November 7, 2006
Material separation with a band saw machine is the beginning of most fabrication and many manufacturing operations, but it doesn't have to cause headaches with the proper preventive maintenance.
The structural materials that welding and fabrication shops must saw are more abusive to band saw blades than any other metal sawing application, but paying attention to band selection, band speed, feed rate, coolant, and maintenance can minimize the abuse.
Band sawing square tubing, round tubing, angles, channel, and I-beams presents unique challenges for band saw blades. These materials are a combination of flat surface areas and vertical walls, each of which normally would require a substantially different tooth pitch to be cut effectively.
For instance, sawing a piece of flat stock 6 inches wide typically requires a varied 3/2 pitch blade, while sawing 1/4-in.-thick material, vised vertically, commonly requires a varied 18/14 tooth pitch. The challenge when sawing a piece of 6-in. square tubing, for instance, is to select a tooth pitch that not only can cut through the flat surface area effectively, but also cut the vertical walls without stripping teeth.
A tooth pitch that's too fine for the flat surface area can cause chip loading in the gullet, which causes the band to ride through material without taking an effective bite. This reduces the effective life of the band dramatically through premature dulling. But a tooth pitch that's too coarse can straddle the tube's vertical wall, causing tooth stripping.
Most fabrication shops saw-cut a range of structural shape wall thicknesses, yet prefer to use one tooth pitch for all applications to reduce inventory as well as the time required for band changes. This makes choosing the correct versatile tooth pitch especially important.
Further, work-hardened chips trapped in the interior of square or round tubing can be picked up in the gullet, often causing teeth to strip as the band exits the outside wall. Yet another problem, particularly when sawing round tubing, is material movement. When stock—round or square —shifts in the vise once the blade teeth are in the material, tooth tips can break off and eventually strip a series of teeth from the band, rendering the blade useless.
Bundle cutting of structural shapes, while increasing production rates, multiplies all of these problems by the number of pieces stacked in the bundle, because more openings are present to collect chips. In addition, it's difficult to achieve a tight bundle even with a top-vise bundle-cutting attachment on the saw. This allows for a greater possibility of material movement, creating tooth damage.
Material separation with a band saw machine is the beginning of most fabrication and many manufacturing operations. Unfortunately, the saw that's responsible for business income usually doesn't receive the preventive maintenance it deserves. Basic housekeeping and preventive maintenance help produce quality parts and maximize band saw blade life.
Metal chips should be cleaned from moving parts, the saw table, and the vise area daily. Band guides should be opened, cleaned, and inspected for wear or damage at each band change. Oil leaks should be investigated and repaired to avoid coolant contamination. Worn band wheel bearings should be replaced to maintain proper band tracking to prevent erratic movement of the band that can cause teeth to strip.
A small $3 chip brush can save hundreds of dollars in band cost. Work-hardened metal chips carried back into the kerf can snag and break tooth tips. Sufficient coolant flow combined with a good chip brush can alleviate this problem.
Cutting fluid is critical when saw-cutting nearly all metals. Usually the most cost-effective coolant for sawing mild steel structural shapes, as well as other metals, is a quality cutting fluid specifically formulated for band sawing applications. Properly mixed to the manufacturer's specification, coolant can extend band life by providing both lubrication and evaporative cooling.
Because coolant also helps flush chips from band gullets, adequate flow is imperative. Some sawyers will attempt to reduce resident deposits that affect paint and other up-line coatings by reducing flow. Selecting a quality cutting fluid specifically formulated to prevent such deposits will help maximize band life.
Often, to avoid the mess created by water-soluble coolants, sawyers will opt to use an oil misting system that provides lubricity but not evaporative cooling. When a misting system is used, some reduction in band speed usually is required.
Band speeds for sawing structural carbon steel shapes are in the range of 325 to 250 surface feet per minute. Smaller shapes can be cut at faster speeds, but materials larger than 6 in. should be cut at the slower speed. It's always best to check with the band manufacturer's speed recommendations for the band in use. The feed rate for structural shapes should be in the light to medium range, depending on the tooth pitch and the material size. Light feed generally is recommended on small material, heavy feed on larger material (see Figure 1).
ing cutting is a major cause of band tooth loss, it's important that the vise, manual or hydraulic, hold the material securely in position.
When bundle cutting, every effort should be made to hold each piece of material securely. Ideally, a top-vise attachment specifically for bundle cutting is most effective. Butt ends of round tubing can be welded together to avoid individual piece rotation.
It's also imperative that any chips that may have accumulated be removed from the sides of the vise before the material is clamped. Vibration created during the cutting process can cause chips to break loose and allow material movement.
Thin-wall shapes up to 1/2 in. can be cut successfully using an 8/5 or a 10/6 wavy set varied tooth pitch. A wavy set band helps reduce tooth stripping and provide longer band life Greater wall thicknesses, up to 2 in., can be cut more efficiently using a 6/4 varied pitch band, and larger shapes with thicker walls can be cut effectively with a 4/3 varied tooth pitch. In these coarser tooth pitches, a modified progressive close-tolerance set pattern helps provide longer blade life and smoother finishes than a standard alternating raker set.
For extruded shapes that tend to close on the band during the cut, bands are available with a wider tooth set, which provides a wider kerf and helps prevent blade damage caused by binding. The new tooth geometry of these wide-kerf bands is suitable for sawing bundled or stacked structural beams and tubing.
It's important to keep in mind that a band saw blade, thin and somewhat narrow, is subject to tremendous force generated during cutting. Heat is the enemy of a band. Excessive heat generated by friction of metal against metal and the heat caused by the shearing of metal can be detrimental to the band. Running the correct band speed and feed rate will help minimize heat generation. Clean and properly mixed coolant will help reduce and dissipate heat.
Housekeeping, preventive maintenance, cutting speed, feed rate, band selection, coolant quality, and proper vising are all interrelated. Ignoring one or more of these factors can create stress on the band.
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