The Fabricator - November 2013

up against the band saw machine’s guides; these tapers cause the blade to oscillate slightly up and down. ;is in turn causes the blade to enter the workpiece in a rocking motion, causing the blade to cut on multiple planes. ;e result: Fewer teeth engage the metal at once.

;is means that it takes less cutting pressure per inch to remove metal, which produces a smooth cut surface. It also improves cutting e;ciency and shortens cycle time.

2. What are my overall cycle times?

;e actual cutting is just one component of the overall cycle time. After cutting, the saw lifts out of the cut, indexes the material, clamps it, and descends the blade to begin the next cut. In recent years machine manufacturers have worked to significantly shorten indexing time, and for good reason. With the actual cut time dropping, all the other elements of a band sawing cycle, including indexing time, become much more significant (see Figure 2).

Similarly, material handling processes have helped automate loading and unloading. Some modern band saw systems have sophisticated magazines that you can load, and programs can be made ahead of time and loaded into the CNC, eliminating programming time during the changeover. O;oading systems have advanced as well. Taken all together, a band saw system now can operate lights-out, unattended—something that not long ago just wasn’t feasible.

A comprehensive understanding of work flow needs to enter the equation too. After o;oading, cut material needs to flow e;ciently to downstream processes. Productivity gains are harder to come by if you’re just overloading a downstream operation. If you do this, your band saw may be twice as e;cient, but you may not be shipping any more products than you did before. For this reason, be sure to keep the entire work flow requirements in mind.

3. How much scrap and rework do I have?

Band saw scrap and rework drive costs up and hamper productivity. Most blade and machine manufacturers have recommended speed and feed charts. Besides characteristics of the blade itself, two primary factors regulate how quickly a saw makes a cut: One is the rate of descent of the sawing head, and the other is the pressure associated with that descent. Many view these as one and the same, but they are not.

Some operators let the rate of descent exceed the blade’s beam strength or ability for the teeth to penetrate. ;is puts excessive pressure on the teeth and causes the teeth to strip or the band to bend. From there the band takes the path of least resistance, deflects left or right slightly, and begins cutting down

Figure 2

;is band saw indexes quickly to bundle-cut rectangular tube. With the actual cut time dropping, all the other elements of a band sawing cycle, including indexing time, become much more important.

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at an angle, causing a crooked cut. Crooked cutting also can be caused by too fine of a tooth pitch for the work being sawed, low blade tension, or worn guides.

A poor or crooked cut may need to be recut, trimmed, filled with weld, or scrapped entirely. For aerospace material in which component weight is critical, the tolerance is zero for crooked cutting, and the cost of that rework and scrap adds up quickly.

In this case, reducing the feed can overcome the problem. Alternatively, a tapered back blade can help, because fewer teeth are simultaneously engaged in the cut. With fewer teeth engaged, you get better penetration and reduce the risk of deflection.

Be aware also that you shouldn’t feed too lightly, either, because that will cause another problem that can lead to premature blade wear and a rough cut finish: premature tooth dulling from rubbing, not cutting. Dulling teeth take the path of least resistance, which often is to either side, and not straight down.