Volume 34, Number 7, July 1999


Metal Curving Do’s & Don’ts

Knowing the Basics Up Front Will Mean Fewer Reject Parts and
Less Field Rework and Assembly Time

by Helmuth Sens

What was once strictly an aerospace metal forming technology has grown to serve the ever-demanding architectural market. Curved metal lends a dramatic element to a design project, and curved sections often simplify the closure of a structure.

Designs now call for everything from curved balustrades to curved windows and skylights. Most people think of aluminum extrusions in bent applications, but brake shapes and other metals are increasingly popular. The use of curved brake metal serves not only as a closure for glazing systems, but is now used to accent a variety of interior finishes.

Brass, bronze, stainless steel and even mild steel are curved to increasingly tight tolerances. The improved tolerances reduce rework in the field and allow more accurate estimation of projects involving curved sections.

Large spans and the pressure for reliable, yet easy-to-install designs, have increased the complexity of the metal systems, which now have multiple snap-together parts, gutters and heavy cross sections that must be bent consistently. Still, the skills of metal curving and glass bending vendors have made it easier for even a small glazing contractor to bid and install a curved system. The following information will help you accurately order bent metal and get you the best price and service. Also see page 55 for an overview of different bending technologies.


Measuring the Opening

The opening you need to fill is probably set by other trades (steel or masonry), by the architect’s plans or some combination of both. Ideally, all trades work to proper prints and you can order from specific measurements.

Unique openings or renovation projects can make it virtually impossible to determine the measurements required to order curved metal. One of the best solutions is tracing a pattern of the opening onto wood, cardboard or paper. The more rigid the material, the better your field crew can trace and the easier it is for metal and glass bending vendors to use the pattern. However, the realities of mailing the pattern tend to dictate your choice of media for it.

Make sure you indicate exactly how the pattern is to be used by each vendor. The glass or insulating glass manufacturer will need to know the actual glass dimensions after the proper deductions have been taken for finishes, glazing materials and framing. The metal curving vendor will need to know the inside radius of the curved part. If the outside radius is specified, make sure you note the metal to be used and the orientation.

The Hard Way or the Easy Way?

The terminology "easy way" or "hard way" was created in the early days of bending technology when Level I (bash to fit or blacksmithing) or Level II (roll bending) were the only choices. Envision trying to bend a piece of metal by hand, then ask yourself "Which way would be easiest?"

Multiple-part extruded assemblies need to be considered one piece at a time. Some parts may have to be bent the hard way and some the easy way so the total assembly fits together to obtain one multiple-piece assembly.

Getting the Best Price and Service

Group bends. Being organized and specific means a lot. When you prepare for a job, gather measurements of all openings with the same radius. Even if the arc lengths are different, the same die and set-up frequently can be used to curve several openings of the same radius.

Combine to 90 or 180 degrees. In some cases, small openings (eighth or quarter circles) can be combined to 90 or 180 degree bends. For example, consider if you have four openings at 45 degrees (eighth of a circle) each.

Method 1: Bend each at 45 degrees. Results: four set-ups for the metal bender, 96 inches of grip required (four pieces by two ends by 12-inch grip).

Method 2: Bend one piece at 180 degrees. Results: 1 set-up for the metal bender, 24-inches of grip required (one piece by two ends by 12-inch grip).

You have saved 72 inches of material!

The ability to combine bends is limited by the total length of material and the maximum length of capacity of your bending vendor. Also, beyond 180 degrees, most metal stretchforming equipment does not operate efficiently. Simple shapes or large quantities (50 or more bends of the same radius) provide more flexibility in technology and tooling. In general, as your requirements for better tolerance or quantities increase, the tooling cost can be adjusted to meet your needs. Large, repetitive jobs generally can be negotiated to make the stretchforming vendor responsible for tooling maintenance with an annual quantity and minimum order lot.

Leave tangents. Leaving a tangent, or straight leg, on a curved part can reduce your fabrication costs and make the structure stronger.

Send stock lengths. Let the experts do their job. Metal bending vendors need an extra length or two to set-up and prove the bending die. Send full stock lengths—don’t cut the material. If you ask, bending vendors will return unused material.

The stretchforming house will generally cut the metal to appropriate lengths for free. This is because the effort to cut the metal is minimal compared to the flexibility the full stock lengths give the bender.

Once a material lot is understood and tested, the learning curve is much easier for the entire job, regardless of the number of bends or radii.

Verify the vendor’s equipment capacity. Stretchforming technology is limited to the length of metal the machine will take. Most vendors approach the equipment limit at 20 to 22 feet of good bend arc, plus minimum of a 12-inch grip on each end of the part. Large parts must be evaluated on a case-by-case basis by the stretchformer. The part design, its geometry and cross section will have an impact on the formability of the material.

Allow for grip and set-up. Stretchforming requires the part to be gripped hydraulically, so at least 12 extra inches of material are required at each end of the part. Some heavy, difficult or very small radius parts may require more grip. Again, the safest bet is to send full stock lengths.

Although the die constrains the shape of the finished part, a trial piece or two generally is required to proof the die and develop forming pressures. The stretch process reduces spring-back, but even careful calculation cannot predict exactly how metal will behave. The same extrusion or break shape can vary between different lots.

Order annealed material. The aluminum vendor can usually supply annealed (Tempered "T-1" or soft) material, saving the cost of annealing the material before stretch bending. Curvetech believes annealed material from the mill or warehouse is generally more consistent than annealing in small lots.

T-42 (half hard) material can be used in many cases unless the radius is tight or the cross-section to be bent is large. Fully hard, or T-6, does not have enough elongation and has too much spring-back to form properly even with stretchforming technology.

Paint after bending. Material should be painted after bending. However, the painting of curved material can prove costly. Most baked-on or Kynar® finishes will survive intact except for very tight radii. The key to determining the feasibility of curving painted material is to evaluate the amount of stretch required at the outside of the bent area versus allowable elongation of the paint. For Kynar-painted pieces the maximum material hardness should be T-42 (half hard) because they cannot be annealed with Kynar paint.

Order debridged material half hard. Some curtainwall systems have snap in plastic pieces to form the thermal break. In this case, no special considerations need to be made. A poured and debridged system, however, offers a few challenges.

The safest method is to get the material before the thermal break has been poured and the metal debridged. If the schedule or small size of the curving job make the debridged material the only realistic option, two actions will help avoid problems. The material should be ordered in half hard or annealed condition—it cannot be annealed after debridging. Extra material should be shipped to the stretchforming vendor in case of failure during forming. The tighter the radius, the more extra material should be supplied. Of course, all unused material will be returned.

Buy first-quality material. All material to be curved must be first-quality. While aluminum is one of the more easily recycled materials, all metal curving vendors have reported serious problems with recycled or secondary material.

Perhaps a lack of material consistency or variations in wall thickness cause the inordinate number of failures of secondary material during the bending operations. The structural integrity of material which tears so easily during the bending process should also be doubted.

Ordering all material from the same lot will also speed your project and reduce the scrap rate. Material can behave very differently between lots even if ordered from the same vendor. Mixing material from the distributor’s shelf supply with material directly from the mill will cause inconsistencies in the bending. The lot of material should come from the same lot at the mill or at least all material from one shelf lot.

Watch brake metal thickness. Generally, all brake metal should be at least .090-inch thick, and the brakes should have at least a .100-inch radius to prevent crazing, which can lead to failures during the stretchbending process. The same allowances for grip apply for brake shapes and extrusions.

These tips apply to both standard and custom extrusions. An increasingly competitive market has lowered the cost of pushing a custom extrusion, which can be designed by some of the more sophisticated stretchbenders.

Helmuth Sens is president of Curvetech Inc., based in Keenesburg, CO.


Three Bending Technologies

Level I: Bash to Fit

Many bending methods have been used in the hope of saving money or time. Some jobs can be bent by hand, but these are far fewer than envisioned by some estimators or project managers. We have seen many cases where field crews have wasted material and precious time trying to bend metal to match an opening.

One cost-conscious customer lowered a 3/4-ton truck onto a multiple piece extrusion to bend it over two cinder blocks. Fourteen stock lengths of material and one injury later, the foreman sent the order to a qualified stretch-forming vendor and finished the job for half the cost of the metal already wasted.

Level II: Roll Bending

Some simple shapes can be economically bent with roll bending. Most roll benders use a pyramid set of rollers. The metal is placed in the rolling machine and increasing pressure bends the metal as the material is rocked through the machine.

For simple extrusions and small quantities, the rolling method can be relatively effective. Set-up operation of the machine is fairly easy. Also, a 2- by 6-inch tube might be rolled the easy way, but would be very difficult to roll the hard way even if wall collapse could be tolerated. The common bosses and gutters would make life even more difficult.

Complex or multiple piece assemblies are not well suited to rolling because of the changes in the cross sections during rolling. In my experience, most parabolic or spiral bends or tangents are not well-suited to rolling technology since no die is used to match the difficult contour.

Level III: Stretch Bending

Stretch bending or radial draw forming was developed for the aerospace industry, but has been used in commercial and architectural markets for more than 20 years. Stretchforming equipment actually stretches the metal as it is wrapped around a die.

The die controls the radius and allows greater consistency. Multiple-piece assemblies are generally bent on one die, with some minor modifications. This allows the assembly to operate in the curved condition as designed or straight.


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