Mechanical Engineering Technology

Topic: Threads and thread cutting operations

WELCOME

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.

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

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.

Diagram of Screw Thread

Definitions of PartsMajor 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

Definitions Cont’dFlank: 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.

ACTIVITY # 1Label the parts represented by the letters on

the diagram below.

KEY FOR ACTIVITYA – Flank I - CrestB – Pitch J - RootC – Helix Angle K - DepthD – Minor Diameter L - ScrewE – Pitch DiameterF – Major DiameterG – NutH – Thread angle

TWO MAIN CATEGORIES OF A THREAD

External Thread: this is the thread on a bolt.

Internal Thread: this is the thread inside a nut.

External Threads

Internal Threads

Types of Thread

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.

THREAD CUTTING OPERATIONSTap and Die MethodLathe Operation Method

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.

Types of Tap

Taper TapSecond or Intermediate TapPlug or Bottoming tap

TYPES OF TAP

Tools used for holding Taps

Chuck Pattern Adjustable Tap Wrench Tap Wrench

How to use wrench and tap together.

Cutting External ThreadsDies are used to cut external threads by hand. Solid Dies, are not adjustable and forthat reason are seldom used. The AdjustableRound Die, are the most widely used die forhand use. The latter has a wide range of adjustmentsand is fitted with guides to keep it true. Diesare available for cutting most standardThreads.

TYPES OF DIE

Circular split Die Rectangular/Half Die

Care in TappingConsiderable 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.

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

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.

Thread Cutting on the LatheThread cutting on the lathe is a process that produces a helical ridge of uniform sectionon a work piece. This is performed by takingsuccessive cuts with a threading toot bit ofthe same shape as the thread formrequired. Work to be threaded may be heldbetween centres or in a chuck. If held in achuck, the work should be turned to sizeand threaded before it is removed.

CUTTING SCREW THREADS ON THE LATHE

Screw threads are used for manyreasons, including: Making adjustmentsAssembly PartsTransmitting MotionApplying PressureMaking Measurements (eg. micrometer)

THE CENTRE LATHE

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.

Steps Cont’d5. 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.

Steps Cont’d8. 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.

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.

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.

QuizWrite “T” if the statement is True and

“F” if the statement is false1. Taps are used for cutting internal threads. T F2. Taps come in sets of three T F3. Dies are made from mild steel T F4. The die is fitted into the tap wrench T F5. The crest is located at the top of the thread. T F 6. The root is located between successive threads

T FClick here for answer

Answers for Quiz

1. T2. T3. F4. F5. T6. F