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Band-Sawing Basics

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Wednesday, June 14, 2017
 
Rising manufacturing costs are forcing manufacturers and machine operators to seek more economical ways to cut steel. Fortunately, sawing technology has improved greatly and kept pace. Modern, high-technology materials have paved the way for new saw-machine designs, while improved saw blades are helping to keep manufacturing costs under control.

While each job presents its own unique challenges, the following band-saw terminology, specifications and usage tips will help fabricators understand and verbalize common problems in order to find practical solutions and maximize cutting efficiency and cost-effectiveness.

Choose the Right Blade

The first step in achieving cost-effective band sawing is choosing the right blade for the material to be cut. Take into account these five key aspects of blade makeup: blade construction, tooth construction, tooth form, tooth set and teeth per inch.

Blade construction. A blade can consist of one or two pieces, depending on the performance and life expectancy required. One-piece blades are made from single pieces of carbon steel with the teeth and/or blade back (body of a blade not including the tooth portion) treated for necessary hardness or softness. Hardened tooth edges and blade back define a hard-back blade, while a flex-back blade has a hardened tooth edges but a soft back to ensure flexibility.

A bi-metal blade is a two-piece blade constructed by electron-beam-welding a high-speed steel-edged material to fatigue-resistant spring-steel backing. An advantage to this construction: It provides the best combination of cutting performance and fatigue life.

Tooth construction. Manufacturers often build up blades by welding carbide to the teeth before grinding down to the desired shape. For example, the welding of carbide-tipped teeth to a high-strength alloy back results in a longer-lasting, smoother cutting blade.

Fig. 1—Tooth forms
Tooth form.
The shape of the tooth’s cutting edge, or tooth form (Fig. 1), affects how efficiently a blade can cut through a piece of material. It also accounts for a number of important factors such as blade life, noise level, smoothness of cut and chip-carrying capacity. Here are some tooth forms, accompanied by their relative advantages: 

• Variable Positive—Variable tooth spacing and gullet capacity (the size of the curved area at the base of the tooth); allows for reduced noise and vibration, while also allowing faster cutting rates, long blade life and smooth cuts. 

• Variable—Offers benefits similar to the variable-positive form; for use at slower cutting rates. 

• Standard—Consistent tooth spacing and gullet capacity; a general-purpose blade design for a range of applications.

• Skip—Consistent wide-gullet design; suitable for nonmetallic applications such as wood, cork, plastics and composite materials.

• Hook—Similar in design to the skip form, but with a higher tooth-rake angle (the angle of the tooth face measured with respect to a line perpendicular to the cutting direction of the saw); can be used for brittle materials that produce a discontinuous chip, such as cast iron, as well as for nonmetallic materials.

Tooth set. This refers to the number of teeth and the angle at which they are offset. The bending of teeth to the right or the left (as seen from above) allows for the clearance of the back of the blade through the cut, impacting cutting efficiency and chip-carrying capacity. The following are some tooth sets and their respective strengths:

• Raker—Three-tooth sequence with a uniform set angle (left, right, straight, repeat). 

• Modified Raker—Five- or seven-tooth sequence with a uniform set angle for greater cutting efficiency and smoother surface finish (left, right, left, right, straight, repeat). The order of set teeth can vary by product.

• Vari-Raker—The tooth sequence is dependent upon the tooth pitch (the distance from the tip of one tooth to the tip of the next tooth) and product family. Typically provides quiet, efficient cutting and a smoother finish with less burr. 

• Alternate—Every tooth is set in an alternating sequence. Used for quick removal of material when finish is not critical.

• Wavy—Groups of teeth set to each side within the overall set pattern. The teeth have varying amounts of set in a controlled pattern. Typically used with fine-pitch products to reduce noise, vibration and burr when cutting thin, interrupted applications. 

• Single-Level Set—The blade geometry has a single tooth-height dimension. Setting this geometry requires bending each tooth at the same position with the same amount of bend on each tooth.

• Dual-Level Set—This blade geometry has variable tooth-height dimensions. Setting this product requires bending each tooth to variable heights and set magnitudes in order to achieve multiple cutting planes. 

• Vari-Set—The tooth height/set pattern varies with product family and pitch. The teeth have varying set magnitudes and set angles, providing for quieter operation with reduced vibration. Efficient for difficult-to-cut materials and larger cross-sections.

Teeth per inch (TPI). Selecting a blade with the right TPI—measured from gullet to gullet—for the material being cut is an
important consideration in maximizing cutting efficiency and cost-per-cut. The size and shape of the material, as well as the desired finish, dictate optimum TPI selection. 

Make the Right Cut

If viewing a blade cutting metal under a microscope, note that the tooth tip penetrates the workpiece and actually pushes, or shears, a continuous chip of metal. The angle at which the material shears off, referred to as the “shear-plane angle,” is the most important factor in obtaining maximum cutting efficiency.

 Fig. 2—Low shear-plane angle
 Fig. 3—High shear-plane angle
Generally, with a given depth of penetration, the lower the shear-plane angle, the thicker the chip becomes and the lower the cutting efficiency (Fig. 2). The higher the shear-plane angle, the thinner the chip becomes and the higher the efficiency (Fig. 3).

Shear-plane angle is affected by a number of factors, including feed and gullet capacity.

Feed refers to the depth of penetration of the tooth into the material being cut. As such, feed helps determine kerf, or the amount of material removed by the cut of the blade. A deeper feed removes more material per tooth and results in a lower shear-plane angle. This results in faster cutting, but reduced blade life. Conversely, a light feed will increase the shear-plane angle, meaning longer blade life and more efficiency, but also a higher cost per cut.

Determining the optimum feed rate/pressure is dependent on machinability of the material being cut and blade life expectancy.

Gullet capacity also impacts cutting efficiency. As the tooth scrapes away material during a cut, the chip curls up into the gullet, or the space between the tooth tip and the inner surface of the blade, a blade with the proper clearance for a cut allows the chip to curl up uniformly and fall away from the gullet. If the amount of material scraped away exceeds the gullet capacity, the chip will become distorted and jam into the gullet area, causing increased resistance, jams and chokes. These mistakes load down the machine, waste energy and can cause damage to the blade.

Proper understanding of feed and gullet capacity can allow for feed-rate optimization. Suppose, for example, that a 4-in. round must be cut. Here, three cutting areas must be considered (Fig. 4.)

 Fig 4—Feed-rate optimization
1) Entering the material, the blade encounters a small material width and therefore meets minimum resistance. Feed rate is the limiting factor here, so a feed setting that maximizes cutting can be used without losing blade life.

2) As the blade moves along, material width increases, so more material fills the gullet area and imposes limitations on feed and depth of penetration. For maximum sawing efficiency in this difficult midsection, the blade must have ample gullet capacity. Otherwise, the feed rate must be reduced accordingly.

3) As the blade moves out of the difficult cutting area and into an area of decreasing material width, feed rate again becomes the important limiting factor, and the feed setting can again be increased. With an understanding of those portions of the cut that affect only the feed rate, the rate can be varied accordingly in order to improve overall cutting efficiency.

Each job is unique, but every cut relies on the same important factors. Understanding which considerations are the most important for a given material, cut, shape or finish will allow fabricators to achieve efficient and economical cutting. FPN

This article was excerpted from the Lenox Guide to Band Sawing, provided by Lenox, East Longmeadow, MA; 877/438-5615, www.lenoxtools.com.

 

As seen in Fabricating Product News: Lenox Saw