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Post by dadealeus on Sept 7, 2019 16:46:08 GMT -5
I'm attempting to perform some kerf cuts on my CNC to bend 3/4" plywood. Has anyone done this before (via CNC *or* manually) that would be willing to share some pointers? I'm using a 1/4" flat end mill to start with (it's the only thing I had readily available) and cutting to leave about 1/8" of an inch left out of the 3/4" plywood, but the wood is definitely doing more breaking than it is bending. Do you think my depth of cut is not enough (I need to leave even less material), or do you think my cuts are too wide (use a more narrow end mill), or possibly something else? Any tips would be appreciated. For reference, I designed this end-table for my girlfriend after watching some YouTube videos on kerf bending, so now I'm trying to make it happen, but not until I get some more positive test results on scrap pieces:
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Post by dadealeus on Sept 7, 2019 16:57:22 GMT -5
After several failures, I had some success immediately after my original post. The piece that sort of worked left only 1 mm of wood and I had to spray it with water to soften it up, but it did bend (albeit with quite a bit of tear-out and other jagged edges): I'm going to try a tapered ball end mill next to see what effect that has and I'll post my results.
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Post by stevem on Sept 7, 2019 17:17:58 GMT -5
I think a V-Bit would work better and leave about 1/16"of the wood.
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Post by dadealeus on Sept 7, 2019 17:29:57 GMT -5
I was thinking the same thing, but it appears that a tapered or v-bit is actually more prone to breaking because the bends occur in the thinnest parts of the wood. So, with the tapered (7.5 degree) ball-nose end mill I just used, the inner parts fit together better, but almost every cut line had a full break in it because it wasn't a wide enough area to bend: Depth of cut seems good though (leaving about 1 mm of wood). 1/16" (~1.5 mm) still seemed too thick, surprisingly. I'm going to see if I can find a tapered flat-nose end mill I can use. I have one, but it only has a 1/2" cutting face, so I can't get it low enough into the 3/4" plywood. Might need to order a new end mill just for this project. I'll update again when I make more progress.
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Post by dadealeus on Sept 7, 2019 18:23:45 GMT -5
Well, I ran out of properly grained test pieces, so I ended up using a piece that had a perpendicular grain to the other test pieces, but I'm out of time for today and just wanted to get this last one in. I used a 5 degree tapered 3/16" flat nose end mill I found. The result is *incredibly* flexible (due mostly to the bend going along the grain of the wood), but it's also very weak for the same reason. A number of cracks appeared immediately, so I'll have to repeat the test tomorrow with properly grained wood. I'll try again tomorrow. I also want to try a 1/4" bull-nosed straight end mill that I have - I'm hoping that the rounded cut will help reinforce the kerfs a little better.
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Post by joeblow on Sept 7, 2019 19:28:02 GMT -5
I've done quite a bit of kerf bending throughout the years on jobs. This calculator kerf spacing calculator is spot on and takes the guess work out of kerf spacing for the desired radius. Your 1st pic shows the depth that I would use on 3/4 ply. I usually go just into the final ply core prior to the finished face. Fill your kerfs with woodglue or construction adhesive and clamp in a jig overnight. Keep in mind it is difficult to sand out the faceted edges on the face. I usually sand smooth and care not if I sand thru the face into ply core as I then apply my own veneer of choice over the now smooth radius. Otherwise, keep your spacing and depth uniform and you be fine. This is def a task more suited to a radial arm saw or decent circular saw but if I were to do this on my cnc I think I would use a .125EM for starts as that is closest to saw kerf.
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johnb
Full Member
New owner @ March 2019, AR16 Elite, Aspire, 4th Axis & Laser
Posts: 326
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Post by johnb on Sept 7, 2019 20:45:46 GMT -5
I'm not entirely sure where you're going with this. The test pieces seem to be "construction grade" plywood (which is going to have a LOT different bending characteristics than a veneered plywood).
The model seems to reflect an edge banded (in the cutouts) veneered plywood. Are you planning to do this with a "pre-veneered" plywood or apply your own veneer "after the fact"?
If you're using the pre-veneered plywood, the only way to figure out what's going to happen is to run tests on that. The splintering may not be an issue...or may be worse. What joeblow said is probably really good info, but adding a veneer "after the fact" (and after "getting to a smooth radius") doesn't sound like a very efficient way to do this.
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Post by dadealeus on Sept 7, 2019 23:20:28 GMT -5
All excellent information. This is my first foray into a project of this type, so I honestly don't know where I'm going! I generally expect my first attempt at something like this to fail multiple times for multiple reasons, but my approach is to usually just jump in and figure it out as I go along to ensure I'm making at least *some* progress.
For the actual project, I picked up some 3/4" sanded maple plywood. I assumed (possibly incorrectly) that I'd be able to sand it to a nice enough finish without any veneer applied as long as I figured out how to space and depth the kerfs. Ultimately, I just need it not to splinter (which, based on these tests, is a goal I should be able to achieve) and then I'll likely finish it with a couple of clear, 2-part epoxy coats.
The kerfed wood will be cut from 3 different pieces (the top and the two bottom edges). I basically hacked around the Fusion 360 sheet-metal forming tools to get it to lay out the shape I made flat and create the cut paths for the kerfs exactly where they need to be.
The black supports will be cut from plywood and then the edges rounded with a hand router before sanding, priming, and priming painting with a glossy black alkyd urethane. I'll likely combine kerfed pieces with the frame via wood glue. I'm not terribly concerned about the proper spacing or width of kerfs since the frame should carry the weight and restrict the shape - I just want it to bend relatively smoothly, so that's my focus with this series of tests.
I'll try again tomorrow with a depth of just into the final ply and see how that series goes.
Thanks, guys!
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Post by redwood on Sept 10, 2019 1:17:42 GMT -5
In my former life as a carpentry contractor, I did a fair amount of curve work. Started by using kerf cuts, there were books at the time that specified things like the kerf calculator mentioned above. Eventually, everything became laminations. More work, but less worrisome and definitely stronger.
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Post by dadealeus on Sept 12, 2019 5:37:11 GMT -5
In my former life as a carpentry contractor, I did a fair amount of curve work. Started by using kerf cuts, there were books at the time that specified things like the kerf calculator mentioned above. Eventually, everything became laminations. More work, but less worrisome and definitely stronger. Hi Redwood, Thanks for the reply. I didn't have the time to put in any additional work on this yet, but I was curious of what you meant regarding "laminations". Are you referring to the steam-bent, wood-glued and clamped bending that I see in many online videos?
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Post by traindriver on Sept 12, 2019 20:05:26 GMT -5
I like the table design! If you make the laminations thin enough, you don't even have to use steam to bend them. While it sounds like it might be one of the books of the Bible, lamination is simply gluing thin pieces of wood together, like making plywood. The idea is that you have a bunch of thin layers that are easy to bend individually, so you apply glue to the thin pieces, put them together and clamp them in place so the glue will dry with them in the bent position. You'll have to make them longer than you need because when you bend them, the ends won't all end up the same length, and you'll have to cut them so they come out even.
You might also consider edge gluing solid maple for the kerf cut method. It will not be as prone to breaking as the pine plywood you are testing with.
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Post by dadealeus on Sept 13, 2019 21:33:37 GMT -5
Thanks for the explanation and suggestions, traindriver!
I have a local lumber mill here that I'll try to run by to see what types of options I have available. Are the really thinly cut pieces of wood typically available at a lumber mill? Or do you think it's something I'd have to purchase from online somewhere? (I don't have access to any tooling I'd need to make my own)
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Post by vintage on Sept 14, 2019 0:48:51 GMT -5
Ever thought of trying wiggle board? It's basically plywood that has all the veneers glued together in the same direction, allowing it to "wiggle". Might be the easy way...
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Post by traindriver on Sept 14, 2019 18:04:56 GMT -5
Thanks for the explanation and suggestions, traindriver! I have a local lumber mill here that I'll try to run by to see what types of options I have available. Are the really thinly cut pieces of wood typically available at a lumber mill? Or do you think it's something I'd have to purchase from online somewhere? (I don't have access to any tooling I'd need to make my own) Not sure about sourcing. I would use my bandsaw to rough cut the strips, then sand them flat.
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Post by redwood on Sept 16, 2019 12:20:49 GMT -5
I would either rip the material myself or for bigger stuff, my lumber yard would rip it for me. I'll admit though, the stuff that I made had much larger radius's, but the methods are the same. The trick of course is determining how thin the material you are using needs to be to make the curves you need. That is done by trial and error, with a little experience mixed in. Your table would be a challenge, to say the least.
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