Help Needed - neck set, bridge height

[darren]

Ok, I have to humbly ask for help here. I've read all the 'theory' i can handle, and still can't be sure what to do at this point, so you guys are going to have to weigh in... This is kind of the tail wagging the dog, but here's the situation.

I'm at the point on #2 where I need to carve my bridge, and am looking at what height it needs to be. I'm afraid that in order to make this thing playable, I'm ending up with a rather short saddle in a short bridge. Here are some measurements:

• Neck angle 2.5 degree
  Fretboard height 7mm
  gap above SB at scale length (neck is straight, ruler on frets) 7mm
  humidity: 60% (tried and tried to get this down, I'll be getting a de-humidifier today to see if the top comes down)
  built following Kinkaid's book and plans, which show a 2mm arc over the length of the X braces, 2degree neck angle, and a 10mm bridge height

So... with action set at 2mm at 12, i'm getting a 4mm saddle on a 7mm bridge.

1. Will this be acceptable and what steps would you take if you were in my shoes? I'm waiting to see if humidity has got this top too high, but I want to plan my course of action.

2. If the bridge indeed ends up this short, would I benefit from a wider footprint to make up some of the lost mass. bridge material is Macassar Ebony.

3. while I'm asking questions, how deep would you slot the saddle slot (kinkaid says to within 3mm of the overall bridge height)

Edit
Thanks in advance for any help you can offer!
/Edit

[kencierp]

2.5 degree neck angle is pretty steep? --- At this point in construction --- "set-up" the instrument needs to be at the actual normal situation humidity not a control condition -- so drying it out is not a good idea, when you place it in the normal everyday humidty the set-up will be off. As for the 1/4" thick bridge -- that's what most Gibsons have so it not a big deal. The saddle slot depth can be almost, but not through the bottom of the bridge. Again, some old Gibsons actually had the slot all the way through and the saddle rested right on the sound-board.

[darren]

Thanks Ken.
Agreed, the neck angle is pretty steep, I realized already when fitting the neck that this problem would come up.
As for humidity, the guitar has been in my basement 'shop', it's now upstairs which will be its actual normal situation.

When laying out a bridge, does anyone account for any 'bellying' when the strings are installed?

[tippie53]

You may have designed a 2 1/2 degree but what does the guitar really need ? I hope Ken chimes in again but you have to look at what your targets are . In the perfect world you want a 1/2 inch string height at the bridge with action at the 12th fret a 7/64 on the Bass and 5/64 on the treble . The bridge should be about .375 and .125 saddle. Allow a 1/16 tolerance and you can then be at .400 bridge max side and .160 max saddle.
You may need to adjust the neck set where this will be what falls out . If you have to make the bridge / saddle area much over 9/16 .563 you may be locking up the top by over stressing it . I have a neck set video available on you tube that may help . http://www.youtube.com/results?search_q ... etail&aq=f and with what Ken will pop up to you , you should be able to find a solution. If you are designing a bridge that takes into account a belly you may not be designing things the right way . I don't design for a belly . The top should be braced well enough to handle the load.

[kencierp]

This puzzle is a little curious in that with the "neck set angle" at 2.5 degrees, the plane of the top of the fingerboard should be WAY high over the bridge --- actually opposite of what you are describing. Is the 2.5 degrees measured along the neck heel and the gluing surface of the FB to the neck? In other words 87.5 degrees. Also, what contour did you sand on the top braces?

[Tom West]

Darren: Not sure what you mean by "short"....? I assume you mean height...? Also I find for the guitars that I build my measurments come out as per John's and I must admit I concur with all of John's post.While I know low bridges are used,I used them myself on some of my early guitars,I think better results can be had with thicker bridges. The bridge is the stiffest brace use on the top and a certain height is needed to get the proper string height, without using a high saddle, to be able to drive the top at it's optimum. Just my two cents worth,your getting enough help from the other folks. One more comment re: compensation for bellying. In set up I allow about 1/16" for the top to rise and the guitar to settle a bit after pressure from stringing. Not sure if that's what you meant. Good luck.
Tom

[tippie53]

I agree that the top settles to a degree. Look at what is happening upon stringing up. The neck wants to rotate forward from the string tension , and the bridge starts pulling on the top along with the torque of the strings and saddle. This also applies a compressive force between the neck block and bridge . Finally the lower part of guitar is forced under tension so you will expect some movement . I look for about a 1/16 also. I use a straight edge from the fretboard to the bridge and I like to see the line off the top of the bridge by 1/16 to 3/32. After about 2 weeks the guitar will need to be adjusted for the load. Often just a saddle tweak.

[kencierp]

Darren,

Just to make you feel a tad bit better, note that at the Martin factory the high line guitars 18’s up, dependant on precise neck set angle, are fitted with a range of bridges (can’t remember exactly) I believe .360” to .410”. So in that regard tweaking the bridge thickness is not unique at all. Also note that these are belly bridges and they indeed taper down close the .25” at the outer ends of saddle. As mentioned early on, the math, science and common building and design practices for the most part, indicate that less soundboard mass/weight is generally and theoretically a good goal, IE brace shaving -- more recently carbon fiber bracing. CF laminates, honey comb etc. The good news is -- changing strings (different overall string tension) and properly chambered bridge pin ramps (string angle of attack to saddle) go a long way in establishing effective energy transfer. And as John mentioned honing in the action with the saddle height after the guitar has been under tension a while is common -- as a matter of fact in the KMG assemble manual we recommend waiting a couple of weeks before making final adjustments.

Back to your specific problem – unfortunately as much as I like the Kinkead publication there are some math errors that simply jump out --- yes one of them is the neck set complimentary angles (neck heel to sound-board) if you literally follow his recommendations one would most certainly build in a 14th fret hump. Another thing that I noticed is that even on the line drawing (which is a rendering since scaling show different dimensions) the fingerboard is somewhat thicker then the standard. Did you make your FB based off the drawing or is if .23” + frets at the high point?

[darren]

Tom, yes bridge height off the soundboard. You got it, I was referring to the top 'rising' on being strung up, not bellying. Wrong terminology on my part. I realize that a lower bridge and saddle combination will drive the top less than a higher combination.


John, I didn't design a 2.5 degree neck, I'm following Kincaid's book and plans which call for a 2 degree neck. (p.81) When fitting the neck i realized that string height was going to be way too low and was able to sand the top another .5 degree and change the angle at the heel to match. Right now it (string height) looks to be .47 at the bridge location. You said about a .125 saddle, and I have .157 calculated now, so it looks like I can take the saddle down if necessary.


Ken, This is a curious puzzle isn't it? Yes, I measured as you described and got 87.5 degrees. The only explanation is that I put too much of an arch on the x braces. The plans call for a 2mm soundboard arch (p. 56 illus. 34) I must have over-did it... sheesh


Thanks for your time guys. I'll keep you posted.

Edit/

Ken posted as I was posting...

yes, I flattened the upper bout to get rid of the hump (He does say to plane the fb from the 14 fret up to reduce the hump) (p. 122) and have noticed he uses and calls for a pretty thick FB - I'll measure mine this am again.

[darren]

FB is 7mm or .275 at highest point. Directions only state to "Plane the surface of the fingerboard straight...to no less than 1/4 in (6.5mm)" (p.122).

In this case would the guitar benefit from the additional height of the fb, giving me an another .5mm from strings to soundboard?...


perhaps this thread needs to be moved to the scratch/Guitars board. anyone?