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Thread: How Flat Does An Assembly Bench Need To Be?

  1. #1
    Join Date
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    How Flat Does An Assembly Bench Need To Be?

    My plans are to build some furniture so I need a really flat assembly bench. I built one out of MDF and surfaced planed it to perfectly level so I decided to build a second one out of sandply since I had the other flat one to build it on. I decided on a second plywood one because I was afraid that the MDF might not hold up. I tried to laminate it but it did not come out exactly level so I ripped off the laminate and surface planned it again with a router sled.

    Much to my supprise the new sandply one is not perfectly flat even though I had it clamped to the MDF one at all times. Across the three foot width it is very flat but lengthwise is where I have the problem. About one third of the way from one end it has a slight dip of .020 of an inch that runs about eighteen inches or so. The .020 of course is at the center of that span. I would plane it as I did the MDF one but the outer layer of the sandply is probably no more that .020 and then I would run into some rough wood.

    My questions are: Is this flat enough? If not, what would you do about it?

  2. #2
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    Quote Originally Posted by Allen Bookout View Post
    My plans are to build some furniture so I need a really flat assembly bench. ...Across the three foot width it is very flat but lengthwise is where I have the problem. About one third of the way from one end it has a slight dip of .020 of an inch that runs about eighteen inches or so. The .020 of course is at the center of that span. ...My questions are: Is this flat enough? If not, what would you do about it?
    Hmmm.. 1/50"... For me, that would be 'close enough.' That much variance would easily be compensated for by virtually any carpeting.

    If you have hardwood floors, tile floors or linoleum floors, have you tried your straightedge across any of them? I'd bet that most - or all - of them are out of 'flat' at least that much somewhere across their span.

    I wouldn't worry about it, Allen.
    Jim D.

  3. #3
    Join Date
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    Ditto! I am hard pressed to measure .02 with a tape measure.
    God grant me the senility to forget the people I never liked anyway,
    the good fortune to run into the ones I do,
    and the eyesight to tell the difference.


    Kudzu Craft Lightweight Skin on frame Kayaks.
    Custom built boats and Kits

  4. #4
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    I'm thinking that's plenty flat. If you have to use a feeler gage to measure the dip then it's flat enough.
    Jim

  5. #5
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    That's a tough one. 0.02 is a little more than 1/64th. You can see 1/64 on some joints, for instance table leg to apron.

    I wonder if there would be a way to add a little to the area where it takes the dip, maybe a coat of shellac or two?

    Having said all that, its awful darned close!

    What I would worry about is whether you will curse yourself in the future for not fixing it, every joint that isn't crisp may haunt you, I know it would me.
    Don't believe everything you think!

  6. #6
    Steve Clardy Guest
    Close enough for me

  7. #7
    Quote Originally Posted by Steve Clardy View Post
    Close enough for me

    Same here.........

    WoodWorking, Crappie Fishing, Colts, Life is good!

  8. #8
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    I'm also in the close enough camp. remember this about anything you build or assemble on top of that table. Wood moves, sometimes a Lot, so I'd say over that size table you can declare it as 'flat'. just my .02c
    -Ned

  9. #9
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    Thanks guys! The good news is that the consensus is that it is flat enough. The bad news is that it still bothers me a bit. I just cannot figure out how to fix it without making more of a mess out of it. I though about flow coating a little epoxy in that area and then coating the whole thing with a couple of coats of epoxy and sanding smooth. That would make it pretty tough. Oh well, I will see how I feel about it in the morning.

  10. #10
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    Allen -

    I guess it depends on how anal you are towards doing something to the absolute best of your ability. If your anything like me... I truely feel sorry for you.

    I made my assembly table from 3/4" MDF as a tortion box - both upper and lower surfaces are faced with an internal honeycomb that's about 8" apart. Due to humidity in my shop, I noticed a slight depression in the MDF that almost clearly outlined the honeycomb core. The amount of sag was something like 1/64 at the very most. Just enough to be annoying, as it probably would have very little effect of projects.

    Anyway, I knew it was there...and just wouldnt be content knowing I was working around this 'flaw'.

    I used a floor leveling compond to level it out using a heavy staight-edge. I then then glued down a 1/8" piece of hardboard. (with lots of MDF and cauls!) Put a few coats of blo/turps then wax. Haven't detected a problem since. Or rather, It's because too chicken to check.


    Like others have said, .02 isnt much to worry about. But understand, everything that you assemble on this table will also risk being .02 off. This is the part that would bug the heck out me. To me, "Good enough" often isn't.

    It's almost like having a TS blade set to 89.9 degrees. I can live with this, or with just a little more effort I can get it set properly. It's my curse.
    Last edited by Timothy Lindgren; 04-23-2007 at 04:30 PM.

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