'69 Martin D-35 reset question

[Chris Scott]

Note - edited for clarity - please re-read if you've already seen this

 

Hey folks, first post here, and glad to find a place to ask stuff a bit more "advanced"...

 

...my question involves a reset on a D-35. No major out-of-the-ordinary going on here - bridge was shaved to approx. 1/4", and even with this having been done, the action  is uncomfortably high, so after a new bridge installed we're talking a fair bit of material to come off the heel.

 

Here's where I'm at; With the new bridge (which is a Stew-Mac replacement with 1/16" bigger footprint already) I've got 3/16" from the leading edge of the saddle slot on the etreble end to the corresponding edge of the bridge, which imo is right on the edge of acceptability - the higher the reveal of the saddle, the more force pushing forward, which will increase the chances of a split. Fwiw, I've already fudged the bridge forward as much as I can without revealing any of the old glue line.

But that's not what got me concerned. I'm actually worried about is this; as I mentioned before, this one's already pretty darned under-set, and by the time it's reset properly, the (as a reference point) 12th fret is going to be a bit farther away from the saddle, as I'm "rotating" the neck AWAY  from the body in the reset process, which will obviously require moving the saddle slot even further forward, burning up more of my already small amount of material seperating the saddle from the leading edge of the bridge - am I making sense here? 

I'm wondering if anyone here has any figures that might show me just how much further away the 12th fret (again, just a reference point on the f-board) will be from the body after the reset procedure, as I'm thinking if it's more than say a 3/2nd, I might have to fabricate a new bridge with more material on the neck-side to compensate, simply to avoid the possibility of a bridge split-out somewhere down the road.

 

Thanks in advance! 

[nathan clark]

Instal a new bridge sans saddle slot, reset the neck, refret, then locate and cut the saddle slot. Stew macs "the intonator" makes a great temporary saddle:

http://www.stewmac.com/shop/Tools/Special_tools_for_Bridges/The_Int...

[Eric Steven Johansen]

You never mentioned what kind of nut to saddle distances you have existing in the current set up. Many times when a neck needs resetting the scale length is already a bit short and correcting the neck angle returns the scale to proper length. If a correction is needed after the angle is set it can be accomplished by evenly removing material from the entire heal to body joint, essentially moving the nut closer to the saddle. I'd first check the existing scale as that might answer your question. Welcome to the site BTW.

[Chris Scott]

Hey, thanks you guys. I already was thinking of just shaving the neck joint closer, but it's just an option at this point.

  - and btw, I arrived at my conclusion w/ a saddlematic. one of the best tools I own...

 

[Eric Steven Johansen]

So what does the saddlematic say? If you're short an 1/8" or so un-reset, then you will likely to be golden when you're angles right. If your scale is right on before reset you'll need to adjust, likely at the joint, although I have been known to favor my saddle shape a hair forward as well if needed.

[Chris Scott]

Thanks Eric. If the reset pulls the nut 3/32" closer, I'll consider the job 100%

 

Spoke to FF today, and he seems to think it won't even be an issue.

 

I'll mock assemble the thing anyway, so we'll know soon enough how much (if at all) it'll be off.

[Mark Pollock]

Okay, I might be TOTALLY off base here, but why would tilting the neck back slightly cause more distance between the nut and saddle?

I think of the top of the dovetail, where the fretboard meets the cheeks of the neck joint, as a fulcrum. Tilting the neck up and down doesn't drastically alter the distance from the saddle. I'm sure we could come up with some kind of logarithmic calculation for what distance would be added.

Rather than thinking of "rotating the neck away from the body", think of "tilting back on a fulcrum", which is closer to what you are doing.

[Jeff Highland]

That's my take on the geometry too Mark.

If you shaved off 1/16" at the end of the heel it would only potentially move the 14th fret  about 1/256" if you tapered the cut to nothing just under the fretboard

In practice - nothing

[Eric Steven Johansen]

The largest change in length will occur between the nut and the saddle, which I think is the distance Chris is concerned about. I agree with the fulcrum concept, but I set my scale length from nut to saddle so that's the measurement that I look at.

[Chris Scott]

 So, after reading all of your thoughtful posts, I just tried something. Went into the shop and grabbed the first 2 objects I saw, which in this case was a couple of frets I removed from the fb extension of said guitar, and I performed the following (very Rube Goldberg-esque) experiment;

In my left hand I placed one of the frets lengthwise underneath my trusty 6" machinist's rule, with one end even-up w/ the "0" end of the rule. Next, I placed the other fret underneath and ACROSS the rule, butting up to the other end of the other fret - a simple "T", under the rule. I have the "cross" fret upside-down with the tang sticking up, which as you might now guess is my indicator, reading quite nicely on the upper 32nd scale.

 

Sooo, as I raise and lower the "0" end of the rule thru an approx. 1" arc, the 2nd fret tang reads a variance of just over a 64th, showing me that for now at least, my theory has merit.

 

Thoughts? - I fully realize that this little exercise might well define the very concept of splitting hairs, yet it (for now at least) seems to show that I may very well end up with something I'll need to deal with, given my limited "safety zone" between the treble-side of the saddle and the corresponding front edge of the bridge...we'll see soon enough.

 

Thanks again everyone, and shoot - keep 'em comin'!

 

   

[Mark Pollock]

I'm trying to figure out the geometry here, and I'm having a heck of a time. I'm not putting this out there to assert myself, but to try to figure out what we're dealing with.

So, first, we establish the point that will remain the same during a traditional reset - that is, the part of the neck where no material will be removed. Looking at the heel sideways, we would remove material from the bottom of the heel, in a diagonal line that ends where the top of the heel at the fretboard meets the body. That end point is the point where the neck rotates as we make the reset, in normal circumstance with correct bridge placement.

Okay, with that point in mind, we can then use that as the center of a circle, and any point on the fretboard as the other end-point of the radius of a circle. This radius starts from a point under the 14th fret where the fretboard meets the body on most guitars, and continues with a line to whatever fret we wish to measure to, proceeding diagonally through the fretboard.

This traces the movement of the frets in relation to the body and bridge. The circle remains constant as we move the neck around, but moves with the neck in relationship to the body and bridge.

Calculating the length of that line is a minor challenge unto itself - it's a diagonal, so if our fretboard is a 25.4 scale length, then the ideal distance that I get from the StewMac fret calculator from nut to 14th fret is 14.086 inches. But we need to account for the thickness of the fretboard as well, so, assuming a 1/4" thick fretboard, and using the good 'ol Pythagorean theorem, I get 14.088 inches from the nut to the point where I've got the fretboard and body meeting. This is really inconsequential, but let's stick with it.

So, we rotate the neck back one degree. The relationship between the nut and other end of the radius does not change, but the relationship between the nut and the saddle does change, because my circle has moved. The nut has moved .122 inches downward.

At the other end, the pivot point measurement to the saddle is 11.314 squared (that is the remaining amount of the scale length), plus the original height of the saddle from the plane of the bottom of the fretboard squared, then the root of those two. After the reset, the value of the height of the saddle will change. This change will then affect the length of the space between the pivot point and the saddle top.

I'm out of time, I need to head out to celebrate Father's Day (and a Happy Father's Day to all of you Dads, btw). I think I'm really barking up the wrong tree here, but I'll try to figure out where I was headed when I started looking at this.

[Mark Pollock]

I figured out how to calculate the change - I'll put together a spreadsheet this evening. Basically, we end up calculating 2 distance - the distance between the pivot point and the saddle, and the distance between the nut (or whatever fret you want) and the pivot point, then we can establish the distance from the nut and the saddle as the third side of a triangle. When we vary the length of the distance between the pivot point and saddle (by tilting the neck and then changing the height of the saddle), we can use that change to determine the change in distance from nut to saddle.

This math is really proving to be tough - I teach 3rd grade for cryin' out loud!!!

[Chris Scott]

Hey thanks a bunch Mark - though it might be a non-issue for most folks, it's something I'm really curious about, so I'm totally appreciative of your kind efforts.

 

I'm cursed with a strange disease called Perfectionitis - perhaps you've heard of it, as it seems to strike luthier types in particular...