Matching Radius of Top Kerfing to Soundboard

[Ken C]

I have built a half dozen guitars over the past two or three years, so my experience is rather limited. For 5 of the 6 guitars, I sanded the top of the rims perfectly flat regardless of the radius put on the soundboard. Some s/b's had a 28' radius and some had a 40' radius. All the guitars sound nice with my latter guitars sounding even better as I had better control of thicknessing the tops. However, last winter I built #4, an OM style guitar from madagascar rosewood using a red spruce top. On this guitar I chose to radius the rim and kerfing to 28' to match the 28' radius of the s/b, pretty much the same approach I use for the backs, which are radiused to 15'. My understanding is that radiusing the top kerfing traditionally is not done, but I did it anyway. The guitar was built specifically for an individual and was sold to him when complete. This guitar has developed a truly exceptional tone--As good as the best guitars of that size that I have heard. The owner called me earlier this week to talk about the guitar. He is trying to understand why it sounds better than his modern collection of Martins, Collins, and Baranick guitars. He said he plays daily and hasn't picked up another guitar in weeks, and he has plenty to chose from.

Yeah, I like the great feedback, and that is not the reason for my post. But this guitar really is unique. He and I talked for some time trying to figure out what makes this guitar stand out so much. I had heard great things about madrose prior to building with it. That may have some influence. I have built with red spruce before, so that was nothing new. My bracing approach was similar to others I have done. I did build this guitar a little deeper, not tapering it so much to the neck joint, and I radiused the top of the rims. We each chase that illusive goal of the perfect guitar, and having built one that turns out to be extra special, I am trying to determine exactly what sets it apart, so I can do it again.

At the time I built the madrose, I had been building only a 28' radius in my tops. By the time I got to this guitar, I started thinking about the stress I was putting on the tops from forcing the outer edges of the tops flat to the kerfing. The single reason I radiused the top of the madrose was to have as little stress on the top joints as possible. I wondered at the time I did it and still wonder today if less stress translates to a more freely moving top, thus better sound. Perhaps forcing the radiused top flat at the edges stiffens the top, requiring more time for the soundboard to mellow. Following the madrose OM, I built the claro walnut FB-185. That guitar was built with a 40' radius and flat top kerfing. This is the only guitar I have built with that shallow of a radius. The FB-185 also has great tone and more closely reflects the quality of the sound of the madrose OM than the other OM's I have built with a 28' radius. Again I wonder if the shallow radius resulted in less stress on the top joints, and thus better sound.

I have two more that I am just getting under way, and am debating how to approach the top radius. I'd be curious to hear specifically about radiusing the top of the rims from you folks that have built a number of guitars. Why isn't this traditionally done? Could this be partly attributable to the improved tone relative to my other OM's with 28' radiused tops? Or are other factors more responsible, such as the wood chosen for the B&S, perhaps a stellar red spruce top, or is the reason simply dumb luck? An answer that we don't do it because it kills the tone goes against what I am seeing with this specific guitar.

Ken

[Freeman]

My experience is limited, but I angle (radius) the kerfing. Now with sanding in the radius dish, earlier with a 25 foot radius sanding board, 16 on the back - if for no other reason to get a better gluing surface.

Why would you want to build the stress into the top by forcing that little edge to be flat while the rest of the top isn't? Do you do the same thing to the back at 16 foot or whatever your dome is? And isn't Taylor now doing some sort of little relief routed around the edge of the top just inside the kerfing to free it up?

[deadedith]

I would like to know the angle difference of the kerfing for the 28' vs. 40' radii.

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[Ken C]

Why would you want to build the stress into the top by forcing that little edge to be flat while the rest of the top isn't?

Freeman, that is precisely why I did it. I have the impression, though, that most don't radius the kerfing, and I was curious as to why.

Ken

[kencierp]

Hoffman does it both ways --- claims the contour on the kerfing increases treble response. But, interestingly changed from contoured to flat sanding -- not the other way around.

Martin, and at least Larrivee and Taylor with all their technology and acoustic testing equipment leave it flat
http://www.youtube.com/watch?v=rAu3Nw4KYdo

Do a real world drawing of top/kerfing intersection and you see how miniscule that contour really is especially at 40' 52' 65' --- so either way there is no gluing compromise. However based on the common X brace layout there's very little/if any actual contouring on the edges -- also the bracing and lack of contoured bracing at the upper bout makes that area pretty flat.

Just my opinion but I am going along with Bob Taylor ---- his very best "R. Taylor" models have flat tops (no contour at all) you just have to wonder what those experts know?

My "Lab Guitar" was built with 25' top radius with contours sanded on the kerfing its one of the oldies (30 years or so) I keep track of --- only one built that way and the only one in the family with top cracks.

I just stay out of the what sounds best conversation --- its subjective. You need a sample size of many (100 at least) that were built exactly the same with the exception of the contoured kerfing and those would have to be blind tested.

I build with flat sanding -- been told my guitars sound pretty good. Most of the guys/gals who have built KMG kits (flat sanded) will tell you their instruments are balanced, loud and lively. But that's subjective too!!

[kencierp]

Dave regarding your question -- a little late for me to be doing math but here goes:

The bevel a 28' radius imparts on the kerfing is 1.7 degrees the inside of the rim is .0094" higher than the outside. Based on .320" thickness = sides + kerfing

The bevel a 40' radius imparts on the kerfing is 1.2 degrees the inside of the rim is .0051" higher than the outside. Based on .320" thickness = sides + kerfing

+/- .005" in woodworking by most is considered zero error so both inner height dims are pretty darn close --- thus I just make them 90 degrees.

[deadedith]

Thanks Ken - intuitively, it seemed to me that the difference would have to be negligible with a 40' radius top. I'm glad the math bears it out - as well as the testimony of Taylor et. al.
Dave B

[kencierp]

Hi Dave,

As mentioned I should not have been doing math last night please revisit the post there are some changes, to make the data more accurate -- the results are similar. Late night and a crashed laptop cad program do not make good science.

Also, here I've attached an Email exchange I had with the Martin factory earlier this year regarding their past and current treatment of this issue -- read from the bottom up.

-----Original Message-----
From: Carmen Cortez
Sent: Tuesday, February 02, 2010 4:19 PM
To: Mark Bickert
Subject: FW: rim sanding
Importance: High



Mark,

Can you help us out on this question?



Regards,

Mr. Carmen A Cortez

CFM Customer Service Manager

-----Original Message-----
From: Chris Thomas
Sent: Monday, February 01, 2010 4:40 PM
To: Carmen Cortez
Subject: FW: rim sanding
Importance: High



Carmen,

Please respond to this customer. I do not know the correct answer.

Hi Ken,

My name is Mark Bickert and I have some clarification on the rimsanding of the guitar sides. For most of our guitars, we sand the back using a radius wheel. The top is sanded using a flat wheel. The top is sanded flat all the way from the rear block to the top edge of the soundhole. Then an angle is sanded from the top edge of the soundhole to the front block. This angle compliments the neck angle and gives us proper bridge height and fingerboard drop.

Sincerely,

Mark


Thanks
chris

------ Forwarded Message
From: Ken Cierpilowski <kencierp@yahoo.com>
Date: Mon, 1 Feb 2010 13:00:42 -0800 (PST)
To: Chris Thomas <cthomas@martinguitar.com>
Subject: Fw: rim sanding

Hi Chris,
This is an important issue that affects your Martin GMC customers, please respond -- If you need to off load please tell me who to contact.

Thank you,
Ken Cierpilowski

--- On Thu, 1/28/10, Ken Cierpilowski <kencierp@yahoo.com> wrote:


From: Ken Cierpilowski <kencierp@yahoo.com>
Subject: rim sanding
To: cthomas@martinguitar.com
Date: Thursday, January 28, 2010, 1:57 PM

Mr. Thomas,

My name is Ken Cierpilowski, I am the owner operator of Kenneth Michael Guitars here in Michigan. Since a portion of our activity is supplying tools and fixtures to hobby builder's. Many of our customers are constructing or thinking about constructing kits purchased from the GMC --- so I am called on regularly to provide clarification regarding what tools and fixtures are needed and how they should proceed with the various assembly processes.

I am assuming that you are the Chris Thomas that provided the tour direction for Primer Guitars in the You Tube video. You are obviously very knowledgeable of the Martin production system. My question is regarding the sanding process for the top edge of the rim. You clearly indicate that the top edge is sanded flat and the back of the rim is sanded to a slight contour. In reviewing Dick Boak's videos he also clearly states flat on the top edge with a slope from the waist to the neck block. And, that is what I personally observed while visiting the factory. Some of the boutique builders actually sand the top of the rim with a contoured disk as well --- roomer has it that Martin has adopted this process and now puts a bevel on the the top edge. It would be appreciated if you could verify or debunk this roomer, or direct me to someone who could shed some light on the subject.

Thank you in advance,

Ken Cierpilowski

[deadedith]

Very interesting.
Thank Ken.
Dave B

[Ken C]

Ken,

Thanks for the insight. Is Martin using a 28' radius on their tops? I probably knew this once, but can't recall now. My first four guitars were all done with a 28' radius. I switched over to a 40' radius for the 5th guitar, the claro walnut 185 due to comments you had made to me previously regarding Bob Taylor and actually playing an R Taylor that sounded amazing. I also used a 40' radius on the EIR OM that has been ready for spraying for a month or two, but as that one has never been strung up, I have no idea how it will sound.

Sounds like radiusing the top of the rims by itself is not something that influences sound or tone greatly just by itself. I was wondering if I had stumbled onto something, but like most other aspects related to this hobby, someone already has been down that path. As you mentioned, the best test would be to build two guitars with components as similar as possible but with the only change being the radius on the top rim. I am guessing even then, I'd have much inconsistency in other areas of the build such as bracing, so any discernible difference in sound may not be entirely due to the radius on the top of the rims.

There is no doubt that my last three guitars sound better than my first two, which as noted above, I largely attribute to being able to better thickness the tops. But perhaps the difference in sound among the last three is more attributable to the woods used (and certainly body style) than whether or not the top of the rims were flat. Differences among these three may be mostly subjective as you say. Wish I had the time to build a dozen guitars just to play around with this, but my builds are an extremely slow evolution and building a dozen guitars would be a four or five year process. By the time I finished the last couple, the first ones would be five years old and thus sound different just due to their age.

...Oh well!!

Ken