The Movable Gantry CNC Machine will have a much larger cutting area than my Movable Bed CNC Machine. So this would, in fact, give me more flexibility in terms of the jobs that I could handle. What I could think of at this time was the use of a drag knife or a tangential knife for cutting thin sheets of cardboard, vinyl, cloth or other similar materials. I would certainly want to try this out when my movable gantry cnc machine was ready.

Gathering information from the internet, I came to the conclusion that there were some distinct differences on how these knives work. The drag knife turns on its own while the tangential knife had to use a stepper motor and programming to turn to the required angle. Since the tangential knife was more complicated, I would not want to delve into it at this moment.

The drag knife worked like the back rollers of a supermarket trolley. The rolling action of the back wheels, being off-centered from the side thrust turning axis, was able to make the wheels rotate in the direction of the side thrust. However, just like the supermarket trolley, there would be a certain position of the wheels when the wheels would not behave as required.

In the drag knife, if the blade were to be caught immovable deep inside the material to be cut and is then required to change direction, there was a likely chance that the blade would break. In order to avoid this phenomenon, the software that generated the gcode had to add in some additional codes before changing direction so that the knife would move up very near to the surface of the material, change the direction, plunge deeper back again, and then proceed with the normal gcode movements.

Since the direction of the drag knife was dependent on the movement of the machine, the knife blade must always touch the material to be cut when the movement was made. Careful selection of the contact depth of the knife blade when changing direction was essential.

The parts of the drag knife was simple. Essentially, it consisted of a vertical shaft with a means to attach an offset knife blade, being free to rotate on ball bearings following the movement of the cnc machine. I have looked at how commercially available drag knifes work but they were too expensive for a hobbyist like me. The Grunblau drag knife available as a plan seemed a good option, but since it still required some aluminium machining-fabrication from me, I decided to design and fabricate my own design.

You can call it the “Yoon Drag Knife”. The essential components of a drag knife was still there, but I felt that making the moving parts out of steel would suit me better since I wanted a strong material with a minimum of machining. I also had an arc welding machine which I felt could be used to make reasonably strong joints. Any other non-stressed features like locators for the knife could be done using metal strips glued by epoxy resin. Rough edges, arc spatter or other visual defects could be cleaned up by a hand grinder or file and any holes could be drilled using my portable drilling machine.

The Yoon Drag Knife was designed to fit in any normal 60 degree angled blade bought from the local hardware store. The shaft was a short length of 8 mm diameter stud which was ground flat at one side in order to coincide with the center line of the knife blade when the blade holder was in place. These were welded in place. The whole assembly would be placed in the bearings and held in place by nuts.

The ball bearings would be clamped in place by bearing housings machined out from 11 mm plywood by nuts and bolts. These would be the only machining that I needed to do on my movable bed cnc machine. Separately, the complete bearing housing unit would then be secured to the z-axis spindle holder of the movable gantry cnc machine by nuts and bolts. Thus, I eliminated the use of a shaft at the opposite end of the drag knife which other designs had, because I felt that this was a weak point. I did not have a lathe to ensure the centering of the shaft was true. When in used, I just needed to remove the spindle and replace it with the drag knife assembly.