The movable gantry cnc machine that I am making would have a bigger working area than my previously made movable bed cnc machine. Therefore, using a drag knife to cut thin pieces of cardboard, plastic sheet and similar materials had become feasible for this machine.

Since this would be the first time that I am using a drag knife, I wanted to find out how the tool path would change according to the direction of drag knife movement.

With a normal rotary milling tool, the cnc tool path would just follow the outline of the design, and the tool would not break no matter where it was heading.



However, with a drag knife, the situation was different. The cutting action in this case would be a slicing action – with the sharp edge of the knife at a tangent to the slicing direction. If the cnc machine made a movement on the work piece with the direction of the knife at broadside position, this would most likely result in the breaking of the knife.

In order to find out whether the g-code generating software, dxf2gcode, could be used for drag knife application, I decided to make a test design, generate the g-code, and then examine the resulting tool path with LinuxCNC software.

As you can see from the video, the result clearly showed that there was some sort of transition in direction of the the tool path whenever there was a change of direction. Following the tool path, the drag knife would rise up slightly, make a small curve, and then plunge down again for the new direction.

In actual run, the knife would be raised up so that it would still be touching lightly the surface of the work piece material to be cut. This height would depend on the thickness of the material to be cut and had to be adjusted accordingly. With the drag knife still lightly touching the work piece material, when the small curve movement was made, the knife would be dragged along by the cnc machine, much like the wheels of a supermarket trolley, to change direction. Once the knife had changed direction, then it would be plunged back into the work piece material again for the next drag cut.

With this test, I am confident that the movable gantry cnc machine would be able to handle any project using a drag knife.