Help cutting plywood

Discussion in 'Wooden Boat Building and Restoration' started by wudenbote, May 11, 2009.

  1. wudenbote
    Joined: Aug 2008
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    wudenbote Junior Member

    I am about to cut my first piece of plywood. The piece for the transom and I AM SCARED! :) It is 3/4" marine ply and being very pricey, I don't want to screw it up. What is the best way to cut plywood to prevent feathering (tearing ?) I want as smooth a cut as possible. I await your valuable help. Thanks much!!!!
  2. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    If is a pretty straight cut uses a circular saw, if it a curve then perhaps a small circular saw, sawall or a jigsaw. Practice on a cheaper piece of plywood. Also remember that blade removes at least a 1/16 of wood. If a very precise fit is needed, cut oversize and sand down. Also use at least moderately good equipment and new blades. I sometimes use 40 teeth per inch, sometime 26 inch depends on wood.
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Tightly clamp a piece of 1/4" plywood or other sacrificial piece over the cut. The rip out will be on the upper side only so you don't need it on the bottom.

    A good method to cut down on this is to always cut with the good side facing down, as this edge will cut cleaner.

    Use a fine tooth blade and make sure it sharp. Use a saw guide to make straight cuts, don't force the saw, let the blade do the work, not your arms. Set the blade so just the teeth and the gap between the teeth show though the cut, no more.

    When worried about edges, it's best to sneak up on them. Cut well outside the line then plane or sand to the line. This prevents under cutting and permits you to fine tune the fit of the piece.

    You can also razor the cut line before making the cut, so that rip out on the surface is precut and you have the razored edge to look at.
    Joined: Mar 2009
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    Location: Ontario, Canada Junior Member

    All of the advice above is excellent. I've used each suggestion myself in the past.

    I'm no qualified woodworker, but I'd like to add that sometimes all it takes is a piece of masking tape laid along the cut.

    Whatever solution you decide to test first, try it on part of the panel that will end up as waste in the end.
  5. Boston

    Boston Previous Member

    place the good side down
    its a transom so you are going to have some arcs to cut
    normally for straight cuts in finish ply I have a special right handed porter cable high speed ( side winder type ) saw with a panel blade in it
    you dont have strait cuts

    first thing you need to do is get a pair of safety glasses and use em
    next thing is fingers
    when your done with the cut
    your primary concern is to have the same number of fingers you started with
    so relax
    your not going to use the finger eating panel saw

    for absolute precision ( Ive cut up things that cost tens of thousands of dollars before ) Ild cut a practice piece out of crappy lumber with a jig saw and sand the edge to fit to perfection ( its going to end up being your jig to cut the final piece )
    your going to make it smaller once you get the fit perfect so if you do screw up a small section you can fix it no worries
    jig saw is best to cut the practice piece, its fast, ugly and effective, will always tear up a finish piece so dont try the jigsaw trick on an expensive piece of wood
    sand it, rasp it, whatever
    once you get it perfectly fitting remove it and go get your router

    I use porter cable routers
    the 1.5 hoarse d handle is best
    get a pattern collar and a straight 3/8 x 2" bit with a half inch throat and get your router set up to do the work for you

    take another practice piece and cut a hole in it with an electric hole saw ( say 3 inches ) and then use that hole to cut another using the first as a guide

    this will tell you how much difference there is between the router blade and the router guide (collar) so you know how much you need to remove from the template

    oh the template is best in 3/4 ply
    1/2 will do but check the depth of the collar non the router cause the ones I use are kinda deep and sometimes 1/2 inch isnt got enough clearance between the dust and the pattern

    the second hole will tell you how much to mark in on the transom template you just made
    measure the diameter of the first hole and the second
    take half the difference and thats how much you are going to take off the
    edge of the template

    flip your finish piece over and check for where you want the grain to end up
    kinda picture it in your mind and flip it back to face down
    be thinking about grain throughout the hole job as a transom is kinda artwork

    take the pattern and mark in the half the amount of the difference in the circles you cut then cut it real carefully with that wood eating jigsaw again
    file it clean as you can cause that pattern is how good your cut will be

    now there is something I forgot earlier and that is the bevel of the transom edge
    leave it on for now
    your going to sneak up on it once your piece is in
    oh most transoms are not flat so the arch adds to the length of the piece
    so a practice piece is normal specially if your working with some expensive laid up piece of say burl maple or something worth not screwing up

    ( although I would have planked up burl on the frame in place, but you know what I mean )

    check that you remember that grain you liked again and this time
    locate the pattern on the back side so that the grain is nice and horizontal and its best character might say show just beneath the name plate

    use clean oil free philips head screws to attach the template to the Finnish piece ( key thing on this is to use screws that are not so long at to pierce the face of the finish side but long enough to get a grip in the ply

    take a deep breath
    think happy thoughts
    you are about to be a finish carpenter
    check your cord on the router and make sure you can get a full swipe on the work without having to stop and readjust your cord
    plugging it in helps a lot to

    the router will spin clockwise so you are going to go counterclockwise round the template

    plunge into the work from about 3/4 of an inch off the template keeping the base plate of the router solidly on the template
    let the other half float over the finish piece while plowing your way to the template
    eventually the collar will make contact and your off
    with a steady hand and long calm breaths gently let the bit do the work and follow the edge of the template with the collar
    keep an eye on how nice and flat you are to the template with the bottom plate of the router and just let it happen
    slowly follow it all the way around watching your cord from time to time
    if you do have to pull off bring the bit out into open wood before you tilt the router up using the base as a pivot
    if it grabs ( almost never does ) no worries just hang on and lift the router up out of the wood
    two hands on the router at all times and have fun with it
    best of luck

    using the depth of the router you can also herringbone the layers of multiple layered hulls into the transom
    makes for a dam strong connection

    once your transom piece is in you can take a fine rasp and following the line of the hull form gradually take off the bevel leaving abut a 1/16 for final sanding into place
    a rasp is not a finish tool
    always rasp into the finish face of the ply
    never away from it or your going to regret it
    if your dam good with a rasp you can glue to it
    if not I use a variable speed belt sander to clean up the bevel

    helps if you pay attention to the direction of the paper and keep it entering the finish side not leaving it

    transoms 101 is now ended
    hope you enjoyed the show
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    That's about the most difficult and time consuming method I've ever heard of cutting transom, Boston. Frankly, it's an absurd way of doing things, particularly knowing the edges will need to be beveled to match deadrise and flare. It would be fine if you had a dozen of exactly the same transom to make. The transom needs to be rough cut, then beveled done to it's final shape. Either you've done this before or your guessing. I've done it before.
  7. Boston

    Boston Previous Member

    Ive built more stuff than God
    if I want it perfect I make a test piece turn it into a template and start slicing
    if its curved it might as well be a test piece that he screws up to the frames rather than put a bunch of holes into his nice finish piece and when withdrawing the screws likely tears out some

    basically he cant screw up the method described
    and it sounds like he has little experience

    appreciate the input though
    nice to know someones keeping me honest


    yes he could clamp, trace it and bludgeon off the excess which could also damage the wood if he didnt use blocks or clamped it tight as he could
    which is a common mistake or had some huge tear out drift into the finish area

    Ive lay'd up transom's plank by plank but I never used much plywood
    I was just trying to give him a fool proof method
    should have told him to lay the finish piece on a few stickers though
    wonder if he now has a lovely transom shaped grove in the middle of his girlfriends favorite antique coffee table
  8. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    There is no way the transom is going to be curved if the ply is 3/4", so just do what PAR has said, think twice, cut once. (IF it is curved mate, then 3/4" thick is nbg anyhow, it will have to be laminated, usually over a template, but can be done insitu if you like)
  9. wudenbote
    Joined: Aug 2008
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    wudenbote Junior Member

    Thanks to all who responded. I hope the heated exchange was good natured. The transom has straight edges except for the top edge which has a gentle curve for aesthetics. The boat is pretty much a utility craft, not a "show boat" with strength and stability being paramount and speed and beauty at the bottom of the list. I purchased a Bosch 40 tooth thin kerf blade, razored the line, set the blade depth as suggested and went to work. It turned out beautiful! Thanks again!
  10. Boston

    Boston Previous Member

    way to go
    and yah its common
    everyone's got there own way of doing things
    depends on the situation, skill level, desired level of quality
    its like explaining a three plate method of western platform framing to someone who has always stick framed

    to each his own
    best of luck with it all
  11. kroberts
    Joined: Mar 2009
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    kroberts Senior Member

    One thing nobody here mentioned is that you should cut as much "along" the grain as possible. If you have a line going diagonally in relation to the grain, if you cut in one direction the big chips/flakes will come off on the piece you want to keep, and if you go the other direction they will come off on the piece you are going to scrap. Keep that fact in mind, and pay attention to it.

    This is plywood, so the layer you want to keep most is the "good" face. The next most important layer is the other face. The layers in the middle are protected by the outside layers, either that or your plywood is junk.

    Using a router to cut a complex curve for a one-time piece is like giving yourself a root canal without pain killers. Yes, you could do it but I can't imagine why anyone would want to. I use a circle jig all the time, or an edge guide or fence to router-cut anything I want to be perfect. It's the most accurate tool I have for cutting. It's not the right tool for this job, IMO.

    No matter how curvy this cut is, it's not so curvy that a good finish blade on a jig saw can't follow it. They make finish blades that will cut with almost nothing chipping off. Go get a package of them. Practice on some scraps so you know how it works. Learn to not press to one side, you can tell because the blade bends over and your cut is not square to the face of the board. Again, don't push the saw. Finish blades are NOT fast blades, it will be a long time before you get to the other end of the plywood, and you might go through more than one blade.
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Look the bottom line are the bevels, so the cut line is just to provide the approximate shape only. Personally I'd cut the bevels at the same time, but I've been around the block a time or two. A novice has to sneak up on the final shape, which can be down with a number of tools, but not a circular or jig saw.

    Tape doesn't work that well, clamping stock over the work does. You can engineer and setup a template, but you're doing the work twice, why bother.

    Glad to hear you got it done Wudenbote. Now how about some pictures? Our exchange wasn't heated, I can show you threads where it's gotten heated.
  13. Boston

    Boston Previous Member

    Ill second that
    and ya send up some picts
    best to all
  14. Anchy
    Joined: Jul 2012
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    Anchy Junior Member

    is this forum still alive?

    Thanks for advice as I am just about to cut marine ply to fit the windows. I wonder if this forum is still going on as I started work for very first time on a boat and is very very tricky.
    Thanks again


  15. liki
    Joined: Nov 2008
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    liki Senior Member

    A note about jig saws more. The blade will wear out quickly and the cut will start to form an angle if you push or turn the blade too much, or cut for too long without pausing. Or maybe I have just had bad luck with blade quality?
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