Is a torsion box really necessary?

[Bulkley]

Various articles and posts on BT3Central point to the advantages of a torsion box under mobile work benches and saw centers. But, I have never seen anyone remark on any disadvantages of a torsion box. I see two offhand: they look like a lot of trouble to construct, and they take up a lot of room underneath that could be used for storage.

Is a modified torsion box practical? Could one leave the lower corners open so that casters could be mounted under the top sheet instead of the bottom? That would lower the box by about 4".

Most of the torsion box articles that I've seen use MDF. I've had trouble gluing MDF. How does it survive over time? It's fine for projects that receive no abuse, but in a workshop things happen. Even vibration can be a problem. I'm looking to build a mobile cabinet under a heavy contractor saw and I wonder if MDF will be strong enough. I don't want to do this job twice.

[RayintheUK]

I can only speak about building a mobile cabinet for my BT3 (details here). The reason I used a torsion box base was in order to ensure that the saw was always going to be perfectly horizontal in both directions. Any slight deviation can cause problems in the accurate operation of the saw - in particular the twisting/racking of the fence rails.

Bear in mind that, on my project, I discarded the use of the stand that came with the saw altogether. This meant installing the saw on a custom base, but that also gave additional storage, not less.

I know that others have mounted their castors through the top of the box instead of the bottom - requiring that the top overhung the bottom at both sides. I chose not to do that, as I was expecting the final project to become very heavy when fully loaded and I didn't want to risk the integrity of the base. As it turned out, this was a good decision in my case, as the mobile cabinet is now very heavy indeed.

The reduction in storage involved in the use of a torsion box was not a factor I even considered. I was able to exactly dictate the overall saw table height and make use of all the available space between that height and the base.

My project used 3/4" MDF throughout. I have never had trouble when gluing MDF - I attribute this to brushing a thin film of glue onto both mating surfaces. Shallow dados and generous use of flat-plate dowels (biscuits) have ensured that my base is as steady and rock-solid today as it was the day I completed it. The surface was protected with Tung oil and has stood up extremely well.

The strength of any torsion box will - to a great extent - rely upon the spacing, number and dimensions of the ribs used. I would build a base for practically anything using this method and not worry about it at all as long as the castors were fit for purpose and could be effectively locked when the saw was in use.

Ray.

[LarryG]

A torsion box is, at heart, a truss. Some trusses do bear on their top chords, which is the situation you'd have with "up-set" casters, but with a wood or MDF assembly it would require careful design to keep the top skin from trying to act like a hinge where it joins the last cross-rib. The loads will place the top skin in tension, which will tend to pull the glue joints apart. With the casters below, the loads are all compressive; i.e., gravity is working in your favor, rather than against you. If I were building a torsion box out of steel or aluminum, I'd be a lot more comfortable with this idea. Using MDF or plywood, I wouldn't do it. IMO you'd be largely be negating the whole point of building the TB in the first place.

MDF can be adequately strong for just about anything, depending on how you join the pieces. Its worst enemy is water and humidity. I don't think vibration is a concern, unless it's severe enough that any glue joint would be hard pressed to withstand it. I will concede that its corners don't stand up well to impacts; i.e., it dents very easily. It'll also split easily if you use ordinary nails and screws as fasteners.

One trick for getting a better glue joint on the somewhat porous edges of MDF is to apply sizing first ... if you're unfamiliar with that term, it means to apply a thin layer of glue, allow it to almost dry, scrape off the excess, and apply glue as normal. The glue sizing acts to seal the porous edge and keep the material from sucking all the glue at the joint away from where it's needed.

[JimD]

Bulkley,

I am not a fan of MDF in a structural application. Is has about the same strength as particle board (to which it is related). For a cheap material that will see load, I like to use waferboard. It's strength and weather resistance are better - it's almost as strong as plywood. It is very ugly but where you do not have to look at it, is a good choice IMHO. I made the torsion box that supports my BT3100 with an outer frame of a clear 2x4 and inner torsion box pieces of 3/4 waferboard, a bottom skin of 7/16 waferboard (painted) and a top skin of 3/4 plywood. The top skin has the vertical section dividers screwed to it before it becomes the top skin of the torsion box. Pictures are in the articles section as "Spruce and Sandeply". If you are interested in the most space for drawers you may get some ideas. I fastened a HF mobile base directly to the 2x4 - only less than an inch of added height versus the 2x4 torsion box alone. Much less loss of height than if I had used casters. I've been using this for awhile and it works fine.

If you made a well contructed cabinet without the torsion box it is probably adequate in strength.

Jim

[ejs1097]

Most speaker boxes are made out of MDF because it doesn't cause vibration and distort the sound, I don't think you'll have to worry about that.

Like you said you don't want to build this twice, use the best product you're comfortable with, if that's birch veener, etc then use that instead of MDF or other material. That said many have used MDF and haven't had a problem with it's integerity.

[JimD]

Eric,

I would avoid concluding that commercially made speakers and speaker kits use MDF because it is the better material for speaker boxes. I think they use it for the same reason as people have used it for torsion boxes - it is cheap and it will work. The last commercial speakers I messed with used particle board (covered by plastic film made to look like wood).

I have used both plywood and waferboard for speaker boxes and they work well. The waferboard is for a 12 inch subwoofer with a 100 watt amp that is buried in the floor trusses and shoots out a fake heat vent. You don't see the box so I went with something cheap and strong. I also added cross bracing because it is important that the sides of the speaker do not move or that they do it out of the range of reproduction of the speaker in question. If I had used MDF, I would have added more bracing due to it's lower strength.

MDF's advantages are cost and, for applications like speakers, it's weight. The disadvantage of it's reaction to getting wet make me want to avoid an application down on the floor of my shop.

Jim

[don_hart]

I have built a few stands out of MDF using a trsion box and have been very pleased with the result. Overall the torsion box was very easy to build and is very solid.

As for gluing MDF I am surprised that you have had any problems. I have found with MDF that a properly glued up joint no longer acts like a joint but acts like a solid piece of MDF. Once glued up and coated you should have no problems with MDF.

[mleichtle]

I just picked up better homes and garden "Wood" "best ever work shops" or somthin like that. They have plans for a mibile base, and they did a torsion box just as you said, they left the the ends open, not just the corners to recess the casters. I would think the cabenets are go into stiffen the base also, therefore given the joints are decent, I don't think there will ever be enough weight on a tool base to comprmise the strength of the glue and joints.

[Bulkley]

I have a copy of the Better Homes and Gardens "Wood" issue that you refer to. Is that a torsion box under their dolly? It has no bottom to it. The construction looked more like a form of trusses to me. They used maple for the trusses and trim. I don't know about you guys, but hardwoods are really expensive here.

MDF has one really important advantage for me: it is flat. I have some 2X6's that I would like to use, but they are a bit warped and I don't have a joiner. (I live in a logging area and the best wood is exported.) MDF is easy to cut in straight lines, with square edges, etc. I love working with it. (It is possible to put a really teriffic finish on MDF using exterior semi-transluscent stain followed by Varathane, but that's another subject.)

BTW, my complements to the BT3 forum people. You have done a great job. This is one of the better woodworking forums.

[JTimmons]

Before buying the house a couple of years ago car audio was my thing. 100 watt amp on a 12" sub is not pushing a whole lot, so I am sure waffer board/ply would have worked quite well. Drop a Rockford Fosgate T212D2 (2,000 watt peak) into your waffer board/ply box and the distortion alone will make you want to turn it down before the thing comes apart.
MDFs density (or the ability to hold and direct the air flow) is why the pros and sub manufactures recommend it's use for speaker boxes. I have never seen the use of plywood at a car audio competition other than to hold amplifiers.
Of course, the big thing now, but very expensive is custom fiberglass boxes.

I am not sure what swelling everyone is referring to when it comes to MDF, I have got several jigs, cabinets and my recent mobile cabinet for my BT made of MDF. I live in Eastern Oklahoma and the humidity levels are exactly the same as those in San Antonio in the summer months 90 - 99% humidity levels all summer, most of the time it's 70 - 80% the rest of the year. I have yet to see any change in the way my MDF has held up. Unless you’re leaving it out in the rain and sun I don't see a problem.

My mobile base is made out of 3/4 inch MDF w/ a 2 x 3 frame built under it, I would not have attempted doing this w/ 3/4 inch plywood unless it was oak, then your talking $$.

As Don said, the gluing factors are very good at least w/ my experience.

Is a torsion box needed, I didn't think so, but that's my feeling. The torsion box is going to be stronger than the method I used, but how much weight are you expecting to put on it. The torsion box made in the above post is pretty well made and could more than likely support my truck.

[Bulkley]

How much weight am I going to put on it? My saw has a cast iron surface of 27" X 40", a 2hp motor and home made 50" rails. It's heavy. I definitely need some kind of frame under the cabinet, unless I can make the cabinet itself part of the frame.

[Habe]

I built my saw stand on a torsion box using rayintheuks idea. I do not regret building it that way and find that it is very sturdy. You can hang one caster off the edge of the shop floor and the stand will stay steady one the other three. Also, remember that you need some toe room under the edge of the stand and the casters will give you that. The minor storage loss to the thickness of the box is offset by the better stability of the stand. Just remember to assemble the torsion box on a straight surface when you build it.

[Bulkley]

Well, I built my dolly last night using an underslung box. I used MDF only for the large flat surface; the rest is all wood. I used spruce which I cut to 1 1/2 X 4". To begin, I made two "L" girders, which is strong and straight. I fastened them in place and added cross braces, ladder style. All joints glued and screwed. The result is heavy and solid.

Thanks for the advice everyone.

Now, to build the cabinets . . . .