MODULE

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ACKSAWS

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MODULE OBJECTIVES

After the completion of this module, student should be able to: Describe the basic hack sawing process.

Identify the parts of a hacksaw.

Name the parts of a saw blade.

Identify the correct blades that should be fitted for different materials.

Demonstrate how to fit and remove a hacksaw blade correctly.

Demonstrate the procedure used when cutting materials.

Identify some defects of saw blades and their causes.

Cut a metal work piece by using a hack saw to a given dimensions.

4.1. INTRODUCTION TO HACK SAWING Hack sawing is a process of cutting metal to size using a

cutting tool called hacksaw.

The typical hacksaw consists of a saw blade supported by a frame fitted with a handle to give a firm grip

4.2 MAIN PARTS OF A HACKSAW

4.3 TYPES OF HACKSAW FRAMESThere are two types of hacksaw frames:

Type 1: Solid hacksaw frame

In this type, only one particular standard length of blade can be used with this frame.

Type 2: Adjustable hacksaw frame

In this type different standard lengths of blades can be fitted to this frame.

4.4 HACKSAW BLADESThe hacksaw blade consists of different parts:

- 2 pin holes- Center line- Side - Back edge

CHARACTERISTICS OF A HACKSAW BLADEThe hacksaw blade has 2 main characteristics

1- Teeth pitch which is the number of teeth per 25 mm.

2- Blade length which is the length between the centers of its pin holes. (See previous Slide)

4.4.2 HACKSAW BLADE PITCH

The pitch of the hacksaw blades are divided into 3 classes:

1- Coarse pitch teeth

2- Medium pitch teeth

3- Fine pitch teeth

The classification depends on the number of teeth per 25 mm also know as pitch

COARSE PITCH TEETH

The coarse pitch blade has 14-18 teeth/25 mm. It is used to cut thick and soft work pieces.

MEDIUM PITCH TEETH

The medium pitch blade has 18-24 teeth/25 mm. It is used to cut medium hard steel and copper alloys.

FINE PITCH TEETH

The medium pitch blade has 24-32 teeth/25 mm. It is used to cut thin and hard work pieces.

 Note: A fine pitch blade has more teeth per 25 mm than a coarse pitch blade.

Number Of teeth per 25 mm (Pitch)

Name of Pitch Class

Thickness Class used for

Material Class used for

14 to 18 Coarse Pitch Teeth

Thick Soft

18 to 24 Medium Pitch Teeth

Medium Hard Steel and Copper Alloys

24 to 32 Fine Pitch Teeth Thin Hard

TOOTH SELECTION

Selecting a blade with the correct number of teeth depends on the thickness and type of the material to be cut.

During the cutting process there should be at least 3 teeth in contact.

For thin sections the highest number of teeth should be selected.

Fine Pitch Teeth

Coarse Pitch Teeth

4.4.3 BLADE SETTING

The teeth are set so that the blade makes a cut wider than itself.

This helps to prevent the blade from jamming (becoming blocked) and provides for good chip clearance when cutting the metal

 THE MOST COMMON TYPES OF SETS USED

Alternate set: The teeth are arranged alternately

Wave set: the teeth are arranged like a wave form

4.4.4 INSTALLING THE BLADE IN THE HACKSAW FRAME.

1- Install the teeth point away from the handle of the hacksaw.

2- Tighten the wing nut until the blade is definitely under tension. This helps make straight cuts.

4.4.5 SAW BLADE DEFECTS AND THEIR CAUSES:

1. Fast Blunting If wrong teeth number is chosen. If saw blade is not tightened properly.

2. Teeth breaking Tooth pitch is too small while cutting thin

elements If material is not tightened well.

The hacksaw blade consists of different parts:

- 2 pin holes- Center line- Side - Back edge

SAW BLADE DEFECTS AND THEIR CAUSES:

3. Breaking of saw blade This occurred when the cutting

material is not tightened properly

4. Curved cutting Wrong fixing of blade in the frame. Blade is insufficiently tightened.

Defects Causes Figures

Fast blunting 1. Wrong teeth number

2. Blade is not tightened

Teeth breaking 1. Pitch too small2. Material not

tightened

Breaking of saw blade

1. Cutting material is not tightened

Curved cutting 1. Wrong fixing of blade

2. Blade not tightened

4.4.6 HACKSAW BLADE CHECKLIST

The following points should be taken in consideration before using the hacksaw

1.Select the correct pitch for the material you want to cut.

2.Ensure that the blade has the correct tension.

3.Ensure that the blade is not twisted or discolored.

4.Ensure that the teeth are pointed away from the handle.

5.Ensure that there are no broken or blunt teeth.

 4.5 HACKSAW SAFETY TIPS

1. Always wear safety goggles while using a hacksaw.

2. Be sure that the hacksaw blade is properly tensioned.

3. Do not brush away chips with your hand; use a brush.

4. Never test the sharpness of a blade by running your fingers across its teeth.

5. Keep the saw blades clean, and use light machine oil on the blade to keep it from overheating and breaking.

4.6 CUTTING WITH THE HACKSAW

1.Hold the hacksaw properly at an angle of 30°

2.When cutting, move your body rather than just your arms.

3.Apply pressure only during the forward stroke (cutting stroke).

4.Use the entire length of the blade in each cutting stroke.

5.The usual cutting speed is from 40 to 50 strokes per minute