Kreg Jig Mini Measurements?

[Dan Coes]

Hi All...

I purchased the Kreg Jig Mini today, planning on joining some 1" (actual thickness) hardwood stock. Is there a chart that lists jig position and depth collar information for stock with an actual thickness other than 3/4", 1/2", and 1 1/2"?.

I've spent the last hour scouring Google and these forums, and cannot for the life of me find any information about this. Without this simple information the jig is basically useless to me :(

I would drill some test holes to try and figure it out, but I'm making a table out of an old door and therefore don't have any extra wood to use for testing...

Thanks for any help you can provide!

-Dan Coes-

[Joe Wickline]

John,

Sorry for posting to this old thread but like others I too having purchased the Kreg mini-jig and set out to solve this trig problem.  I was in process of doing a search to see if jig dimensions (those that you provided as “z” - 70 mm and “x” – 35.45 mm) were public knowledge anywhere to ensure my calculations were correct when I came across your post.  I believe there is an error in your sketch however in that you should reverse positions of “w” and “≈x” where they appear along the drill bit centerline such that the sketch will align with the assumption you stated as:

• “that the depth of wood left from the bottom of the pocket to the timber face is equal to the thickness of the timber”

The way you have it sketched is showing the depth of wood removed as equal to thickness of lumber when in fact the “face” being referred to in the assumption is the face of the joint and not the face where the bit first enters the wood.  Reversing positions of these two variables makes sense because it is the wood that remains (not wood removed) that resist tear out of the joint.  It is also why the problem is not as simple as adding 70mm to the thickness of wood being used as one reader suggested (although that reason would align with the drawing provided).  In fact ≈x must be calculated as I supsect you have done.

The other assumption needed to solve this problem with certainty (which you show in your sketch and use in the equations but did not explicitly state) is:

• “the longitudinal axis of the screw should pass through the midpoint of the drilled wood’s thickness”

Starting with dimensions you provided together with both assumptions it’s easy to visualize that Y is defined by the position necessary to make the centerline of the bit pass through the midpoint of thickness (using mm for constant and degrees for the angle*):

• Y= ((1/2W) / Tan(15)) – 35.45 mm
• * noting that Excel trig functions require conversion to radians in place of angles

then, having defined Y, solve for “≈x” necessary to add to 70 mm such that it leaves “w” length of wood to resist tear out (again using mm for the constant and degrees for the angle)

•  ≈x = ((x + y) / Cos(15)) – w
• Collar offset = ≈x + 70 mm

Finally these calculated values give the ideal dimensions for setting the stop collar and jig offset which will be rounded (down for ≈x  and up for Y to avoid screw protruding through second workpiece) to the nearest measuring tape increment.  Then screw choice would be the longest standard screw available that does not exceed 2 times “w”.

I’ve restated equations above since I come up with slightly different values than your original spreadsheet.  I wasn’t sure if those differences though were attributable to equation differences or simply due to rounding choices (metric vs. imperial or decimal vs. measuring increments) and hence the request that you confirm the above is what your spreadsheet uses or repost a new version of your very well formatted spreadsheet as may be appropriate.  I'll attach my more rudimentary spreadsheet in case it makes that easier to do.

KregMini_Joe.xlsx

John James said:

Hi Peter again,

Having cracked my spreadsheet passwords, you're quite right; I had mis-typed the two formulae noted on the spreadsheet, though the actual calculation was right*.  I've updated the spreadsheet, and uploaded the revised version together with the diagram, with this reply.

*  it still gives -11mm - rather than Kreg's own figure of -6mm - for 13mm or ½inch timber.

[Ken Kortch]

Using simple linear regression (fitting a line of best fit through points on a graph):

Thickness = Material Thickness

Distance_from_Edge = -1.076923077 + (1.538461538 x Thickness)

Drill_Collar_Depth = 2.814903846 + (0.951923077 x Thickness)

Screw_Length = 0.826923077 + (0.461538462 x Thickness)

A negative Distance_from_Edge means that the jig is hanging over the material.

Actual:

Thkns, Edge, Colr, Scrw

0.5, -0.25, 3.3125, 1

0.75, 0, 3.5, 1.25,

1.5, 1.25, 4.25, 1.5

Estimated:

Thkns, Edge, Colr, Scrw,

0.5, -0.31, 3.29, 1.06

0.75, 0.08, 3.53, 1.17

1.5, 1.23, 4.24, 1.52

Not too bad. I hope this helps you until/if Kreg can give better information.

Ken