Another Shop Cabinet (Torsion Box ?)

[Dave Richards]

http://www.woodshopphotos.com/albums/Dave-R1s-Album/millcab1.jpg
I'm planning to make a rolling cabinet for my ornamental mill. Above is an early sketch. I have a couple of questions regarding the construction of torsion boxes.

As I understand them, the skin needs to be firmly attached to the entire framework. If the skins are to be plywood, should the framework be something more inert like MDF? I'm planning to use carefully dried 2x lumber milled down to 1-1/4" x 3 or 5 for the cabinet frames. would this be suitable for the torsion box frames?

There will be a torsion box for the base with a piece of 3/4 MDF for the top skin (I think). Or would it be alright to use 3/8" plywood for that skin? Can the skins be of differing thicknesses?

The panel in between the cabinets will also be a torsion box to add some stiffness. For the heck of it, I decided to add the arched trim to it along with the arching the bottom of the front spreader. Any opinions regarding the way I intend to do that trim?

Finally, going back to the base, I'd like to put an arch in the front rail. I am considering a couple of options. First, arch the front rail and stop the bottom skin before at the next rail behind it. Second, cut an arch in all the longitudinal rails and bend the plywood skin to match. Thirdjust add a decorative piece with the arch to the front rail and paint the front rail black so it is less noticeable. Again, any opinions?

[Michael Stafford]

The one and only torsion box that I made was made from plans that called for strips of 3/4" plywood to be used as the internal framework to which the plywood skins were attached. The plywood was ripped to 3 1/2" and half-lapped egg crate fashion. Then I used 1/4" plywood for the external skins. Then I wrapped the entire thing in 3/4" poplar around the exterior and ripped those pieces 1/2 wider than necessary so that I could use 1/4" hardboard skins as my actual work surface. These were tacked in place so that they could be replaced as necessary. Oh, by the way glue on the plywood skins to your internal egg crate frame- much stronger.

[Chris Padilla]

Mike pretty much has the construction down, Dave.

I made a torsion box all out of 3/4" MDF very similarly to how Mike made his:

3/4" MDF half-lapped grid (Mike called it egg crate)
3/4" MDF wrapped the grid
3/4" MDF skins on both the top/bottom of the wrapped grid (all glued/screwed)

My torsion box is heavy...heavy, heavy, heavy. You can make much lighter ones that are still incredibly strong with smaller dimensioned MDF or plywood. Try all 1/2" plywood or MDF (maybe the lower density MDF if you can find it).

What was nice about the MDF is it is dead on for its thickness dimension. It is also cheaper than plywood but quite heavy (did I mention mine was heavy?). I was able to dial-in the half-laps so close that I didn't even need to put glue in the half-laps. I knocked the half-laps together with a rubber mallet and I couldn't get them apart again so I said forget the glue!

Keep in mind that a torsion box will only be as flat as the surface you build it upon. I made this workbench and the bottom is a torsion box. I built the boxes/cabinets first being careful to get them dead-on so I could build the torsion box on top of them. Glue up is interesting for a torsion box: pile everything you can find on top of it...concrete cinder blocks make great 'clamps'.

If you want to attach wheels to the torsion box (assuming it is on the bottom of the bench/cabinet), be sure to fill in the cavities of the grid that a lag bolt can bite into. I just glued in a bunch of scrap plywood in various spots that I expected to attach a caster to.

My whole bench is MDF...mistake. MDF for a torsion box, I think, is a great idea as it is cheap and heavy and dead flat. MDF for the rest of the bench is a bad idea as it is too heavy and not structurally sound. From all the clamping I've done, most of my bench edges are out of square. One of these days I will redo the whole bench--from the torsion box up.

[John Miliunas]

My torsion box is heavy...heavy, heavy, heavy. You can make much lighter ones that are still incredibly strong with smaller dimensioned MDF or plywood. Try all 1/2" plywood or MDF (maybe the lower density MDF if you can find it).



I don't know, Chris. I'm thinking you like "heavy". Just look at those towel racks on the right! :D :eek: :cool:

[Jamie Buxton]

As I understand them, the skin needs to be firmly attached to the entire framework. If the skins are to be plywood, should the framework be something more inert like MDF? I'm planning to use carefully dried 2x lumber milled down to 1-1/4" x 3 or 5 for the cabinet frames. would this be suitable for the torsion box frames?

You can use lumber this bulky for the spacer grid inside the torsion box, but you don't have to. The real strength of the torsion box is in the skins, not the grid. You could make the grid with 1/4" ply or even 1/8". I make the grid by running the strips the full width and length of the box, and cutting slots half-way through the strips at each point they cross. I tape the strips together and gang-cut the slots on the tablesaw. If you use 1/8 ply, one pass with a full-width blade cuts a slot of the required width.

There will be a torsion box for the base with a piece of 3/4 MDF for the top skin (I think). Or would it be alright to use 3/8" plywood for that skin? Can the skins be of differing thicknesses?

The skins can be different thickness. I prefer plywood over MDF because it doesn't stretch as much under load.

Finally, going back to the base, I'd like to put an arch in the front rail. I am considering a couple of options. First, arch the front rail and stop the bottom skin before at the next rail behind it. Second, cut an arch in all the longitudinal rails and bend the plywood skin to match. Thirdjust add a decorative piece with the arch to the front rail and paint the front rail black so it is less noticeable. Again, any opinions?

[COLOR=Blue] You can make an arched torsion box. However, it is a lot more difficult than a flat one. On a flat one, you can be pretty sloppy about the postioning of the elements in the spacer grid; the only constraint is that the thickness of the gird is constant. On a curved one, the elements in the grid need uniform, and fit on the curve exactly. Furthermore, gluing the second skin on is very challenging. Before you glue the second skin on, the box can be twisted fairly easily. After you glue the second skin on, there's no way to remove twist. On a flat torsion box, there are ways to observe and remove twist before you glue the second skin on. (Crossed strings is my favorite.) On a curved torsion box, it is more difficult.
[COLOR=Black]

[Jamie Buxton]

Well, I don't know why the coloring in my previous post didn't work. Oh, well.

The crossed-string trick I mentioned is quite useful. You run one string from one corner of the partially-assembled box to the opposite corner. You run a second string between the other two corners. They cross in the middle of the box. I put shims at the corners which are the string thickness under the upper string. If the box has no twist, the strings will just kiss. For precision work, I use monofilament line, and use lead fishing weights to tension both lines the same. If you have a sense of pitch, you can pluck the strings and listen to the tone; if they have the same pitch, they have the same tension.

If you do it carefully, you can easily observe the twisting effect of inserting one business card under one corner of the box.

[Dave Richards]

Thanks guys. That stuff was a big help. Here's where I'm at now. I ended up drawing the mill (although I haven't finished it yet) so I could see the relationship of the cabinet to the mill. I still need to draw the DW625, too.
http://www.woodshopphotos.com/albums/Dave-R1s-Album/legacycabinet.jpg

And here it is in the shop.
http://www.woodshopphotos.com/albums/Dave-R1s-Album/shop1.jpg

Behind it is the cabinet built from shop scraps. I still need to build the upper part of that. Above the cabinet is the Jet air filtration unit. The brown box on the wall is the cabinets I took down from the kitchen and to the left is the new mount for the WoodRat which I haven't drawn yet.

Sooner or later I'll manage to get the rest of the shop drawn to scale and I can play with moving the tools around without hurting my back.

[Bart Leetch]

Here is something I found during my search. A article by David Marks on building a Torsion box.


http://www.diynet.com/diy/ww_materials_products/article/0,2049,DIY_14442_2278181,00.html

[Chris Padilla]

Hmm, I see David doesn't bother with half-lap joints for the grid.

[Jamie Buxton]

You're right, he doesn't do the half-lap clearance thing. As a result, he's got lots more pieces to handle, and worse, he's nailing each and every blessed joint. Seems a lot of extra work to me. It also creates an opportunity for a screw-up. If he lets one of those short cross pieces get up off his reference surface just before he puts a nail in it, he's got a problem. The top skin won't be flat -- which is exactly why he's bothering to make a torsion box. Me, if I were hand-holding and nailing that many pieces, some would wind up where they shouldn't be.

[Greg Hairston]

Dave,
What software are you using for the drawings. I would like to know what others are using also

Greg

[Dave Richards]

Greg, it's called SketchUp. Here's a link to the website: http://sketchup.com/


According to Todd Burch's poll there are six of us here on the Creek who use it. It is very intuitive to use. Simpler than any of the half dozen or so CAD programs I've tried and will allow you to do both working drawings as well as studies and drawings to show customers if that's your deal. It was intended for doing architectural stuff but it is well-suited for furniture and cabinetry work as well.

You can download a demo of it. the demo is the full version with a clock attached to it. After 8 hours of use it locks you out. But until then you have full access to all of its capabilities. There are also some very good video tutorials on the site under Training.

If you want to see more drawings of mine, go here: My Drawings (http://www.woodshopphotos.com/gallery/Dave-R1s-Album)

[Greg Hairston]

Dave,

Ok you have me interested. How long did it take you to draw the shop drawing you posted?

Greg

[Dave Richards]

Greg, I don't know exactly how long but I spent. Not real long. The Legacy mill might have taken me an hour to draw. The cabinet underneath it and the one behind might actually have a bit more time because I did a fair amount of draw and erase. I was designing those pieces as I went and just like drawing on paper, you end up changing your mind on things. The mount for the WoodRat on the left was probably a 15 or 20 minute piece. The upper cabinets on the wall, maybe 30 seconds. The air filtration unit, maybe 10 minutes.

The mill, it's cabinet, the one behind it and the mount for the WoodRat were all drawn separately and then combined. The rest were done in that combined drawing.

The nice thing is that you can start with a rough sketch and then add as much detail as needed. If I was planning only the shop drawing I wouldn't have added the details on the other side of the big cabinet. Since I drew the big cabinet to use as a plan for building it, I did add the details for the front of it.

You might have seen this already but here's a drawing with even more detail.
http://www.woodshopphotos.com/albums/Dave-R1s-Album/tbuckles.jpg

[Jim Fancher]

Thanks for the heads up on SketchUp. I'm going to give it a shot.