Power hacksaw blades differ from hand hacksaw blades in that they are generally heavier, made in longer sizes, and have fewer teeth per inch. Hacksaw blades are discarded when they become dull; sharpening is not practical.
Materials commonly used in manufacturing power hacksaw blades are high-speed tungsten steel and high-speed molybdenum steel. On some blades only the teeth are hardened, leaving the body of the blade flexible. Other blades are hardened throughout.
The set is the amount of bend given the teeth. The set makes it possible for a saw to cut a kerf or slot wider than the thickness of the band back (gage), thus providing side clearance.
This is the pattern in which the teeth are set. There are three set patterns: raker, wave, and straight, as shown in Figure 7-4.
The pitch of hacksaw blade teeth (Figure 7-5) is expressed as the number of teeth per linear inch of blade. For example, a blade having 10 teeth per inch is said to be 10 pitch.
Power hacksaw blades are coarser in pitch (fewer teeth per inch) than hand hacksaw blades. Common pitches for power hacksaw blades range from 4 to 14 teeth per inch.
The following are guidelines for the selection of power hacksaw blades.
Select power hacksaw blades for material to be cut.
Soft materials require a coarser blade to provide adequate spaces between the teeth for removal of chips. Hard material requires a finer blade to distribute the cutting pressure to a greater number of teeth, thereby reducing wear to the blade.
At least three teeth must be in contact with the workpiece at all times or the blade will snag on the workpiece and break teeth from the blade. Therefore, a blade must be selected with sufficient pitch so that three or more teeth will be in contact with the workpiece, no matter what type of material is being cut.
Figure 7-5 is provided to assist in the proper selection oxf power hacksaw blades. Note that sheet metal and tubing are listed separately from solid stock. It is assumed that solid stock will be sufficiently thick that three or more teeth will be in contact with the stock at all times.
Figure 7-4. Set pattern.
Figure 7-5. Selection of power bandsaw blade.
tubing. If the sheet metal is too thin to meet this requirement with the finest tooth blade available, place the metal between plywood fiberboard, or thicker metal. Figure 6-7 is a guide for selecting the proper pitch band saw blade for different metals and metal thickness.
The finish depends largely upon the saw pitch. The faster the saw speed and the finer the sawpitch, the finer the finish. Lubricating helps to improve the finish. A fine saw pitch, high velocity, and light feed produce the finest finish.
Bandsaw Blade Wear
Bandsaw blades naturally become dull from prolonged use, but some conditions promote greater than normal wear on the blades. Blades dull quickly if used at too high a speed for the material being cut. Also, if the material to be cut is too hard for the pitch of the blade, abnormal wear will result. The most common cause of premature blade dulling occurs from using too fine a pitch blade and from feeding too heavily.
The following symptoms indicate a dull bandsaw blade. When these symptoms are noticed, the blade should be replaced.
It becomes difficult to follow a line, the blade being forced to one side or the other.
The chips are granular (except for cast iron, which produces granular chips with both sharp and dull blades). The bandsaw blade cuts slowly or not at all when the workpiece is fed by hand.
With the machine stopped or the bandsaw blade removed, run a finger slowly over the teeth in the cutting direction. If sharp edges are not felt the blade is dull.
Figure 7-6. Saw hand selection for various radii.
Figure 7-7. Selection of bandsaw blades.
The bandsawing machine is adapted for filing by use of the band file attachment. A band file is fitted over the drive and idler wheels and in place of the bandsaw blade. The band is made up of several parts or segments which are riveted at one end (the leading end) to a spring steel band. The trailing end of each segment is free to lift during the time when the band bends over the drive and idler wheels of the band saw. When the band straightens out, the segments lock together.
Figure 7-8 shows the construction of and terminology for file band parts.
Note that the gate segment (a segment at one end of the band that is specially designed to allow the two band ends to be locked together) has a shoulder rivet and a dowel rivet protruding from beneath it. The shoulder rivet locks into the other file band end, and the dowel rivet aligns the two end segments and prevents the shoulder rivet from sliding out of the locked position during tiling. The gate segment of a file band is identified by yellow paint.
Cut Of File Teeth
File bands are either coarse or bastard cut and normally range in pitch from 10 to 20 teeth per inch. The coarse 10-pitch bands are used for filing softer metals such as aluminum, brass, copper, and cast iron. A bastard-cut 14-pitch band is a good choice for general steel tiling, while 16 to 20 pitch bastards are recommended for filing tool steel.
Figure 7-8. Construation and parts of a file band.Selection of Band Files
Choose band files on the basis of workpiece thickness and type of material to be filed. In general, the thicker the workpiece, the coarser the file should be. This is due to a large; chip accumulation from the larger area of the workpiece, thus requiring additional space for the chips between the teeth. On thin sheet metal, a fine pitch file is required to prevent chatter. Use fine pitch files for filing tough carbon and alloy steels; use coarser pitch files for filing softer metals.
Figure 7-9 is provided to aid in selecting the proper file for filing specific materials.
Figure 7-9. Selection of band files.
Care and Cleaning of Band Files
Clean the file often, using a stiff brush or a file card. Move the brush in the direction of each cut of the file to dislodge all particles hidden between the teeth.
The file band should not be coiled into more than three loops. The best means of storing file bands is in a cabinet looped over a 16-inch radius support with the ends hanging free.
Band File Attachment
A band file attachment (Figure 7-10) is provided with most bandsaw machines to permit the use of band files. A typical band file attachment consists of a band file guide and upper and lower guide supports that attach to the frame and part of the band saw. A special filing filler plate is provided to adapt the table slot to the extra width and depth required for the band file and file band guide.
Figure 7-10. Band file attachment installed on bandsawing machine.
Polishing can be performed on the bandsaw using a olishing attachment and polishing band. The polishing and is usually 1 inch wide and has a heavy fabric backing.
Types of Polishing Bands
Polishing bands for bandsawing machines are usually supplied in various grain sizes of aluminum-oxide or silicone carbide abrasive: No 50 grain (coarse) for heavy stock removal and soft material, No 80 (medium) for general surface finishing, and No 120 or No 150 grain (fine) for high polishing and light stock removal.
Selection of Polishing Bands
Polishing bands should be selected according to the particular job to be performed, For removing tool marks and deburring edges, use the No 50 grain polishing band. Finer grain polishing bands should not be used on soft metals like aluminum or cast iron because the band will quickly fill with metal particles, reducing the cutting action
The polishing attachment (Figure 7-11), similar to the band file attachment, provides support for the polishing band. The polishing band plate acts as a solid backing for the polishing band to prevent stretching and distorting the band when the workpiece is held against it. Use a polishing band filler plate to fill the table slot so the workpiece can be supported close to the polishing band.
Figure 7-11. Polishing attachment installed on band sawing machine.
Use the disc-cutting attachment (Figure 7-12) to saw internal or external circles and discs. The diameter of the circle that can be cut is limited to the length of the cylindrical bar on the attachment or to the throat depth of the machine. The disc-cutting attachment consists of three main parts a clamp and cylindrical bar, which is fastened to the saw guidepost; an adjustable arm, which slides on the cylindrical bar; and a pivot or centering pin. The disc must be laid out and center-drilled to a depth of 1/8 inch to 3/16 inch to provide a pivot point for the centering pin. The centerline of the centering pin must be in line with the front edge of the saw teeth and at the desired distance from the saw band.
Figure 7-12. Disc-cutting attachment.
ANGULAR BLADE GUIDE ATTACHMENT
This attachment (Figure 7-13) twists the blade so that long workpieces that would not normally clear the machine column can be cut. The blade is twisted to a 30 degree angle on most machines.
MITER GUIDE ATTACHMENT
A typical miter guide attachment is illustrated in Figure 7-14. The workpiece is supported against the miter head which attaches to the slide arm. The attachment can be set at an angle with a protractor, using the table slot as a reference line. A gage rod can be extended from the attachment and used as a stop when identical lengths are sawed. When not in use, swing the attachment on the slide rod so that it hangs below the table.
Figure 7-13. Angular saw guides.
Figure 7-14. Miter guide attachment.
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