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
Certainly a rabbet will help, but you might want to consider using
some blocking in the corners--this is what you're going
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
: 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.
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
Let it cure a couple of days before using.
The plywood will turn to compost before the joint fails.
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
They won't stack but are simple to build, strong, and stable.
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.)
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
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. :)
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.
I have seen the wood boxes made with internal braces of steel. They were
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.
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?
BTW, I've decided to use boxed corner joint and a traingular blocking.
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