Understanding coil straightening in stamping operations
May 29, 2003
Before coiled material can pass through a die to produce an acceptable part, it must be straightened. Coil straightening is accomplished by bending a strip of material around sets of rollers that alternately stretch and compress the upper and lower surfaces so that the material's yield point is exceeded.
Before coiled material can pass through a die to produce an acceptable part, it must be straightened. Coil straightening is accomplished by bending a strip of material around sets of rollers that alternately stretch and compress the upper and lower surfaces so that the material's yield point is exceeded. Surfaces then are the same length after springback, which results in flat material. Straightening requirements vary depending on material defects, die design, and part requirements.
Modern straightening equipment falls into two categories: straighteners and precision levelers. Straighteners, or flatteners, are used more in the stamping process. These machines use between five and 11 rolls, depending on the material's width, thickness, strength, and type. Their large-diameter rollers are widely spaced, which removes coil set rather than levels material.
Straighteners are available in powered or pull-through versions. In powered units, an electric or hydraulic motor moves the material to the next operation. In pull-through units, the feed equipment pulls the material through the straightener, which is nonpowered.
The second classification of straightening devices is precision levelers, which, like straighteners, remove coil set but also can remove other material imperfections, such as camber, wavy edges, center buckles, and trapped stresses. Levelers are distinguished by closely spaced, small-diameter rollers with backups.
Normally, a precision leveler has more rollers than a straightener. Rollers have the ability to flex and work the material harder. Unlike straighteners, precision levelers are always powered because of the amount of work they must do to the material.
Understanding basic straightening principles is necessary to obtain consistent results in the setup and operation of straightening equipment. In theory, three staggered rolls should be enough to straighten most materials. This approach will work if the amount of coil set in the material is constant throughout the coil.
Depending on material composition (thickness and yield strength), the amount of coil set can dramatically increase as the coil is depleted. In most cases, coil set is induced in the material during a previous process, such as slitting, edge conditioning, or finishing.
The outer wraps of the coil are placed under the tension and compression required to bend the material around the coil's OD, usually from 54 to 72 inches. The coil's inner wraps are placed under the tension and compression required to bend the material around the ID of the coil, usually from 16 to 24 in.
The range between the inside and outside diameters of a coil can result in a dramatic change in the amount of coil set. Using only three staggered rollers, the operator has to adjust the machine constantly to obtain an acceptable level of flatness.
Power straighteners are built with multiple work rolls to deal with varying coil set. As more work rolls are used in a straightener, the amount of coil set that can be removed becomes greater.
Thick materials require large-diameter rollers. The center distance spacing of these rollers can be relatively large and still do an effective job of back-bending the material. Thick materials normally require fewer straightener rollers. As the material thickness increases, the roller diameters and support journal diameters also must increase. The work rolls must be able to withstand the forces required to back-bend the material without excessive deflection across their width.
Conversely, thin materials require small-diameter rollers. The rollers' center distance spacing should be shorter to stretch and compress the material. Thin materials normally require more rollers to remove varying amounts of coil set in the material.
A consideration also should be given to the support journal diameters of work rolls on light-gauge applications. As the material and machine widths increase, the tendency for small-diameter rollers to flex and deflect also increases.
Stampers struggle to increase volume, quality, and production efficiency while retaining a high level of flexibility. Most companies don't have the luxury of building their capital equipment to meet the needs of a single product line. Therefore, flexibility and capacity should be maximized to meet the demands of changing product lines, customers, and markets.
When choosing a straightener, stampers must think about the trade-offs between producing a single product versus producing a range of products. These options should be explored at the onset of a search for straightening equipment. Some fundamental decisions must be made concerning the level of the straightener's flexibility and the breadth of the demands it must meet.
When determining the level of flexibility needed for the range of materials a straightener will process, stampers must evaluate the maximum material width and the machine along with the range of material thicknesses that will be run. As the width of a straightener increases, its ability to process a material with a given thickness and width is compromised.
The tendency for work rolls and end journals to deflect becomes greater as the machine width increases. For example, a 12-in.-wide straightener with 3-in.-diameter work rolls located on 5-in. centers can process 6-in.-wide, 0.187-in.-thick material. The same straightener configuration but 36 in. wide cannot effectively process the same material because of potential roll deflection. Excessive roll deflection may result in a loss of contact surface area, decreased straightening efficiency, material slippage through the straightener, and broken work rolls.
Because of variations in material type, thickness, and width, a single straightener can't meet the demands of every application. Therefore, during the equipment specification process, all variables associated with the straightening process, such as yield strength and surface finish, should be considered.
Stampers using nonpowered pull-off coil reels, which rely on a straightener's horsepower to pull the material off of the coil and through the straightener, should consider ID, OD, coil width, and maximum coil weight variables. However, for all straightening applications, the maximum line speed must be defined.
Stampers evaluating straightening equipment may find that the more detail they can offer in defining coil variables for a specific application, the easier it will be to work with a machine supplier to select the best straightening equipment.
Bruce Grant is research and development manager for Coe Press Equipment, 40549 Brentwood, Sterling Heights, MI 48310, 586-979-4400, fax 586-979-2970, www.cpec.com. Coe manufactures pressroom feed equipment, including servo roll feeds, power straighteners, coil reels and cradles, cut-to-length systems, and complete coil processing systems.