Our Sites

Lean manufacturing in producing square, retangular tubing

Cage forming process elininates tooling changes, improves uptime

Just-in-time delivery. Quick die change. Waste elimination. Non-value-added step reduction. These aren’t just buzzwords. As manufacturing becomes more competitive, any manufacturer that doesn’t take steps to improve operating efficiencies is at risk of losing market share, falling revenues, and potentially an irreversible decline. Continuous improvement is a matter of survival.

Maximizing efficiency is critical for every manufacturer, but not equally so. Manufacturers that move more material have more to gain, and those that produce tube, pipe, and profile move a lot of material. On one hand, they usually move hundreds of feet of material per minute. On the other hand, a mill stopped for tool change moves no material and therefore makes no money. Reducing the time needed for tooling changeover makes the mill more efficient and can help in solidifying customer relationships by maintaining aggressive just-in-time delivery goals.

At the same time, tube, pipe, and profile manufacturers try to minimize inventory levels for any of several reasons—space restrictions, capital restrictions, and to guard against raw material price volatility. This leads to smaller and smaller batch sizes, meaning more frequent tooling changes.

For tube, pipe, and profile production, reducing the downtime between production runs is one of the most effective ways to reach lean manufacturing objectives. Flexible Cold Forming® (FCF) is a cage forming process that provides an alternative to the conventional forming process. It reduces mill downtime and can help a tube producer remain viable.

How It Works

Like conventional forming, cage forming pulls a continuous strip of material through tooling rolls to change its shape. The main difference concerns how the tooling works. The conventional process uses one set of roll tools for each diameter produced on the mill. Cage forming uses one set of tooling for every diameter produced on the mill. In other words, it uses a universal tool set. This means that when the operator switches from one diameter to another, he doesn’t have to go through a lengthy tool-change process. In a few minutes one operator can adjust the tooling to new positions for the next production run.

Unlike the conventional process, which uses tooling mounted to paired roll stands, cage forming uses forming blocks. The tooling is mounted to these blocks and the tooling position is adjusted by drive motors. Switching from one product size to another is a software function; the operator doesn’t change the tooling, but rather calls up new tooling locations by calling up the settings at the mill’s control panel. The software and the drive motors take care of the rest.

The tooling itself isn’t driven. In cage forming, the forming blocks have pinch roller stands that propel the strip forward. Like the forming blocks, the pinch rollers use a single set of rollers for the entire product range. The first and third forming blocks have such a pinch roller stand in front of them. The pinch rolls as well as the lower rolls are driven at precise speeds by AC servomotors and a distribution gearbox.

Forming the Forms

Shaped tubes typically are produced by making a round tube and then forming it into a square, rectangular, or other shape. The cage forming process doesn’t form round first, then the final shape; instead, the material goes from flat directly to the desired shape. Although this process is fundamentally different from conventional forming, it still meets the commonly accepted requirements regarding tolerances, radii, and quality. For making squares and rectangles, the forming process uses three forming blocks for three process steps (see Figure 1).

The FCF process uses paired tooling, but the pairs on forming blocks 1 and 2 aren’t opposite each other. They are offset and thereby form the left and right strip edges in an alternating arrangement (see Figure 2).

FCF also imparts less cold work. The bigger stand distances on each side enable a gentle shaping of the profile, imparting less tension than they would otherwise. Reducing the edge tension means the resulting edges are essentially flat and free from the irregularities that can develop when the strip edges are overstressed. This is a particular advantage if the tube or pipe will be bent in a subsequent process.

Figure 1
Forming block 1 forms the strip edges in several steps to an 80-degree angle, making the top of the rectangle or square. At the end of this stage, the strip has a U shape. Forming block 2 forms the sides in several steps up to a 60-degree angle. At the end of this stage, the strip is essentially a C shape. Forming block 3 closes the tube for welding.

Additional Notes on the Tooling and Processes. Like the forming rolls, all squeeze rolls are universal. Two side rolls, a bottom roll, and two tilted upper rolls put pressure on the shape, which can be measured and digitally displayed.

FCF sizing stands, equipped with AC servomotors, and the Turk’s head stands calibrate the square and rectangular shapes to their final tolerances. A universal set of four cylindrical tool rolls, which can be adjusted both horizontally and vertically, finalize the size and shape of cross-section and corner radii.

Most FCF mills use just one set of forming blocks and tooling. However, if the manufacturer needs to increase the diameter range of the products an FCF mill can make, the operator removes the initial set of forming blocks (with tooling) and installs a new set.

The process does have a limitation. For rectangular tubing, the ratio of the long side to the short side should not exceed 2.5-to-1. If the ratio gets much larger than this, cage forming becomes less cost-effective because the tooling becomes significantly more expensive.

Calculating Time and Material Saved

The big difference is in mill uptime. Consider a conventional production facility that runs one shift 48 weeks per year and performs seven tooling changeovers every week. Five of the tooling changes require 1.5 hours (partial change) and the remaining two require 3 hours (full change).

  • Number of hours/year: 48 weeks x 40 hours/week = 1,920 hours
  • Downtime for tooling changes: (48 x 5 x 1.5) + (48 x 2 x 3) = 648 hours
  • Uptime: 1,920 hours - 648 hours = 1,272 hours
  • Uptime percentage: (1,920-648)/1,920 = 66 percent
For an FCF mill running one shift 48 weeks per year, five of the automated changeovers each week require 0.25 hours (wall thickness adjustment); the other two require 0.5 hours (dimensional change).

  • Number of hours/year: 1,920
  • Downtime for tooling adjustments: (48 x 5 x 0.25) + (48 x 2 x 0.5) = 108 hours
  • Uptime: 1,920 hours - 108 hours = 1,812 hours
  • Uptime percentage: (1,920-108)/1,920 = 94 percent
Therefore, FCF results in 540 more productive hours per year (based on the difference in downtime: 648 hours-108 hours = 540 hours).

Using that time equally to make two products, 4- by 4-in. square and 2- by 2-in. square adds 270 hours of production capacity for each product, being able to sell 50 percent generates an extra $488,430 in gross profit per year (assuming steel at $500/ton and a profit margin of $60/ton):

  • 4 by 4 sq. tubing, 0.250-in. wall, 13 lbs./foot, produced at 120 FPM: 13 lbs. x 120 FPM x 60 min. x 270 hours / 2,000 lbs. per ton = 12,636 tons = > 50 percent = $379,080
  • 2 by 2 sq. tubing, 11 ga., 3 lbs./foot, produced at 150 FPM: 3 lbs. x 150 FPM x 60 min. x 270 hours / 2,000 lbs. per ton = 3,645 tons = > 50 percent = $109,350
  • The gross profit increase is $379,080+ 109,350 = $488,430
Forming the inside corner radii with consistent wall thickness saves about 3 percent of the strip width, which results in a cost reduction. Using half of the mill’s additional uptime (1,542 x 0.5 = 771 hours) per product yields a material savings of $697,370:

  • 4 by 4 sq. tubing: 3 percent x 13 lbs. x 120 FPM x 60 min. x 771 hours / 2,000 lbs./ton = 1,082 tons
  • 2 by 2 sq. tubing: 3 percent x 3 lbs. x 150 FPM x 60 min. x 771 hours / 2,000 lbs. per ton = 312 tons
  • Material saved = 1,394 tons
  • Cost of material saved = 1,394 tons x $500/ton = $697,370
Adding the additional profit generated by adding capacity and the cost avoided by material savings yields an annual total of $488,430 + $697,370 = $1,185,800.

Figure 2
Cage forming uses alternating tools to form the strip (top). This decreases the amount of stress on the material, reducing the likelihood of undulations (wavy edges) that make good welds difficult to accomplish. For improved wall thickness consistency, the FCF mill upper rolls are inclined (left) and therefore form inside radii more precisely. In conventional forming, the wall thickness tends to increase in the inside corners, wasting 3 to 5 percent of the strip width.

About the Author

Jurgen Jost

Vice President & CEO

11405 Grooms Road

Cincinnati, OH 45242

513-985-0500