The Skillful Art of Coil Joining

BACK TO BASICS

A review of the basics helps us understand the newest technology

Through the years, the art and science of coil joining has greatly changed. As technology has advanced and the use of programmable logic controllers (PLCs) has increased, the degree of automation and the repetitiveness of weld parameters have brought about coil joiners that are truly automated and require no operators.
Before discussing the truly automated units, this article will first review the various methods that are used today for joining coil ends for various strip processes, such as stamping lines, roll formers, tube and pipe mills, and continuous process lines.

TIG Welding

Tungsten inert gas (TIG) welding is a process in which the two coil ends are sheared square and generally butted tightly together. A torch with a tungsten electrode is then traversed across the material.
An arc is ignited between the electrode and the material. The material melts together and then solidifies, creating a “cast” weld. This process is generally used for the following:

Material ....…….Most weldable alloys, copper, brass, aluminum, steel, stainless steel
Thickness …...,.. .006 inch to .200 inch
Welding speed .. .90 inches per minute (IPM) to 6 IPM

TIG welds are usually very close to the same strength as parent metal, and since no filler wire is used, the weld is approximately the same thickens as parent metal.

TIG Applications

Typical applications for TIG welding coil ends include stamping presses, roll form lines, traverse winding lines, small tube mills, coil buildup lines, and rolling mills for the nonferrous industry.
Since the weld is usually very close to parent metal thickness, it can generally pass through die and forming rolls without damage to these items. Also, quite often, the weld can be sent through a reducing mill without failure.

MIG Welding

In the metal inert gas (MIG) welding process, the two coil ends are generally aligned with a slight gap between them. Instead of a tungsten electrode, the torch uses a wire which is fed from a spool.
The wire is actually melted and fills into the gap. It also fuses with the parent metal of both coil ends.
MIG welding is generally used with the following application:

Material …......……. Hot-rolled steel, stainless steel
Thickness …...,……. .030 inch to .750 inch
Welding speed ....... 70 IPM to 15 IPM

Depending on weld over-thickness and the material being joined, MIG welds can be 80 to 95 percent of parent metal strength. Compared to TIG, MIG is much faster, and strip fitup is generally not quite as critical.
Also, since a filler wire is added to the joint, there is usually a buildup in the weld area. This buildup can range anywhere from 5 to 90 percent of parent metal thickness.
MIG welding applications are generally found in large tube and pipe mills; annealing, pickling, and coil buildup lines for the stainless steel industry; and certain continuous process lines, such as heavy-gauge galvanizing.
If a very simple, fast weld used for transport only is required, an overlap MIG weld can be made. The weld would have to be cut out at the exit end of the line.

Plasma Welding

Plasma welding is similar to TIG in that wire is usually not inserted into the weld joint, and the two coil ends are tightly butted together.
The arc is actually passed though a plasma, gas heated to a high temperature. This produces a small cylinder-type arc instead of the come-type arc created by TIG. This results in better penetration, aster weld speeds, and narrower heat-affected zones (HAZs).
The plasma process can be used on the following:

Material …........ Most weldable alloys
Thickness …...... .015 inch to .280 inch
Welding speed ... 30 IPM to 8 IPM

Most plasma welding systems are found in stainless steel strip processing lines and on some nonferrous applications. Because no wire is generally used and the strength of the weld can be quite high, this process can be used for coil buildup, even if re-rolling (reducing) will be required.

Laser Welding

The process of laser welding can uses the same coil end locating procedures as the TIG welding process, with more attention to precise fit of the butted coil ends.
A laser welding head containing a focusing lens and a shielding gas nozzle is rapidly traversed across the coil ends. Infrared energy, the same wavelength, produced by the sun, is supplied from a welding head, where it is focused to a .006-inch to 0.12-inch spot.
The resulting narrow weld will be the closest to parent metal strength of any of the welding processes. The small HAZ and ideal weld bead geometry from the laser weld gives the best forming results of any of the joining processes. As with TIG welding, no filler wire is required.
Current coil joining equipment uses CO, lasers in the 1- to 6-kilowatt power range. It can be used on the following:

Material ....…….....Most ferrous metals
Thickness ……….006 inch to .300 inch
Welding speed …240 IPM to 40 IPM

Compact Seam Welding

This is a resistance welding process where the coil ends are very slightly overlapped after shearing. The amount of overlap is about the same as the strip thickness.
Once lapped, copper welding wheels pass above and below the joint, and electric current passes through, joining the overlapped material using the resistance weld theory.
This process produces high-quality welds at a very high rate of speed. Figure 1 shows a typical compact seam weld.

Figure 1 - This is a typical compact weld.

Figure 2 - The typical lap resistance weld has double thickness at the seam.

General strip parameters are below:
Material ................Cold-rolled steel, stainless steel, hot-rolled pickled and oiled (HRPO)
Thickness …….. .008 inch to .135 inch
Welding speed …400 IPM to 40 IPM

In almost all cases, these units are self-contained and require only one electrical and one pneumatic connection to be fully operation.
These devices generally consist of a TIG weld power supply, pneumatic shear, manual weld clamps, a simple weld carriage which supports the TIG torch, and a variable-speed DC motor used to traverse the torch across the strip.
In addition, fixed rear edge guides are usually provided to align the back side of the strip. Since these machines are generally portable, they should be designed so they can operate with either a left-right or right-left strip flow
without increasing the operator’s effort. Years ago, a gauge bar was generally used to align the strip in the weld clamps so the joint would properly align with the weld carriage (torch) for proper tracking. However, newer machines have been
developed which eliminate this loose piece. The operation of these units requires the operator to manually position the tail and lead end into the shear area and then move these ends to the weld clamp position.
Because this is manual operation, the strip size is usually limited to material approximately .125 inch by 10 inches wide. This is done not because of machine capability, but because of the effort required by the operator.
As the material size increases, the distance from the coil joiner to the mill and to the uncoiler must also increase to give the operator a chance to position, move, and align the strip.

The larger, faster, higher-producing process lines, with entry and exit strip accumulators, require more automation in their coil joining system so the weld time can be reduced, and the consistency of the weld remains the same with little
or no operator involvement. These fully-automatic machines can be used on any size strip but are usually found on large anneal and pickle lines, large tube and pipe mills, or occasionally on coil buildup lines, where the strip can be as thick as .800
inch and as wide as 100 inches. Depending on the actual strip process, either the TIG, MIG, plasma, or even laser is used on these units.
Because of the degree of automation required on these units, the older single-cut-type machine is seldom used for these coil joining applications. Today, these machines are almost always a double-cut type, using a roller shear to cut
both strip ends simultaneously. In operation, generally, the tail end leaves the uncoiler, is cropped at the cropping shear, and then is indexed to the shear welder.
During the tail-out process, the exit side guides of the welder are automatically closed on the tail end, assuring the strip is centered. In the ideal situation, a hump roll is also provided to allow for easier tail end centering.
After the tail end is centered, the exit weld clamp closes, containing the tail end strip.
While all of this is happening to the tail end, the new coil is out on the uncoiler, cropped, and automatically indexed to the shear welder. During this indexing process, the entry side guides of the shear welder are automatically closed on the new lead end.
After the lead end is centered and stops in the shear welder, the entry weld clamp closes. As soon as the entry clamp closes, the double-cut shear cuts both strip ends at the same time.
Since most of these units have a high degree of automation, and since the sheared scrap piece is very heavy, the ideal situation is to have automated scrap discharge. With this, the operator never touches the strip.
After the shear returns, the clamps containing the strip index toward each other (either one or both clamps index), the backup bar is positioned, and the torch is ready to make the weld.
After the weld is complete, the machine is reset, and the entry end of the process line is ready to run. All of this occurs without the operator at the weld control station.
Because time is very important on these lines, two torches are sometime used simultaneously to cut the welding time in half. A complete machine’s cycle, including the welding, can range from two to four minutes on this type of machine.
Several important items must be mentioned about these fully-automated units. They include torch tracking, quality of joint, condition of sheared edge, and repeatability.
Since the operator may not need to be at the coil joiner during operation, the machine must assure that the gap (MIG) and butt (TIG, plasma) of the joints is exactly what is required for the weld process.
Also, the shear welder design must guarantee that the torch or torches will always travel along the seam without the operator constantly making adjustments.
In addition to these items, the shear welder must also have a uniform sheared edge across the strip so that the proper weld arc is maintained. The sequencing of a typical fully-automatic shear welder is shown in Figure 3.

Resistance welding is generally used on process lines where high-strength, very fast welds are required. The simplest form of this type of welding is the wide overlap type.
These machines generally include entry and exit weld clamps, an easily-replaceable weld wheel and backup bar, and an off-line weld wheel reconditioning unit. Auxiliary items include side guides, a shear, and a strip positioning device.
The usual operation of these units (see Figure 4) is simple. The tail end strip is stopped somewhere below both weld clamps, and the exit clamp is closed. This clamp contains the tail end piece and also acts as a gauge bar for the new coil end.

Then, the new end is fed into the unit, adjusts the exit clamp, and the entry clamp is closed. The weld wheel traverses across the overlapped strip, and the weld is made.
New technology in this field has allowed the welding wheel to be easily removed. Thus, the reconditioning of the wheel – actually machining the wheel on the outside diameter (OFD) – can be done off-line instead of at the computer.
By doing this, the copper chips and dirt are associated on the machine or the strip. This improves weld quality and strip condition. Since the reconditioning process is simple, the wheel tends to stay in good condition, and better welds are achieved without additional effort.
The highest degree of technology in resistance welding is the compact scam process. With this process, the weld produced is high quality and can actually pass through a temper mill.
Because the welds are generally made on light-gauge material (.010 inch to .125 inch), all aspects of the weld seam are critical. This is especially true with the tail and lead end strip positioning and the actual welding operation.
As with any strip welding, the most critical areas are usually the front and rear edge of the strip where the weld starts and stops.
In an effort to reduce the tension on either the start of stop areas, many advances have been made in the entry and exit guides of the strip when using this process.
The latest design actually calls for a pair of edge guides, a hump roll, and a pinch roll on each side of the seam welder. With this equipment plus internal adjustments within the weld clamps, the lead and tail end centerline are parallel within less than one degree of each other.
Also, the strip edge can be within ±.025 inch of each other. The operator does not have to touch one push button.
Although the strip alignment and strop overlap is important, a successful weld will not be made unless the weld wheels operate properly. To assure that this happens, the weld wheels must be easily accessible and easily replaceable.
The newest machine in use today has weld wheels that can be replaced in less than one minute. This helps assure that the weld wheels are always in optimum condition. Also, the weld wheel force, which is very critical, must be easily and precisely controlled to assure constant resistance across the seam.
The sequencing of a typical compact seam weld is shown in Figure 5.

 

Flash butt welders are the most ex-pensive and exotic of the coil joiners used in process lines today. They also produce the fastest, highest-quality welds for the strip processing industry.
Because of the high-quality of weld joint required, there is continued upgrading of the flash butt process. The current thought is that a V-type unit offers more control and precision than the older-type machines (see Figure 6).

 

Several of the more significant advantages of the V-type unit are as follows:
1. Since the platen pivots instead of moving linearly, less friction is associated with the movement.
2. Since the pivot is below the strip and the picot actuator is above the strip, there is a 2:1 revolution from the weld, because the cylinder stroke is actually twice that of the flash stroke.
3. Since the pivot bearing is eccentrically mounted, when welding difficult strip thickness together, the strip center-line (in terms of thickness of the strip) is aligned rather than the bottom edge of each strip. This is beneficial if the weld will be passed through a rolling mill.
Incorporated into almost all new flash butt welders are entry and exit shears used for the last precise cut prior

o welding. Also, flash trimmers are present to cut the excess flash on top and bottom of the joint after the welding operation is complete.
Virtually every coil processing operation can use a coil joiner. Whether or not any of the previously-described methods can be used profitably on a specific application will take a certain amount of research. However, as a product quality becomes more important, and as the need to reduce or eliminate scrap becomes more critical, the use of coil joining can be more easily justified.
Many examples can show this. Suppose a tube mill is operating at 100 feet per minute (FPM) and is running material .250inch thick. If the coils used are of relatively common size – 24 inches inside diameter (ID) and 60 inches OD – then the total coil is about 800 feet long. With this being the case, the coil will pay off approximately every eight minutes.
It is not uncommon for a rethread on a tube mill this size to take 15 minutes. However, if a coil joiner is used, the coil ends can be joined in less that five minutes.
Running without a coil joiner requires 23 minutes (8 + 15) per coil, whereas running with a coil joiner requires 13 minutes (8 + 5) per coil. Using the coil joiners can increase productivity by nearly seventy percent.
In addition to the above process, there are other methods for coil joining, including overlap TIG and MIG, spot welding, tapping and stitching. The actual method required depends upon many variables, including type of line, line speed, quality of weld required, type of material to be joined, and quality of weld required.

The information presented in this article was prepared by Lee Kothera, Vice President/Sales, Guild International Inc., Bedford, Ohio. Reprinted from the December 1991 issue of the Fabricator with their permission.



Innovation and Progress for the Strip Processing Industry

Guild International, inc. is the world leader in design and manufacture of coil strip joining and accumulating machinery. This equipment has increased productivity and yields on installations throughout the world on virtually all types of processing lines and materials. Typical Coil Joining Applications:

• Annealing Lines
• Coil Build Up Lines
• Galvanizing Lines
• Grinding Lines
• Paint Lines
• Pickling Lines
• Roll Forming Lines
• Shot blast Lines
• Spiral Pipe Mills
• Stamping Press Lines
• Tension Level Lines
• Traverse Winding Lines
• Tube & Pipe Mills