Mechanical Engineering

Power Hacksaw

ObjectivesState safety precautions associated with

power sawIdentify the main parts of the sawSet up work and select speeds and feedsPerform cutting operations (square cutting,

angular cutting)

Power SawsPower saw are used to cut stock to the desired

length. It is more accurate than hand saws.

Power HacksawPower hacksaws are used to cut large sizes

(sections) of metals such as steel. Cutting diameters of more than 10/15mm is very hard work with a normal hand held hacksaw. Therefore power hacksaws have been developed to carry out the difficult and time consuming work. The power hacksaws main feature is its reciprocating frame. The heavy ‘arm’ moves backwards and forwards, cutting on the backwards stroke with the help of a downward force.

Parts of the Power HacksawBaseA mechanism causing the saw frame to

reciprocateClamping ViceFrameTableFeed knobCoolant pump

Types of Power HacksawsDirect Mechanical driveHydraulic drive

Direct Mechanical Drive

Hydraulic Drive

Feed MechanismThe feed mechanism regulates the amount

of downward pressure the blade applies on the material during the forward stroke.

Types of Feed PressureMechanical Feed- light

feed pressure on soft materials and work with small cross-section.

Hydraulic Feed- constant pressure but designed that when hard spots encountered, the feed stop or decrease the pressure until the hard spot has been cut through

Gravity Feed- which provides for the weights on the saw frame. These weights can be shifted to increase or decrease the pressure

Operating the Power Hacksaw

The metal to be cut is held in a machine vice which is an integral part of the base. Turning the handle tightens or loosens the vice. The vice is very powerful and locks the metal in position.

Operating the SawWhen cutting is taking place, the metal and especially the blade heats up quickly. Coolant should be fed onto the blade, cooling it down and lubricating it as it cuts through the metal.Without the use of coolant the blade will over heat and break/snap. This can be dangerous as the blade can break with powerful force, shattering.

When the metal is placed and fixed in the vice, the blade is lowered onto its top surface. The diagram below shows the ‘arm’ being lowered with the ‘adjusting handle’.

Driving MechanismCircular motion is transformed into linear

motion. The crank which is the rotating disc, the slider which slides inside the tube and the connecting rod which joints the parts together.

Driving Mechanism

If the belt is placed on the smaller pulley wheel the speed of cut will be fast. Changing the belt so that it runs round the larger pulley wheel will reduce the speed

Types Of BladeWhen selecting the

blade at least three teeth must be in contact with the work at all times.

Flexible- used for cutting odd shapes and is shatter proff.

All hard- straight and accurate cutting but shatter when breaks.

Blade PitchThe correct pitch of teeth is determined by:The size of the section,The material to be cut

Type of PitchCoarse- soft steel, cast iron and bronzeRegular- anneal high carbon steel and high

speed steelMedium- for solid brass stock, iron pipe,

and heavy tubingFine- thin tubing and sheet metals

SafetyNever attempt to mount, measure or remove

work unless machine is turned offGuard long materials at both endsUse cutting fluid whenever possibleHold thin materials flat in the vice to

prevent breaking of blades