Recurve on flattops

Carl1Mayer · #1 03-03-2019, 07:14 AM

Do any of yall have experience scraping a recurve into a flat top guitar?

One thing I like about making archtops is being able to fine tune the responsiveness and emphasize certain modes after its strung up by taking more or less out of the recurve from the outside.

For flat tops it seems like the approach is to mostly just glue it together and trust the design is correct, maybe reach in and shave some braces for a coarse adjustment. This is obviously sufficient when working with a tried and true design or if you do enough iterations to get the design right, but since I rarely use the same design twice it would be advantageous to have another means of fine-tuning if necessary.

I dont see any technical reason not to scrape a recurve a flat top, other than maybe aesthetics and the extra labor. Any other reasons yall can think of?

bostosh · #2 03-03-2019, 08:38 AM

The soundboard can be too thin to begin with?

Cosmetics? of the top outer surface?

MC5C · #3 03-03-2019, 08:56 AM

I have read that certain master luthiers of classical guitars would reduce the thickness of the outer edge of their tops to gain responsiveness. That sounds like a recurve to me. In any case, it depends on the rest of your bracing evolution. A lot of classical guitar fan braces don't reach the outer perimiter of the top - they already are promoting a more flexible area. A lot of flat-top bracing technique seems to take the major braces and terminate them in the kerfing of the side - pretty much trying to couple the top to the sides. I think the idea of loosening up the perimiter of the top has value, and could be a neat thing to try.

iim7V7IM7 · #4 03-03-2019, 09:09 AM

A couple thoughts:

Many builders start by understanding the density and stiffness of a top set and consider the lower bout size of the guitar. The top plates are frequently thickness based on this.

Builder’s have different approaches to arching their tops from very flat. (Say 50’ radius dish) to quite arched (say 30’ radius dish). Other builders use a tight lateral arch (say 5’ radius) or others are completely flat in the lower bout with a severe dish in the upper bout (say 22’ radius) for structure.

Some builders keep their tops uniform in thickness, but others do thin their top plates near the rims. I have seen tops about 0.135” near the bridge, but thinned to 0.095”-0.105” near the rims. I have seen some create create a recurve by having a differential between the top dish and the radius applied to the rims (say a 30’ radius to the top but a 60’ radius on the rims creating a recurve once glued).

The bracing and adjustment of it is also done in addition to this.

Bruce Sexauer · #5 03-03-2019, 10:51 AM

IMO the OP has a misconception of the physics of a flat top guitar. I view the top as a membrane, and while the top must have enough integrity to function as as a single plane, the actual structure comes from the bracing. Thus “recurve”, as it were, comes from manipulating the braces rather than the top. The OP is not alone in this misconception, and fortunately there appears to be quite a bit of wiggle room where the workability of the flat top acoustic guitar is concerned. Having started with IMO, I will finish with IMO.

charles Tauber · #6 03-03-2019, 10:55 AM

Bruce, does you post imply that you do not thin the edges of your guitar tops, as you see no advantage to that?

bostosh · #7 03-05-2019, 10:50 AM

Check this subtle small (about 3/16) "groove" along the lower bout,
made to thin the edge, to create a "diaphragm" like structure.

Taylor patented, you can search it

https://drive.google.com/file/d/0B_p...ew?usp=sharing

Bruce Sexauer · #8 03-05-2019, 11:19 AM

Quote:

Originally Posted by charles Tauber

Bruce, does you post imply that you do not thin the edges of your guitar tops, as you see no advantage to that?

Didn’t mean to imply at all: I do not intentionally thin my plate edges. I think most historical evidence of others doing so is actually just over sanding of the binding. I make my tops, backs, and sides the thickness they should be (IMO) and then do my best to maintain the dimensions. I view the top plate as a membrane which must have a precise level of integrity, no more or less, and think it is very important, particularly below the sound hole, for the membrane to be as close to 100% consistent as possible in order to use the braces to control the structure. If the membrane becomes thin enough to allow inconsistent integrity, the bracing must become more dense locally in order to control what I call “local resonance nodes” which are the major culprit behind imbalanced response.

So, the alleged advantage of thinning the top rim would probably be a more responsive instrument. IMO, if that is so, the difficulty would be that the increase would be uneven, and any attempt to control this unevenness would be beyond my ability, or at the least impractical. Further, if the top is at its ideal membrane thickness and the bracing is distributed workably, there is no lack of response.

All opinion, and it seems to be working for me.

Howard Klepper · #9 03-05-2019, 11:45 AM

I do some tuning of the completed guitar body by thinning toward the top edges. But never a recurve (actually, on a flattop it would just be a curve, but I get what you are talking about). I do this to adjust the pitch of the box's Helmholtz resonance downward if I think that is needed. And maybe sometimes just for a good tap tone if I think it would help.