Drag Knife Plotters - Edward Mathias & Co

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Drag Knife Plotters

Info. & Technical
DRAG KNIFE CUTTERS
Sometimes called swivel knife or trailing knife cutters.
Tangential plotters are far superior but drag knife plotters are more common because they have fewer moving parts, are less expensive to manufacture and therefore cheaper to buy - their ONLY advantage.

DISADVANTAGES OF DRAG KNIFE CUTTERS
Drag knife cutters are inherently inaccurate because unlike tangential cutters they have no ability to “steer” the blade when it is cutting and no ability to rotate it when it is lifted out of the media.

HOW THE DISADVANTAGE IS OVERCOME
To make them follow the cut path drag machines use blades which are designed to castor freely and their cutting points are offset from their axial centres so just as flags and windsocks are dragged downwind of their poles (their axial centres) drag blades are dragged into line by the cutting resistance of media.
An example of this action is the castors on a chair which are offset from their centre of rotation and will change direction to “trail” as you “drag” the chair around the floor.

SOLVING ONE PROBLEM CREATES ANOTHER
Offsetting the cutting point of a blade and allowing it to castor freely solves the problem of making it follow the cut path but it creates a different problem - offset blades cannot cut sharp corners.
Unless they are cleverley manoeuvred to compensate for the inaccuracy that offset creates.

WHY?
When a drag blade reaches a corner its axial centre (the point around which it rotates) arrives before the cutting point and unless there is some means of causing it to do so the cutting part of the blade would never reach the corner and would truncate it thus creating a radius instead of a sharp corner.
Just as the rear wheels of your car will mount the kerb at corners if you fail to compensate for the distance by which they are offset from the front wheels.

SOLUTION
Drag knife plotters have a built in routine to manoeuvre the blade in a way that compensates for blade offset.
 
BUT ONLY IF YOU SET IT CORRECTLY - - - - See:- CUTTER OFFSET
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