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Band saw filing
Band saws in timber mills range in size from about (4" x 22ga x 10') to (16" x 11ga x 62'). They are run for 4 to 24 hours then sent to the sawfiler for maintenance.
The sawfiler inspects the saw for needed repairs then gumms, fits and benches the saw as necessary.
Gumming involves grinding the gullets of the saw teeth to a particular shape. The sawfiler uses a semi-automatic grinding machine for this. Band saws operate under high stress and heat and in the presence of wood chips. Carbon migrates into the steel from the wood. Gumming prevents case hardening and fatigue cracking of the band saw gullets. Resaw Band saws (teeth on one side only) may be left or right handed, depending on which way the teeth are pointing and which way the plank falls from the log when cutting. Double cut saws (teeth on both sides) are always gummed right hand teeth first.
A precise tooth profile of the gullet must be maintained for proper saw operation and wood chip removal. Ease of cutting greatly depends on this. The shape is determined by the type of wood and cutting conditions. A sawfiler will maintain the gullet shape by manually shaping the grinding wheel with an abrasive brick, and the set-up of his grinding machine. Variations include face angle, face length, back angle, gullet width and depth, and a frost notch (if necessary). Typical band saw tooth dimensions are 1-3/4" tooth space x 3/4" gullet depth x 3/4" gullet width (grinding wheel width) x 30deg face angle x 16deg back angle.
Fitting means tooth dressing and involves; swaging, shaping, gauging, and grinding. The tip of the saw tooth is swaged to a flair, then the sides are compressed in slightly with a shaper tool to an exact kerf. Then a final grinding pass is made. The usual gauged tolerance is +/- .005" in kerf, and < .003" side to side variation. The same grinding machinery used for gumming is used for fitting.
The saw kerf is usually made this way from the base saw metal. Sometimes, however, the kerf is made with stellite or carbide tips, in which case swaging and shaping isn't needed, although gumming is still required. The kerf may also be 'set' with a punch and hammer, with the teeth bent left, right, left... Set teeth are rarely used.
Benching is the leveling and tensioning of the saw. When a band saw is run on a mill is it stretched with thousands of pounds of force, and during operation the cutting edge heats up. These forces and temperatures cause the saw to deform. Benching deforms an un-mounted saw in a way that counteracts the operating stresses, and allows the saw to pull flat and cut straight when in use. It takes months to learn benching.
Benching is done in a dark room with a stretcher-roller machine and flat anvil (see picture above). A single light at the benchman’s work station, along with ground gauges, allows the sawfiler to measure level and tension.
Leveling is done with a crossface hammer and stretcher-roller adjustments. Cross face hammers are available in left and right hand versions. Each filer has his own hammer which he carefully dresses.
Tensioning is done with the stretcher-roller. This machine has hardened rollers above and below the saw. They rotate slowly and pinch the saw when a lever is cranked, rolling a thin strip through the length of the saw, stretching the metal where it was rolled. Careful placement and force of the rolls deform the metal in a way that counteracts the forces the saw sees during operation. More rolls are placed in the mid-section of the saw. Resaws have the back pulled to counteract the uneven heating of the cutting edge. This is done by rolling the back (non cutting edge) of the saw. The back is measured with a three pin gauge, and is usually around .003" per three feet curved.
Benching involves the simultaneous solution of multiple deformations introduced to the saw to counteract the predicted stresses of the saw in operation. Benchman can easily recognize variations in steel batches.
Other band saw duties include welding broken teeth, fixing cracks, and trouble shooting operating problems.
CNC equipment is starting to evolve to the point of being able to do some benching and fitting tasks.