Proper Way to Hold a Duct Board Kerfing Tool

Kerfing tools are designed for hand fabrication of fiberglass duct board to make HVAC ducts out of fiberglass insulation.  Kerfing tools contain sharp blades which are run through the interior layer (black side) of the fiberglass board to make grooves or shiplap cuts.  Once all of the cuts are made, the fiberglass board is then folded together and stapled/taped shut.  There are a few different ways of making this fabricated duct board, but this short blog post is to help correct or teach users the proper way to hold a duct board kerfing tool.

Incorrect Way to Hold a Duct Board Tool:

When you open up the box and remove the colorful tool you will notice that the tool has a convenient handle.  Your first thought is going to grab the tool by the handle like you would do with any door or drawer handle.  This is, however, not the proper way to hold a duct board kerfing tool when using it. Running a duct board through the fiberglass board takes force and pressure to keep the tool flat against the table.  If you held the tool as shown in the image below (on right) you will find that the tool will want to dip from front to back when running through the glass.  This will cause undesirable wavy cuts.  You will also wake up the next day with severe pain in your thumb and the inside grip of your fingers the next day.

Correct Way to Hold a Duct Board Tool:

Rather than grabbing the tool by the welcoming handle, you will want to cup the tool in your hand with the bottom of your palm on the bottom half of the handle.  The pointer finger should be positioned on the top half of the handle with your thumb and fingers grasping the sides of the tool.  When running the tool through the glass keep in mind that you want to keep the tool as flat to the board as possible while using your weight to push the tool through the cut.  At the very end of the cut near the top of the board it is important to keep the tool flat and not ease up and allow the blades to lift.  Allow the blade to run through the very edge of the board before pulling up on the tool.