A guillotine shears or shears is a motorized or hand-operated machine tool for cutting of metal sheets , plastic sheets or cardboard boxes . When cutting, the material to be cut is sheared between a fixed and a movable knife in one operation.
The special thing about the guillotine shears is that they enable long, straight cuts. The cutting length is usually two meters, but up to 15 meters can be achieved. The material thicknesses that can be cut are between fractions of a millimeter and 200 mm, depending on the type of machine.
Before cutting through, the material lies on a straight surface - the machine table - and is clamped onto the table by the hold-down device in the cutting position . The lower knife is mounted flush with the table, while the upper knife moves straight down with link- guided scissors or is guided on a circular path with swing-cut scissors .
It is important that there is a cutting gap between the lower and upper knife . The smaller the gap, the better the cut surface, but the necessary machining forces and thus the load on the cutting tools also increase .
In order to obtain a cut surface at right angles to the material surface , the upper knife must shear through the sheet with an inclination of 2 ° to 3 °, since the line of action of the shear force is also not perpendicular. With swing cut shears this can easily be ensured by the position of the axis of rotation. Sheet metal that is pressed down on the front of the board remains very flat, while the section that falls down at the back, especially if it is narrow compared to the cut length, is bent and twisted. If this section is to be used, it usually has to be addressed. It is then also beneficial to use an inclined board to reduce the height of the fall.
The position of the cut on the sheet metal can be indicated by the shadow of a tensioned wire. Cutting gap and cutting angle can be set on good machines depending on the material properties (aluminum, steel) and thickness.
In the case of lever guillotine shears , the upper knife is screwed onto a long movable lever. Cutting is done by swiftly swiveling the lever down by hand - the required high instantaneous power can only be achieved through momentum (moment of inertia). Balancing the lever in its upper position (scissors open - risk of accident) is done by a counterweight, which at the same time increases the flywheel. On motorized scissors with a flywheel, the re-acceleration can sometimes be heard immediately after the cut.
To protect against slipping when cutting, the material is clamped by a hold-down device. The hold-down device can be driven by hand or by machine (pneumatic, hydraulic). Plate shears have a (sometimes motorized) adjustable stop up to which the sheet is pushed before it is held down.
- Reimund Neugebauer: Influences on the cut part quality when cutting sheet metal on guillotine shears . EFB, Hanover 1999
- Heinz Tschätsch: Machine tools for non-cutting and cutting shaping . Carl Hanser Verlag, Munich / Vienna 2003, ISBN 3-446-22141-7 , p. 361