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Lean manufacturing in producing square, retangular tubing
Cage forming process elininates tooling changes, improves uptime
- By Jurgen Jost
- June 24, 2015
- Article
- Shop Management
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.
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
- 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
- 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
- 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
About the Author
Jurgen Jost
11405 Grooms Road
Cincinnati, OH 45242
513-985-0500
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The Tube and Pipe Journal became the first magazine dedicated to serving the metal tube and pipe industry in 1990. Today, it remains the only North American publication devoted to this industry, and it has become the most trusted source of information for tube and pipe professionals.
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