Getting more from your miter
August 10, 2004
As a structural steel fabricator, you may have operated a miter saw. The fact is, however, in many other shops the miter feature has rarely been used because, traditionally, most steel buildings—from the skyscraper to the humble warehouse—have been designed without miter cuts. Beam ends and clips have been obscured in finished buildings. After all, the steelwork really fulfilled only a perfunctory function as the skeleton that supported the "flesh" of the building. So, you might ask, "Why spend extra time and money cutting on a miter?"
Because architects increasingly are becoming bolder in their use of fabricated steel as a visual element—even as the principal feature of their designs. As such, modern steelwork is expected not only to function structurally, but also to add aesthetic appeal to the entire building. Therefore, miter cuts are becoming used more frequently to increase eye appeal, even if the miter is not a structural necessity.
Whether you use band or circular saws, the introduction of a miter angle adds an extra level of complication. So what can you do to speed up the process and take the hassle out of miter cutting? Much of the perceived trouble in cutting miters has more to do with handling and measuring the material than setting up the machine and making the cut.
Measuring. If you use an integral measuring system and your machine does not rotate on the datum point, a length offset will occur when you are miter cutting. The bad news is that this offset can be different on the input and the output sides, depending on the actual position of the point of rotation. The good news is that whatever the offset is, it will always be the same, point-to-point, on the miter. To save time and avoid on-the-job measuring, calculate the offset distances for the most common miter angles—45, 30, 15, and 221¼2 degrees. If the machine miters both ways, check positive and negative miter offsets, just in case. Make up a chart, laminate it, and stick it on the machine or in full view of the operating position.
Handling. A material handling system can dramatically improve work flow, and thus enhance production throughput—and not only for miter cutting. In particular, large structural sections in long stock lengths are difficult to move and align quickly and accurately. A material handling system that incorporates input and output cross transfer arms and powered roller tables in front of and behind the machine enables steel to be fed into and removed from the cutting zone efficiently and easily.
A stand-alone saw tends to spend a disproportionately large amount of time unproductively waiting for stock to be presented or sawn parts to be removed—a production bottleneck.
Bridging the Gap. One side effect of using a miter cutting setup is that the gaps that are necessary between the machine and the input/output roller tables, which allow the machine to rotate, create problems when the machine is set for straight cutting.
For example, when the machine is set for 90-degree straight cuts, short components may not be able to bridge these gaps. If the system is powered, this situation might be a sticking point—literally. The steeper the miter angle the machine accommodates, and the larger the machine's width capacity, the bigger these gaps will be.
Ideally, when you are making straight cuts, you should use removable or extendable fillet roller units in front of and behind the machine to bridge these gaps. Some systems incorporate these roller units into the machine's control system so that they automatically retract or extend as required. Whether such a system is manual, powered, or automatic, it enables a sawing line to be converted easily and rapidly between straight and miter cutting mode, boosting shop efficiency and taking much of the hassle out of sawing.
One consideration often overlooked is that the width of engagement of the blade becomes considerably longer during miter cutting. For example, a 45-degree cut is nearly one and a half times as long (1.4142135 times, to be precise) as a straight cut on the same material. This means more teeth will be in the miter cut than in a straight cut.
Of course, when you are cutting structural sections, the number of teeth in the cut varies dramatically anyway. Calculating blade pitch and geometry is a complicated equation, but simply put, if you are running at the limit of blade pitch when straight cutting, you will tend to overload the gullets on a miter cut. Because this causes either blade damage or runout—or both—you should choose a rougher blade pitch or cut back the feed rate accordingly.
Do not change the blade speed. It should be left at the manufacturer's recommended speed rate. Just back off the feed regulator to achieve the desired results.
Naturally, an increased number of teeth in the cut means an increase in cutting forces and an increase in friction. Coolants and lubricants play a vital role in reducing friction in any metal cutting process—and sawing is no different—but it becomes doubly critical when you are miter cutting, because of the increased number of teeth in the cut. So check coolant levels and mix ratios before you get into trouble. If you are using an atomizer system, click the pumps up a notch or two for miter cuts on heavier sections.
|Whereas structural band saws offer good performance and flexibility on small sections, circular cold saws perform well on medium-to-heavy sections.|
The increase in cutting forces is compounded by the fact that those cutting forces are now acting at an angle. They are no longer at right angles to the fixed jaw and the base of the machine. Any weakness in the machine, whether because of its design, age, lack of maintenance, or damage, will be exacerbated during miter cutting.
Basically, if anything is loose, now is the time to tighten it. If your machine is not equipped with automatic hydraulic band tensioning, check the tension before doing miter cuts. Likewise, check the machine-clamping devices and, if applicable, the blade guidance system. You will be rewarded with better cuts and longer tool life.
The ever-present band saw versus circular saw debate gains a new dimension in the face of the rapidly expanding global economy. Whereas structural band saws offer good performance and flexibility, the latest structural cold saws, while not as flexible as band saws on small sections, perform well on medium to heavy sections. Simply put, while the choice between band saws and circular saws remains largely a question of tonnages, today's "lean" mentality requires structural steel fabricators to draw upon every advantage. Surely, any technology that claims to double throughput, if only in some areas, merits consideration.
Such is the case with circular sawing on large structural sections. On beam sizes up to 12 inches, circular saws are only a little faster than band saws. However, the larger the beam size, the greater the advantage circular saws offer. On a 36-in. by 256-lb./ft. beam, for instance, a good circular saw tends to be twice as fast as even the very best band saw, based on feeds and speeds sustainable in production.
Circular cuts also exhibit less runout and better surface finish. In addition, runout does not increase as blade life diminishes—an important consideration in high-rise projects.
So you might ask how a circular machine cuts larger members faster. It is simply a matter of mechanical strength. The larger, wider, stronger circular blade absorbs much higher cutting forces without mechanical failure. A circular blade runs optimally at feed rates that would strip the teeth off any band saw. This greater intrinsic strength allows the circular saw to cut slower peripheral speeds, while maintaining a chip load many times greater than that mechanically possible with a band saw. A slower peripheral speed generates less heat for a longer blade life. Of course, the machine running the blade also must be designed and built to endure the powerful cutting forces generated by a slow-running blade making a very heavy and intermittent cut.
If you intend to cut miters on beams 24 in. and wider on a band saw, the topic of extra-wide-set band saws—or whatever proprietary nomenclature this type of band saw goes by—will sooner or later come to the fore. As the name suggests, the set—the amount a tooth is bent off the centerline of the band—is increased to provide a wider kerf to prevent the band backing from being "nipped" by the stresses inherent in rolled-steel sections. Such a band seizure generally is unrecoverable. The cut closes on the back of the band with huge force, the band stops dead in the cut, and it will not come out. Your only recourse is to grind the band off and start again. Once a band seizes in the cut, there is no going back.
Some newer machines are designed to catch this problem as it occurs, and shut the main drive motor off before the machine sustains damage. Using wide-set bands offers a solution, albeit at some cost to cut quality and band life.
It is interesting to note that circular saws do not normally suffer from blade seizures for two reasons: Their higher feed rate and larger kerf allow material stresses to dissipate more effectively. Also, their higher torque keeps the tool moving, even when the going gets tough.
By adopting a modicum of common sense and allowing for the increased and modified forces acting on the blade and machine during miter cutting, you can obtain good results without subjecting machinery and blade to undue stress, and without incurring huge time or quality penalties. A little extra care and attention to detail upfront will result in improved product quality, reduced scrap, longer blade life, and overall enhanced shop efficiency.