Keep on stamping
Correcting material-equipment induced process problems
To be competitive in a global economy, U.S. stampers must provide defect-free parts, reduce waste, and eliminate downtime while contending with increasing raw material costs and limited availability of some metals. Because stampers have no control over the supply side of the market, they must focus on the process side.
Many problems in the production process arise because of the material condition. Some common coil material challenges are:
• Coil set is the amount of curvature present in the material in the length direction (perpendicular to the material width). Winding material into a coil causes this condition. Coil set can be removed with a straightener.
• Crossbow, the opposite of coil set, is a curvature in the direction of a material's width. A straightener can improve this condition, depending on the material's thickness and yield strength. If a straightener is used to work with crossbow material, the life of the machine will be significantly reduced.
• Crowning occurs when material thickness varies across the width of the strip. A straightener cannot correct this condition. A leveler is best for correcting crowning.
• Oil canning—when the center strip is longer than the edges—can be corrected best with a leveler.
• Camber, which is induced during slitting and rewinding at the mill, is a condition in which one edge of the strip becomes curved and longer than the other. Although a straightener may achieve flatness, it will not remove camber. Cambered material also tends to "walk" to one side as it runs through the straightener, causing damage to strip edges. When too much pinch roll pressure is put on thin stock or silicon steels, the existing camber can be magnified. Straighteners do not cause camber, but can make it more pronounced.
• Edge wave is caused by excessive pressure from pinch rolls on thin stock or overpenetration of straightener rolls, giving a material's edges a wavy appearance. Some steels, such as silicon steel, are susceptible to this condition. Checking the straightening rolls to verify their individual positions and adjusting pressures can eliminate edge wave.
Most material distortion is caused by improper pinch roll pressure. The amount of pressure needed is the amount that allows the straightener to pull the material off the uncoiler. Usually entrance and exit pinch pressure are set the same, although the exit takes less than the entrance.
• High-memory material contains too much memory and springback after the flattening process. As with crossbowed material, high-memory steel requires extreme bending, which will reduce the life of a straightener.
Equipment Problems and Solutions
To remove coil set, material must be bent beyond its yield point. This can be accomplished by back-bending the strip over several properly spaced rolls. The first set of straightening rolls does the most bending. It gradually decreases as the strip is pulled through the straightener. If the leading edge of the strip needs an upturn or down-curl, the last straightener roll can achieve this.
The amount of material penetration varies with each coil because of thickness and yield strength. Depending on a material's thickness and yield, the amount of penetration is subject to change as the coil depletes, because coil set increases as the OD decreases. The tension and compression of 60-in.-OD coil is less than the bend tension and compression of a 20-in.-ID coil.
Each type of material defect usually requires a particular piece of equipment. A straightener can remove only coil set, which is usually sufficient for most applications. A precision leveler can remove edge wave, camber, center buckles, twist, and trapped stresses within the material, as well as coil set.
Catenaries comprise several rolls designed in a radius to support material as it leaves the exit pinch rolls without reintroducing coil set. The radius cannot be less than the material's minimum allowable bend radius or a reverse bend will be induced when material enters the loop area.
The catenary on the exit end of the straightener and the entry end of the feed should be set to the proper radius. To avoid reintroducing coil set, the horizontal loop length should be approximately 1,440 times the maximum material thickness the line will run. In some instances coil set is thought to be a straightener problem, but it could be the catenary settings.
A threading table spans the distance between the straightener and feed and provides hands-free material threading. A table prevents material from falling into the pit and damaging loop control mechanisms. A threading table keeps material off the floor and away from contaminants, which can damage dies and equipment.
It also acts as a barrier to the looping area while in a lowered position. The looping area should be designed to provide a free loop of material without reintroducing coil set. The purpose of providing a free loop is to accumulate slack, and all slack is accumulated in a vertical direction, not horizontal. A guideline for figuring the radius of the material within the loop is 360 times the material thickness. A guideline for calculating the loop area is four times the radius or 1,440 times the material thickness. High-yield-strength material normally has a tighter radius, while material with lower yield strength will require a larger radius.
Identifying and correcting potential problems induced by each piece of equipment can help stampers reduce downtime. This list presents equipment troubleshooting tips for common equipment-induced problems.
Single- or Double-end Reel (Uncoiler)
|Problem:||Material is unwinding off-center.|
|Solution:||Reposition coil on-center of mandrel and straightener.|
|Problem:||Expansion does not pick up coil.|
|Solution:||Reposition coil on-center of mandrel and straightener.|
|Problem:||Expansion does not pick up coil.|
|Solution:||Rotate coil shoe to the 12:00 position.|
|Problem:||Expansion does not stay constant in manual mode.|
|Solution:||Replace check valve.|
|Problem:||Coil shoes have lateral movement.|
|Solution:||Check center guide bolt located in center of coil shoe. If the bolt is loose, remove it, apply thread-locking material (blue for less than 5/6 in. and red for 3/8 in. and larger), and refasten.|
|Problem:||Mandrel is coasting.|
|Solution:||Increase brake (air) pressure, or turn up adjustment for electrical braking systems.|
|Problem:||Inner coil wraps are distorted.|
|Solution:||• Limit expansion range with manual adjustment.|
• Replace existing shoes with new coils with full-radius design.
• Use bolt-on shoes with existing mandrel segments if expansion range and coil ID allow.
• Use reels supplied with automatic expansion regulation based on coil's diameter.
|Problem:||Restrictor arm won't stay up.|
|Solution:||Replace the check valve, because a cylinder is leaking internally or externally.|
|Problem:||Double-end reel won't rotate.|
|Solution:||• Check that restrictor arm is in the up position.|
• Make sure shot pin is retracted.
• If inline with a straightener, verify that peeler blade is retracted to its home position.
• Peeler table should be in home position.
|Problem:||Material won't track on-center.|
|Solution:||• Position straightener on-center of coil and/or reel. |
• Position stock guides on-center of straightener.
• Make pinch rolls parallel to each other.
• Make upper straightening rolls parallel.
• Adjust the pinch roll pressure.
|Problem:||Motorized head adjust trips out.|
|Solution:||• Set torque limits properly. Motors generally do not trip out, but torque limiters will slip if not properly set. |
• Retighten limiters to run material.
• Adjust straightener head so it is not penetrating material too far.
• Set amount of penetration for thickness, yield strength, and material type.
|Problem:||Surface is getting scratched.|
|Solution:||Clean pinch or straightener rolls with solvent or emory cloth.|
|Problem:||Rolls are walking to one side.|
|Solution:||• Loosen drive chain. If the drive chain is too tight, rolls tend to walk toward the sprocket side in forward rotation.|
• Adjust pinch rolls so they are parallel.
• Check stock guides and position of coil on reel. This is especially important when excessive penetration is needed to flatten the stock.
|Problem:||Drive chain is breaking.|
|Solution:||• Make sure sprockets have not moved. Retighten set screws.|
• Verify that penetration is appropriate for material. Excessive roll penetration will cause too much torque on the drive.
Sonic, Photoelectric Loop Control
|Problem:||Material won't stop running.|
|Solution:||• Because atmospheric noise or ambient temperature changes can cause problems with any sonar device, calibrate device and evaluate environment for excessive noise or sudden temperature changes such as open doors. |
• Make sure photo-eyes are clean, and check eyes before start of each shift.
• Reset eyes because settings can change if an object strikes the eyes.
|Problem:||Material from straightener can't keep up with the feed.|
|Solution:||• Adjust speed from sonar control.|
• Check voltage output relative to straightener drive and adjust as needed.
Rack and Pinion, Electronic Feed
|Problem:||Feed lengths are inconsistent.|
|Solution:||Brakes may be loose or contaminated. Replace brake lining or readjust pads.|
|Problem:||Backlash is excessive between rack and pinion gears.|
|Solution:||Reshim rack to tighten backlash.|
|Problem:||Keys are loose.|
|Solution:||Replace loose keys in rolls and couplings if needed.|
|Problem:||Roll gear has backlash.|
|Solution:||Replace roll gears if teeth are worn.|
|Problem:||Clutch has backlash.|
|Solution:||Flush and refill. If backlash is still present, clutch may have to be rebuilt or replaced.|
|Problem:||Roll lift timing is incorrect.|
|Solution:||Retime lifters and check cam switch.|
|Problem:||Crankpin-adjusting screw has backlash.|
|Solution:||Replace crankpin and adjusting screw.|
|Problem:||Feed lengths are short and material is slipping on rolls.|
|Solution:||• Check roll surface for wear. If the finish is worn off, resurface rolls. |
• If upper roll pressure is weak, tighten springs for spring hold-down or increase air pressure for air hold-down.
• On an electronic feed, check for a fault in drive diagnostics.
|Problem:||Material tracking off-center of die.|
|Solution:||Recenter stock guides if needed.|
|Problem:||Upper roll not parallel with lower roll.|
|Solution:||• Make sure upper rolls move up and down freely. |
• Equalize roll pressure on both ends.
|Problem:||Coil is not on centerline of press.|
|Solution:||• Locate the coil properly on reel. If coil is not on centerline, adjust it. |
• Locate reel and straightener on centerline of press.
One last critical component in reducing equipment downtime is a preventive maintenance program.
A preventive maintenance program can keep your equipment operating optimally and identify a problem before a major breakdown occurs.
Identifying the problem is only part of the equation, however; the problem also must be corrected. Often it's common knowledge that a piece of equipment has a problem, but the line cannot be shut down because of production commitments. However, a couple of hours of downtime is better than having a piece of equipment down for days or weeks.
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.