Part of what makes the CNC machine so versatile is the ability to incorporate different tools for various manufacturing needs. The most common piece for making cuts is CNC router bits, but you can use other tools to work with even more materials. One such tool is the drag knife. Learn more about the CNC drag knife, its many applications, and how to use it with your CNC router.
What Is a Drag Knife?
A drag knife cuts how you would use your dominant hand to wield a utility knife. It features a blade resting on a swiveling mount that goes into the spindle collet. When you turn on your CNC machine, the blade will swing around the bearing as it moves through the intended line of cut.
What Can You Cut with a Drag Knife?
Combined with traditional router bits, CNC machines enable users to cut wood, plastic, and non-ferrous metal. With a CNC drag knife, you can cut cardboard, sandblast mask, veneer, vinyl, and many other materials.
There are many types of drag knives available, and each allows machinists to complete different tasks. For example, Donek Tools is one of the most reputable drag knife brands. Some unique applications of the Donek Tools Drag Knife include cutting leather, wood veneer for inlays, and carbon fiber pre-peg laminates.
What Is a CNC Drag Knife Used For?
From experienced home hobbyists to shop owners with small and large-scale operations, machinists with different goals rely on the CNC drag knife. The following are just some of the uses for these cutting tools:
- Automotive, aerospace, and military applications
- Custom packaging
- Design and artwork
- Bases for snowboards and skis
About Donek Drag Knives
This guide discusses using a Donek drag knife with your CNC machine. Donek drag knives range from D1 to D4 with different capabilities.
The D1 drag knife has a ½ in. shank and has a maximum cutting depth of 1/16 in. and a minimum cut radius of 1/16 in. The D3 drag knife has the same material and cutting specifications but features a ½ in. shank.
The D2 drag knife has a ½ in. shank and minimum cut radius of 1/8 in. in thin material 1/8 in. or smaller. For materials thicker than 1/8 in., the minimum cut radius equals the material thickness. This drag knife is best suited for thicker materials like Coroplast. The D4 drag knife has the same parameters for material thickness and cutting but has a ¼ in. shank.
While all of these knives come with standard utility knife blades, it’s recommended you switch them out for higher quality, bi-metal blades. This can ensure your CNC drag knife doesn’t buckle under the stress of use.
Steps to Using a Drag Knife for CNC Applications
Knife cutting with a CNC router provides many benefits, from speeding up production to helping you reduce waste. Following these steps can facilitate a smoother experience and ensure you make the most of your CNC drag knife.
Preparing the Spoilboard 
To start, get a new spoilboard. This will be a temporary spoilboard put over your existing spoilboard. Secure the spoilboard with hold-downs, covering T-tracks and fastener holes. Then, you’ll want to make a fly-cut to guarantee flatness, since even small bumps can affect outcomes.
Setting the Spindle 
Unplug the CNC router and set your spindle RPM to zero, as indicated in the above photo.
Gradually lower the drag knife, sliding the paper back and forth while lowering the Z. Once the knife’s tip reaches the paper, zero the Z-axis. This step is important even if you use a touch plate because touch plates can break the knife’s tip.
Programming Your Toolpath
Software helps you map out your project. A popular program is the Drag Knife Toolpath gadget from VCarve Pro, as it makes it easier to use the CNC drag knife. With this gadget, the depth of cut is equal to the thickness of the material, and the swivel depth is about half the thickness of the material. If you use Donek drag knives, you’ll receive a blade offset.
To better understand the importance of swivel depth and tolerance, watch this video from Donek Tools. It also explains how to add your drag knife to your Tool Database.
If you don’t have VCarve Pro or prefer to use different software, you can still set up standard toolpaths for your CNC drag knife.
Working Within the Software
Once you choose your design, open the 2D Profile Toolpath. Select the drag knife and appropriate cut depth for the material thickness. Then set Machine Vectors to cut along the design’s outline. 
Next, you want to add a lead to your toolpath. Leads ease the blade into the toolpath, aligning the knife with your intended direction for the cut. 
You can view your toolpath to see the lead and how the drag knife will cut away at the material. Once the CNC drag knife begins cutting, the spindle collet will swivel the knife to align it with your toolpath.
Getting Square Corners
If you want to cut a square design, you need to adjust the toolpath because the 2D Profile Toolpath may not give you sharp outside corners. Follow these steps:
- Open the Fillets tool
- Choose the Plasma/Drag Knife Fillet and set a ¼ in. Fillet/Tool Radius
- Click each corner to add an external loop so the drag knife can follow the toolpath and create sharp corners
- Utilize the 2D Profile Toolpath to add a lead
- Preview the lead and corner sweeps using the Preview Toolpath feature to verify the design
Get More Tips for CNC Machining from Laguna Tools
With a drag knife, CNC machining becomes more advantageous for business owners and expert hobbyists. A CNC drag knife gives you the flexibility needed to expand your capabilities, whether you want to add veneer inlays for woodworking projects, cut leather for furniture, or incorporate lettering. For more on knife cutting with CNC machines, check out blogs from Laguna Tools. You can also contact us today to learn more about our product lineup.
