July 10, 2007
Using a band saw to cut bundles of structural shapes is good for productivity but bad for band saw blade wear. Knowledge of application-specific tooth designs, tooth pitch, band tension, band speeds, and cutting fluid will help maintain blade life.
Band saw cutting of square, rectangular, or round tubing in bundles allows you to cut a high number of parts per shift, resulting in increased production. This part volume efficiency, unfortunately, is offset by a 20 percent to 25 percent reduction in band life. Therefore, a thorough evaluation of your equipment, operator skills, and throughput is essential to ensure maximum blade life. A band saw with a bundling attachment—normally an added cost option for applying top-clamping force to eliminate material movement—will not add appreciable labor cost to a bundle-sawing operation.
A common problem associated with sawing square and rectangle tubing is that the blade is cutting a larger surface area as it travels through the upper and lower surfaces of a bundle than when it is cutting through vertical walls. To lessen this problem, some manufacturers offer vertical saws with frames canted forward or inclined bands on twin-post saws. Scissors-type saws do not encounter the problem previously mentioned until the saw head reaches a nearly horizontal position at the end of the cut.
The saw used for bundle cutting should have positive controlled feed to maintain a consistent feed rate throughout the bundle. A gravity-fed saw has a tendency to fluctuate as it travels through vertical wall surfaces. This inconsistent feed rate causes teeth to chip or break, eventually stripping many sections of teeth.
The easiest way to secure bundles of tubing is with a bundle cutting attachment, which essentially is an additional adjustable vise. This reduces the chance for round tubing to spin and square or rectangular tubing to shift, diminishing the major cause of teeth stripping from the band.
Band saw cutting of tube is itself abusive to the band saw blades, because most damage is caused as the teeth enter and exit a workpiece. Damage potential is immediately doubled for hollow tubing because the blade must enter and exit two walls each time it cuts. When multiplied by the number of pieces in your bundle, it is easy to understand how band life is reduced 20 percent to 25 percent.
Band manufacturers have developed application-specific tooth designs and patterns to help optimize tool life when sawing tubing and structural shapes. Unlike solid materials, tubing usually does not produce long, curled chips when it is cut; therefore, a short, strong tooth with a modified progressive set and a long, shallow gullet to carry chips is effective in reducing the chance of broken teeth.
Bundled tubing, as well as larger structural shapes, often pinch the band, damaging the band body or affecting the set. A heavier, wider set than normal provides additional kerf width during the cut, allowing clearance to prevent pinching.
A wavy-set band is suitable for sawing thin-wall-tubing bundles, as well as for maintenance department applications. This undulating set allows each tooth to protect the following tooth, reducing tooth breakage.
Bimetal bands, while more expensive than carbon bands, always should be used for bundle cutting because they remain sharp for long periods of time and provide a strong profile and tooth face that help withstand frictional heat buildup that is common when sawing stacked bundles. Band width also affects productivity performance. A wide band provides high beam strength, reducing upward deflection and the possibility of band body stress breaks. Always use the widest band possible, and always keep guide arms as close to the workpiece as possible.
Workpiece size determines tooth pitch. It is recommended that a tooth pitch allow at least three teeth in the workpiece at all times. A 14/10 or 12/8 varied tooth pitch should be used on very thin-wall tubing; a 10/6 or 8/5 varied pitch should be used on average wall thicknesses; and a 6/4 or 4/3 tooth pitch should be used on thick-wall workpieces.
Correct band tension, as recommended by the band manufacturer, is critical for maintaining straight cuts and avoiding upward band deflection. In general, bimetal bands up to 1 inch wide require around 25,000 pounds per square inch (PSI) of tension. Bands wider than 1 in. wide require about 30,000 PSI.
Band speeds for bundle cutting are determined by the material to be cut. Mild steel speed requirements are from 250 to 275 surface feet per minute (SFPM), while stainless steel requires between 90 and 135 SFPM. These suggestions assume a saw that is in good mechanical condition with proper band tension and effective cutting fluid. The saw should travel through the work at a light to medium feed rate. If the feed rate is too slow, teeth will dull quickly. If the feed rate is too fast, it will cause tooth tips to chip or teeth to break off the band.
Bundled materials with surprising hard inclusions, multiple interrupted cuts, and hardened chips lodged between stock and in the hollows of tubing all add up to amplified stress and increased blade wear. To make matters worse, tubing usually is not perfectly straight, and dimensions are not always consistent. These conditions can allow individual pieces to shift or spin within the bundle. Material movement is a primary cause of teeth stripping that results in reduced blade life.
Workpieces must be tightly secured to minimize movement and vibration as much as possible. The most effective method to hold material in place is to use a top clamp, commonly referred to as a bundle-cutting attachment, to secure the bundle near the cut. Quite often round stock will spin within the bundle unless it is tack welded at the butt end (seeFigure 1).
Bundles often are secured with metal straps. However, strapping does not hold round material tightly and is labor-intensive because the strap must be placed around the bundle just behind the intended saw cut location each time the material is advanced.
Metal-on-metal frictional heat created during saw cutting, combined with heat created by the shearing of chips, can damage even a bimetal blade. Using a water-soluble synthetic coolant that combines lubricity and evaporative cooling is the best way to reduce this heat damage. Unfortunately, bundle cutting with flood coolant is usually a messy operation with liquid running through the tubing and out both ends. There seems to be no easy way to collect this coolant runoff. Containers set in place at the tail end of the tubing must be moved each time material is advanced for the next cut.
An often utilized alternative to flood coolant for bundle-cutting operations is a lubricating misting system. Though not as effective as flooding, misting eliminates waste and pools of cutting fluid around the saw. A combination of compressed air and concentrated lubricant flowing through separate tubes and mixing in a vortex nozzle aimed at the blade helps reduce heat and improve tooth penetration. The nozzle must be aimed directly at the face of the teeth and lubricant applied to the tooth tips as well as to both sides of the teeth for effective performance.
Bundle cutting of tubing does improve production by allowing more cut pieces per shift. Bundle cutting also makes sawing more difficult because of material vibration, wide guide arm spacing, difficult cooling of the band teeth, and work-hardened chips.
Regular machine maintenance should be performed and weekly checks made of the guide condition, blade tracking, fluid levels, and cutting fluid flow.
Sawing tubing and structural shapes presents the most challenging and potentially destructive application for a saw blade. A saw in poor condition, the wrong band for the application inattention to detail, and careless saw operation will easily negate any savings that should have been realized from bundle cutting.
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