Rim sanding and neck angle

[Tj Turner]

Afternoon everyone…

Seems like I always get stuck scratching my head on this one, and I would have thought I'd have it straight on guitar #3, but I think I fudged and got lucky on the first two. So before I make a mistake that can't be undone…

I've got my rims all together with tail block and head block. I sanded the top with my 28 foot radius disk, and I now need to get the 1.5 degree in for the neck angle. I thought I remembered a discussion a while back about the 28 foot radius disk giving us just about a 1.5 degree angle…is that right? Certainly the peak height on the rims is about at the waist, so there is some angle already imparted by the radius disk. Also, I believe Ken recommends using a flat profile (as opposed to the 28' radius) from the sound hole to the top of the guitar, where the 1.5 degree angle is sanded into the rims. Am I right about that? If so, do I follow the 28 foot sanding with a flat sanding at 1.5 degrees?

I don't have a mega mold yet, so I know my technique may be old-school compared to the slick new way you guys are doing things here!

Thanks!

[ken cierp]

Ken -- recommends making this slope using the same parameters as the Martin factory, Jim Olson and Charles Hoffman, Larrivee and others. The geometry is what it is --- The process as well as the machines used can be found in this link

http://www.kennethmichaelguitars.com/neckangle.html

I should add that two of the most prominant vintage replicators, Preston Thompson (he uses KMG tooling) and John Grevens also use the same process. And you will not find a radius dish of any kind/dimension in Wayne Henderson's shop!! -- a 28' bevel on the rim has little if anything to do with the precision of the final requirements or expected results relative to neck set, kind of a crap shoot (what if you tilt the rim or dish fore /aft / left /right or a combination while sanding?). In fact beveling the rim on the top edge is thought to constrain the sound board in such a way that bass response is limited a bit which gives the preception of increased treble response.

Doming the sound board a little does incease the surface area relative to the perimeter of the rim -- so if the SB shrinks do to lack of moisture it will cave inward before it cracks.

[Tj Turner]

Thanks Ken…

Looks like one way to handle it then, with the tools I have, is to use a flat radius dish (which I have), and then sand the entire top flat. Then sand in a 1.3 degree down-slope from the neck block to the top of the sound hole? That should get me the same thing that Martin is using. Then the dome in the top is completely controlled by the bracing, and the is glued down to the kerning which are actually squared to the sides, not with a radius in them. Is that right?

Time for a megamold I guess!

Tj

[ken cierp]

That is correct -- all said and done its the relationship of the neck heel to the rim at the neck block. That needs to create the projection angle of the finger-board plane "slightly over" the bridge.

The Mega Mold certainly is not the only way to get this right -- but it does allow one to process based on math and science rather than trial and error.

[Tim Benware]

Thanks Ken…

Looks like one way to handle it then, with the tools I have, is to use a flat radius dish (which I have), and then sand the entire top flat. Then sand in a 1.3 degree down-slope from the neck block to the top of the sound hole? That should get me the same thing that Martin is using. Then the dome in the top is completely controlled by the bracing, and the is glued down to the kerning which are actually squared to the sides, not with a radius in them. Is that right?

Time for a megamold I guess!

Tj

I would add one caveat. You only sand in a 1.3* (91.3*) angle if you neck has an 88.7* angle. If your neck has any other angle you want the complimentary angle for your 180* straight line.

[ken cierp]

Yes! Excellent point Tim