Page 39 - MetalForming June 2012
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strip will be snapped tight, possibly stretching or damaging the material. Excessive drag-brake strength, on the other hand, may cause material slippage through the straighten- er or lead to excessive tension on the material.
Establishing Roller Position
Most straighteners include a simple calibrated scale and pointer combination to establish roller position. The amount of work-roll penetration required to back-bend the materi- al to an acceptable level of flatness varies with material thickness and type, roller diameter and roller center-distance spacing. With the optimum depth setting established for a specific material, the stamper must ensure that the work rolls return consistently to this position each time the job runs.
For those applications requiring more accurate position- ing, digital roll-height indicators get the call. The upper work rolls of most straighteners are contained in precision- guiding slide-block assemblies. Methods for raising and lowering the rollers within the slide-block assemblies include fine-threaded screw and nut combinations, worm gear and screw mechanisms, and precision screw jacks.
Most often, coils are unwound from the top of the coil, so that the induced coil set naturally gives the material a down- ward bend. Stock straighteners typically are equipped with an odd number of work rolls, with the extra (or odd) work roll in the lower fixed bank of rolls. With proper setup, this con- figuration creates a slight upward bend in the material as it leaves the straightener, which helps the material slide across the die surface with a minimum amount of friction.
The guidelines for establishing proper work-roll depth set- tings tend to vary as much as the potential variations in material types, thickness and width. In addition, different machine builders recommend different setup practices for effective use of their machines. Here we’ll assume use of a seven-roll straightener with three adjustable upper work rolls. Position the first upper work roll to a setting that alter- nately stretches and compresses the upper and lower surfaces so that 60 to 70 percent of the material crosssection exceeds its yield point. Then position the second upper work roll so that 30 to 40 percent of the material crosssection exceeds yield point. Lastly, position the third upper work roll to bring the material back to a flat condition.
Use the least amount of roll penetration that produces an acceptable level of flatness. Excessive roll penetration will inhibit straightener efficiency, cause material to slip across the straightener, and place unnecessary strain on the machine’s drive components. To set roll penetration, conduct a quick visual check of material flatness before the material runs into the loop area. Then, using the threading table or similar device to support the leading edge of the material as it exits the straightener, fine tune work-roll settings to the minimum depth required to give the leading edge a slight upward bend. Document these settings for reference, to ensure the same setup is used each time the job runs. MF
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