Presentation on theme: "Product specification Dimensioning and tolerancing It is impossible to make a perfect component so when we design a part we specify the acceptable range."— Presentation transcript:
Product specification Dimensioning and tolerancing It 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.
IE 316 Manufacturing Engineering I - Processes Chapter 2 Suppliment DIMENSIONS, TOLERANCES, AND SURFACES Dimensions, Tolerances, and Related Attributes Surfaces ASME Y14.5 Form Geometry Effect of Manufacturing Processes
THE DESIGN PROCESS Product Engineering Design Process Off-road bicycle that Conceptualization 2. Synthesis 3. Analysis 4. Evaluation 5. Representation Design Process How can this be accomplished? 1. Clarification of the task 2. Conceptual design 3. Embodiment design 4. Detailed design Functional requirement -> Design Steps 1 & 2 Select material and properties, begin geometric modeling (needs creativity, sketch is sufficient) 3 mathematical, engineering analysis 4 simulation, cost, physical model 5 formal drawing or modeling
DESIGN REPRESENTATION Design Engineering Representation Manufac- turing Verbal Sketch Multi-view orthographic drawing (drafting) CAD drafting CAD 3D & surface model Solid model Feature based design Requirement of the representation method precisely convey the design concept easy to use
A FREE-HAND SKETCH Orthographic Projection
A FORMAL 3-VIEW DRAWING " 4 holes 1/4" dia around 2" dia, first hole at 45° A 0.001
DESIGN DRAFTING Third angle projection Profile plane Y Z X III Horizontal Frontal plane I IV II Drafting in the third angle
INTERPRETING A DRAWING
DESIGN DRAFTING Partial view Cut off view and auxiliary view Provide more local details A-A
TOLERANCE Dimensional tolerance - conventional Geometric tolerance - modern unilateral bilateral nominal dimension tolerance means a range
TOLERANCE STACKING "TOLERANCE IS ALWAYS ADDITIVE" why? What is the expected dimension and tolerances? d = = 3.00 t = ± ( ) = ± '± '± '±0.01 ? 1. Check that the tolerance & dimension specifications are reasonable - for assembly. 2. Check there is no over or under specification.
TOLERANCE STACKING (ii) What is the expected dimension and tolerances? d = = 1.00 t = ± ( ) = ± '± '± '±0.01 ?
TOLERANCE STACKING (iii) Maximum x length = = 1.03 Minimum x length = = 0.97 Therefore x = 1.00 ± '± '± '±0.01 ? x
TOLERANCE GRAPH G(N,d,t) N: a set of reference lines, sequenced nodes d: a set of dimensions, arcs t: a set of tolerances, arcs A B C D E d,t d : dimension between references i & j t : tolerance between references i & j ij Reference i is in front of reference j in the sequence.
EXAMPLE TOLERANCE GRAPH A B C D E d,t different properties between d & t
OVER SPECIFICATION If one or more cycles can be detected in the graph, we say that the dimension and tolerance are over specified. A B C A B C d1d2 d3d1,t1d2,t2 d3,t3 t1t2 t3 Redundant dimension Over constraining tolerance (impossible to satisfy) why?
UNDER SPECIFICATION A B C D E A B C D E d1 d2 d3 C D is disconnected from the rest of the graph. No way to find When one or more nodes are disconnected from the graph, the dimension or tolerance is under specified.
PROPERLY TOLERANCED A B C D E A B C D E d,t
TOLERANCE ANALYSIS For two or three dimensional tolerance analysis: i. Only dimensional tolerance Do one dimension at a time. Decompose into X,Y,Z, three one dimensional problems. ii. with geometric tolerance ? Don't have a good solution yet. Use simulation? true position diameter & tolerance A circular tolerance zone, the size is influenced by the diameter of the hole. The shape of the hole is also defined by a geometric tolerance.
3-D GEOMETRIC TOLERANCE PROBLEMS ± t datum surface datum surface Reference frame perpendicularity
TOLERANCE ASSIGNMENT Tolerance is money Specify as large a tolerance as possible as long as functional and assembly requirements can be satisfied. (ref. Tuguchi, ElSayed, Hsiang, Quality Engineering in Production Systems, McGraw Hill, 1989.) function cost Tolerance value d (nominal dimension) Quality Cost -t +t Quality cost
REASON OF HAVING TOLERANCE 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.
EFFECTS OF TOLERANCE (I) 1. Functional constraints e.g. d ± t flow rate Diameter of the tube affects the flow. What is the allowed flow rate variation (tolerance)?
EFFECTS OF TOLERANCE (II) 2. Assembly constraints e.g. peg-in-a-hole dp dh How to maintain the clearance? Compound fitting The dimension of each segment affects others.
RELATION BETWEEN PRODUCT & PROCESS TOLERANCES Setup locators ±0.005 ±0.005 ±0.005 Design specifications Process tolerance Machine uses the locators as the reference. The distances from the machine coordinate system to the locators are known. The machining tolerance is measured from the locators. In order to achieve the 0.01 tolerances, the process tolerance must be or better. When multiple setups are used, the setup error need to be taken into consideration. A ±0.01 tolerances
TOLERANCE CHARTING A method to allocate process tolerance and verify that the process sequence and machine selection can satisfy the design tolerance. blue print Operation sequence Not shown are process tolerance assignment and balance produced tolerances: process tol of 10 + process tol of 12 process tol of 20 + process tol 22 process tol of 22 + setup tol
PROBLEMS WITH DIMENSIONAL TOLERANCE ALONE ± ±0.001 As designed: As manufactured: Will you accept the part at right? Problem is the control of straightness. How to eliminate the ambiguity?
GEOMETRIC TOLERANCES FORM straightness flatness Circularity cylindricity ORIENTATION perpendicularity angularity parallelism LOCATION concentricity true position symmetry RUNOUT circular runout total runout PROFILE profile profile of a line ANSI Y14.5M-1977 GD&T (ISO 1101, geometric tolerancing; ISO 5458 positional tolerancing; ISO 5459 datums; and others), ASME Y Squareness roundness
DATUM & FEATURE CONTROL FRAME Datum: a reference plane, point, line, axis where usually a plane where you can base your measurement. Symbol: Even a hole pattern can be used as datum. Feature: specific component portions of a part and may include one or more surfaces such as holes, faces, screw threads, profiles, or slots. Feature Control Frame: A // M A symbol tolerance value modifier datum
MODIFIERS MMaximum material conditionMMCassembly Regardless of feature size RFS(implied unless specified) LLeast material conditionLMCless frequently used PProjected tolerance zone ODiametrical tolerance zone TTangent plane FFree state maintain critical wall thickness or critical location of features. MMC, RFS, LMC MMC, RFS RFS
SOME TERMS MMC : Maximum Material Condition Smallest hole or largest peg (more material left on the part) LMC : Least Material Condition Largest hole or smallest peg (less material left on the part) Virtual condition: Collective effect of all tolerances specified on a feature. Datum target points: Specify on the drawing exactly where the datum contact points should be located. Three for primary datum, two for secondary datum and one or tertiary datum.
DATUM REFERENCE FRAME Three perfect planes used to locate the imperfect part. a. Three point contact on the primary plane b. two point contact on the secondary plane c. one point contact on the tertiary plane Secondary. Primary Secondary T e rt ia ry A B C O M A B C primary Tertiary
STRAIGHTNESS Value must be smaller than the size tolerance '± Measured error Š '± DesignMeaning Tolerance zone between two straightness lines.
IE 316 Manufacturing Engineering I - Processes Dimensions and Tolerances 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
IE 316 Manufacturing Engineering I - Processes Surfaces Nominal surface - 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
IE 316 Manufacturing Engineering I - 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 Smooth surfaces make better electrical contacts
IE 316 Manufacturing Engineering I - Processes Surface Technology Concerned with: – Defining the characteristics of a surface – Surface texture – Surface integrity – Relationship between manufacturing processes and characteristics of resulting surface
IE 316 Manufacturing Engineering I - Processes Figure 5.2 ‑ A magnified cross ‑ section of a typical metallic part surface
IE 316 Manufacturing Engineering I - 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
IE 316 Manufacturing Engineering I - 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
IE 316 Manufacturing Engineering I - Processes Surface Texture Repetitive and/or random deviations from the nominal surface of an object Figure 5.3 ‑ Surface texture features
IE 316 Manufacturing Engineering I - Processes Four Elements of Surface Texture 1.Roughness - small, finely ‑ spaced deviations from nominal surface determined by material characteristics and process that formed the surface 2.Waviness - deviations of much larger spacing; they occur due to work deflection, vibration, heat treatment, and similar factors – Roughness is superimposed on waviness
IE 316 Manufacturing Engineering I - Processes 3.Lay - predominant direction or pattern of the surface texture Figure 5.4 ‑ Possible lays of a surface
IE 316 Manufacturing Engineering I - Processes 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
IE 316 Manufacturing Engineering I - 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
IE 316 Manufacturing Engineering I - Processes Surface Roughness Average of vertical deviations from nominal surface over a specified surface length Figure 5.5 ‑ Deviations from nominal surface used in the two definitions of surface roughness
IE 316 Manufacturing Engineering I - Processes Surface Roughness Equation Arithmetic average (AA) is generally used, based on absolute values of deviations, and is referred to as average roughness where R a = average roughness; y = vertical deviation from nominal surface (absolute value); and L m = specified distance over which the surface deviations are measured
IE 316 Manufacturing Engineering I - Processes An Alternative Surface Roughness Equation Approximation of previous equation is perhaps easier to comprehend: where R a has the same meaning as above; y i = vertical deviations (absolute value) identified by subscript i; and n = number of deviations included in L m
IE 316 Manufacturing Engineering I - Processes Cutoff Length A problem with the R a 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 width eliminates waviness deviations and only includes roughness deviations
IE 316 Manufacturing Engineering I - Processes Figure 5.6 ‑ Surface texture symbols in engineering drawings: (a)the symbol, and (b) symbol with identification labels Values of R a are given in microinches; units for other measures are given in inches Designers do not always specify all of the parameters on engineering drawings
TRUE POSITION 1.20 ± ± Tolerance zone 0.01dia O 0.01 M A B O.80 ± 0.02 Dimensional tolerance True position tolerance Hole center tolerance zone A B Tolerance zone
HOLE TOLERANCE ZONE Tolerance zone for dimensional toleranced hole is not a circle. This causes some assembly problems. For a hole using true position tolerance the tolerance zone is a circular zone.
TOLERANCE VALUE MODIFICATION ProducedTrue Pos tol hole size 0.97 out of diametric tolerance out of diametric tolerance O 0.01 M A B O 1.00 ± 0.02 M L S The default modifier for true position is MMC. MMC LMC For M the allowable tolerance = specified tolerance + (produced hole size - MMC hole size) A B
MMC HOLE Given the same peg (MMC peg), when the produced hole size is greater than the MMC hole, the hole axis true position tolerance zone can be enlarged by the amount of difference between the produced hole size and the MMC hole size.,
PROJECTED TOLERANCE ZONE Applied for threaded holes or press fit holes to ensure interchangeability between parts. The height of the projected tolerance zone is the thickness of the mating part. O.010 M A B C.250 p UNC - 2B
IE 316 Manufacturing Engineering I - Processes 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
IE 316 Manufacturing Engineering I - Processes Surface Changes Caused by Processing Surface changes are caused by the application of various forms of energy during processing – Example: Mechanical energy is the most common form in manufacturing. Processes include metal forming (e.g., forging, extrusion), pressworking, and machining – Although primary function is to change geometry of workpart, mechanical energy can also cause residual stresses, work hardening, and cracks in the surface layers
IE 316 Manufacturing Engineering I - Processes Surface Changes Caused by Mechanical Energy Residual stresses in subsurface layer Cracks ‑ microscopic and macroscopic Laps, folds, or seams Voids or inclusions introduced mechanically Hardness variations (e.g., work hardening)
IE 316 Manufacturing Engineering I - Processes Surface Changes Caused by Thermal Energy Metallurgical changes (recrystallization, grain size changes, phase changes at surface) Redeposited or resolidified material (e.g., welding or casting) Heat ‑ affected zone in welding (includes some of the metallurgical changes listed above) Hardness changes
IE 316 Manufacturing Engineering I - Processes Surface Changes Caused by Chemical Energy Intergranular attack Chemical contamination Absorption of certain elements such as H and Cl in metal surface Corrosion, pitting, and etching Dissolving of microconstituents Alloy depletion and resulting hardness changes
IE 316 Manufacturing Engineering I - Processes Surface Changes Caused by Electrical Energy Changes in conductivity and/or magnetism Craters resulting from short circuits during certain electrical processing techniques
IE 316 Manufacturing Engineering I - Processes 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 ( in.) or better – Sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts must be specified
IE 316 Manufacturing Engineering I - Processes Surfaces and Manufacturing Processes Some processes are inherently capable of producing better surfaces than others – In general, processing cost increases with improvement in surface finish because additional operations and more time are usually required to obtain increasingly better surfaces – Processes noted for providing superior finishes include honing, lapping, polishing, and superfinishing