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I'm building my first acoustic with the aid of the Grellier plans and the Cumpiano/Natelson book.
I'm using the book as a general construction guide, and building with very simple tools.
I did deviate from Cumpiano in that I built a body mold instead of using a workboard.
But I am at the point now where I need to think about trimming the sides to depth, establishing the taper, kerfed lining glue up and such.
The book is confusing at times because all of the information for a process is generally spread around. For instance, the depth of the sides is in a location earlier in thebook from actually trimming the sides, gluing kerfing in, etc. Not a real problem, but i find myself spending a lot of time searching for stuff i need during the build.
On the sides, it looks to me like he is using a flat sanding board to level the sides/linings top and back for gluing. I can understand a fairly flat side for joining the top to the sides, since the radius is very shallow/flat. The edge of the top should conform under clamping pressure.
But the back is a tighter radius, yet it seems to me he is not advocating putting any sort of radius to the sides and linings where the back meets the sides. I could be wrong. Perhaps I missed where and how he does this, but i don't think so. Is this correct, and will the edges of the back conform to a flat side edge? I am having nightmares about closing the box, then routing for binding and having the back fall off because it is only glued to a very small contact area on the top of the sides.
Also, for body depth of the sides he specifies 4 inches, tapering to 3 and some- odd inches (don't remember at the moment) at the heel block. I would like to actually deepen the depth to around 4.5", more like the dreads I own, a few of which are about 1/16" shy of 5".
I am trying to figure out if that will change the angle at which the heel block needs to be tapered for the back arch. If I add half an inch to both the heel and tail depth, I think that angle will remain constant, with the heel and tail blocks just being a little taller. Is this correct as far as you can see?
Any insight into these issues is welcome.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Mike--

On the fear that the back will not glue well to the sides without a matching radius:

Keep in mind that the Cumpiano/Natelson book advocates gluing the back on with a sliced up inner tube. Whether you use that, or some other elastic material, that elasticity will pull the back down flush to the sides pretty well. No need to panic.

On the taper of the back of the headblock:

I think your plan makes sense. Plus, keep in mind that you are going to sand the back of the headblock along with the sides with a big flat piece of plywood covered in sandpaper (if you follow the book) prior to gluing on the back, so whatever angle exists at the cutting stage gets adjusted during sanding.

Don, thanks.
I plan to make a caul in the shape of the guitar, sort of a 1" "frame" shaped like the guitar, with no center. It will just sit around the edges of the back or top, and I'll clamp that. I think that will do the same.
Thanks for the quick reply. Much appreciated, and a load off of my mind.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

You have to be really careful about mixing methods on your first couple guitars. It has been a while since I read the Cumpiano book but if I remember correctly, the top geometry for the fretboard plane is built in with the workboard. If you are using an outside mold, you have to establish that geometry another way.

This is not inherently obvious until you have done it so let me (try to) explain. The top in the upper bout has to have an angle that works with whatever dome you choose to allow for the fretboard to lay flat on the upper bout with the neck angle you are using. You need the plane of the upper bout to allow for the thickness of the FB and frets plus action to deliver the strings about 1/2" above the soundboard dome at the bridge location. There are lots of ways to do this, but if you don't know going in, you can get a surprise later.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Bryan, thanks. I think I have a handle on that. I have restored a couple of acoustics, literally took them apart and re assembled, because of fire damage in one case and being left out on a covered porch for a decade or more in the other. Both of those caases were instruments with great sentimental value(granpa's guitar, etc.).As a repairman, I've retopped, reset the necks(dovetsails), etc., etc.
We will see, but I think I have a good understanding of how that will work in my case.
Really appreciate your input. Thanks.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Mike--

One thing about the caul idea that you describe: Make sure that the caul is not too rigid. The contour of the back at the back/sides joint is undulating. So, the caul needs to be able to conform to that undulation. On the other hand, it needs to be solid enough to spread the clamp pressure out a bit. Maybe a good way to do it is to cut some vertical kerfs in the caul so that short sections are fairly rigid, but overall the caul can conform to the undulation.

Or you can just switch to spool clamps.

I also used the Cumpiano book on my first build. I still use parts of it today. I also varied from the book on tools and techniques used with no problems.

For side depth I would tend to stay close to what is on your plans.

As far as side profiling goes, if my memory serves me, he makes his sides parallel from the end block to a point about 2" short of the CL of the sound hole, then tapers the back up to the thickness at the heel block. Kind of like this:

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Joe Beaver
Maker of Sawdust

Mike, I'm thinking back to my first couple of guitars that I build mostly following Cumpiano. I did use an outside mold to keep the sides aligned and square, but I did not at that time use a radius dish or workboard. I don't have a lot of pictures but I can kind of talk my way thru it.

First, I braced the top with the normal arch - 24 foot or so. With the rim in the mold I kerfed the top and sanded the top of the rim perfectly flat. I then made two curved sanding blocks by tracing 24 and 16 foot curves (tie a pencil on a piece of string that long, tack it down and strike the arc). Put some sticky back 120 grit sand paper on it and sand a little angle into the top rim.

The back is more or less flat from the tail block to the waist, then it starts dropping down to the neck block. I just put a piece of masking tape on the sides to form the rough curve from the waist to the neck and planed the back of the sides. After I got it close I glued the kerfing on the back. After that I used the 16 foot sanding block and kept working the around the sides - rest the block on one side of the guitar and keep working until you get smooth transitions at the head and tail blocks and all around.





Keep testing the fit of the back - it should take a little pressure to hold it down but you don't want to be forcing it so much that it might crack.

I made a couple of cauls out of 1/4 inch MDF - you can barely see them in the background of the second picture. I've tried lots of different methods of clamping with the limited number of clamps that I own - here is a cobbled up scheme with my little go bar and some cauls, including the MDF one on the top



As I've built more and more guitars I've added some radius dishes which I think do a better job of both sanding and clamping. I also now make my molds with a much smaller outside shape so I can get in with various quick clamps and such

edit to add - you asked about making the sides deeper - let me give you a caution based on experience. Build to fit commercial cases if you can, custom ones can get very expensive. I built an 000 shaped 12 string but got the bright idea of making it deeper and a longer scale - the custom case is beautiful but cost me $350 and took 4 months to get here.

That is a good idea on kerfing the caul.
Concerning spool clamps, which I can make pretty cheaply, I would have to take the sides out of the mold to use them; do you foresee any issues with this?

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Joe, that's it.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

The way Cumpiano shows is basically the traditional method used for years and years. It's what Gibson, Martin, and Harmony did for many many years. "Radius dish" construction is a fairly recent innovation.... It turns out that the backs and tops were "curved" - and that curve was just a curve.... Not a radius.... And even better - it's often a "different radius" in different places on the top and back...

The reason the side geometry looks whacky is that you are trying to unwrap a 3D shape into a flat sheet.... And so for example - the end near the head block looks nearly flat, then it flares out towards the waist, then tapers up and flattens out again near the tail....

A good way to visualize it is to simply trace the side of a guitar onto a big piece of craft paper.....

Freeman, thanks for all of that. I considered the 16' sanding beam, but did not know for certain that was the radius he used.
As far as a case, this will no doubt be a personal player. I will have to look at the depth dimensions on currently available cases, but.....for one instrument, I'll build my own case if I have to.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Well, first, I question the assumption that you for sure have to ditch the mold to use spool clamps. If you want to use both, it can be done. Just drill holes through the mold (top to bottom), about 1/8" in from the inside face, about 1.5" apart, all around the mold. Fish the spool clamp rods through the holes. You probably want to take all the tops off the spool clamps before you start gluing the back on, so that you can move faster when you want to apply them.

Second, assuming you need to or want to ditch the mold, it will be important that the back braces "click" into the notches that you cut in the kerfed linings. If the back fits the sides snugly, you should be fine without the mold.

16 for the back and 22 or 24 or so for the top are pretty standard. You are probably right in your first post that the angle is too small to matter on the top but I think it matters a lot on the back. One nice thing about making the radius sanding beams is that you can use them to make your braces, then clamp against them when you brace the top and back. You have to move them and do one brace at a time but with a couple of deep clamps you can get by without a go bar deck and radius dish - I built a couple of guitars that way.

Another trick as you move forward but related to these top and back angles - when you get ready to route your binding channels you can put little wedges on the base of your router with double sticky tape and if you are very careful, route the bindings more or less perpendicular to the sides. I bought a fancy floating router thingie when I started making guitars with radically curved tops but for the first ones a little ingenuity will get you by.

And just commenting about the case. I have built three odd ball guitar that required custom cases, expensive but nice. You can alway use a gig bag or pad an oversized case however I believe that a hand made instrument, even your first, needs a good safe (hydrated) home.

Traditional guitar cases have a very limited range of fit due to the shaped waist area and bouts, so just minor changes to shape can have big impacts on fit. Hiscox cases accept a much larger range of instruments, with the standard dreadnaught sized case taking 12 and 14 fret Martin dreads (including the 12 strings), square and round-shouldered Gibsons (including the wider early jumbos up to 16-1/4" width), small jumbo (J-185 and similar 16" wide narrow waisted guitars), and 0000/J/M sized 16" grand auditoriums. Because the waist is not shaped and the fit strips on the Pro-II cases create just four points of contact on the sides of the guitar, even oddly shaped (intentionally or otherwise!) instruments can be accommodated. Worth checking with Hiscox on their dimensions when considering something off the beaten path.

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For the times they are a changin'

- Bob Dylan

I built at least half a dozen guitars using the method for arching the back described in the Cumpiano book and really had no luck at all.I would get flat spots and it was never really what I wanted sometimes the back actually came out concave.
As soon as I switched to the dome sending dish,everything started working out beautifully so that’s the method I would recommend.It’s basically foolproof


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I use spool clamps and do it out of the mold. I considered putting holes in my molds to feed the clamps though but I didn't love the idea of taking them all apart to put them in and again to take them out (I'm lazy). I take the rim out of the mold but since I use solid (laminated really, three strips each at about side thickness) linings my rim doesn't move at all. With regular kerfed linings, I would worry about the body shape shifting and ending up too long, or too short, or too asymmetrical. I could be wrong since I haven't done it. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Concavity of the plate overall is a result of humidity changes.

It is not too effected by the geometry of the rim.

The OP question was relating to the mating surface of the plate to the sides with kerfed lining.
That and the desire to increase the depth of the body.

I think it got answered.

Something worth mentioning though is an issue that got inserted here.

What Truckjohn wrote is important. Go back and read it, I urge you.

No comment here is wrong in the sense that it is technically incorrect.

However upon searching for a solution to a guitar making problem a future search might lead here.

Bracing a plate be it a back or top, the gluing together the brace and plate needs to take into account the potential change in relative humidity where the finished instrument will be. If there is a chance, which there will always be, of changing humidity the plate needs to be curved. This really has nothing to do with the rim. There are at least two ways to curve the plate so that it won't go concave or the opposite. The first is to curve the brace and glue it to the plate and the second is to glue a straight brace to the plate in a curved caul. In the second case once dry the plate will be fixed in a curve. In these two cases curving the brace is most desirable than the second which is usually called Sprung in brace because it relies on the glue Strength.










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Ken

I have assembled 7 boxes now with solaria, using GTT as guide. My problem has been lack(little/no) of proper back arch, side to side as arch front to back is too great. I made a15' radius dish and now problem solved. More came out/off in the middle.


I have stuck with GTT procedures to avoid confusion. Variations have already caused problems. Book is hard to follow but still best for me.

I still use the Cumpiano back profile method. I have a concern that when you go to a straight taper from the heel to endpin then drive the bus, you are giving up valuable air volume in the lower bout. Plus you are changing the angle of reflection of sound waves off the back. My suspicion is that keeping the back parallel, at least almost to the soundhome, were part of the sound of the Golden Age guitars.

Perhaps just crazy thoughts of a idle mind.

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Joe Beaver
Maker of Sawdust

Another thing to do if you don't want to pay for radius dishes....

Bend the sides and install head/tail blocks. But don't install the linings.

Plane the sides down flush at the head and tail blocks.

Brace up the top and back.

Now chalk-fit the top and back to the sides to help match the curves together. You can start going by eyeball - it's pretty obvious at first when it's hitting super high at the head and tail ends. Once it's pretty close - install the linings and chalk fit to get everything carefully fitted up. Then plane or sand the linings flush to the sides and glue everything up.

If you feel the back doesn't end up with enough curve when test fitting braces - just bend the ruler a bit more when tracing the fair curve..... Or just plane a little more off the ends of the braces until the spring of the curved back matches the brace curve...