Design issues

[dr_scrimshaw]

I started experimenting in the shop with various ideas for a neck that can stay straight with 10 or more strings. I am a Civil Engineer, so I may think differently than most luthiers! Basic structural analysis tells us that thicker is less flexible, and if you look at steel beams, or floor joists you see that you don't need much material in the middle, just the top (in compression) and the bottom (in tension). So I was thinking of a fairly thick instrument, but not solid so it isn't too heavy. My first question is - how thick is too thick?
Here is a sketch of what I am thinking. The whole thing is about 2 inches thick. I cut the webs out of white oak. They are pretty stiff, and when I glue the top and back on they will be even stiffer. The plan is to glue them to a thin board that will be behind the fretboard. Then I will install truss rods in the spaces between the webs - I will have to glue in short sections of wood at both ends of the neck for the truss rods to bear on. Then I put on the back (the sketch shows it as if the sides go on next, but I think the back should go on at this point). Now I cut the whole neck at the angle shown for the sides. Next the sides are put on. At this point it is a rigid box, much like a stressed skin panel. I will run the top face over a jointer to true it up. Finally the fretboard will be put on.
I have a lot of details I am working on as far as the head, area for the pickups, etc., but I like the structure of the neck. Any thoughts on either the thickness or on the effect that long hollow channels might have on sound?
I'll stop here, this is long enough for one post.

[dr_scrimshaw]

Too dumb to attach a sketch I guess.
http://www.fidnet.com/~showmo/Tap_body.GIF

[traktor]

Hi,

Well, what you have is certainly interesting. I am assuming that we are viewing the neck from the body end, for example. Of course, if you just mentally fill in all the holes (except for the truss-rods channels) with wood, then what you have is a conventional neck.

To wit: a fretboard glued above, with a hemispherical cross-section of wood below, into which are cut truss-rod channels.

Several million instruments have now been constructed on this general design, and what's been learned by that experience is that the hemispherical neck needs only be an inch or less thick.

My guess is that making it two inches thick, even with holes, would weigh more than making it an inch thick and solid. It might also be a neck easier to hold, though I'm not so sure on that point. A thick neck might feel really good.

Of course, if your sketch is meant to represent a side view, then your diagonal and bottom pieces are doing the work that an original-design truss-rod does, but a metal truss rod would have more longitudinal strenth than would wood.

Emmett once built a very long instrument for Randy Strom, and because at that time he had no adjustable truss rod, it bent like a bow. To fix it, on the rear surface of the neck he installed eye-bolts near head and body, with wire strung between, and passing over two small metal bridges. Acting like an inverted suspension-bridge, the truss-wire brought the instrument straight.

Felt odd to play it, though, with that wire in the way of your thumb, and it was hell to get it into the case.

[rjgoos]

Dr. Scrimshaw,

What have you got to lose, friend...build it, string it up, and see if it holds!!! It was the only way for me to find out whether my neck design would hold (and hold it does...well, at least for six strings...).

A thicker neck, I'm guessing, would be better for "uncrossed" technique than for "crossed" hand technique. A very thin neck, while desirable for many reasons, can pose hand cramping problems with uncrossed technique.

Since you are an engineer, you may appreciate the following treatise on young's modulus, etc., as applied to ukulele necks:

http://www.ukuleles.com/Technology/neck.html

http://www.ukuleles.com/Technology/statmeas1.html

http://www.ukuleles.com/Technology/dynmeas1.html

On the Internet, various stiffness modulii are available for many kinds of wood, as are the string tensions for various kinds of strings. Statics class all over again!!! Unlike a bridge, though, you don't need a safety factor of 10, nobody dies if a neck warps (at least we hope so).


Jay

[dr_scrimshaw]

Thanks for the feedback already. The sketch is a cross section through the neck. I am saving some weight by not making it solid, and I don't have to route out channels for the truss rods, as they can go in the spaces.
I checked out the ukelele page. Interesting information, a more analytical stage by far, but generally where I am headed. I see that I should swap the white oak "webs" for hickory, both of which I have in the shop since they are cheap here in Missouri.
I should also note that I am trying to figure out how to make this as simple as possible to build. So far it's only table saw work, no routing.
Thanks for the encouragement. I think I will build it and string it up, and we'll see what happens.

[rjgoos]

There must be a material better than wood for the internal stiffness elements....carbon fiber? Aluminum?

How about two internal elements of 1/2 by 3/4 inch rectangular aluminum bar? Would weigh about 2.5 lbs for a total length of about six feet.

Just a thought...OK, OK, maybe even a stupid one, but don't be afraid to think beyond wood.


Jay


Rolla, MO??? You know Mike Brewer or Tom Shipley?? I have all of their records/CDs.

[dr_scrimshaw]

I've tried flexing aluminum bars by hand, I don't think you gain much. Carbon or glass fiber is better. I can still cut a slot and install something stiffer, and I will have a steel truss rod in it.

I glued up some of the parts this morning, I'll test it for stiffness tomorrow and decide if it needs something added.
The university built a bridge with carbon fiber reinforcing, I have a bunch if 1/4 inch rod to use if I need it.

I can 't say I know Tom Shipley, but he does work here at the University. My son is a keyboard player, Tom's son is a drummer, and I have taken my son to their house to jam.

[dr_scrimshaw]

The design pretty heavy, but I started building and the neck is very stiff. I haven't put on the back or side pieces, just the 3 internal webs and the top, plus 2 lengths of 1/4 inch all thread rod (located near the bottom of the empty channels in my sketch). I plan to put on the sides, skip the back for now, put on a fretboard and temporary bridge, string it up and just see how it feels. Then I'll worry about the pickups. I have been taking photos, I'll stick them up on my website and post a link as the work progresses.

I am thinking if it is too heavy to hold for long it might need a support leg like a stand up bass or cello.

By the way, I found an 8 (eight) inch single blade pickup at:
http://www.windworld.com/emi/products/catalog.htm#hardware

[rjgoos]

Originally posted by dr_scrimshaw


By the way, I found an 8 (eight) inch single blade pickup at:
http://www.windworld.com/emi/products/catalog.htm#hardware


Yowza...8 inches? Now THAT's wide!!

I was thinking of also using a blade-type pickup for my next prototype. The EMG HZ series bass pickup available from stewmac.com seems to be about what I want. The EMG site has a detailed pdf on this series, it is a blade-type inside.

The only question I have is...will a bass pickup do OK for the melody strings?

Jay

[sobear2003]

My luthier built the neck of my axe in a very secure fashion.

He used 9 veneers of maple placed so that the grains ran perpendicular and 2 truss rods.

So far no untoward bending and no warpage.

Tom
www.tommcmahon.ca (http://www.tommcmahon.ca)

[Uberbass]

Sounds like the Kubicki necks ... that approach (w/ multiple maple veneers) makes for a super solid neck. I surprised more manufacturers don't do that.

b

<<My luthier built the neck of my axe in a very secure fashion.

He used 9 veneers of maple placed so that the grains ran perpendicular and 2 truss rods.

So far no untoward bending and no warpage.>>