formula for diff cuts??

[TheWesternCanuk]

Hey guys, is there a formula for figuring out diff types of wood thicknesses and bits. I want to use a 3/4" 14 degree bit. My wood thickness is 7/8. If someone could point me in the right direction to do some reading that would be great. Thanks guys.

[microsuffer]

I looked up some router bits and found that for a 14 degree 3/4 bit you have a 3/4 cutting depth. Not long enough for through dovetails in 7/8 material. It looks as though you need a bit at least as wide as the material is thick. Half blind is another matter and the template pattern strip for HB in 3/4 material should work.

Google-ing I found a program that will generate templates for the Incra jigs from user inputs, including material thickness, dovetail bit, etc.. It has checks built in to keep you from asking for spacings and pin/tail sizes not possible because of the 1/32 increments forced by the design of the jigs. The link is http://www.internetwoodworking.com/w5/dovetail.html. Sorry, but I haven't tried this out.

By the way, if you are working with Douglas plywood, use double wide pins and tails and backer boards (front and back for through cuts). DP seriously doesn't like dovetails.

[billw57]

This post got me thinking about dovetail spacing and calculations. A couple of sketches and some trig calcs later, I came up with this.
Given the bit diameter (D), bit angle (A), and spacing (S), the depth of cut (C) is

C = (D - (S/2)) * TAN(90 - A)

With the Incra jig you need to start with the spacing, and then adjust the depth of cut, since the depth is "infinitely" adjustable, but the spacing must be in increments of 1/16". I checked this against the Incra templates and it works.

So, for a 7/8" 14 degree bit, the spacing is 1-1/8" and the depth of cut is 0.7520". Two thou deeper than the bit can cut. Looks like you need a 1" bit, at least for through dovetails. For half-blind with the 7/8" bit, a spacing of 1-1/4" gives a depth of 0.5013".

With a 1" 14 degree bit the spacing is 1-5/8" and the depth of cut is again 0.7520"

BTW, the Incra template is for a 7 degree bit, and won't work with the 14 degree bit.

-- Bill

[microsuffer]

Is the term S/2 because the Incras use 1/32 increment tracks for position locking?

[billw57]

Is the term S/2 because the Incras use 1/32 increment tracks for position locking?

No, that's just how the math works out.

The multiples of 1/16" spacing is a result of the Incra 1/32" stops.

-- Bill