Engineering Drawing and Design Seventh Edition Chapter 10 Chapter 10 Threaded Fasteners 10-1 Simplified Thread Representation • • • • Two Basic types of fasteners: Permanent & removable Permanent- Examples: Rivets & welds Removable- Examples: bolts, screws, nuts, pins, rings keys Standardization reduces the costs of fasteners Screw Threads • Definition: a SCREW THREAD is a ridge of uniform section in the form of a helix on the external or internal surface of a cylinder • Definition: the PITCH of a thread P is the distance from a point on the thread form to the corresponding point on the next form, measured parallel to the axis • Definition: the LEAD L is the distance the threaded part would move parallel to the axis during one complete rotation in relation to a fixed mating part (the distance a screw would enter a threaded hole in one turn) THREADED FASTENERS SCREW THREAD TERMS THREADED FASTENERS THREAD FORMS THREADED FASTENERS THREAD FORMS Three types of thread representation used on working drawings • • • • Simplified Detailed Schematic Figure in text shows the three representations THREADED FASTENERS SYMBOLIC THREAD REPRESENTATION Right- and Left-Hand threads • Threads are assumed to be right hand unless otherwise designated • A bolt being threaded into a tapped hole would be turned in a right-hand (clockwise) direction THREADED FASTENERS RIGHT- AND LEFT-HAND THREADS THREADED FASTENERS LEAD = 1P SINGLE THREAD THREADED FASTENERS LEAD = 2P DOUBLE THREAD THREADED FASTENERS LEAD = 3P TRIPLE THREAD Inch Threads • Definition: PITCH =1/#threads per inch • Definition: THREAD SERIES are for fasteners with different diameters and the same #threads per inch • UNC Unified Course Thread Series • UNF Unified Fine Thread Series (Table appendix) • UNEF extra fine thread series THREADED FASTENERS BASIC THREAD CALLOUT • • • • Thread Class 1A,2A,3A External 1B,2B,3B Internal 1A,1B: loosest fit 2A,2B: ordinary good grade of commercial product 3A,3B: High-grade commercial products Example: .750-10 UNC-2A class of fit • Nominal Diameter thread series #threads/inch (Diameter, #threads/inch, thread form & series, class of fit) THREADED FASTENERS EXTERNAL THREADS THREADED FASTENERS BLIND HOLE INTERNAL THREAD Metric Threads • Definition: Pitch for metric threads is the distance between corresponding points on adjacent teeth. • Course-thread is intended for general engineering work and commercial applications • Fine-thread series is for general use. Compared with coarse, a fine-thread screw is stronger in both tensile and torsional strength • Definition: the FIT of a screw thread is the amount of clearance between the internal and external threads when they are assembled • Tolerance grades: the larger the number, the larger the tolerance – Tolerance position e (large allowance) – Tolerance position g (small allowance) – Tolerance position h (no allowance) – For internal threads: • Tolerance position G (small allowance) • Tolerance position H (no allowance) THREADED FASTENERS BASIC THREAD CALLOUT METRIC THREADS ISO Metric Screw Thread Designation • Nominal size (basic major diameter) and pitch expressed in millimeters. • Example: M16 x 1.5 – 5g6g Metric designation Nominal diameter Pitch Tolerance position & grade • Example: M10 x 1.5 x 25 ะค10mm 1.5 pitch 25 mm long • Example: M16 (pitch may be omitted for coarse threads) Metric Threads continued • For external threads, the length of the thread may be given as a dimension of the drawing. • For threaded holes that go all the way through the part, the term THRU is sometimes added • If no depth is given, the hole is assumed to go THRU • For threaded holes that do not go all the way thru, the depth is given in a note (minimum depth of full thread) Ex. M12 x 1.75 x 20 Pipe Threads • Inch: 4-8NPT or 4 NPT • Metric: 4 IN. – 8 NPT or 4 IN.-NPT • Nominal diameter and pipe thickness specify inch-sized pipe (typical) • • • • • • 4=nominal diameter 8=number of threads/inch N=American Standard P=pipe S=straight pipe thread T=taper pipe thread