fitting back to rims

[tim crain 54]

your illistration is how the front went on, very little clamping pressure needed. The front was braced using your 25' radius dish and the rims/kirfings were angled 2% by hand. I have your 15' rectangular radius caul but not the dish. I guess what I don't get is that when bracing the back using the rectangular block you are only creating a curve across the width of the back, the length is straight thus when place on the rims that have been shaped in the dish don't you end up with the same problem I am facing though to a lesser degree? The back is just slightly over .100, it is .103. Wait, the light bulb is coming on. When I glued up the top the only place that there was a slight gap was at the waist. Since the rims were flat and angled by hand that left the inside curve at the waist low!! Why didn't I use the radius dish?? Being flat and only a 25' radius the problem was minimal as opposed to a 15' radius with a curve to boot. Looks like I will be buying a radius dish. Thanks for the guiding hand, Tim

[ken cierp]

don't you end up with the same problem I am facing though to a lesser degree?

NO --- again your back plate is fine your rim is not the correct profile,

The little tiny surface of the brace wood edge cannot "shape" the 20" length of the back plate -- how could that happen? Its the properly shaped and beveled rim as you have said, that defines the length curvature and refines the overall dome shape of the back.

[dave d]

Wait, the light bulb is coming on. When I glued up the top the only place that there was a slight gap was at the waist. Since the rims were flat and angled by hand that left the inside curve at the waist low!!

A eureka moment! Yes, that sounds like the origin of the problem. Conceptually, I certainly encountered the same challenge at that stage. Don't know if this helps, but in 2 dimensions I think of it like this.

sideview.jpg (37.62 KiB) Viewed 2013 times

The neck-to-tail curvature of the dome is handled by the side profiling and sloping of the rim with the radius dish/bar, and the widthwise curvature of the dome is handled by the 15'radius braces and the rim sloping.
To get rid of the gap at the waist, you need to remove material from the high points along the rim, as John said.

[John Parchem]

One comment you wrote stuck in my head, I have read it before as well:

" I have read that you don't want to be adding stress to the instrument while you are constructing it"

To clamp the back to the rims head to tail is going to require that the back is bent to the vertical curve sanded into the rims. There will be stress.

Clamping a flat back plate to curved braces adds stress

Clamping a flat top plate to curved braces adds stress

Stringing the guitar adds a bunch of stress that actually powers the guitar.

So by design we add stress to the top, stress to the back, stress to the whole guitar with the strings

what is this comment about not adding stress ???

[ken cierp]

Ditto John -- a guitar correctly constructed is "stressed" and tensioned to the point where the energy from the slightest string motion is transferred and transformed into a bigger more elequent sound by the top and the guitar body in general.

However, there is a point where a "forced fit" is solely a matter of craftsmanship -- to me this issue/problem falls in that catagory and really needs correction.

[John Parchem]

Ditto John -- a guitar correctly constructed is "stressed" and tensioned to the point where the energy from the slightest string motion is transferred and transformed into a bigger more elequent sound by the top and the guitar body in general.

However, there is a point where a "forced fit" is solely a matter of craftsmanship -- to me this issue/problem falls in that catagory and really needs correction.

I agree that in this case the rims need properly profiled before gluing on the back.

[tim crain 54]

Thanks for all the help getting this into my head. Just ordered my 15' radius dish, I'll let you know how everything works out. I do have one question that came to mind, when using the radius dish to radius the top of the rims to match the top, do you have to go back and flatten the heel block? I'm understanding that it should be flat at that point so the fingerboard will lay flat on the top, is this right? Thanks again, Tim