Camber-free slitting for successful stamping

In the competitive world of progressive die stamping, slitting-induced strip camber can quickly turn a profitable job into a losing venture.

This can be especially true for large-volume part runs. Problems such as misfeeds, off-center hits, and inadequate transfer webbing can be caused by camber. Stamping press downtime caused by camber problems quickly can become a costly situation.

The production of camber-free slit coils has evolved to a science in recent years. Proper material selection, tooling, techniques, and inspection practices can provide a level of process control that helps to eliminate slitting-induced camber.

Slitting Equipment

While both pull slitting (nonlooping) and loop slitting machines are capable of producing camber-free material, there is little doubt that loop slitters are much more capable machines.

After you select the proper material, the first critical area to control is the slitting arbor. The size of the arbor will determine how many cuts of a specific material thickness may be taken. If you take too many cuts on a smaller arbor, significant deflection may occur. It helps to know the equipment's limitations. The outside diameter (OD) and length of the arbor both are determining factors for the maximum number of cuts possible. Arbor deflection will affect clearance and can result in cambered strip.

The ability to control back tension from the uncoiler during a slitting operation also is critical. If material tension is allowed to vary widely, the strip may wander, which will promote a cambered shape.

Similarly, the other tensioning devices on the slitter also must be controlled, such as the drag board and any anti-skid devices. Too much drag or tension can "taffy-pull" the material, resulting in both camber and thickness variations. If you have too little tension, the material also may wander.

As with many pieces of equipment, total machine alignment is critical. Poor alignment, from coil payoff to slitting arbor and knives to tension devices and the final recoiler and coil separators, will cause cambered material and other coil shape problems.

Slitter Tooling

Even if all the equipment mentioned so far is in excellent shape and operated with good process controls, slitter tooling can create camber. By far, the most critical areas to control and maintain are the slitting tool complement slitting knives, and rubber stripper rings.

The slitting knives must be the correct type for the material to be slit, the correct width, and maintained for maximum life and edge condition for a clean, camber-free slit edge.

The process of knife rotation, gradually moving knives that are wearing to thicker and wider widths until machine sharpening is required, can ensure good slitting quality and maximum knife life between sharpenings. A good edge on all slitting knives is required for camber- and burr-free material.

Rubber stripper rings often are overlooked as a critical component for high-quality slitting. Of particular importance is the OD size relative to that of the slitting knives for the specific material and thickness to be slit.

The male and female stripper rings should match correctly, and the durometer value, or hardness, of the rings must be correct. An incorrect OD, match, or durometer value will result in the material being pushed off the slitting knives incorrectly, causing knife marks and camber.

Material Properties

After checking the slitting equipment, make sure the coil itself won't cause camber problems. Additional information about the material is required to set knife clearances for camber-free slit coils.

First you should know the general yield and tensile strength of the material. Although you might purchase many coils without specific certified properties, you should consult your material supplier or a metallurgical expert to determine strength levels. Then from these values you can calculate the correct knife clearance for a clean break on the slit edge.

Most modern slitters use computer-assisted tooling programs to determine the correct clearance value if the material parameters are known. If you simply guess, the clearance might be too large, causing rough edges and a lot of burrs. If the clearance is set too tight, the slit edges can be overstressed and form a cambered condition.

Different materials' properties also should be considered in the slitting process. Stainless steels often match yield strength levels of mild carbon steels, but the high chromium and nickel contents will produce higher tensile strengths and elongation values.

This condition usually requires different slitting knives and clearance values. Also, coated materials such as hot-dip-aluminized or -galvanized steels can present additional clearance concerns because of coating properties and thicknesses.

Think about the material's surface when you set the process controls. A dry surface, especially on a coated material, will create problems in the tooling area, such as coating residue on the slitter knives. Several remedies such as vanishing oils and fluids are available for this condition that will result in a dry surface when the product is shipped to the stamper. In most cases, an oiled surface is preferred on the slitter.

Slitting Techniques

The correct slitting knife clearance is important for camber-free slitting. In addition to the material properties, the surface quality, and the material grade, you also must look at actual material thickness and the required number of cuts.

The correct penetration of the arbor in contrast with the material thickness allows for minimal arbor deflection. Combined with optimal overarm pressure on the recoiler as the slit coils are rewound and the overall straight-line material flow through the machine, control of these pressure parameters can lead to a quality slitting process.

Slitting Inspection

Once the correct equipment, techniques, tools, and materials are in place and the slitting process is proceeding well, you still need to know that the results will be satisfactory to your customers.

Although process controls continue to advance, making detection inspection procedures less important over time, it's a good practice to perform a cursory inspection for slit width and shape or camber.

Because camber can be induced at several points in the slitting process, you should evaluate camber at those points. In most cases, you should program a stop of the slitting run to measure camber at the slitting head, after material has left the tensioning devices, and after recoiling.

For a thorough evaluation, the material must be loosened, with all tension removed from the coils. Aligning a calibrated straight edge along a slit coil edge will indicate if a camber problem exists. Once the coils are rewound, your customer will do the next evaluation for camber—not the place you want camber to be detected for the first time.

Material Storage

Even though the slitting operation is completed and all inspections have proven the slit coils are camber-free, the end user still could encounter a camber problem. Particularly on narrow, thin-gauge coils—traditionally the most camber-prone slit material—incorrect packaging and storage can result in camber. This can occur at the slitting facility or at the customer's facility.

For most narrow, thin-gauge slit product, several coils are stacked vertically and banded together on a wooden or composite pallet or skid. Because the stamper will lift each coil individually, coil separators are inserted between each coil.

It's important that the separators are lined up with each other vertically in the stack. Misaligned separators and multiple-skid stacking at your or your customer's storage facility can cause a strip to camber, even after a perfect slitting process.

A high-quality, camber-free slitting system is an important part of a successful stamping process. Both the slitter and the stamper should work together to ensure that all material requirements have been covered, including stamping process criteria for material dimensions and shape and final product characteristics that may require particular attention during the coil slitting process.

Fred Barrera is the vice president of manufacturing and Ed Basta is the vice president of technology and market development for Atlas Steel Products Co., 7990 Bavaria Road, Twinsburg, OH 44087, phone 800-444-1682, fax 330-425-1611, e-mail service@atlassteel.com, Web site www.atlassteel.com. Atlas Steel Products Co. is a steel service center and welded tubing manufacturer specializing in stainless steels, aluminized and other coated steels, and tubing products in these alloys.