Filling in the blanks
What to look for in an in-house blank shearing line
Because of sluggish economies and uncertain markets, the need to hone a competitive edge is more sharply defined. Many stampers are doing this by taking control of their material inventory and production schedules by adding a cut-to-length blank shearing line.
An in-house blank shearing line allows stampers to pay less for steel because they can purchase it in bulk, monitor coil condition, and run blanks on their own production schedules instead of a supplier's.
A basic line will unwind, straighten, feed, and shear-to-length materials from 6 inches to 60 in. wide and 0.030 to 0.135 in. thick.At the beginning of every blanking line is a feed line that sends material into a shear instead of a press. However, before the material gets to the shear and stacking system, it has to go through other equipment at the front end of the line. If these pieces of equipment are not given careful consideration, the new line can become a cost instead of a profit. Here's where it all starts.
Stampers have two choices for unwinding coil: a coil cradle or a coil reel. A coil cradle may limit the variety of material that can run through the blanking line because the coil rests on two rollers in the bed of the cradle. A high coil-weight-to-stock-thickness ratio can damage the material's surface or deform the stock to an unacceptable condition.
Coil reels come in two versions: powered or pull-off. The type of coil reel needed depends on the line's straightener. Typically, a pull-off coil reel is used with a powered straightener, and a powered reel is necessary with a pull-through straightener.
Regardless of which reel type is chosen, several commonalities emerge for all blanking lines. First, the most overlooked item in reel is its braking system. This may not sound important, but problems can arise by using the wrong reel braking system.
The brake should allow for gradual application without jerking the material, which prevents damage such as skidding. In all cases, a fail-safe brake must be used that locks the mandrel upon loss of air pressure, which reduces the chance of costly mishaps.
The mandrel and mandrel caps are another area to research. If a coil's ID is not concentric during the run, the last few wraps could be damaged. A full-radius mandrel cap or a drum-type cap should be considered. Choosing the right hold-down arm is another consideration. Sometimes a nonpowered hold-down wheel will fit an application.
In job shops where frequent changeovers and batch runs are the norm, a coil load car allows prestaging of the next coil for faster changeovers. Coil car options such as a rotating coil deck that turns 90 degrees can enhance the loading process in tight spaces. When a full coil is not processed in a single run, the smaller OD of the partial coil makes offloading and reloading difficult. Though most coil cars have a lift of 18 in., an extended lift of 24 in. may be specified to handle a range of coil sizes.
Straightening It Out
Most equipment manufacturers offer five, seven, or nine straightening rolls. The application will dictate how many rolls are needed. However, more important than the number of rolls is an independent roll adjustment, which allows each set of rolls to be set and fine-tuned to the needs of each job.
Guiding is needed to keep material centered and tracking properly in the straightener. It's a good idea to have self-centering or independently adjustable hardened vertical roller guides. Straight-edge guides sometimes can damage material. In a line in which keeping the material square is important, stampers should inquire about which kind of guiding is offered.
Another option for most blanking line straighteners are flash chrome rolls. Many coils have edge burrs, which tend to groove the rolls. Flash chrome rolls can reduce roll damage and keep maintenance costs down.
A powered straightener pulls material from the coil reel through the straightening rolls and out into the loop area. If the material requires a cosmetic surface or is sensitive to damage, a powered straightener is a good choice. A standard seven-roll machine with independent roll adjustment works well for surface-sensitive materials.
A nonpowered pull-through straightener usually is mounted on the entry side of the servo feed unit, letting the feed do the work. Nonpowered pull-through straighteners normally are available with five, seven, or nine rolls. Stampers should remember that more is not always better. A powered coil reel and a servo feed with a pull-through straightener configuration often is all that's necessary for a line to send material into the shear.
Stampers should research a few additional items when considering a pull-through straightener. First are entry stock support rolls to maintain the correct loop radius. Second are powered entry pinch rolls for moving the material through the straightener and into the servo during the initial threading process.
Pinch rolls are a set of rolls in front of the straightening rolls. Because straightening rolls are not powered, 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 keep the material square before it goes into the servo feed and are critical when using a pull-through straightener.
Another point to consider when using a pull-through straightener is power requirements. The servo feed unit has to be built to heavy-duty engineering standards, have the drive power to straighten, feed accurately, and not suffer from premature wear and tear. Drive specifications must be more than adequate for anticipated work loads.
The smooth acceleration and deceleration speeds of sophisticated motor drives help protect fragile materials from kinks and damage. Another key is the proper material loop control to provide smooth material flow. Although sonic systems are popular, a set of electronic eyes placed in the loop area is the most precise way of controlling the material.
A true curved stock support forming the radius of the loop either at the exit of the powered straightener or at the entry into the servo feed or servo/pull-through affects blanking quality. If the stock support and the loop do not match, coil set is put back into the material, defeating the straightening process and the blanking operation. Sometimes the support or loop control may only require an adjustment.
This is the heartbeat of the line, where accuracy, repeatability, and reliability are paramount. This is the point at which application elements such as memory for different job settings, push-button control, and readout of settings and adjustments must be considered. The feed typically provides the widest latitude, from simplicity to sophistication, for the job. Usually the right location for secondary operations such as notching and punching is after the feed.
Shear and Stack. Hydraulics can provide an economical power supply for the shear. This equipment offers easy adjustments, fewer moving parts, and less vibration than a mechanical shear, which can result in quality cuts and less downtime.
Without question, it's important to guide material and keep it square through the line. The secret to guarantee a square blank is an upper bow tie blade, shaped like an inverted V. Instead of cutting from one side to the other, which applies a unilateral force, as a rake angle blade would do, the bow tie applies simultaneous forces and cuts from each side to the center, keeping the material centered and square.
Stackers range from the standard drop type to fully automatic with hydraulic lift tables and skid loaders. But the most widely used for in-house blanking is the standard drop type with manual unload, because only one operator is required for the full line, reducing additional labor costs. Custom units also can be supplied for special operations or blank shapes.
Many optional features are offered today, but the elements highlighted are those that will most affect the outcome of the blank. The idea, however, is to keep it simple and economical. If implemented carefully, a blanking line achieves the best of both worlds—the speed, productivity, plus labor and cost savings of a coil-fed line with the accuracy and flexibility of blank processing.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.