Chapter 08 Thread Fastener

7/24/2019 Chapter 08 Thread Fastener

1/19

Thai Nguyen University of Technology Introduction to Engineering Drawing

Division of English Taught Mechanical Engineering ME11- 3 Credits

Eng. Phan Thi Phuong Thao 1

Email: [email protected]

CHAPTER 8: THREAD FASTENER

Mechanical assembly uses various methods to mechanically attach two (or

more) parts together. In most cases, the method involves the use of discrete hardware

components, called fasteners, that are added to the parts during the assemblyoperation. In other cases, the method involves the shaping or reshaping of one of the

components being assembled, and no separate fasteners are required. Many

consumer products are produced using mechanical assembly: automobiles, large and

small appliances, telephones, furniture, etc. In addition, industrial products such as

airplanes, machine tools, and construction equipment almost always involve

mechanical assembly.

Mechanical fastening methods can be divided into 2 major classes: (1) those

that allow for disassembly, and (2) those that create a permanent joint. Thread

fasteners, for example screw, bolts, nuts, washers, keys, pins, are examples of the

first class, and rivets illustrate the second.

Mechanical assembly is usually accomplished by unskilled workers with a

minimum of special tooling and in a relatively short time. The technology is simple,

and the results are easily inspected. These factors are advantageous not only in the

factory, but also during field installation. Large products that are too big and heavy

to be transported completely assembled can be shipped in smaller subassemblies and

then put together at the customers site. Ease of disassembly applies, of course, only

to the mechanical fastening methods that permit disassembly. Periodic disassembly

is required for many products so that maintenance and repair can be performed;

today, they are designed for hundreds of different purpose, the three basis

applications being (1) to hold parts together, (2) adjust parts with reference to each

other, and (3) to transmit power.

Figure 8.1: Disassembly (1) and permanent joint (2)

(1) (2)


2/19





1. Thread terminology

* Thread application:

- To hold parts together

- To move parts relative to others

1.1. Thread definitions

- Screw thread: a ridge of uniform section in the form of a helix (Figure),on the

external and internal surface of a cylinder.

- External thread- A thread cut on

the outside of a member, as on the

shaft.

- Internal thread- A thread on the

inside of a member, as in a hole.

- Comparision between external and

internal

- Right-hand thread- Thread that will assemble when turned clockwise.

- Left-hand thread - Thread that will assemble when turned counter-clockwise.

Figure 8.2: Screw thread


3/19





- Crest- the top surface joining the 2 sides of a thread.

- Root- The bottom surface joining the sides of 2 adjacent threads.

- Thread angle- The angle included between the sides of the thread measured in a

plane through the axis of the screw.

- Major diameter- The largest diameter of a screw thread (applied to both external

and internal thread)

Figure 8.3: Right-hand thread and Left-hand thread

Figure 8.4: Crest, root and thread angle


4/19





- Minor diameter- The smallest diameter of a screw thread (applied to both external

and internal thread)

- Pitch- The distance from a

point on a screw thread to a

corresponding point on the

next thread measured

parallel to the axis. The

pitch is equal to 1 divided

by the number of threads

per inch.

- Lead- The distance a screw will advancewhen turned 360 degrees.

- Thread form- the cross section of thread

cut by a plane containing the axis.

Figure 8.5: Major diameter and minor diameter

Figure 8.6: Pitch in external and internal thread

Fi ure 8.7: Thread form


5/19





1.2. Thread manufacturing

a. Thread manufacturing by hand

* External threads- For producing external threads, the hand operated dies are

commonly use, which are coaxially rotated around the pre-machined rod like blank

with the help of handle or die stock.

+ Solid die: used for making threads of usually small pitch

and diameter in one pass.

+ Spring die: the die ring is provided with a slit, the width of

which is adjustable by a screw to enable elastically slight

reduction in the bore and thus cut the thread in number of passes

with lesser force on hands.

+ Split die: the die is made in 2 pieces, one fixed and one movable within the

cavity of the handle or wrench to enable cut relatively larger threads or fine threads

on harder blanks easily in number of passes, the die pieces can be replaced by

another pair for cutting different threads within small range of variation in size and

pitch.

+ Pie die: Pipe threads of larger diameter but smaller pitch are cut by manually

rotating the larger wrench (stock) in which the die is fitted through a guide bush.


6/19





* Internal threads- small size are cut manually in

plates, blocks, machine parts, etc. By using taps

which look and behave like a screw but made oftool steel and have shaft cutting edges produced by

axial grooving over the threads. Three taps namely,

taper tap, plug taps and bottoming tap are used

consecutively after drilling a tap size hole through

which the taps are axially pushed helically with the

help pg a handle or wrench.

b. Thread manufacturing by machine tools

+ By lathe- this is done by means ofthe lead screw, which is driven by a train

of gears from the spindle. Modern lathes

are fitted with change gears boxes, by

means of which any thread pitches can be

cut without working out and setting the

change gears. However, there are some

machines on which change gears must be

fitted for screw cutting.


7/19





Machining threads in semiautomatic lates:

+ Thread chasing process of cutting a thread on a lathe with a chasing toll that

comprises several sing-point tools banked together in a single tool called a chaser.

Chasing is used for the

production of threads

that are too large in

diameter for a die head.

Thread chasing reduces

the threading time by

50% compared to a

single-point threading.

However, thread chasing is a relatively slow method of cutting a thread, as a small

depth of cut is used per pass.


8/19





+ Thread milling- used a single form or multiple form milling cutter. Thread milling

makes smoother and more accurate threads than a tap or die. It is more efficient than

using a single cutting-point-tool in a lather.

2. Thread drawing

2.1. Detailed representation- representation is a close approximation of the

appearance of an actual screw thread.

- Use slanting lines to represent crest and roots.

- Roots and crest are drawn in sharp V

- Advantage: Looks good and clearly represents a thread

- Disadvantage: Takes a long time to draw


9/19





2.2. Schematic representation

- Use alternate long and short lines for representing crests and roots of the thread,

respectively. Should not be used for hidden internal threads or sections of external

threads.

- Advantage: Nearly as effective as the detailed representation and easier to draw.

- Disadvantage: Still takes some time to draw.

2.3. Simplified representation


10/19





- Use thick continuous lines for representing crest and thin continuous lines for

representing root of the thread, respectively.

- Advantage: Simple and fast to draw.

- Disadvantage: Does not look like a real thread.


11/19





2.3.1. Drawing steps of external thread

(1) Draw thread axis, major and minor diameter, starting position, thread length,

minor diameter, 45 degrees chamfer, line which is made 30 degrees with thread axis.

(2) Draw an arc that represents as a root, draw a circle that represents at a crest

2.3.2. Drawing steps of threaded hole

a. Through threaded hole:

(1) Draw thread axis, major diameter, minor diameter, draw a root and crest.

(2) Draw an arc that represents as a root, draw a circle that represents at a crest.


12/19





b. Blinded thread hole

(1) Draw thread axis, major diameter and minor diameter, thread depth, hole depth.

(2) Draw an arc that represents as a root, draw a circle that represents at a crest.

Section view of blinded threaded hole

3. Thread symbol

- Thread tables in the appendix can be used up value for the: pitch, minor diameter,

tap drill diameter.

- If screw thread tables are not available the minor diameter can be approximated as

75% of the major diameter.


13/19





a. Pitch diameter: The pitch diameter cuts threads at a point where the distance of

the spaces equal the distance of the threads.

- Metric thread note component


14/19





+ Metric form: Placing an Mbefore the major diameter indicates the metric thread

form.

+ Major diameter: the largest diameter.

+ Pitch (P): millimeters per thread.

+ Tolerance class: It describes the looseness or tightness of fit between the internal

and external threads (Number= tolerance guide; Letter= tolerance position).

+ Tolerance class: Tolerance grade: smaller numbers indicate a tighter fit; Tolerance

position: specifies the amount of allowance (upper case letter= internal threads;

lower case letter= external threads). 2 classes of metric thread fits are generally used:

6H/6g= general purpose or not specified; 6H/5g6g= Closer fit.

+ Right handed (RH) or Left handed thread (LH)+ Depth of thread: it indicates the thread depth for internal threads, not the tap drill

depth.

Dimensioning in external and and thread

hole

- Use local note to specify: - thread form,

nominal size, pitch (if it is a fine thread)

- Use typical method to specify: - threadlength.

- Use local note specify: tap drill size,

drill depth, thread form, nominal size, pitch,

thread depth.


15/19





4. Thread fastener

4.1. Bolt- a threaded cylinder with a head

Drawing steps:

- Draw bolt axis

-

Draw an end view of the head

-

Draw a bolt head

-

Draw body of a bolt


16/19





4.2. Nut

Drawing steps:

- Draw an end view of the nut

- Dash lines represent a threaded hole are omitted for clarity

Application of bolt and nut


17/19





4.3. Stud- headless bolt, threaded at both ends

Drawing steps

- Draw stud axis

- Major, minor diameter

- Thread length

- Draw 45 degrees chamfer and draw line making 30 degrees with

axis.

Application of stud


18/19





4.4. Cap screw- similar to bolt but has a longer thread than a bolt.

- Counterbore hole:

- Countersink hole:


19/19




Email thaophan@tn t ed n

4.5. Set screw- a thread cylinder used to prevent rotation or movement between

parts.

5. Keys, pins, rivets and springs- Self study

Chapter 08 Thread Fastener

Documents

Transcript of Chapter 08 Thread Fastener