January 2012 / Volume 64 / Issue 1|
By Susan Woods, Contributing Editor
Courtesy of Behringer Whether using a bandsaw or a circular saw, mastery of the basics makes operators more productive.
Even the most complex machining operations start with a seemingly simple operation—sawing. Shops looking to optimize their processes shouldn’t ignore this step, because small improvements in sawing can pay big dividends down the part manufacturing line.
Metal suppliers use saws to process tubing, bar stock and solid materials for end users, and shops saw those materials into smaller workpieces. Band and circular saws are the two most common types found in shops. The bandsaw is a versatile machine that can be used for many different applications. For people who production-cut a variety of materials, a bandsaw is a good choice.
A circular saw has the ability to cut much faster than a bandsaw and provide a more accurate cut with a better finish. A circular saw performs a specific job more efficiently than a bandsaw, but is not nearly as flexible.
“A bandsaw is like a pickup truck—it can do almost anything,” said Dave McCorry, president of Structural Machinery Solutions, Columbus, Ind., a distributor of metal sawing and structural steel fabrication machinery. “The circular saw is more like a racecar. If it can be tuned to the application at hand, the circular saw can be more accurate and faster. So it is a question of flexibility vs. out-and-out production.”
Circular saws generally cut smaller workpieces. For example, most circular saws cut material from ½ " to 8 " in diameter, whereas a bandsaw can be made to cut material of pretty much any size.
A 4 "-dia. round solid bar of mild steel is the classic breakeven point between a bandsaw and a circular saw, McCorry noted. “They will both cut that material in approximately 1 minute,” he said. “Anything below that could be cut quicker with a circular, assuming reasonable materials, while anything above that would be quicker and cheaper with a bandsaw.
“But you get to a point where circular saw blades are just too big and heavy to go through really hard material,” he continued. “If you are cutting titanium or Inconel, for example, you can only take a very thin chip. When in the cut, the circular blade takes 3 or 4 times the width of that chip, so it takes much more power to produce the same effect as a bandsaw blade. That is why a bandsaw tends to be better on harder materials.”Automatic or Not
Both saw types are available in manual, semi and fully automatic models. Manual is for users who are just cutting a few pieces periodically.
“Semiautomatic saws are for shops running different lengths of material and generally smaller quantities,” said Matthew Klipp, sales and marketing associate for saw manufacturer Behringer Saws Inc., Morgantown, Pa. “They can measure manually or utilize a material handling system that allows them to run material into a stop or feed it into a unit that will measure the length. They just have to input the length into a CNC, cut it and immediately change it by inputting a different length. So they can cut one piece at 2 ', one at 4 ' and so on.”
Courtesy of Structural Machinery Solutions
Automatic saws, according to Klipp, allow companies that have high quantities of the same size to run efficiently. “They can take long lengths of material, program the quantity into the CNC, and then it is automatically and continuously fed into the machine and cut to length.”
Even more sophisticated CNC versions of bandsaws and circular saws are available, right up to a fully programmable sawing cell. “You preprogram (the sawing cell) and it allows you to cut as many parts as you want at 12 " long, 10 " long, 6 " long and so on,” McCorry said. “You can load different types of material into a magazine and program every single compartment. You can program as many parts as you want in each type of material, so you have full operational flexibility.” He noted that this type of system is only used by about 1 percent of his customers because of its high cost.Cool Cutting
Coolant plays a major role in band and circular sawing. It keeps the saw blade cool and lubricates the face and gullets of the teeth so the chips will form easily in the gullet area and be carried out of the cut and drop off into a chip bin or be knocked off by a blade brush.
In both processes, users can apply flood coolant or spray mist, or cut dry, depending on what they want to accomplish, according to Klipp. “Some like to have clean material after they cut it so they have fewer secondary steps and therefore use mist instead of flood. The mist, in some cases, can lower blade life somewhat because it doesn’t keep the material and blade as cool as flood coolant. But in a lot of applications, it is still a good alternative as it will keep the shop floor and material cleaner.”
For shops cutting soft or thin-walled materials and aluminum, spray mist provides cut times and quality just as good as flood coolant. But Klipp recommends flood coolant for stainless steel and Inconel because cutting them generates a high level of heat.
Behringer has developed a system for applying multiple coolants—flood coolant, spray mist and air mist—for its HCS series circular saws. “Those three can be used individually or in any combination,” Klipp said. “Because circular sawing is typically performed at high speeds (up to 1,100 sfm), you want to make sure you are cooling effectively for good blade life and excellent cutting. The ability to cool effectively when cutting more difficult materials, such as chrome molybdenum and nickel, has been a hurdle in increasing the cutting speed. But by being able to utilize flood coolant in this typical spray-mist application, you are taking more heat away from the blade and material, and allowing for faster cutting.”Bandsaw Blade Characteristics
Most metalcutting bandsaw blades are bimetal, featuring a thin strip of HSS that is welded to a wide strip of spring-steel backing material. The spring steel makes the blade flexible so it can bend around the machine’s wheels and twist into the guides. Different types of HSS are used, but M-42 is the most common because it is said to be the most versatile for bandsaw applications.
Once the two types of steel are welded together, the teeth are milled or ground into the HSS edge. “The teeth are then set before the final heat-treating process,” said Glenn Tatro, director of sales for Lenox, Industrial Products & Services, East Longmeadow, Mass. “This means the teeth are bent left and right in a pattern.” Various set patterns are used by different manufacturers.
Tooth setting prevents the bandsaw blade from binding in the cut. “The set provides a slot (kerf) for the back of the blade to move through the cut freely,” Tatro said. “For example, on a 1 "×0.035 " blade, if you set the teeth 0.011 " on each side, the total slot you can cut would be 0.057 ". If the slot is the same width as the thickness of the blade (0.035 "), the material can expand or stress-relieve itself and close in on the blade, and the blade can get hung up in the cut.”
Variable-tooth bandsaw blades are the most common type used for metalcutting. Variable tooth means there is a variable number of teeth on the blade in a designated length of that blade and the tooth pitch changes from one tooth to the next. This helps eliminate vibration, one of the major enemies of blade life.
Knowing how many teeth to have on a bandsaw blade depends on a variety of factors, but there is a simple rule of thumb that can be a good starting point.
“The 3, 6, 12, 24 rule is very simple,” Tatro said. “You want at least three teeth on the work all the time, you don’t want more than 24 teeth on the work at any one time, and what you’d like is between six and 12 teeth on the work all the time.”
Take, for example, cutting a 2 " solid square with a 6/10 variable-tooth blade. “The average amount of teeth per inch on a 6/10 variable tooth is around eight teeth,” Tatro said. “So if you have an average of 8 tpi and you have 2 " of blade in that 2 " square material, that would put about 16 teeth on the work. But the rule says ideally you want between six and 12. So go down to the next size. A 4/6 variable tooth, with an average of about 5 tpi, would be a great choice for the application because you would have 10 teeth on the work.”
Courtesy of Lenox
Carbide-tipped bandsaw blades are also available. A carbide blade retains its sharpness longer and imparts a finer surface finish than a bimetal blade.
Blade coatings, which can be applied on bimetal and carbide-tipped bandsaw blades, also play an important role in bandsaw cutting. A coating shields against heat. “Heat is another major enemy of saw blade life,” Tatro said. “When the teeth on the blade get hot, they lose their hardness to the point where they are no longer effective. When that happens, the operator tends to push the blade harder to make it cut and eventually the blade can strip the teeth off.”
Courtesy of Behringer
Coated carbide-tipped bandsaw blades can reduce the time it takes to make the cut. For example, the AlTiN-coated Lenox Armor Black can take a much higher level of heat generated by mild steels, alloy steels and stainless steels than a bimetal blade, according to Tatro.
“Because of this, band speeds can be increased up to three times faster than the recommended speed for a bimetal blade,” he said. “Feed rates and pressures also can be increased to the point that cutting a piece of steel that would normally take 25 minutes to cut with a bimetal blade can be cut in 45 seconds with the Armor Black.” He added that up to 90 percent of the overall cost in a bandsawing operation is how long it takes to make the cut.Circular Blades
The standard circular saw blade is made of solid HSS. The teeth are machined along the periphery of the blade. The other types are carbide- and cermet-tipped ones, with the body being alloyed steel hardened to 38 to 40 HRC.
“Circular blades do not have a set,” McCorry said. “The teeth are all in line and the cut is divided between the teeth in a different fashion.” Generally, one tooth is square and one is “roofed,” or chamfered. Two teeth then make up a pair; the first tooth cuts the center portion and the second tooth rakes out the corners, breaking up the chip. “If you made all the teeth the same width, the chip would have difficulty evacuating from the cut as it would be the full width of the kerf.”
The number of teeth in the cut for a circular blade is typically two to three for HSS, carbide and ceramic blades. “When cutting solid materials or heavy-walled tubing or pipes, the operator should use a triple-chip-grind saw blade where you have a tooth that is making the first cut, the second one is raking out the chip and the third is making the cut again,” said Richard Otter, sales manager for Tru-Cut Saw Inc., Brunswick, Ohio. “When cutting thin-walled tubing or pipes, the customer should use a blade with a notch grind where all of the teeth are ground equal so you are cutting with all of the teeth. You are constantly taking the chip and cleaning it out of the way.”
With carbide- and cermet-tipped circular blades, only a certain tooth count can be used because the pitch has to be correct for the size of the carbide inserts, according to Otter. The limitation is the distance from tip to tip for the carbide to slide into the blade and for the brazing equipment to operate effectively. “For example, to cut thin-walled tubing with an OD of 250mm with 200 teeth, the pitch would be 3.9mm, which would be too close for most current brazing equipment,” Otter said. “If the customer wants to use a circular blade in this application, they would have to use a HSS blade.”
Tru-Cut applies a PVD coating to all of its saw blades. “In most cases, the coating does the work a coolant would do,” Otter said. “If you are cutting 300 series stainless, we would recommend a full TiCN coating on the tooth forms. Using a TiCN-coated circular blade with flood coolant would be the best way to cut 300 stainless steel. But if you are cutting mild steel, you could get away with dry or spray-mist cutting and go with a TiN or AlTiN coating based on the sawing equipment. As the blade heats up, the aluminum in the coating activates and acts as a heat barrier.”
Unlike bandsaw blades, which are disposable, HSS and carbide-tipped circular blades can be resharpened repeatedly. Cermet-tipped blades cannot be reground because fracture cracks invisible to the naked eye develop during use.Blade Tension is Key
Maintaining the proper tension on a bandsaw blade keeps it rigid and straight in the cut, preventing deflection, according to Behringer’s Klipp. “Saw manufacturers typically recommend 30,000 to 40,000 psi on the blade,” he said. “The tighter the blade, the less vibration and the more pressure you are going to get so it stays square going into the material.”
Maintaining tension on a circular saw blade is different. It is not controlled by the sawing machine. “If the blade is what we call ‘stiff and neutral’ (which means a flat steel plate with no movement in it during and after the manufacturing process), as soon as the blade generates heat on its rim, that heat goes into the body of the blade and makes it wobble,” Otter said. “You have to have a saw smith who can roller-tension the blade into the right position to make the blade flexible so when it heats up in the cutting process, it tightens the steel naturally and becomes straight.
“If you have too much tension, when the operator makes the first cut the blade will become loose and cut out of square,” he continued. “This may also happen if he isn’t using the right speed and feed for the material. It can be done by hand by a saw smith or done on a roller tension machine.”Breaking In
With any new saw blade, the tips are sharp and need to be broken in. “You have to cut with slow feed rates for a certain period of time to wear that sharp point on the tooth off and put a radius on it,” Lenox’s Tatro said. “Just like a pencil.” Many operators do not properly break in a new blade, reducing blade life up to 50 percent.
The break-in process differs, depending on the workpiece material. Recommendations exist. “If you are cutting soft steel, you should not be cutting at your normal feed rate until you have cut approximately 100 square inches of that material,” Tatro said. “If you cutting a 2 "×2 " square bar of soft steel (of any length), it should take about 25 cuts before you are up to the recommended cutting rate.
“On hard material, you should cut about 50 square inches of material before you are at the recommended cutting rate,” he continued. “You have to cut more aggressively on the break-in cut on hard materials to continuously remove a chip and not workharden the material, so you are forming the radius on the tooth tip much sooner than on soft material.”
Another issue to avoid is running the blades too hard. The biggest problem Otter encounters is a customer running the saw blade 20 or 30 percent over the correct speed. “They just end up using more blades through premature wear,” he said. “Everyone wants to cut faster, but it ends up costing more when you go through blades faster than you should. I would say if the operator is not running at the correct speed and feed, it will reduce blade life by 50 to 70 percent.” CTEAbout the Author: Susan Woods is a contributing editor for CTE. Contact her at (224) 225-6120 or by e-mail at firstname.lastname@example.org.
Courtesy of Crow
Shop saw runs hard, may be upgraded
Behringer Saws Inc.
Structural Machinery Solutions
Tru-Cut Saw Inc.
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