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recalcitrant flatback

Discussion in 'Setup & Repair [DB]' started by Matthew Tucker, Feb 22, 2008.


  1. Matthew Tucker

    Matthew Tucker Commercial User

    Aug 21, 2002
    Sydney, Australia
    Owner: Bresque Basses, Sydney Basses and Cellos
    I have the back of my cornerless bass glued up and rough shaped, with the break already bent. No braces glued on yet - I've been waiting for drier weather.

    The back is cut from nicely quartersawn wood, and has been stored upright on my bass rack with free air circulation all around

    However, the whole back seems to want to bend such that the back would be concave if glued on as-is. I can force it to lie flat, and in fact my braces are carved with a slight convex curve in them to force the back to remain slightly convex when glued on.

    I can make it lie flat by spraying the concave side with water, but left to its own devices it just seems to want to curve the other way, and will return that way when it dries out again.

    i can't really use it the other way around either, a) cos I have bent the break already and 2) the bass is slightly assymetrical and it won't fit the other way around, even if i cut through the bend and make a butt joint at the top brace.

    What I'm hoping is that Arnold or Bob have come across this sort of thing before; am I asking for trouble if I glue the braces and force the thing to lie flat or very slightly curved the other way, knowing that in its heart of hearts, the wood wants to be elsewhere?

    Or do you reckon, it'll be OK.
     
  2. 1st Bass

    1st Bass

    May 26, 2005
    Forest Grove, OR
    Have you considered heat? As in steam, then dry heat to lock it in place? Antique wooden canes originally wanted to be straight (Bentwood rockers even more so), but judicious use of heat and moisture has persuaded them to remain in the curled position for centuries, in some cases. So it CAN be done.

    Remember my bass was bent-back, then carved...but it has only been around for a year or so, so I didn't want to use it for an example.
     
  3. arnoldschnitzer

    arnoldschnitzer AES Fine Instruments

    Feb 16, 2002
    New Mexico. USA
    Anyone who builds a flat-back bass with cross bars is asking for trouble! But seriously, if it's a small amount of warp, I would not worry too much. Spray the concave side, let it flatten and then glue on all the cross bars while it is relaxed. Wait a while before you carve them down. It's a good idea to use a curved caul that matches the arch of the crossbar, on the outside of the back for the glue-up. Leave clamped for 24-48 hours so the moisture from the glue has fully dissipated. While you are waiting for dry weather, keep the back clamped between two flat boards if you can, with some newspaper between the back and the flattening boards.
     
  4. 1st Bass

    1st Bass

    May 26, 2005
    Forest Grove, OR
    Arnold, I understand the problems with flatbacks and crossbars...what do you see as the best alternative? X-bracing of some sort, like a dreadnaught guitar? Or something still more exotic? If you don't mind sharing...?
     
  5. KSB - Ken Smith

    KSB - Ken Smith Banned Commercial User

    Mar 1, 2002
    Perkasie, PA USA
    Owner: Ken Smith Basses, Ltd.
    Some old Flatbacks have cracked a lot, some a little and some not at all. Something has to give when the Bass moves. A common thing and the least damaging is when the center seam opens instead of the Back splitting. Crossbars also pop off as well but the main concern is the integrity of the Back wood.

    Shown below is my Loveri 1873 (now restored but new pics to come later) with knotty type wood and several cracks. My old Cornerless c.1780 with several cracks, more than can be seen in the photo. My Mystery Bass c.1800? with only 3 cracks (not all can be seen and the upper and lower Block areas 'toasted' but since repaired). Finally, my Hart c.1830 with the center seam re-joined with a small shim of wood from opening up but not a single lateral crack. Just a tiny one across the treble side of the angle break.

    Good wood, dry wood, good care and good thickness ratio to hardness helps the stability and integrity of the Flatbacks as I have seen in my life. Both the Loveri and Mystery Bass have had a small strip inlaid in the outside center seam from shrinkage. The Loveri though was just split down the center, re-joined and a wider center strip added using some old Walnut I had in stock (pics to follow next month on my website). The Mystery Bass has a 3 pc X-Brace without the lap joint. This is still fine so we left it in the Bass. Other wood was added in the Back for support and cleats as well on the cracks during its on-going restoration.

    [​IMG][​IMG][​IMG][​IMG]

    Crossbars are usually replaced when Bass Backs are restored so it's not a permanent part of the Bass. I see Crossbars replaced as much or more than Bassbars but that depends on the repairs needed. In a way, they are just as replaceable. Blocks though are not replaced as much but often repaired and saved in some older Basses. I prefer saving what's there whenever possible.
     
  6. Assuming you've made the back as thin as most conventional flatbacks and you can easily press it back into shape with light pressure, I doubt if it is going to be a problem. Spraying the concave side as Arnold suggested is a good idea. It's always best to avoid building in stresses, but unsupported thin wood as big as a bass back is always going to have a little bend in it. I would be reluctant to use heat to bend it, and would only use it in a worse case senario.
     
  7. Matthew Tucker

    Matthew Tucker Commercial User

    Aug 21, 2002
    Sydney, Australia
    Owner: Bresque Basses, Sydney Basses and Cellos
    thanks Arnold, what you suggested was my plan A ... but I wanted a crystal ball! I had seriously considered X braces actually for this bass, but you lot scared me off them!

    yup and i tried using my heat blanket to bend the thing back. clamped the centre to a long block then bent one side, then the other, with heat and bags of shot to over-bend the wood the other way. Seemed to work. I was chuffed. Released all the clamps. Yep, worked.

    Next morning ... back the other way again :(

    i might try this again with no moisture, just heat ...
     
  8. Matthew Tucker

    Matthew Tucker Commercial User

    Aug 21, 2002
    Sydney, Australia
    Owner: Bresque Basses, Sydney Basses and Cellos
    and now I'm not sure .. .:confused:
     
  9. KSB - Ken Smith

    KSB - Ken Smith Banned Commercial User

    Mar 1, 2002
    Perkasie, PA USA
    Owner: Ken Smith Basses, Ltd.
    Dry clamp it flat with the Cross bars oversized (on both sides like a press), un-trimmed and leave it for a week or so. Let the Back settle to the shape you want it in before gluing the cross bars in.
     
  10. Darren Molnar

    Darren Molnar

    Nov 6, 2007
    saskatoon Saskatchewan Canada
    humble instrument maker
    Just a shot in the dark, which might inspire a better idea. Would it help to glue on strategically placed strips of linen in places, taking advantage of linens facility to shrink and pull?
     
  11. If repeated efforts to flatten it out don't work, maybe it should just be used as is. I mean, plenty of flat back basses end up a little concave anyway, so while it might not be optimal, at least you wouldn't be building stress into the bass by forcing the back to conform. Assuming, of course that we're just talking about a small amount of concave, not something more extreme.

    If I were buying a new bass and the situation was explained like that, I wouldn't balk at a little concave; it is, after all, a bass and not a Strad model violin destined for judging...

    :bag:
     
  12. did the wood come from N.of the equator?:rolleyes:trying to return to the northern hemisphere;)
    when all said and done can the back be massaged back to plumb over time w/ soundpost?
     
  13. Matthew Tucker

    Matthew Tucker Commercial User

    Aug 21, 2002
    Sydney, Australia
    Owner: Bresque Basses, Sydney Basses and Cellos
    Its local wood, and I *couldn't* live with a concave back, it's just so wrong for all sorts of reasons. And linen? Well I'll try everything else first.

    Well, for better or for worse,I "sandbagged" it for a while in a carving cradle using my heat blanket and some bags of lead shot. No water this time. After an hour it still looked OK, but now I've locked myself out of the workshop so I won't know how it is until my wife returns tonight with a spare key. DOH.
     
  14. Matthew Tucker

    Matthew Tucker Commercial User

    Aug 21, 2002
    Sydney, Australia
    Owner: Bresque Basses, Sydney Basses and Cellos
    I found my keys.

    I found the back still curved the right way except for a bit of the upper bout.

    I moved my heat blanket up over the centre of the back, protected the centre glue join with another strip of wood, and bagged away. Sure enough, it took a bend. turned it off and left it to cool

    The I got called away by my 13y/o daughter for lunch scrambled eggs.

    When i got back -- disaster. Obviously I hadn't protected the centre joint from the heat enough (sorry Bob, you did warn me) and the centre joint has now split from the button to the top of the lower bout, and splitting a bit of the wood too.

    Aaaargh!!!!

    Well, not to worry. Now I can learn how to repair a centre joint ....
     
  15. Darren Molnar

    Darren Molnar

    Nov 6, 2007
    saskatoon Saskatchewan Canada
    humble instrument maker
    Linen has been used by restorer type guys to correct concave ribs. as the glue dries, the linen shrinks, pulling the ribs into a flat, or convex shape. In case you thought I was beeing a nut-job. Picture the inside of you're back, with the braces ( that you carved convex ) attached. And in between the braces, linen strips running the width of the unsuported back areas. pulling them at least flatish, if not convex.
     
  16. Matthew Tucker

    Matthew Tucker Commercial User

    Aug 21, 2002
    Sydney, Australia
    Owner: Bresque Basses, Sydney Basses and Cellos
    No, yep, no you're right Darren. Linen does that, (brown paper too!) but unless I want to rip my shirts up i won't be using linen this weekend. The back is now behaving itself, with a few weights to stop it running off, and I'm trying to be patient (who, me?) and let the glue dry really well before I touch it again.
     
  17. Matthew Tucker

    Matthew Tucker Commercial User

    Aug 21, 2002
    Sydney, Australia
    Owner: Bresque Basses, Sydney Basses and Cellos
    Well this weekend it has been the driest for a while so after all my drama s I decided to move things along a bit and start gluing up the braces.

    Wish i had more big clamps, but hey, have to make do with what I've got.

    Oh and by the way, the back IS supported in the middle with a caul under the plate, below that bag of shot.
     

    Attached Files:

  18. KSB - Ken Smith

    KSB - Ken Smith Banned Commercial User

    Mar 1, 2002
    Perkasie, PA USA
    Owner: Ken Smith Basses, Ltd.
    Is this Bass made with the same materials/woods as the last Bass you made?

    Please remember to give us a complete report on the differences in tone, color of sound, depth, volume etc as compared to the last Bass you made.

    Also, I don't recall seeing all the measurements posted on the other Bass you made. Please show us both sets of numbers for comparison.
     
  19. This is what I would do also except more like a couple months.

    I am having quite a bit of trouble with my cornerless bass wood. The side wood is buckling and the back is potato chipped. My back is not joined yet and I have it clamped flat with air on each side. I will see what happens in a season or so. My maple is flat sawn wood and that is not good some times.

    Darren, have you done the linen thing? I am going to contact you for advice.

    Matthew, Can you use your ceiling for go bars? Just make sure it is extra dry when you glue the bars even if you have to use a hair dryer right before gluing. Another tip is to place it in black plastic trash bags and put it in the sun for a few minutes each side. This will dry it to low ECM and that is what you want. it will also give you an idea of what the wood wants to do in low humidity.
     
  20. Darren Molnar

    Darren Molnar

    Nov 6, 2007
    saskatoon Saskatchewan Canada
    humble instrument maker
    Hi Ken, I've done it on cello ribs a couple times, and it worked out really nice. And as far as bases, I did it to a school orchestra bass about four years ago, to bring up a sunken ply wood top that really should be tossed in the bin, for all the things wrong with it. That worked out better than I expected.

    I took the top off, used some moisture and a heat gun to try to get it back in shape. After that I let it sit for a week or so with a buch of weight on it. Then I glued linen strips all width wise across the top, with really strong h-glue on the kitchen stove. I was surprised how much the shrining linen thing helped. that's why I mentioned it here.

    Haven't done it on a flat back, But I have no doubt it would contribute to the cause, even if just to hold the shape after the other tricks are used.

    Hey ken, since you're up in the snow belt as well, do you find Februarys give you the most trouble with wood movements? for me, I think I'm seeing a trend that this month always sucks for making center joins, or watching fresh carved plates potato chip in front of my eyes.
     

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