Typical weld samples after forming

Fig. 1--Typical TIG shear and end welder. This is a stationary machine and remains in-line at all times with the strip passing through it continuously.

Accumulate Uptime by Joining Coils

By Len Steinmeyer, Kent Corporation, Cleveland, OH

The average coil fed production line typically loses from 20 to 30 percent of its productive day because of coil changes and rethreading. Elimination of downtime between coils by using a combination of end-joining methods and strip accumlators results in increased productivity and other dividends.

Running at higher speeds generally brings with it process modifications. It means changing the existing drive system and cut-off capabilities while updating coil handling at the entry end. The need to address exit handling problems and the possibility of requiring increased labor also must be taken into account. Running at faster processing speeds means that more coils are processed and more coils mean more downtime between coils.

Eliminating downtime between coils will provide the same result as increasing line speed, if that were truly a viable option. This can be done in two steps. First, join the end of the expended coil to the end of the new coil. This eliminates rethreading the line and saves time as well as scrap. A strip accumulator then can be installed to store enough material to allow the processing line to continue running while an end weld is made.

Joining Coil Ends Together

On a typical processing line the uncoiler is the only entry equipment employed. When the end of the coil is reached, the line has to be stopped while another coil is put in place and the line is rethreaded.

How often is this done and how long does it take? Typical running speeds for press lines, roll formers and tube mills are as follows:

Average downtime between coils for each event by industry:

If your company's performance reflects any of these statistics, you are losing valuable production time rethreading the line after each coil. Closer examination may prove that you are often down longer because stopping the line can cause other problems.

The number of coils processed per day varies significantly between industries. Users of coil stock can expect to average:

Production averages based on 60 in. OD x 20 in. ID material shows that heavier gauges result in more coils being processed during a day. For instance, 0.049 in. thick material may run for 40 min. at 100 fpm; 0.095 in. thick coil may only run for 19 min. at 100 fpm. So, depending upon the gauge, a typical company changes coils between 12 and 25 times per day on a one-shift basis.

By computing typical profit per foot of product, it is evident that production and profits increase by eliminating downtime between coils. To illustrate, multiply coils/day x lost minutes/coil change x 250 days/year x speed (fpm) of the line/by profit per foot). Example: 5 coils/day x 10 min. x 250 days/year x 40 fpm x 0.05 profit = 500,000 x 0.05 = $25,000. The larger the number of coils processed, the more is saved.

Common Coil End Welding Methods

Several methods of welding coil ends exist to help manufacturers speed production. The most common include:

Oxy acetylene is simply holding the two ends of the strip together and heating them with a torch until they reach a semi-molten state and fuse. This method is common and has the advantage of being inexpensive and easy to accomplish. Its major disadvantage is it is not a reliable weld and it presents a thicker weld than parent material. This, of course, is not good for tooling.

Spot welding requires that the two strips are overlapped. An electrode is then put above and below the double thickness as current is passed through the electrode and the two strips in several spots, generally 1/4 in. to 3/8 in. diameter. The two strips then are fused together at each spot. This method also is inexpensive, but has the disadvantage of overlapping material, which is not advisable for most tooling applications. Moreover, it is not a 100 percent weld and often breaks in forming.

Mash seam welding is overlapping the material about 1/16 in. to 1/8 in. and then mashing the overlap with a half-moon-type copper electrode as current passes through, fusing the two together. Its advantage is quickness and a good weld. Its disadvantage is metal build up, typically 10 to 15 percent of metal thickness. This also is not good for tooling.

Flash butt welding is an excellent method of joining narrow width coils. Each end of the strip is set into a copper clamp. When current passes through the clamps and the strips are pressed together the weld is completed. However, flash build-up, a by-product of the process, must be ground down. Advantages include cost-effectiveness in narrow widths and weld integrity. The disadvantage is the grinding required, which often produces a section that is not the same thickness as the parent metal.

G.T.A.W. welding (TIG) is when the two strips are butted together and a tungsten electrode is moved along the seam. An electric arc passes from the electrode to the strip, melting the material. The arc is protected by an inert gas and the two edges are melted and solidify instantly forming a cast weld. This is one of the best methods for welding coils together.

Mash seam and planish is probably the ultimate in butt-welding. The strips are overlapped, then mash welded with a rotary electrode that applies weld power and force. The seam then is planished to parent metal thickness. This is the best of the methods discussed. However, it is also the most expensive.

Fig. 2--Portable TIG shear and end welder. This welder can move from one line to another. It is easily pushed and only weighs about 1000 lbs.

TIG Advantage

Kent Corp., North Royalton, OH offers both portable and stationary coil end welding devices using the latest TIG technology, Figs. 1 and 2. TIG welds are advantageous because they do not add any metal to complicate downstream operations. Therefore, the chances of tooling damage as a result of metal buildup are negligible. Also, using the automatic weld cycle gives reliability and repeatability without hardness buildup as long as carbon content is in the standard low-carbon steel range.

In practice, after the two strips are butted together, a tungsten electrode is moved along the seam. An electric arc passes from the electrode to the strip, melting the material. The arc is protected by an inert gas and the two edges are melted and solidify instantly, forming a cast weld.

The quality and integrity of the TIG welding process can be expected to be high because the process is user friendly and does not require a great degree of operator expertise. Coil end joiners can be equipped with manual, semi-automatic or fully automatic features to weld a variety of metals including aluminum, mild steel, prepainted, galvanized stainless, aluminum, brass, copper and other ferrous and nonferrous materials.

Overall cycle time is between one and three minutes for most metals and usually averages approximately two minutes.

The portable coil end welding machine is completely self contained, meaning installation costs are low. Its versatility is enhanced because it can be used on more than one press line, on various materials and can be used for stick electrode welding around the shop.

Strip Accumulators

Joining coil ends is only half of the solution to minimizing downtime between coils. There also has to be a method of accumulating stock to allow time to perform the weld without halting production. For all practical purposes, the state-of-the-art up until 1973 was to provide two minutes of storage time to make an end weld. This meant using fast, completely automatic, and expensive end welders.

There are several frequently used methods of accumulating strip available today:

Fig. 3--Strip accumulation in a deep pit.

The pit-type accumulator, Fig. 3, is filled so that the strip loop is at position A. As the processing line removes strip, the loop will stay at position A, and the accumulator will remain full. When the supply end is stopped (because the supply coil has been depleted) and the processing line continues its strip removal, the loop will rise to position B as a new supply coil is end-joined to the trailing end of the depleted coil.

This joining can readily be accomplished because the entering strip is not in motion. As soon as the end joining has been completed, the supply to the accumulator can be restarted and strip can be entered at a higher speed than the strip being removed by the processing line. Under this condition, the loop will return to position A as the accumulator fills.

The length of strip in the pit between position A and B represents the capacity of this accumulator and is equal to or greater than the length of the strip removed during the time that is required to perform the end joining operation.

For example, if the processing line runs at 100 ft./min. and it requires 4 min. to complete the end joining operation, 400 ft. of strip will be consumed by the line while end joining occurs. The accumulator must therefore have a capacity of at least 400 ft.

Fig. 4--Vertical "festoon" accumulator.

The vertical tower or festoon, Fig. 4, most often is used in industries where continuous processing lines are employed. This includes galvanizing lines, paint lines, annealing lines, etc. This solution is limited by ceiling height, and it also is expensive to configure.

In practice, it is a variation of the pit in that several strands are combined and extended downward and upward. A festoon uses a number of fixed rolls attached to a vertically moving structure.

Strip accumulation occurs as the moving rolls move away from the fixed rolls in a vertical direction. A large accumulation can be obtained from a much smaller total vertical movement than the pit type. These accumulators are capable of storing more stock than the pit-type accumulators but not as much as the rotary-free loop accumulator or rotary horizontal accumulators.

Fig. 5--Overhead horizontal loop.

The overhead-loop method, Fig. 5, was widely used until 1973. It used a movable, overhead carriage that pulls two or four strands of strip along a track in the ceiling. The track, generally 100 to 150 ft. long, stores 300 to 600 ft.

Fig. 6--Spiral accumulator.

With a spiral accumulator, Fig. 6, the strip is formed in a loop at position B and the incoming strip is entered at a higher speed than the processing line. Loop B progresses in a guided spiral path (using roller guides) to position A to fill the accumulator.

When the entering strip stops to end-join a new coil, the accumulator is depleted and the loop moves from A towards B. This accumulator has found limited application on light and medium gauges (0.118 and less) narrow strip (2 in. to 12 in.) with a low-yield strength.

Fig. 7--Horizontal rotary accumulator.

The horizontal rotary accumulator, Fig. 7, differs from other horizontal types in three significant areas:

• The feed pinch rolls are located outside of the machine for ease in threading and maintenance.
• The strip is fed into the machine on the outside coil as opposed to strip entered on the inner coil.
• The support for the strip is a driven rotary table that is synchronized with the pinch roll as opposed to driven support rollers on other models.

The operating principal is simple. The strip is fed into the accumulator by a set of pinch rolls. As in all horizontal accumulators, the strip is turned 90 deg. before entering the pinch rolls of the machine.

A DC motor drives the pinch rolls at two to three times line speed. As the strip enters the accumulator, it is supported immediately by a rotary table. A separate DC motor drives the rotating table and the outside diameter is synchronized with the pinch roll. Therefore, there is no resistance to the entering strip and the driven support table helps pull and guide the strip into the accumulator. This greatly reduces the chances of strip cobbling, jam ups, marking, tension, etc. The inside wrap of the accumulated coil is pulled to the center take out arbor by the strip processing line.

The drive has three modes of operation--stopped, line synchronized and high-speed fill. Once the accumulator reaches the full condition, the drive automatically goes into the synchronized mode and the accumulator stays full until the supply of strip from the uncoiler is depleted. The operator or an auto end detector senses the end of the coil and the pinch roll stops pulling strip in.

The processing line then feeds from the accumulator by pulling the inner wrap of the outer coil around the take out arbor and into the mill.

When the end weld is complete, the operator signals the pinch rolls to start filling. The accumulator then fills at two to three times the line speed until it is full. The drive then enters the synchronized mode until the new coil is depleted.

The H-coil accumulator is simple in design, has few moving parts, and stores a lot of strip in a small space. However, it does take more floor space than the vertical rotary accumulator because the strip must be turned 90 deg. before entering the machine and 90. deg. after exiting.

Fig. 8--Free loop accumulator.

Kent Corp. developed the FLOOP^® (Free Loop) strip accumulator after two years of research and development. It combines the best features of all accumulator types and none of the disadvantages. Fig. 8 shows the device during its first filling cycle. The velocity into the FLOOP is set by driven pinch rolls and is represented by V₁.

The FLOOP is threaded with one turn and the velocity out is the speed of the processing line, represented by V₀. Because V₁ is greater than V₀, the strip must go into the free loop. By creating a differential in speed, strip is accumulated.

Notice that there is no strip scrubbing. As V₁ makes one complete revolution, it lays on top of the previous strip, which also has an input velocity of V₁. The same holds true of V₀ or the line speed.

Regardless of line speed, even if the operator changes it during a run, the V₀ does not create any scrubbing. Nor does it affect incoming velocity. The differential in speed always goes into the free loop.

Since the entering strip always is the outside wrap, the OD is continuously getting bigger. Therefore, the outer basket rolls expand in a circle together. Every time the free loops make one revolution, the outer basket expands one strip thickness. In the meantime, the processing line takes a strip from the inner wrap of the inside coil.

Every time one wrap is taken from the inside coil, the ID of that coil gets larger by one strip thickness. Therefore, to keep the inside coil of material in a perfect circle, it is necessary to continuously expand the inner basket of rolls. The expansion of the inner and outer basket is done automatically through cylinders, valves and switches.

There is one more component of the device that enables it to function automatically. It is called the electronic coil depletion sensor (ECDS), which provides signals to the FLOOP control to tell the pinch rolls when to fill and when to stop, depending on the size of the coil. How does this type of device compare with other methods of strip accumulation?

• It requires no costly overhead structure.
• It does not have relative motion between strips.
• It can store as much as 3000 ft. or more of strip in light gauges, and often more than 1000 ft. in heavier gauges. This means that you have more time for end welding and can use a less sophisticated end-welding machine.
• It requires no costly pull out pinch roll system with the corresponding DC drive synchronization problem.
• It is a portable system, so it can be moved if the processing line is moved.
• Minimum back tension reduces the wear on the next piece of equipment.

Besides increased production, users notice less scrap, less tooling wear and better quality of parts by running continuously. MF