How to Keep Cut-to-Length Lines Within Tolerances

(Cut-to-Length (CTL) lines - Source: Herr-Voss)

Cut-to-Length (CTL) metal cutting operations have customers placing increasingly tight tolerance requirements upon them. Formerly the cut-to-length lines only need the resulting metal blanks to conform to the final product dimensional needs. Now there are other machines and operations that expect metal blanks to arrive within +/- .015 inch (0.380 mm) or even as tight as +/- .005 inch (0.130 mm). This new demand for precision means that cut-to-length metal slitting lines have to apply tighter tolerances throughout along with specific technology for keeping the blank shapes consistent throughout.

Gripper feeds are one of the main tools for achieving higher tolerances. We will cover how gripper feeds assist with consistent production accuracy after we delve into what we mean by tolerances on cut-to-length production lines.

Cut-to-Length Metal Slitting Line Tolerance Parameters

There are no perfect parts from metal slitting lines, whether they are cut-to-length operations, or full coil operations. All the products have some degree of camber deformation.

Camber for coil operations shows the devil is in the details. Camber is how far off the side edge of a strip from a straight edge. There are different types of camber that affect semi-continuous metal slitting operations.

Minimizing Camber for Sweep and Snake for CTL operations

Sweep is a more or less uniform curvature along the edge of a metal strip. Snake, as you might imagine, snakes back and forth along the strip edge. Wider metal slitting lines usually experience the sweep form of deformation.

Snaking is a problem that tends towards narrow slitting operations, and is generally less common. Both of these imperfections come from strips with uneven edge lengths.

Cut-to-Length Problem Blanks

There are no exact squares coming off metal coil slitting cut-to-length lines. Every blank either has some property of a  parallelogram or is cambered with a definite curvature.

A parallelogram metal blank has width and lengths even, but the diagonal measurements are different. Cambered parts have a constant width but the edge lengths are different.

Both of theses forms of deformed parts will be present depending on the errors within the coil or the line. Camber is always present making a straight parallelogram impossible.

Achieving Close Cut-to-Length Tolerances

The American Society for Testing and Materials (ASTM) defines out of square tolerances based on half the difference of one side's deviations from a 90 degree line at each corner.

The more usual way of achieving this square tolerance level is to simply measure the diagonals and make sure they are equal within tolerances. ASTM's standard for camber from a coil larger than 12 inches is 1 inch in any given 20 feet (25 mm in 6.1 m).

Grip Feed Measuring for CTL Lines

Sometimes referred to as hitch or side feed systems, these grip feed measuring systems have been around for years. Aside from assuring accurately produced cut-to-length metal blanks, they also prevent damage to the presses or shear.

These systems can provide similar tolerances as automatic press feed devices. Theses work by hydraulic actuated and mechanical linked grippers delivering a precise sheet geometry to the high-speed CTL/blanking lines.

Grip feed systems are able to achieve 250 fpm +  feed speeds at the accuracy level required.

Understanding how to keep CTL within tolerances, as well as what the common errors are, can keep precise blank production flowing as the metal slitting line was designed to do.

To return from this Cut-To-Length page to the Strip Metal Coil Processing Home page, click here.

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