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Thread: How would you cnc a corner joint

  1. #1
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    How would you cnc a corner joint

    I have a question to ask

    If you have two pieces of BB ply 3" wide say 5/8" thick and you want to cnc a corner 90 degree joint with some mechanical strength using only a staight cutter that leaves a .125" radius what type of joint would you cut that would not leave gaps in the joint.

    Thanks for any suggestions.
    cheers

  2. #2
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    I guess I'm struggling to picture what joinery you're describing ... so i'll go over the ones i can imagine...


    So let's say a shelf style joint - for that, i'd just do a rabbet or dado - no concerns or round cutters in that situation.


    If it's a mortise into one piece, like say a table leg and stretcher style joint, then I'd cut the mortise with what are called fillets in the corners. These are just little parts that the cutter extends past the actual square corner since it can't actually cut a square corner. This extra clearance allows something like a tenon with square corners to fit right into the mortise. While you said no gaps, this leaves very minimal gaps only in the corners and if you cut your tenons with enough of a shoulder, the gaps are completely concealed.



    Now let's say something like a picture frame. here you can get creative and do some kind of fancy half-lap type joint with geometric shapes or something creative (think puzzle pieces, or old time wooden railroad track sections). These are pretty nifty.



    Ok now let's talk what I think you might be thinking of ... a box type arrangement - where the two parts are mated at their ends like a drawer or box. If your CNC can machine the end of the board, you could cut some pretty nifty dovetail type joinery or box joints. But most machines can't. So let's go with what a "flat" machine could do ... box joints with those fillets above would definitely leave gaps. Fancy interlocking joints could be made up but it gets tricky going in 3D because one board lays flat and involves mostly Z-axis movements while the other board lays flat and involves mostly X/Y axis movement so the geometry would have to be drawn such that it allows for bit diameter (or strategic shallow rabbets that cover any fillets needed).

    A simpler method, though, might just be a rabbet. That would be the simplest, no-gap, joinery method.
    Jason Beam
    Sacramento, CA

  3. #3
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    My bad thanks Jason your last version like a box or dovetail joint is the kind I am thinking of. I don't see how any mechanical joint can be cut in the two parts flat on xy axis only without gaps to accommodated the cutter radius.
    cheers

  4. #4
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    I had planned to test doing standard dove tails on my cnc, but have some issues with the screw rods and alignment I need to work out. With that working, it would have been as simple as a clamping jig at the end where the head overhangs to cut them using a dovetail bit. Laying one side flat the other would be clamped on the end for the mating piece.

    Similar to this, just wouldn't need the template.
    Click image for larger version. 

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    Darren

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  5. #5
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    For Rob. 1/4" spiral cutter. Radius irrelevant

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    ++++++

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  6. #6
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    Thanks all for the input, I found a process that works in the flat. It would require cutting fingers like one would have on a box joint but longer than normal and sanding off extended ends on both sides. Darren in this case it has to be flat. I like your idea Carol especially given it can be fine tuned to be a tight fit / Have done that on my table saw i call it Alan B joint from Texas, my way of remembering it.
    cheers

  7. #7
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    Quote Originally Posted by Rob Keeble View Post
    Have done that on my table saw i call it Alan B joint from Texas, my way of remembering it.
    Ah, I know which one you're referring to now.
    Darren

    To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault

  8. #8
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    [QUOTE=Rob Keeble;414330]I have a question to ask

    If you have two pieces of BB ply 3" wide say 5/8" thick and you
    want to cnc a corner 90 degree joint
    Hi Rob; Kindly explain your meaning to "cnc" the joint. Thanks!
    Mack C. in Brooklin ON
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  9. #9
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    Mack i am having a box made essentially like the four sides of a draw.
    The 4 pieces are to be cut from sheet goods in this case BB plywood. It is intended that a company with a large cnc router which is only able to cut in the x y axis (straight router bit cutting at 90 degrees to the ply) will cut the pieces for me.
    During this process of cutting out the individual pieces i want it to cut the edges of each piece where the joinery would be required. So as it stands their smallest router bit the use leaves a radius cut of 3mm. .125" .
    Tradditional finger joints are square cut. If the fingers are cut at 90 degrees to the side (ie while lying flat on the table) the bottom corners of the fingers are rounded resulting in gaps when assembled.
    The answer i have discovered requires that the finger is cut longer with a round end and when assembled the rounded bit sticking out is either sanded off or left as a feature depending on cosmetic need. Sorry i dont have picture.


    Sent from my SGH-I337M using Tapatalk
    cheers

  10. #10
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    Gotcha now Rob! I didn't understand you were having someone cut the pieces for you on a cnc router. I know a guy over near Guelph that could cut those pieces by laser. That would give square edges.
    Mack C. in Brooklin ON
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    It feels even greater to give one to a friend!

    If your presence doesn't make an impact;
    Your absence won't make a difference!


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