Cutting Tool Applications, Chapter 15: Saws and Sawing

Once, sawing was considered a secondary machining process and saws were used mostly for cutting bar stock in preparation for other machining operations. In recent years, the development of new types of saws and better blade materials have made metal sawing a much more effective, versatile and economical process. In many cases bandsaws are now being used as the primary means of shaping certain types of metal parts.

When the proper sawing machines and blades are used, sawing is one of the most economical means of cutting metal. The saw cut (kerf) is narrow and relatively few chips are produced in making a cut. When a bandsaw is used for cutting the contours of a complex shape only a small portion of the metal is removed in the form of chips. Therefore, only minimal power is used in removing large amounts of waste metal.

Sawing Sawing is a process wherein a narrow slit is cut into the workpiece by a tool consisting of a series of narrowly spaced teeth, called a saw blade. Sawing is used to separate work parts into two or more pieces, or to cut off an unwanted section of a part. These processes are often called cut-off operations and since many manufacturing projects require cut-off operations at some point in the production sequence, sawing is an important manufacturing process. Basically, sawing is a simple process: As the blade moves past the work, each tooth takes a cut. Depending on the thickness or diameter of the work, the number of teeth cutting at one time varies from two to ten or more. Saws may be of the continuous cutting (band or rotary) or reciprocating type.

All saw blades have certain common characteristics and terminology:
• Rake angles: Rake angles are 0 degrees or neutral rake on most saw blades. Some have a positive rake angle.
• Width: The width of a saw blade is its total width, including the teeth.
• Set: The set of a saw blade means the offsetting of some teeth so that the back of the blade clears the cut. The "raker" set is most frequently used and is furnished with all hacksaws and band saws unless otherwise specified.
• Kerf: The kerf is the width of the cut made by the saw blade or the material cut away. The thickness of the blade is called the gage.
• Pitch: The pitch of a saw blade is the distance between the tops of two adjacent teeth. This is specified in teeth per inch.

Saw Blade Material
Saw blades are made from various materials, including:
• Carbon steel: General utility for small lot, low-speed work. The least expensive blade, these may have a hard "back" for greater wear.
• High-speed steel (HSS): This costs two to three times as much as carbon steel, but it is much longer wearing and is a necessity for the "difficult-to-machine" metals. • High-speed edge: This is a carbon steel blade, which has a narrow strip with HSS teeth welded on. This is a tough blade, intermediately priced, and widely used for most materials.
• Tungsten carbide-tipped blades: Available in a few sizes. Used only on large, very rigid sawing machines for high-production sawing of difficult materials.

Saw blade selection — The process of choosing the best bandsaw blade for a particular job must start with an evaluation of the material to be cut. Such factors as hardness, machinability, cross-sectional shape and area must be considered.

After the material to be cut has been properly identified, the selector on the machine can be used to help select the proper blade and cutting speed, tables and selectors are helpful, but the operator often must make choices that affect the three variables present in every sawing operation: cutting rate, tool life and accuracy. Generally, increasing any one variable results in a decrease in one or both of the others. For example, an increase in cutting rate always reduces tool life and may affect accuracy.

Saw blade welding — Nearly all vertical metalcutting bandsaws have an attachment for butt-welding blades electrically. It is usually set on the column of the machine at the operator's left and consists of a blade cutter, a small grinding wheel and the butt-welding machine. The blade welding attachment can be used for making saw bands from bulk sawblade stock or for welding bands that have been cut and inserted into a hole in a workpiece that is to be band-sawed internally.

The importance of making good welds in saw blades cannot be overemphasized. Breakage caused by poor welding, improper joint finishing or improper heat treatment is time consuming and potentially dangerous.

The resistance-type butt welders found on almost all vertical bandsaws operate by causing electrical current to flow through the ends of the bandsaw blade while pressure is being applied. The high resistance where the blade-ends meet causes the metal to become white hot momentarily, and the blade-ends fuse. Provision is made for annealing (softening) the welded joint. As the operator presses the anneal button for a very short time, current flows through the completed joint until the joint heats to a dull red. The joint then anneals as it cools slowly.

Sawing Equipment
In most sawing operations, the work is held stationary and the saw blade is moved relative to it. As shown above, there are three basic types of sawing operations, according to the saw blade motion involved.

Hacksawing — Hacksawing involves a linear reciprocating motion of the saw against the workpiece. This method of sawing is often used in cut-off operations. Cutting takes place only on the forward stroke of the saw blade. Because of this intermittent cutting action, hacksawing is less efficient than other sawing methods. Hacksawing can be done manually or with a power hacksaw. A power hacksaw provides a drive mechanism to operate the saw blade at a desired speed and feedrate.

Power hacksaw: The power hacksaw is the original and least expensive saw for the work. These saws work the same as a hand hacksaw: They cut on the forward stroke and then lift slightly so that the blade does not drag on the return stroke.

The size of a power hacksaw is the cross section of the largest piece of stock that it can cut. Typical sizes are 6x6 inches to 24x24 inches. The motors used will vary from 1 to 10 hp.

The speed of these saws is expressed as “strokes per minute.” This may range from 30 strokes per minute for large cuts with heavy saws on difficult materials, up to 165 strokes per minute on carbon steels and nonferrous materials. The hacksaw usually has four to six different speeds available.

Feed may be a positive advance per stroke or may be gauged by a friction or pressure drive. The smaller power hacksaws feed about 0.006 ips (inches per stroke) and the larger ones 0.012 to 0.030 ips. Feed pressures will be 450 to 750 pounds on the blades. Work is held in a built-in vise, which may be hand or power operated.

Bandsawing — Bandsawing involves a linear continuous motion, using a bandsaw blade made in the form of an endless loop. The bandsaw provides a pulley-like drive mechanism to continuously move and guide the bandsaw blade past the work. Bandsaws are classified as vertical or horizontal. This designation refers to the direction of saw blade motion during cutting. Vertical bandsaws are used for cut-off and other operations such as contouring and slotting. Horizontal bandsaws are normally used for cut-off operations as alternatives to power hacksaws.

The choice of which type of cutoff saw to buy often is influenced by custom or habit. However, there are definite factors that can be considered:
• Cost: A hacksaw is much less expensive, often about half the cost of a band saw of equal size and power.
• Saw blades: The hacksaw blades may cost one-half to one-quarter the price of a bandsaw blade. However, the hacksaw will become dull in one-half to one-quarter the number of cuts that the band saw will make. The hacksaw blade is almost unbreakable and is somewhat less likely to have its teeth stripped off by hard spots in the material being cut.
• Kerf: The bandsaw blade is thinner than the hacksaw blade, especially for the larger sizes. Thus less metal is wasted in the cut. However, this "saving" is often lost because of the 2" to 6" long "stub end," which is thrown into the scrap bin when the bar of stock is used up.
• Speed: The bandsaw will cut off stock up to twice as fast as the hacksaw. However, it takes more care and more time to change bandsaw blades, adjust saw guides and regulate feeds. Thus, less-experienced operators can use the plain hacksaw.

Bandsawing Operations
The types of work described here account for most of the band sawing operations used in metalworking.

Cut-off Sawing — Although cut-off sawing can be done on any type of vertical or horizontal bandsaw, the majority of cut-off sawing is done on powerful horizontal machines. A variety of workholding devices and fixtures can be used to hold tubing, angle iron and other shapes.

Blade selection is important in terms of economy and the finish on the material being cut. The precision tooth type blade is used extensively with the recommended pitch ranging from 10 teeth per inch for sections up to 3/8-inch thickness to four teeth per inch for material over 3-inches thick. Manufacturers' manuals should be consulted when heavy cuts are being attempted. The claw tooth type of blade is used when cutting some tough steels because the tooth penetrates the surface of the work more easily.

Stock feeders are often used on cutoff machines, along with an indexing mechanism that allows the operator to automatically repeat cuts of pre-selected lengths. Almost all cut-off operations are done with a liquid coolant delivered to the saw cut by a pump.

Contour Sawing — Contour sawing, both internal and external, is one of the most versatile operations that can be done with a bandsaw. It may range from simple shapes cut on a fractional horsepower machine to complex internal cuts made with tilting table machines. Blade selection is important when cutting complex contours, especially when small radii or corners are involved. Select the widest blade that will allow turns of the proper radius.

For internal work, a hole must be drilled so that the blade can be passed through it and re-welded. For plain contouring, the hole is drilled perpendicular to the face of the workpiece. When the internal shape has corners, holes must be drilled at the corners so that the blade can be turned and the cut started in another direction.

Friction Sawing — Friction sawing is a unique process. A bandsaw blade with dull teeth traveling at very high speed, 6000 to 15000 SFPM (surface feet per minute), is used to cut both hard and soft ferrous metals. Friction sawing works particularly well on metals that have poor heat conductivity because the heat-affected zone remains very small. It is the fastest method of cutting ferrous metals less than 1-inch thick.

As the blade contacts the work, the metal at the point of contact immediately becomes white hot and is carried out by the teeth. The blade itself remains relatively cool because during its operating cycle it is in contact with hot metal for only a short time.

Circular Sawing
Circular sawing uses a rotating saw blade to provide a continuous motion of the tool past the work. Circular sawing is often used to cut long bars and tubes to specific lengths. The cutting action is similar to slot milling, except that the saw blade is thinner and contains more cutting teeth. Circular sawing machines have power spindles to rotate the saw blade and a feeding mechanism to drive the rotating blade into the work. Shown above is a semiautomatic circular saw.

Bandsaws as well as circular saws have advanced to be highly automated and many of their functions are computer controlled.

Cold Sawing
Most cold saws, regardless of size, consist of a, base; drive mechanism, blade arbor, vise, feed mechanism, and necessary guards and switches. On some small saws the blade is fed into the work by hand.

On larger machines the feed mechanism is pneumatically or hydraulically operated. The operator controls the rate of feed.

The base of the machine or the vise can be swiveled to make angular cuts. In some cases, two machines can be set up on a single work stand for production.

Cold Saw Blades — Blades smaller than 18-inches in diameter are cut directly in the rim of the saw disk. For cutting soft materials, the teeth are spaced farther apart, as in the case of bandsaw and power hacksaw blades, so that the gullet (the space between the teeth) will be large enough to accommodate large chips. When cutting thin tubing or other thin materials, use saw blades with closely spaced teeth to avoid chattering and tooth breakage. Cold saw blades with teeth cut directly on the periphery of the disk may be made of high-carbon or high-speed steel.

Larger blades usually have segmented teeth. The body of the blade is made of rough, resilient alloy steel, and the inserted teeth are made of high-speed steel or tungsten carbide. The individual teeth or segments of three or four teeth are wedged or riveted to the blade and can be easily replaced if a tooth is damaged or broken. Larger cold saw blades can cut a kerf as wide as 1/4-inch and remove metal rapidly.

Abrasive Cut-Off Machines
Abrasive cut-off machines are used in many shops to cut metallic and nonmetallic materials. Because an abrasive (usually aluminum oxide) is used as the cutting tool, hardened steel can be cut without being annealed. The cutting action here is faster than on other types of cut-off machines.

Abrasive cut-off machines may be of the wet or dry type. The flow of coolant, usually water and an anti-rust chemical of some type, is controlled by the operator. The coolant tank is separate or built into the base of the machine.

Some larger cut-off machines have powered feed mechanisms and oscillators. The oscillator moves the abrasive disk back and forth in the cut as feed pressure is applied. This reduces the amount of blade in contact with the work at any given time and reduces the power input required to cut solid bar stock of a given cross-sectional area.

The abrasive disk usually will have a resinoid bond agent, although rubber can be used on smaller wheels. Glass fiber is sometimes impregnated in the disk to increase its strength. Abrasive disks work efficiently at surface speeds of 12,000-15,000 surface feet per minute.

George Schneider, Jr., is the author of Cutting Tool Applications, a handbook to machine tool materials, principles, and designs. He is the Professor Emeritus of Engineering Technology at Lawrence Technological University, and former Chairman of the Detroit Chapter of the Society of Manufacturing Engineers.