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  1. #16
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    Theoretically not.

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  3. #17
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    I theorised that.

  4. #18
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    Quote Originally Posted by Wongo View Post

    Good Morning Wongo

    When I read your succinct* post last week I thought that you had cracked it.

    On reflection, I am not so sure. If you look at the absolute outside egdes on the convex side, and the dead centre of the concave side, in the jointing and thicknessing stages you are removing zero thickness. I have never been able to joint anything by removing zero material.

    Looking from another perspective; If the 12 mm plank was perfectly flat and square initially (absolutely no cancavity or convexity or twist or any other normality) and you jointed it, you would lose 1 mm. Then you thickness the other side and lose another 1 mm.

    In summation, would not Mr Sinjin's most perplexing plank finish up around 9 mm thick ????


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    Graeme
    ( * succinct = zero words.)

  5. #19
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    Quote Originally Posted by GraemeCook View Post
    Good Morning Wongo

    When I read your succinct* post last week I thought that you had cracked it.

    On reflection, I am not so sure. If you look at the absolute outside egdes on the convex side, and the dead centre of the concave side, in the jointing and thicknessing stages you are removing zero thickness. I have never been able to joint anything by removing zero material.
    That is precisely how jointing works; you take the high bits off and, ideally, remove zero from the low bits.

    Looking from another perspective; If the 12 mm plank was perfectly flat and square initially (absolutely no cancavity or convexity or twist or any other normality) and you jointed it, you would lose 1 mm. Then you thickness the other side and lose another 1 mm.
    That is a non-sensical statement. If it is flat already it can't be jointed, by definition.

    As shown perfectly in the diagram, when you joint a board you lose the height of the bow (in the case of an already jointed board you still lose the height of the bow: NOTHING). That is how it works.

  6. #20
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    Well might be the tips and the centre of curve are only points. A single point has zero length so removing zero thickness is possible.

    Of course it is different in reality.
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  7. #21
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    I'll explain something....many yrs ago i was a wooden boatbuilder and timber that was warped or twisted was common place. Planing a face flat before you ran it through a thicknesser always presented issues. Least waste of the wood was always a concern and joiner width (not wide enough or not long enough)was often a problem not to mention sharpness of blades. etc etc

    So moving on many decades i now oversee a very large NC Mill. nearly 2 cubic meter capacity and I'm a mechanical designer/ project manger for that machine....since the advent of importing many products that were once manufactured in Aust are now from China its no surprise Aluminum is one such product. Myself i think its rubbish but that doesn't matter.
    What does matter is 99% of the time sheets are not flat. Recently we had to machine 8 sheet 1800 x 900 x 12mm flat. The usual worry is once you cut the outer layer of the Alu it stress relives and moves even more.
    The other issue is bent out of the box at no extra charge and complaining gets you no where. So the usual is to clamp sheets flat to the table skinning the surface which does not help you get it flat. I then told the machinist it's no big deal we will use and old boatbuilding technique. Basically we put masking tape over the surface of the bed of the mill and use car body filler in big dollop's in about 12 points the tape stops the filler sticking to the Mill table. We then carefully lower the sheet onto the body filler and allow it to go off....maybe 15 minutes tops and then machine is flat my skimming the high points till the entire surface i machined. Filler has very lower tensile strength so to pop it off the table top is very easy and the cutting forces are quite low in shear stress the sheets once machined were perfectly flat and parallel to within 3 thou,



    So starting with 12 a mm sheet which is bent by 1mm. Alex was on the money from the start and Wongo's simply and accurate sketch says it all.


    I use this same technique at home but instead of a NC Mill is use a router when i need to get a bark to bark planks flat which may have a little bend or twist or both.. Basically i have a well supported sheet of Melamine which is planer i guess to within 1mm over 1.5mm sq meters. Which is not so bad but after going through my 20" thicknesser a couple of times flipping either face and then placing a straight edge over the faces it very good.

    But i would advise people to think about body filly as a support of warped boards. You can experiment...tape the surface yr working on and tape the wood yr using so the filler doesn't stick to anything other than the tape.....sometimes i don't bother if i know i have heaps to machine off i just use a sander to remove any filler before i pass it through the Planner. The filler you use in dollopes and you need to use enough just to support the board so it doesn't rock or move in any way.

    My router table is basically a half sheet of MDF attached to a torsion box maybe 150mm thick with a couple of simple rails attached and in my case it just 200mm by 25mm thick play attached to the side on the torsion box. Simple piece of play spans the width of the torsion box with a large router attached. I then glue the warp board on the torsion box with the filler. lower the router cutter till it hits the high spot...run over the surface enough so you feel you will have good contact when it goes through your planer. So you don't need a 100% Routed flat face...I have made a few and once they get knocked around i chop them up and make a new one when i have a job o do. The traditional method of hand planing with winding sticks works the same just slower and i won't get into the hand tool as opposed to machines.
    Sinjin

    Sinjin

  8. #22
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    Sinjin, how long does it take to put the tape on the table? I think if you wipe wax (cheapest available) instead, you'll find it's much quicker for the same effect.
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  9. #23
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    Hmm..we use 75mm wide tape....i'm guessing maybe a minute. And tape is used to stop filler etc getting in the T-slotts.

  10. #24
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    Fair nuff.
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