Band Sawing With Interrupted Cuts

When the blade is new, the first time its teeth hit the workpiece is the most crucial point in that blade’s life.

A band saw blade tooth is essentially a single-point cutter. The blade in its entirety actually contains a series of single-point cutters that have been precision-milled, set, and heat-treated and are held in place by backing material.

The number of teeth per inch (TPI), tooth set, and the blade's gullet depth all are engineered to ensure that a proper cutting environment is created. When employed at the optimum band speed and feed rate, the band saw blade should be a reliable and predictable cutting tool.

However, if care is not taken, failure can occur, especially in difficult conditions like interrupted cutting.

"People have long understood the relationship between proper speeds and feeds and successful metal removal in operations other than sawing. The fact of the matter is that a saw tooth is much smaller in size than many other cutting tools. Therefore, the combination of using the correct tooth and running the correct speeds and feeds in sawing is extremely critical," explained Glenn Tatro, director of technical sales for LENOX® band saw blades.

In a milling operation, for example, if the tool is moved into the material too quickly, with too much force at the wrong speed for the material being cut, it can fracture or break. A band saw blade reacts the same way.

"In most instances, people don't consider their saw blade as a precision cutting tool, when indeed it is," Tatro said. "Blade changes due to premature blade failure are very costly. To have a successful sawing operation, you can't be changing blades all the time. That's how productivity and money are lost."

According to Tatro, in a typical shop environment, 90 percent of the total cost of a band sawing operation is generated by the length of time it takes to complete the job. Included in that total cost are capital equipment and labor. Seven percent of the overall cost is for coolants, including their disposal. The most common part of the cost equation that people spend a lot of time analyzing is the price of their blade, but it is only 3 percent of the total cost of the operation. If you want to lower your costs, cut 50 pieces today and throw the blade away, don't cut 25 pieces today and 25 pieces tomorrow and feel good about your blade life. After all, the blade is only 3 percent of the cost equation, while time of cut is 90 percent. (These numbers could vary from one operation to another.)

A price-versus-cost comparison can be helpful in determining the performance of the blade you are using currently.

"Price-versus-cost is an interesting comparison," said Tatro. "Price is the size of the check you write today; cost is how many times you have to write that check."

While maximizing productivity is important in any cutting operation, in difficult sawing operations, blade failure may occur more often, requiring more blade changes. Interrupted cutting is one example.

In all sawing operations, the two major enemies are heat and vibration.

Interrupted Sawing

Interrupted cutting can be as hard on a band saw blade as cutting difficult-to-cut materials, such as heat-treated metal or metal with a high nickel or high chrome content.

Over the years band saw blade manufacturers have learned how to deal with many cutting scenarios, but one that is still very difficult to deal with is interrupted cutting.

"Interrupted cutting can be very hard on a band saw blade," Tatro said. "Because you don't have a consistent cross section through the material, you need to be aware of a couple of key areas of concern when performing interrupted cutting, and tooth strippage is very common."

Cut at the Proper Speed

When tooth strippage occurs during interrupted cutting, the equipment operator can focus on four probable culprits: improper TPI, improper speed/feed, improper break-in procedure, or poor coolant condition. It could be any combination of the four.

In many interrupted-cutting operations, the operator has a tendency to slow the blade down when tooth strippage first occurs. This is actually the worst thing that can be done, Tatro said.

When cutting moderately soft materials, such as A51 and other structural grades, if the blade is going too slow, the teeth aren't moving through the material quickly enough, and they create a chip that is too thick. This overloads the tooth and causes strippage.

Tooth geometry, size, and set are also important variables in interrupted band saw cutting.

"If you have teeth that are too small for the size of material you are cutting, by the time they get halfway across the material, the gullet is packed," explained Tatro. "Then, as the head of the saw continues to push the teeth into the material, the gullet will become so packed that pressure will build on a single tooth until it breaks off and gets stuck in the cut. The rest of the teeth will come in contact with the broken tooth that is stuck in the cut, and the number of teeth breaking off continues to multiply. Very rarely will a blade lose only one tooth. "

Blade Break-In

Tooth strippage in interrupted-cutting applications often can be traced back to the fact that the break-in procedure was performed improperly or not at all.

An extremely sharp milled or ground tooth tip will break the first time it hits the material being cut. A radius gradually formed on that tip will produce the optimal blade life and most efficient cut times for the life of that blade.

Because there isn’t a consistent cross section through the material when performing interrupted cutting, tooth strippage is very common.

"When the blade is new, the first time the teeth hit the workpiece is the most crucial point in that blade's life," Tatro said. "If the operator doesn't run the correct speed for the material being cut and backs way off on the feed rate, the operator will see the tips of the teeth break off immediately. Essentially, 50 percent of the life of that blade has been lost in a matter of seconds."

Break-in is very important to get the full life of the blade, Tatro said. Also, many times people fail to understand that the break-in period differs, depending on the material being cut.

For example, for difficult-to-cut materials, a heavier break-in feed pressure is used to ensure that the teeth are constantly pulling a chip and not just sliding over the material so that it becomes work-hardened. This type of material generally requires about 50 sq. in. of cutting before the blade is fully broken in.

The opposite effect takes place when cutting soft material; you have to use a light feed pressure so that the tooth doesn't dig in too deep. In these situations, the break-in should be performed on about 100 sq. in. of material.

Successful Cutting

The success or failure of any sawing operation depends on many factors. One variable that is easily controllable is the condition of the band saw itself.

"Saws are often the most abused piece of equipment in the shop," Tatro said. "In most instances, it is the first operation in the entire manufacturing process. Nothing happens or things happen very inefficiently if the saw is not in the best possible condition for its age. This means that adhering to proper preventive maintenance procedures on the saw is extremely important. I can't stress enough how important it is."

In addition to the wheels, guides, and other parts of the band saw, coolant also needs to be maintained properly.

"People who are in the business of sawing for a living and understand cost per cut will change their coolant out twice per year," Tatro said.

In the interim period, a refractometer should be used to check the mixture of the coolant on a daily basis.

"What happens over time, especially in warm conditions, is that evaporation will affect the mixture ratios of the coolant," Tatro said. "It doesn't matter if it's a synthetic- or oil-based coolant; the water content will be affected by evaporation."

Don't just top up the tank with water either, he added. Maintain the proper mixture.

"If you just keep adding water to maintain the proper level, your mixture ratios can quickly rise to 30-to-1 or even 50-to-1," he said. "While this means that you will probably still have a coolant, you will not have a proper lubricant."

Blade Technology

Coatings, which were introduced to the band saw blade industry in the mid-1990s, can play an important role in band saw cutting.

"Heat and vibration are the two major enemies of a saw blade. The purpose of a coating is to provide a shield against the heat generated by the cutting process," Tatro said. "A bimetal blade can take only 1,250 degrees of heat, which is an easy temperature to obtain when cutting hard materials. Once it rises above that temperature, the blade will begin to deteriorate and no longer be effective. Therefore, there are distinct limitations to how fast you can cut with a bimetal blade."

The two coatings available for band saw blades today are titanium nitride (TiN) and aluminum titanium nitride (AlTiN).

TiN coatings typically are used for difficult-to-cut materials when blade life is the main goal. AlTiN coatings can be used for cutting various hard or soft materials when cut speed is critical.

"We see more AlTiN applications than TiN applications," he added. "If a shop understands cost per cut, cost per square inch of removal, and are in the business of separating materials for a living, these blades will change their operation."

For more information, visit www.lenoxtools.com.