Slides and Extenders

[julvr]

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.

Thanks,

John

[Robert Bonomi]

WAX. a good "hard" wax. e.g. canning paraffin.

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'.

*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.

[Dave Hinz]

I have got a _case_ of paraffin wax slabs. (auctions.....you know...) Is this something people use a lot of anywhere?

>

[Doug Miller]

For canning. Specifically for sealing jams and jellies.

-- Regards, 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?

[loutent]

I've used half-a-slab over a 7-8 year period for waxing my 3-inch screws...

Lou

[rcook5]

Home canners use it by the case. Which is why at the right time of the year some grocery stores stock it by the case.

==RC

Knowledge is knowing a tomato is a fruit; Wisdom is not putting it in a fruit salad

-- Suzie B

[Dave Mundt]

Actually, wood turners use a chunk of it too. It is a great thing for coating blocks of freshly cut wood, to limit the loss of water to a slow enough rate that the wood will not check or split. Regards Dave Mundt

[seelyjv]

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. Jim Seelye

[Dave Hinz]

Any turners in or near Milwaukee: I'll cut off a chunk of the stuff on request. Standing offer.

Dave Hinz