Part 1-Section 3 Dimensions and Notes

Unit 17 Representing and Dimensioning External Screw

Threads

• Profile of Unified and the American National Threads– Thread form-refers to shape– U.S. Standard, American National, and Unified

Threads– National Screw Thread Commission, American

Standards Association, U.S. Standards Institute

• Profile of Unified and the American National Threads– Unified (UN)– American National (N)– Crest-shape of the top of the thread– Root-shape of the bottom of the thread

• American national Standard & Unified Screw Threads– Sharp - V– Unified – UN– American Standard – N– 60 Stub– ASME Straight Pipe Threads– NPS– ASME Taper Pipe Threads- NPT

• Other Common Thread Forms– Square threads-transmits great power of force– ACME thread-included angle of 29– Buttress thread-transmit power in one direction

• Thread Series– Standard number of threads per inch

– Thread series• Fine• Coarse

• Thread Series1. Coarse-UNC and UNCR

2. Fine-UNF, UNRF and NF

3. Extra fine-UNEF, UNERF, and NEF

4. Nonstandard or special threads-UNS-(American National Standard Special Series)

5. Constant pitch series-fixed number of threads for range of diameters

• Representing External Screw Threads– Two basic types

1. External threads2. Internal threads

– Representation1. Pictorial2. Schematic3. Simplified

• Dimensioning External Screw Threads– Dimensions– Letters– Numbers– Specifications

• Pipe Thread– Straight – NPS– Tapered-NPT– Dryseal pressure-tight- NPTF and NPSF

Spindle Shaft (BP-17)

Print Reading Assignment-Unit 17

1. What material is used for the part?

2. Give the basic overall length of the shaft?

A

3. What system of representation is used for the threaded portions?

4. At how many places are threads cut?

5. Start at the bottom of the part and give all the thread diameters.

6. Name the three thread series that the letters UNC, UNRF, and NC specify.

7. How many threads per inch are to be cut on the 3/4”, 1 1/4”, and 1” diameters?

8. Give dimensions , and .

B

C D

9. Give the upper and lower limit diameter for the 3/4” threaded portion.

10. What is the length of the 3/4” threads? The 1 ¼” threads?

11. Give the upper and lower limit dimensions of the diameter portion. F

12. What angles are the chamfers at the starting end of each thread cut?

13. What tolerance is specified for angular dimensions?

14. What is the upper and lower limit of size on the 1 1/8” diameter?

15. How long is the part of the shaft that has the 11/4” -12 thread?

16. Give the angle of the countersink.

17. What is the largest diameter that the 2.25” diameter can be machined to?

18. Give the diameter to which the center holes are sunk.

19. How deep is the undercut on the 1 ¼” diameter?

20. Name the final machining operation for all diameters after the part is heat treated.

21. Classify the tolerance for ground diameters and .E F