2 Module ObjectivesAfter 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.
3 4.1. Introduction to hack sawing Hack sawing is a process of cutting metal to size using a cutting tool called hacksaw.
4 4.2 Main parts of a hacksawThe typical hacksaw consists of a saw blade supported by a frame fitted with a handle to give a firm grip
5 4.3 Types of hacksaw frames There 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.
6 Type 2: Adjustable hacksaw frame In this type different standard lengths of blades can be fitted to this frame.
7 4.4 Hacksaw blades 2 pin holes Center line Side Back edge The hacksaw blade consists of different parts:2 pin holesCenter lineSideBack edge
8 Characteristics of a hacksaw blade The 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)
9 4.4.2 Hacksaw blade pitch 1- Coarse pitch teeth 2- Medium pitch teeth The pitch of the hacksaw blades are divided into 3 classes:1- Coarse pitch teeth2- Medium pitch teeth3- Fine pitch teethThe classification depends on the number of teeth per 25 mm also know as pitch
10 Coarse pitch teethThe coarse pitch blade has teeth/25 mm. It is used to cut thick and soft work pieces.
11 Medium pitch teethThe medium pitch blade has teeth/25 mm. It is used to cut medium hard steel and copper alloys.
12 Fine pitch teethThe medium pitch blade has 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.
13 Number Of teeth per 25 mm (Pitch) Name of Pitch ClassThickness Class used forMaterial Class used for14 to 18Coarse Pitch TeethThickSoft18 to 24Medium Pitch TeethMediumHard Steel and Copper Alloys24 to 32Fine Pitch TeethThinHard
14 Tooth selectionSelecting 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.
17 4.4.3 Blade settingThe 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
18 The most common types of sets used Alternate set: The teeth are arranged alternately
19 Wave set: the teeth are arranged like a wave form
20 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.
21 4.4.5 Saw blade defects and their causes: Fast BluntingIf wrong teeth number is chosen.If saw blade is not tightened properly.Teeth breakingTooth pitch is too small while cutting thin elementsIf material is not tightened well.The hacksaw blade consists of different parts:2 pin holesCenter lineSideBack edge
22 Saw blade defects and their causes: 3. Breaking of saw bladeThis occurred when the cutting material is not tightened properly4. Curved cuttingWrong fixing of blade in the frame.Blade is insufficiently tightened.
23 Defects Causes Figures Fast bluntingWrong teeth numberBlade is not tightenedTeeth breakingPitch too smallMaterial not tightenedBreaking of saw blade1. Cutting material is not tightenedCurved cuttingWrong fixing of bladeBlade not tightened
24 4.4.6 Hacksaw blade checklist The following points should be taken in consideration before using the hacksawSelect the correct pitch for the material you want to cut.Ensure that the blade has the correct tension.Ensure that the blade is not twisted or discolored.Ensure that the teeth are pointed away from the handle.Ensure that there are no broken or blunt teeth.
25 4.5 Hacksaw safety tipsAlways wear safety goggles while using a hacksaw.Be sure that the hacksaw blade is properly tensioned.Do not brush away chips with your hand; use a brush.Never test the sharpness of a blade by running your fingers across its teeth.Keep the saw blades clean, and use light machine oil on the blade to keep it from overheating and breaking.
26 4.6 Cutting with the hacksaw Hold the hacksaw properly at an angle of 30°When cutting, move your body rather than just your arms.Apply pressure only during the forward stroke (cutting stroke).Use the entire length of the blade in each cutting stroke.The usual cutting speed is from 40 to 50 strokes per minute