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5th January 2019, 03:12 PM #1SENIOR MEMBER
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I might be crazy..... MDF torsion box assembly table
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I kind of suspect I might be, but hey, if you don't give it a try, you'll never know.
Baulking at the price of nice straight ply at our local merchant, and the crap state of most ply at the big box, I figured, 'hey, it's $100 for 3 sheets of 18mm MDF, give it a go Bob.'
So far some conclusions even mid-stream -
1, It's blinkin heavy. I'll need a couple of mates just to turn the torsion box over to glue and screw the top face on, let alone get it off the ground onto a base, WHEW!
2, I'll need to coat the surfaces with the old 50/50 mix of poly and Shellite, that's a given.
As for whether MDF will achieve the desired objective of giving me a stable and straight work surface, well, time will tell, and it will only cost $100 and a lot of sweat to find out . Will replace the old desk and chop saw/outfeed table setup I made a few years ago as I don't use the chop saw at all.
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5th January 2019 03:12 PM # ADSGoogle Adsense Advertisement
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5th January 2019, 03:13 PM #2SENIOR MEMBER
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By the way, there is more ribbing going in there.
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5th January 2019, 08:04 PM #3
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,
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5th January 2019, 08:05 PM #4
The stiffeners will work better in a diamond pattern. That's how it's done in steel.
See Blodgett, "Design of Weldments."
Cheers,
JoeOf course truth is stranger than fiction.
Fiction has to make sense. - Mark Twain
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5th January 2019, 08:24 PM #5SENIOR MEMBER
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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
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5th January 2019, 09:35 PM #6
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6th January 2019, 12:13 AM #7
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
The person who never made a mistake never made anything
Cheers
Ray
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6th January 2019, 12:24 AM #8GOLD MEMBER
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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.
CHRIS
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6th January 2019, 07:57 AM #9SENIOR MEMBER
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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
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6th January 2019, 07:58 AM #10SENIOR MEMBER
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Joe, going to take your advice on board and put the remaining stiffeners in with a radiating pattern. i can see the logic there!
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6th January 2019, 11:35 AM #11GOLD MEMBER
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6th January 2019, 03:07 PM #12
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7th January 2019, 06:28 AM #13SENIOR MEMBER
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7th January 2019, 06:34 AM #14SENIOR MEMBER
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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.
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7th January 2019, 07:28 AM #15
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.regards from Alberta, Canada
ian
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