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Thread: Clamp calculations

  1. #1
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    Clamp calculations

    Trying to see if I have enough clamps for my table top. Read about this 45 degree rule in spacing them out. Will probably cut it down to 8’ (currently 107”). For the three panels, the distance to first glue line is average of 11 to 12”. The width will be about 34”.

  2. #2
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    Tony, in your favour you only have two glue lines. Also in your favour the boards at 11” or so and fairly thick no doubt are very stiff, the clamp force applied will spread evenly.
    What you do need because of the stiff boards is very straight well matched glue line. The clamps are not going to close errors, however many you have.

    I see the boards stacked on edge with a lamp behind them and much fine tuning. When you are ready, my table top experience says a clamp 2” from each end and a clamp every foot is plenty. A rope wrap can be added with the clamps if you are short.
    ​You can do a lot with very little! You can do a little more with a lot!

  3. #3
    The 45˚ thing is fine, but it depends on the strength of the clamps as well.

    Rubber bands spaced a certain distance will produce even pressure, just not very much of it.

  4. #4
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    Quote Originally Posted by Cameron Wood View Post
    The 45˚ thing is fine, but it depends on the strength of the clamps as well.

    Rubber bands spaced a certain distance will produce even pressure, just not very much of it.
    Most of my long clamps are Bessey and Jorgensen parallel clamps.

  5. #5
    If your top is 1" thick, and 8' long, then at 250 psi clamping pressure (quite low in the range of recommendations)
    there should be 24,000 lbs pressure. with one clamp every foot- 9 total, each clamp should produce ~2,600 lbs force- about the weight of a car every foot.

  6. #6
    Quote Originally Posted by Cameron Wood View Post
    If your top is 1" thick, and 8' long, then at 250 psi clamping pressure (quite low in the range of recommendations)
    there should be 24,000 lbs pressure. with one clamp every foot- 9 total, each clamp should produce ~2,600 lbs force- about the weight of a car every foot.
    You can safely ignore those pressure guidelines developed for industry. All you need is enough pressure to pull the joints up tight, and if the joints are accurate it doesn't take that much force. For what you are talking about I would typically use 6-8 aluminum bar clamps but with such wide boards could probably work with 4 or 5. For reference, Jorgensen aluminum bar clamps are spec'd at a maximum force of 1,000#, their heavy I-beam clamps at 7,000# and light duty Pony F clamps at 300#. I've glued up many a door panel with those Pony clamps without issue.

  7. #7
    I'd argue that you don't need any pressure if the joints are perfect. At least on smaller projects I've gotten good results with rubbed joints. So I wonder where the idea of huge amounts of pressure comes from.

  8. #8
    Quote Originally Posted by Richard Hutchings View Post
    I'd argue that you don't need any pressure if the joints are perfect. At least on smaller projects I've gotten good results with rubbed joints. So I wonder where the idea of huge amounts of pressure comes from.
    It comes from the glue manufacturers for industrial situations where the mating surfaces are less than perfect and hydraulic presses are used to achieve the specified pressures. The successful use of vacuum presses with maximum pressure of about 11 psi demonstrates that the industrial guidelines are not gospel.

  9. #9
    I've been woodworking for a long time, I never heard of the 45 degree rule.
    I looked it up, IMO, it's just common sense to spread your clamps out for even pressure.

  10. #10
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    The better the edges are matched to each other, the lesser the need for an inordinate number of clamps or high pressures.

  11. #11
    I put that clamp pressure thing up for shock value, as it runs so counter to most folks' experience,

    but for that example, using pipe clamps, which are said to produce about 1,000 lbs force, one would use 24- about one every 4".

  12. #12
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    Quote Originally Posted by Rafael Herrera View Post
    The better the edges are matched to each other, the lesser the need for an inordinate number of clamps or high pressures.
    Which goes back to my skill with a number 7 :rolls eyes: :exasperated look to heaven emoji:

  13. #13
    I've accidentally glued enough things together in my life to know you don't need any clamping pressure to form an incredibly strong glue joint. Now, I'm not saying I don't use clamps or that they're not necessary. In fact, I use a lot of clamps every time I glue up! But, I don't worry about math or theory when it comes to clamps, and it's never come back to haunt me. I just eyeball it and if it looks fine, it'll be fine. Works every time!

    I do have one rule, however. Whenever I start a new project, I buy 2 new clamps for it. You'll always be 2 clamps short of what you think you need, and you can never have enough clamps.

  14. #14
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    Quote Originally Posted by Tony Wilkins View Post
    Which goes back to my skill with a number 7 :rolls eyes: :exasperated look to heaven emoji:
    I don't really know what your skills w a jointer are, so I don't understand why you're exasperated.

    An old joining technique uses pinch dogs, which can still be bought, to glue up two boards.

    I have to admit I've not used them, but I do have a set for when I decide to use them. I assume that if the joint has no gaps, a dog on each end does the trick.

    Nail-Dogs-e1456100215714-576x1024.jpg

  15. #15
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    Quote Originally Posted by Rafael Herrera View Post
    I don't really know what your skills w a jointer are, so I don't understand why you're exasperated.

    An old joining technique uses pinch dogs, which can still be bought, to glue up two boards.

    I have to admit I've not used them, but I do have a set for when I decide to use them. I assume that if the joint has no gaps, a dog on each end does the trick.

    Nail-Dogs-e1456100215714-576x1024.jpg
    I actually have some pinch dogs somewhere (added to anLV order once upon a time). I could use them on this, especially since I plan on breadboard ends. My exasperation is trying to get perfectly straight over 9 foot of length of each board. I’m getting better as I go along but I’m a perfectionist. I’d like to do rub joints with hot hide hut I don’t trust my ability to get test perfect with the edges.

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