December 8, 2010
For inline cutoff, tube and pipe producers typically use a mechanically driven, single-blade device. This type of cutoff unit is good for most applications, but in many specialized cases, it might not produce good results. Square or rectangular tubes, heavy walls, and lockseam tubing present cutoff challenges that call for a hydraulic unit or a swing blade.
Tube and pipe mills usually produce standard lengths of tube or pipe (for subsequent recutting), either for in-house fabrication or for fabrication at a customer’s facility. Conventional tube mill cutoff systems typically are single-blade devices, but alternatives abound, including flying high-speed saws, abrasive, roller/rotary, lathe, and dual-blade shears. Determining the best cutoff process is based on the best possible operation for the product, how the operator’s business is set up to process products, and in-house equipment availability. Each method listed has its own distinctive advantages and disadvantages when cutting various wall thicknesses and materials.
The single-blade cutoff die is the most common and basic design; it has been used since the inception of cutoff dies (see Figure 1). The die tooling consists of two pairs of jaws used to clamp the tube or pipe in place (one pair of jaws on each side of the blade). The die jaws are manufactured to conform to the outside surface of the tube or pipe and allow the material to pass through freely because just before the cutting process starts, two vertical-acting cams clamp the jaws in place, securing the material; then a single blade passes between the die jaws in a vertical motion, removing a section of material (a slug).
The main advantage of a single-blade cutoff die is that it can handle any shape, closed (tubular) or open (profile), from light gauge to heavy gauge. In most cases, the cut will be satisfactory (as long as the cutting surfaces are sharp), because the process is uncomplicated. The basic die has two die sections, one incoming and one outgoing, and one blade for simple, similar shapes. Replaceable inserts and blades that fit a common die set make this sort of cutoff system relatively inexpensive. Another advantage of the single-blade cutoff die is that it allows the system designer to combine punching and forming in the same die set. It also makes it easy to coordinate cutoff with pre-punching operations.
One disadvantage is that, when they are cutting round tubing (or square or rectangular, with the blade approaching a flat), the blade leaves a dimple where it contacts the tube (see Figure 2). The amount of deformation depends on the type of material, diameter, wall thickness, ram speed, blade thickness, and blade configuration.
In some cases, preventing damage to the tube or pipe is simple. First, verify that the cutting sections match the tube or pipe (see Figure 3). Second, slow the ram when cutting heavy-wall material. Third, verify that the press has enough tonnage. The tonnage rating of a crank-type press is based on the force it develops near the bottom of the stroke; if cutting a 3-inch-diameter section on a 4-in.- stroke press, the tonnage at the start of the cut is much less than the tonnage at the end. If the material’s yield strength exceeds the press’s tonnage, the press will stall.
Other problems can be more difficult to solve. In some cases, it takes a different approach to cutting to prevent the tube or pipe from dimpling or collapsing.
When a square or rectangle shape is formed with an angle in the 12 o’clock position (corner up), or some other position so that the blade contacts a corner, the part often has sufficient strength to resist dimpling (see Figure 4). However, in some cases the corner of the tube isn’t accessible, making it impossible to use a good cutting angle.
One alternative is a swing-blade cutoff die. This die type typically is used with a square or rectangular part that does not have a corner at the 12 o’clock position (see Figure 5); it’s also used on lockseam tubing. If designed properly, the swing blade will reduce the tendency for the tube to collapse. Also, the swing-blade cutoff changes the cut angle; in the case of lockseam tubing, it cuts up into the wall of the tube. This helps keep the seam from popping, a typical problem when using a conventional single-blade cutoff die, which makes a single vertical cut.
Another cutoff type is a self-contained hydraulic cutoff die (see Figure 6). A hydraulic unit is versatile because it allows any attack angle, so the designer can choose an angle that produces the best cut quality. This provides the benefits of a swing-blade cutoff with one advantage—the slug is ejected downward, so gravity assists in it’s disposal. Also, a hydraulic system delivers the full tonnage throughout the stroke, so the designer doesn’t have to consider the varying tonnage a mechanical system delivers.
When a part does not have an accessible corner for initial blade impact, another strategy is to use a prenotch operation to add a blade access opening (see Figure 7). This opening allows the blade to cut without distortion. The key is to notch the part in an area that will not affect the welding or lockseam operation. The prenotch operation can use a stationary punching die with press and feeder and free loop out, or flying prenotch die in a tight line configuration. Along with a blade access opening, other part features can be added to the prenotch operation such as holes, forms, and nut piercing. The only downside is the potential for a mismatch between the width of the prenotched opening and width of the cutoff blade. If the difference is large, you will likely notice a step between the area that was notched and the area that was cut.
Although cutoff dies are available in many variations, one thing most have in common is their function: cutting a tubular shape to length. In most cases, this is all that is required. Sometimes, however, suppliers are looking to take the next step and add value to their products; adding subsequent operations can go a long way (see lead photo).
Determining the right die for the operation is based on the following criteria:
Only experience can tell you what can or cannot be done and what types of part cutoff operations are achievable. Much depends on the part’s contour, required end condition, forming operation (welded, lockseam, overlap), and value-added operations such as adding notches, holes, and forms.
The maximum benefits and profits are achieved when dies are used not only for cutoff, but also to punch or form the part on the mill, eliminating all secondary operations.
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