When building a box with the port flared...

cut until it bends where you need it.

It's actually pretty simple to calculate. You need to find the difference between the circumference of the outside flare and the inside flare. Using the formula pi*diameter=circumference and an MDF thickness of 3/4", the difference in circuference will always be (1.5"*pi)/4 because of 1.5" difference in MDF thickness (over a 360 degree circle) and divided by 4 because a 90 degree bend is 1/4 of the cirumference. That number is the difference between the inner and outer faces of a 90 degree flare and remains constant no matter what the flare radius. To find the number of kerf cuts needed, simply divide the difference by the kerf width of the blade. If we assume the standard blade kerf width of 1/8", then simply divde (1.5*pi"/4) by 1/8" to find how many kerf cuts you need. If you do the math, the answer is about 9.4, which we can safely round up to 10 kerf cuts for a 90 degree bend.

To find the proper spacing, simply take the inner circumference of 90 degree bend which is (2*flare radius*pi/4) and divide that by the number of segments that will be cut. If we make 10 cuts, then that will make 9 segments. If we are doing a 2.75" inner flare radius then the math would be 5.5*pi/4/9 = ~0.48. We can safely round this up to 1/2".

So in conclusion, to make a 2.75" inner radius flare (3.5" outer radius), make 10 cuts spaced approximately 0.5" apart. Of course it does take a little trial and error to really get it perfect. BJ and Thumpper really have this down to an art. You need to figure out the proper depth of the kerf cut (about 5/8" to 11/6" deep). Sometimes you may want to make more than 10 cuts, maybe 12 or so, which will allow you to avoid bending the wood all the way, but leave wider grooves in the inner kerfs (which can be filled with resin to increase strength). Some people may use differential kerf spacing (for example using closer spacing between the two kerfs at the beginning and ends of the inner flare) to really dial it in to perfection. You need experiment with scraps so you know exactly how it will turn out. But with a little basic math and a little applied logic, it is not difficult to fairly accurately predict the necessary number of cuts needed to make a certain kerf flare.

The spacing of the cuts and the amount determine the radius of the flare. 10 cuts @ 1/2" spacing gives you a 3.5" radius.

I calculate it using sketchup. I use 10 cuts per 90* bend and I get the spacing of it by putting in kerf cuts to the 3D drawing spaced out evenly. It has worked well for me but Brian's math isn't complicated and it's accurate.

awesome info here

It won't change the tuning but it changes the way you calculate it.

It really depends on the flare and the angle of the flare but it is safe to say that about 1/2 of the length of the port flare will still act like the port. If you account too much for the port flare, the tuning will be higher than expected. A recent enclosure I built was supposed to be tuned around 22hz and was tuned to 25hz roughly. I basically accounted too much of the flare to be part of the port.