Considerations when sawing round and square tubing
April 24, 2001
All of the components of a tube cutting job—the workpiece, the blade, the saw, and the cutting fluid included—have to work together optimally to maximize your productivity.
If you are sawing tube or pipe in substantial quantities, you most likely have a cutoff saw to do the work. As with all of your machinery, your goal is to get the most out of it, and that takes some special consideration.
Tubes and pipes are hollow workpieces, so each cut consists of three sawing operations—solid, thin-wall, and solid. As the blade starts the cut, it is sawing a solid; then it makes two thin-wall cuts simultaneously as it passes through the midsection; and last, it saws a solid again as the blade passes through the bottom. This operation sometimes is called an interrupted cut.
The workpiece parameters affect not only the best saw design choice for tube and pipe, but also the blade, the cutting fluid, and its application. All of the components must work together to achieve the lowest cost per cut, thereby maximizing the productivity of the sawing operation.
The best saws for cutting hollow workpieces are configured so that the blade cuts the workpiece at a slight angle (see Figure 1). This is especially important when cutting nested square tubing stock, which presents an extended flat surface on which the saw blade starts cutting.
The saw head on a general-purpose or production power saw is mounted to one side of the frame so the saw blade starts cutting at an angle to, rather than parallel with, the surface of the workpiece. Starting at an angle puts less stress on the blade's teeth.
When sawing is started on a wide surface, as occurs when the blade is parallel to the workpiece, the blade's teeth and drive motor are stressed as they pull a heavy chip load through the cut. Starting and finishing the cut at an angle, with the blade canted as little as 3 degrees, reduces the width of the workpiece to a single point rather than a wide surface.
Most general-purpose cutoff saws and production power saws have canted blades as a result of their design. The saw head holding the blade is attached to one side of the frame and pivots down through the cut with a scissorslike action, so the sawing starts at an angle.
A dual-column or guillotine-type saw whose saw head is mounted between two columns may or may not have a canted head, depending on the machine. Tilt-frame saws also can be designed with canted heads.
Other features on cutoff saws that can be helpful for tube cutting are automatic indexing, which moves the stock forward and positions it for the next cut; variable vise pressure control to reduce the chance of crushing thin-walled tubing; and a swivel head for quick setup of angle cuts.
A split front vise secures the stock on either side of the blade so vibration is minimized for more accurate cuts. In addition, the potential for a burr to form at the end of the cut is reduced.
A split front vise (see Figure 2) is an important feature on saws designed for high-volume production sawing. The front vise is split so it grips the stock on both sides of the cut. By holding both the tailstock and the workpiece, the front vise minimizes vibration during sawing for a more accurate cut and reduces the potential for a burr occurring as the cut is finished.
Tubes, pipes, and structural pieces such as I beams and angle iron do not have the mass that solids have, so they are more likely to vibrate while they are being cut. Held in a saw's vise, the workpiece acts as a tuning fork as a segment is being cut. To minimize vibration and the accompanying squeal, special multipitch saw blades have been developed.
Pitch is the word used to describe tooth spacing and is represented numerically as number of teeth per inch. Thus, a 4-pitch blade, called a single-pitch blade, has four teeth per inch. Multipitch blades vary tooth spacing and are designated by the coarsest pitch followed by the finest pitch; for instance, a 4-6-pitch blade has from four to six teeth per inch.
With a multipitch blade, the harmonic vibration frequently caused by a single-pitch blade is minimized, and the squeal that can occur is either decreased or eliminated in many instances.
The tube or pipe wall thickness dictates the minimum number of teeth used in a multipitch blade. If only a few teeth are in contact with the walls after the blade has penetrated the top solid portion, pressure on them can be excessive. The teeth can be stripped from the blade or prematurely dulled. Manufacturers recommend that a minimum of three teeth always be in contact with the workpiece as it passes through the thinner-wall sides of a hollow workpiece. Figure 3shows the minimum pitch suggested for most tube and pipe.
Manufacturers also suggest that a maximum of 25 teeth be in contact with the workpiece at any given point in the sawing cycle. This helps to ensure that each tooth is taking a large enough chip to maintain efficient sawing while not creating excessive drag by pulling too many teeth through the cut or loading the teeth with an excessive number of chips that will overload the gullets.
In addition, the teeth should have a wide set to give the back of the blade plenty of clearance, especially on thinner stock that can bend and pinch the blade.
For sawing solids, most manufacturers suggest flooding the cut with as much fluid as possible. With hollow workpieces, however, this can cause problems. High volumes of fluid can flow inside the cavities and collect there or drain out the back of tubes and pipes, which creates a housekeeping mess. Shops have been known to stuff rags in the ends of the tubes to keep the collected fluid from pouring out onto the shop floor or to attach buckets to the ends of hollow tailstock.
The cleaner alternative to flood coolant is to install a mist lubricator. It sprays a fine mist onto the blade that is sufficient to lubricate the cut and prevent chips from welding in the blade's gullets.
While misting may not cool the blade and workpiece as effectively as a flood coolant system, and blade life can be shortened, the increase in blade costs can be offset by the savings in worker time spent cleaning the area around the saw. In addition, a misting system uses only 2 to 3 ounces of cutting fluid for continuous sawing in an eight-hour shift, so fluid costs can decrease.
Either a vegetable-based or synthetic cutting fluid specifically formulated for misting applications is recommended. Both are easy to use because they do not require mixing and are safe for the operator and environment. Depending on further manufacturing requirements, formulations are available that leave either little residue or a protective coating on the cut piece.
The most obvious way to maximize a saw's utility, particularly when the requirement is for a large quantity of short pieces, is to nest the stock so the saw is cutting many workpieces simultaneously. While fixed costs remain the same, this technique can save money because time is not lost waiting for the saw to recycle. Furthermore, variable costs for blades and cutting fluid do not increase appreciably when measured against the number of pieces cut.
One thing that may cause trouble, however, is that the saw blade's guide arms must be set wider apart to clear the nested stack. The greater width means the blade has less support, and beam strength is reduced. Diminished blade rigidity translates into greater deflection and can result in less accurate cuts. Testing can determine whether nesting your raw materials will give you acceptable accuracy; usually it does.
Figure 4was prepared with a proprietary computer program designed to illustrate the cost impact of nesting. In this case, the only change was the number of hollow round and square pieces being cut. It was assumed that 5-inch pieces of mild steel were being cut with a general-purpose cutoff saw using a variable-pitch, bimetal blade. Feed rate, blade speed, and all other variables were held constant. The chart shows that by nesting the stock, cost per cut can be nearly halved.
The chart also compares round tubing to square tubing. Round tube with a 1.5-inch outside diameter and 0.125-inch wall thickness has 0.54 square inch of metal versus 0.69 square inch for square tube with the same outside dimension and wall thickness. The difference is 28 percent more area to be cut on the square tube, which is reflected in the proportionally greater cost; in other words, it costs 28 percent more to cut the square tube than to cut the round tube.
New metal sawing techniques, blades, and fluids are being continuously developed and improved. Take the time to learn how your application can be made more efficient. Computerized sawing models can simplify your investigation of blade, cutting fluid, and saw operation alternatives and help you find the right combination to maximize the return on your investment.