Exploring the friction alternative
September 4, 2001
There are many ways to cut sheet, plate, tubing, and structural shapes, ranging from a hand hacksaw to power shears and lasers.
There are many ways to cut sheet, plate, tubing, and structural shapes, ranging from a hand hacksaw to power shears and lasers. The best choice depends on the situation at hand, but the goal always is to achieve the best cost, quality, and schedule results.
While considering the newest options, it is important not to lose sight of some of the basic process methods and technologies. Friction sawing is one such "oldie but goodie" process.
For most generally used materials and mainstream applications, conventional sawing or other chip-removal cutting methods are better than friction sawing. Friction sawing can be considered a solution with advantages for special (but common) situations.
In true friction sawing, a metal band or circular blade does the cutting, but at much higher speeds than conventional sawing. Instead of shearing small chips of solid material, the high speed creates frictional heat that actually softens the contact surface to the point of melting.
At the highest speeds, a dual action sweeps away the molten surface layer as the teeth of the saw blade further penetrate and cut the advancing softened solid layer underneath. A smaller tooth loading or depth of cut is involved, but this is more than offset by the high speed that provides an aggressive rate of penetration into the stock.
When friction sawing is performed properly, heat does not build up in the workpiece. Instead, most of the heat is carried off by the metal particles that exit the saw kerf (the narrow slit left by the blades passing through the part). Parts can be handled comfortably immediately after cutting, and there is no significant heat-affected zone.
In friction sawing of titanium, such as in trimming hot-formed aircraft parts, a ferrous-contaminated surface layer of about 0.005 inch (0.127 millimeter) may need to be removed, but this is less than that produced with some other cutting methods.
Friction sawing cuts quickly through conventional materials and uses basic equipment to cut metals such as white cast iron or tool steel of more than 60 Rockwell hardness C. It can be used to cut such materials as thin sheet and single or bundled thin-wall tubing without snagging on the material.
Friction band sawing is a good process for several applications, including cutoff of hard, thin-wall tubing and contour cutting of titanium sheet or other difficult parts (see Figure 1). Particularly if anything other than a straight cut is required, this can be a suitable choice.
The basic key to friction band sawing is to attain the high speed necessary to generate sufficient heat. Several band saw manufacturers offer machines designed specifically for friction sawing. A friction band saw typically has a large-diameter wheel to generate blade surface speeds of up to 20,000 surface feet per minute (6,100 surface meters per minute). Its components are balanced to handle these high speeds, and a disk brake system can stop the machine within a few seconds if the blade breaks.
The band blades are made typically of high-carbon steel with a high silicon content for heat resistance. The teeth have a wide set that helps to generate frictional heat at the tooth while providing a wide kerf for reduced friction on the sides of the band and long wear life. A tooth pitch of 10 teeth per inch is common, which generates high heat by putting several teeth on the work surface.
Some sources indicate the practical limit on material thickness typically is about 1/2 in. (13 mm), although others state that thicknesses of 3/4 in. (19 mm) may be cut effectively. In extreme cases, material up to 1 in. (25 mm) may be friction-sawed, but it requires a struggle involving rocking the part to reduce the effective contact area.
It is possible to do a limited amount of friction band sawing with a conventional vertical metal cutting, or even woodworking, band saw. Some people do small jobs by using a worn-out regular blade, sometimes even reversing it to generate more heat with the flat rear edge.
However, these machines do not have the combination of power, speed, and special safety features of a true friction sawing machine. They generally are limited to about 5,000 SFPM (1,525 SMPM), which is the bottom end of the speed range for effective friction sawing. And if the saw or work area has oil, wood, or other flammable residue, a friction sawing operation could ignite those materials.
For simple stock cutoff or straight-line sawing, true friction sawing circular blades and machines are available. Some manufacturers make machines with blades as large as 96 in. (2.5 m) in diameter for steel mill work, but the more typical size for a fabrication shop is about 24 in. (0.61 m).
A variety of machine styles are available with speeds of up to 25,000 SFPM (7,600 SMPM). The blades usually are made of alloy steel rather than carbon steel and have a tooth design featuring a flat land for friction generation, rather than a normal point. The high rotational forces can create a dangerous situation if a blade breaks, so particularly good safety guarding is required.
In friction circular sawing, the life of blades is reduced and fatigue cracking can be more common, so blade suppliers recommend establishing a disciplined blade change schedule. Another key factor is the use of flood coolant water of as much as 350 pounds per square inch (24.5 kilograms per square centimeter). The coolant system must be well-designed, -applied, and -maintained to avoid blade distortion and to flush metal wastes without cooling the friction contact between the teeth and the workpiece.
Both friction band and circular sawing operations generate a lot of noise. Special consideration must be given to personal protective equipment for hearing, and engineering controls such as sound-attenuating shields or machine enclosures may be needed to keep exposure to sound levels below Occupational Safety and Health Administration-required maximums.
Like circular saws, abrasive disk sawing equipment is available in types ranging from small manual cutoff saws to very large, computer numerically controlled gantry-style machines used to cut plate up to 6 in. (152 mm) thick.
Unlike true friction sawing, abrasive sawing does not rely on heat to soften the hard work material. Instead, it uses ceramic grains that are harder than the metal being cut, so that the cutting edges of each abrasive crystal cut a small chip at rates of 20 cubic in. (5,460 cubic mm) or more per minute (see Figure 2).
As the individual crystals dull, forces increase until they are torn out of the bonding matrix, exposing new, sharp crystals underneath. This keeps the blade sharp but leads to a continuous reduction in blade diameter and requires fairly frequent replacement.
Machine and blade technologies have improved over the years, so the abrasive process has become increasingly competitive. The ratio of metal removed to blade loss continues to improve, and advanced controls can optically monitor the blade diameter and increase the revolutions per minute to maintain a constant surface speed. As with circular sawing, flood coolant is important, but the noise level may be lower. A variety of blade compositions are available, so it is important to seek expert guidance in selection for each application.
Friction sawing and the related abrasive sawing methods have been around for a long time and continue to advance. These processes cannot be judged now on how the equipment performed a decade or so ago. In many situations, friction or abrasive sawing can provide effective results from relatively low-tech, low-cost equipment that may require no programming or operator skill.