Thread Cutting on Lathe

ELEMENTS OF A SCREW THREAD:-

OD

out side dia or major dia of the thread. A screw thread is specified by its out side diameter, type of thread and its pitch Ex: M20 X 1.5 Means metric thread out side dia 20 mm and pitch 1.5 mm. “3/4” out side dia. If pitch is not specified, it is to be taken as standard pitch.

RD:-

Root diameter or core diameter or minor diameter of the thread. For cutting internal threads a hole equal to root diameter is to be drilled or bored first

Core dia= OD- 2 X Depth of thread

Pitch diameter (PD):-

The diameter of a thread is an imaginary diameter which passes through the mid point of the thread depth . It is also called as effective diameter and is the basis for all thread calculations.

Thread profile angle :-

The included angle between the two flanks of the thread is the thread profile angle. It depends on type of thread.

Thread axis:-

Thread axis passes through the centre of the thread along it length. It is the reference for the thread angle.

Helix angle:-

For screw threads the angle is measured between a perpendicular to the tangent axis and a line tangent to the flank of the thread.

Pitch:-

The distance between the consecutive threads parallel to the axis is called pitch.

Lead:-

The linear distance a nut or a screw travels in one full rotation is called lead. In single start threads the leads is equal to the pitch. In multi start threads the lead is equal to the pitch of the thread multiplied by the no.of.starts

TYPES OF THREADS:

METRIC THREADS

BRITISH STANDARD THREADS WITH WORTH THREADS

SQUARE THREADS

ACME THREAS

TREPIZOIDAL THREADS

BUTTERS THREADS

SAW TOOTH THREADS

KNUCKLE THREAD

TAPER THREAD

RIGHT HAND THREAD

LEFT HAND THREAD

MULTI START THREAD

1. METRIC THREADS:-

This is a v-shaped threads. Angle of the thread is 60° pitch of the thread is specified in mm. used

2. BSW threads:-

Like metric threads this also a v- shaped thread . Angle of the thread is 55°. Pitch is specified in inches as 7p( Thread pitch). Used for general lpurpose.

Example:- ¾”

3.Square threads:-

Form of the thread is a square. Depth of thread and witdth of thread is equal to ½ pitch. Pitch may be in mm or inches. Used when stronger thread in required like in vices, hand presses etc.

4. ACME THREAD:-

Form of the thread is trepezoidal. This is modified form of square thread. Angle of the thread is 29°. Used for heavy loads and where frequent engagement and disengagement of the screw and nut are required. Pitch may be in inches or mm.

5. TREPEZOIDAL THREAD:-

This is a metric version of Acme thread. Shape of the thread is trapezoidal angle of the thread is 30° pitch is in mm. Used where heavy loads are to be transmitted.

6. BUTTRESS THREAD:-

Shape of the thread is like a saw tooth. Angle of the thread is 45° used where one sided loads act like in collects. One side of the thread is straight and other side is inclined at 45°to the axis. Pitch is in inches.

7. SAW TOOTH THREAD:-

This is a metric version of buttress thread is inclined by 3° to the vertical to the axis and the other side is inclined by 30° to the vertical in the opposite direction. Pitch is in mm. Used for one sided loads.

8. KNUCKLE THREAD:-

Shape of the thread is curved. Also called as pipe thread. This is mainly used for pipe joint . Effective against leakages in gas and liquid flow.

9. TAPER THREAD:-

Taper threads are useful for self locking in one direction and easy removal in the opposite direction. Threads are cut on a cone, Usually v- shaped. Generally used on buffing and polishing machines.

10. LEFT HAND AND RIGHT HAND THREADS:-

These may have any shape and may be in mm or inch thread i.e., any thread can be cut in right hand or left hand direction. A screw having right hand thread moves away from the operation when rotated in a nut in clock wise direction and a left hand threaded screw will move towards in a nut is clock wise direction and a left handed threaded screw will move towards the operation when rotated in a nut in clock wise direction. These are used according to necessity.

11. MULTI START THREADS:-

A multi start thread is a thread whose lead is a multiple of its pitch. For example the lead is a multiple of its pitch thread is 3 X 2 =6 mm. These are used where a longer distance is moved with the same strength of a single start thread. At such large leads, a single start thread would require large depth resulting in weakening of thread of root diameter.

Change Gear Calculations

Introduction

External and internal Threads can be cut on a Lathe with the help of the lead screw and gear wheels. For each revolution of the work pieces, the threading tool has to move a certain linear distance equal to the lead of the thread to be cut. To accomplish this, the spindle has to be connected to lead screw with gear calculations.

The tool gets its motion from the lead screw, and the lead screw is rotated by the spindle gears. The gear ratio between the spindle and the lead screw depends on the pitch of the thread to be cut anc pitch of the lead screw.

The relation is expressed by the formula

Where

Z1= No. of teeth on driving gear (Spindle Gear)

Z2= No. of teeth on driven gear (Lead Screw Gear)

Pw= Pitch of Work Piece

Pl= Pitch of Lead Screw

Cutting British Threads on British Lathe

The pitch of a BSW thread is specified either directly in inches or TPI. The Pitch of the lead screw in a British Lathe is also Specified in same Way

In a case where the pitch of the work piece is in inches while the pitch of the lead screw is in mm. we have to use following formula

Cutting metric threads on British Lathe

In this case the pitch of work piece is in mm while the lead screw is in Inches. So we need to use the following formula

Multi Start Threads

Multi start threads are used where a long travelling distance axial direction is required with a small turn. For example, Pen caps, Bottle caps. For such a big Pitch, Single start threads need to be turned so much.

Change Gears

Lead= Pitch * No. of Starts