Strong corners

I am building a plyometric jumping box using 3/4 plywood with about a 7 degree taper (to avoid tip over) with a platform size of 20 x 24. There will be 5 total boards; front, rear, right, left and a center for extra strength. I am thinking about dadoing the corners and the center on the front and rear boards for the sides to slide into to gain extra strength and stability as opposed to just butting the ends together with glue and nails.
Is dadoing a better option? I know it's probably not the strongest connection compared to a dovetail or something else, but I don't have a dovetail template. Therefore, will the dado be sufficient or is something else better?
Thank you
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Perhaps "rabbeting" is also prefered over "dadoing".
Thanks

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SBH wrote:

Certainly a rabbet will help, but you might want to consider using some blocking in the corners--this is what you're going for--http://www.aircraftspruce.com/catalog/wppages/cornerblock.php
According to my CAD program, for your 7 degree corners you need an angle on the "square" side of 89.14 degrees. If you leave it at 90, over a 3/4 inch section that should put you out about .01 inch, so it's worthwhile to cut the angle--if you can hit 89.14 that's good but just going for 89 should put you close enough for an adequate glue bond (you want the bond line to be between .003 and .006 inch with most adhesives).
--
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--John
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: SBH wrote:
: > wrote: : Certainly a rabbet will help, but you might want to consider using : some blocking in the corners [...]
: According to my CAD program, for your 7 degree corners you need an : angle on the "square" side of 89.14 degrees. If you leave it at 90, : over a 3/4 inch section that should put you out about .01 inch, so : it's worthwhile to cut the angle [...]
Here's what I'd do:
The important thing is that the angle on the blocks matches the angle on the sides for a good glue joint. I wouldn't trust myself to be able to measure anything down to less than a degree, but I can handle "the same". You're going to need to cut the sides with the compound angles, so I'd attach blocks to the corners before cutting, and cut the blocks and sides at the same time. Or cut the blocks using the same setup as the sides.
--- Chip
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"SBH" wrote:

If I were going to build these, I would use epoxy, fiberglass and fairing putty made using epoxy and microballoons.
Position the pieces using butt joints that are "close" (they don't have to be exact), then apply fairing putty.
Next day, sand off rough spots, then apply a layer of glass wet out with epoxy.
Let it cure a couple of days before using.
The plywood will turn to compost before the joint fails.
Lew
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"SBH" wrote:

Another approach:
Cut two (2) identical trapazoidal shaped pieces complete with a slot in the middle the width of the plywood thickness cut halfway thru.
Slot is cut from top on one piece, from the bottom on the other piece.
Now assemble pieces at the slots to form an intersecting cross, much like an egg crate.
Fit a square piece of ply on the top and secure with glue blocks and screws.
They won't stack but are simple to build, strong, and stable.
Lew
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Hi,
I'm not quite sure what a jujping box is, but I assume that you will be jumping on it.
The force of jumping on it is going to try to push the two adjacent sides apart. A rabbet joint doesn't give you any more long wood grain surfaces in contact than does a butt joint, if you think about it.
I'd cut a dado in the top for the sides. That will keep the side panels from spreading apart, at the top. Then fasten the side to side joint with glue and screws, and at the bottom of the side to side join, on the inside, put a glue block to keep the bottom from spreading when a person jumps on it.
The 7 degree taper will make it interesting to cut dados and glue blocks that are tight fitting, but it should be possible with a few mental gymnastics. (If it were me, I have to cut two test joints first, to spoil them in figuring it out.)
Good luck.
Old Guy

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"SBH" wrote in message

Just some FWIW, off the cuff, thoughts:
If I understand your concept, and since these things are normally made of welded metal due to the forces they encounter, the "taper" appears to be the 'Achilles heel' as far as strength/resistance to the corner joints spreading under force is concerned.
IMO, if you're going to rely solely on woodworking joints to hold up under the intended use, it would behoove you to use a joint exhibiting the strongest inherent mechanical properties.
If dovetails are out of the question, my next choice would probably be a well glued "locking rabbet" joint for the four corners.
This type of joint can be cut with either a router bit set made for the purpose, or on a table saw, and will give you more strength along than a rebate joint.
I would also consider insetting/gluing the top "platform" in a dado, which would increase the strength a bit more as well and/or, if weight is not an issue, insetting a bottom, just like the top.

Due to the taper, and the force from jumping, you might want to consider interior cross bracing, to tie the tapered sides together, as well.
... I have a severe tendency to overbuild/engineer, so you might want to take that into consideration as well. :)
--
www.e-woodshop.net
Last update: 3/27/08
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I have seen these plyometric boxes made of doubled 3/4" plyood. Esentially an inch and a half plywood boards. That was strong and heavy.

attached with carriage bolts. This made for a smooth appearance and no jagged edges on the outside of the boxes.

Nothing wrong with that. Particularly if human flesh and bone hangs in the balance. I have built a lot of gym equipment over the years and my stuff is strong enough to support elephants.
True story; I was on a fitness newsgroup when somebody asked me to help him with "step racks". This is just a simple plywood and 2 X 4 rack to protect you if you fail at the bench press or squat movements. I made a crude drawing in Corel Draw, and gave him some feedback to insure he built it strong enough. He was intimidated by the project and had a carpenter friend build it for him.
He became emboldened with his new safety devices and pushed himself beyond his abilities to recover safely while doing the bench press. He lost the lift and the weight crashed into the step racks. He ended up with a minor bruise on his chest. He had a good scare and learned much from the experience. Nothing hurt but his pride though.
He had his carpenter friend check out the rack after the big crash. It got a clean bill of health and comment on he could now see why I emphasized all the extra bracing and adhesive. I built a lot of these step racks over the years and it felt good that I was able to prevent an injury in this case. Sooooo....., there ain't no such thing as overbuilding when it comes to flesh and bone.
Lee Michaels
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Thank you for all the advice and am considering all of them. I do have one more question, which the words from the advice have made me think.
The platform, as I stated, will be a 24 x 20 rectangle, of course. Obviously, the idea to taper is purely to eliminate the potential to tip over, especially at 30" in height. In general, I would believe a square box with straight 90 degree sides, would/could tip over. But since this is rectangle, I was thinking of making two sides at the tapered angle while the ends of the 24" side would be straight (90 degree), since I beleive the potential to tip over is limited, or non existence, on a 2 foot length. The main reason I think of changing is to add strength to the box, as some have stated when jumping/landing on top. Thus, the straight 90 degree sides will provide a structure with greater integrity and the front and back tapered sides will eliminate tipping over.
Any extra thoughts?
Thank you
BTW, I've decided to use boxed corner joint and a traingular blocking.
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