Need Input on My "Torsion Box" Idea

My 1000mm xcarve is on its way and I want to build a good table to mount it to. I have been brainstorming and figured I would reach out to the community and get some input on my ideas. I have looked into torsion boxes but I dont have the tools to make a bunch of accurate cuts for the internal honeycomb structure. So, I would like to limit my cutting to what the guys at Lowes will do.

Heres what I am thinking:

I will buy 2 8’x4’ sheets of .75" MDF and have them cut one in half making 4’x4’ squares for the top and bottom of my table. From the other 8’x4’ sheet I will have them cut a piece to be used as my wasteboard. With the remaining piece I will have them cut 4 thin 47.25"x1"x 0.75" pieces that will run along the inside perimeter of the table top/bottom forming a thin box.

The construction will take place on top of my thick glass kitchen table which I will assume is flat. I will start building my table by constructing a 5 sided box (48"x48"x1.75") and leaving the top off. Next I’ll coat the inside of the box with a thin coat of silicone sealant and then fill it completely with GFRC cement. At this point I will have already assembled the xcarve on top of the remaining MDF table top. I’ll mark the spots where the frame needs to be bolted down and drill holes through the MDF, glue some bolts into the holes so that they are coming through the board and out of the top, and apply silicone sealant to the entire under surface on the bolt head side. Then i’ll put the top on the box, screw it down, and let the concrete cure.

It seems to me like an easy way to make a solid table but Im worried that the water from the concrete could swell and warp the MDF. I’m also worried that a thin 1" layer of concrete will be cracked easily if the table is stressed. Unfortunately 1" is the maximum thickness of the concrete layer because I dont want my table top to exceed 200lbs (which at 150lbs/ft^3 a 1"x48"x48" concrete slab will be almost exactly 200lbs). Although it should weigh less than that since I’m using GFRC cement but I really want it as light as possible.

Would really appreciate anyones input!


Cement Im thinking about using:

Personally I think your idea is very creative but overkill. Maybe see if someone in this forum lives near you and will help you out to cut strips for inside the box. Or give the guy at lowes a tip to cut all the pieces you need to make a torsion box.

If you do go the concert route don’t use mdf. Use melamine. they sell it in sheets and planks. I use it for concrete molds all the time and it holds up very well. You could use it over and over if you want. Message me if your gonna do this ill send you some more tips.

That’s an interesting idea. I can’t say how well that will work, but I do have a couple of thoughts:

Don’t forget the thickness of the saw kerf. You’ll probably want to cut 4’ sections from each sheet of MDF.

I do like Joe’s suggestion to use melamine instead of sealing the MDF. Then you just have to caulk the seams.

Regarding cracking, consider some reinforcement in the concrete. I would try cutting down a steel stock panel and positioning it with spacers before the pour.

You may want something to vibrate the concrete to ensure it settles properly, especially if you put in reinforcement.

Since we are brain storming:

Concrete harden over a very long time (months) and is not dimensionally stable during this time.
Epoxy “granite” (Laminating epoxy and sand/aggregate) is much better in that regard, but pricier.
Weight is a good thing for the CNC, bad thing for the guy mowing it :wink:

A torsion box can work very well with simpler materials, I built my own TB out of 3x1-1/2" stringers and 3/4" particle board intended for wet room sub flooring (water resistant). Inside the TB I only have 4 “cells”.

The evenness of the TB is not critical as you should skim cut the top surface board once the Xcarve is installed. I would suggest a 3rd board on top to act as a sacrificial board.

good morning my name is johan I was reading all your project and I only recommend using structural aluminum bars and you do not have to cut anything just screws and the project is finished.

Typically MDF sheets are 49x97


Luke originally posted

have them cut one in half making 4’x4’ squares

The point is don’t assume cutting a sheet advertised as 8’x4’ in half will equal two 4’x4’ pieces. Instructing the hardware store employee to “cut it in half” or “cut it at 4 feet” won’t get the desired result. Three (if you can get them to stack the 48"x49" pieces) or four cuts would be required on a hardware store panel saw to make a 49"x97" sheet into two 4’x4’ squares.

Thank you all for your replies! I Was really busy last week and forgot to respond.
After reading your responses and thinking a bit, I realized my table concept was overkill, would be a pain to move around and build, and could potentially waste a lot of money if it didn’t work. So, I went with a basic design that required no cutting outside of Lowe’s.
I also really appreciate you all for making sure I account for kerf.
Unfortunately I had a couple of issues in my build…

  1. the Lowe’s saw was off by 1/4” between a 48” top to bottom cut!! I know it’s a “rough cut” but dang.

  2. I accidentally bought 2.5” wide boards for the inside of my box instead of 3”. Dumb… just dumb.

Wasn’t too worried since the table doesn’t have to be perfect… but my wasteboard is a 1/4 longer on one side than other. One side fits perfectly inside the rails and the other is loose by 1/4”. Will this cause issues or be fine once I mount it?

I put it all together because I just couldn’t wait to get carving… basically the top of my table has beams running through it that are 2.5” tall and the MDF sides of the table are 3”, so the bottom of the table does not touch them. Disappointing mistake but I already had lowes cut the wood so I went with it and it feels pretty sturdy.

I’ll post some pics in a few minutes.

Thanks again!

Luke, One reason torsion boxes are made using engineered plywood like MDF, is that it is more stable across a range of temps and humidity levels vs dimensional lumber. Those 2x’s you used could twist, warp and/or bend causing the entire box to twist and or warp.

Accurate cuts CAN be made with a circular saw. You lay one sheet of MDF on top of the other and use the factory edge as a straight edge to guide the saw.

Then use plenty of glue and screws to put the thing together.

Also, the guy at the store does not get paid enough to care how accurate his cuts are. Add in the fact that that panel saw probably isn’t tuned to be square by any stretch of the imagination.

I know it sounds ridiculous, but if it were me, I would spring for more material and make the table right the first time. It will equate to better end results. Trust me.

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Your idea of using concrete as part of the foundation to make the frame more rigid is common practice. Large industrial machines will either use massive slabs of steel or they use massive slabs of black granite that is perfectly flat. One of the keys to accuracy is getting the frame rigid simply using gussets, screws and nuts does not always to the trick. These machines create a lot of vibration which in turn creates accuracy issues. Want to make the machine more accurate then start with increasing the weight of the base and work your way towards anything else that deals with motion and the frame
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