The importance of straighteners in stamping operations
December 13, 2005
The straightener, when set properly, removes the coil set, or curvature of the rolled material. If the machine is used or set incorrectly, the coil set can remain in the material, even after being struck in presses with capacities to hundreds of tons. This can cause a variety of problems: out-of-tolerance parts, difficult feed operations that can disrupt and slow performance, and, to some extent, additional wear and tear on feed line components and tooling.
Stamping presses and coil feed lines—a match made in productivity heaven. When running smoothly and well synchronized, few manufacturing processes can outperform the stamping line for its cost efficiencies and proficiency.
Then again, when individual pieces of equipment comprising the press and line are not in harmony, the outcome can be devastating. A feed that isn't fast enough to meet the press's strokes per minute or accurate enough to deliver necessary tolerances, or a decoiling cradle that damages thin stock, can result in lost productivity.
Within the feed line, one of the most critical devices is the powered or pull-through straightener. The straightener, when set properly, removes the coil set, or curvature, of the rolled material. If the machine is used or set incorrectly, the coil set can remain in the material, even after being struck in presses with capacities to hundreds of tons. This can cause a variety of problems: out-of-tolerance parts, difficult feed operations that can disrupt and slow performance, and, to some extent, additional wear and tear on feed line components and tooling.
Before deciding which style of straightener to use, you need to consider a few basic factors pertinent to either powered or pull-through styles.
First is the number of rolls required. Most manufacturers offer five, seven, or nine rolls, depending on the material and thickness. However, the application should dictate what number is right, and more is not always better. More important to a straightener's performance than the number of rolls is that it be equipped with independent roll adjustment, allowing each set of rolls to be set and fine-tuned to the specific needs of each job and, further, to each coil (see Figure 1).
The roll material also is important. Chrome-plated rolls, regardless of the material used in their construction, can help reduce the need for roll repair and cleaning.
In this straightener, the upper roll fixture is mechanically locked in the bottom or straightening position.
Guiding is required to keep the material centered and tracking properly in the straightener. If the application requires the material to remain square, the type of guide used is important. While straight-edge guides often are chosen to keep costs down, self-centering or independently adjustable, hardened vertical roller guides can adapt to various operational conditions and help minimize material damage.
Of the two straightener technologies, the powered straightener is probably the most commonly used. In operation, this mechanism pulls material from the coil reel through the straightening rolls and out into the loop area. A seven-roll machine is most common.
In applications involving surface-critical material, a powered straightener can minimize marring. This type of straightener also generally is suitable for high-speed jobs, allowing for the use of small, more economical feed units. Also, it can remove coil set efficiently and thoroughly.
Typically, powered straighteners require a large amount of floor space to accommodate the loop of material between the straightener and feed unit. Also, the length of the line, including the feed, loop, and press, increases the amount of material that has to be run through and wasted when straightener adjustments are made.
The pull-through straightener usually is not powered and typically is mounted on the entry side of the servo feed unit, letting the feed do the driving work. Pull-through straighteners typically are available with five, seven, or nine rolls.
With this type of straightener, entry stock support rolls maintain the correct loop radius. Powered entry pinch rolls, a set of rolls in front of the straightening rolls, move the material through the straightener and into the servo during the initial threading process. Because the straightening rolls are not geared, the pinch rolls grip the material and send it through the straightener and into the feed. Once the feed rolls contact the material, the pinch rolls are raised, and the feed rolls take over.
Entry and exit stock guides, which keep the material square before it goes into the servo feed, are critical when using a pull-through straightener.
Another feature is a piloting head straightener, which often is used when lateral or side-to-side positioning is required within the die. Before each stroke of the press, as the pilot pins enter the material, the straightener's rolls are opened enough to release their pressure on the material.
Two piloting head designs are a true piloting configuration and a pivoting, or alligator-style, system. A true piloting unit is suitable for lines that straighten and feed materials from 0.030 to 0.500 inch thick, while clamshell designs work well with thinner-gauge stock. The true pilot concept means that all upper rolls of the mechanism are raised an equal height, releasing the tension between the upper and lower rollers of the straightener during the feed operation's pilot pin orientation of the material.
With a conventional pneumatic- or hydraulic-actuated pivoting or alligator style, the exit end of the straightener is hinged, and the entry end is opened and closed via an air or hydraulic cylinder.
Pull-through units require less floor space than powered straighteners because there is no loop between the straightener and feed. Also, adjustments to the straightener rolls are quickly apparent because the straightener and feed are located right next to the press. They are most suitable for slow- to medium-speed applications.
Pull-through straighteners are not appropriate for applications in which the surface finish of the stock is critical. The material in the pull-through system starts and stops with each feed cycle, unlike the continuous operation of the powered straightener, and this interrupting action can negatively affect surface finish.
Whichever straightening system is right for your particular application—powered or pull-through, piloting or standard—the straightener is an essential element to the success of all coil-fed stamping operations.
Brian Landry is regional sales manager with Dallas Industries Inc., 103 Park St., Troy, MI 48083-2770, 248-583-9400, fax 248-583-9402, www.dallasindustries.com.