Edward Mathias
& Company

ADVANTAGES OF DRAG KNIFE CUTTERS

Sometimes called swivel knife or trailing knife cutters.

Ideally all plotters would cut tangentially but drag knife plotters have become more common
because they have fewer moving parts and are less expensive to design and manufacture.

Also, because they do not take time to lift, rotate and drop the blade at corners some drag

knife plotters may cut faster than some older tangential plotters.

DISADVANTAGES OF DRAG KNIFE CUTTERS

They have no ability to rotate the blade when it is out of the media.

To make them follow the cut drag blades are designed so that 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.

Another example of this action is the castors on a chair which are offset from their centre of rotation  
and will change direction to “trail” behind as you “drag” the chair around the floor.

Offset is a physical characteristic of a blade that causes it to follow the path of the cut.

To ensure it follows the cut path a drag blade must be free to rotate/swivel/castor.

And its cutting point must be offset from its axial centre.

Without offset a drag blade would rip media by cutting backwards or sideways.

BUT - - -  OFFSET BLADES CANNOT CUT SHARP CORNERS

Unless  they are cleverly manoeuvred to compensate for their offset

When a drag blade reaches a corner its axial centre arrives before the cutting point and unless there is some means of causing it to do so [offset] 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 trail behind the front wheels.

Drag cutting is an inherently inaccurate process

Cutter offset compensates for most of that inaccuracy.

BUT ONLY IF YOU SET IT CORRECTLY - - - - See:- CUTTER OFFSET