Re; Alcoa Aluminum Double Bass Neck / Fingerboard Failure, Design Flaw
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... https://picasaweb.google.com/MEandKC...eat=directlink
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.
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