Flat back crossbars

Discussion in 'Setup & Repair [DB]' started by bengreen, Jun 17, 2016.

  1. bengreen

    bengreen

    Jan 26, 2016
    San Diego
    Hi again,

    Further adventures making John Lott Sr. copy bass.

    Last exciting episode, purfling the back! The heroes of TalkBass aided our intrepid neophyte maker through the perils of slicing and dicing a $400 slab of flamed maple. I'm happy to report all involved survived the ordeal with sanity and structural integrity mostly intact.

    Next up: the flip side...crossbars!

    Leaving aside the issue of diagonal vs horizontal for the next bass because I only have the lumber for the latter:

    1. This bass has four crossbars. The plans call for the lower two to be sprung: 3mm for the soundpost crossbar and 5mm for the one spanning the wide point of the lower bout.

    Do you like to do this or is there the same contention as in springing bassbars?

    2. Do you make any attempt to tune the crossbars a la Chuck Traeger?

    Thanks,

    Ben
     
  2. 1st Bass

    1st Bass

    May 26, 2005
    Forest Grove, OR
    I've never made a flatback, Ben, so I will sit this one out...but there are bunches of fellows here who regularly make them, so it will be interesting to see the replies.
     
  3. bengreen

    bengreen

    Jan 26, 2016
    San Diego
    Well here's a thing: I was searching back through old posts about crossbars, with the inevitable back and forth about horizontal vs diagonal, humidity at time of gluing, strength of glue, margins between rib and bars, inducing arching...I get that.

    And I do understand the wood movement issues, that you're restraining the inner face of the back which has relatively large movement as it gains and loses moisture with a crossbar which has virtually no movement in the direction that the back is moving because along-the-grain movement is tiny compared to movement tangential to the grain. That restraint leads to differential movement between inside and outside surfaces which can split the back.

    Somebody asked a question in the thread which I was intensely curious to hear an answer to, but which was never addressed at the time (about a dozen years ago).

    He asked, what if horizontal crossbars were either not glued continuously across the back or if spaced cleats were placed between the crossbar and the back, would that give the back more space to move but still provide enough rigidity?

    I had a variant of the same idea, but instead of cleats between crossbar and back, my thought was to saw a series of kerfs, either vertical or diagonally, along the length of the gluing surface of the crossbar to allow some movement between glued and not glued parts of the back.

    The back bend in Roger Hargrave's and Rene Zaal's bass pdf and that of the the bass made at Ft. Collins ISB had only cleats running across it. But really I'm mainly concerned with the other three, especially the sound post crossbar.

    Too weird?

    Any answers may not be in time to do me any good because I intend to glue up tomorrow around noon (hi temp/low humidity forecast). But I'm curious anyway.

    Unless someone stops me, I probably will spring the lower two bars. But it's going to make me nuts if my carefully flattened ribs and linings don't fit as a result.
     
  4. Ortsom

    Ortsom Inactive

    Mar 23, 2016
    I can not sensibly advise you either on this Ben, other than by this motherhood statement: If you want to make a copy, best follow the plan.
    My flatback had 3 bars, all fully glued & un-sprung, but was not to a conventional plan.
     
  5. bengreen

    bengreen

    Jan 26, 2016
    San Diego
    Crossbars are glued.

    Ended up not springing them.

    Couldn't convince myself that that they would actually arc the back out without having the ribs/lining/corner blocks inclined as well.

    Then stuck trying to match and glue compound curved surfaces.

    Plus made a test brace with a 5mm spring and bent the back against it. It put substantial tension in the back both pulling down the edges and preventing the center from rising above the brace.

    Couldn't make myself comfortable with it so just glued flat without tension.

    JLott Crossbars Clamped.jpg JLott Crossbars Untrimmed.jpg

    Next on my checklist from Arnold S is putting center seam cleats in.

    I used to take really good notes. So good that I never had to crack a book to revise after the initial reading. Just study my notes.

    What happened to that guy? Total gibberish! Arnold told us how many and how big but you'd never tell from my scrawl.

    So...help! I know that's something you dealt with Chet. And there was also talk of using linen instead. The only online pics I could find was a Thomas Martin restoration of a Lott bass in which he seemed to be using continuous vertical strips running between all of the crossbars.

    I have spruce patches both square and diamond about 30mm edge to edge and 5mm thick.

    Also, tuning: Traeger talks about tuning the braces to the surrounding back pitch. But when I can actually hear a pitch that seems the same two times in a row, I'm hearing about a minor sixth difference between the lower and the upper back. I even tried Arnold's deal of standing on my left leg and borrowing my sister the doctor's rubber reflex hammer to whack it with!

    I'd been planning on gluing the back to the ribs today, but I'm starting to think it might make more sense to take it with me to Oberlin and see if Arnold wants me to stand on my right leg instead.

    Arnold's checklist called for gluing the back, letting everything relax for several days then glue the front linings and outline the top.

    No time now. Gotta leave Tuesday. Think I'll do the top linings out of sequence today. The inner mold's still keeping everything rigid. Then I'll have a final outline ready at Oberlin and can dive right into the top. And hopefully get the back brace trimming sorted as well.
     
    Last edited: Jun 19, 2016
    eh_train and james condino like this.
  6. I'm no builder, but I've laid a cleat or two.

    Shen uses linen for top and back seams, and quite successfully in my experience.

    Denim will also work, if you'd prefer something more rustic, but at the risk of some yet-unborn luthier tsk-tsking your thinking decades in the future.
     
  7. james condino

    james condino Spruce dork Supporting Member Commercial User

    Sep 30, 2007
    asheville, nc
    Lookin' good Ben!

    Don't stress over the little details. Remember that you can always take the back off and modify things later on. Of all things about flatbacks, probably number one is that sooner or later it is going to crack; they all do, no matter how well you brace it. The trade off is you just saved yourself 100+ hours carving a giant 3" thick chunk of curly maple and close to $1000 more for the wood!

    On a slight detour, how common is it for flatback bass cross braces to be coupled into the ribs- basically having the linings lock them down to the back. On flatback guitars that approach is very common and results in two things: the braces tend to not pop loose over time and there is a bit more power and focus to the projection. I usually see them on basses uncoupled from the edge of the ribs. I've made them completely flat and they held up fine and I've also made them with an arch and a few years later the back moved so much the arch disappeared and from the outside it was almost concave.....never ending.....

    j.
    www.condino.com
    www.kaybassrepair.com
     
  8. bengreen

    bengreen

    Jan 26, 2016
    San Diego
    Thanks guys. Don't have any linen (and never could wear denim with my bod) but have to get some eventually. If I remember right, it's called for in strips on the ribs and to cover the ends of the bass bar. So yeah, I'll just throw some cleats down for now as insurance for the trip and the huge change in humidity.

    There was a pointed warning in Traeger about trapping the crossbars under the linings, so I hadn't considered it. I've got a quarter inch between the bars and the linings.