Metal Slitting and Rewinding Line

Overview

A metal slitting and rewinding line takes a wide master coil, runs it through a bank of circular knives that cut it lengthwise into several narrow strips, and winds those strips back into separate finished coils. It is the standard way a steel service centre turns a mill coil of, say, 1250 mm width into the 60 mm or 120 mm widths that a stamping plant or a tube mill actually buys. The line works as a continuous web process: the strip is unwound at one end, kept under steady tension through the middle, slit on the fly, and rewound at the other end, so the whole coil passes through without stopping once it is threaded.

In process order the strip passes through the Double-Mandrel Decoiler, the Entry Pinch and Leveller pinch and leveller, the Slitter Head, the Tension Stand and Loop Pit and its loop pit, the Overarm Separator, and finally the Recoiler. The Scrap Winder takes the edge trim away, the Main Drives turn the machinery, the Hydraulic Power Pack powers the clamping and lifting cylinders, and the PLC Control Desk coordinates speed and tension across every section.

Decoiling and entry

The master coil is loaded onto the Double-Mandrel Decoiler, a turret that carries two Expanding Mandrel units so the operator can load the next coil while the current one runs. A Coil Car lifts the coil from the bay and aligns its bore to the mandrel, which then expands hydraulically through its Mandrel Segment leaves to grip the bore from inside. A Peeler and Threading Table lifts the lead end off the wrap and feeds it onto the threading table.

From there the strip enters the Entry Pinch and Leveller section. A pair of Entry Pinch Roll rolls draw it off the decoiler, and the Roller Leveller flattens it. The leveller bends the strip past its yield point back and forth between staggered Leveller Work Roll rolls, which removes coil set and edge wave so the strip lies flat going into the knives. Flatness matters because a strip that is not flat will not feed squarely and the slit edges will not be true.

The slitter head

The Slitter Head does the cutting. It holds two Arbor Shaft shafts, one above the other, each loaded with a stack of Circular Slitting Knife knives, Arbor Spacer rings, and rubber Stripper Ring rings. The knives on the two arbors interleave so that an upper and a lower knife overlap slightly at each cut line. The strip is sheared between the two edges in the same way scissors shear paper, not torn or ground, so the slit edge is clean if the knife clearance and overlap are set right.

Slit width is set by the spacers: the widths of the spacer rings between the knives fix where each knife sits along the arbor, and so the width of each strip. Changing a job means restacking the arbors with a different spacer and knife arrangement, which is why the head uses a Knife Cassette that can be preset off the machine and swapped in. The stripper rings hold the strip down and push each cut ribbon off the knife so it does not climb and jam. The outermost cuts trim a strip off each edge to square the master coil; this edge trim is pulled off by the Scrap Winder and wound into scrap coils.

Tension and recoiling

Slit ribbons must be wound under even tension or the finished coils will be loose, telescoped, or uneven across the set. Between the slitter and the recoiler the strip runs through the Tension Stand and Loop Pit, where Bridle Roll rolls build running tension by friction and a Loop Pit lets the strip hang in a free loop. The loop and its Dancer Roll absorb the small speed differences between slitting and rewinding so neither section starves or slacks. A bank of Felt Tension Pad pads adds a light, even drag to each ribbon so all of them wind at matched tension even though they are now slightly different lengths.

Before the recoiler the Overarm Separator fans the ribbons apart with grooved Separator Disc discs so neighbours do not rub. The Recoiler then winds them. Its Recoiler Mandrel expands to grip the strips, a Strip Separator keeps each ribbon in its own track, and an Outboard Support swings up to carry the free end of the mandrel so it does not deflect under a 25 t coil. A Pressure Roll presses the strips onto the building coil to wind it tight and flat.

Drives and control

The line is driven by the Main Drives. A Slitter Drive Motor turns the arbors through the Reduction Gearbox, built from a Gearbox Housing and Helical Gear Pair sets that drop motor speed and raise torque. The Decoiler Motor and the Recoiler Motor hold tension at the two ends, with the recoiler tapering its torque as the coil grows so tension stays constant from empty mandrel to full coil.

All of this is run from the PLC Control Desk. A PLC sequences threading, running, and coil change. A set of Variable-Frequency Drive drives set the speed of each motor, and the Tension Controller reads the Tension Load Cell cells and the dancer position to trim torque in a closed loop. Line Encoder encoders report speed and strip length so the line stops a coil at the set length, and the operator works from the HMI Touchscreen screen.

Materials and use

Slitting lines run carbon steel, stainless, aluminium, copper, and coated stock from about 0.3 mm to 3 mm thick. The knives are hardened Circular Slitting Knife discs of D2 or similar tool steel, since they shear millions of metres of strip between regrinds; soft or sticky metals like aluminium need different knife clearances and sometimes lubrication. Service centres run these lines to convert mill-width coils into the exact widths their customers order, which is cheaper and faster than buying narrow coil from the mill. The same line feeds tube and pipe mills, roll-forming lines, and stamping plants, all of which need strip cut to a tolerance the master coil does not hold. Output quality is judged by burr height on the slit edge, by camber along the strip, and by how tight the finished coils wind, all of which trace back to knife condition, clearance setting, and the tension held across the line.