fitting back to rims

[tim crain 54]

I want to make sure that I understand the fitting of the back to the rims before I proceed. I have Ken's back and rim contouring system and have the back bent to a 15' radius. The rims have been tapered per the J185 plans that I am working from and then contoured to the 15' radius. As I understand the radius is set by the bracing being glued to the back in the caul and the curvature is achieved when the back is glued to the rims. I have the rims notched for the braces on the back but the amount of bend that is needed to curve the back to the rims seems quite a lot. The top fit very nicely with very little effort to get it to mate with the top. I guess my question is this, is the amount of gap that I am seeing on the back normal? I have read that you don't want to be adding stress to the instrument while you are constructing it and this seems like it would add a huge amount of stress. Thanks for any insight and thanks for the response on getting my pics attached. Tim

[ken cierp]

Is that the AGP16 plan? If so I don't see any side profiling pattern on that drawing and that might explain the problem. Although it slopes from the tail to the neck block the 2D side profile is not just a straight line. There are some sweeping curves, but most important is the rise that is necessary at the inner most point of the waist to accommodate and shape the spherical back plate dome, especially important with tight wasited styles like the J185. Bottom line is -- the back edge of the rim looks to be way too flat.

Here's a demo of our side profiling layout system, note the undulations in the final pattern

http://www.kennethmichaelguitars.com/si ... lates.html

[tim crain 54]

Yes I am working from the Scott Antes AGP 16 plan. There is no side profile so I went on information from the Jonathan Kinkead book which describes tapering the rims by removing the appropriate amount of material from the heel end and making a smooth curve starting at the mid point of the lower bout working up to the heel. I am still not clear on what the difference is, I went over the rims with the contour bar as described. I would think that it would achieve the same thing right?? When the top went on so well I thought this is a piece of cake, Hah!! Well how bad is it?? I sure appreciate having a place to turn for some of these questions. Thanks much for the help, Tim

[ken cierp]

Unfortunately -- you have been tripped up by the flaw in Jonathan's process -- it does not produce the correct geometry -- its discussed here:

viewtopic.php?f=29&t=34

[tim crain 54]

Well I can see I have a problem, any ideas as to how to salvage my project? I would think that if the back is formed in a 15' radius dish and the rims shaped in the same dish we should be at least in the ball park. I know on your rectangular contouring form you state that the geometry of the braces is not correct to a radius dish, I would think that if you sanded the braces in a circular or figure 8 pattern in a radius dish that they would end up with the same geometry as the dish and should fit perfectly when gluing them to the back while it is pressed into the same radius dish. With the rims sanded to the same radius it all should fit right? Isn't this basically the same process I have seen in videos of the Taylor factory where they hold the rims against a rotating radiused sanding disk?? What part of this puzzle am I missing?? Thanks for all the input and patients, Tim

[ken cierp]

There is nothing wrong with your back plate or the back braces. Swirling the braces in a figure 8 on the retangular abrasive dish, radius dish or any other place will not correct the geometry and will only make a bigger mess. That tends to end up rounding over the gluing surface of the braces which makes the fit less than perfect -- the glued edge needs to be contoured and perfectly flat across the width (just like or KMG Kits and all the factory made guitars that don't want warranty problems) The problem here is with the back your rim -- it has no profile, its flat. Since you mention the rotating disk I believe you have this visulized. The back plate is a dome getting higher as you move toward the center thus the inner perimeter (waist curve) has to get higher to match that dome. Also, the back plate should only be about .100" thick. Do you have access to a 15' radius dish? If you place the rim in the dish you'll see how it touches at the ends and not at the waist. You can make the rim usable by rotating it on center against a radius (circular) abrasive dish. A rotating 15' abrasive bar could also be used but has to be positively located -- any tilting or tipping will ruin the plane. A lot of material needs to be removed. Do not try either of these methods until you fully understand what needs to be accomplished. The back shown below has a 15' radius you can see how little pressure is need to clamp it to the properly shaped rim.

[John Parchem]

I agree with Ken that you can correct that your problems with a 15" radius dish. Looking at your pictures the only area where I am sure I see an issue is in the last picture where the waist has a band on either side and there is a gap at the inside of the waist.
As the braces do not provide a heel to tail curve, the pictures showing the tail lifted while the neck block is clamped is normal. It does look like you will need to loose a bunch of material from the rims before you close the gap in the rims near the waist.
To repeat Ken's advise get a 15' radius dish with sand paper and just place the rims in it. Look at how the rims fit in the dish. There should be no gap between the rims and the dish anywhere. If there are the areas that are touch the dish need to be sanded until there is no gap.