Without a scratch: special technology reduces rejects when cutting highly delicate strip for visible and functional parts
Metal strips with elaborately machined surfaces – from brushed to mirror-polished – are nowadays demanded for ever more applications. The range extends from visible parts for presentation purposes such as façades or trim strips, which must be flawless due to their external impact, to functional components such as conductor strips, which would no longer be able to fulfil their required function if damaged. Since reworking such parts would hardly be economically viable, these products become rejects if they have even the smallest of scratches. Therefore machining, particularly cutting the plates into strips of the desired width, must not damage the material. Ideally any contact with the surface should be kept to a minimum. Depending on the width and weight of the strips, two different slitting system designs are possible – with a loop and brake system after the shears, or with no loop.
"In general when cutting delicate surfaces, you have to be able to rely on the system absolutely, as the strip is coiled up immediately after machining", explains Thomas Baral, CEO of cutting and straightening machine manufacturer Burghardt + Schmidt. Although tolerances for visual appearance and functionality are becoming ever more stringent in all sectors and even small discrepancies that occur during machining could make an entire batch worthless, subsequent polishing or burnishing of defects is almost entirely precluded. Even subsequent inspection cannot be performed on more than a spot check basis. "Broadly speaking, either the quality is acceptable or it isn't. There can be no compromises when it comes to compliance with quality standards", says expert Baral. This applies to trim components and stainless steel panels as well as for strip conductors or reflectors in lights. Even in the production of crown caps, attention is now paid to achieving a flawless surface, in order to present the customer with a high-end, attractive product image. Soft materials, such as aluminium and copper, or materials which are already brushed, coated or polished in advance pose a challenge here.
Conventional slitting shears cannot be used for such applications, because rough rollers, unequal strip tensions and speeds as well as mechanical brake systems would lead to pressure marks, scratches, or torn-up areas, as Baral remarks: "Strip tension and brakes are the main problems in this area. If the tension is not constant, this may result in relative movements in the shears, which makes it impossible to achieve a clean cut and could produce marks on the surface." Even the established indirect tension measurement process, which estimates the forces applied to the motor output is insufficient in this case, as it fails to consider additional effects such as friction. Even attempts to brake the strips with brake rollers coated in non-woven fabric are hardly productive: "Such systems require a slight draw in any case, which requires contact with the surface – and this holds a risk of damaging the material."
Constant tension and sensitive brakes protect the surface
This is why at Burghardt + Schmidt we recommend two different system concepts for delicate surfaces, according to requirements: for very narrow and thin strips, we produce slitting systems without loops before or after the shears. This means that the machines work with direct tension, so that brakes can be done away with altogether. This means that the only remaining critical factor is the uniformity of the strip tension on the unwinder and the winder side. To ensure consistent values for this, an electronic dancer control before the slitting shears is combined with a direct tension measurement after it. The measurement is made by strain gauges in loadcells integrated into the bearing of the deflection rollers before the winders, which can be used to determine the actual tension value. This is reconciled by the software with the preset values, and if necessary automatic fine adjustment is performed. In addition this process has the benefit that the permanent tension adjustment also has a positive effect on the quality of the cut. The cut strips can then be wound onto the coilers, either individually or several at a time depending on how delicate they are.
In the case of wide input materials, large coil diameters, and high throughput speeds up to 400 m/min, on the other hand, it is not possible to do without a looping pit after the shears, otherwise it will not be possible to wind up the different lengths of strips with the same tightness. Such differences in length can be compensated for by the strips hanging down by different depths into the pit, though this will also require a brake before the coilers. For this purpose, in addition to the dancer control, B+S supplies special vacuum brake rollers, which do not come into contact with the top of the strips. Instead of clamping the material, as on tension pads, which could damage the surface and will certainly rub off coatings, the bottom of the strips is pulled onto the roller by negative pressure. This in turn uses regenerative braking, which hardly results in any relative movements between the strip and the roller. "This is the only technology that can guarantee that the surface will remain free of damage", says Baral.
Precision cutting and measuring systems for clean cross-sections
As another measure to protect the delicate product, all rollers which come into contact with the product on both system variants are coated with polyurethane, to prevent metal-on-metal contact. Moreover which slitting shears are used has a deciding role to play in the quality of the cut and the careful machining of the material. To this end. the precision shears from Burghardt + Schmidt are not only adapted to the material in question in advance, they are also equipped with highly-precise roller bearings and a play-free overcut setting, which ensures precise positioning of the blade at all times.
"This design for the shears, in conjunction with careful guidance of the material and the controlled strip tensions ensures that the quality of the outgoing strips is always equal to that of the incoming plates – regardless of how high the surface quality of the input material is," sums up the expert Baral. To check the quality of the cut – including burr, edge indentation, and deformation – Baral also recommends taking measurements with the newly developed EP 300 measuring instrument. Modern measuring technology now makes it possible to test samples of different lengths and thicknesses on site without extended preparation and without contact with the product. The systems are also under continuous development, so that a broader spectrum of samples can be tested with maximum precision. Already today it is possible to achieve resolutions of 2.2 x 10 µm to an accuracy of 1 µm using the Lateral Chromatic Imaging (LCI) .process. And thus it can be ensured that strips will satisfy the customer's wishes in spite of delicate surfaces and low tolerances.
(additional information is available online at: www.b-s-germany.de)