Reducing scrap, tooling damage with coil end welding
May 30, 2002
Exploring the benefits of welding coils in roll forming and progressive stamping operations. Welding coils can reduce scrap, downtime, and tooling damage. A variety of coil end joiners are described, such as portable, stationary, single-and double-cut, fully automatic, and semiautomatic.
Welding coil ends is a common practice in the tube producing, strip processing, roll forming, and stamping industries. Coil end joiners are used most often in tube mills because they run high volumes of coil at fast speeds.
When a coil end joiner is used in conjunction with a strip accumulator on a tube mill, the mill runs continuously with no downtime because of seamless coil changeovers.
Accumulating the strip from the uncoiler at two to three times faster than the press, roll forming line, or tube mill is running allows the line to run continuously, even while the accumulator is filling. Once the accumulator is full and the coil is depleted, the operator loads a new coil and makes an end weld, while the line continues to run.
Coil end welding is gaining acceptance for roll forming and progressive stamping press lines because of these lines' higher run speeds. The welds are of high enough quality to pass easily through the tooling, contributing to faster coil changes, reduced tooling damage, and reduced scrap from the tail end of coils.
The overview shown in Figure 1indicates the increase in uptime resulting from welding coils instead of rethreading them.
Coil welding is suitable for progressive die presses because these presses take longer to rethread than blanking presses and have a greater chance of mis-hitting as the new coil is fed. Also, a progressive die is longer than a blanking die, so the amount of scrap produced is greater. Welding coils together can reduce coil end scrap by 8 to 15 feet.
By welding coils together, those 8 to 15 ft. can be stamped into additional parts. The savings can be measured by subtracting the scrap value from the value of the parts.
Repairing or replacing a damaged tool is a common cause of press downtime. Threading the lead end of a new coil into a press can damage the dies. Welding coils together can eliminate approximately 90 percent of this die damage and the related downtime. A gas tungsten arc weld (GTAW) easily passes through a die.
The welded part usually is identified and thrown out by the operator. Some manufacturers identify the stage of the process by the number of hits and shut down the press as the welded part moves into the last die station. The operator can remove the part or it can be discarded automatically.
Welding PreparationProper shearing, clamping, and torch travel are needed to make a smooth, strong weld quickly.
Shearing. Both coil ends must be sheared for a burr-free, distortion-free cut. The shear blade's cutting angle is also important. With a large shear rake angle, the blade approaches the strip edge first instead of coming down on top of the strip.
Forces applied on an angle at the edge of the strip result in a pushing action. Large cutting angles can induce camber in the coil ends after welding.
If the cutting angle is more than a few degrees, the strip must be clamped close to or directly on the lower shear blade to prevent movement. A large cutting angle also creates breakthrough burr at the edge.
This edge burr creates width tolerance problems, which can cause a misfeed as the strip enters the die. The best cutting angle typically is 1 to 2 degrees, with strip clamping pads used directly on the lower cutting blade.
Clamping. Coil ends are butted together manually or automatically, depending on the welding machine style. The clamps, which hold the material during welding, are made of copper and act as a heat sink for GTAW. Therefore, it is important to have equal clamping pressure throughout the width of the weld.
Many clamp styles are available. They should hold the material securely during welding to prevent movement of the strip caused by heat or the weight of the strip in the loop area.
Torch Travel. To get a smooth weld close to the edges, the torch should be driven automatically by an AC or DC motor with a ball screw or threadless nut design. It is important that the torch is held steady and moved across the seam accurately. Therefore, the fixture and drive mechanism must have a tight tolerance, with no slop or movement.
Many stampers use smaller, portable coil end joiners with a threadless rod drive system. The drive shaft has a nut with bearings that are spring-loaded to take up any slop, so the torch moves accurately and stops and starts precisely.
A triple-guided system can be used on machines larger than 20 in. wide. Because the torch assembly and travel distance are larger, two ball screw bushings and guide shafts hold the torch steadily and accurately, with one drive screw moving the torch across the seam.
A variety of coil end joiners are available, such as portable, stationary, single- and double-cut, fully automatic, and semiautomatic. Many types of welding methods can be used. However, GTAW is the most common because it's generally inexpensive and leaves a smooth weld to pass through tooling.
Portable Coil End Joiners. Portable units are used primarily on light-gauge, narrow strip. After shearing, the strips are moved manually to the clamping and welding station, where they are welded with an automatic torch.
A three-blade shear produces a distortion-free cut. All of the distortion from shearing goes into a center slug, leaving the coil ends flat and distortion-free. A wedge-style clamp holds securely and uniformly for consistent heat sink throughout the weld area and eliminates hard spots.
Stationary Single-cut Coil End Joiners. This style is designed to butt up against the exit end of a straightener. The tail end of the strip comes off the uncoiler and is positioned by the stamping press or an optional exit pinch roll.
After the strip is positioned, an operator uses a two-hand control to activate the shear. Clamp pads hold the strip while it is crop-sheared. The shear has a 1.7-degree cutting angle, which produces a camber-free cut and a weld joint that passes easily through tooling. The tail-end scrap is removed manually or drops out of the bottom of the machine automatically.
Automatic scrap discharge typically is limited to 1 to 3 ft., depending on the coil joiner's size. The lead end of the new strip is jogged into the welding machine by the straightener. The shear crops off the lead-end scrap. Both coil ends remain stationary, which helps to ensure a tight fit-up.
The straightener and exit pinch roll hold the coil ends in place while the shear assembly moves out and the weld clamp assembly takes its place. The coil ends are welded together, and the torch returns to home position.
A stationary single-cut unit works best with wider, heavier coils because more material must be sheared off of these coils, which can be time-consuming on a double-cut model. Several feet of scrap can be sheared off the lead end of the new coil with one cut and automatically discarded.
Stationary Double-cut Coil End Joiners. The most common stationary double-cut unit has an L-style, or scissor, shear. With this machine, two coil ends are brought into the shear's 3- to 4-in. opening manually for light gauges or by the process line or straightener for heavier gauges. Some units have openings as large as 12 in.; however, the farther the coil is indexed, the more chance there is for slippage and improper fit-up.
Larger openings also require several more inches of slack strip on both sides of the coil end joiner, which is difficult to obtain if it is close to a straightener or the entry of a roll form line.
Once activated, the shear cuts off about 2 in. from both ends and discards the scrap out the bottom of the machine. After the coil ends are indexed, the weld backup bar secures the strips against the upper clamp pads for welding. The weld torch travels across the seam, welding the coils together.
Because the coil ends are indexed together, this unit requires slack strip on the entry and exit ends. Therefore, a few feet of space may be needed on both ends between the welding machine and the next piece of equipment, such as a straightener. This process can be fast because both coil ends are cut at the same time and the scrap is removed automatically.
However, on heavier gauges, more than 2 in. may need to be cut off the tail end, so two cuts may be required. This makes the cycle time the same or longer than a single-cut joiner.
Also, the unit's large cutting angle may induce camber in the coil ends. This coil joiner style works well in tube mills and roll form lines when tight die tolerances and camber are not big issues. Upper and lower tube mill rolls can take a buildup of as much as 30 percent, and a small amount of camber won't cause a misfeed.
The reductions in downtime, scrap, and tooling damage make coil end welding justifiable for progressive die stamping presses. Coil end welding is becoming more popular for stampers as a method of increasing uptime, which improves the bottom line.
Mark Costello is a vice president with Kent Corporation/Tesgo Inc., 9601 York Alpha Drive, North Royalton, OH 44133, phone 440-582-3400, fax 440-237-5368, e-mail email@example.com, Web site www.continuouscoil.com. Kent/Tesgo is a manufacturer of coil processing equipment.