Torsion Box problem

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Forum topic by MrRon posted 01-05-2012 06:46 PM 3044 views 0 times favorited 31 replies Add to Favorites Watch
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3888 posts in 2660 days

01-05-2012 06:46 PM

I made a torsion box, 36” x 84” from 3/4” MDF. The total depth is 4”. I made the mistake of not having a level surface to begin with and as a result, it has a dip in the middle of about 1/16”. I tried to jack it up in the center while pulling down on the ends, but it won’t budge. I need this box to be perfectly flat as it’s the base for the CNC router I’m building. The solution I have in mind is to trowl a leveling compound over the top and put down a new piece of MDF, making the total thickness 4-3/4”. Will this work or should I just scrap it and start anew. I wish my leg was a little bit longer so I could kick myself in the you know where.

31 replies so far

View TheDane's profile


4930 posts in 3080 days

#1 posted 01-05-2012 06:55 PM

MrRon—I would try the leveling compound idea with another layer of MDF first. That will probably fix it for you without having to start over. If it doesn’t work, you really haven’t lost that much.


-- Gerry -- "I don't plan to ever really grow up ... I'm just going to learn how to act in public!"

View DrDirt's profile


4133 posts in 3159 days

#2 posted 01-05-2012 06:58 PM

Give it a try – the “do over” is always an option later on

Upside is you know how rigid the torsion box is and how it will resist flexing when you WANT it to stay flat.

-- 'Political correctness is fascism pretending to be manners' ~George Carlin

View dannelson's profile


181 posts in 1788 days

#3 posted 01-05-2012 07:17 PM

add on to the box and surface it with your machine using a fly cutter, be sure to bury the screws , you will probably be using a spoil boad that has to be re surfaced anyway.

-- nelson woodcrafters

View SPalm's profile


5249 posts in 3299 days

#4 posted 01-05-2012 07:55 PM

Like others mentioned, try it, or just do over.
And wow, 3 feet by 7 feet! You don’t mess around.
If you do make it over, you can save some money by making all the ribs out of 1/2 inch.
I just used the little small pieces, glue and a brad nailer techinque. It is a faster build IMHO.


-- -- I'm no rocket surgeon

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3888 posts in 2660 days

#5 posted 01-06-2012 08:37 PM

SPalm: I used 3/4” for the top and bottom sheets. The leftovers made up the interior structure. Two sheets were used.

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3888 posts in 2660 days

#6 posted 01-06-2012 09:33 PM

I made this sketch to show how I’m going to attempt to salvage the torsion box. The 1/4” wood strip will be glued all around the edge. I will plane the strip until it is flat. The thickness will vary from 1/4” at the low point to around 3/16” at the high point. I will then trowl a floor leveling compound to the top and strike it with a straight edged board. After it is set, I will attach the new MDF top using a tile adhesive and drywall screws. What do you think? The sketch is not to scale.

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19135 posts in 2092 days

#7 posted 01-06-2012 10:01 PM

I agree with dannelson’s suggestion!
The CNC is designed for the accuracy that you are looking to achieve anyway. Let the machine help build itself!

I would think the leveling compound would/could break up & desinegrate into a powder over time?

-- Randy-- I may not be good...but I am slow! If good things come to those who wait.... Why is procrastination a bad thing?

View IrreverentJack's profile


724 posts in 2260 days

#8 posted 01-06-2012 10:02 PM

I like dannelson’s suggestion too. -Jack

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3888 posts in 2660 days

#9 posted 01-06-2012 10:14 PM

Dannelson: The CNC router is not built yet. I’m trying to get the base flat first. The rails for the X-axis are mounted on the top, so if the top has a sag, the rails will follow the sag and so will the Y-axis gantry.

View Sylvain's profile


637 posts in 1916 days

#10 posted 01-06-2012 11:15 PM


I would think that what you need for the base is something dimensionally stable (the torsion box), whatever its shape….

Then you put the rails for the X axis on it.
Why not “simply” shimming the rails every “so many inches” to have them straight and parallel ? (not converging and in a same plane)

If you were building a railroad track (even for a high speed train like the French TGV) you would add ballast where it is needed.

Of course it would have been (or is it ‘have seemed’ ?) easier with a “perfect” torsion box (if there is such a thing as a perfect torsion box)

Look at this web site to see the trick with the strings to verify two rails are parallel :

How to Flatten a Workbench with a Router :

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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637 posts in 1916 days

#11 posted 01-06-2012 11:26 PM

Just an afterthought,

you fix two wooden rails to your torsion box as it is (fitting the sagging on the underside)
and then you use the “flatten a Workbench with a router” method to have the top of your two rails perfectly straight and parallel.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

View DS's profile


2145 posts in 1837 days

#12 posted 01-06-2012 11:35 PM

You might try getting it flat using a wide-belt sander. You’d have to look around to see who might allow you to use it.

My local Woodworkers’ Source has a 52”w two-head machine by the hour.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View gfolley's profile


14 posts in 2774 days

#13 posted 01-07-2012 12:10 AM

I would try flipping the box over so now your dip becomes a high spot. Then you could take the high spot down so everything is flat. You could use a bet sander or router on rails to get it flat then you could glue your second sheet of MDF over that if needed.

-- When tempted to fight fire with fire, remember that the Fire Department usually uses water. Gfolley, Ohio

View cabmaker's profile


1470 posts in 2226 days

#14 posted 01-07-2012 12:20 AM

Were talking about mdf here right ? Two sheets ? one hour labor if you take a break ? Just build another.Good luck JB

View Sylvain's profile


637 posts in 1916 days

#15 posted 01-07-2012 01:02 AM

Correct me if i am wrong.
If I understand correctly, as it is not to be an assembly table but the base of a CNC machine, your base does not need to be flat, only the top of your X rails need to be in a plane and parallel; then your Y rails need to guide your router perpendicular to the X rails
(and the Z rails perpendicular simultaneously to the X and Y rails).

If you look to a metal lathe with a cast iron base only the rail is machined.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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