Geometry on a brand new OM28
 

Hey guys. Just brought home my first "real" Martin a few days ago, a factory-fresh OM28. It looks, sounds, and plays amazing, but I've been reading entirely too many forum threads about proper guitar geometry. Naturally, I was hoping to get some input on measurements I took on my new guitar.

Action:
Low E: 7/64
High E: 6/64
Relief: .005

Saddle Protrusion:
Low E: 9/64
High E: 8/64

The low E measures dead on 1/2" off the soundboard at the bridge. My only concern is that, when doing the "straight edge test" for neck angle, I hit the bridge about a 64th or so low.

Everything else seems about right to me. If I had to guess on the straight-edge test results, I'd chalk it up to the guitar being factory fresh, and the humidity in my home hovering around 60% (a little high) since I brought the guitar home. I want to bring the action down a hair, but I do believe I have plenty of saddle to work with.

Aside from that, I'm thrilled with the guitar. Better safe than sorry though, any input is greatly appreciated.

If it plays great, then the rest is irrelevant. Try not to look for things wrong with it.

If you want the action lowered, and you have never done it, then see your local luthier. Always good to have it setup professionally for the first time, that way you can see the potential of the guitar.

When I say get it setup by a luthier, I mean, ask around find someone with a good rep, people hang shingles outside and call themselves luthiers with little to no experience.

Congrats on a nice guitar

Steve

I agree with you on "If it plays great, then the rest is irrelevant" but it's never hurt me to seek out the knowledge of greater minds :) Thanks for the input!

I would consider that to be medium/high action but it sounds like you have plenty of room left to get a good set up. I would check the relive too. A lot of times guitars are shipped without ever having tightened the truss rods and you might get excessive relief which will cause the action at the 12th fret to be too high.

I know everyone says a truss rod is not designed to adjust the action but... it actually does in many cases, so check that first. To check it, press down or capo the first fret and then press and hold the 14th fret. You are using the string to create a straight edge. Look for the air space over the 7th fret, there should be barely a bit of airspace there, about the height of a business card maybe. Do it on both E-Strings so that you can get some idea if there is a twist in the neck. If it's really high there then tweak the truss rod about 1/2 turn and do it again till its right. Doing that might actually bring your action down to where it should be and in any case it should be done right anyway.

When I got it the relief did need tweaking, but not much. I brought it down to .005, which is where I usually like it.

My only slight concern with the instrument is the slightly low result of the straight-edge test. Everything else seems perfect to me. Just wanted to get some thoughts on that.

A straight edge hitting 1/64" below the top of the bridge is no concern and is nearly perfect (I consider flush with the bridge surface to be about perfect). If the straight edge hit below the halfway point of the bridge thickness or at the bottom of the bridge, then you probably have a neck angle issue.

You might be a little high at the saddle. Most people go for 6/64" on the bass side and 4/64" on the treble side, but that depends on your playing style, how hard you strum the strings, etc. A blue grass player might like your set up exactly as-is, and a gentle finger picker might want it lower. Taking 1/32" off the saddle would lower the action by 1/64" at the 12th fret. I always save the OEM saddle and use a new one to make adjustments. That way, when/if I mess up it is easy to restore the original condition and start over. And wait a month or so to do the set up, once the guitar acclimates to its new home. Otherwise you might be chasing your best right set-up.

I also vote for "don't mess with success", since you like the way it plays and sounds now.

When you are fussing with thousandths of an inch (1/64" is .016" more or less) the neck has to be adjusted dead straight for the "straight edge test" to be completely accurate. I am curious about one thing - the new Martin's I've looked at in stores have a little hang-tag with the action measurements on them, and they always say 4/62 high E and 6/64 low E. Yours is obviously higher than that at 6/64 and 7/64. 7/64 is within their factory spec window, but for high E the maximum factory spec is 5/64 (or so I am told by Google). What did your hang-tag say?

Brian

I'm new to this and don't have near the experience of most of those who frequent this part of the board, so feel free to jump me, but that Martin's factory-fresh and its straightedge already lands below the bridge top? That, and his action's already where Martin says a reset is due (7/64 max was their number, at least a while back) with an unmodified saddle? I don't know the purchase circumstances, but I wouldn't be comfortable with that geometry. If it were 5+ years old and priced accordingly, then sure, but not brand new.

I just sent Taylor my 520 to correct a finish issue. While it was there, and though I didn't request it because I didn't consider it an issue, Taylor reset the neck (easy-peasey with Taylor, so not a big deal). When the guitar left me in June, the straightedge *just* kissed the apex top of the bridge - it wouldn't slide over it, but neither was it 1/64 below it, as is the OP's brand new Martin. When Taylor was done, the straightedge cleared the bridge by 1/32, which, from everything I've read, is the goal for new guitars, and for well-done resets on older guitars.

Guitars can come from the factory "less than perfect." Necks can be off-center, and neck set geometry can range from excellent to "it passes QC, box it up." When Godin builds a guitar, they set the neck first, then choose from different-thickness bridges to get their final geometry to match the neck angle so everything lines up correctly. Gibson LPs are supposed to have a 4 degree angle (backward, off center) to allow for normal tailpiece height - for years, Gibson pumped out LPs with 5-6 degree angles, which is why you see so many with their tailpieces well off the deck, or cranked down to the deck with the string tension and break angle bending the bridge posts toward the neck.

What I've said here corresponds to what I've read in Dan E's books on neck angle, and from what I've read on multiple luthiers' sites about where they want neck angle to end up on a fresh reset. Why would you allow a worse neck angle on a new guitar than on a fresh reset? The neck angle will only get worse over time and this guitar will need a reset quicker than most, all else being equal.

My knowledge is all book learnin', so I welcome any schooling on this point, but if I was the OP and in position to swap that guitar back, I'd head back with my straightedge toot sweet and find one that allowed the straightedge to clear the bridge (and sounded great).

The straight edge test is also done with the strings off, no tension, for the very reason that it might pull up with string tension and become 'perfect.'

Sounds to me like you got nothing to worry about. String height at the bridge over the soundboard is ideally 1/2 in plus or minus something-ish Probably 1/16th.

So you are fine. Take it in for a good set up and you will be happy.

Did you do the straight edge to bridge with the strings at tension or with no tension ?

The test was done at full tension.

What is the source of the info re: "no tension"? I check "with tension," and so does Kimsey - http://www.bryankimsey.com/necksets/reinstall.htm - looks like Frank does, too - http://www.frets.com/FretsPages/Musi...neckangle.html

I agree the OP can get it set up and be happy, for now. That's a $2500 guitar, it's new, and in his place I'd want to be as happy as possible for as long as possible. That might mean exchanging for a better example. That said, I understand excess humidity (undefined) will raise the top/action, which may be the reason the guitar doesn't presently pass the straightedge test w/higher action. I agree w/OP re: letting it sit in lower humidity environment a few days and trying it again.

Let's work backwards.

What we want is a desirable string height as measured at he 12th fret. We also want there to be "sufficient" vertical distance between the top of the guitar top and the underside of the strings at the bridge. This vertical height provides torque/moment on the bridge. Different manufacturers have different ideal vertical heights. Martin's is usually nominally 1/2".

Next, we want to have sufficient break angle of the strings over the saddle that there is sufficient downward pressure by the strings on the saddle. A number somewhere greater than about 6 degrees seems to be the minimum. Practically, this is probably no less than 1/16" of saddle projection.

Next, we want to have sufficient saddle height projecting from the bridge that, should it be required in the future, for a variety of possible reasons, there is enough there to accommodate that change in saddle height AND still have a sufficient break angle of the strings over the saddle. Many luthiers/manufacturers want to see a minimum of 1/8" to 3/16" at the bass E, a little less, perhaps, at the high E.

Taken together, the vertical height of the strings from the top, the projection of the saddle from the bridge, at the desired string height at the 12th fret - from top of fret to bottom of strings - will leave a gap. That gap is filled by the thickness of the bridge. Put another way, a given string height at 12th and bridge, and a chosen saddle projection, determines the thickness of the bridge. (If you are making the guitar, the thickness of the fingerboard, the height of frets and how much the top will distort under tension also factor into it.)

That's the geometry we are after. Whether or not we have that desired geometry on an existing guitar can be determined with the following measurements performed while the strings are at full tension:

1. vertical string height from guitar top to bottom of strings at the bridge
2. projection of saddle from top of bridge
3. string height from top of frets to bottom of strings at 12th fret
4. thickness of the bridge.

As a quick test, people often put a straight edge on top of the frets that projects from the nut to the bridge. A rule of thumb for Martin style guitars is that, at tension, with a straight neck, the bottom of the ruler should be at the top of the bridge. There are a variety of factors that figure into interpreting the result of that test. They include all of the measurements above, 1 to 4.

For example, consider the case where you place a straight edge on the frets, as above, and it rests just on top of the bridge. If one blindly states, "Yup, that's good, it passes", one might be wrong. If the string height is much too high at the 12th fret AND there isn't sufficient saddle projecting from the bridge to lower the saddle to achieve desired 12th fret height, the straight-edge test "passes" but the desired result isn't obtained - a good playing action and good break angle. One interpretation might be that the bridge is thicker than it ought to be and might be a candidate for thinning to increase the amount by which the saddle projects from the top of the bridge.

My point is that the "straight-edge test" only provides meaningful results if one understands the context, the geometry one is attempting to verify.

Charles, I think I understand, and agree with your points. Left out, however, is another concern, one that I would have (and the OP may have) on a new guitar, and that is, how much useful life is left in the guitar without major surgery (including bridge height reduction)? I'd expect a "new" guitar to have maximum intended life ahead of it.

The measurement goals you mention can all be achieved on a guitar that is well down the road to its first reset, which is fine if playability is the only goal. From a "new" guitar, however, I expect more, including longevity, containment of unnecessary expense and work to make it "right" (work beyond a good setup, that is), and, to an extent commensurate with normal wear/use, maximum resale value. I may be under or over-thinking this one, however. I'd love to learn I'm concerned about nothing!

It's an age old builders trick. I guess coming from the perspective of building a guitar, i.e. it's never had strings on it yet. In determining the proper neck angle you lay a straight edge across the fretted fretboard to the exact location of the saddle and measure that distance and set it so that the height is about 1/32nd inch over your target bridge height and to achieve the goal of getting about 1/2in string height over the soundboard at the bridge.

So I use that same method when repairing guitars as well. Just try it anyway and it will be an interesting experiment. Take the string tension all the way off and do the straight edge test. If it's now flush with the bridge or even a hair higher then imho you have a perfect thing going on, a responsive top. Most guitars are overbuilt so you hardly see any change under string tension.