Understanding Kerf

General Description

AMS defines Kerf as the amount of material that is removed during the shear process. Often called the “slug”, this small amount of scrap material has to be accounted for by the microprocessor (controller) on each part, or part lengths will be wrong. Even though part lengths may be wrong, they’ll be consistently wrong.

In order to simplify part programming for operators, AMS includes a Kerf parameter in all AMS controller models. Once the Kerf amount is known, the number is entered into the controller and the operator only has to program finished part lengths without having to calculate for the loss of material. Each time a different die is put into the press, a different Kerf number should be entered, unless the dies use the exact same Kerf numbers down to the thousandth of an inch.

Finding Kerf

Kerf is often measured from the slug, the blade thickness, or the slot the slug is pushed through. These methods are incorrect. The only accurate way of measuring Kerf is to step through the following procedure:

1. Jog some amount of material through the shear and cut it out through a standing crop. The amount of material cut in this manner should be easily measurable down to the thousandth of an inch. The length is not important. It should be short enough to fit in whatever measurement device will be used, but long enough to be easily handled.
2. Measure the piece. Accurate measurement is important for accurate part lengths.
3. Back the material away from the die (if possible) or cut the material away.
4. Manually push the measured piece back through the cutting die.
5. Perform another manual crop with the measured piece sitting inside the die. This should leave two shorter pieces.
6. Measure the two shorter pieces as accurately as possible and record their lengths.
7. Add the lengths of the two short pieces together and subtract their total length from the length of the first piece. This is the number that should be entered into the Kerf parameter.

Symptoms and Troubleshooting

A classic Kerf problem results from entering the wrong Kerf amount into the controller. The problem manifests itself as a length problem that is correctable, but only for one length. For example;

The operator finds that 50” parts are coming out 0.25” short. He calibrates the machine for that length and finds that the parts come out consistently 50”. He then changes the length to 100” and finds that all the parts come out 0.5” long. He calibrates the machine to the new length and finds that the parts come out consistently 100”. He then goes back to the old length of 50” and finds all his parts are coming out 0.5” short.

When the Kerf is entered incorrectly, part lengths will come out consistently, but they will be consistently wrong. This error can be corrected with a length calibration, but the calibration is only good for one length. Once the length changes, the measured length will be out of tolerance, again.

If this is the case, check to be sure the correct number is entered for Kerf, or step back through the above procedure for measuring Kerf and enter the correct value into the controller.