Hi, I'm currently in the process of designing an X table -- a folding
table whose legs form an X at the bottom. Like all my designs, it's
overly complicated, but it will have some neat features that you can't
get with other folding tables (should fold and store in less than 10
secs, adjustable height, etc).
The design is a lot like an ironing board. Unlike the ironing board
though, both legs will slide at the top -- not just one. This keeps
the 'X' centered to the table. I have three design issues with this
that I'd appreciate advise on (actually, four, but I'll post the
fourth as a seperate thread):
1) Making a sliding dovetail slide: In order for this to work, I
need to make several peices of wood slide together. I was thinking
of using my router with a dovetail bit, and make the male portion a
bit smaller (say 1/16") than the female slot. Is there any way to
treat the wood once cut so that it slides easily? Is there a
preferential type of wood to use for this?
2) Keeping things even: In order for the table to work, with both
slides have to move the same amount. That is, if one slide moves an
inch one way, the other slide has to move an inch in the opposite
direction. I can think of two ways to do this: One is to use a gear
system (similar to leigh valley's table extension hardware), the other
is to use two wires and two pulleys. The thing is, that this
mechanism is going to be weight bearing (It's not like I'll be
standing on the thing, but it does have to hold a 20"x40" peice of
plywood). Any advise on which would be better?
3) Slide Stoppers: When the slides have moved the correct amount, I
want the slides to lock into place to prevent the table from
collapsing). I want to use a spring loaded mechanism to press
against the sides of both slides to prevent them from moving. The
problem is that I don't think friction will be strong enough to hold
it in place. So what I want is some sort of locking hardware, like
say two complementing gear tracks. But, I have no clue where to get
something like that. Any ideas on that would be great too.
If anyone has any advise on any of these three, I'd love to hear it.
I will post some designs once I have built the prototype.
a good, fine-grain, hard, hardwood. e.g. Maple, Birch.
Things like the Lee Valley system work when you *don't* have longitudinal
stresses (i.e. 'along' the gear track) on the piece.
Where you *do* have that kind of loading, you need a _worm_gear_ mechanism.
See many of the light-weight automotive "sissors jacks" for an example of
how they work.
As you said, 'overly complicated'. <grin>
*IF* you don't need to be able to adjust the height of the table _while_
it is set-up, I'll suggest a simpler design: *Four* 'X' legs -- one from
the corner going towards the center of the table.. Then, you can make the
top of the 'X' at the corner of the table a *fixed* piece. simply on a
hinge. run a 'longeron' from the corner of the table towards the center.
Put some cross-wise slots in the longeron, that mate with the top of the
other X member.
You have your quick set-up/take-down, and adjustable height (although *not*
'continuous', it _is_ trivial to make sure that you can hit any specific
desired height when planning the location of the 'slots'.
Incidentally, with this design, you've *always* got a 'foot' of each leg set
directly under each outside corner of the table. This helps significantly
with the over-all stability of the table.
For canning. Specifically for sealing jams and jellies.
Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?
You don't say how big this sliding dovetail is going to be or how much load
it is going to carry - but, I think 1/16th inch clearance in the dovetail is
probably way too much to let it work properly. Before you make your
assembly, you may want to take a look at the wooden sliding dovetail
assemblies used for dining tables. They are usually two or three extension
assemblies with each piece of the slide around 1-1/2 inches thick by about 3
inches high. Most use a dowel arrangement to limit the extension of each
piece. They are capable of carrying a considerable load and still slide
easily. These fit pretty tightly - much less than 1/16th clearance -
usually a "snug" hand fit when first made. Lubrication is usually not
recommended. I have found that if one fits too tightly, it can be lightly
scraped, waxed with a paste wax (Johnson's yellow can)- then buffed dry
before re-assembly. Smaller versions of these slides can be made with
"normal" sized router bits. I recommend using a tight grain and naturally
oily wood. If you can't do that, use poplar or hard maple. Whatever you
use, the wood must be very stable so it doesn't "warp" after assembly - even
a little curve or twist will make the assembly bind.
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