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A PRACTICAL BANDSAW CUTTING GUIDE THAT COVERS PHYSICAL OPERATING FACTORS AND MAXIMIZING CUTTING EFFICIENCY
Assuming an unusually capable saw and ideal conditions, it is possible to cut at a maximum rate of approximately 30 square inches per minute. (laboratory tests have attained up to 90 square inches per minute with a bandsaw, but this is not practical� for real world cutting operations) this rate could be obtained only on� a material which is easy to cut, such as C1018 cold finish bar. It would also require the correct blade tooth and spacing, the right blade speed and feed rate, and an appropriate high quality coolant.
Under more normal conditions a cutting rate of 15 inches per minute is practical and readily obtained when using a high speed electron welded blade. When working with more difficult materials, of course, slower cutting rates may be required. Each type of material has its own characteristics and some require unusual measures to obtain satisfactory cutting performance.
There are many factors which affect cutting performance.
The major ones are:
- Material composition
- Material size and shape
- Guide spacing
- Blade selection
- Blade sharpness
- Blade speed and feed rate
- Blade tension
- Blade Vibration
- Saw design & construction
As the material machinability lowers, so does the cutting rate. For example, stainless steel is slower to cut than C 1018, which in turn is slower than B 113. Surface conditions will also affect the cutting rate. If there are places on the surface or in the material which are hard, a slower blade speed will be required or blade damage may result. Tubing will be slower to cut than solids, because the blade must enter the material twice, and because coolant will not follow the blade as well. Tough or abrasive materials are much harder to cut than their machinability rating would indicate.
Material Size and Shape
Each blade configuration will have an optimum width of material to be cut. Below this width, tooth loading may become excessive and the cutting rate must be reduced. But when the material is wider than the optimum width, blade control begins to be lost, as will be discussed below. For example, for a band saw blade 1 inch wide by� .035 thick, the optimum width is between 4 and 5 inches. But a 1.25 inch blade .042 thick will have optimum cutting in stock which is about 6 inches wide. This is because the heavier blade has nearly twice the beam strength, which allows higher pressure and straighter cutting in heavier material.
Since the blade �sees � only the material actually being cut, the shape of the stock being cut will also affect cutting speeds, particularly if the piece is excessively wide or if it varies in the dimensions being cut.
Cutting tubing presents special problems.
The actual area of the cut can be found by using the following formula:
However, there are additional complications, such as the fact that the blade must enter the material twice and that maintaining adequate coolant flow on the blade as it enters the second side in nearly impossible. This, whenever the inside diameter begins to approach 50% or less of the outside diameter, it is best for practical purposes to treat the material as a solid. In other words, as well thickness increases, the tubing begins to more and more closely resemble a solid in terms of cutting speed.
The rigidity of the blade is a function of the of guide spacing, with rigidity being reduced to the third power as the distance between the guides increases. For example, with guides spaced 2 inches apart, blade deflection might be approximately 0.2�. Under the same conditions, but with the guides spaced a 4 inches apart, blade deflection would be approximately 0.8�.
- Y = Blade Deflection
- W = Load on Blade
- L = Spacing of Guides
- E = Modulus of Elasticity
- I = Moment of Inertia
This is a much-simplified version of the fourmula, because it does not consider band tension or guide design. It is important to recognize, for example, that rollers are sonsidered as a pivotal contact, whereas carbide faces could be considered as anchored supports. A more complete derivation, including band tension and guide design, is included in Roark�s Handbook, �Formulas for Stress and Strain"
Thus, the greater the distance between the guides, the greater the probability of a crooked cut. The solution is to reduce cutting pressure. However, if the material is hard or tough, cutting may stop altogether. Thus, when cutting wide stock, a compromise between too much and too little cutting pressure must be found. Trial and error may be the only satisfactory method.
There are five types of blade materials generally used:
Carbon, Hardback Carbon, Semi-High Speed, High Speed, and the Electron Welded Blades.
Carbon blades cannot be generally recommended because the back of the blade is not sufficiently strong to stand adequate tension, and because it has poor resistance to heat and abrasion. The hard-back carbon blade�s teeth do not have re-hardness, but if the blade is run slowly it can be very economical in some applications. The semi-high speed will allow greater blade speed, but is still relatively economical in applications requiring great toughness, such as in the cutting of structural shapes. The high speed blade, very popular a few years ago, is now being replaced with more economical electron welded blades. Electron welded blades, which although the most expensive are also the best blades, come in many configurations. However, they generally follow the same basic construction. This consists of welding special tool steel teeth of appropriate size and shape to a very tough black back, using special welding process. The teeth are most commonly made of M-2 tool steel, but many other types are also available for special purposes. These special teeth may be either particularly hard, to permit very high surface speeds, or extremely tough, for use in particularly difficult material, such as irregular or large shapes in which vibration is a problem. There are electron welded blades suitable to almost any type of cutting.
Tooth Form and Spacing
The selection of a tooth form is generally determined by the material to be cut.
There are three general factors to consider:
- Tooth form, the style or shape of the teeth;
- tooth spacing, the number of teeth to the inch;
- tooth set, which provides clearance for the body of the blade
Three styles of teeth are shown below
In general, a coarse, hook tooth blade is the most efficient in materials where it can be used. Mild steel and aluminum would be appropriate applications. In wide cuts, a skip tooth blade would be effective, since it simply reduces the number of teeth per inch. The standard tooth blade is, of course, a blade for general applications or where a variety of materials are being cut. It is also particularly useful in cutting fragile materials, such as castings, brass, and so on.
Tooth pitch, or spacing is generally determined by the material and its thickness in cross-section.
It is generally specified in �teeth per inch�, as indicated here:
When cutting narrow shapes, more teeth per inch will be required to prevent damaging the blade. Similarly, softer materials will also require more teeth per inch. Wider shapes and harder materials will require a coarse blade with fewer teeth per inch.
A relatively new development is blades with variable tooth spacing. On blades of this type the tooth spacing might, for example, vary from 3 to 6 teeth per inch on a particular blade. Or, on a less coarse tooth blade, it might vary from 6 to 10 teeth per inch. The purpose of this type of tooth spacing is to prevent vibration, which will be discussed in more detail below.
Tooth set prevents the blade from binding in the cut. It may be either a �Regular Set� (also called a �Raker Set�) or a �Wavy Set�. The regular or raker set is most common and consists of a pattern of one tooth to the left, one to the right and one (the �raker�) which is straight, or unset. This type of set is generally used where the material to be cut is uniform in size, and for contour cutting. Wavy set has groups of teeth set alternately to right and left, forming a wave-like pattern. This reduces the stress on each individual tooth. Making it suitable for cutting thin materials or a variety of materials where blade changing is impractical. Wavy set is often used where tooth breakage is a problem.
It comes as no surprise that a dull blade will cause problems but it is also true that a very sharp blade can be a source of difficulty; vibration, to be exact. What happens is this: When a very sharp point enters the material, it immediately begins to dig itself into the material. At some point, it gets too deep and �bounces� up. The next tooth does the same thing, and results in vibration. Excessive vibration will greatly reduce blade life, and will also cause excessive wear on other parts of the saw. As the blade begins to dull just slightly, the points of the teeth stop digging in and the vibration stops. Now the teeth must be pushed into the material by the saw, permitting proper cutting pressure to be applied.
This �honing� process is best accomplished by careful breaking in of the new blade immediately after installation. Certain blade manufacturers actually sandblast their blades to remove the very sharp points. This may be an advantage in situations involving inexpert saw operators and difficult materials. But careful break-in of a new blade is by far the best method of obtaining the maximum blade life.
A dull blade, on the other hand, cannot be expected to cut straight. For example, picture a 10 pitch blade with a .001� flat on each tooth. One thousandths of an inch, smaller than the naked eye can detect (a human hair is generally from .0025� to .003�). If you were cutting a piece 4� wide you would have forty teeth engaged in the material at one time. That is a total of .040� of flat pressing into the material. Now imagine trying to cut the same material with a chisel with a .040� flat on the point. What degree of accuracy would you have?
In addition, a dull blade will not cut efficiently. As the blade gets dull, it penetrates more slowly and generates more heat which will quickly dull the blade as it becomes duller still, generating more heat, and so on. Soon the teeth will fail won�t cut at all.
Since a dull tooth cannot be detected by the naked eye, cutting time is the best indication of a dull blade. Typically as a blade begins to dull, the cutting time will begin to show a significant increase. It is possible, but un-economical to leave the blade until cutting time has increased two, or even three times the normal time. Maximum efficiency and straight cutting require that the blade be changed as soon as dulling begins to become significant for the material being cut.
It is worth noting, however, that a blade which is too dull to cut stainless or similar materials efficiently will still be satisfactory in mild steel. However, a blade which is too dull for mild steel will not be satisfactory in aluminum.
Blade Speed and Feed Rate
Blade speed is generally limited by vibration and the ability to keep the blade cool to avoid dulling the teeth. A blade which is running fast and taking a very light cut will dull quickly because the tips of the teeth will overheat from the rubbing action. If, however, we force the blade teeth deeper into the material, the blade will be less sensitive to heat, because the teeth are cutting more and rubbing less. This increased pressure may also prevent vibration. Thus, up to a point, a higher pressure on the blade may actually permit higher blade speeds.
If we have a sharp tooth with a .0002 radius on the tip, and we apply only enough force to cause penetration of .0002, the tooth will not penetrate and cut. If, however, we apply enough force to cause penetration of .001, the tooth still has .0008 of a sharp edge to cut with. This is similar to the �dull tip effect� observed frequently in lath and milling operations. When taking a finish cut with a dull tool, a fine adjustment may make no cut at all, but an additional fine adjustment will cause the tool to dig in deeply.
If, on the other hand, we apply too much penetrating force the teeth will be ripped out of the blade. The maximum feed rate is determined by the saw, material size and shape, guide spacing, coolant, and the size and shape of the teeth. The greater the blade speed, the greater the feed rate can be, up to the limits imposed by the factors.
Thus, for each blade and material being cut, there is an optimum balance between the blade speed and feed rate. This rate will give maximum blade life and most satisfactory cutting.
- General recommendation:
- Coarse tooth blade, so that each tooth has adequate force
- Guides set close to the work to permit relatively heavy feed pressure and still control the blade.
- Carefully controlled feed rate to prevent the teeth from tearing out.
Blade tension is an important factor in blade rigidity. Adequate tension prevents the center of the blade from being deflected to the side, causing a crooked cut. It also prevents the blade from achieving reduced penetration of the teeth in the center of the cut. From the cutting standpoint, the more tension the better. The limiting factor is blade fatigue.
Blade vibration is caused by a blade tooth entering the material. Force is required to penetrate the material, while resisting force causes the blade to rise slightly at the time of contact. Raising and lowering of the blade causes vibration, and if allowed to build up, will affect blade fatigue life. This might cause the blade to break. To eliminate blade vibration, increase blade tension, feed rate, blade speed, or use a different tooth form. Blades with variable tooth spacing may be helpful in eliminating vibration in some applications.
Spacing the guides farther apart will allow the blade to vibrate freely in the cut without this vibration being transferred to the sawing machine. Thus, the vibration will appear to stop, but will actually continue. And, of course, blade control is lost with wider spacing.
Coolant is so important it cannot be overstressed. A good quality coolant in a band saw is one of the most important factors in straight cutting. Coolant keeps blade teeth cool,� prevents chips from welding to the tooth and also lubricates the chips, allowing them to move easily through the cut.
If coolant is unable to cool the blade teeth, they will soften and become dull. If the coolant is distributed to only one side of the blade, the opposite side will become dull. This will cause the blade to move toward the side which has the most coolant and the cut will be crooked.
If we compare sawing to milling, we immediately see that in sawing there is much less room for the chip. The chip must lodge in the small space between the teeth and be carried smoothly out of the cut.
- Without proper coolant, either of two things will happen:
- 1. The chip may become welded to the tooth. This will change the form of the tooth, which in turn changes the amount of force required for the blade to cut.
The result is an unbalanced blade which will produce a crooked cut.
- 2. The chip will wedge in the cut. Since the chip is work-hardened and harder than the stock from which it came, the blade will cut into the stock beside the chip. Again, the result is a crooked cut and dulled blade.
In selecting a coolant, pick one which is of highg quality. Avoid thinly mixed soluble oils. Some of the new synthetic oils are highly satisfactory in difficult operations.