THREADS AND FASTENERS - Goodheart-Willcox

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17/32 – 1.25 5/8 X 82° 5/8 – 11UNC – 2B .75 Figure 11-8. Additional specifications for a thread may include thread length, hole size, or chamfer. fine threads are designated by listing the pitch as a suffix. For example, in Figure 11-9, M8 is a coarse thread designation representing a nominal M8 Figure 11-9. Metric threads are specified slightly different than other threads. Unit 11 Threads and Fasteners 137 thread diameter of 8mm with a standardized pitch of 1.25mm understood. A fine thread designation for the part is M8×1.0. This indicates an 8mm diameter with a pitch of 1.0mm. The most common metric thread is coarse. Metric coarse threads are generally between Unified coarse and fine series for a comparable diameter. The specification for metric series threads does not indicate the number of threads per unit of length, such as threads per millimeter. Instead, the pitch is indicated in the specification. For example, M8×1.0 designates the pitch is 1mm. The tolerance and class of fit for metric threads are designated by adding numbers and letters in a certain sequence to the callout. The thread designation in Figure 11-10 calls for a 20mm diameter fine thread with a pitch of .75mm. In addition, the thread has a pitch diameter tolerance of grade 6 with an allowance “h” and a crest diameter tolerance grade 6 with an allowance “g.” See Unit 13 for a discussion of these tolerancing symbols. Pipe Threads The three forms of pipe threads used in industry are regular, aeronautical, and dryseal M8 X 1.0 138 Print Reading for Industry METRIC THREAD DESIGNATION M20X1.5–6h6g MAJOR DIAMETER Figure 11-10. The tolerance class is added as a suffix to the thread note. pipe threads. The regular pipe thread is the standard for the plumbing trade. The aeronautical pipe thread is the standard in the aerospace industry. Regular and aeronautical pipe thread forms must be filled with a lute or sealer to prevent leakage in the joint. The dryseal pipe thread is the standard for automotive, refrigeration, and hydraulic tube/pipe fittings. Dryseal pipe threads do not allow leakage, even without the use of sealer. This is due to the metal-to-metal contact at the crest and root of the threads. Representation and Specification Straight and taper pipe threads are represented graphically in a similar manner as other screw threads. However, pipe threads are shown tapered at an angle of approximately 3° to the A B C D 1/4 – 18 DRYSEAL NPTF Figure 11-11. Pipe threads are drawn similar to normal threads, but at a slight angle to the centerline axis. PITCH (FINE THREAD) PITCH DIAMETER TOLERANCE 6 = GRADE g = POSITION (ALLOWANCE) CREST DIAMETER TOLERANCE 6 = GRADE g = POSITION (ALLOWANCE) TOLERANCE CLASS centerline axis. See Figure 11-11. The specifications for American Standard Pipe Threads are listed in sequence of the nominal size, number of threads per inch, and the symbols for the thread series and form. For example, the thread specification 1/2-14NPT designates a 1/2″ nominal size, 14 threads per inch, and an American Standard taper pipe thread. Pipe Thread Symbols The following symbols are used to designate American Standard pipe threads. • NPT. American Standard taper pipe thread. • NPTR. American Standard taper pipe thread for railing joints. • NPSC. American Standard pipe thread for couplings. • NPSM. American Standard pipe thread for free-fitting mechanical joints. • NPSL. American Standard pipe thread for loose-fitting mechanical joints with locknuts. • NPSH. American Standard pipe thread for hose couplings. Symbols for dryseal pipe threads are designated
as follows: • NPTF. Dryseal American Standard pipe thread. • PTF-SAE SHORT. Dryseal SAE short taper pipe thread. • NPSF. Dryseal American Standard fuel internal straight pipe thread. • NPSI. Dryseal American Standard intermediate internal straight pipe thread. A typical specification for dryseal pipe thread is shown in Figure 11-11. This specification includes: A) Nominal size B) Number of threads per inch C) Form (dryseal) D) Symbol Unit 11 Threads and Fasteners 139 Print Reading Activities In the activities in this unit, you will read some actual industry prints. Many companies have their own drafting standards. As such, some items shown on industry prints may not exactly conform to the standards taught in this text. However, an important aspect of print reading is being able to correctly interpret all prints, including prints drawn to older or company-specific standards. Additional advanced print activities for this unit, APR11-1 and APR11-2, are located at the back of the book. 140 Print Reading for Industry Print Reading Activity 11-1 External Threads Refer to the print in PR11-1 and answer the following questions. 1. What is the name of the part? 2. What is the drawing number? 3. As given in the title block, what is the scale of the drawing? 4. What material is required? 5. What is the maximum allowed finished length? 6. What size is the hole drilled through the piece? 7. Interpret the thread specification for A. .750 10 UNC 2A 8. Give the dimensions for the groove on the left-hand side of the part. 9. Interpret the thread specification for B. 1 8 UNC 2A 1.25 LG 10. What does UNC stand for? 1. ______________________________________ 2. ______________________________________ 3. ______________________________________ 4. ______________________________________ 5. ______________________________________ 6. ______________________________________ 7. ______________________________________ ______________________________________ ______________________________________ ______________________________________ 8. ______________________________________ 9. ______________________________________ ______________________________________ ______________________________________ ______________________________________ ______________________________________ 10. ______________________________________
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