Luthier Tips du Jour - Kerfing

Uploaded by OBrienGuitars on 10.05.2012

Kerfing is a small strip of wood that has small cuts or kerfs
evenly placed along it allowing it to bend or conform to the shape of your sides.
It is glued to the sides or rims of the guitar
and the back and top are then glued to this to hold them in place.
In this video I will talk about the two most common types of kerfing
and show how to install them.
There are a couple of types of kerfing, regular kerfing,
where the cut is on the side facing the inside.
The solid side then is glued to the rims of the instrument.
And reverse kerfing where the cut side is glued to the rims of the instrument
and the solid side faces the inside of the guitar.
Both types have their advantages and disadvantages while installing
but the reverse kerfing will stiffen the sides much more than the regular kerfing.
This will arguably isolate the top and back more allowing more of the energy
to stay concentrated in them rather than dissipate throughout the sides.
Tonally speaking this could be that extra advantage you need
to take your guitar from a good guitar to a great one.
As my friend Kent Everett says, making a great guitar is a study in subtleties
and kerfing could be one of those subtleties that helps you make a great guitar.
To install the kerfing first you need to find the correct length you need for your body size.
This is easily done by placing one of the pieces of kerfing in position
and very carefully use it to determine how long it needs to be.
To reduce the risk of breaking it during this procedure
you can remove the upper bout portion to reduce stress at the waist.
Use this piece to cut the other pieces to length as well.
If you are installing reverse kerfing I like to wet it
to help keep it from breaking while installing.
This is not necessary with regular kerfing.
If it breaks it is not as noticeable as the reverse kerfing.
After the reverse kerfing is moistened
I just clamp it to the outside of my rims as it dries.
This allows the fibers to stretch and take shape without breaking.
If you are in a hurry you can use a hair dryer to speed up the drying process.
The kerfing is now pre-shaped and much less likely to break while installing.
When applying glue less is definitely more.
Don’t get happy with the glue or you will have a lot of squeeze out
to clean up on the inside of your guitar sides.
Regular kerfing is easy to apply the glue.
Just put it on and use your high tech glue spreading device to spread it.
However, on reverse kerfing you need to lightly bounce
your high tech glue spreading device to spread the glue,
if not you just push it down into the openings.
This could result in a weak glue joint as well as a mess to clean up later.
I start by installing the kerfing always from the heel block
going towards the rear of the guitar.
This way if your kerfing is a skoshe short the gap is at the end of the guitar
instead of by the heel block where you can see it through the soundhole.
On the back side of the rims I install the kerfing a skoshe proud of the side.
This allows you to then later radius the more pronounced back radius
without removing anything from your pretapered sides.
On the top I leave the kerfing flush with the rims.
I am using some oversized high tech lutherie fastening devices
to secure the kerfing to the side as the glue dries.
Commercially made kerfing clamps are available from lutherie supply warehouses.
Notice how I pull the kerfing up against the side with one hand
while simultaneously installing the high tech lutherie fastening device with the other hand.
This may take a little practice for some of you.
Roll the fastening device forward
so you get good clamping pressure at the bottom of the kerfing.
Continue clamping until you arrive at the end block
and then do the other side the same way.
I only do both sides of the top or back at once.
It doesn’t matter which one you do first.
After the kerfing is clamped I then put the form in my bench vice
and take a quick look to see if I need to clean out any squeeze out.
If necessary, I use a razor blade to clean it up.
On reverse kerfing I even clean out the glue that is in the kerf.
I also see if there are any gaps between the side and the kerfing.
This can happen in the tighter curves
and is easily fixed by adding a stronger clamp.
Sometimes an industrial strength can of Guitar Builder’s Whoop Ass is needed
to coax the kerfing up tight against the sides in these problem areas.
After 30 minutes of dry time you can remove the clamps
and install the kerfing on the other side in the same manner I have just shown.