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Dimensioning & Fastener Specification
Lecture 06 Dimensioning & Fastener Specification 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Assignment: HW 06 Reading Skim CH 9, 11 Review pp. 380 – 381 (pp. 314 – 318 in old text) (Dimensioning Do’s & Don’ts) Assignment: Ex. 9.2, p. 387 (Fig. 9.67, p.327 in old text): Sketch dimensioned views for b and d, only Scale drawing so that the parts are roughly double size on your paper (smallest hole diameter is 2 squares in width) Draw orthographic, multi-view with straight edge (not CAD) Use English units – 1 square is .20 inches / side 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Dimensioning Why dimension drawings? Historical practicality Production inaccuracies Clarity of: Size Position Geometric Form How to dimension Know dimensioning techniques Select dimension placement Choose dimensions shown to convey: Functionality of part (very similar to design intent) Manufacturing steps for part 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Dimensioning Terms Nominal Size The approximate size used for naming purposes Limits The extreme allowable sizes for a feature Basic Size The theoretical exact size from which limits for permissible variation are derived Basic Dimension The theoretical exact location, to which tolerances are applied so as to limit permissible geometry variation Datum A feature that is assumed to be exact for the purposes of manufacture and inspection Tolerance The total amount the feature is allowed to vary (upper limit - lower limit) 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Dimensioning Terms Allowance The minimum space between mating parts Purposes of Dimensions Specify the size of a feature Specify the location of a feature Purpose of Tolerances Specify the allowable variation of a feature Variation in size Variation in position Variation in form (shape) Dimensions & Tolerances specify relationships Functional relationships Manufacturing relationships Inspection relationships 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Dimensioning Lines Dimensioning Line Types Dimension Line Thin, dark, solid, with arrowheads at ends Extension (Witness) Line Thin, dark, solid, starting a small distance from the feature and running to just past the dimension line Centerline Thin, dark, long-short-long pattern; may be extended for dimensioning Leader Thin, dark, solid, with a single arrowhead/dot, at a large angle with respect to feature being referenced 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Dimensioning Systems Dimension Text Directions Aligned Unidirectional Dual Dimensioning Metric on top / left, English on bottom / right Metric in square brackets, position consistent Decimal Dimensioning Systems Complete decimal dimensioning Combination dimensioning mixed decimal and nominal dimensions Number of decimals shown reflects tolerance 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Dimension Rounding Three part rule: If the number following the rounding position is less than 5, the positional digit is unchanged If the number following the rounding position is exactly 5, round the position to the closest even digit Otherwise, increment the positional digit Exception: Diameters and other dimensions likely to be divided by 2 should be rounded to an even digit, unless the precision is critical for strength, fit, etc. 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Dimensional Values Clarity is most important Decimal point is bold Avoid crowding Always break lines crossing dimension values Metric Has leading zeros on fractions less than one mm There are no right zeros in fractions unless it is expressing the tolerance English Has NO leading zeros on fractions less than 1 inch The number of right zeros in fractions are the same as the feature tolerance 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Placement Dimension lines may NOT cross other dimension lines Extension lines may cross other extension lines and visible lines, but must be broken if crossing a dimension line. There is a small gap between an extension line and the object it references Centerlines may be used as extension lines for dimensioning purposes Leaders may be used to specify radii and hole diameters, but NOT protrusion diameters 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Rules for Dimensions Prime Directive: Dimensions should not be shown on a view unless they promote the clarity of the drawing Important Rules: Show dimensions where shapes are shown in profile and contours are shown true size Align dimension lines and group dimensions as much as possible for reading ease Prefer placing dimensions between views Dimension lines are drawn at right angles to extension / centerlines whenever possible, but always parallel to the measured distance in any case 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Standard Symbols Counterbore / Spotface Countersink (not a finish mark) Depth (blind hole) Diameter / Round shape Radius Places / times / by Reference Square shape R ( ) 4/22/2017 Engineering Graphics & 3-D Modeling
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Manufacturing Considerations
Dimension from datums (finished measuring surfaces) Finish Mark (examples) Avoid tolerance stacks Chain vs. Datum dimensioning Do not over-dimension Leave one dimension missing in a locational chain Do not over-specify tolerances / conditions Over specification can result in excessive cost Design for manufacturability / assembly / X See Figure 9.51, p. 314 (old text, only) Append notes to clarify intent 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Assignment: HW 06 Reading Skim CH 9, 11 Review pp. 380 – 381 (pp. 314 – 318 in old text) (Dimensioning Do’s & Don’ts) Assignment: Ex. 9.2, p. 387 (Fig. 9.67, p.327 in old text): Sketch dimensioned views for b and d, only Scale drawing so that the parts are roughly double size on your paper (smallest hole diameter is 2 squares in width) Draw orthographic, multi-view with straight edge (not CAD) Use English units – 1 square is .20 inches / side 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Fastener Terminology Screw Thread a helical ridge of uniform section on the curved surface of a cylinder (internal or external) External Thread a thread on the outside of a member (shaft) Internal Thread a thread on the inside of a member (hole) Major Diameter the largest diameter of a screw thread Minor Diameter the smallest diameter of a screw thread 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Fastener Terminology Angle of Thread the angle between the two sides of a thread, measured on a plane through the axis of the screw Crest the top surface/edge joining the thread sides Root the bottom surface/edge joining the thread sides Side the surface connecting the crest with the root 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Fastener Terminology Pitch the distance from one thread to the next, measured parallel to the axis. The reciprocal of the number of threads per inch. Pitch Diameter the diameter of a cylinder if it passed through the point on the thread where the area of the threads and the space between threads is equal. Lead the distance a thread advances axially in one turn. 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Fastener Terminology Axis of Screw the longitudinal centerline through the screw Depth of Thread distance from crest to root measured normal to the axis Form of Thread cross section of thread cut by a plane through the axis Series of Thread standard number of threads per inch and based on the screw diameter 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Thread Forms Sharp V Whitworth Standard American National Unified Metric Standard Worm Square Acme Knuckle Buttress 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Thread Notes Thread Series Coarse Fine Extra Fine 8-pitch 12-pitch 16-pitch Thread Fits Class 1 Class 2 Class 3 Internal / External A - external B - internal Thread Engagement S - short N - normal L - long Multiple Threads DOUBLE TRIPLE QUADRUPLE Left / Right Hand RH assumed LH must be noted Rule of Thumb: point thumb into material, and fingers will curl in turning direction 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Coding a Thread Note American National Major Diameter Threads Per Inch Form & Series Fit Class LH/RH Thread Depth Metric Metric Designation Pitch Tolerance Grades Unified National Major Diameter Threads Per Inch Form & Series Fit Class LH/RH Thread Depth Examples 3/4-10NC-2A LH-1.00 .75-8UNF-3B DOUBLE-.50 DEEP M10X1.5-6H/5g6g LH 4/22/2017 Engineering Graphics & 3-D Modeling
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Thread Representation
Detailed shows thread profile approximate helix by straight lines between crests Simplified shows thread depth by hidden lines parallel to the axis length is determined by thread length Schematic show alternating crest (thin) lines and root (thick) lines perpendicular to the axis 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Standard Fasteners Screws Nuts & Bolts Washers Keys Rivets (note spelling!) Pins Comments: Text gives typical shapes & dimensions for the heads Appendices give standard dimensions for nominal sizes of fastener geometry 4/22/2017 Engineering Graphics & 3-D Modeling
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Engineering Graphics & 3-D Modeling
Specifying a Fastener Bolts & Nuts: Nominal body size Thread spec/note Length Finish Head style Name* CAP SCR MACH SCR SET SCR NUT Etc. Examples: M10 X 1.5 X 12 HEX HD SCR M10 X 1.5 X 12 HEX SOC HD .375 X 1.25 SQ HD SS 10-32 X 5/8 SLOT PAN HD MACH SCR Abbreviations: See Appendix 4, pp. a21 - a 24 4/22/2017 Engineering Graphics & 3-D Modeling
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