Screw Penetration Calculations

[Ken Kortch]

So bear with me on this ... I got a new Kreg Jig Master System for Christmas and I've yet to get to use it, but I was playing with some calculations ...

I was trying to calculate how deep the screw penetration is for the different board thicknesses. 

From what I can find online the angle of the pocket hole is 15 degrees. From measurements on the kreg jig, the narrow tip on the drill bit is 15/32" long, and the tip tends to stop about 1/16" from the bottom of the jig. 

So, that means 15/32" + 1/16" = 17/32" of the screw length is embedded in the pocket hole side of the joint.

For a  1-3/8" thick board I should use a 2" screw.

The 2" screw would penetrate 2" - 17/32" = 47/32"   or 1-15/32" or 1.4688" into the lower board (that has the screw thread - not the screw head).  

Because of the 15 degree angle, the vertical distance of the screw penetration (in the direction of the thickness of the board) is less than the actual screw penetration. It is 96.6% of the actual screw penetration, which means 0.966x(1.4688") = 1.4187".

But a 1-3/8" thick board is 1.375" thick. So that would mean that the screw with 1.4187" depth would actually pop out the bottom of the board. Huh??

Sorry for all the math, but what am I doing wrong? 

... and feel free to make fun of my silly calculations if you wish. I deserve it.  :-)

[John Osborne]

Ken,

 

Look on a Discussion on Jan 28, 2010 "Scew size fo different sized lumber" and scroll down to Ken's answer on 1/31/2010 @ 12:20.  Ken is with Kreg.  He has posted a screw/lumber chart that will help you out.

 

JohnO

[Hugh Clare]

I wouldn't dream of making fun of your calculations. If you tell me that you're an engineer, though, it's game on...... :)

I don't have the benefit of any training in trigonometry and my last geometry class was more than 30 years ago, but I'll take a SWAG at it.

Assuming your measurements are correct, a 15° deflection would net a 1:3 deflection/penetration ratio. That would make 94.9% ratio (perpendicular depth/screw penetration). 1.4688 X .949 = 1.394. Still .019 too deep. HMMM. That is scarcely over 1/64, but we'll keep running.

You said the bit "tends to stop about 1/16 inch from the bottom of the jig". How did you arrive at that dimension? What are you measuring with? What's its resolution and repeatability? Is it calibrated? I could go on....... Please don't take what I'm saying as mockery. I am being absurd to make a point.

What I think you're doing "wrong" is applying too much precision to the tool. Certainly you want to keep in mind that 3/4 plywood isn't really .75. Adjust your stop collar to compensate for that. Remember that screw penetration is also a function of the torque applied to the screw. The density of your wood is a factor. Those factors will render your calculations moot. 

I work on turbine engines. Measuring to four or five decimal places is necessary. I unwind by working with wood. My brother is a drywall contractor. Precision in that field is 1/8 inch. He's always reminding me that I'm "not building an airplane". I just use the chart that the Kreg engineers published and make my "tribal knowledge" notes there. Let your hair down and build something. :)

[Jay Boutwell]

For better that 20 years I have used the kreg jig in a professional cabinet business.  What I have see over this last two or three years is the set up of a jig drill bit has become a complicated item of which users keep confussing themselves about how to set the debth collar.  My method is simple as can be and has not let me down in all the years of using it and believe me I have drilled litterly thousands of holes in all types and thicknesses of wood.  I have found that using a nickel to set the debth of the bit is the most fool proof method yet.  I doesn't matter the model of the jig as they all perform the same task.  If your drill bit exits the material you are boring it is too deep,  If you set the bit up using my method it will not exit the stock that is properly set on clean  base of the jig.  The reason you do not want the bit to exit the stock is not only to prevent it from boring into the jig base but prevent the bit from making a dimpel on your stock that causes your mating surface to have an obstruction on the end.  This obstruction prevents a percise joint and had been the reason for some joints to be less than perfect.

Hugh is correct when he says "applying too much percission to the tool".   I like Hugh have mechanical experience as I was an automotive mechanic-machinest where I  built many high performance engines and transmissions.  I can relate to what Hugh is saying.  Sometimes you have to alter the information you are given with the tool in order to make it work.  In building high performance engine I could not set up a race engine to the manufactures specs as it would fail.   Simply there are those whom write the books and thoes whom have to make it work.  Somtimes the two do not work the same way.  From experience I'm telling you that if you take a nickel coin from your pocket set it on the base of the jig and lower the bit into the jig and set that stop collar at the top of the jig you will have what you are looking for a drill bit debth. 

If you screw is still exiting the bottom of your stock and your stop collar is still correct as explained above, then the screw you are using is wrong for the thickness of the stock.  Since there is limited lengths of screws made for the kreg jig I simply shorten the screw to fit the thickness of the stock by nipping off the end of the screw.

[Ken Kortch]

Hi, thanks for the insight. 

I was actually using a dial caliper for measurements - its graduated in 1/64ths of an inch ... roughly :-)

I measured the 1/16" distance from the drill bit tip to the bottom of the jig by just dry fitting the bit in the jig for several different width settings.

I think that is what Jay B. is talking about with the nickel - just lay a nickle at the bottom of the jig, put the bit in so it touches the nickle, then tighten the collar so it touches the top of the jig. BTW, measured a nickle and its 1/16"+1/128" thick (I know this because on the caliper dial it is half a notch above the 1/16" mark. That's a great idea. Any concern about dulling the bit tip doing that??

What I was focused on is whether or not the screw would poke out the other side of the lower board. I suppose in the end I need to leave all the math behind and just have faith in Kreg's recommendations.

[Ken Darga]

Ken Kortch

When you made the measurement,

was the dimensional reading "below" or ''above'' the line-mark on the scale?

[Ken Kortch]

For the measurements I took above it didn't involve measuring to any jig lines.

I just measured the length of the narrow bit tip (15/32") and the gap from the tip to the bottom of the jig (about 1/16") when dry fitting the bit with collar attached . The screw length is from the charts.

The 96.6% comes from the fact that (for example) the tip of a screw that penetrates 1" into a board at a 15 degree angle will be 0.966" from the top of the board (that is the hypotenuse of the right triangle).

BTW, I hated math in school, but now I like it - its like a puzzle to me. I couldn't do it without the help of internet angle calculators and such.

[Jens Jensen]

sure is hard to beleave that a simple screw can affect a persons woodworking . Just set the jig and go to work

[Ken Kortch]

Good advice!  Thanks

[Stanley C. Pearse]

Here is a response from Kreg Tool for to a similar question that I posted in 2009.

 http://kregjig.ning.com/forum/topics/is-there-a-kreg-jig-screw?comm...

[KregRep]

See this PDF for a bit more information on this topic.

[Jay Boutwell]

Kreg Rep, that is one of the better explaination that I have seen in a while about screw penetration and debth.  It was well presented and is great documented chart to make a shop copy to refer to.  I also viewed the video and found some good information on joining different thickness of stock.  Thank you for posting this.
 
KregRep said:

See this PDF for a bit more information on this topic.

[Ken Darga]

Thanks Much!

This will be a great help to many.

KregRep said:

See this PDF for a bit more information on this topic.

.....>>>>