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Thread: Keeping bedding compound out of screw holes - how?

  1. #1
    Join Date
    Dec 2001
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    Mansfield, MA USA
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    Angry

    That pretty much says it all!

    I started fastening the frames together last night. I was advised to predrill, then apply bedding compound, then screw together because drilling through bedding compound would potentially ruin my fuller bit.

    Got them all aligned and clamped. Marked the screw locations, and drilled. The bit still got smoking hot, presumably because with the stop-collar in place, the countersink didn't clear the wood shaving very efficiently.

    Then unclamped, applied bedding compound and tried to keep it out of the screw holes. Is there an incantation I should chant as I do this? I am afraid that even the little bit in the holes will lead to significant hydraulic pressure tha could split the frames. In fact, it did. It was definately my fault - I drilled too close to the end and started to see splitting. Or would you just drill through the Dolphinite?

    Also, would a thin glue like superglue that could creep up the split and hold it be a major faux pas?

    [ 06-04-2002, 09:46 AM: Message edited by: Jeff Evans ]

  2. #2
    Join Date
    Feb 2000
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    Northeastern USA
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    Post

    I can't think of any reason to try to keep the bedding compound out of the screw holes. In fact, when I use bedding compound, I try to get some IN the hole to seal out water.

    Hydraulic pressure from bedding compound is a new concept for me. I suspect if the frame was that close to splitting, it would have split with or without the compound. You were lucky to find out now rather than later.

  3. #3
    Join Date
    Dec 2001
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    Post

    the problem is that if there is more than just a very thin coating of bedding compound in the hole, the screw won't go into the hole all the way and pull the frames together - liquids are not compressible.

  4. #4
    Join Date
    May 2001
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    Seattle, WA
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    Post

    What exactly are you attempting to do? Assemble double sawn frames or assemble laminated frames using bedding compound instead of glue? How heavy of an application of the bedding compound are you using? Skim coat or something akin to a peanut butter sandwich built by a four year old? Are you retracting your drill bit to clear shavings as you progressively drill deeper or are you trying to drill each hole with one smoking heroic effort? I wonder if your drill/countersink is mis-matched for your screws or improperly set. Double check all that with some scrap wood. Get rid of the stop collar/training wheel from your countersink, get used to working without them. Screwing frames together is a good place to practice countersinking. Is your wood drilled sufficiently to accept the shank diameter of your screws? Some people prefer to use "stepped" drills and countersinks rather than the tapered ones such as Fuller's. Remember kiln dried wood is much more brittle than air dried so you have to take extra precautions with it. Good luck.

    [ 06-04-2002, 11:25 AM: Message edited by: RGM ]

  5. #5
    Join Date
    Dec 2001
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    Mansfield, MA USA
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    Post

    Thanks for the reply. Training wheels, huh? Well when you put it that way....

    Many questions there:

    Assembling dory frames from matched knees. Scewing them together where they overlap across the bottom of the boat.

    Yes, retracting the bit to clear it, but the countersink gets clogged. maybe getting rid of the collar would be good.

    The wood is air dried hackmatack.

    According to the Hamilton's Marine catalog, the bit is appropriate for #12 screws in softwood. They set in nicely except where I got googe in the holes.

    I am smearing the bedding compound on thin but with enough extra that a little squishes out the sides when I tighten the screws. I scrape this off and return it to the can.

    Maybe a few more practice pieces are in order before I attack the remaining three frames.

  6. #6
    Join Date
    Mar 2001
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    Riley, Mi U.S.A.
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    Post

    If you're splitting frames, the hole is too small, I think. Sometimes even a recommended hole feels too small. You'll know as you try to drive a screw. This is a decent sizing chart-- http://www.wwforum.com/faqs_articles/DrillChart.html

    If the bit is hot, it could be dull, or you need to clear chips more often. I don't bother with tapered bits anymore. They wear out fast and aren't practical to sharpen, though my Dad seems to be able to sharpen anything. [img]smile.gif[/img] Old bits with worn-out margins, or new bits that have margins ground wrong can generate a lot of heat just from friction with the wall of the hole.

    An OBG around here taught me to use either wax or bedding compound on the screw just to make 'em easier to drive. I wouldn't worry about hydraulics.

    Rich

  7. #7
    Join Date
    May 2001
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    Post

    Since you're dealing with a lap joint did you give any thought to using bolts (carriage)? They draw up nice and would probably work better in that situation.

  8. #8
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    Jul 2001
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    Mission Viejo, CA
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    Post

    As I remember the article on silicon bronze screw breakage in the current issue of Wooden Boat, there is anecdotal evidence that the problem seems to be worse in tapered holes than in holes step-drilled (once for the thread and once for the screw shank). I'll double check that when I get home tonight.
    Frank

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