Drag Knife Plotters
Info. & Technical
DRAG KNIFE CUTTERS
Sometimes called swivel knife or trailing knife cutters.
Tangential plotters are superior but drag knife plotters are more common because they have fewer moving parts, are less expensive to manufacture and therefore less expensive to purchase - their ONLY advantage.
DISADVANTAGE OF DRAG KNIFE CUTTERS
Tangential cutters are inherently more accurate because they have mechanisms to “steer” the blade to point in the correct direction.
The disadvantage of drag knife cutters is that they need to point the blade in the correct direction but they do not have mechanisms to "steer" it.
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 another problem - offset blades cannot cut sharp corners unless they are correctly manoeuvred to compensate for the inaccuracy that results from using blades that cut behind the point about which they rotate.
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 thereby 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
BUT ONLY IF YOU SET IT CORRECTLY - - - - See:- CUTTER OFFSET