Building a Better Mousetrap . . . err Bass Neck

Discussion in 'Luthier's Corner' started by 41hzthunder, Aug 4, 2020.


  1. Don't know if this will draw much interest, but here we go. A couple months ago a thread on neck wood and grain showed up spurring discussion on what wood and grain patterns were best and whether solid or laminated necks were better. It got me thinking about neck design. For this thread, we are talking bolt-on necks. There are die-hard fans of one piece and two piece necks. Two piece meaning a fretboard and another piece of wood for the rest of the neck. These have been around a long time and there's nothing wrong with that design. It just won't be the focus of this thread. I am interested in looking at laminated necks in a new way.

    If you've built a bass neck, you know wood is difficult to predict. Rip a piece of wood and hidden tension can be released, causing a straight and true piece to curve like a banana. Or it can twist like spiral pasta. Old growth and tight growth rings may limit these issues, but they can't guarantee stability. Add in the time and money of finding really stable wood and it gets frustrating quickly. I am not a fan of paying $100.00 per board foot and still not knowing how it will behave. There's always a risk. With certain complex wood patterns, favored for asthetics, its a real game of chance. The more figured, the more curves in the grain.

    One solution to create predictability has been to laminate smaller pieces together and arrange the orientation of the growth rings so these tensions are cancelled or minimized. A common pattern is to laminate pieces side by side and glue the fretboard on top. There's a wide variation in the number of laminated pieces and types of wood used. Often it appears asthetics may become more of a concern than the structure and behavior of the neck. After all, there's a truss rod to correct just about all neck relief ailments!

    So, this thread is about exploring one or perhaps more different patterns of lamination and identifying the reasons it may work or not work. My plan is to build the neck presented, or a version of it, and see how it behaves. My inspriration is the engineering in woodstrip canoes and WW2 airplane wings. What does that have to do with bass necks? Currently, not much. But, both use a monoque concept where one side of the object is dealing with compressive force and the other a stretching (tensile strength) force. Weight, strength and stability were/are extremely important and that sounds similar to bass neck construction.

    Attached is a sketch of a 6 piece laminate concept. The pieces marked 1, 6, 3 and 4 are of the most interest. 1 - the fretboard, will be subjected to compression, but also perpendicular force by the truss rod. 6 - the back of the neck will be subjected to forces pulling along the grain and perpendicular force by the ends of the truss rod. 3 and 4 are essentially spacers, but could be more. It might be possible to make the neck more stiff and less flexible or more flexible, better for truss rod adjustment, by changing the species of each part and the orientation of the grain.

    An unexpected benefit is that the truss rod slot could be made to exactly fit the truss rod without any routing.

    Here's the main concept of pieces:

    laminate neck concept (2).jpg
     
  2. chinjazz

    chinjazz Supporting Member

    Sep 11, 2002
    Atlantic Beach, FL
    This is a very interesting subject. All the necks I've made thus far multi-laminate. Definitely subbed!

    I'm interested in seeing where this goes. In whatever design you make for this research, I would think factoring in how much the truss rod can adjust. Another way to think about it: If the neck turns out too stiff (non-adjustability).
     
  3. You are reading my mind! I'm ording a couple truss rods today and plan to measure the upward force exerted when tightened. Also plan to test the amount of weight and deflection required to move (deflect) the neck.

    The neck will be either a P-bass or Jazz, just so there's a lot of examples to compare it against.
     
  4. Slidlow

    Slidlow Supporting Member

    Apr 15, 2009
    Oshawa, Canada
    I would consider keeping "6" equal thickness through the neck by tapering 2,3,4 and 5.
     
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  5. dwizum

    dwizum

    Dec 21, 2018
    I'm excited to see where this goes. After going through engineering school I can't help but see everything in a structural context.

    That said I'm a little lost reading your post. What problem exactly are you trying to solve? You talked about stability, and predictability, is that it? I think it's important to define what you mean by "better" when you're trying to do something better.
     
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  6. Beej

    Beej

    Feb 10, 2007
    Vancouver Island
    One factor I didn't see mentioned but I think is important, is how seasoned the timber is that you're using. I like to use lumber I've personally had stored indoors for at least two years, plus I rip the stringers and let them sit for at least a season before using them in a neck. If a stringer is still straight after sitting for several months, I'll use it. :thumbsup:
     
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  7. I'm curious what you're doing with the stringers you don't use. ;)
    #ImJustAcrossTheBay
     
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  8. I thought about how "6" would likely taper if the thicknesses didn't change. Definitely would have to build some jigs to do this. It would probably look nice!
     
  9. Those are really good questions. The first goal is a stiffer neck that won't require as much tension on the truss rod. The second is to keep the neck light. I could build a very stiff neck out of sapele, but would pay a price in weight. The third goal is to eliminate uneven deflection. I had a Fender Jazz that had grain pattern change at the 5th fret that was a week spot that dipped. I really couldn't correct it with the truss rod. I ultimately sold it.

    Based on your question, it seems a good idea to build some test blocks with different woods and measure how they deflect to see if there is any advantage. The blocks could be .9" X .9" X 20" with three layers. I have some ash, maple, bubinga, sapele and walnut lying around and could work with that. So to test the first goal, I'll work on putting some test blocks together and see how much they deflect with different weights: 10, 20 and 30 pounds perhaps. Looks like I have a project for this weekend!

    I've found some publications on wood strength on the web and will use that information when choosing the combinations of wood. I may have to look for the data on bubinga and sapele.
     
  10. I've read that the elasticity of wood is also effected by the moisture content. Here in California, the humidity gets pretty low in summer and I have some wood that's been in the shop for three years, so it should be good to go. I don't have a moisture meter to measure more precisely.
     
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  11. I was able to get into the shop and start building some test blocks to see if I can measure how some different woods perform. All will have a 3/8" maple core (3 and 4 in the design) and differing species of wood to represent the fretboard (1) and the tension side (6) of the neck. Here's what I decided to test:

    "Fretboard" tension side comp #s
    1. maple walnut 7830, 7,580
    2. walnut walnut, 7580, 7,580
    3. sapele sapele 8160, 8,160
    4. ash ash 7,410, 7,410
    5. sapele walnut 8160, 7,580
    6. ash walnut 7,410, 7,580

    The number after the tension side column is the lbs per square inch of compression parrellel to the grain of the species before failure as listed in the Wood Handbook, Chapter 4. There's little information on tensile strengths of wood species, but the tensile strength for sugar maple was almost double the compression strength. Based on this, I would expect the sapele/sapele to perform (bend) the least and ash/ash to perform the worst.

    The samples are clear, near vertical grain and cut from the same pieces of wood. Thicknesses are 1/4" and run through the planer at the same time, then measured. The glue up is between a 2X6 of maple jointed flat and a 5/4 X 2 maple block to make the test blocks flat during glue up. Each block will be cut to 20" and dimensioned side to side after glue up. A small piece of maple will be glued on the "fretboard" and provide a surface to clamp against and place the tension above the "fretboard" like strings. Here's the concept and some photos:

    test blank (2).jpg quarter inch measurement (2).jpg
     
  12. Played around with the test blocks working on getting a good system for measuring how much they bend and how to measure the force on them. In just playing with them in my hands there were some that seemed more springy (is that a word?) than others. The sapele was definitely the most stiff feeling.

    There's no way for me to measure the actual compression a bar clamp is exerting, at least with any tools I've got. There is a point when setting the blocks in the bar clamp where the passive end sort of locks in place. You can see it and feel it. It appears to happen at a consistent pressure. So I think I will set the blocks to that point and give 1/8, 1/4, 3/8 and 1/2 turn, if they will take that much. I can then measure the bend at each point. I'll have to measure the starting point using a straight edge accross the clamping blocks and measure the distance at the middle of the blank.

    I toyed with this method, but want to refine it a bit. Its amazing to watch how the blanks bend under the stress. It mimics a neck well, but in fast motion. A 1mm bend looks like a lot more when you see it happen. The unofficial results showed most of the wood combinations bending around 1mm at a 1/4 turn on the clamp. The sapele, maple, sapele blank only bent about .6mm and seemed to take a lot more force to get to 1/4 turn. It made me think I may want to use a torque wrench on the clamp as another way to measure. What was interesting is the walnut, maple, sapele (FB) bent as much or really close to the majority. The sapele on the tension side (back of neck) seems to make a real difference.

    More as I refine this. Hope to have a chart.
     
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  13. wraub

    wraub

    Apr 9, 2004
    ennui, az
    I believe @Bruce Johnson has done numerous tests on deflection and compression, perhaps he'll join in...
    :)
     
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  14. Beej

    Beej

    Feb 10, 2007
    Vancouver Island
    Eventually everything gets used. :roflmao:
     
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  15. 5tring

    5tring

    Sep 16, 2018
    UK
    Impressed with your commitment to guitar science!

    This looks like a great test but you need to either apply a known load and measure the deflection, or vice versa, to get an idea of load/deflection

    By doing ‘number of turns’ you are applying the same displacement to each sample, and then measuring the deflection - you are bound to get similar results each time

    if you have a torque wrench handy that would give you a better result as you would be able to vary the applied load and measure the deflection
     
  16. Same thing I was thinking all day. 1/4 turn gives a certain amount of movement, not load. Some of the test pieces were harder to turn than others, so you get a "feel". But, that's not quantitative. I have a couple torque wrenches and the small one is inch/pounds. I just have to figure out how to get it connected to the clamp.

    I do have some weights that I can apply on the center of the blanks while supported on the ends. That would give a way to measure the deflection per pound perpendicular to the grain. That's pretty easy to set up, so I'll start on that in the next day or so.

    I'm also realizing that the numbers for compression online are to the failure point. It doesn't tell you how the wood behaves below that point. I don't know if the fibers expand easier than they compress. Different species may behave differently.
     
  17. If he's done some of this work, it would be great to know what he found out.
     
  18. JayGunn

    JayGunn Supporting Member

    Jan 24, 2010
    Chapel Hill NC
    I did this experiment about 5 years ago, using (2) 1/8" x 1/2" carbon fiber rectangles in the place where you have pieces 3 and 4. For 1 and 6 I used wenge and for 2 and 5, the outside middle pieces, plus the center piece that gets the truss rod mounted in it I used the lightest (recycled) Honduras mahogany I could find.

    End result: Very stable neck, meaning no need for truss rod adjustments for the seasons, although I do expect to tweak it if I change strings to a different overall tension. Based on this sample size of only 1, I'm thinking that your layout will produce a very stable neck.

    I chose a very stiff wood for fretboard and backing piece (#1 and 6) and a very light, not stiff filler section (#2 and 5 plus the wood in the center with the truss rod route) because I was trying to reproduce the effect of an I-beam, with the carbon fiber as the web(s) of the beam and the wenge as the flanges. Also to keep overall weight down, because wenge is heavier (and stiffer and more stable) than maple.

    I'll happily keep an eye on your research and progress.
     
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  19. Arie X

    Arie X

    Oct 19, 2015
    not ideal to have the seam from #6 on the back of the neck.
     
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  20. Jon Clegg

    Jon Clegg Supporting Member

    Feb 9, 2015
    Northern Virginia
    Yes, laminated necks typically use an odd number of pieces. I've seen some 2-piece necks develop a split from the TR blowing out at the seam.
     
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Jun 19, 2021

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