Lecture 06

1. Lecture 06 Dimensioning & Fastener Specification Engineering Graphics & 3-D Modeling

2. Assignment: HW 06 • Ex. 9.2, p. 387 (Fig. 9.67, p.327 in older texts): • Use two sheets of EP Paper, with the title blocks filled out. • Sketch dimensioned views for b and d, only one part per page • 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) • Measure units from Assignment Page with Scale (transfer using note card) • Use millimeter dimensions for (b) • Use inch dimensions for (d) Engineering Graphics & 3-D Modeling

3. 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 Engineering Graphics & 3-D Modeling

4. 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) Engineering Graphics & 3-D Modeling

5. Dimensioning Terms • Allowance • The minimum space between mating parts • Purposes of Dimensions • Specify the perfect size of a feature • Specify the perfect 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 Engineering Graphics & 3-D Modeling

6. 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 Engineering Graphics & 3-D Modeling

7. 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 • Number of decimals shown reflects tolerance Engineering Graphics & 3-D Modeling

8. 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. Engineering Graphics & 3-D Modeling

9. 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 Engineering Graphics & 3-D Modeling

10. 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 Engineering Graphics & 3-D Modeling

11. 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 Engineering Graphics & 3-D Modeling

12. Standard Symbols R ( ) • Counterbore / Spotface • Countersink (not a finish mark) • Depth (blind hole) • Diameter / Round shape • Radius • Places / times / by • Reference • Square shape Engineering Graphics & 3-D Modeling

13. 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 • Append notes to clarify intent Engineering Graphics & 3-D Modeling

14. FastenerTerminology • 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 Engineering Graphics & 3-D Modeling

15. 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 Engineering Graphics & 3-D Modeling

16. 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. Engineering Graphics & 3-D Modeling

17. 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 Engineering Graphics & 3-D Modeling

18. Thread Forms • Sharp V • Whitworth Standard • American National • Unified • Metric • Standard Worm • Square • Acme • Knuckle • Buttress Engineering Graphics & 3-D Modeling

19. 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 Thread Notes Engineering Graphics & 3-D Modeling

20. American National Major Diameter Threads Per Inch Form & Series Fit Class LH/RH Thread Depth Metric Metric Designation Major Diameter Pitch Tolerance Grades LH/RH 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 Coding a Thread Note Engineering Graphics & 3-D Modeling

21. 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 • length is determined by thread length Engineering Graphics & 3-D Modeling

22. 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 Engineering Graphics & 3-D Modeling

23. 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 Specifying a Fastener Engineering Graphics & 3-D Modeling