Cutting to the chase
December 13, 2001
This article examines common fabrication processes for structural and architectural tube. It specifically focuses on cutting, sawing, miter cutting, bundle sawing, and cambering.
The circular saw is a commonly used option for cutting structural materials, including (top) solid H beams, and both round (middle) and rectangular (bottom) hollow materials.
Architectural tubing is something of a mixed blessing: On the one hand, the material is relatively easy to work with, and it has an aesthetic appeal; on the other hand, today's architects are asking for increasingly complicated structures to be fabricated from tube.
A large part of structural tube processing for architectural applications involves cutting it. The two principal types of metal sawing equipment are circular saws and band saws. Each of these tools offers different advantages. The band saw provides flexibility and the option to cut round tubes in bundles, whereas the circular machine offers speed, as well as superior surface finish and better squareness tolerances (see Figure 1).
In sawing, tubes basically fall into two categories: round, and everything else. The old adage in sawing is, "If we can clamp it, we can cut it." By their very nature, round tubes will spin if the cutting forces exceed the clamping force. Also, on thinner-walled material, clamping forces may have to be reduced to avoid deformation, and the feed rate may have to be backed off accordingly to reduce cutting forces.
A clamping alternative for round tubes is a form jaw made to match the outer diameter of the tube to allow sufficiently high clamping pressure without causing deformation. Unfortunately, each diameter requires a different form jaw.
Bundle cutting—for straight cuts only—is very common on tube. In bundling, several layers of tubes are clamped and sawed together (see Figure 2). In many cases, this can result in increased production, but at the cost of tolerances and tool life.
Bundling is most effective on square and rectangular tube, because they nest well and will not spin. Round tubes also can be bundle-cut, usually on band saws, but greater care must be taken to prevent spinning, which invariably damages the tool and, in severe cases, the machine.
Also, considerable downtime is incurred to change the band, set up the job again, and perform a fresh trim cut.
In the past bundle cutting was a messy job, with coolant flooding everywhere. Now atomized coolant systems, which apply highly refined oil in a compressed-air stream right to the tooth tip, are an environmentally acceptable and commercially viable alternative to flood cooling. These systems can be fitted to almost any machine.
Sawing through a tube creates some interesting challenges. For instance, many more teeth come in contact with the metal at some points of the cut than at others. This phenomenon is more obvious on square and rectangular tubes.
Consider a 4- by 4-in. by ¼-in.-wall tube, for example. At one point in the cutting the saw band is engaged with the material over the full 4 in., but at other points there is only ½ in. of engagement (two ¼-in. wall thicknesses). This is why band saws specifically designed for structural work have an inclined band, usually between 3 and 10 degrees. This incline reduces the maximum width of engagement. The idea is that the left-hand side of the band makes it through the wall before the right-hand side is in full engagement.
In bundle cutting, a very common tube sawing practice, several layers of tubes are clamped and sawed together. In many cases, this can result in increased production, but at the cost of tolerances and tool life.
Exactly how steep this angle needs to be is primarily a design issue with the individual manufacturer, but it is related to the maximum width capacity of the machine and the degree of sophistication of the automatic feed back-off system used in most machines to help accommodate varying engagement widths. These systems control the rate at which the tool advances through the material by detecting increased back pressure when more saw teeth enter the material and adjusting the feed rate.
Circular saws are affected less by width of engagement difficulties because the arc of the circular blade creates much the same effect as an inclined saw band and therefore helps to even out the number of teeth in the cut.
Miter cutting (cutting at angles) is a frequent requirement on tube (see Figure 3). Again, round tube is the most difficult to miter-cut. If miter cuts at opposite ends of round tube are not to be offset axially (that is, if the opposite ends are to be cut at the same angle and in the same plane, so that the finished tube looks like a parallelogram when viewed from the side), then the tube must be clamped at all times during the complete cycle, otherwise the material can roll off-axis. In the rare case when an axial offset is the aim, some kind of jigging always is needed, because the round tube does not automatically give a neat 90-degree offset simply by turning it onto the next face, as is the case on square or rectangular tube.
Miter cutting (cutting at angles) is a frequent requirement on tube.
Of course, because of rolling tolerances, square tube is not always square; parallelogram might be a better description. This has to be taken into account when mitering square and rectangular tubing, because the parallelogram effect turns a miter cut into a compound miter cut. Frameworks cut from such material would not match up correctly. The effects can be minimized by identifying one face of the material and orienting all components of the frame with reference to that face.
To enhance the aesthetic appeal of tubing in architecture, modern designs increasingly call for cambered structures.
As it is almost impossible to cut tube with any accuracy before cambering (which naturally causes considerable deformation at the tube ends), the sawyer faces the unenviable task of cutting it afterward, in the bent condition. Because few shops have roller tables on their saw that are wide enough (or on an arc) to accommodate the camber, these sections often are cut to finished length by other means.
Where a saw can be used, supporting the material requires a certain amount of improvisation, often with a crane holding up the far end. Fortunately, saws are forgiving, flexible machines, and structural fabricators somehow always find a way.
As steel structures become more aesthetically pleasing, and more sophisticated, machinery manufacturers and the fabricators who use their equipment must continue to work together to create the means to make them. One only needs to look at some of the structures recently erected to see that the effort is well worthwhile.