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Kerfing - why?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=41362		 Page 1 of 3
Author:  absrec [ Tue Sep 10, 2013 6:08 pm ]
Post subject:  Kerfing - why?
What is the purpose of kerfing? The only info I've found says it provides rigidity and stability to the corners of a guitar while providing a slightly wider gluing surface for the top and back. But what I was wondering is if it also helps to maintain the shape of the bent sides even if you remove them from the mold without gluing up the top and back? I just bent my first pair of sides and they spring back a lot. Just wondering if the kerfing will help that.
Author:  theguitarwhisperer [ Tue Sep 10, 2013 6:21 pm ]
Post subject:  Re: Kerfing - why?
http://en.wikipedia.org/wiki/Guitar#Lin ... d_purfling

Section 3.15 has the answer.

:D

But no, the don't really add the kind of rigidity you are asking about, although they do increase the overall stiffness of the sides.
Author:  the Padma [ Tue Sep 10, 2013 6:25 pm ]
Post subject:  Re: Kerfing - why?
Kerfing is a verb. the act of cutting a series of kerfs (or cuts) in a piece of wood in close proximity, so the wood can be curved.

see...Image

Loofers sometimes use linings on the edges of the ribs. We do not use curfs cuz a curf in reality is a pile of sawdust cut from a chunk of wood.

However some loofers do use kurfed linings.

Regarding usage of solid and curfed linings is mainly a matter of preference.
Author:  Josh H [ Tue Sep 10, 2013 6:29 pm ]
Post subject:  Re: Kerfing - why?
absrec wrote:
The only info I've found says it provides rigidity and stability to the corners of a guitar while providing a slightly wider gluing surface for the top and back.


While these two things may be the only function of liners they are both vitally important. Liners will help your sides maintain their shape, but if the spring back is really bad you may want to bend your sides a little more before gluing them up. What type of wood are the sides and how thick are they?

Josh
Author:  meddlingfool [ Tue Sep 10, 2013 6:42 pm ]
Post subject:  Re: Kerfing - why?
I have always thought kerfing should be placed in the lexicon to honour all the unacknowledged kerfs that have fallen in the service of guitar making. Where else can you see so many kerfs in active service?

Anyway, primary purpose to give gluing surface for the top and back...

Solid linings and reverse kerf linings also add rigidity to the frame before the top and back are glued on, but once the top and back are glued on, I can't see them making much difference....
Author:  absrec [ Tue Sep 10, 2013 6:44 pm ]
Post subject:  Re: Kerfing - why?
Sounds like I need to practice bending sides. How much spring back is too much? I can get them in the molds without snapping but it requires a good bit of clamping pressure. Having never done this before, I don't know how much spring back is acceptable. Do they need to be closer to their intended shape before gluing up? Maybe within an inch or two of the shape of the top?
Author:  absrec [ Tue Sep 10, 2013 6:46 pm ]
Post subject:  Re: Kerfing - why?
Josh H wrote:
absrec wrote:
The only info I've found says it provides rigidity and stability to the corners of a guitar while providing a slightly wider gluing surface for the top and back.


While these two things may be the only function of liners they are both vitally important. Liners will help your sides maintain their shape, but if the spring back is really bad you may want to bend your sides a little more before gluing them up. What type of wood are the sides and how thick are they?

Josh

Bloodwood - .100" thick
Author:  absrec [ Tue Sep 10, 2013 8:08 pm ]
Post subject:  Re: Kerfing - why?
Anyone? :roll:

I understand it's not the easiest species to bend but...
Author:  Eric Reid [ Tue Sep 10, 2013 9:18 pm ]
Post subject:  Re: Kerfing - why?
I was taught to bend the sides accurately to shape, and not rely on the mold. I'm sure good guitars have been built in many ways, but it seems logical to me that the plates would be less likely to crack, and would be freer to vibrate, if they are not also being tugged on by the sides.

Maybe I'm dead wrong about that. We deliberately build in stress when we arch the plates. What's one more stress?

I think I'll go on doing it the way I was taught. I bend on an iron, and I enjoy the struggle to get all the curves in place. If you bend with a form, you'll probably still need to touch things up on the iron.

One suggestion: get some easy to bend wood (some oak, or ash, or walnut), and do some practice bending. Even if you're bending on a form, you'll learn a lot by bending a bunch of sides for the wood stove. If they turn out too good for the stove, you can always make another guitar.
Author:  nyazzip [ Tue Sep 10, 2013 9:48 pm ]
Post subject:  Re: Kerfing - why?
Quote:
while providing a slightly wider gluing surface

it's not exactly "slight": just approximating, it is in the neighborhood of a 10-15x wider gluing surface.....
Author:  Spyder [ Tue Sep 10, 2013 10:40 pm ]
Post subject:  Re: Kerfing - why?
One thing to keep in mind is the binding on the outside of the guitar. When you rout the channel for the binding, it is not uncommon to cut completely through the joint between say the top and the side. When this happens, the only thing holding the guitar together until the binding goes on is the lining.
Author:  Bryan Bear [ Tue Sep 10, 2013 11:15 pm ]
Post subject:  Re: Kerfing - why?
0.1 inch is pretty thick. Keep in mind I have never worked with bloodwood so there may be something I am not considering. I like to bend around 0.85 or a bit less. That small amount of difference in thickness makes a big difference in stiffness.

Kerfed linings (unless they are reverse kerfed) will add minimal rigidity to the rim. Many of us use solid (laminated actually in my case) linings that do make the rim very stiff before the plates go on. That is really a matter of preference and process. If you are having to do a lot of clamping into the mold, I would rework the bends.
Author:  Trevor Gore [ Wed Sep 11, 2013 4:55 am ]
Post subject:  Re: Kerfing - why?
First, get your sides to fit the mould properly. Bloodwood is not particularly hard to bend, (it bends somewhat like glass though!) so it needs to be very hot first. If you re-bend lots of times to get it right and "cook" them, the wood will get very difficult to bend. When cool, blooodwood sets up really hard, so you then need a lot of force to make a fit if it's not bent right. IME it will bend OK at the thickness you have, but thinner is certain easier.

The idea of a mould is to keep the end blocks in register so that you have a predictable neck angle when you lock the geometry in by adding the back and top, not to compensate for sides that don't really fit. Reverse kerfed and solid linings certainly add rigidity to the side assembly, but reverse kerfed may well "pop" if you have poorly bent sides and you remove the assembly from the mould.

The linings add both stiffness and mass as well as gluing area. The amount of stiffness and mass depends on the type of lining, with sparse tentalones being one extreme and deep solid (laminated) linings being the other. As well as the more obvious structural implications there are also acoustic implications. Stiffer, heavier linings create a greater impedance mismatch to bending waves in the top panel as they reach the rims and reflect more energy back into the top. Sparse tentalones are the opposite extreme and pass a lot of energy through to the sides, which is why flamenco blancas use that method of construction (the objective is for the guitar to "bark" then stop, not the usual design criteria for most guitars).
Author:  absrec [ Wed Sep 11, 2013 6:10 am ]
Post subject:  Re: Kerfing - why?
Thanks for all the great info. I don't know why I have to learn on such difficult exotic woods but I do it every time. I guess it's just my way. :) I built my first neck ever from Zebrawood. I learned a lot and it made everything else that followed seen kinda easy though. Like running with a back pack.

Anyway, I'm gonna go back and touch up with my pipe/torch setup and see if I can't get these things a little closer. Right now, when the one sits in the mold, the waist touches and the upper and lower bouts spring back around 3/4" - 1". I can squeeze it in with my hands but it doesn't stay. I've attempted to bend this one a few times. Maybe the pipe will allow me to over bend slightly so it springs back into the correct shape?
Author:  Tony_in_NYC [ Wed Sep 11, 2013 6:59 am ]
Post subject:  Re: Kerfing - why?
absrec wrote:
Bloodwood - .100" thick


That is thick for an easy bending wood. I'm surprised you didn't snap those sides. You want them to be as close to the mold shape as possible, so I would suggest bending the curves tighter so that they spring back much less. If you glue in the linings, they will add a bit of rigidity, but not enough to make getting the rim back in the mold very easy with a lot of spring back. Reverse kerfed linings add more rigidity, so I would use those to help hold the shape. If you are using a pipe to bend, get back to work and tighten up those curves!
Author:  absrec [ Wed Sep 11, 2013 7:12 am ]
Post subject:  Re: Kerfing - why?
Tony_in_NYC wrote:
absrec wrote:
Bloodwood - .100" thick


That is thick for an easy bending wood.

I'm confused. If it's an easy bending wood, wouldn't thicker be ok? People have said it wasn't an easy bending wood.

Anyway, if I'm picking up what everyone is laying down, .080" - .085" is probably a better pre bend thickness than .100".
Author:  absrec [ Wed Sep 11, 2013 7:19 am ]
Post subject:  Re: Kerfing - why?
When thicknessing the sides, do you use flexibility as a gauge to know when you have the correct thickness?
Author:  Trevor Gore [ Wed Sep 11, 2013 8:24 am ]
Post subject:  Re: Kerfing - why?
absrec wrote:
I'm confused. If it's an easy bending wood, wouldn't thicker be ok? People have said it wasn't an easy bending wood.

It's not an easy bending wood like straight grained maple, EIR or straight grained Aus. blackwood. The major difference is you have to get it very hot, just short of scorching and then it bends fine (and has a banana smell!). But you don't get many chances to re-bend if you don't get close quickly, because it "cooks", glazes and gets really difficult to bend.

I bent the cutaway on this guitar and this guitar at 2.0mm (0.080") for the cutaway section. For a guitar without a cutaway, 2.5mm (0.100") is possible, but, of course, easier when thinner.
absrec wrote:
When thicknessing the sides, do you use flexibility as a gauge to know when you have the correct thickness?

I don't. I use ~2.2mm as standard for non-cutaways, thinning to 2.0mm to 1.8mm for the cutaway area. If experience shows that a wood is difficult, I might thin it ~0.2mm or so more next time I use it.
Author:  Tony_in_NYC [ Wed Sep 11, 2013 9:16 am ]
Post subject:  Re: Kerfing - why?
absrec wrote:
Tony_in_NYC wrote:
absrec wrote:
Bloodwood - .100" thick


That is thick for an easy bending wood.

I'm confused. If it's an easy bending wood, wouldn't thicker be ok? People have said it wasn't an easy bending wood.

Anyway, if I'm picking up what everyone is laying down, .080" - .085" is probably a better pre bend thickness than .100".



HA!! When I read it, I didn't realize it was my quote and I said to myself, "That doesn't make sense!"
What I meant was, even if you were bending wood that is easy to bend, like walnut, .100" is pretty thick. Bloodwood can be difficult and prone to cracking, so you would want it thinner. I usually bend at .085" as well but have gone thicker on cooperative wood like walnut or black limba. Since your bloodwood bent at .100", that is fine, just tighten up the bends.
Author:  absrec [ Wed Sep 11, 2013 12:37 pm ]
Post subject:  Re: Kerfing - why?
Tony_in_NYC wrote:
Bloodwood can be difficult and prone to cracking, so you would want it thinner. I usually bend at .085" as well but have gone thicker on cooperative wood like walnut or black limba. Since your bloodwood bent at .100", that is fine, just tighten up the bends.
Well... It really didn't. I soaked it in the bath tub for almost a day and that's probably why it bent as much as it did. Since I didn't get it on the first shot, tightening up the bends wasn't really possible. I ended up scorching the crap out of it. I don't get how to get it hot enough to bend and not burn it. I guess I still suck at it. Until now, I've only bent bindings. Those are easier for a number of reasons. Mainly, if I did burn it, the burn was either hidden or scraped away.
Author:  Rodger Knox [ Wed Sep 11, 2013 1:04 pm ]
Post subject:  Re: Kerfing - why?
I've done a couple of bloodwood guitars, and is is NOT an easy wood to bend. The first was about 0.08" thick, and I scorched it badly and still barely got it close enough to shape to get the sides into the mold. The second set was much easier, but I thinned them down to around 0.06".
As for linings, solid laminated linings is the way to go. Kerfed linings add almost no stiffness to the sides, reversed kerfed linings do add some stiffness to the sides, but laminated solid linings make the side RIGID. I use 4 or 5 ply at .06" thick, the first two are 3/8" tall and the others are 1/4" tall.

Scroll down to the middle of the second page to see the bending/lining photos of the second one.
viewtopic.php?f=10133&t=37283&start=25
Author:  Dave Fifield [ Wed Sep 11, 2013 3:12 pm ]
Post subject:  Re: Kerfing - why?
absrec wrote:
Well... It really didn't. I soaked it in the bath tub for almost a day and that's probably why it bent as much as it did. Since I didn't get it on the first shot, tightening up the bends wasn't really possible. I ended up scorching the crap out of it. I don't get how to get it hot enough to bend and not burn it. I guess I still suck at it. Until now, I've only bent bindings. Those are easier for a number of reasons. Mainly, if I did burn it, the burn was either hidden or scraped away.


wow7-eyes wow7-eyes wow7-eyes

You soaked the sides in a bath tub??? Why?
Make yourself a Fox style bending machine with heating blankets, then you only have to spritz the sides with water right before bending.

Using my bending machine, I never have any significant springback. The trick is to heat, then bend, then allow to set (total time = 15 minutes), then leave the side/s in the machine and allow them to come all the way back to room temp again. Then heat the side/s up again for 10 mins, then turn off the heat and allow them to return to room temp again (overnight preferably). The second heat/cool cycle locks in the shape good and solid.

HTH,
Dave F.
Author:  Michael.N. [ Wed Sep 11, 2013 3:55 pm ]
Post subject:  Re: Kerfing - why?
absrec wrote:
Tony_in_NYC wrote:
Bloodwood can be difficult and prone to cracking, so you would want it thinner. I usually bend at .085" as well but have gone thicker on cooperative wood like walnut or black limba. Since your bloodwood bent at .100", that is fine, just tighten up the bends.
Well... It really didn't. I soaked it in the bath tub for almost a day and that's probably why it bent as much as it did. Since I didn't get it on the first shot, tightening up the bends wasn't really possible. I ended up scorching the crap out of it. I don't get how to get it hot enough to bend and not burn it. I guess I still suck at it. Until now, I've only bent bindings. Those are easier for a number of reasons. Mainly, if I did burn it, the burn was either hidden or scraped away.


There comes a point when you have applied so much heat that you have no moisture left in the wood. It becomes brittle and very difficult to bend without breaking it. If I reach that point and I haven't achieved the right curve I will leave the side for a day or so to allow it to re-hydrate. Every single time I have done this it's easier to make the bend.
Either that or use my foil idea from the start. The foil helps to prevent the moisture from being driven off. On certain woods it can work too well but it's a good technique for those woods that are difficult to bend.
Author:  Tony_in_NYC [ Wed Sep 11, 2013 4:30 pm ]
Post subject:  Kerfing - why?
Did you have a wet rag on the pipe? That helps. I use a machine so I am only regurgitating info I have read here, but it's good info so I'm not afraid to puke it up for your consumption. As gross as that sounds.

Edited to conform with the proper location of a wet rag when bending.
Author:  Bryan Bear [ Wed Sep 11, 2013 4:36 pm ]
Post subject:  Re: Kerfing - why?
Tony meant to type ON the pipe not IN the pipe. . .
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