Hey guys, Mobile here.
I wanted to do a write-up of kerf ports because I know some people have issues with knowing the length to make them for multiple panel usage for bass reflex style enclosures.
So, what is a kerf and what is it's purpose?
The kerf port is a bend within the dimensional length of a panel used on the enclosure to allow for increased compression and velocity with the ability to lower distortion and resonance build up within the enclosures port created from port noise. It also allows the ability to reduce resonance peaks within the compression chamber if the other side of the port consists of a compression volume, normally where the drivers are installed.
So, the basic look of a kerf is shown here:
This is utilizing a kerf on both sides in and out of the enclosure, much like a precision port does, but using a slot style layout. SO, in order to do this kerf on each side, you will see in the pic, that the front and the ports are actual one piece on each side. SO, each piece involves 2 kerfs on a single panel!
Now, if you have a CADD program of the such, this will make this much easier, but for hand sketches, a compass will work just fine as well, with a transparent plastic ruler.
We need to know the angle that these kerfs are to be formed into. These angles in the example are both 90 degrees, which is the most common kerf bend used.
We know that (2)90degree bends are needed. Now, we need to know the radius of those bends. This can be sketched up using 12 sections of equal length to make up the 90 degree angle composing of 13 cuts. 12 sections usually give a good distance between each cut to allow for proper flexibility and stability of the wood. SO, using 12 sections, we have to seperate them by length. We will say that we need a 2" radius kerf on the bend.
Now, there is a template I put together that accounts for this bend and the length at the same time. Here is the example I sketched of the template.
It has a 1.4% error on average for each inch of the kerf's radius. So, by the time you do say a 12" radius kerf, you are still only looking at less than 17% error. That is a pretty big kerf. The error only exists due to the simplicity of the template for ease of use. This error is a fraction of a quarter of an inch in most cases.
So, say in the example that the wood before the kerf begins is 6" and the wood after the bend is 4". We know that is 10" already, so we just have to account for the bend length. The reason for this is to know how long to make the panel so we account for the length in the bend by not cutting is too short or too long, though cutting it too long is a better deal, it is best to get it as accurate as possible.
So, if you notice in that pic, the 1 2/16" (1 1/8) dimension. That is the dimension from that point to the center of the circle, and then continues the 2" radius of the circle to the point where the 6" length stops and the kerf begins. This was calculated by taking the inner circle to half of the radius of the outer circle, and then drawing a line from the end of the kerf (at the bottom part of the pic), to the circles edge and continuing until it touches the horizontal line. This is where the measurement of the kerf starts. So, this is saying to add 3 2/16 to the length of 10" to make it 13.125" long. This will have a 2.8% error from the actual bend, which is a fraction of a fraction.
The lengths of the cuts are found by taking the length of the actual kerf, and dividing it by 12. In this case, the length is 3 2/16(3 1/8) or (3.125"). So, the cuts should be about 1/4" apart for this to bend correctly.
Now, we do the second kerf bend, say if it is the same radius at the first, then we will have another 3.125" to add and end up being a total length of 16.25 inches long to account for both kerfs.
At this point, you then have to measure the distance between the kerfs to begin the cuts. Say if the example pic has a distance of 7" between the bends, then that distance is accounted for between the beginning and end of the kerfs respectively.
In the example pic above of the double kerf ports, the layout is as follows if you follow the template and the 12 cuts for each 90 degree bend:
The front is 14 inches, the kerfs are 4 inch radius, and the space between is 7 inches and the part after the inner kerf is 2.25 inches (example). Then it would look like this:
Hope that helps.