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Thread: How much strength do fillets add?

  1. #1
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    Default How much strength do fillets add?

    I’m building some shutters out of 3/4 ply. They are fairly large (eg 50”x50”). And they will be wrapped in metal and remotely mounted. I hope to never touch them again so I need a long useful life. The outside edge of the plywood is doubled and face glued with titebond 2.

    I can go back and add fillets if that would help with strength and longevity. Or are they mostly cosmetic, or perhaps more important on a joint that we experience more stress than these?

    Any thoughts appreciated.

    Tom

  2. #2
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    Default Re: How much strength do fillets add?

    Quote Originally Posted by bluedog225 View Post
    I’m building some shutters out of 3/4 ply. They are fairly large (eg 50”x50”). And they will be wrapped in metal and remotely mounted. I hope to never touch them again so I need a long useful life. The outside edge of the plywood is doubled and face glued with titebond 2.

    I can go back and add fillets if that would help with strength and longevity. Or are they mostly cosmetic, or perhaps more important on a joint that we experience more stress than these?

    Any thoughts appreciated.

    Tom
    I can’t speak scientifically, but when a joint is glued with epoxy an additional fillet does add an additional amount of strength - I don’t know how much % wise but I’ve tested a few filleted epoxy joints against a non filleted epoxy joints and found the former quite noticeably stronger. But it also provides a curve to carry a strip of fibreglass across both pieces being joined and that provides substantially more strength.
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  3. #3
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    Default Re: How much strength do fillets add?

    I do not understand where filets would go on a flat sheet of ply.
    I do not understand what is "the outside face doubled and face glued".

  4. #4
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    Default Re: How much strength do fillets add?

    Quote Originally Posted by wizbang 13 View Post
    I do not understand where filets would go on a flat sheet of ply.
    I do not understand what is "the outside face doubled and face glued".
    The OP is hard to visualize. Photos would be helpful.
    There are a zillion different fillers for making fillets with epoxy. Some are very low strength, some very high. The answer here is not a simple yes/no.

  5. #5
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    Default Re: How much strength do fillets add?

    Where'd fillets be placed?

    Typically they do the job where additional surface area of contact gains structural integrity from the additional material. Adding 'glass on top also helps with flexibility not weakening joint over time.

    I'd think with 17-1/3 sq. ft. of 3/4" softwood ply (~ 30 lbs.) wrapped in metal, outside, your biggest issue's gonna be rot. Those shutters aren't going to be moving once installed. At 40+ lbs they need to be mounted well too if anywhere up off the ground.

  6. #6
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    Default Re: How much strength do fillets add?

    IMG_20210221_094556842.jpgl

    IMG_20210221_093514337.jpg

    The first one is about 65" x 53".

  7. #7
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    Default Re: How much strength do fillets add?

    I think putting epoxy filets on the plywood cleats would be a waste of time and material.
    I might have just made them in 2 full layers and staggered the seams,if strength is the concern.
    Blast them with wood preservative before sheathing if longevity is the concern.

  8. #8
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    Default Re: How much strength do fillets add?

    I doubt fillets will help much with those. If you're wrapping face & sides with metal, do a solid job of sealing every bit of the wood before adding the sheet metal. Maybe add some smallish drainage holes in the metal cladding along the bottom edge, near the back.

    w/o doubling that panel's 24 sq. ft., about 46 lbs. Likely 60+ once ready to hang so puh-leeze mount 'em SOLID!

  9. #9
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    Default Re: How much strength do fillets add?

    They will be heavy. I’m in a bit of a rush to get them done while the crew is around with the scaffolding. The barn door hardware is rated to about 450lbs. So I should be good even with 2 doors per track.

    On the wood preservative, any suggestions? My choices seem to be deck stain, something like Australian timber oil and paint.

  10. #10
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    Default Re: How much strength do fillets add?

    Don't know what that ATO's like but pretty much any paint'd be better than nothing. Tung oil might be good too if you can source that.

    Boiled linseed oil with melted paraffin (hard petroleum wax) added's a good preservative too unless the soaked object's gonna be in direct sunshine half the time 'cause the wax'll melt out & get all over everything underneath.

    Oil needs to be fairly warm when adding the melted paraffin, both of which are flammable to some degree so use due caution when doing the warming: no open flames anywhere around.

    Needs to be applied warm too so have your shutters done before getting to this step.

    That original formulation includes pentachlorophenol which isn't really necessary for stuff that's not directly exposed to the weather. It's not something you need to have this mixture work, besides it's toxic and may not be available or approved for use where you live & work. Also colorants which are irrelevant for your application. Mineral spirits thins it out, makes it easier to apply but is the most flammable of the three main ingredients. I'd not bother adding it for your project.

    Leave some time open too for this stuff to soak in & cure before adding your cladding. Otherwise it'll be a messy job.

  11. #11
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    Default Re: How much strength do fillets add?

    In our local Depot/Lowes, the green or brown wood presevative is kept out of the paint department.
    It is next to the fence posts.

  12. #12
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    Default Re: How much strength do fillets add?

    Mix as much Boraxo as will dissolve in hot water and slobber it all over all of the surfaces. Every time it get's damp due to condensation on the inside of the metal the borax will soak in a little deeper.

    When you mount the hardware bed it in enough paintable 40 year acrylic latex with silicone that it squeezes out everywhere to keep rain water from finding it's way in.

    Fillets work where 2 pieces of wood join edge to face. Your's are face to face. Save your time and money.

  13. #13
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    Default Re: How much strength do fillets add?

    Quote Originally Posted by wizbang 13 View Post
    In our local Depot/Lowes, the green or brown wood presevative is kept out of the paint department.
    It is next to the fence posts.
    interesting and good to know. Thanks

  14. #14
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    Default Re: How much strength do fillets add?

    I always feel much stronger after eating a good filet. A porterhouse is OK, as is a T-Bone - but they don't give me quite the boost a filet does.
    "If it ain't broke, you're not trying." - Red Green

  15. #15
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    Default Re: How much strength do fillets add?

    Fillets spread a point load, as opposed to a sharp corner,which concentrates the load.
    They prevent cracks on many different types of metal parts. Look on any crankshaft for many fillets.

    You don't have a point load on that build up.

    Will you be looking at the inside of those shutters from your windows?
    You could put art on the inside.
    Maybe do some nice shadow boxes.
    Or hire amish rob to do something REALLY cool.
    R
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  16. #16
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    Default Re: How much strength do fillets add?

    Quote Originally Posted by Ron Williamson View Post
    Fillets spread a point load, as opposed to a sharp corner,which concentrates the load.
    They prevent cracks on many different types of metal parts. Look on any crankshaft for many fillets.
    ...
    R
    This is spot-on correct.

    Fillets add material where a stress concentration occurs --- so the load is spread and the stress (force/area) is reduced. This also adds stiffness. Nautical knees are a macro example (transom, lodging, hanging etc).

  17. #17
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    Default Re: How much strength do fillets add?

    Oh they are plenty strong...in the right place.
    Many here know this...my Bingo is built with epoxy filets only...no FG tape .Absolute minimum of fastenings.
    It's all that holds the boat to the bottom.
    This is not an 8 foot sailing pram, I beat the daylights outta 'er.

    White stuff in the corners..WEST and 403. haha...the rule pump from wizbang crash is even epoxied back together

    Wizbang was the same. I crashed her into a rocky shore going fast, flipped her and tore a big gouge out, but no filets broke loose



    I drove her 15 miles home like this
    Last edited by wizbang 13; 02-22-2021 at 10:20 AM.

  18. #18
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    Default Re: How much strength do fillets add?

    I knew that, Bruce. You “ruined” my life. I don’t remember the last fillet I taped.

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