Official Luthiers Forum!

My heads gonna pop!

[QUOTE=mgcain]

thanks so much for all the good information - As usual, I'll probably study this to death before i put any wood in harm's way, but at least I know I've come to the right place to learn...

wow.

Mitch

[/QUOTE]

Mitch buddy you asked a great question and many of us are having a great time discussing this topic!

This has been said before here and I am sure else where but don't worry about putting any wood in harms way - after all it does grow on trees.....

Actually no Arnt, the doming was the deciding factor. Agreed controlled environment helped but once the guitar is outside the shop I no longer can control that.
The doming allows the extra movement. Whether you put the doming into the braces or compound the sides and braces , that is the key.
   If doming had no effect , why are most cracks on the top ? and back that are radiused seldom crack other than from damage?
   Here in my area we have huge RH swings through the year from 15% in winter to 100% in the summer.
john hall

John, I guess in an effort to save type typing, I am sometimes unclear. Unfortunately, I learned in 1979 that wood expands and contracts at a vastly different rate parallel to the grain than it does across the grain. I built a large walnut coffee table in high school shop class and I framed it with a mitered end cap that was glued all the way across the end grain of the top. In my case, the panel expanded and the miters I had worked so hard on popped wide open in about a month!

And yes, when you have a thin, flexible piece of wood that is braced across it's grain and held around it's perimeter by the rims, it will want to bow upwards as it expands from taking on moisture and flatten or bow downwards as it looses moisture. In theory and maybe to some extent in fact, building in a bow may give it more leeway to contract without splitting in some areas. But there are many areas of the top that are still unable to move freely with changes in humidity. Those include anything adjacent to the rims, the bridge, or the fingerboard. Those areas are locked in by rigid cross grain members and I can't see how they will be any less prone to cracks if the top is domed. And in my limited repair experience, those are the areas that crack most frequently.

Mitch, sorry if I sound like I have pent up angst! I didn't mean to come across that way and I'm certainly not angry at anyone here. But I admit, this is a bit of a pet peeve for me in general.

And Arnt, thanks for the nod of agreement! This is often a lonely position to take. And by the way, a radiused top with flat rims is one of the variations I was referring to but I didn't say it specifically.

For those of you who have not played around with these kinds of changes, I highly recommend it. It is a good tool for customizing sound.

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http://www.chassonguitars.com

If doming had no effect , why are most cracks on the top ? and back that are radiused seldom crack other than from damage?

Because the woods they tend to be made from are so much weaker in their ability to resist splitting. Because of the stress concentrations at the bridge and fingerboard. Because most top woods have greater dimensional changes than most back woods with changes in humidity.

By the way, here's a cool program for calculating that http://justwoodworking.com/software/wood_move.php

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http://www.chassonguitars.com

Kent - my comment wasn't directed at you - or any one person in particular - no worries! I was just shocked that what I thought was a rookie question inspired such great debate!!

I almost feel special!

 

There will always be great debate where there is difference in opinion. Some times we as humans hold our perceptions so close that fight the input of difference. This trait can be a two edged sword. It can protect you from facing unknown dilemmas thereby maintaining your beliefs but at the same time block the acquisition of knowledge that may bring your perceptions crumbling down around you.

To learn for knowledge sake requires you to be willing to let go of current perceptions and embrace change.

[QUOTE=zehley] How do you glue up a brace thats not either all flat or all radiused?
[/QUOTE]
There are a couple of options:
If you have deep-throat clamps, you can use a flexible caul underneath and just clamp the brace to the plate- see any of the early books on building (Sloane, David Russell Young) for details.
If you are using go-bars and a workboard, just make up a workboard something like this (mine needs some contouring before use- this is just roughed out):


I think the Larrivee idea is that the lower bout will be pulled into a dome by string tension, so the braces need not be radiused in the lower bout. I had this concept backwards when I first read it- thanks to Tony Karol for setting me straight on this!

Cheers
John

Kent, that's a bummer about the coffee table, but considering my kitchen table (which is 32" wide and 72" long, 4 pieces of AAAAA curly red maple and banded with an 1" wide frame of popcorn maple) has never exhibited anything like that I wonder....

Said table was made before I was aware of such concepts of raised panel door construction, where you never glue/nail the panel in place and always leave ~1/8" of float (more for alder) on the panels before you glue up the rails and stiles...to the point, my table which is 18 years old now still has tight miters on the banding with no signs of any cracking in the finish (automotive lacquer).  So I wonder if the walnut you used was still green, or perhaps overly dry when you made it.  Perhaps red maple doesn't move as much as walnut?  I did use biscuits and lots of Titebond II, but that shouldn't make that much difference on the subject of wood expansion/contraction.

Mike, sounds like a nice table. Don't know what your secret is but I wish I had some. Maybe it's the glue and biscuits holding the banding together. But based on the calculator I mentioned above (which only gives approximations), it would need to have the strength to compress that maple about 1/4" given a 20% rise in humidity. Who knows.

The walnut I used was kiln dried but it was in NC where the summers can be 90% humidity.

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http://www.chassonguitars.com

This is from Stew-mac's excellent free-for-download dread kit instructions.  It's very much what John described, a flexible board/caul under the top, cam clamps, etc.

Here's another radiused top related issue. Does one radius the bottom of the bridge to match the top surface before gluing the bridge on or just let it flex to the soundboard upon gluing? I'm trying to scrape a radius into the bottom of my bridge to conform to the dome of the soundboard but I don't have a precise method. Dare I go by feel? I've seen some complex jigs to sand the bottom to shape but its beyond me to reconstruct what I've seen. Are the Martin style bridges provided with the Stew Mac kits radiused on the bottom to fit the contour of the soundboard? I didn't concern myself when I put together my first kit but now I wonder...?  Excellent topic. Greg

I shape the bottom of the bridge to match the curvature of the top. My method for creating the correct radius is not elaborate at all; I quickly plane some of the material away, then true the curvature up on a very simple jig. It is just a piece of wood where one edge is radiused to the correct convex curvature with a piece of sand paper is glued on it. I scrape the bottom of the bridge to a smooth surface immediately before gluing it to the top.

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Rian Gitar og Mandolin

Rogue you want to shape the bottom of the bridge to match the exact radius of your top.  And the Martin style bridges that come with Stew-Mac kits are not pre-radiused to fit, nor is any bridge for that matter.

Regardless of what radius we built our tops to the reality may be that the actual radius that we achieve is different from the spec, if ever so slight an amount.

So... here is how you can easily radius the bottom of your bridge.

Using low tack tape or tape that you stick a few times to a clean towel to reduce the stickiness tape some 120 paper over the bridge area of your guitar.



Notice the little pieces of tape on the sides?  These are a marker of sorts as to where the bridge will be located forward to aft and where we want to sand on the paper.  The latex gloves just help to hold onto the bridge and are certainly not required.

Make some marks on the bottom of your bridge and the goal is to sand all the marks away.



Move the bridge on the sand paper, forward and aft, and in no time you will begin to see the marks disappear.  This is the beginning of your radius being sanded into the bridge bottom.



Just continue the process until all marks are gone and then place your bridge in it's exact location to be and check the fit.  You will see that it now fits perfectly on your top.  If you want to ever so slightly scrape the bottom prior to gluing this will provide a better glue joint but only do it with in 15 minutes of actually gluing the bridge.

Lastly, your dome on your guitar and your radiused bridge may change if you don't have humidity control in your shop so as always build, including fit, in a stable RH environment.

Hope this helps.

Sorry for the diversion folks, just trying to give an complete answer to a fellow OLFer.

As a follow-up to Hesh's method-
If you have a stationary belt sander, you can use the same idea Hesh has described, for roughing out your bridges. Take some 1/4" ply and fashion an 'arched' insert which can be temporarily fastened to the platen of your sander, under the belt. With the belt moving along an arched profile, a quick touch of the bridge to the belt will do the bulk of the necessary contouring.
Finish up with the 'sandpaper on the guitar' method, or just by using a sharp scraper and fitting by hand.

Cheers
John

I like that one, John, will try it on my next. Thanks!

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Wes McMillian
Santo, TX
http://www.wesmcmillian.com

If you mark the bottom of your bridge with a pencil like Hesh shows, and rub it on a piece of sand paper on your domed guitar top, in most cases the sanding marks in the middle of the bridge will disappear first.   

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Rian Gitar og Mandolin

BTW Greg wondered why the material was not coming off the center of the bridge bottom first....  Great catch Greg.

It's because I fake all my photos and guitars and they are nothing more than virtual creations in Photoshop.....

Seriously this bridge, in the pic, was CNC'd for me by Sir John Watkins and in doing so he was kind enough, and talented enough, to pre-radius the bottom to 25'.  So my top has less then 25' radius apparently hence the wings sanding first.

Yea I was puzzled as why Hesh's bridge was out of plane. Arnt is right here the deepest part of the valley will be the center of the bridge so the sanding will happen there first. I don't even make marks on the center section. Instead I paint a light coat of white tempera paint on the wings and sand till it is gone on both sides. (evenly) You want to watch as you sand that the removal is happen concentrically about the centerline of the bridge, at least if you are as buttal about the bridge being level to the top as I am