Official Luthiers Forum!
http://www.luthiersforum.com/forum/

Sound Board Geometry Question
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=40244		 Page 1 of 2
Author:  LarryH [ Thu May 02, 2013 6:09 pm ]
Post subject:  Sound Board Geometry Question
I use a 25 ft. radius for my sound boards and then glue the UTB flat which has created an almost perfect neck angle on my last 2 guitars. The current tops have a much more severe angle on the fret board area of the top and can't seem to figure out a better strategy to lessen that angle - either during brace glue up or rim preparation.

The attached pics are showing a much more extreme condition than last night as we're experiencing a Santa Ana (very dry) condition right now but the rise over the bridge is about 5 - 7 mm instead of the 2 - 3 mm I'd like to see. Any ideas?
Author:  DennisK [ Thu May 02, 2013 6:26 pm ]
Post subject:  Re: Sound Board Geometry Question
...time to try the raised neck/cantilevered fingerboard style?
Author:  LarryH [ Thu May 02, 2013 6:37 pm ]
Post subject:  Re: Sound Board Geometry Question
Yikes - I'm having a hard enough time with the 'brace the top hope it works out' technique - but thanks a pant load any way. [:Y:] :D :D
Author:  LarryH [ Thu May 02, 2013 6:50 pm ]
Post subject:  Re: Sound Board Geometry Question
So I'm wondering, is that angle all figured out before gluing the UTB? Do you work up a template to check the angle and adjust the UTB's radius to conform to what you'd like to see there? I guess I just lucked out on my first couple. gaah
Author:  Jim Watts [ Thu May 02, 2013 7:09 pm ]
Post subject:  Re: Sound Board Geometry Question
Larry you mention it's very dry, I think that's exaggerated the doming as the wood fibers are shrinking and the edges are free, thus contracting the top and increasing the radius.
I'd get it stable to the RH you braced it at and check it again.
Author:  LarryH [ Thu May 02, 2013 7:36 pm ]
Post subject:  Re: Sound Board Geometry Question
True enough Jim and that change happened today. Last night BOTH my new tops exhibited the same behavior but not as exaggerated. The rise over the bridge location was about 5 - 7mm and I'm looking for about 2 - 3mm. Maybe a better question, ignoring the pictures is, "How do I dial in the angle that I want while bracing a top?"
Author:  Fred Tellier [ Thu May 02, 2013 8:08 pm ]
Post subject:  Re: Sound Board Geometry Question
If you sand the upper bout of the rim flat at the angle that will match the needed neck angle when the top is glued to the rim it will all match. Hesh had a tutorial on this http://www.lenaweelutherie.info/page6/page6.html see flattening the upper bout.

Fred
Author:  LarryH [ Thu May 02, 2013 9:53 pm ]
Post subject:  Re: Sound Board Geometry Question
Thanks Fred - I've seen that tutorial but I thought there was a down side to it (can't remember what that was) and other methods to achieve the same thing. Is that the method you use?
Author:  bluescreek [ Fri May 03, 2013 9:46 am ]
Post subject:  Re: Sound Board Geometry Question
Setting the geometry is one of the most critical parts to get a good action height. I use a 28 dish as the 25 is a bit much for my liking , but I am a martin style traditionalist. Drying a braced top will lower the that radius not raise it. I would say you may want to take the braces off and redo. That is way too much radius for my liking .
prepping the rim may help and you can always check it by dry clamping.
Author:  LarryH [ Fri May 03, 2013 10:26 am ]
Post subject:  Re: Sound Board Geometry Question
Thanks John. The radius definitely increased as the weather dried - for whatever reason. I checked again last night and the cool, more humid evening had reduced the rise over the bridge location back down to around 5 - 7mm. Still too high for my liking but the movement was pretty dramatic and noticeable. I'll find a strategy that works in the future, for now I'll try and sand the rims.

I wish I had more experience to comment on top radius preferences but the 25 ft' radius is the only dish I have so I'll build with that for a while longer till it proves to be (more) problematic.

How do you set the angle of the top under the fret board? Do you sand the rims? Or do your radius choices get you there 'automatically'
Author:  LarryH [ Fri May 03, 2013 11:57 am ]
Post subject:  Re: Sound Board Geometry Question
Understood Todd but I thought I read where you use a radius dish for your sound board and a 60' radius for your UTB which always gets you close to the proper angle for the top under the fretboard at the upper bout.

I use a 25' radius for the sound board and flat UTB and have been very, very close on two previous tops and way off on these latter two. So I'm trying to understand the process a bit better so I can develop a system that gets me closer than these last two tops.

I was able to modify the first top, and rim, that had too much radius and got it just right but I really thrashed and struggled to get it there. Trying to stop thrashing quite so much - but that might be a distant dream.
Author:  Glen H [ Fri May 03, 2013 12:01 pm ]
Post subject:  Re: Sound Board Geometry Question
Todd Stock wrote:
You won't get anything close to accurate until it's on the rim and the box is closed.


I could not agree more. At a minimum, set it all up in the go-bar deck (if you use one) - dry - like just before you are ready to glue the top to the rims. Then use straight edges and rulers to see how your neck set angle is. If it is off, sand the correction in the upper bout area of the rims.
Author:  LarryH [ Fri May 03, 2013 12:09 pm ]
Post subject:  Re: Sound Board Geometry Question
Thanks Glen. That's what I'm doing now but was wondering, do you use a 'system' of sorts to get you close to start with? Do you use the same method to sand the rims as shown in the tutorial above? I thought I read where it's not so great to have that angle break in the top when using the method shown in the tutorial and that there were other, better methods.
Author:  Greg B [ Fri May 03, 2013 1:29 pm ]
Post subject:  Re: Sound Board Geometry Question
If the Santa Ana winds are an issue, I assume you are in southern California. The humidity is 5% right now where I am. In these parts it can go from 8% to 85% in a matter of hours, which is something people from other states don't have to deal with - I don't think.

That top is not going to stay still until it is glued to the rim. In a way, you're lucky it's not moving the other way, that's the usual problem. Typically a top will go flat or concave as the humidity drops to bone dry. I've found it's best to brace at around 25%. My goal is that the top will never descend past dead flat as the humidity approaches zero, preferably maintaining a small amount of arch.

IME bracing at 45% will leave you seriously screwed when the Santa Ana comes.
Author:  Alan Carruth [ Fri May 03, 2013 1:43 pm ]
Post subject:  Re: Sound Board Geometry Question
Greg B wrote:
"The humidity is 5% right now where I am. In these parts it can go from 8% to 85% in a matter of hours, which is something people from other states don't have to deal with - I don't think. "

You've never lived in New England....
Author:  LarryH [ Fri May 03, 2013 2:22 pm ]
Post subject:  Re: Sound Board Geometry Question
Todd Stock wrote:
LarryH wrote:

I use a 25' radius for the sound board and flat UTB and have been very, very close on two previous tops and way off on these latter two.


Are you saying you have closed two boxes and have issues, or you think you will have issues based on what you've seen with tops free of rims? If you are looking at free tops, there's certainly going to be some distortion with RH shifts, but if the top and back go on at the bracing RH (I do everything at 40%), should be OK.


I'm finding asking the right question most difficult.

I've closed two boxes with the 25' radius dish and flat UTB and all the geometry, both free tops before, and tops after gluing, was basically right on. Right on to me is having a 2 -3mm rise at the bridge location. And I guess I'm saying I have no idea how that happened. I thought it was repeatable with the 25' radius and flat UTB but not so much.

This time the free top has 5-7mm over the bridge location, previous tops had my desired 2-3mm. Again no real clue why. The RH is not a factor as I won't glue until it goes back up but it still leaves me with that 5 -7mm that I don't know how I got (remember previous tops came out at 2 -3mm). Should I add some radius to the UTB next time? Or is it kind of a crap shoot and adjustments are just part of the process?
Author:  LarryH [ Fri May 03, 2013 2:26 pm ]
Post subject:  Re: Sound Board Geometry Question
Greg B wrote:
If the Santa Ana winds are an issue, I assume you are in southern California. The humidity is 5% right now where I am. In these parts it can go from 8% to 85% in a matter of hours, which is something people from other states don't have to deal with - I don't think.

That top is not going to stay still until it is glued to the rim. In a way, you're lucky it's not moving the other way, that's the usual problem. Typically a top will go flat or concave as the humidity drops to bone dry. I've found it's best to brace at around 25%. My goal is that the top will never descend past dead flat as the humidity approaches zero, preferably maintaining a small amount of arch.

IME bracing at 45% will leave you seriously screwed when the Santa Ana comes.


Yeah wicked dry here in San Diego too. I'll wait until the RH stabilizes before gluing top to rim and from experience that will be 45 - 50% around these parts. The bracing was probably glued at that same 45 - 50% and as long as I keep the garage door closed I can keep the RH from dropping into single digits.
Author:  Goodin [ Fri May 03, 2013 2:53 pm ]
Post subject:  Re: Sound Board Geometry Question
Question for Todd and John Hall: I've been searching through old threads on this subject and read some of your informative posts but still a bit fuzzy. Here is what I gather for building up the geometry for the top. Please let me know if I'm on the right track. (Larry, I hope you don't mind me butting in here, but it's right on subject with what you are dealing with!)...

X braces radiused the whole length of the brace to 28'.
Tone bars and finger braces have no radius.
Upper transverse brace radiused to 60'. (or is it 42' for Martin's)?
Sides/lining edges are trued up with a 28' radius dish.

This schedule will get a 1 1/2 degree angle? What I don't get is how the sides are able to glue up accurately with all the different radii on the top braces.
Author:  Goodin [ Fri May 03, 2013 4:26 pm ]
Post subject:  Re: Sound Board Geometry Question
You radius the bridge plate?
Author:  Tom West [ Fri May 03, 2013 4:48 pm ]
Post subject:  Re: Sound Board Geometry Question
Check here.
viewtopic.php?f=10117&t=25931
Tom
Author:  LarryH [ Fri May 03, 2013 5:17 pm ]
Post subject:  Re: Sound Board Geometry Question
Goodin wrote:
You radius the bridge plate?


Interesting - I never have.
Author:  Tom West [ Fri May 03, 2013 5:26 pm ]
Post subject:  Re: Sound Board Geometry Question
Some folks radius the plate and some don't. I'm in the don't camp. I put the X on first and the plate next and at .100" or so for the plate, it easily conforms to the top radius. I think either way is acceptable.
Tom
Author:  Goodin [ Fri May 03, 2013 8:19 pm ]
Post subject:  Re: Sound Board Geometry Question
Perhaps, just glueing on the bridge plate in the radius dish is what TOdd meant. THanks Todd.
Author:  Joe Beaver [ Fri May 03, 2013 9:55 pm ]
Post subject:  Re: Sound Board Geometry Question
Larry, I would let the RF return to the normal dry, as opposed to the current severe conditions we have now. There is a chance of rain later in the weekend. Give it until Tuesday or Wednesday and see what you have. I have had to work around the So Cal RH myself. I did close one up when I shouldn't have. It never got right, had to take the top off and remove/reglue the braces. It was a mess!
Author:  LarryH [ Sun May 05, 2013 3:07 pm ]
Post subject:  Re: Sound Board Geometry Question
Just a quick follow up on this issue and one of the things I HATE about hobby building. The part I hate is I can never remember the process from build to build so It's like I'm constantly starting over.

Anyway after I braced the tops I measured the free top angle and it measured too high or 'over set'. I placed the free top on the rims and cut the bracing notches, pressed the top down and checked again. Still too high an angle. Then I trimmed the waist and the UTB areas to lessen the angle and got it right on EXCEPT I was measuring this on a flat surface which was in essence moving the upper and lower bout edges down with the radius of the back distorting the angle.

Then when I glued the top on, this time on the 15' back radius dish, in the mold, without that distortion, the angle was now too little. SIGH. And of course now I remembered the process I used BEFORE to get it right, which I had forgotten because that was months ago. Sigh. I sanded the upper bout to get the angle back but now understand the initial free top angle was probably about right but I need to test the top in the mold and over the back radius dish. Sigh.

I'll write down the process and try and remember the next time and maybe this post will help someone else.
Page 1 of 2 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/