Many months ago I got lots of helpful advice on this group for building
a router table. Thanks guys.
So to add to the collection, here's what I ended up doing, what worked
for me and what didn't..


TABLE TOP
Approx 27" x 32" ?
I decided I didn't trust MDF not to sag; and I didn't think I could
laminate layers of MDF with contact adhesive and ensure they were flat
at the same time. So I:

built a shallow (~3") torsion box out of 1/2" birch ply. I knew that
despite my best efforts it wouldn't come out completely flat, but it
would be (and was) easy to tune with my #8.

used a piece of 3/4" MDF for the working surface, with Formica contact
glued both sides. Bolting it to the top of the torsion box ensured
flatness (I inset 6mm T nuts before adding the Formica).

The whole was bolted to the base from below via 8mm T nuts in the top of
the torsion box.

Insert plate - I used a phenolic Kreg one.

Insert plate height adjustment - I embedded 6mm T nuts in the underlying
ply and threaded long grubscrews into them, such that the plate corners
sit on the grubscrews.

Finished size approx 27" x 32" ?


FENCE
3/4" birch ply sub-fence, approx 7.5" x 8" (x 32" long); 3/4"
Formica-laminated MDF fences with 6mm T-nuts embedded for holding it to
the sub-fence.

The fence is in 3 parts: 2 pieces approx 4" high which can be moved
apart/together according to the size of the router cutter; above that a
full width piece with a piece of 3/4" T track in it for mounting feather
boards.

The sub-fence fixes to the table top with a clamp at each end:
adjustable, and avoided the bother of insetting T track just to clamo
the fence down.


BASE
1/2" birch ply box, with 3 compartments open to the front. The router
sits in the middle compartment.

I routed dust extraction through the base - my vacuum extraction (~2"
diameter tube) plugs in on one side, and is routed to the router and the
fence separately via a Y connector (ABS waste pipe) and pieces of vacuum
hose.

I made 2 butterfly valves in the Y connector (4mm s/steel rod +
aluminium sheet) so I can route dust extraction according to where it's
best placed. Rather elegantly, imho, those valves are controlled by a
pair of levers sticking out the side, which connect to the butterfly
spindles via a pair of 2:1 ratio plastic gear wheels (to reduce the
angle through which the levers turn to 45 degrees). OK, a touch OTT, but
fun to make, and it does work well

I was planning to extract dust from below the insert plate by extracting
from the compartment as a whole, but all that happened was a massive
build up of dust in that chamber (it works a bit like a bag-less vacuum
cleaner). So I routed a piece of vacuum cleaner hose to the router
dust-hood thing itself.

Oh, and an NVR switch.


WHAT'S GOOD
It works!
It's solid and doesn't move;
It's flat enough (variations in the glue thickness between MDF and
Formica are apparent - probably my poor application technique; I used
liquid glue, and I suspect I could have applied a spray more consistently).
The router is quieter - the ply base seems to absorb the nasty
high-pitch whine of the motor.
The fence is excellent.
Dust extraction is good enough - there's always a bit of dust on the
table that seems to escape but most is caught. It may be that my vacuum
unit (Trend T30A) might not be quite powerful enough.


WHAT I'D DO DIFFERENTLY
I sized the whole thing so that it would fit under my neander workbench
for when I'm enjoying proper tools, but it's too heavy and cumbersome to
move. Not sure what I'm going to do about this as I don't have room to
leave it out all the time. And I need my sawhorse/workmate back.

Not use the Kreg insert plate, or at least measure more, cut less - I
spent *ages* attaching the router such that the spindle comes out
"exactly" in the centre of the 30mm guide bush hole. Then I found that
that guide bush hole wasn't actually in the centre of the reducing ring,
off by ~0.5mm. Hmmm.

Use a different router - I used a Freud FT3000VCE because it was
reasonably priced and had above table height adjustment and cutter
changing. However it also had these problems specific to use in a table:
- the base wasn't flat, or square to the spindle;
- the above table height adjustment involves an access hole in the
insert plate positioned right underneath the fence - not useful. So I
don't actually have above-table height adjustment, grrr;
- the template provided on the Freud website for insert plate fitting
has the holes marked in the wrong places!
- the dust extraction attachment relies on the router's plastic sub-base
being used. As well as not being remotely flat, it also prevents the
collet nut from protruding above the table sufficiently to enable
above-table bit changing. So I made a dust take off hood out of ABS
plastic sheet (with much use of a heat gun and glue (the type plumbers
use for ABS pipework). Yawn.
- the plunge depth stop turret is sprung - I mean the thing with the
pillars on it for setting different plunge depths - it's located using a
spring and ball bearing, which works well, BUT the spring is right
underneath the point where the adjustable rod thingy presses on it. So
the plunge depth varies a mm or so according to how forcefull the router
is pushed down. OK, not an issue in the table, but it's so useless I had
to mention it!
- one of the plunge pillars has a bit of play in it.
Had I known the total set of issues I would have returned it, but by
then I'd already flattened the base (coarse silicon carbide paper on
thick glass), and drilled the insert plate. It'll do.


WHAT I'VE YET TO DO (SOMETIME, OR NEVER)
Height adjustment. I'm toying with the idea of using a stepper motor
attached to the router to drive the "above table" threaded rod
mechanism. With a nice digipot control of course

Fine tune fence - I'll probably fit the fence with a pivot around the
clamp on one end, and at the other with a piece of threaded bar.

Buy another router - I intended to use the router interchangeably in/out
of table, but it's a faff to change it around (unscrew dust hood, remove
sub-base, attach my dust hood, screw to insert plate, insert into table,
attach dust hose) so if I find myself having to swap it around a lot I
might end up leaving it there, and spending more money On a router
where the plunge depth stop turret isn't sprung!


SOURCE OF T-TRACK
Finally, T-slot is easy to get in the UK - best value I found was
Tilgear. But the T-bar that goes in it seems unavailable in the UK
except in overpriced little kits. So along with a mate I imported some
48" lengths from peachtree USA. Even with the postage and import duty it
was still cost effective. Now I can make mitre guides etc as I choose.

Phew! That was longer than I as expecting.

regards all,
g.