Putting Teeth into Cutting
May 20, 2010
Consider the simple saw. As you ponder it, you realize that although we take saws for granted, they may not be as simple as they seem. There are a variety of different types of saws used for various functions. The basic categories are handsaws and mechanically powered saws.
Handsaws fall into three types based on how they hold the blade stiff. Basic handsaws use either a blade thick enough to be stiff, or cut on the pull stroke, which reduces the stiffness requirement. The types are as follows: crosscut, rip, floorboard, Japanese, keyhole and two-man. Backsaws are a second category of handsaw that keep a thinner blade stiff by reinforcing it with a steel or brass back. The most common in this category are the mitre, carcase, tenon and dovetail saws. The third category of handsaws stiffen the blade by placing it in tension, hence the name tension saws. Four tension saws are most common and they are the bow saw, hacksaw, bucksaw and coping saw.
Mechanically powered saws are the second major category, and they are categorized by how they move the teeth past the product to be cut. The circular blade saws are as follows: circular, table, cabinet, radial arm, rotary and chop saw. The reciprocating blade saws are the jigsaw, reciprocating and scroll saw. The final type is the continuous band saw, and these are the band saw and the chain saw.
Having now finished reviewing the basic types of saws, it’s important to understand the influence heat treatment plays. When we examine this, we find that there are almost as many heat-treatment variations as there are types of saws!
The basic saw blade is manufactured from carbon steel, and the teeth are hardened by flame hardening or induction while the blade back remains flexible. These types of blades are generally used for cutting wood or soft metals. A second blade type is made from high-carbon alloy steel, and the entire blade is hardened and tempered. Sometimes this is also followed by a second hardening process of the teeth alone. A third type is called bi-metal, and it is produced by joining a high-speed steel cutting edge to a tough, flexible, carbon-alloy steel back for fatigue resistance and durability.
Many of the saws used by those of us in the metalworking industry (band saws, reciprocating saws, hacksaws and hole saws) are the bi-metal type, so it would be of interest to examine the manufacturing process for these blades. A high-speed edge wire and backing strip are joined by electron-beam or laser welding. This process is performed on coil stock, and following welding, the coil is annealed to soften and relieve stess. The coil then proceeds to a rolling operation where the weld is flattened and the material is rolled to the proper thickness. Next, the teeth are formed by milling or grinding, and they are then “set” to get the required relief angles for most effective cutting. The blade, still in coil form, proceeds to heat treatment in strip furnaces, where it is typically hardened at 1900-2200°F in a protective atmosphere, followed by multiple tempers. Tempers are done as coil. The resultant tooth-tip hardness is typically 66-69 Rc, while the backing steel is 42-50 Rc.
The final step in the process is putting the blade into its usable form. For hacksaws and reciprocating saws this happens by stamping to the final shape. Band saws are cut to length and flash butt-welded followed by a localized grind and anneal.
After considering the saw, it’s not at all simple. The heat treatment varies as much as the variety of saws. Without effective heat treatment, however, a saw just couldn’t cut it. IH