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Based on an earlier post I spent some time trying to determine how to calculate the screw penetration depth given the screw length. This is what I came up with:

Penetration Depth =SIN(RADIANS(75))*(ScrewLength-(0.073/SIN(RADIANS(75)))-0.48)

The formula is shown using Excel format, which also works for Google (Docs) Sheets.

This is the depth 90 degrees to the material surface - not along the angle of the screw, assuming that the Drill Bit Depth Collar is set by placing a U.S. nickel (5 cent coin) on the bottom of the jig base - between the bottom of the base and the tip of the drill bit. Note that this tip-to-base distance can vary considerably if you're using the Depth Collar guide on the jig (I have the K4 jig).

I checked the results of this formula against an earlier post that give depth as a function of screw length, and the results were close, but I think the method of using a nickel to set the Drill Bit Depth Collar provides a more predictable result.

ASSUMPTIONS

  • The published Kreg drill angle between the path of the drill/screw and the Kreg Jig fence is 15 degrees.
  • The angle between the path of the screw and the surface of the material is (90 - 15) = 75 degrees.
  • The Screw Length is from the flat base of the head to the tip, in inches.
  • The thickness of the 1976 U.S. Nickel that I measured was 0.073 inches. The official U.S. Mint thickness is 0.077, but that is not what I measured using a dial caliper. Maybe my nickel was a bit worn. I measured a 2013 nickel and found it to vary from 0.074 to 0.077. More likely my cheap dial caliper was varying, but you get the idea.
  • The length of the tip of the Kreg Jig drill bit from the tip to the step shoulder is 0.48 inches.
  • The calculated penetration depth is in inches.

Ken

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Here is the formula for the horizontal screw travel distance from the point where the screw exists the pocket hole material - the point where the screw enters the material that receives the screw - toward the direction away from the Kreg Jig base fence.

HorizontalScrewTravel =(SIN(RADIANS(75))*(ScrewLength-(0.073/SIN(RADIANS(75)))-0.48))/TAN(RADIANS(75))

The distances are in inches.

Again, this formula uses Excel and Google Sheets format.

If you are end-joining material, and the HorizontalScrewTravel is greater than half of the thickness of the material that receives the screw, then screw is predicted to exit the material on the side opposite the Kreg Jig base fence.

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