts required Dimensioning Techniques Chain Dimensioning - use this method only when the possible accumulation of tolerances does not endanger the function of the part Baseline / Parallel Dimensioning - based on a common datum, selected datum is the left - hand side Running Dimensioning - A simplified method of baseline dimensioning, requiring less space, take reference from a single datum marked with a narrow circle Combined Dimensioning - Chain dimensioning + Baseline dimensioning (dimensions are arranged in a straight line) Dimensioning a “Reference/Auxiliary Dimension” - Simplified method of baseline dimensioning, requiring less space. Dimensioning of features not drawn to scale - This method underlines a particular dimension with a wide line Dimensioning a Theoretically exact dimension (TED) - Always used with geometrical control Dimensioning Squares or Flat Surfaces - A square machined on the end of a shaft so that it can be turned by the means of a spanner. The narrow diagonal lines are added to indicate the flat surface Dimensioning Holes - M10: Nominal Diameter or size of screw hole 1.5 Pitch (distance between adjacent threads) 6H: Tolerance Drafting & Dimensioning Screw Threads Male Thread (External Thread) - found on components like bolts, screws, studs Female Thread (Internal Thread) - usually found inside a hole or cavity Dimensioning Counterbores 1. Create a recess for a screw head 2. Flat surface obtained by spotfacing 3. Dimensioning counterbores Dimensioning Countersunk Holes - Commonly used angles (60 and 90 degrees) Unidirectional Dimensions - Writing parallel with the bottom of the drawing sheet - ‘Notes’ use this method Aligned Dimensions - Writings parallel with the related dimension line - Must be readable from the bottom or right-handed side of the drawing Superfluous Dimensioning - Useful for convenience (overall length), must be indicated as ‘Reference Dimension’ Guidelines for Constructing a Drawing 1. Dimension and extension lines are narrow continuous lines 0.35mm thick 2. Keep extension lines away from the drawing feature’s outline, leaving a small 2-3mm gap 3. Arrowheads should be approximately triangular 4. Limits of size are quoted; adequate space must be left between rows of dimensions 5. Center Lines cannot be used as dimension lines 6. To ensure clear readability, position figures to be readable from the bottom or by turning the drawing clockwise to read from the right-hand side 7. Leader lines are used to indicate where specific indication apply General Rules of Dimensioning 1. Place dimension on the clearest view, specifically in orthographic projection 2. Place dimension outside the outline of the part 3. Circular features should be dimensioned from the front 4. When several dimensions are placed along a side, put the shortest closest to the part. Tolerances - The amount that a feature is allowed to vary from what is specified by the drawing or model dimension Milling machine can produce parts within a tolerance range 1. Design requirement 2. Production drawing 3. Choice of machine Nominal Dimensions Limits and tolerances are based on ‘nominal dimensions’ which are target dimensions No such thing as a nominal dimension, since no part can be manufactured to a theoretical exact size Limits Maximum and minimum dimensions for a given feature (20 ± 0.1) Upper and lower limits of size are 20.1 and 19.9mm Tolerance Algebraic difference between the upper and lower limit of size (0.2mm) tolerance is the amount of variation permitted Tolerance Stacking A location of a surface may be affected by more than one tolerance value (cumulative tolerance) Chained/continuous Dimensioning Lead to tolerance stacking Useful if location between the intent is for the relative position between features Useful for ensuring mating with other parts Baseline Dimensioning Avoid tolerance stacking Useful to limit dimensional variation from a single datum Useful in CNC programming Coordinate Dimensioning More suitable for ensuring mating with another part Used for precision holes required for proper mating/fitting Limits & Fits Fits are controlled by limits (of dimensions) 1. In practice, no components can manufacturer to an exact size 2. Designers need to decide the upper and lower limits of size to be achieved in fabrication 3. Fabricated parts must fall between the limits (to give the pre-designed type of fitting) 4. Each dimension in a drawing shall have a tolerance Purpose of Limits & Fits To ensure that an assembly will function correctly, its component parts must fit together as designed Terminology used in Fits & Tolerances Types of Fits 1. Clearance Fit (Shaft is always smaller than the hole) 2. Interference Fit (Shaft is always bigger than the hole) 3. Transition Fit (Shaft may be either bigger or smaller than the hole) Hole-basis Systems Hole size remains constant Shaft can be machined to different size diameters for a specified fit Drilling or reaming is challenging to produce precise hole diameters, whilst precisions turning of shafts is very accurate Shaft-basis Systems Shaft size remains constant Hold diameter varies for a specified fit Difficult and expensive to precisely machine holes - used only when the shaft diameter cannot be changed Selecting Hole-basis system Fundamental deviation of hole is zero Shaft is varied to suit Only be chosen for general use Used for practical reasons - holes produced using fixed tools such as drills, reamers, broaches Easier to machine a shaft to any specific size Selecting Fit system Fundamental deviation of shaft is zero Holes made to suit the shaft Used when the shaft is the standard Only be used where it will convey unquestionable economical advantage Selecting Fits Past experience shows that most fit conditions required for normal engineering products can be provided by a quite limited selection of tolerances ISO Standards Using ANSI Standards Expressing Tolerances in Drawings 1. Individual Tolerances (Max & Min sizes specified as part of the dimensions) 2. Unilateral Tolerances (Varies in one direction) 3. Bilateral Tolerances (Varies in both direction) Geometric Dimensioning & Tolerancing (GD&T) 1. The function of the part 2. How this part functions with related parts Allows a more defined feature more accurately, without increasing tolerances GD&T is a system (a symbolic language) that uses standard symbols to indicate tolerances that based in the feature’s geometry Geometric Tolerance Specify the maximum error of a component’s geometrical characteristic, over its whole dimensioned length or surface Defines a zone in which the feature may lie Tolerance Zone A tolerance zone is the space in which any deviation of the feature must be contained Space within a circle Space within a cylinder Space between two concentric circles Space between two coaxial cylinders Space between two equidistant lines or two Space between two equidistant surfaces or parallel straight lines two parallel planes Space within a sphere Application of Geometric Tolerances Applied over and above normal dimensional tolerances to control more precisely the form or shape of a manufactured part. They are applied due to a particular duty that the part has to perform. Important consideration for surfaces which meet other parts 1. Should be applied only when the design requires 2. Indiscriminate use of them can increase costs 3. Tolerances should be as wide as possible as the design function permits 4. Use of them does not involve or imply any method or manufacture All 3 shafts are subjected to: 1. Conical 2. Concave/convex 3. Bending errors Tolerance Specification 1. Traditional Dimensional tolerance specification 2. Exaggerated view of imperfect manufactured specimen 3. Exaggerated view of imperfect manufactured specimen  4. Not all features are dimensional in nature, where dimensional tolerances can be specified Terminology used in Geometric Tolerancing 1. Feature (Used to identify part of a portion of a component e.g. surface, axis of a cylinder) 2. Datum (A point, line, plane or other surfaces from which dimensions are measured or to which geometric tolerances are referenced) 3. Virtual Size (Dimensions of the overall envelope of perfect form which touches the highest point of a feature) Maximum Material Condition (MMC) Condition where a size feature contains the maximum amount of material within the stated limits of size. (largest shaft and smallest hole) Least Material Condition (LMC) Condition where a size feature contains the least amount of material within stated limits of size (smallest shaft and largest hole) Symbols related to tolerancing Additional symbols in tolerancing Theoretically Exact Dimensions (TEDs) Dimensions are identified by enclosure in rectangular boxes Known as basic dimension, boxed dimension, true position dimension Define the true position of features (hole, slot, boss profile) Never individually toleranced Accompanied by a positional or zone tolerance specified within tolerance frame Always associated with feature control frame (or datum target) Applying Tolerance Frame to Toleranced Feature Methods Surface or plane Axis or median plane Datums and Datum Systems are the basis for establishing the geometric relationship of related feature of a workpiece Datums A datum surface should be accurately finished Virtual Size The dimensions of the overall envelope of perfect form which touches the highest point of a feature Straightness tolerance may be used to control Straightness of a line Straightness of an axis in a single plain Straightness of the axes of solids of revolution Flatness Flatness tolerances control the divergence or departure of a surface from a true plane Flatness tolerances is the specified zone between 2 parallel planes Circularity (Roundness) Any point on a feature’s continuous curved surface is equidistant from its center in the same plane Circularity tolerances controls the divergence of the feature, with the tolerance zone defined by the difference between the radii of the two coplanar concentric circles Cylindricity Combination of parallelism, circularity and straightness when applied to the surface of a cylinder. Tolerance zone is the annular space between 2 coaxial cylinders, with the radial difference being the specified tolerance Parallelism (Squareness) Two parallel lines or surfaces are uniformly separated, and may need to align with datum planes or axes Tolerance zones can be between parallel lines or surfaces, or the space contained within a cylinder positioned parallel to its datum Tolerance value’s magnitude = Distance between parallel lines or surfaces or the cylinder diameter Perpendicularity When a line, plane or surface is at right angles to a datum feature Tolerance zone is the space between two parallel lines or surfaces or the space contained within a cylinder All tolerance zones are perpendicular to the datum feature Tolerance magnitude’s value = Distance between parallel lines or surfaces or the diameter of the cylinder Angularity Defines a condition between two related planes, surfaces or lines Angularity tolerances control this relationship Circular Run-Out Unique geometrical tolerance Composite form control relating two or more characteristics  Requires a practical test where part is rotated through 360 about its own axis Total Run-Out 3D control for rotating parts relative to a datum axis. When applied to cylindrical parts, controls circularity, concentricity, straightness taper and surface profile Position Positional tolerance controls the location of one feature from another feature or datum Zone defines the permissible deviation of a specified feature from a theoretically exact position (TED) Tolerance value = Distance between parallel lines or planes or the diameters of the circle or cylinder Concentricity & Coaxiality Two circles are said to be concentric when their centers are coincident Two cylinders are said to be coaxial when their axes are coincident Maximum Material Condition (MNC) Condition of a part or feature which contains the maximum amount of material (e.g. minimum-size hole or maximum-size shaft) Permits additional geometric tolerance, as the considered feature departs from its maximum material condition Allows an increase in the specified geometrical tolerance (loosening of tolerance) Functionally acceptable and advantageous during process of manufacturing Requirement for Positional Tolerance Bigger hole allows for more deviation from the hole center, still allowing a bolt to go through Consider other design considerations Stating the positional tolerance with MMC, the tolerance is loosened to an extent, depending on actual size resulted upon fabrication Benefits manufacturing process by giving flexibility between hole size tolerance and positional tolerance Tolerancing for Function 1. Select datum features (Start with primary datum) 2. Control datum features (Control primary datum e.g. flatness) 3. Dimensions the sizes of features (Use plus/minus tolerances) 4. Geometrically control all functional features (Locate the theoretically exact position of each feature) 5. Refine the dimensioning & geometrical tolerancing (Within specification (functional), beyond specification (not functional) Engineered Surface A surface that has been produced either by a forming process or by a machining process Importance of surface finishing 1. Friction and wear depend on surface characteristics (e.g. journal bearing surfaces, dies and molds, brake drums) 2. Surfaces affect mechanical and physical properties (e.g. fatigue strength) 3. High precision or optional components (e.g. reflectors, lenses, require optical surface textures) 4. Assembly of parts is affected by their surfaces 5. Smooth surfaces make better electrical contacts 6. Aesthetic reasons Surface Texture Refers to topography and geometric features of the surface When highly magnified, the surface has roughness, waviness and flaws Possesses a pattern or a direction resulting from machining process that produced it Has repetitive or random deviations from the nominal surface of an object Roughess (Small, finely-spaced deviations from nominal surface) Waviness (Deviations of a much larger spacing, occur due to work deflection, vibration, heat treatment, roughness is superimposed on waviness) Flaw (Irregularities that occur occasionally on the surface, includes cracks, scratches, defects) Lay (Predominant direction or pattern of the surface texture) Surface Profile Parameters  Maximum Profile Peak Height, Rp Arithmetic average, Ra (Based on absolute values of deviations, referred to average roughness) Root Mean Square, Rq (RMS) Cutoff Length Used as a filter to separate waviness from roughness deviations A sampling distance along the surface If sampling distance is shorter than waviness eliminates waviness deviations and only include roughness deviations Relationship between surfaces and processes: Some processes are better of producing better surfaces than others Processing cost increases with improvement in surface finish Additional operations and time are usually needed to have better surfaces Processes for superior finishing (Honing, Lapping, Polishing, Diamond Turning) Measure Surface Roughness 1. Comparative Method (Roughness comparison specimens) 2. Contact Stylus Method (Transducer is used to convert the vertical movement of the stylus into a electrical signal, touching and moving across the surface) 3. Optical Method (Vertical Scanning Interferometer, Confocal Microscopy, Focus Variation) What is the definition of a feature in geometric tolerancing? a) The general term used to identify part of or a portion of a component b) The point, line, plane, or other surface from which dimensions are measured c) The condition where a size feature contains the maximum amount of material within the stated limits of size d) The dimension of the overall envelope of perfect form which touches the highest point of a feature What is the definition of a datum in geometric tolerancing? a) The general term used to identify part of or a portion of a component b) The point, line, plane, or other surface from which dimensions are measured c) The condition where a size feature contains the maximum amount of material within the stated limits of size d) The dimension of the overall envelope of perfect form which touches the highest point of a feature What are Theoretically eXACT Dimensions (TEDs) commonly known as? a)Geometric Tolerance b)Basic Dimensions c)Boxed Dimensions d)True position dimensions What are Datum and Datum Systems used for? a)Establishing the geometric relationship of related features of a workpiece b)Finishing the surface of a workpiece c) Indicating the letters A as a datum surface Positioning the main outline of a feature Which of the following are Dimensioning Techniques 1. Baseline/Parallel Dimensioning 2. Combined Dimensioning 3. Dimensioning of Extrusion 4. Dimensioning Squares a) 1 only b) 1,2 and 3 c) 1,2 and 4 d) all of the above Based on the following image, what are the maximum and minimum limits of the dimension? a) 0.476, 0.474 b) 0.4752, 0.4751 c), 0.5, 0.4 d) 0.001, -0.001 “Tolerance is the ____ between the upper and lower limit of size tolerance” a) Algebraic Difference b) Dimensioning c) Numerical difference d) Depth What does the terminology “Minimum limit of size” mean? a) The algebraic difference between two sizes b) Line of zero deviation c) The smaller of the two limits of size d) The tolerance between two dimensions must be small Which of the following is a type of fit? a) Clearance fit b) Drilling fit c) Shaft fit d) Hole fit What standards are commonly followed by engineers when determining hole and shaft tolerances? a) OSI and ANDI standards b) General Practices standards c) Tolerances Global Standards d) ISO and ANSI standards Which of the following describes Bilateral Tolerances best? a) Max and Min sizes specified as part of the dimensions b) Varies in both direction c) Varies in one direction d) Varies in all directions Which of the following describes Individual Tolerances best? a) Varies in one direction b) Varies in all directions c) Varies in both directions d) Max & Min sizes specified as part of the dimensions “GD&T is a system that uses ____ to indicate tolerances that are based in the features geometry” a) Universal symbols b) Standard symbols c) Dimension symbols d) Tolerance symbols Which of the following are tolerance zones? 1. Space within a circle 2. Space within a sphere 3. Space within a solid 4. Space between two coaxial cylinders a) 1 and 4 b) All of the above c) 1,2 and 3 d) 1,2 and 4 What does the following symbol mean? a) Total run out b) Concentricity and coaxiality c) Circular run out d) Perpendicularity What does the following symbol mean? a) Roundness b) Cylindricity c) Positioning d) Circular run out What does the following symbol mean? a) Datum feature indication b) Toleranced feature indication c) Theoretically exact dimension d) Datum target indication Which of the following statements are true about Theoretically Exact Dimensions (TEDs) 1. Dimensions are identified by enclosure in rectangular boxes 2. Known as basic dimension, boxed dimension, true position dimension 3. Define the true position of features (hole, slot, boss profile) 4. Never individually toleranced a) 1, 2 and 3 b) 3 and 4 c) 2 and 3 d) All of the above Based on the photo below, determine which datum does each letters, C A and M represent C- A- M- Ans: C - Primary, A - Secondary, M - Tertiary