Drag Knifes are really handy for cutting soft materials like vinyl, cardboard, mat board, foam, wood veneer, leather, and many others.  With flat sheets, you might use one to create some custom packaging for some product, for example, or at least to prototype the packaging. Commercial drag knifes are readily made, but can be a bit expensive for the hobbyist.  I recently came across a great CNCZone post for this drag knife:

DragKnife1

Drag Knife by Grunblau…

Here it is broken down into component parts:

DragKnife2

Drag Knife components…

As you can see, it’s pretty simple and would be pretty easy to build.  The main thing in designing one is to hold the blade so it has a vertical dull edge that is fairly close to the axis of the shaft but still off axis enough to have a little castering effect.  There are fancier drag knives available that even put a 4th axis servo to control knife direction, but you can do a lot with a self-steering drag knife.  Sometimes the simpler solutions are best.

Programming one of these drag knives may require you to perform what’s known as a “corner action”.  In order to make a tight corner, you want to raise the blade up so the tip is barely touching the material–just enough so the material can barely grip it as it turns in the bearings. Now program the knife to move in a small circle around the tip to re-position it in the direction you want to move.  Drop back down to cut depth and you’re ready to head out.

If you’re willing to radius all the corners, there’s no need for the special corner action moves.  Without corner actions, most any CAM software will give you a decent result with a Drag Knife.  Just use an engraving style toolpath that follows a line in your drawing.

We’ll close with a nice YouTube video of the project:

Note his use of WD-40 when cutting aluminum.  Good idea!

Tip: Check our library for free DXF files that you can cut out with a drag knife.

No time to make a drag knife?  

No worries, they’re available commercially.  Donek is probably the best known brand.

The video demo shows some pretty amazingly sharp cuts being made at high speeds because the knife orients itself.  It takes ordinary utility knife blades and comes in 2 sizes.

Donek founder Sean Martin

Donek founder Sean Martin invented the Donek Drag Knife to cut graphics for the custom snow boards he was making on his Shopbot CNC Router.

 

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