Dead flat light work table?

Hi,
Real man's DIY question here...
I don't have a proper workshop (yet), but I need to start making stuff for the house. I have a shed that can take an 8x4' sheet on its side for storage, just to give an idea of what I do have.
I don't have any level surfaces either outside. I do have a block paved drive to work off, but its curved, unlevel but solid. So I'm thinking of making a worktable top that can be sat on adjustable feet trestles to work off.
It needs to be:
1) dead flat for gluing up;
2) water resistant (ie will not warp if it gets rained on);
3) not insanely heavy for carrying back to the shed.
3 rules out glueing up loads of 2x4" which is the traditional way to make a monster solid bench.
2 rules out any use of MDF.
So I was wondering if anyone had any ideas? 2x4" frame with 3/4" ply glued/screwed to the top maybe?
And how can I test that a surface is fully in one plane (ie dead flat)? Measure it with a level in every direction (assuming we build on the target tresles and that these can been set up first to be level). Or is there another way :)
I'd also like the option to make this a modular saw bench and router bench with a removable ply panel say around 1.5x2' which can be replaced by identical panels that are customised to hold a circular saw and router upside down. I guess this is just a matter of adding some extra 2x4" framing to form a window in the surface that the panel can site on and be made solid with bolts into nutserts.
I would want to then start looking to evolve this with guide rails and/or T-track to make it more versatile - but I'll probably start with a plunge saw track of some sort.
All random thoughts welcome :)
Cheers,
Tim
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On 17/05/17 11:12, Tim Watts wrote:

Resolving this to its most basic properties you want 1/. light 2/. water resistant 3/. stiff
That leads me directly to polystyrene or polyisocyanurate foam.
4"-6" thick. Used in floor and other insulation.
What you skin it with is another matter.
Polyiso is much ,more resistant to (celluelose) solvent attack, but is much more expensive.
I've used smaller boards very successfully for building model aircraft on. it stays flat really well
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On 17/05/17 11:45, The Natural Philosopher wrote:

Interesting idea - thanks :)
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On Wed, 17 May 2017 11:45:29 +0100, The Natural Philosopher wrote:

Polyiso is being used in new houses nearby. The sheets are laid on the base and the men are walking all over them and using heavily laden barrows. I don't know if the sheets are those 'structural' ones, but some were in over the void a couple of hours after I first went past and there couldn't have been much time to put in any over(under?)all support.
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On 17/05/17 15:20, PeterC wrote:

here they laid 6" thick polystrene blocks between concrete beams and then covered that with another 6" of poly that they tagged the UFH pipes to.
Then they chucked about 4" of concrete over the lot.
Its very strong is polystyrene of reasonable density.
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On Thu, 18 May 2017 02:13:41 +0100, The Natural Philosopher wrote:

Especially with a well-spread, 'flat', load and in compression. ISTR it has been used under roads across soft ground. What happens if petrol or diesel - or many other solvents - get in could be interesting!
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On 18/05/2017 02:13, The Natural Philosopher wrote:

Its not the strength its the low load factor that a mere 4" of concrete has. You could easily push a piece of wood into the expanded polystyrene or just the pegs they use to secure UFH pipe. I assume they chucked in some weld mesh somewhere to stop the concrete cracking or maybe some chopped glass?
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On Wednesday, 17 May 2017 11:30:06 UTC+1, Tim Watts wrote:

Measure it with a straight stick in every direction, don't need a level. There is a very simple solution: a small pallet. It'll need sanding to get it dead level. Having used one with an added vice as a small workbench I much preferred it to a gapless sheet worktop.
NT
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On 17/05/2017 11:12, Tim Watts wrote:

Cement board is pretty weather-proof. Ideal if you are going to build a shower later and use it for your work table in the meantime.
Define what you mean by dead flat, btw.
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On 17/05/17 11:48, GB wrote:

+/-1mm across the whole surface ideally, +/-2mm at worst.
Any non flatness will transfer to anything made on it.
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On 17/05/2017 11:12, Tim Watts wrote: 8<

A flush door? They are light and stiff and cheap.
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On 5/17/2017 2:00 PM, dennis@home wrote:

But not particularly strong. I was going to suggest a length of kitchen worktop (which can always be left outside if you get caught out by a shower). But the door gives you more width and won't sag much even if supported near the ends.
Depends a bit on what you might be doing. Door would be OK for a lot of carpentry, not so good if you were stripping an engine.
Rather than adjustable trestles, another option is a couple of cheap workmates. Then clamp suitable offcuts of 18 mm ply or whatever in them to provide two straight horizontal edges at the same height on which you can rest your worktop.
My FIL had a small slab path with a couple of dwarf walls at right angles, he made a clever adaptor resting on them which supported his table saw firmly, level, and at the correct height.
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On 17/05/17 14:36, newshound wrote:

Also a great idea. Why did I not think of that...

This is carpentry - I've been inspired by a series of american woodworkers - most have massive workshops, but one guy specialises in using home made benches that have running tracks flushed in to act as a table saw, router table and stuff. If I had a workshop, I'd do the 4x2" laminated to make a 4" thick top (after truing with a planer) - but for now I need portable.
I might even try to make it up as 2 4x4' or 4x3' tops that can be dowelled together when needed.

With my ground, even 4 feet won't sit right...

:)
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On 5/17/2017 4:38 PM, Tim Watts wrote:

If you spin the workmate about the vertical axis as well as moving it from side to side and back and forward (if you see what I mean) you can usually find a secure point, if the surface is not too bad. I used to have to do this on my old and very irregular patio.
If it is really bad, you could modify a cheap workmate by removing the bottom of two legs, and adding a third foot in between them (and still have a system which would fold for storage). On one of mine, each pair of legs is fitted to a cross beam about 4 inches above the ground. You could screw a triangle of 18 mm ply to this, using the "point" of the triangle as the ground contact point. Rounded off a bit, of course. Or a bit of chequer plate if you have some.
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On 17/05/17 14:00, dennis@home wrote:

That's actually a rather good idea... I could still face it with ply to get a machinable surface to take track. But you're right - very stiff and very likely dead flat.
Cheers!
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On 17/05/2017 11:12, Tim Watts wrote:

Yup, torsion box design - 1/2" WBP ply would probably be ok. You can make the frame with ply braces as well if you want.

You can use a long straight edge to test for flatness in two axis, then make a set of winding stick you check that the surface really is all in one plane.
See
https://paulsellers.com/2014/05/making-winding-sticks-by-hand/

Yup, sounds doable... or even a side mount jig that hooks onto an edge.

There are also ready made options to do this kind of stuff:
These with a couple of 4x2s will give you a decent surface - but not always coplaner: http://www.axminster.co.uk/axminster-trade-saw-horses-pair-103349
Ready made torsion box:
http://www.axminster.co.uk/ujk-technology-multifunction-workbench-for-parf-dogs-717978
--
Cheers,

John.
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On 17/05/17 15:41, John Rumm wrote:

Thanks :)

Thanks John. I have never heard of these.

I did have something like that in mind for the legs - really like the look of those...

I wonder if that is water resistant - but it gives me some ideas...

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On 17/05/2017 16:46, Tim Watts wrote:

Paul has some videos on you tube on their use....
The basic idea is that they allow you to sight one over the top of the other and easily see any twist in what they are sat on.
You can also do the same trick with a couple of parallel battens on the surface and running tight fine thread corner to corner - the threads should just touch but not bend around each other.

I suppose you could make up an ali framed box, and just bolt your ply surface down to it. The wood could be sacrificial anyway - just replace it when it gets too knackered.
--
Cheers,

John.
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On 17/05/2017 18:25, John Rumm wrote:

I just put a couple of spirit level across. I can even put the digital angle gauge on the levels and get an accurate measure if needed.
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On 17/05/17 15:41, John Rumm wrote:

Hi John,
I'm going to be bankrupt and it's your fault! ;-)
I've been drooling over their CMS modular system that goes with the Festool version of the above:
http://www.axminster.co.uk/festool-mft-3-multifunctional-table-basic-508565
You could blow 3 grand by the time you've got the router and saw!
I'm looking at that for ideas (or to do a very slow acquisition of that, like a module a year).
It has some good points - folds down, top is HDF Valchromat which is claimed to be "moisture resistant" by the portugese manufacturer (I assume it is very heavily loaded with resin compared to MDF).
You could do that table easily enough - but the modules? Perhaps not so well. As soon as you start screwing saws upside down to bits of board, you lose the ease of adjustment - and in the case of the saw, you tend to end up with a very wide slot to allow tilting.
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