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Thread: Building tower speakers using pre-veneered MDF-- input and suggestions re lock miters

  1. #1

    Building tower speakers using pre-veneered MDF-- input and suggestions re lock miters

    I am working on building a set of tower speakers and decided to use pre-veneered MDF-- I am saving learning how to veneer for another day. Anyhow, I bought a lock miter bit and had success setting it up to make a scaled down sample with plain MDF, but am not happy with the results. My router is underpowered and the table and fence work but are finicky. Pulling off this joint doesn't allow for any error.

    I recently got a Grizzly 0690 table saw and the more I contemplated my next move, the more tempted I became to just make plain a old miters using the table saw and just hope I don't screw up the glue up. But I recently came across an old article showing the use of locked miters made on the table saw, which I realize is a fair amount of work but with my choices of tools, seemed like a better idea. The alternative would be to do a splined miter or something similar to assist in glue up. It will be four sides with bracing (installed using stop dadoes) so I really want to use an invisible joint (miter) that will fit together well for the glue up since I am notoriously oafish and not a fast worker when it comes to executing glue ups.

    I'd appreciate any suggestions or insight from anyone have experience making these joins using similar material on the table saw as opposed to the router.

    Thanks in advance!

  2. #2
    was it something is said?!? Or just that I made the mistake of admitting I was using MDF in the title?!?!

  3. #3
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    No shame in mdf. I am a strict pragmatist and use what materials suit the job. Anyone who criticises mdf had better stop using ply....
    For speakers and similar projects, I simply use a mitre joint with no other machining. However, I fit glue blocks inside the joints to reinforce them. The blocks aren't seen, you can make them neatly and they don't hurt acoustic performance. This is a perfect application for titebond 3 or a similar good quality glue.

    I use ratchet straps for clamping mitred boxes. They are cheap, they apply pressure evenly and they don't damage the surface. They clamp all 4 joints at once and self align with a bit of gentle persuasion with a rubber mallet if necessary. Cheers

  4. #4
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    MDF is perfect for speaker boxes due to it's high density.
    As Wayne mentioned, a simple miter joint is plenty strong but to keep the sides aligned and from sliding around during glue up it would be beneficial to insert splines in the miters. Simply cut a shallow saw kerf perpendicular to the miter faces and insert a strip of 1/8" plywood.

  5. #5
    Thanks both of you. I have one set of the corner band clamps from HF I got in anticipation of this glue up. I was thinking a splined miter would work but can you do that so it is blind? I know it's usually decorative for frames and the like but I want the veneer to all match up and joints.to be invisible....

  6. #6
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    As was stated before MDF is the correct material. It's dense and reduces spurious audio junk.

    As for joints, mitres are fine. Make sure your tablesaw work is as close to FLAWLESS as you can manage. Square cuts. Accurate bevels on 45deg. And make sure the resulting beveled boards are 100% accurately sized too.

    NB - Band clamps are a great trick. But do yourself a favour. First buy enough mitre clamps (like 8) to hold everything in place BEFORE you try putting on the band clamps. Having everything clamped in place first will eliminate the boards sliding around on their mitres while you try getting the band onto your project.

    It's less nerve-wracking that way and definitely worth it.

    Howard
    Howard Rosenberg

  7. #7
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    MDF is pretty much persona non grata in the speaker building community these days, supplemented by high quality BB plywood and occasionally more exotic materials but if MDF is your choice then MDF it is.

    When you say tower speaker I am guessing something on the 3 cu ft + range and therefore will have a ton of internal bracing or it will ring like a drum. With a lot of internal bracing simple mitered corners will me fine, though I would personally use a lapped miter joint in MDF, lock miters are a poor choice for MDF. Dadoed bracing will help a lot with cabinet resonance and make keeping everything aligned during glue up easy.

    Out of curiosity what transducers are you using and are you going active or passive with the Xover?
    Of all the laws Brandolini's may be the most universally true.

    Deep thought for the day:

    Your bandsaw weighs more when you leave the spring compressed instead of relieving the tension.

  8. #8
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    The reason for plain mitres with veneered mdf is that you have scope to get the joint aligned even if your machining is not perfect. Variations and additions to mitres make them self aligning but your machining has to be perfect.

    There is endless debate over speaker enclosure materials. Make the box rigid and toss a handful of polyester wadding inside. It really upsets the purists that your handmade boxes sound as good as their mega dollar ones. Yes, speaker building is on the CV as well. Cheers

  9. #9
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    For clamps, I would recommend a strap type clamp for the center portions of your boxes. I have some of these and they work great from boxes and odd shaped pieces.
    Lee Schierer
    USNA '71
    Go Navy!

    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  10. #10
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    well ,,, i've had a lot of luck just taping mitres together with blue masking. lay the pieces face up and tape across the joint then one long length on the joint . three times , flip over , glue and fold. glue and tape the last corner with careful alignment. corner blocks or other reinforcement later.
    wipe down the panel to get off sawdust.

  11. #11
    The way I would do this is cut the case miters on the table saw, and reinforce with biscuits. I have built many drawers and other boxes this way and never seen a failure. The biscuits are a spline hidden inside the joint and they keep everything from sliding around when gluing up. The spline is not running across (perpendicular) to the miter, it is running parallel to it, along it's length. I place the biscuits every 4-6".

    No biscuit joiner? No interest in messing with splines? No problem. There is an article in FWW from Mar/Apr 1998 by Steve Latta called Strong No Clamp-up Corner Joints. In it he cuts the miters at the table saw using a nice set-up, and then uses clear packing tape for assembly and glue-up, with no splines or other reinforcement. If you can find that article, I think it will be helpful to you. If you cannot find it send me a PM and I'll see if I can dig it up for you.

  12. #12
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    I use the tape method all the time for mitered corners on boxes, never have seen a failure. As long as you've sized the sides correctly and cut the miteres exactly, you'll see glue squeeze out when you fold the box together.

    I've been using this tape lately:

    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

    It has a bit of stretch to it, yet it's stronger than regular masking tape. Just be sure to wipe down with mineral spirits to get rid of any stickum before finishing.

  13. #13
    The more I consider everyone's comments, the more I am convinced that the lapped (rabbeted?) miter is the best compromise-- it provides a seamless miter joint with a registration for both mating sides, without the fuss and frailty of the lock miter. I was having trouble not damaging the MDF when running the pieces through a second time, for example. I'll search for some info on making those joints using the table saw.

    I am building the ZDT3.5 speakers from Zaph Audio, so the plans are very well thought out and thoroughly documented HERE. Also, I've build guitar amps and (amp) speaker cabs before, but have always used BB plywood since they were being covered by tolex. As I mentioned in my OP, I wanted these to be "wood" but did not want to get into veneering. There is plenty of bracing, so that will help. I plan to cut dadoes for the shelved bracing but wasn't sure whether it would be best to glue them all to one side, the rear panel, or different sides to help stabilize the final glue up. Thoughts?

  14. #14
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    Except for some very high class brands, most speaker manufacturers use MDF, so just go ahead. OTOH - I would very much like to see a sketch on how to make lock miters on the table saw. As for the glue-up using regular miters, line up all the sides edge-to-edge and fix the joints with tape on the outside. Then fold together like folding a cardboard box...

  15. #15
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    I know this conversation is about MDF, but I'm building some speakers for the garage, probably based on Paul Carmody's "Classix II" design. I was hoping to use 3/4" BB with lock miters (haven't used my lock miter bit in a while.) Any advice? Plan is to rabbet in the front and back and cut the holes with a jigsaw and then clean them up with a little sanding.

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