I have checked out the paulk torsion box table, and (1) it is WAY overbuilt and (2) too heavy and (3) too expensive.
Aircraft wings are constructed thus:
Spar
Ribs
Covering
The spar bears ALL the bending loads
Torsional loads are borne by the covering (or by internal bracing)
The shear web keeps the spar caps apart, providing the bending rigidity.
The wing’s rigidity is a function of the fact that the bottom and top spar caps strongly resist both compression and elongation. To provide rigidity, these two spar caps have to be kept apart. This is where the shear web comes in. Typically, the spar caps are 20mm x 20mm spruce, and the shear web is 3mm plywood. The wing skins are 1mm plywood. That’s all. With a depth of about 7 inches, such an arrangement is capable of withstanding in the region of 1,800 KILOGRAMS of deflection force.
So to build the shear webs in paulk’s box out of beefy plywood is a complete waste of weight and expense and rigidity.
Instead, I will build my torsion box out of 3mm plywood (with no cut-outs), and reinforce this with 20mm x 20mm pine.
2400 length, with 400mm space between the “ribs” will allow me to use only 3 full sheets of 3mm plywood. You should be able to pick this up with one hand, and have it be amazingly rigid.
While I appreciate and respect your understanding of aircraft engineering, wings are not meant to be used as work tables. My understanding of the Paulk box is that it is meant to be a portable WORK table.
Weight engineering can be an advantage. My mpcnc is on a pretty solid oak entertainment stand but still shakes with the rapid direction changes.
I agree that a torsion box is a great approach to building a base for the LR or MPCNC. In either machine, the base is part of the design and needs to be rigid. I built the base for my MPCNC with 12mm plywood because I had it lying around, and it isn’t so big that weight was a primary concern.
Of course, you’ll need an equally well-designed support for the platform, as well.
I’m also looking to build a lightweight torsion box… just a few thoughts and questions…
If I understand right, each webbing is three pieces… the webbing and a spar cap on either end… are these all glued/clamped? Is it a lot of work?
Where will your reinforcements be?
Also, don’t forget if you have a 7" box you will need to make it a bit narrower than you want and add runners on the side for the carriage since it can’t handle more than 4 inches. Also, it is helpful to have everything very flat maybe to a greater extent than you would need with an airplane wing (fractions of a millimeter)
I’ve seen aircraft wings ruined with a soft faced hammer.(incorrect way to deice…) You also need the beef to clamp stuff to it, take the brunt of the forces when you put your materials on it, trust me, you’re not going to delicately place a sheet of plywood on there. It’s going to get knocked around a bit. Even my table, which is a Polk knock off, moves around a bit, and it’s attached to a 2X6 table frame and legs, with an extra sheet of half inch mdf as a spoil board.
Thanks for the feedback. The basic job of the torsion box is to provide a rigid base, not a work surface. It will need a full sheet of plywood or MDF to provide this surface. My main point is that to overbuild the base is unnecessary. Additional weight adds to the inertia of the entire table, which can be a good thing, so I plan to add a full sheet of plywood and a sheet of MDF. The stand also needs to be pretty solid so I’m going to place my torsion box on the oversized wooden desk I have in the shop. That aught to do me quite nicely I think.
I’m not a mechanical engineer, so I don’t have the knowledge to be able to predict how well a design will work by numbers. I think my torsion box is 3/4" ply, 3" wide, and 16" on center, I think. Mine is full width, but only 30" or so long, not the full 9 feet. It is very rigid, and I made two extras (without the skin) because I was planning on building expansions. It used less than a full sheet of 3/4".
I agree the paulk uses more material than necessary, but weight is not a large concern for everyone. I don’t think taking the exact equation from wing building makes sense either, because weight is such a huge factor and flex is not. I am encouraged with the thought of a lower material usage design.
I would encourage you to make it your way, take some measurements, and share. Your results will be well received if you 1) have tried it and are sharing results and 2) focus on the positives instead of other design’s negatives.
The two long pieces are 7 inch pieces of ply with the 20mm x 20mm spruce (or good pine) bonded along all edges. Use something like epoxy for a really strong bond. You could screw the reinforcements to the ply, but it isn’t necessary. Maher the “ribs” in exactly the same way. Clamp with LIGHT pressure. You don’t want to squeeze all the glue/epoxy out. Plastic clamps work fine. Doesn’t take long at all.
Honestly, we’re way over thinking things. The amount of torque, pressure, stress that the MPCNC can exert on the work surface is nowhere near strong enough to warrant a torsion box of any type.
The only thing we’re combating with the torsion box is possibly the ability of the surface to move over time due to moisture or other factors.
Two pieces of MDF laminated together and thrown onto some saw horses would probably suffice for 95% of the work most of us are doing with our machines.
Aircraft wings are designed to flex. They are not a rigid structure, but a dynamic one as wing loading changes constantly throughout a flight. A CNC table needs to be rigid.
With all that being said. I did a small torsion box. I used 3/4" mdf with 2 ribs in each direction inside my 3’x3’ box. It’s over kill. It’s heavy. I had the materials, so why not.
Duncan… there is a plate that runs along the side of the table with some fixed bearings on the bottom of it to guide the pipes you see going vertically in the pictures. These bearings surround the pipe and partially go underneath the table. It’s all fine if your table is 4" or less but if it’s thicker the bearings that hang down 4" will have no spot under the table to exist since there will be table there.
Most people with thicker tables attach some kind of metal or wood railing to the side for the wheels to go on so the bottom bearings will sit out from the table a little farther. It doesn’t reduce your usable work area since the cutting bit can’t actually travel to the edge of the tables anyways.
It could also be possible to design a taller Y plate to handle a 7" table but I’m not sure what the pros an cons of that would be.
My LR2 is a bit over 4’ x 4’ work space. I built a torsion box out of 7/16 OSB. I used 4 sheets but only used 5 feet of each sheet and was left with (4) 3’ x 4’ cutoffs. I glued it together with construction adhesive. It weighs about 115 pounds and cost around $42 including the strongest adhesive I could find. It is heavy but 2 people can move it around and it stays very rigid.
My goal was not perfectly flat. However, I did want something very rigid, stable, weather resistant, and close to flat. My plan was to use the CNC to make a flat spot to put new waste boards on. However, the box is flat enough that I don’t think I will ever need to do that.
I have been recording my build and will be uploading as soon as I have some time. I plan on using a dial gauge to check how close to flat (measured from the router carriage) the table is. Once I do that I will upload the results.
I posted something yesterday which does not show up. I no doubt did something wrong.
Anyway, what I said was, an easy, cheap and effective way to make an extremely stable torsion box would be to sandwich two sheets of 3mm ply on either side of some 100mm blue foam. Heck, even 50mm foam would be fine.
Ever pick up a piece of 3mm foam with ordinary PAPER bonded on either side? The stuff is incredibly rigid. 3mm ply and 50mm foam would be amazingly rigid. And very easy to build.
Duncan, I bet that would work really well as long as you can get the wood bonded to the foam and not have any glue bumps since there is not any place for the squeeze-out to go. 3M makes an adhesive spray for gluing laminate down on counter tops that might work well if it plays nice with the foam.
I’m looking to build a table that can cut full 4x8 sheets and the largest material I can find for skins is 4x10 so with a seam on my skins not sure how I could make that work. Maybe if I did two layers of thin skins going 90 degrees to each other. Plus they only sell rigid insulation in 1", 2", 4" here. That’s a lot of layers to try and keep flat.
Check out hardboard and thin MDF for the skin too… they are more dimensionally stable than plywood.
Also you need spots on the sides to attach the belt holders so it would need wood sides.