Drag Knife

A while back, we decided to order a drag knife for the CNC after seeing it being used online. Originally we talked about cutting some leather to make bags that could be cut on the CNC with the drag knife and then sewed together on the Sailrite machine which can handle thicker materials / multiple layers of fabric. But sometime inspiration strikes and you start somewhere else....

A teacher stopped me at lunch earlier this week and asked about creating scenery and props using the large format Roland Tru-Vis printer we have in the makerspace. They imagined using cardboard as a backing material which usually would be cut on the laser cutter. The downside to that idea is that the maximum size piece of cardboard we can cut is 32" x 20". That meant we would need to attach multiple sheets together and reinforce them somehow. On a walk later in the day, I was thinking about the project and an idea to use a large sheet foam core board as the backing material and cutting it with the drag knife emerged.

I decided to start by learning to install and set up the drag knife "bit" on the CNC. The Donek drag knife had some simple printed instructions as well as some online documentation. VCarve was something I have just started learning and recently completed a cutting board with inlay milled using a v-bit so the tool was becoming more familiar. VCarve has a gadget menu with extensions and offers a drag knife tool that made it fairly easy to set up the tool. The first challenge was learning how to avoid having the drag knife spin. Typically bits are run at 18,000 RPM and in this case the goal was zero. VCarve didn't seem to allow setting the speed to 0, but did allow for 1 RPM. When I generated the first test toolpath and loaded it on the CNC, the bit definitely was spinning at a low speed. I was able to manually turn that off in the Velocity software so it followed the toolpath, but didn't spin which worked great. The first test cutting through cardboard worked fairly well.

An additional challenge with the Drag Knife gadget in VCarve was getting the toolpath to handle sharp angles and control how the tool enters the material. I kept having toolpaths that started inside the desired section of material to be cutter rather than the desired outside of the part. I was not able to find a way to alter the toolpath or gadget to get the path to flip to the outside. I think it may be possible to get this to work by adding an additional shape outside of the desired shape. I plan to try this later....

11/2/2022 Update: Did some research and learned to change where the drag knife begins cutting. Interestingly, by moving the entry point to the right side of the object to be cut, the blade starts outside of the desired artwork. Yesterday, I worked with one of the 5th grade teachers to create the large tree (shown at right) that will serve as a background set piece for their musical. We were able to successfully print and mount an image of a tree on to a piece of foam core board. The drag knife worked beautifully to cut out the tree in two pass of 0.1" each (0.2" total). The biggest ongoing challenge continues to be setting the exact location of x and y starting point becuase the blade swivels and is offset from directly under the spindle. For now, the best option is to get as close as possible and to move the cutting vector line slightly inside the desired cut so that the entire cut is slightly smaller than the target image. This worked fairly well for this tree and reduces the amount of background image that could potentially be missed by the blade due to inaccurate setting of the origin.