Biscuits in MDF?

Discussion in 'Home Theater Projects' started by Nick Hallett, Apr 22, 2004.

  1. Nick Hallett

    Nick Hallett Agent

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    For those of you who are more accomplished woodworkers than I, how well does MDF hold biscuit joints? Does the fact that there is no grain structure in MDF pose a problem with strength?

    Nick Hallett
     
  2. Allen Ross

    Allen Ross Supporting Actor

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    I have used biscuits on my tempest and my 4X10 cab and have had no problem with them. As for the strength, i am not sure, you have to realize that the cool thing with biscuits is the fact that they expand with the moisture from the glue so it tightens the joint and adds to the rigidity of the joint more then then glue it self.

    Personally when I created my enclosures I had plenty of time to work on it and I said what the hell and tossed them in
     
  3. Nick Hallett

    Nick Hallett Agent

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    I'm not too worried about the strength, just wondering in case it was one of those "absolutely not" scenarios.
     
  4. MikeWh

    MikeWh Second Unit

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    Biscuits are ideal for MDF, IMHO. I use biscuits in all my MDF projects. Typically, I use size "0" or "10", depending on the thickness of the MDF and the stress on the joint. The fact that there is no grain structure is actually why biscuits are preferred--- fasteners, like screws, won't hold nearly as well.

    The biggest problem with strength is prior to glue up. During a dry fitting, you must take special care keep any butt joints from racking. Pieces of the biscuited end will easily break out, if the biscuit is in place (the biscuit will stay in place and cause the edge of the MDF to break out). This is especially true of thinner MDF, like 1/2".

    After glue up, you are just as likely to crack the biscuit as you are to break out the MDF. The edges of MDF absorb glue very well, and the joints are extra strong with the biscuits in place. I am sometimes amazed at how easily I can mistreat a well-glued/well-joined MDF cabinet, without it simply falling apart on me.
     
  5. Doug_M_Fraser

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    Biscuits work great in MDF. I usually use #20 in 3/4 in. When I laminate 2 pieces of 3/4 together to make a 1.5" baffle, I use use 2 rows of #20 biscuits to joint the walls to the baffle. This is overkill but my motto is, "If a little is good then alot is better" [​IMG]

    An alternative to clamping the biscuited pieces is to screw them together. The screws simple provide the pressure until the glue and biscuits dry (make sure you don't try to scew through a biscuit). I use dry wall screws into predrilled holes (to avoid splitting the MDF). When dry, I use some spackel (or other filler, like autobody bondo, or spot filler) to fill the recessed screw holes, sand smooth and bobs your uncle.

    Doug
     
  6. Bob K

    Bob K Stunt Coordinator

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    Nick,

    I join Mike and Doug: biscuit are GREAT for MDF. They strengthen the joint (although that's unnecessary, since yellow glue applied to a butt join alone is stronger than the MDF). More important, they make alignment perfect every time.

    Like Doug, I use No. 20s on 3/4" MDF, although it sounds like I use fewer than he does -- just enough to give myself some alignment points. I don't use screws, just clamps -- who needs to fill those holes?

    You don't have to be an "accomplished woodworker" to use a biscuit joiner. A biscuit joiner is probably THE easiest tool to use. I'll be shot for this by the Woodworkers' Union, but with biscuits I don't even dry fit any more, because they're so foolproof.

    Tempest and Pi Construction Pix: http://www.audiocircle.com/circles/m...view_album.php
     
  7. Pete Mazz

    Pete Mazz Supporting Actor

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    Too much glue in the bisquit slot and it could wind up splitting the MDF. You may also want to allow some extra dry time before final flat sanding to insure you don't wind up with "bisquit sized" depressions from the bisquit/MDF swelling and causing a bulge. If flat sanding is done too early, you may wind up with a depression when it finally returns to a stable moisture content.

    Pete
     
  8. Allen Ross

    Allen Ross Supporting Actor

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    i have loaded biscuit joints with tons of glue and never had one split the MDF
     
  9. Bob K

    Bob K Stunt Coordinator

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  10. Nick Hallett

    Nick Hallett Agent

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    cool, I got a brand new (OEM) Makita biscuit joiner off ebay for 8 bucks, so I was curious about using it for my speakerbuilding too. hopefully it will work fine while I'm still practicing mortise and tenon joints
    [​IMG]

    nick hallett
     

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