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Old 11-03-2012   #1
 
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Default How to Kerf bend MDF for your subwoofer port!

Kerfing 101-

What is Kerfing?
“Kerf” by definition means “width of a saw cut”. This is the width of the actual saw tooth itself as opposed to the width of the blade as a saw tooth flares out and away from a blade. “Kerfing” is a term that many woodworkers have adopted to define the process of making several cuts within a piece of wood to achieve the removal of material. Often this removal of material will allow one to then bend the wood stock making a nice professional and appealing curve that can be incorporated into the project.

Kerfing and Car Audio:
What can Kerfing do for you in your quest of Car audio domination? Well first of all it is important to note that in any ported subwoofer box hard corners are the devil. They do not allow efficient air flow which can result in less output and more port noise. Kerfing your port will allow you to maximize efficiency and create the most non-turbulent flow for the air moving in and out of the enclosure.

Secondly Kerfing can be used to make an enclosure or other part of your Build more appealing to the eye. You could use it for corners of an amp rack, or to make a custom arm rest more smooth. Kerfing is a process you need to learn to take your builds to the next level.

What you need:
This is outlined in the video but you will need the following (* means you don’t HAVE to have it but it is recommended):
  • Circular Saw (Or table saw but the process will differ from the video) Disclaimer: My Saw blade is 1/8” wide. I recommend the same width!
  • T-Square (Or table saw fence)
  • MDF
  • Wood Glue
  • Safety Glasses
  • Tape Measure
  • Marker/Pencil
  • Clamps
  • Fiberglass Resin*
  • Fiberglass Chop Mat*
  • Body Filler*
  • Sand Paper*
The steps:





Watch the video! Once the kerfed section is made load it up with wood glue and clamp it in place. Finish with fiberglass resin and chop mat and for that added professional touch apply body filler to the area and sand!

The Math:
I go through the math in the video but in case you are having a hard time understanding this should help. Please note this is used for slot porting applications so when I reference “diameter” I am NOT talking about a circular port, I am talking about the curve of the kerf if you were to look at it along its cross section. See the photo below:



Step 1: Determine the diameter of your curve. Note I said “Diameter NOT radius” if you have your radius just multiply it by 2 and you now have your diameter).

Step 2: Find the circumference of a circle using this diameter. The equation is:



Step 3: You should now have a value for your circumference. Take this value and divide by 4 for a 90 degree turn or 2 for a 180 degree turn, this will give you the “length of kerf”.



Step 4: Record your value for the length of the kerf. This is how much material along your mdf “linearly” must be used to make your curved kerfed section. This is the value you want to “mark off’ for your kerf on the mdf. (Note that this method can also be used to find the “length” of this part of your port so that you can Build your port to the target tuning length. Instead of using the diameter of your kerf arc, use the diameter of the arc that is the centerline of the port).

Step 5: Now that you have marked off where your kerf will be on the mdf you are going to take your “length of kerf value” and divide it by 12. This will give you your length between cuts.



Now in the video I show a trick for making these divisions perfect. That way if your value is something like .6384 you don’t have to guess on a tape measure. My method is also much faster as it doesn’t require this steps math. Once you have marked out all the distances between cuts you can proceed with cutting and making your kerfs!

Other notes: These are other notes that have been brought up on other forums and I feel are helpful.
  • I made the mistake of wearing gloves in the video. Never wear gloves with rotary tools!
  • It is not recommended to apply screws in the kerfed section of the mdf, it will likely split, instead use clamps (or heavy objects) and wood glue.
  • If you are blade in not 1/8 you will need to do some math to determine how many cuts to make. I generally remove an inch and a half of material, so if your blade is smaller, more cuts, wider, less cuts. Note that fewer cuts will lead to a rougher curve and more a smoother curve but with less strength.

Pictures:

Design of an enclosure using a Kerfed Port:

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Old 11-04-2012   #2
 
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Default Re: How to Kerf bend MDF for your subwoofer port!

Good info, Thanks.
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Old 11-04-2012   #3
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Default Re: How to Kerf bend MDF for your subwoofer port!

very nice, thank you for taking time to share.

Listen to the music through the stereo, not the stereo through the music.
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Old 11-05-2012   #4
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Default Re: How to Kerf bend MDF for your subwoofer port!

I played with kerfing a little bit a few months ago. works great.

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Old 11-06-2012   #5
 
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Default Re: How to Kerf bend MDF for your subwoofer port!

Any cabinet or furniture maker can order flexboard or 'kerfcore' material. It's a thin layer of masonite with kerfed particleboard on the back. 3/4" thickness is pretty standard and it's available with the kerfs going in either the long or short direction. Saves a lot of work and gives a smoother external curve without visible peaks. Used extensively in millwork and store fixturing.
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Old 11-06-2012   #6
 
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Default Re: How to Kerf bend MDF for your subwoofer port!

yeah and we can order premade sub enclosures and premade home speakers. This forum is for us that want to do it ourselves, no matter how crappy the finished product comes out in the beginning lol. Great article!

I'm wondering now, what calculated losses are with square bends in the ports. On average, would it be 30%? 20%? 1%? Anyone know?
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Old 11-06-2012   #7
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Default Re: How to Kerf bend MDF for your subwoofer port!

That is a really nice write up and video...box too.
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Old 11-06-2012   #8
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Default Re: How to Kerf bend MDF for your subwoofer port!

The bends some...but not as much as the right angles in the length of the port itself.

I did see some hard data on port losses years back, but don't remember where I saw it...was on the internet. It's a measurable difference, but not audible. You can see the differences in vent speed between round and square ports of the same area in a program like winISD.

Quote:
Originally Posted by ds1919 View Post
yeah and we can order premade sub enclosures and premade home speakers. This forum is for us that want to do it ourselves, no matter how crappy the finished product comes out in the beginning lol. Great article!

I'm wondering now, what calculated losses are with square bends in the ports. On average, would it be 30%? 20%? 1%? Anyone know?
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Old 12-27-2012   #9
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Default Re: How to Kerf bend MDF for your subwoofer port!



Average velocity shouldn't change much with a sharp 90 degree port, but local velocity around the bend will be absolutely horrible. If I had to put a number on it, I'd say a sharp 90 is less than 50% efficient at that particular cross section. What that does is create a bottleneck.

How this equates into real world performance of a port that doesn't exceed .05 mach, I can't say.. my experience is with flow hovering around .5 mach.
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