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Details to look for during new and used machinery inspection:
Table tilt v
SPM (if hack)
Throat capacity (if band)
Tool guides, welder, auxiliary table, power feed, filling attach., air jet, work light, vise, bar feed accessories, wet cutting system
Saws and filing machines execute the most basic of all metalworking procedures, the cutting of bar stock to proper lengths for machining. It is done in the simplest of ways, substituting mechanically or hydraulically powered motion for hand or arm motion. Saws used on powered sawing machines are made as thin as possible in order to be consistent with tool strength and rigidness. The width of a cut will be close to the width of the saw, this allows the individual teeth of the saw to deepen the cut made by each preceding tooth as it moves through what’s being cut. Straight or curved cuts can be achieved by controlling the direction of feed.
The principal parts on a circular saw machine are:
Bed - The foundation of the machine, provides strength.
Speed change hand wheel - Performs speed changes through selective gearing, pickoff gears or sheave changes.
Power vertical & Power horizontal clamp - Saw blade to be fed into the work.
Automatic bar feed - Feeds work in and out of the machine, smaller-sized machines furnished with auto bar feeds.
Feed control - May be mechanical or hydraulic or a combination of both.
Stock stop - Provides for positive positioning of the work.
Saw head unit - The main drive mechanism. Wideface, closely fitted, ruggedly supported spiral or herringbone gearing is generally used for the final drive to worms and wormgears may also be used for the input drive to eliminate chatter.
The principal parts on a vertical band saw are:
Base - A box-like casting housing the main drive unit, speed control mechanism and gear shift.
Head Assembly - Accommodates the upper band wheel assembly, job selector mechanism, speed indicator gauge and the band tension indicator.
Tension adjustment - Changes the distance between the two wheels carrying the band. Tension on the band makes it more rigid comparable to the more heavier tools.
Table - Supports the work as it’s fed into the blade.
Chip blower - An air system that removes chips from the cutting areas.
Blade shear, welder & grinder - These are provided so the machine can perform internal cutting of shapes.
TYPES OF SAWING MACHINES
There are 3 basic types of sawing machines:
the power hack saw, the circular saw & the continuous blade band saw.
The power hack saws are depicted by the reciprocating motion of the blade as it cuts. The cutting takes place in only one direction and the saw becomes idle on the return stroke. Hack saws basically follow the same setup: a base and table support the work which is held stationary while sawing, and a C-frame that runs the saw blade. Hydraulic or mechanical drives with speed selection are employed, and three different types of feeds are used.The weight of the C-frame itself maintains pressure that feeds the saw into the cut in gravity feeding. In mechanical friction or hydraulic pressure feeding, the saw is forced into the cut for faster cutting. In a ratchet mechanism, feed may be executed by means of a screw or pawl. Since the stroke is intermittent, hack saws do not have a very fast method of cutting off stock. Their advantage, though, is they are simple in design, easily interchangeable from job to job, and relatively inexpensive to operate and maintain. Hack saws are also available in a wide range of models and sizes, ranging anywhere from manual clamping of a single cut to the more complex automated machines.
The circular sawing machine functions on a milling principle, however unlike conventional milling machines, the diameter of the spindle gear will be smaller than that of the saw blade. This requires careful design characteristics implemented into the machine to guarantee a smooth, powerful drive to the blade without hardly any backlash. That is why wide-face, closely fitted spiral or herringbone gears are used for the final drive, and hourglass worms and wormgears are often used for the input drive to eliminate chatter. Circular sawing machines are distinguished by a round or circular saw blade that is mounted on a power-driven arbor and rotated through the cut. These machines are divided into 3 types: the cold saw, abrasive cutoff machine and the friction saw. Cold saws are the most sturdy and powerful of the saws. Their direct geared drive allows the application of increased cutting speeds, lending itself well to automation and to combine with other machining units. Friction saws operate at high speeds and develop intense frictional heat when coupled with heavy feed pressures and actually melt or burn the metal away as it touches the blade. Teeth, if supplied, serve mainly to carry oxygen into the cut.
Band saws apply a very thin continuous steel loop with hundreds of cutting teeth on one edge. The band is carried on the rims of two or three wheels, one of which is powered by the drive. Friction is created between the band and the wheel prevents slippage. Adjusting of the tension on the band is possible, it gives the band saw a rigidness comparable to that of much heavier tools. There are two basic types of band saw: vertical and horizontal. The vertical saw has one wheel located above the other and a horizontal work table where the band passes. It is commonly recommended for contour sawing, notching, slotting, splitting, serrating and other cutoff operations. The horizontal saw has both wheels in line, so not much additional machining, if any, is required.
While demand for standard controlled bandsaws holds steady, demand for CNC production bandsaws are on the rise. For CNC bandsaws, operators are able to enter the number and length of parts into the CNC and go to other work as the saw cuts the material unattended. This increase in sawing time is also causing a shift in blade requirements. Operators now need blades that last longer to make unattended sawing more productive. Although a blade on modern saws only require two or three minutes to change, the cost of the blade and time to change it add up. Any prolonging of the chipmaking time in between blade changing lowers the cost per cut.
When selecting a saw it is important to consider the proper size and capacity for efficient production. For example, selecting the right cutoff machine can result in significant cost reductions by eliminating waste, and reducing machining time and labor costs. The capacity of hack or band saws is designated by the maximum square section that can be accommodated by the machine. Standard cutoff saw capacities range from anywhere from 6”x6” up until 24”x24”. Band saw size is designated by throat clearance, the distance between the cutting blade and the rear column which supports the upper band wheel. Standard sizes range from 16” to 60”. The size of a circular saw machine is made by the diameter of the circular blade for which the machine is designed. Sizes range from 10” to 120”.
Vertical & horizontal band saws
When selecting a vertical or horizontal band saw, you must consider the specific advantages of each machine. User production requirements will then determine which of the two machines is more suitable for your needs. The vertical band saw is recommended for shaping work, simply because it can remove unwanted material both inside and outside. Since the cutting force is uniform on the vertical band machine, clamps and other fixtures are usually not required. However, machining time is low, the downward cutting force is a slicing action, therefore is best used when soft, spongy or honeycomb materials need to be cut without distortion. Vertical band saws come in a range of sizes from small toolroom machines to large production models. Throat depths range from 16” to 60” with band speeds from 35 to 15,000 sfpm and the horsepower from 1 to 15. The horizontal band saw is recommended when speed, high accuracy, low scrap losses and versatility are special priorities in the cutoff operation.
Check machine’s bed and structure support components for cracks, breaks or welded repairs. Breaks, even if repaired, can affect the machine’s ability to turn out precision work.
Look in gear boxes and confirm the gears are not chipped or worn down. Worn or faulty gears can cause slippage in the drive and feed mechanisms.
Check all ways and slides for signs of excessive wear. Also, check the machine’s table and saw arm.
Listen carefully to all gear boxes while machine is running. Proper coordinating of gears is important for chatter-free work.
Look for backlash in the saw blade, minimum back lash is evident that the drive blade is Smooth and rigid.
Check the clamping mechanism, make sure it works properly for accurate cutting.
Make sure the stock stops operate as they should.
Verify the lift roller mechanism is working properly so that work may be easily moved in and out of the machine.
Check the automatic bar feeder.
Examine the chip clearance system and see that it functions properly.
On a power hack saw, see that the force feed and quick return mechanism are properly working.
For hydraulic equipment, look for leaks, noisy valves and pumps.
Run the machine through its complete cycle. See that all feed and speed controls and electrical controls function properly. Make sure the variable speed adjustment on hydraulic machines are working and that the speeds are adjusted correctly.
After running some work, check the smoothness and uniformity of the finished workpiece and determine if the machine fits your needs.
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:
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�.
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:
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.
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.
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.
Factors Affecting Cutting Speed
Assuming material is the limiting factor, there are several rules of thumb which may be used to determine cutting rates:
Factors Affecting Blade Performance
Factors Affecting Machine Performance
Inaccurate cutting and short blade life usually have simple causes. The following points need to be checked frequently.
As previously stated, dull blades and loose guides cause most crooked cuts.
You may be faced with excessive blade breakage and other equipment problems.
We cant overemphasize the necessity of a good coolant. It is vital that the blade gets adequately cooled and lubricated. Changing from a poor coolant to a good one typically doubles blade life. The difference between controlled, accurate cutting and sloppy sawing can often be traced directly to the coolant.
*This is one article in a series of How to Buy Metalworking Equipment. Each article showcases and explains a particular type of metalworking machine. They were originally published in the Metalworking Machinery Mailer published by the Tade Publishing Group.