FRICTION CUTTING

The cutting action is produced by friction of metal disc. The metal disc is a large diameter circular blade rotating at high speed or a bandsaw moving at anywhere between 20 - 76 m/s, to create friction instead of chips. The frictional energy is converted into heat, it heats the workpiece to a temperature between red heat and its melting point and rapidly soften a narrow zone in the workpiece [7]. With the material in this condition, the weakened surface can no longer resist the sliding action of the blade and the material is literally wiped away [47]. The disc which is provided with teeth or notches, pulls and ejects the soften metal from the cutting zone. A path or cut is formed by the wiping action of the rotating disc.

The melting point and structural characteristics of the workpiece material is critically important. Oxygen carried in the gullets between the blade teeth causes the blade to bum its way through the work. The resulting action is similar to that of a cutting torch. Coolant is required to prevent excessive heat build up at the rotating disk. Many materials that cannot be flame cut may be friction cut.

In bandsaw friction cutting, it is suitable for sawing plastics, wood and non-ferrous metals. Even though friction sawing generates tremendous heat, it is confined to an area immediately ahead of the saw, and only to a very small extent to the sides of the kerf [13]. The resulting heat penetration is very shallow, seldom more than 0.05 mm deep [13]. In contrast with the temperature at the point of cut, the saw band remains relatively cool. The reason is that the band is long and tooth or the high speed band is only in momentary contact with the workpiece [41]. In its long journey around the saw band carrier wheels each tooth has ample time to cool before it again engages the work.

A limiting factor to the process is workpiece thickness. This is because high pressures between the saw band and the work must be maintained to generate the heat required, friction sawing is practical for any workpiece not more than 25 mm in thickness.

Because friction sawing softens and removes material just ahead of the saw band by creating frictional heat, tooth sharpness is not a critical factor as it is in conventional sawing. In fact, the heavy feeding pressure and fast band speed required to produce the frictional heat require dull teeth. This dullness actually makes the teeth more efficient heat generators because a blunt surface produces greater friction than a sharp one. A low carbon flexible friction cutting blade is better than a very hard one since it is less likely to crack from flexing over the drive wheels. Because the blade is moving fast, coarser tooth pitches can be used on thin materials than in conventional sawing, with little danger of tearing out teeth.

Blade life is determined more by set or blade waviness than by tooth sharpness. When the set has been worn off, the blade will still friction cut but it is difficult to guide it accurately. Special friction cutting bandsaw blades are available with increased set to minimise this problem.

Friction cutting can also be done on circular sawing machines as long as the surface speed is fast enough [7], Most circular friction machines, however, are specialised pieces of equipment and have veiy high power, accurate speed control and a high pressure water pump for blade cooling. Although convention air currents set up by the blade rotation do help in cooling the blade, this often is not enough. Compared with a friction cutting bandsaw blade, a circular blade has much more metal and therefore retains heat better [13].

The circular blades operates at sufficiently high uniform velocity and under sufficient pressure to generate heat by friction on the contact surface of the section at a rate that is faster than the rate at which the surface can absorb heat. Heat increases directly as the length of contact between the workpiece and the blade increases. This contact arc can be controlled somewhat by the saw arrangement. When the work reaches a temperature at which it is red hot, its tensile strength is reduced rapidly. At some pint above red heat but below the melting point, the weakened surface can no longer resist the sliding action of the blade periphery and is cut away. A downstroke machine most often used for cutting bar, rod angle and similar stock, is set up for the most efficient friction cutting with the contact arc between the blade and the work nearly horizontal.

Application

It is used for rapid cutting action. This process is several times faster than ordinary circular sawing operations. The workpiece structure characteristics is not important for subsequent usage because heat will cause distortion in the workpiece structure. It is suitable for hard ferrous metals such as steel and iron, etc. and reinforced plastic. Some plastics, for example the thermosetting type filled with hard materials such as glass and mica, can be successfully friction cut, thermoplastics can lose sufficient

strength without melting. It is not suitable for non-ferrous metals because they have a tendency to stick to the blade.

Advantage

1. It is the fastest cutting method than the other power driven saws.

Limitation

1. The process is fast but leaves a heavy burr which can be chipped off easily[l]. 2. The cutting surface is less accurate than tooth cutting does.

3. It is not suitable for non ferrous metals because they have a tendency to stick to the blade [48]. 4. Care must be taken with cast iron because the structure has a tendency to break down before

the appropriate tensile-strength-breakdown temperature is reached [41]. 5. Thermoplastics loss of strength when cut by friction sawing [41].