Information on thread cutting operation

1. Mechanical Engineering
Technology
Topic: Threads and thread cutting
operations
WELCOME

2. OBJECTIVES
By the end of this lesson you will be
able to:
Correctly identify and state the
different parts of the screw thread.
State the correct order and
conditions to successfully
complete a threading operation.

3. Objectives cont’d.
Know the different ways in which
screw threads can be produced.
Know all safety precautions in
relation to producing screw threads.

4. What is a Thread?
Thread: it is the locus of a
point which moves
around and along a
cylinder at a constant
rate. The helix is the basis
of all screw threads.

5. Diagram of Screw Thread

6. Definitions of Parts
Major Diameter or Nominal Diameter: is
the outside diameter across the crests.
Minor Diameter or Core Diameter: is
the inside diameter from root to root.
Crest: the outside part of the thread.
The term applies to both internal and
external threads.
Root: This is the lowest portion of the
groove between two adjacent thread
forms

7. Definitions Cont’d
Flank: This is the straight side of the
thread joining the root and the crest
Pitch: the distance between two adjacent
crests on a thread. The pitch equals the
leads for single start threads, half the lead
for two start thread and a third of the lead
for three start threads and so.
Lead: is the distance a nut will travel in
one revolution along a bolt. On single start
threads the pitch is equal to the lead but
they become different on multiple start
threads.

8. ACTIVITY # 1
Label the parts represented by the letters on
the diagram below.

9. KEY FOR ACTIVITY
A – Flank I - Crest
B – Pitch J - Root
C – Helix Angle K - Depth
D – Minor Diameter L - Screw
E – Pitch Diameter
F – Major Diameter
G – Nut
H – Thread angle

10. TWO MAIN CATEGORIES OF A
THREAD
External Thread: this is the
thread on a bolt.
Internal Thread: this is the thread
inside a nut.

11. External Threads

12. Internal Threads

13. Types of Thread

14. APPLICATIONS OF THREADS
 Acme Thread: it is a modified form of square
thread. It is easier to machine than a square
thread. It is used for producing transverse
motion in machine tools.
 Buttress Thread: it is used for transmitting
heavy loads in one direction only. It is used
in screw vices such as the woodwork vice.
 Square Thread: it presents a robust screw
for controlled movement in machine vices,
the lead screws of lathes, presses, screw
jacks and spindles of valves.

15. THREAD CUTTING
OPERATIONS
Tap and Die Method
Lathe Operation Method

16. Tap and Die
Cutting Internal Threads
Internal Threads are made by using a
tap. Taps are made of Carbon Steel
or High Speed Steel (HSS) and are
carefully hardened and tempered for
long life. Taps are quite brittle and are
easily damaged if not handled
properly.

17. Types of Tap
Taper Tap
Second or Intermediate Tap
Plug or Bottoming tap

18. TYPES OF TAP

19. Tools used for holding
Taps
Chuck Pattern Adjustable
Tap Wrench Tap Wrench

20. How to use wrench and tap together.

21. Cutting External Threads
Dies are used to cut external threads by
hand. Solid Dies, are not adjustable and for
that reason are seldom used. The Adjustable
Round Die, are the most widely used die for
hand use.
The latter has a wide range of adjustments
and is fitted with guides to keep it true. Dies
are available for cutting most standard
Threads.

22. TYPES OF DIE
Circular split Die Rectangular/Half Die

23. Care in Tapping
Considerable care must be exercised
when tapping.
Use the tap before the die because the
taps are non-adjustable for size, the
dies are
Chamfer the rod end before you start
using the die.
Start the tap square.
Use a cutting solution where
necessary.

24. Care in Tapping Cont’d
 Ensure that a blind hole is drilled
deeper than the depth of thread
required to allow clearance for
chipping.
 Do not force a tap to cut. Remove
the chips as necessary.
 Give backward turns to break off
the chippings

25. Hand Threading Safety
 When using the tap or die use compressed air to
clean the piece that you are threading.25
 Chips from the work can be sharp so use a brush
to remove them.
 Wash hands after using cutting fluid because it
can cause skin disorder.
 Be sure that die is securely clamped in the die
stock because it can fall and cause painful
injuries.
 Be careful when handling broken taps because
they have sharp edges.

26. Thread Cutting on the Lathe
Thread cutting on the lathe is a process that
produces a helical ridge of uniform section
on a work piece. This is performed by taking
successive cuts with a threading toot bit of
the same shape as the thread form
required. Work to be threaded may be held
between centres or in a chuck. If held in a
chuck, the work should be turned to size
and threaded before it is removed.

27. CUTTING SCREW THREADS ON
THE LATHE
Screw threads are used for many
reasons, including:
Making adjustments
Assembly Parts
Transmitting Motion
Applying Pressure
Making Measurements (eg. micrometer)

28. THE CENTRE LATHE

29. Steps involved in cutting Threads on
the Lathe
1. Obtain the required threads per inch
(tpi) by gearing up the machine using
the split nut.
2. Sharpen the cutting tool to the
required thread angle, for e.g. 60º
3. Set up the work and the tool in correct
position, with the compound slide
swiveled to half the thread angle.
4. Engage a slow speed on the machine.

30. Steps Cont’d
5. Engage the split nut lever (automatic
feed) with the thread chasing dial at
the correct datum.
6. Advance the compound slide a small
amount for the depth of cut.
7. Start the lathe and engage the split
nut lever. The tool now will move
along the work cutting a helical ridge,
the pitch of which will be dependent
on the gear train that has been set up.

31. Steps Cont’d
8. At the end of the cut, disengage the
split nut lever and bring the cross
slide forward the same time.
9. Reposition the carriage for a second
cut, bring the cross slide back to its
original setting and advance the cut
on the compound slide.
10. This should be done until the correct
size thread is obtained.

32. Safety Precautions when Cutting
Threads on the Lathe
Ensure that coolant and shields are
in place.
Stop the lathe before taking
measurements of any kind.
Never use callipers or gauges on a
rotating work piece.

33. Safety Precautions Cont’d
 Make sure that the lead screw is
cleaned and oiled before starting
operation.
 Apply cutting fluid/coolant while
cutting the thread.
 Make sure that the machine oil is at
the required level.

34. Quiz
Write “T” if the statement is True
and “F” if the statement is false
1. Taps are used for cutting internal threads. T F
2. Taps come in sets of three T F
3. Dies are made from mild steel T F
4. The die is fitted into the tap wrench T F
5. The crest is located at the top of the thread. T F
6. The root is located between successive threads
T F
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35. Answers for Quiz
1. T
2. T
3. F
4. F
5. T
6. F