ALMOST ALL METALS commonlyfound in workshops-iron, mild

steel, cast iron, brass, copper-can becut easily by hand if the correct sawis used. The exceptions are steelswhich have been hardened andtempered for special purposes.

The metal saw differs from thewood saw in that it employs separateblades which have been speciallyhardened and tempered for metalcutting. Because of this, the bladescannot be sharpened with files likewood saws, so when worn out theymust be renewed. As they are hard,or semi-hard, they are liable to crackor break if twisted or wrung exces-sively.

Tubular frames for rigidityFrames to take metal saws, or

hacksaws as they are known, are madein a variety of sizes and designs forblades 9 in., 10 in. and 12 in. long.The standard non-adjustable frametakes blades of one length, but thereare adjustable models of similar de-sign which take all lengths of blade.Tubular frames are, in general, morerigid than those of flat section.

For many purposes, the depth ofcut with a hacksaw is limited by thedepth of the frame+-i.e., the distancefrom the blade to the back of theframe. Special frames are availablefor exceptional depths of cut, however;these are called girder frames. Alight frame, known as a junior, andformed from solid rod takes short finetoothed blades.

Under no tensionA pad handle is made to take a

standard hacksaw blade? or portion ofblade, the projection bemg adjustable.This has the advantage that it can beentered in awkward places where astandard or junior frame would beobstructed, and can also be used forcutting down wide sheets of metal. Asthe blade is not held in tension, morecare is necessary when using the padhandle.

Standard hacksaw blades have teeth

1 SEPTEMBER 1955

of 14, 18, 24 and 32 pitch. This term“ pitch ” means the number of teethin 1 in. Blades with the smallernumbers of teeth are called coarsepitch, and those with the greaternumbers of teeth are of fine pitch.Pitches of 18 and 24 are commonlyused for general work; 14 pitch canbe used if the metal is very thick;32 pitch can be used for sheet metalor thin-walled tubing.

CORRECTCOARSE PITCH

CORRECTFlNE PITCH

The right and wrong ways of cutting

Junior blades are 6 in. long of 32pitch, and have a pin at each end formounting in the frame. Circular bladeshave a spiral tooth, and these aremounted in a standard hacksaw frameemploying clips. These enable one tocut as with a fretsaw but in metal.

There are other types of saws andframes for particular purposes. Saws

249

which are parallel in vertical sectionand without any set on the teeth areslitting saws; these produce a straightsmooth-sided cut, but should not beused for general sawing. Hacksaws,junior saws, and similar blades, havea set on the teeth which providesclearance in the cut.

Choosing the bladesWhen choosing a blade for a par-

ticular job remember a coarse pitchof tooth for large sections and softmetals such as aluminium; and a finepitch for harder metals such as caststeel or silver steel, and for thin sheetmetal and thin-walled tubing. Shorterblades will serve the same purpose aslong ones in many cases and aremore economical. Long blades arebetter if the section of metal is large.

The blade should lie snugly againstthe side of its attachments, with thepins firmly in the holes, and should beput in tension so as not to whip orbend when in use ; the blade shouldremain firm and straight and shouldnot be over-tensioned. For makinglong cuts down the sides of sheets,the blade can be mounted at right-angles to the frame, by turning thefittings through 90 degrees beforemounting the blade.

How to make a cutA cut should be started with short.

strokes, guiding the blade with thumbor fmger. When a start has beenmade, the strokes should be long andfirm using the full length of the blade.

This method ensures that all theteeth are equally worn and therefore asmooth cut wiIl result. If only themiddle of the blade is used for a longtime, when the ends of the blade arebrought into use they tend to “ seize.”This is due to the set not being worn.For this reason a new blade should notfollow in the cut made by an oldblade.

Cutting metals is a matter of know-ledge rather than knack and the correctand incorrect ways of cutting areillustrated.

MODEL ENGINEER