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Precision sawing: Mind the miter

In precision miter cutting of tube, beams, and other material, the costs of inaccuracy can mount quickly

If you are a fabricator, or a supplier to a fabricator, you are unquestionably in the business of sawing steel, including steel tubes and pipes. Further, a sizable percentage, if not the majority, of that sawing requires miter cutting, be it on a cold saw or band saw.

This in and of itself should not create any particular problems, but surprisingly, saw manufacturers often hear about the additional challenges fabricators face when trying to saw miters accurately, consistently, and efficiently. Incorrect miter angles, wrong cut lengths, and cutting out of square are just a sampling of the errors that occur. Put two of these errors on top of each other and what was supposed to be a simple welded frame is now completely out of alignment. Many of these same issues can occur with straight cutting as well; however, they are exacerbated when dealing with intricate miter cutting and the tighter tolerances that miter cuts often require.

Fortunately, because of the continued technology developments in both saws and saw controls, the need for detailed calculations, tape measures, marking, and manual alignment of the material under the blade is eliminated. Today, with virtually the touch of a button, a quality miter saw can produce finished parts that are cut efficiently and within extremely tight tolerances. Depending on the cut quality required, accurate miter cutting can eliminate secondary operations that address finish and squareness.

So what has enabled such gains in efficiency and accuracy? In a word, automation.

Accuracy and Efficiency

To put the importance of miter cutting accuracy in perspective, consider a 6-inch square tube with a 45-degree angle at both ends. If the miter angle is off by just 1 degree—that is, the miter cut is 46 degrees—this part will be too long by 0.21 in. With the same angle error at the opposing end of the square tube, the cut now will be long by 0.42 in. For the majority of fabricators, this variance is far too great. It is generally outside of a customer’s specifications, as well as a waste of material. And because we’re working with tube, it can be extremely costly. If the angle is off by 2 degrees on both ends, the error will generate a part that is too long by nearly 1 in. Adding a length error on top of this further compounds the problem.

In most cases, a short part is worse than a long part. A short part cannot be recut to accommodate the correct length, and an expensive piece of material will need to be scrapped.

If the fabricator is building kits for a framework, these errors can throw off a system design dramatically. Say a fabricator is working with structural tube. A single assembly can involve dozens of precision miter cuts. It doesn’t take much of an error in one miter cut to force other miter cuts in the tubular assembly out of tolerance. For this reason, the saw’s feeding gripper must position the material at the appropriate length, and the cutting head must move to the proper miter angle.

Sawing usually occurs early in manufacturing. If miter cutting errors aren’t caught, parts will continue to flow downstream, where rework and especially scrapped parts become even costlier. This is particularly true when an operation involves advanced processes, like automated laser welding. Laser welded components must be held in perfect contact along the weld area, and the joint to be welded should have no gap (see Figure 1).

Positioning Systems

To ensure accurate length, some saws use material positioning systems (feeding grippers) that allow for accuracy and repeatability within ±0.005 in. In addition, the ability to control head positioning effectively generates miter accuracy as tight as 1⁄10 of 1 degree, ensuring that both miter angle and length tolerance are extremely accurate. Finally, more sophisticated control systems, including full PC control options, offer speed, efficiency, and flexibility.

It is imperative that the saw knows the actual width of the material being cut, and not simply utilize the nominal mill width. Although the material may be well within mill tolerance, any variance in actual width will affect the cut part length.

Figure 1
Precision miter cutting, as shown in these images, requires extreme accuracy. When the individual components (on the left) are mated, no gap between the workpieces remains. Photo courtesy of Behringer Saws Inc.

To demonstrate this, let’s use square tubing with a nominal mill width of 4 in. but with an actual width of 4.025 in., which is within structural tubing tolerances. In this case, we are cutting a trapezoid frame with structural tubes having 45-degree angles. If we program in the nominal width of 4 in., the cut will generate a long length error of 0.05 in. (2 × 0.025). This is not in any way an issue with the saw as it is simply calculating feed length based on the programmed width (see Figures 2 and 3).

PC Controls, Automation Efficiency

PC control of miter sawing allows for dramatic efficiency and overall performance gains (see Figure 4). Such controls automatically calculate the length positioning of the material based on the selected miter degree and the type of angle, such as a parallelogram, double miter, or arrowhead. The control incorporates the standard programming for material optimization calculation (nesting) to ensure the best utilization of raw materials.

Such controls also allow for remote programming, fast communications and diagnostics, a variety of interface options, greater memory and information processing, and data tracking. And they offer networking capabilities to enterprise resource planning and warehouse management systems.

Integrated with a sawing system with an automatic loading magazine, the control allows multiple jobs with different materials and lengths to be cut and sorted and processes them utilizing the most efficient material optimization possible. It allows the saw to know exactly what material (or job) is located in each of the magazine compartments.

All of these miter sawing capabilities are applicable to both band saws and circular cold saws. Which to use for a mitering application depends on the material type, sawing capacity requirements, volumes, and desired finish.

Never Overlook the Saw

Accurate calculations (whether automated or manual), material positioning, and miter head alignment are the key criteria of precise miter sawing, but certainly not the only criteria. All of the bells-and-whistles features in the world will offer little return if they are not built on top of a solid, rigid sawing platform.

The blade must have proper tensioning and be precisely guided close to the material via a solid guide arm and carbide clamping. A control that senses downfeed pressure, adjusts it accordingly, and monitors blade deflection will help ensure quality miter cutting, as will good coolant application and chip removal.

The Real Costs of Inaccurate Miter Cutting

Manually setting up complicated miter cuts, like double bevels or arrowheads on a rectangular tube, requires significant time. All that time, the saw is not making good parts. This slows throughput for the entire shop and, at worst, starves expensive downstream operations, like laser welding, of the parts they need. Not only is the saw sitting idle longer than it should, so is the laser welding cell.

Consider again the trapezoidal assembly using miter-cut square tubes with a nominal diameter of 4 in.—only the diameter is really 4.025 in., which means the programmed miter cut ends up being too long by 0.05 in. How much does this error really cost? This depends in part on when and where the error is caught. Discovering the tubular assembly doesn’t fit in a welding fixture may cost more than catching the error immediately after sawing. The fabricator then has the cost of rework or creating scrap.

But what if the welder “makes it work,” makes some adjustments and welds the tubular assembly? That “slightly off” assembly could make it into the customer’s hands, which opens up the potential of the tubular assembly failing in service. Scrapping a tubular workpiece in the shop costs money, but shipping bad parts and damaging the fab shop’s reputation can be far costlier in the long run.

Figure 2
A square tube like this may be within the supplier’s dimensional tolerance range. But if the saw does not know the workpiece’s actual dimensions, it may cut a miter that could throw even a simple welded assembly out of tolerance. Photo courtesy of Getty Images.

This again is where automation and control at the saw fill a need. No longer must fabricators rely on the operator’s ability to calculate, measure, align, and position material in the appropriate configuration or order. Today the operator can select a job that may have been designed and loaded from an office remotely, put the proper material in the gripper, and push start. The saw will find the leading edge, generate a trim cut if required, and then process the jobs.

Regardless of how they are measured, improved accuracy and repeatability in miter sawing will certainly generate substantial cost savings and quality parts.