What’s a Drag Knife?
When you think about using your CNC router you probably think about cutting wood, non-ferrous metal, or plastic using a router bit. Did you know you can install a drag knife in your CNC and cut veneer, plastic, sandblast mask, cardboard, and more?A drag knife consists of a knife blade on a swiveling mount that goes into the spindle collet. The spindle is shut off for this operation (like a diamond drag). As the CNC moves through the X and Y axes the knife swivels to keep the blade parallel to the tool-path’s line of cut.
In this article, I’ll be discussing the Donek drag knives. There are other drag knives available, but I’ve had great luck with these products. This article provides an overview of drag knifes' capabilities and some insight into setup. We’ll look at specific cutting applications in future articles.
The Donek D3 Drag Knife has a ¼” shank. It’ll cut material up to 1/16” thick and will cut a minimum radius of 1/16”. The D1 Drag Knife has the same material and cutting specs, but comes with a ½” shank.
The D4 Drag Knife has a ¼” shank. It can cut material up to ¼” thick and cuts a minimum radius of 1/8” in materials 1/8” thick or thinner. In thicker material, the minimum cut radius equals the material thickness. The D2 Drag Knifehas the same material thickness and cutting parameters but has a ½” shank. Use a ½” shank drag knife when cutting tough, dense materials like Coroplast.
All of these drag knives use standard utility knife blades, but Donek recommends high quality bi-metal blades so they don’t break under the stress of use.
Setting up your CNC
Add a spoilboard to your spoilboard. It’s best to bridge over the t-tracks in your machine with a continuous spoilboard. After you have it secured with hold-downs, fly cut the new spoilboard to guarantee it’s perfectly flat. Since we’re cutting very thin material, small variations in the flatness of the spoilboard can make a difference. Fly cut an area large enough for the material you’ll be working on, but don’t run into the hold-downs.
Set the spindle speed to zero. Or, if you’re using a router, make sure it’s unplugged.
Even if you own a touch plate, set Z zero to the machine bed using paper. Slowly lower the knife in small increments, sliding the paper back and forth as you lower the Z. When you feel the tip of the knife drag on the paper, zero the Z-axis. The tip of the knife can break if you use a touch plate.
VCarve Pro includes a Drag Knife Toolpath gadget. This makes it very simple to use the drag knife. Cut depth is equal to the thickness of the material. Swivel depth is approximately half the thickness of the material. Blade offset is provided by Donek for each drag knife. This video does a great job of explaining the significance of Swivel depth and Tolerance angle and discusses adding the drag knife to your Tool Database.
If you don’t have VCarve Pro you can still use the drag knife by setting up standard tool-paths.
Select your design and open the 2D Profile Toolpath. Select the drag knife and set Cut Depth to the material thickness. Set Machine Vectors to cut on the line.
Click on the Leads tab and add a lead to the toolpath. A lead is like an on-ramp on a highway. It eases the cutter into the tool-path. This gives the swiveling knife the opportunity to align with the direction of the toolpath prior to cutting your project.
When you view the toolpath, you can see the lead and how the drag knife will engage in the material away from your project first. By the time it’s cutting your shape the movement will have swiveled the knife into alignment with the toolpath.
Getting square corners
Cutting a square won’t work with the 2D Profile Toolpath as it may not give you sharp outside corners unless we make some accommodations.
Open the Fillets tool
Select the Plasma/Drag Knife Fillet and set a ¼” Fillet/Tool Radius
Click on each corner of the square to add an external loop. The drag knife will follow this path, producing perfectly sharp corners.
Use the 2D Profile Toolpath, again adding a Lead to the toolpath.
Using Preview Toolpath (the drag knife was assigned a diameter of .03” in the Tool Database) you can see the lead and the sweeps around the corners.
Watch for future articles that will show you how to cut specific materials with a drag knife.