Shear Slitting and Roll/Coil Shear Slitting are the two most common methods used to Shear Stress Fracture metal webs. Both systems have two knives: one commonly called a Top Knife and the other a Bottom Knife.

Shear Slitting is the process of using two very sharp rotating knives to slice both faces of a moving web at the Cut Point, where the two knives are in contact with each other.

Roll/Coil Shear Slitting uses two less sharp and much wider rotating knives, not in contact with each other, to grip a moving web and shear fracture it, similar to tearing a piece of paper with your fingers.

Shear Slitting’s major contribution to the converting industry is that it has the ability to slit many more materials than any other slitting method, including thin metal foils, whereas Roll/Coil Shear Slitting’s major contribution is fracturing most metal industry webs, even up to 1 in. or 2.5 cm thickness. It also is good at slitting some stiff plastic webs.

Shear Slitting Top Knives more commonly are non-driven when mounted on individual pneumatic knifeholders. Their operating rotation is created by contact friction from the motor-driven Bottom Knives.

There are some web materials that require more precise Top Knife rotating speeds to meet tight finished roll specifications. Individually driven top knifeholders are available to deliver these highly accurate speeds with less rotation contact friction for longer knife life. They also can be mounted along high-precision manufactured mechanical or air-locking motor-driven knife shafts.

The Bottom Knives usually are mounted on and driven by a high-precision shaft as above or sometimes on individual motor-driven bottom knifeholders mounted on a linear rail positioning system.

Roll/Coil Shear Slitting top and bottom knives more commonly are stack-mounted on motor-driven, high-precision shafts rather than individual knifeholders.

There are many different Shear Slitting Top Knife designs to meet many different finished roll specifications. The two most commonly used knives are machined Flat Knives and stamped Dished Knives. A third less used but very effective design is the Wide Rim Machined Knife. These all use one or two ground bevels to lessen web contact deformation when fracturing a web.

The Shear Bottom Knives support the running web during the slitting process and also have a machined cutting-edge bevel (see Figure 1). Also seen is the increase of web-to-Top Knife contact at different Overlap depths.

For all slitting methods, a ‘to-be-slit’ web material’s composition determines where and how the slitting tools need to be set to meet customer slit-edge quality specifications. The composition includes its density, toughness, thickness, stiffness and elongation inclination.

A major difference between Shear Slitting and Roll/Coil Shear Slitting is the required dimensional set up accuracies employed for the two-knife web fracturing relationship. The knife-contacting Shear Slitting Positive Overlap settings commonly are made within one-thousandths-of-an-inch accuracy.

The non-knife-contacting Roll/Coil Shear Slitting ‘gap’ setting and adjusting range is within ten-thousandths-of-an-inch accuracy, and the Overlap can be Positive or Negative (see Figure 2).

Obviously, this article provides only a few major differences between the two metal slitting methods. Contact slitting equipment suppliers if interested in more detail. 

Dave Rumson
860-256-5658; drumson@maine.rr.com
ARC Member, ARC TV Presenter,
R2R Presenter, Converting School Educator