KMG MM used with Obrien/LMII neck jig...

[RandyG]

Hey guys, great resource you folks have created for us Sunday morning deep thinking couch sitters/coffee drinkers...

Anyway, I am more or less just sitting here drinking coffee and doing some "thought experiments" dealing with building guitar bodies and necks from scratch using both the KMG MM and the Robbie Obrien/LMII neck angle jig.

It seems logical that if both jigs work as easily as advertised, a builder could readily use the KMG MM to build the guitar body to a pre-specified angle at the neck block, and then also be able to use the Obrien/LMII neck angle gig to build the neck with the same angle used when building the body with the MM.

Since both the body and the neck are built incorporating the same angles, it seems that they should simply slip together perfectly with very little,if any, adjustment needed.

Just sitting here on the couch drinking coffee, it would seem that the only undefined variable would be what neck block angle is necessary to cause the extension of the fret board to slightly graze above the top of the saddle-less bridge.

What are the obvious oversights in my "thinking" ?

Thanks,

Randy

[ken cierp]

With a 1/4" thick finger-board and moderate dome in the sound board (52' - 40') 1.3 degree or so slope will work just fine. But here is the magic involved in achieving that "mythical" straight edge brushing the bridge construction strategy (which in itself is a variable). The major makers have a range of bridges that vary in thickness, that way the string plane can be matched to the optimum height and action dictated by the mandatory straight line finger board path. Tolerance component matching is the key to producing accurate high quality assemblies and is a basic in all manufacturing arenas. Also, the saddle height can also be adjusted as well. Now really how simple is that!

I can assure you that plenty of guitar projects have been really screwed up by the makers needlessly carving away at the neck joint. Trying to rectify this "brushing/grazing" notion.

[Tim Benware]

I would add to what Ken has said and tell you that I use both of those jigs and have built over 40 acoustic guitars now. I use a 25' radius on my tops BRACING below the sound hole (x braces and tone braces not the finger braces or bridge plate), I sand the top rim flat and the angle from the neck block to about the top of the sound hole to a 1.3* angle.

Each guitar comes out slightly different requiring minor adjustment to make the fit nice and tight (after all wood is wood and has different tolerances to each piece). I place the bridge on last and custom fit it's thickness. As Ken stated, and I can attest to as I have been to the Martin factory to see it, they have bins full of bridges with different thicknesses. When the body comes to that station they have a jig to measure that thickness and pull a bridge out of the corresponding bin, voila. Custom thicknessing the bridge at the end of the build has the same result.

[RandyG]

Probably am missing something huge here, but it seems that overall, both of the above replies sort of confirm my couch sitting/coffee sipping hypothesis that given an appropriate fixed bridge thickness, a guitar builder could simply dial in a 1.3 degree slope on Ken's MM for the top of the box's geometry, and then dial in a corresponding 1.3 degree angle on Obrien's neck jig for the neck heel geometry and, viola, just like magic, you have a guitar with a perfect dovetail, dialed in with perfect predetermine action.

How easy can it get fellas?

[Dave Bagwill]

You can try, but do remember that box of bridges the Martin techs use - if it was easy, they would not need that box.
I always end up fiddling a bit with things, though the bolt-on m&t is a little easier I think; it's what I use anywhoo.

[ken cierp]

Wood is organic -- always subject to subtle changes

Ditto what Dave said --- Martin has millions of dollars invested in the best computer controlled machines and a factory with a constant 45% humidity --- but still to get it right, they have to tolerance match their parts.

[Tim Benware]

The above two responses is what I was trying to convey (Ken & Dave). There will be some fitting needed on most builds. If you start with a fixed thickness on your bridge you may get lucky and have it fit exactly right, but experience tells me that is the exception not the rule. I think I had that happen twice early on before I realized it is easier to complete the thicknessing of the bridge just before glue up.