Thread: I might be crazy..... MDF torsion box assembly table

By the way, there is more ribbing going in there.

I like the idea but possibly to late now.
Use your MDF which I’m a big fan off,nice and flat and cheap.
But the internal ribs I would use 19/42 pine from Bunnings.
Comes as DAR off you go.

Cheers Matt,

The stiffeners will work better in a diamond pattern. That's how it's done in steel.

See Blodgett, "Design of Weldments."

Cheers,
Joe

Thanks guys, all good grist for the mill next time round. I checked the density of MDF by the way, 850 kg per cubic metre. WOW, that is heavy stuff. No wonder my 66 yo muscles are aching tonight

Originally Posted by Bendigo Bob

Thanks guys, all good grist for the mill next time round. I checked the density of MDF by the way, 850 kg per cubic metre. WOW, that is heavy stuff. No wonder my 66 yo muscles are aching tonight

They do a sheet of MDF 1200/2400 32 mm
Now that gets heavy [emoji849]

For 1000th time I HATE MDF!!! but I do understand your logic. one problem, doesn't matter what you seal/coat it with, the very first scratch /hole in it and it almost immediately expands the surface around it, after that its all downhill from my experiences of the dreaded weetbix c..p

Myself and DaveTTC made a 3.5 metre x 600mm x about 60mm deep torsion box and we could both pick it up easily it was so light. It is a sewing room bench with heaps of machines on it and is only supported at the ends and surprisingly has not sagged though I haven't measured it for a year or so. All made from 6mm skins with 3mm web from memory.

Originally Posted by rwbuild

For 1000th time I HATE MDF!!! but I do understand your logic. one problem, doesn't matter what you seal/coat it with, the very first scratch /hole in it and it almost immediately expands the surface around it, after that its all downhill from my experiences of the dreaded weetbix c..p

I understand.

As for the sealing, I sealed my Wood Knight table saw sled (also out of MDF) with a 50/50 mix of poly and Shellite. It soaks in deep and hardens the surface amazingly well. I use 2 coats, the second uses very little.

I do kind of regret using the 18mm for the skins now. Even 12mm would have eased the load a wee bit

Joe, going to take your advice on board and put the remaining stiffeners in with a radiating pattern. i can see the logic there!

Originally Posted by Bendigo Bob

I understand.

As for the sealing, I sealed my Wood Knight table saw sled (also out of MDF) with a 50/50 mix of poly and Shellite. It soaks in deep and hardens the surface amazingly well. I use 2 coats, the second uses very little.

What/Who is this Poly and Shellie you speak of, Bob ��? My Paulk workbench is about to get 19mm MDF tops, and it would be good to seal it up once I’ve made 4.7 million holes in it

Originally Posted by Chris Parks

Myself and DaveTTC made a 3.5 metre x 600mm x about 60mm deep torsion box and we could both pick it up easily it was so light. It is a sewing room bench with heaps of machines on it and is only supported at the ends and surprisingly has not sagged though I haven't measured it for a year or so. All made from 6mm skins with 3mm web from memory.

18 mm for the skins and webs is more than overkill

Originally Posted by ian

18 mm for the skins and webs is more than overkill

Indeed, as I found out the hard way

Originally Posted by Bernmc

What/Who is this Poly and Shellie you speak of, Bob ��? My Paulk workbench is about to get 19mm MDF tops, and it would be good to seal it up once I’ve made 4.7 million holes in it

If you've got some polyurethane buy a litre of Shellite from Bunnies and make a 50/50 mix with it Bern. I'd suggest just a cup of each in a new container, make it up as needed. Test it out for yourself, paint it on liberally and it soaks in, quite deeply actually, and forms a hard durable skin. I sand lightly with 400 grit and then a day or so later give another coat, which of course won't take as much mix to cover.

I initially did some testing with sanding sealer and then the poly mix. That's no good, nowhere near as durable. The poly needs to be able to soak into the MDF and the high alcohol content of the Shellite allows that. My mates and I used various implements to test the surfaces of our first test pieces and by far the simple application of two coats of this mix weas the most durable.

My sled has been in use for about 8 months now constantly, on and off the saw table and still looks like new.

Originally Posted by joe greiner

The stiffeners will work better in a diamond pattern. That's how it's done in steel.

Hi Joe
a torsion box is a slightly different beast to what you use steel stiffeners for.
with a torsion box, the strength is in the skin -- almost the sole purpose of the ribs is to keep the skins a consistent distance apart. The secondary purpose is to prevent the top sagging -- but this is achieved by the spacing of the ribs, not their thickness or layout pattern.