The machine is a natural for integrating into complete work-handling and cutting systems. Stock feeding, automatic work handling, and CNC are features that combine to automate bandsawing operations.
There’s a top end in most categories of machine tools, and we expect special performance from them to justify their price. But, as with most machine tools, the lowest per-part costs rarely are achieved with the lowest-cost machines. The performance we associate with higher quality machines usually includes high productivity and long life. In the end, they can be the real bargain.
Bandsaws don’t often get the kind of attention and scrutiny we give to, say, laser cutters, but they perform an essential process in many segments of fabricating. So FAB Shop thought we should put one on the operating table and dissect the construction features and performance objectives of a line of saws that clearly is in the top category: the German-made Behringer, available in a variety of types and sizes ranging up to just over 82 in. x 82 in. capacity.
There are different features on the different models, especially different levels of automation, and we’re not picking on one particular model. Rather, we’re looking at some highlight features represented on different models in the line.
The first things you notice is that these machines are built from castings, rather than weldments, and that they are massive. A moderate-size model, the HBM 440A, which has a cutting capacity of 17.3 in., weighs 10,100 lb. Rigidity and vibration-damping are the objectives, as one would expect from any massive, casting-based machine tool. Behringer can quote performance figures but the end result is obvious: higher blade tension without breaking blades, straighter cuts, better finishes and faster cut times. Rigidity is especially important when cutting with carbide-tipped blades, and these saws are built to handle carbide.
Automatic hydraulic blade tensioning provides, on average, 35,000 to 45, 000 psi of tension, uniformity, and automatic shutdown if the blade brakes. When the machine is idle, it relieves blade tension.
Supporting the “bowframe” from two sides of the cut, a dual-column downfeed produces more uniform feed pressure on the blade. Cutting paths are shorter than on swing-frame designs, although that varies with the dimensions of the workpieces.
Two circumferential Turcite bearings guide the frame on each column, resisting back-and-forth tipping forces on the frame.
The structure and the guide system make the other performance features possible. Cutting with carbide, especially in structural shapes but also in plain rounds and rectangles, requires consistent chip loads to avoid work-hardening, on one hand, and to avoid breaking blades or chipping teeth, on the other. Controlling feed pressure alone won’t always do it, especially when cutting stainless or high-nickel alloys.
With four bearings riding on dual columns, the structure of Behringer saws provides stiffness and uniform downfeed.
A platform for controls
A rigid, vibration-damped structure allows the application of sensors and controls, without having to compensate for flexibility in the machine structure. Behringer has taken advantage of this with several interrelated features.
On top of the blade are carbide guides and pressure sensors, which feed back to the machine’s CNC. The object is to maintain a consistent chip load, which is a function of feedrate. But, as blades wear, or when hard spots or general hardness inconsistency shows up in the workpieces, feeding blindly into the work, without recognizing how much force is being applied to the blade, is going to result in excessive blade wear or, likely, a broken blade. So pressure has to be monitored to allow positive feedrates (“positive” here contrasting with simple pressure feed, wherein the feedrate is an indirect function of pressure on the blade) without causing other damage.
The machine incorporates cutting-pressure regulation by means of electronic control, taking readings from the pressure sensors, processing that data in conjunction with the programmed blade speed and feedrate, and maintaining a consistent chip load by varying the actual blade speed and feed to avoid excessive stress on the blade. Particularly with materials that readily work-harden, including stainless and high-nickel alloys, this is an effective way to get maximum cutting speeds consistent with good blade life. With pressure control only, stainless can develop a “skin” of work-hardened material, over which blade teeth can skate. With feedrate control only, teeth can chip and blades can break.
Behringer offers customers a database of materials.“This is what we call our auto feed control, or AFC,” says Joe Suydam, Behringer’s inside sales associate. “It draws upon material data stored in the CNC. You could pull up from data memory, say, a 1045 material. It’s going to give you an approximate feed and speed for that particular grade of material.
“Then the operator can adjust speeds and feeds on the fly if necessary. He’ll watch for the chip quality and the chip color, but the AFC control gives him a starting point. He’ll adjust settings manually if required to optimize the cut, whether it’s minimum cut time, if he’s trying to get the job out to the customer quickly; or better blade life; or some combination of both.”
Program the feedrate; monitor the pressure
It’s easy to see the advantage of this combination of feedrate programming and pressure feedback and control when sawing structural shapes and tubing. The load on the blade varies widely as the width of the cut varies from, say, the width of an I-beam flange to the width of the web. Without pressure control, feedrate or blade speed would have to be set too low; without feedrate programming, the chip load would vary tremendously from cutting the flange to cutting the web.
“One related feature,” says Suydam, “is a fixed angular bias of 3 degrees that’s available on some of our machines. From left to right, the blade is angled at 3 degrees from horizontal. It makes the cut length closer to uniform when you’re cutting structural shapes.” Because the head and the blade travel at a uniform angle on the dual-post design, the bias stays the same throughout the cut.
Machine stiffness also is exploited in other ways. Blade length, for example, is about 30% longer than usual, leading to longer blade life. Behringer utilizes a low-hp motor in conjunction with a hi-torque gearbox and a system of sensors and controls that apply the motor in the most efficient way.
We should note that on this model Behringer is using ballscrew feed drive, and the direct control of the ballscrew is felt to be an advantage in getting the best combination of feedrate and pressure.
Controllable features extend beyond this function-oriented list, including programmable workpiece length control, which can be set to automatically make a certain number of, say, 5-in.-long pieces, then another number of 10-in. lengths, and so on. The highest level of automation in the company’s line is extensive.
That’s one company’s design approach to building a bandsaw that delivers high-end functions and performance. We used Behringer not as an example of the only way to do it, but rather as a clear example of the level of performance that can be achieved with bandsaws of this class. It’s a far cry from the basic level of gravity-fed bandsaws that have very attractive price points. But, as any fabricator knows, low-priced machines are not often the ones that produce the lowest cost per part.