1. Threads and Fasteners

2. Screw Threads Threads act as an incline plane wrapped around a surface
They work by forcing objects back and forth by rotating an internal thread through an external thread
Most basic threads follow the “American National Standard for Unified Screw Threads” UN-
Other forms are Worm thread or Acme Thread

3. Terminology
Actually, the crests and roots are flattened or rounded during forming process

4. Metric M and MJ threads
Metric is similar to UN series threads but uses all metric sizing, each type is not interchangeable
H=0.5p/tan 300
(0.25p)/(0.5H)=tan 300

5. Vocabulary Pitch- Distance between each thread or
P=1 / no. of threads per inch
Lead- Distance the threaded part moves against a fixed part in one full revolution
Root- Bottom of the Thread
Crest- Top of the thread
Nominal Dia.- “Major” outside diameter of thread
Minor Dia.- Diameter from root to root
Thread angle- angle of thread between crests
Depth- perpendicular distance from crest to root

6. Types of thread drawing
Schematic
Simplified
Detailed

7. Detailed Pictorial display of threads Applications
Sales catalog
Display drawing
Time consuming to draw

8. Schematic
Not commonly used in industry
Require extra drawing time

9. Simplified Preferred method of thread representation
Describe threads clearly
Easy and quick to draw
Versatile
Can be used in all situations

10. Thread standards - 6 types
UNC or NC- Unified coarse thread, general use
UNF- Unified Fine thread, automotive or aircraft
UNEF- Unified Extra Fine, aircraft or other applications where very fine threads are needed.
Sizes based on inches and have a standard thread count depending on type

11. Reading threads Classes of fit 1, 2, 3 – 1 is least accurate
Direction- A is external, B is internal
Example 7/8”-9 UNC 3” -2B (RH)
is the major dia.
9 - is the threads per inch
UNC - is the standard
2 - is the class of fit
B - is the direction
3” is the length in inches
RH- right handed threads, assumed

12. Bolts and Nuts

13. Bolts and Nuts

14. Thread Profiles General use Power screws & machine tool threads
Thread profiles. (a) ACME; (b) UN (unified; American National )
Text Reference: Figure 15.3, page 668

15. (a) Single-, (b) double-, and (c) triple threaded screws.
Threads
l=p
l=2p
l=3p
l; lead p: pitch
(a) Single-, (b) double-, and (c) triple threaded screws.
Multiple threaded screws
Text Reference: Figure 15.2, page 667

16. Types of Threaded Fasteners
Three types of threaded fastener. (a) Bolt and nut; (c) Cap screw; (c) stud.
Text Reference: Figure 15.8, page 679

17. F or W= width across flats = 1.5 D H= Height of Bolt head = 2/3 D
Threaded Fasteners TL
Hexagon head bolt
F or W= width across flats = 1.5 D
H= Height of Bolt head = 2/3 D
T = Height of nut = 7/8 D
TL=thread length=2 x D + ¼”

18. (a) Fillister head (b) flat head (c) hexagonal socket head

19. As bolt holes may have burrs or sharp edge after drilling that can increase stress concentration, so use washer under the bolt head, and also under the nuts

20. Hexagonal nuts (a) end view, (b) washer faced regular nut, (c) regular nut chamfered on both sides, (d) jam nut with washer face, (e) jam nut chamfered on both sides
During tightening, the first thread of the nut tends to take the entire load. But yielding occurs, with some strengthening due to cold work that takes place, the load is eventually divided over three nut threads. Don’t’ reuse nuts; it can be dangerous

21. Screw CAD Steps- Side view
Draw Major dia
Draw Head Width Dia, 1.5 x MD
Draw polygon head shape- circumscribed
Vertical flats – hex head
Angled flats - square head
Transfer screw head intersections

22. Screw CAD Steps- Front (Bolt Head)
Create head Height- 2/3 MD
Trim shape from transferred screw head intersections
Create tangent arcs from 60* intersections on bolt faces
Draw 30* chamfers on outside corners line up with head diameter
Trim outsides

23. Screw CAD Steps- Front (Bolt Threads)
Carry over MD and create screw length
Determine thread depth 2 x MD + ¼”
Offset 1 pitch= 1/TPI
Draw 60* thread to create mD depth, carry to side view.
Mirror thread to other side
Shift thread left or right ½ P for direction
Connect crest and root
Array # of threads to fit thread depth
Chamfer first thread 45*