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Re Alcoa Aluminum Double Bass Neck / Fingerboard Failure, Design Flaw

Discussion in 'Setup & Repair [DB]' started by kcandme, Jul 31, 2012.

  1. Re; Alcoa Aluminum Double Bass Neck / Fingerboard Failure, Design Flaw
    An Opinion...

    I have owned four Alcoa Double Basses, and one Alcoa Violin. I have had the opportunity to examine in person, ten Alcoa Double Basses. I have done lots of research, and benefited from the research of others, including James Condino of Kay Bass Repair dot com, in SC. I have restored one Alcoa bass to original specifications, (excluding any restoration of the finish paint), and am in the process of restoring two more.

    There are as many opinions about the Alcoa double bass as there are people who have seen one of these instruments. Most of the opinions I have seen or been exposed to are novel at best. However most of those opinions have come from the point of view of people comparing an Alcoa to any other wooden double bass. These comparisons are primarily based on sound alone, with a some factor given to physical feel in the hand.

    Generally, very little consideration is given to the original reason or idea behind the development of the instruments. That of stability and resistance to the environmental factors that so affect wooden instruments.

    The instruments were originally conceived and designed in the mid to late 1920s for use by orchestral students in a summer camp setting, and therefore to be able to withstand the changes in weather of a Northern Michigan environment throughout the seasons in a non-climate controlled setting.
    To address this issue, the concept promised to be very effective. And initially, it was. The trade-off in compromise of sound vs wood instruments was accepted and considered nominal in this setting in the early 1930s.

    However, over the course of time, a problem surfaced which I have yet seen addressed adequately or effectively. That of the neck/fingerboard failure.

    I have yet to see an Alcoa that has not had a neck/fingerboard repair of one sort or another performed on it. Even the best cleanest examples I have seen, have had the neck/fingerboard repaired.
    After studying the design of the neck on these instruments for some time in the process of doing my restoration work on the couple of examples I have owned, I have come to the conclusion that there is a slight flaw in the execution and implementation of the original design.

    The design of the Alcoa Viol instruments is in my opinion, very well conceived and executed, except for the way the fingerboard/neck system was put together.
    The design of the system is good. It was in the execution of the design that the weakness surfaces.

    The design of the system consists of; The Neck Structure of the instrument, The neck 'Core', and The Fingerboard.

    The neck of the instrument is made of the same aluminum that the body of the instrument is made of. It is of a formed 'U' shaped shell, and welded construction with a formed and welded scroll and pegbox welded to the neck. The assembly is welded to the body of the instrument. Once all aluminum parts of the neck are welded together, the neck shell is quite strong. However in and of itself, it is not strong enough to withstand the force of string tension.

    The 'Core' of the neck/fingerboard system, is a piece of Hard Maple stock, shaped and formed so as to fit into the 'U' shaped shell of the aluminum 'neck'. This core being fastened into the neck with machine type screws, is intended to provide structural integrity to the aluminum shell of the neck, as well as providing a surface upon which to attach the fingerboard.

    The fingerboard of the system utilizes a standard fingerboard attached in the usual manner with glue upon the maple core to complete the system.

    When the three components are solidly connected together the system is complete and is structurally sound and strong enough to carry the tension of the strings.

    (one question I have is whether the instrument was originally intended to utilize Gut strings. And do the traditional gut strings produce less tension. Were steel strings available in the late 20s - early 30s...)

    In order to fasten the core piece to the aluminum shell with screws, aluminum 'screw bosses' are welded to the interior of the neck. The design called for three of these bosses to be used, welded equally spaced lengthways along the interior of the neck shell.
    The neck core is drilled in the corresponding positions to allow the core to fit cleanly and snugly into the neck shell, and then be screwed to the neck.

    The problem arises because the combination of the size of the screw bosses and the taper of the neck and core cause the structural integrity aspect of the system to be totally defeated. This is because the bosses being so tall in the neck cavity, cause the relief hole in the core to be drilled almost completely through the core piece. This makes the portion of the core piece where the screw holds the core to the neck so weak that it fails, resulting in the string tension overwhelming the system and bending the aluminum neck, separating the fingerboard from alignment with the neck. The neck bending usually results in stress cracking of the neck at various points on the edges.

    The inherent problem of the system, is that the screw bosses are of inordinate size in relation to the neck cavity space available.
    Specifically, the total depth of the neck cavity from tail end to head end at the placement points of the screw bosses, is; 29/32, 26/32, 22/32, + or - from instrument to instrument.
    The screw bosses themselves are 16/32 tall.
    This provides at best, 13/32 of thickness of wood upon which a screw is to gain purchase.
    This is diminished further by the need to use countersunk screws, the average depth of height of the tapered part of the #10-32 mach screws used being 3/32.
    At the head end of the neck, the total available thickness of wood would be 6/32. And from this amount a countersink bore is cut.

    These factors, along with the difficulty of drilling the wood core pieces to a depth within the tolerances necessary to afford the greatest amount of wood available for fastener strength, while at the same time ensure a proper neck cavity fit, all combine to produce near impossible circumstances upon which to achieve a structurally sound neck system. This is what I believe to be the downfall of the design execution.

    The neck core pieces that I have examined, all have the screw holes broke through at the head, or nut end of the core. Most also have the middle screw hole broke through, and some have had the tail end screw hole broke through as well.



    I have lots of pics if people are interested...


    The attempts at repair of this system that I have observed include;

    1) Replacement core pieces attempting to replicate the original design. In fact, I believe that all of the instruments I have personally worked on have had the core piece replaced. One can only guess how many times over 80 or so years.

    2) Various metal washer/plate insert pieces fitted into the core over the screw holes in attempt to provide strength at the fastening point.

    3) Screws inserted into and through the fingerboard into the bosses. (This could be a good repair if done properly, with a forstner hole cut, then countersunk, a screw installed and then a plug fit into the fingerboard and planed to the board surface. However it would be near permanent and irreversable. Difficult to make any further repair replacement of the fingerboard.)

    4) Wood screws inserted from the back of the neck through the aluminum into the core, and even through the core into the fingerboard.

    Re repair;

    My thought on a way to remedy this problem, is to drill and tap the screw bosses to their full depth. Maybe even to accomodate larger than #10-32 machine screws. Then file the bosses shorter to a height corresponding to their placement in the neck taper.
    This could allow a replacement core piece to be made leaving a greater thickness at the critical screw attachment points. Perhaps providing enough structural integrity to the core piece to hold up to the string tension.
    Removal and re-positioning of the center boss, and addition of another boss would help also. So that there were four bosses rather than three.

    I'm working on #112 and making the outlined mod to the original design with the original three bosses. We'll see how it works.

    Thanks to All for any consideration and comments
    Povl Carstensen likes this.
  2. bssist


    Jun 23, 2007
    St. Louis, MO USA
    Very interesting. I am not a luthier or an engineer but I have heard/read much about cf rod being used to strengthen standard db necks. Would it not be equally feasable to run a cf rod up the neck on each side of center to reinforce the maple structure and prevent breakage?
  3. Hmmmm...
    Breakage of the maple occurs at the screw fastening points, in the center of the maple piece.

    I actually have one of those carbon fiber fingerboards. I was going to use it on #71, which I intend to keep. I plan on polishing that one and using CF fingerboard and tailpiece for accent effect. Like this one...


    Maybe more like this one... But with better bridge fer sure...


    I've thought about some type of reinforcement rod or bar in the neck cavity also. My thought is that once a piece of maple is securely affixed to the aluminum neck, without likelihood of failure, the problem is solved.
  4. james condino

    james condino Spruce dork Supporting Member Commercial User

    Sep 30, 2007
    asheville, nc

    'Good to see that you are still at it. When your hotrod went up for sale last year I thought the basses might be next.

    That seems to be a chronic issue with the AlCoA necks; I've seen it come up on about 50% of them. They break loose and then start to rattle inside. They are also made out of some fairly soft early aluminum, nothing like the strong modern alloys. I've thought about having a larger beam or T section welded right down the center of the neck and then screwing the wooden core to it. The carbon fiber fingerboard sounds pretty cool. You might be able to attach it using a very strong modern epoxy. LMI used to sell a green epoxy that they claimed was used in the aircraft industry to attach aluminum wings and such in areas where the tolerances were too fine or difficult to reach to weld. I could never get it to set properly unless I heated it up to about 180 degrees ( that's right- all the parts went into the oven while still in the clamps).
  5. Thank You JC
    I'll have some progress pics soon.
  6. Hey KC,
    Great info. I found the patent drawings somewhere if you dont have them.
    I am insterested in what strings may have been used originally as well.
    I have #72 about 10 years or so after it passed thru James's hands. Havent played it to much, need to find better strings.
    Would love some pictures too when you get a chance.
  7. MostroDB

    MostroDB Guest

    Apr 18, 2012

    I have no experience with these specific instruments, but the first EUB I built (35 years ago) was also based on a alu U-shaped beam.

    From your description & photos, I can suggest 2 design weaknesses. Firstly, using just a few screws to attach the core to the U-beam results in concentrating the load on just those points, which may lead to mechanical failure, as well as rattling over the sections that are not attached. But these things were mentioned.

    Secondly, this construction combines materials with significantly different expansion coefficients. Aluminium is typically 22, and wood typically 5 um/m/K. That means that if the instrument is exposed (over the course of its life) to a nice summer day out in the sunshine, as well as a cold freezing winter night, you may have up to 0.5mm of difference in length between the core and the neck. That will cause bending, possibly leading to fatigue, and a firm long-term fixation also seems improbable (I believe in aircraft the glues are used between the same material).

    It may not be what you want (quite an outstep from the original), but I suggest an alu reinforcement, best welded as outlined in post #4 above. As welding may be a bit definitive for old instruments like this, there may be another elegant way to fixate it. However, that alu reinforcement should strengthen the neck sufficiently to carry all load, I suggest not to add any substantial wood, at least no long lengths. So that means also an essentially alu fb, maybe covered with veneer or a thin strip of wood, like on guitars.

    For your consideration.

    Best of luck with these instruments!
  8. james condino

    james condino Spruce dork Supporting Member Commercial User

    Sep 30, 2007
    asheville, nc
    When I sold #72, the fellow who bought it was a pretty good welder and talked at length about eventually welding an aluminum fingerboard the the neck core and polishing the entire bass.

    Since it was asked earlier to provide a few images of the discussion at hand, I'll cautiously offer this link in the spirit of knowledge and information sharing with the community here in direct response to a question. Scroll down and you'll see the complete factory spec insides of an AlCoA neck as well as a lot of other information. If anyone here is offended or worked up in some manner, then please post another source of images that can offer insight into a complex problem so far only described in words for an obscure instrument design conundrum from 80 years ago that will surpass this in history, design, and construction with detailed imagery as compiled from quite a few active talkbass members here over the years.:)

  9. MostroDB

    MostroDB Guest

    Apr 18, 2012
    Even ignoring acoustic aspects, alu may be too soft & quick wearing for a fb, but you could of course hard-chrome or nikasil it to last forever. The low friction may make it more difficult to play though.

    Anyway, I can see aesthetically why someone would want a shiny fb, though such an instrument would not at all be my choice.
  10. powerbass

    powerbass Supporting Member

    Nov 2, 2006
    western MA
    If I was doing this project this is how I would approach it:

    A wooden fingerboard needs to be planed and replaced eventually and is a preferred material. I would make a fingerboard with an added piece of wood (like a subfloor) that has metal fasteners (T-nuts) installed as well as carbon fiber rods, this would be glued onto the back of the fingerboard with hide glue. This wood could be maple or an super stiff exotic wood like ipe etc. I would not put any metal into the fingerboard (you could add another CF rod just to the fb?). The neck would bolt onto the structural piece (subfloor) under the fingerboard. This solves all the problems. The stiffness of the neck is now transferred to the fingerboard/subfloor/CF structure. The fingerboard is now removable for repair and adjustment in a matter of seconds all via hide glue or metal connectors. All connectors are metal/metal - no wood screws or rust. This process avoids welding or modifying the aluminum neck.
  11. Cool! You have #72...
    I have a copy of the patent document also.

    I should just provide all my pictures...
    I have lots.
    I have some old ads too.
  12. Good thoughts Mostro and Powerbass.
    Thank You for your input.
  13. BG6tNyH.
    james condino likes this.

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