Torsion Box Top for my new CNC

Just getting into CNC work, so this is all new to me. Decided to build the top for my Sience Labs 30" Longmill by creating a 4' X 4' X 5 1/4" thick torsion box.

The gridwork is made from 5/8" MDF, saddle jointed together. I himmed and hawwed over whether to glue the saddle joints together or not, as I was concered about the tight joints swelling during assembly, and making it very difficult to close the 8 separate joints in each row together without having to beat them to death to close them. In the end I decided to fit them dry, but glued and screwed the rim pieces (which were also dado'd) as well as the top and bottom panels, which I felt would provide the needed rigidy.

The top and bottom pieces were 3/4" MDF, which were biscuit'd to the rims using #20 biscuits. Knowing from experience how dry MDF is, I used watered-down PVA glue to preseal the gridwork prior to glueing the top and bottom pieces on. (I used a foam roller to apply this thinned glue). Once dry, I started by glueing the biscuits into the rim pieces, then applied a healthy bead of Titebond III to the gridwork and to the biscuit slots in the underside of the bottom panel. I flipped the assembly over, and moving as quickly as I could, repeated the same assembly steps I'd followed for the bottom panel.

I clamped the entire assembly onto my extremely flat assembly table (which is also an MDF torsion box) and weighed down the centre sections using 320 lbs of 1 gallon cans of finish that I had on hand. Yes, I know I could have made clamping cauls, but the finish tins were handy, so I went with them.

Of course, the proof is in the pudding, so this morning after I removed the clamps and weights, I checked the torsion box for flatness using my 36" Veritas straightedge (accurate to within .0015" over its length). I was really happy with how flat it was, and how rigid it is.

Now, as soon as my friend has the 2" box steel supporting framework welded up, I'll get to work assembling my CNC!

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I am having a friend weld up the supporting frame of 2" box steel tube.

That's a nice stout surface for your machine!

Well done, Marty. I plan on making a torsion box top for my CNC machine as well.

I like those little corner fillers you made.

John

Are you aware that no material I know of expands and contracts more with humidistatic changes than MDF? Make sure that you surface often

Gary, I think this is just a sturdy table for the CNC machine the OP acquired, not the spoilboard.

From Wison Timbers:

When Standard MDF is exposed to changes in relative humidity, it changes in length about 0.03 - 0.06% for every 1% change in moisture content. In thickness, the panel will change by 0.3 - 0.5% for each change in moisture content. These values relate to a linear hygro expansion of 0.3% from 30% to 90% relative humidity and a thickness expansion of 6% from 30% to 90% relative humidity.

Plain sawn lumber expands/contracts roughly 0.25% in width for every 1% change in moisture content, almost nothing in length.

MDF can be a poor choice of long moldings, but it's a great choice for sheet goods where flatness and stability are needed.

John

Gary, I also thought he was going to use the torsion box for his spoil board :)
My first thought was how could anyone give up so much of their Z clearance.
Right over my head........

In my own defense I dislike MDF, don't want the stuff in my shop for any reason. The humidity here on the coast is not conducive to using MDF IMO.

That is a solid box. You should probably thoroughly seal the top and bottom surfaces or at least ensure that both surfaces are equally exposed to the atmosphere. Plastic laminate, epoxy and shellac are good moisture barriers.

For future builds it's not necessary to glue the lap joints or fasten the core grid to the outer frame. The strength comes from bonding the skins to the core.

There's no harm in sealing MDF, or plywood, or anything else, but MDF is stable in the normal indoor humidity range of 30 - 70% If your shop spends a lot of time outside of that range then maybe it's time to look at some environmental controls. The first horizontal router mortiser I built was with MDF. It's now about 10 years old. The components are still straight and true and the X/Y tables still slide smoothly on their maple runners.

MDF is THE material of choice for spoil boards. Cheap, flat, stable. Yes, good dust collection is important, but that's true with any CNC work.

John