Torsion Box Q

[Scott Hildenbrand]

So I'm going to build a torsion box table top that I can use as an outfeed table. Seems like a no brainer to me, so long as it's flat.

Question is, I see these things built and they always seem to be sheeted on both sides with 3/4" material. Does the bottom HAVE to be that thick?

The whole point is the torsion box is that the inner structure and complete glueup of the inner webbing to the sheeting material is what gives the strength and keeps it from flexing.

Reason I ask is 1) I don't want to have the top too awful thick. 2" max and 2) I have some 1/4" sheet laying around.

Well, sure there's more reasons, just can't think of them this early.. Long night.

Anyway.. top sheet I'm wanting to use 3/4" MDF. For the inner webbing, 3/4" MDF cut into 1" wide strips.. Bottom sheet of 1/4" ply.. Wrapped all up in a skirt of oak with a hardboard topper held in place with w few dabs of silicon to keep it from shifting.

Since my shop is small, trying to get the most bang for the buck of a work table.. So it needs to handle outfeed, be good for a glueup build table and also act as storage for those common table saw items like blades, various ZCIs, push sticks and whatnot.

Thoughts?

[sean m. titmas]

Take a look at interior hollow core doors, they have very thin skins yet have tremendous strength to weight ratios. I see no reason that you couldn't use 3/4" on the top a 1/4" on the bottom. just make sure to have plenty of ribs and use sufficient glue and screws/staples to attach the 3/4" and 1/4" skins respectively.

DONT use MDF on edge as ribs for edge nailing because it has no strength at all. Instead, cut up some poplar for the webbing because its an economical wood that is strong enough for your project.

Whats the hardwood topper for? to protect the 3/4" skin from saw tracks?

[glenn bradley]

I have seen several with 1/2" and even 1/4" skins done by some of the pros. I would think 3/4" for the top so you could put miter grooves in it but, the bottom could be thinner. I assume a thinner skin would benefit from a denser grid/glue pattern.

[Scott Hildenbrand]

Whats the hardwood topper for? to protect the 3/4" skin from saw tracks?

I was planning on using a 1/4" hardboard skin laid on top of the 3/4" MDF to act as a replaceable top. I'd really like to build this thing once and have it at least last me MY lifetime. So if I ever screw the top up I can pop it out and just slip another in its place.

[Doug Carpenter]

My out feed table is my general do everything work srface. It has a 3/4" melamine top and 1/4 luan on the bottom. It is 20+ years old. I wound up laying another peice of 3/4 melamine on top because it got beat up and unfortunately I didn't make the top easily replacable. Something to consider when making yours.

[Scott Hildenbrand]

*points up one post* :D

[Bob Wingard]

I think you're gonna have a tough time meeting your size constraints and still making a real torsion box, considering your materials.

1/4" top sheet
3/4" MDF top
1/4" bottom
1" MDF ribs

You're already at 2-1/4" and you have a nearly solid material box/top.

Lighten up .. really .. lighten up .. that thing will weigh a ton/squeare foot, and you've lost the advantage of a torsion box.

1/4" - 3/8" sheet materials are more than enough to make a strong, rigid, lightweight torsion box. If you go with your original materials, why not just eliminate the 1" X 3/4" MDF ribs, and glue in a solid sheet ?? You'd have your 2" dimensional spec, with a lot less work.

[Mike Henderson]

I'm no mechanical engineer, but this is what I understand of torsion boxes. Maybe a mechanical engineer will chime in.

A torsion box is essentially a structure of I-beams in two dimensions. I-beams are composed of flanges (the outside parts) and the web, which holds the flanges apart.

The strength of an I-beam comes from the flanges ability to resist tension and compression loads - the only purpose of the web is to hold the two flanges apart and to keep them from slipping relative to each other.

So for your torsion box: Make the webs much thinner - 1/4" material is fine and will give you a lighter box. The spacing of the webs will be determined by the thickness of the skins and how much flexing you can tolerate (that is, thin skins will bend inward between the webs when you put a load on the box).

The best thing you can do to increase the stiffness of the box is to make the webs deeper.

So to summarize: The strength of a torsion box is in the skins, and in the depth of the webs (not the thickness of the webs). The spacing (and number) of the webs affects the strength, and also the amount of depression of the top skin between the webs when you put a load on it.

Mike

[Scott Hildenbrand]

Hmmmm... You may be right.. Perhaps I'm over doing it. My thought on the 3/4" MDF for the inner webbing is that it has no predisposition for uneven movement.

As far as thickness, I stated 2" tops, but forgot I'd planned to do the 1/4" hardboard replaceable topper.

Now, lightness of the top is really a null point. I'm NOT building a plane, where torsion boxes were originally developed for. It NEEDS to be able to take a few swift hits of a hammer. So that begs the question.... IF I do a 1/4" ply skin on the bottom AND top, will the 1/4" ply + 1/4" hardboard be enough mass to be able to take a beating?

Like I said, it will be a "do as much as possible" table handling outfeed, glueup and quite likely some hand tool work. It may end up having recesses routed in the top to allow for a bench clamp to be used. I've got a Kregg Bench Klamp standing by in fact.

So, that being given.. I think a minimum requirement for materials would be a poplar core, 1/4" bottom skin and a 1/2" MDF top skin to give enough mass for an all around table. Add to the top skin the 1/4" hardboard topper and that finishes it up.

Anyone have 2 cents more to throw into the pile? :)

[Jamie Buxton]

I have an assembly table that is a torsion box which is 6" tall by 40" by 72". It has 1/8" ply for the ribs, 1/4" ply for the top, and 1/8" ply for the bottom. If I put the short ends on sawhorses, a straightedge across the top, and then sit on it, I can barely see the deflection -- about 10 thousandths or so. It weighs about 20 pounds, so I can easily pick it up and hang it on a wall above my head.

[Scott Hildenbrand]

So for your torsion box: Make the webs much thinner - 1/4" material is fine and will give you a lighter box.

I had actually thought about that, making the inner webbing out of LONG slats of 1/4" strips with inter locking slots in them. Much like the cardboard you find separating mason jars for canning.

My only concern for that, however it is INDEED a valid idea and has been used, is that will the glue joint between the inner webbing and the skin possibly break with heavy use, or hitting it wrong with a hammer.. :)

As far as stability of the core webbing goes, I could indeed use 1/4" MDF. My fear of using ply for the core however is that HALF of the glue strength will be lost as half the 1/4" thickness will end up being end grain. Granted, I could use an epoxy style of glue, so long as I bust my rear to get the glue down quick and lay the top. Not alot of working time, no?

[Scott Hildenbrand]

It has 1/8" ply for the ribs

Wow... I'm assuming that the ribs are as I stated above, long slats with slots cut into them?

I'd say post a pic of it, but I'm sure you don't have any of the inside.. Granted would still love to see it none the less.

OK... Reason for wanting it to be 2" thick is that I want it to be portable in a very specific manner. In fact, a modification of this idea is what I'm shooting for.

http://www.youtube.com/watch?v=KZyguUCdw4s

I have very... very little space to work with, so this is really a great way to not only multi task a table, but also make moving things out of the way much easier.

I also have very... very little headroom... so hanging stuff over my head doesn't work.. Else I'd surely make a 20lb table and just hang it from the joists above. ;)

Further thoughts?

[brett gallmeyer]

i just finished a 6'x4' torsion box that is the top of my outfeed table. i made the entire thing out of 1/2" mdf, trimed in oak. works great! ill b uploading pics soon of it and other recent prodjects soon.

[Mike Henderson]

Wow... I'm assuming that the ribs are as I stated above, long slats with slots cut into them?
I made several torsion boxes, but for a different purpose. Here's a couple of pictures of the insides. The webs are much taller than what you plan. If I recall, this is 1/8" material.

I glued the webs together where they cross, but others have told me that's not necessary. One thing to look out for - you need a really slow glue because it takes a long time to apply the glue to all the webs.

Mike

[sean m. titmas]

My only concern for that, however it is INDEED a valid idea and has been used, is that will the glue joint between the inner webbing and the skin possibly break with heavy use, or hitting it wrong with a hammer..
thats why its futile to want to build these things with the intent of having them last forever. build them simple, cheap and strong and than use, abuse and work them to death. when it no longer functions as desired you can just toss it out and build another. keep in mind that this is not a traditional wood work bench, its just an accessory table. :)

As far as stability of the core webbing goes, I could indeed use 1/4" MDF MDF is a bad choice due to its LOW strength to weight ratio.. My fear of using ply for the core however is that HALF of the glue strength will be lost as half the 1/4" thickness will end up being end grain. That is a mute point because the strength of those individual flitches of wood when glued up into a sheet are greater than the sum of their parts. Granted, I could use an epoxy style of glue, so long as I bust my rear to get the glue down quick and lay the top. Not alot of working time, no?

If you want an ov erall thickness of 2" than use .25" ply for the bottom skin, .5" ply for the top skin and 1.25" poplar ribs. if you assemble the skins with glue, screws and .25 crown staples that table should be able to take quite a lot of abuse before needing replacement. if you do need to cut on it with a circ saw than first lay down a sheet of .75" pink Polyisocyanurate insulation to protect the plywood skin from the blade.

[Scott Hildenbrand]

Top notch, Mike.. Thanks for the visual.. That's just what I was thinking of doing for thin material.

Q, looks like hardboard to me, is it?

I've got a pile of MDF core laminate flooring laying in a corner, was thinking I might try using it as the core. It'd be thicker for sure, around 1/4", though have not measured it.

EDIT:: Hmmm... Hmmmm.. Hmmmm.... So much information to soak in... I think I'll just let y'all add in your cents and then count it up at the end.. :D

Sean, thanks alot for the added info. Will be taken under consideration for sure.

[Sean Nagle]

This is a timely discussion since I'm in the planning stages for building an outfeed table myself (33" x 60").

Before committing to a torsion box top, you should design the rest of the table first. You might just have enough support for the top to throw down a sheet of 3/4" MDF and then a 1/2" or 3/4" sheet of melamine instead of going to all the work of building a torsion box. If you're not going to be removing the top, you don't need to make it light in weight. You'll also want enough solid material to route dados for miter slots.

[Scott Hildenbrand]

Before committing to a torsion box top, you should design the rest of the table first. You might just have enough support for the top to throw down a sheet of 3/4" MDF and then a 1/2" or 3/4" sheet of melamine instead of going to all the work of building a torsion box. If you're not going to be removing the top, you don't need to make it light in weight. You'll also want enough solid material to route dados for miter slots.

Perhaps you're right.. It would certainly simplify things.. So long as it did not develop a sag over time it might be just as well to go with a standard top, considering I need to route slots and install clamp bases.

This does not count out making a torsion table top though, for anyone thinking "OMG, he just wasted my time!" ;)

All in all, it's been a tremendous amount of information on the subject and has been very helpful. Learned alot about torsion boxes, for sure.

[Tony Bilello]

I was planning on using a 1/4" hardboard skin laid on top of the 3/4" MDF to act as a replaceable top. I'd really like to build this thing once and have it at least last me MY lifetime. So if I ever screw the top up I can pop it out and just slip another in its place.

There is a potential problem here with the small height of the webbing. I dont know what a minimum should be but I surely think that 1" webbing is way too small. The stiffness is dependent on the webbing size. If you use 1/4" hardboard, 3/4" mdf for top and 1/4" mdf for bottom that only leaves you with 3/4" webbing.

First question.......why are you limited to 2" thickness?
Second question.... why would you use poplar instead of mdf for the webbing? MDF is definitley more stable than poplar. I have used MDF for torsion boxes and never had a problem. I also use MDF for columns that are covered with plastic laminate. MDF glues up very well on end grain.
3rd Question.... why not 1/4" bottom. 1/2" top and 1/2" thick mdf for webbing and whatever height you choose.

Lastly, if at all possible, reconsider the total thickness of top to give you a higher webbing.

[scott vroom]

I see only one mention in the above posts of using a door for a table top. I can buy a perfectly flat solid core door with hardboard skins for $75 dollars at my local lumber yard (the above poster suggested using a hollow core door, which I believe incorporates torsion construction and would be less expensive). Attach to a simple fir frame and it's done. Can anyone tell my why this is not a good idea?

[Scott Hildenbrand]

I dont know what a minimum should be but I surely think that 1" webbing is way too small. The stiffness is dependent on the webbing size.

The realization that it's the webbing which gives the strength and the inherent fact that having it deeper is better is what's swaying me to make a separate torsion box table with a good 6" rib depth.


First question.......why are you limited to 2" thickness?

The 2" depth was due to the table design itself, in that it stores OVER the table saw. There's only so much lift one can get with flipping casters.


Second question.... why would you use poplar instead of mdf for the webbing? MDF is definitley more stable than poplar.

That was indeed my thought as well RE MDF, was just considering poplar due to material suggestions.


3rd Question.... why not 1/4" bottom. 1/2" top and 1/2" thick mdf for webbing and whatever height you choose.

I really think at this point the short webbing will prove to be no benefit and feel that it defeats the purpose of a torsion box, in that the strength itself comes from the core and not the skin.


Lastly, if at all possible, reconsider the total thickness of top to give you a higher webbing.

As stated above, not really.. BUT, that doesn't count out making a dedicated, light weight, stash easily, 1/4" or less skin torsion box which can be used strictly for assembly.

[Scott Hildenbrand]

I see only one mention in the above posts of using a door for a table top. I can buy a perfectly flat solid core door with hardboard skins for $75 dollars at my local lumber yard (the above poster suggested using a hollow core door, which I believe incorporates torsion construction and would be less expensive). Attach to a simple fir frame and it's done. Can anyone tell my why this is not a good idea?

This is indeed a valid idea. The hollow core of slab doors are made of a honeycomb of cardboard which is glued up to the skin and pine frame.

This would have been my first idea if it was not for that I needed it to be able to take serious abuse and be so many types of tables in one. IE: outfeed, buildup, glueup, etc.

[Ben Hatcher]

Scott, you don’t need to worry too much about the security of the webbing to the skins. Regular wood glue will be fine, even if you use thin stock for the webs. The reason for this is that while in use, the webs are experiencing a compression force for the most part.

My first assembly table was a simple hollow core door. It was free, very light, and nearly indestructible once I put an MDF skin on the top. I still use a hollow core door and a pair of saw horses if I ever need extra assembly/bench space.

[Tony Bilello]

I was just about to suggest the hollow core door when I looked up and Scott Vroom beat me to it. If it needs to take some abuse, but not too serious like outfeed, glue-up and assembly, a sheet of 1/4" MDF glued on top should do the trick. MDF is fairly tough stuff especially if glued down to a flat surface such as a door.
This would still be light enough to be easily managed by one man. As a matter of fact, the first torsion box table top I ever built was started on 2 hollow core doors side by side for flatness. After that the other torsion box tops were made off of the original.
I also covered my top with formica so that I can draw with a pencil and layout my stuff on top of it. It washes off easily as the excess glue from furniture assembly comes off easily also.

[Ben Hatcher]

Scott, I think that you'd be surprised what amount of abuse a hollow core door will take if you do two things.
1. The attachment method of the base needs to have a large surface area. I think that mine was about 12" wide and just under the width of the door in length.
2. Cover the top with something like 1/2" MDF that will distribute the force of a blow over a wider area.

As for toughness, I split retaining wall bricks with a sledge hammer and chisel on mine without any problem. Whatever way you go, if you plan on using it for a long time the sacrifical top is a must have.

[Doug Shepard]

The need for use as a glue-up table is the one that I think makes the hollow core door problematic depending on the type of clamps you plan to use. I put a HC door to use when i was making my bench top and it worked well BUT you need to be careful where you locate clamps and what kind. If you're clamping down to the HC door and you get the clamp screw pad in between the grids, you'll probably crack the outer skin. Something like a parallel jaw clamps with longer flat faces that span the internal grid dont have this problem and larger scrap platens under standard clamp screws would work OK to. But I think something more like a 3/8" skin on the bottom of a torsion box would be even better.

[Cliff Holmes]

the webs are experiencing a compression force for the most part.

Actually, the web will mostly experience a shearing force from the bending moment forces. This shear will be strongest at the glue joint. If a torsion box is going to fail, it'll be at the glue joint between the web and the skin.

[scott vroom]

Great thread....lots of good ideas.

Has anyone used a solid core door? Seems this would solve the clamp problem. My local lumber yard guy recommended solid core for a table, says they're dead flat. The added weight vs hollow core would add stability.

[Brian Walter]

Actually, the web will mostly experience a shearing force from the bending moment forces. This shear will be strongest at the glue joint. If a torsion box is going to fail, it'll be at the glue joint between the web and the skin.

I agree with your comment, but I think a little clarification may be in order. You are correct in that the web will experience a shearing force from an applied bending moment, such as might occur from putting a heavy weight on the table between the supporting legs, but forces from clamping to the table or weight applied directly over one of the legs could easily result in more of a compression force in the web. So in conclusion, I think it would be safe to say that the web could experience either shear or compression, depending on the applied loads.

Brian Walter

[Alan Schaffter]

Wow- a lot of good info here, though some is incomplete, and some conflicting. I think Mike Henderson hit all the majors point dead on.

First, you need to decide what you are really going to use the outfeed table for (outfeed, glue-up and clamping, etc. and will it have miter slots, etc. etc. and what kind of abuse you will give it. Do you want it to last forever? Unless you plan to subject it to point loads (clamping and hammering) the top skin can be thin as long as it doesn't stretch or compress along its surface- e.g. thin aluminum airplane wing skin. If the loads will not be great, you can even omit one skin, though a very thin skin is much better than no skin.

For my large, 2" thick, outfeed and extension tables, I used a half a torsion box with 1/2" x 1-1/2" hardwood ribs, a 1/2" MDF top skin covered with laminate, and an open web bottom (no bottom skin), all joined with yellow glue and brads. Works great as an outfeed table, no discernible sag. I don't load it and it is not subject to point loads (no clamping, no hammering)

http://www.ncwoodworker.net/pp/data/500/medium/P5220039.JPG

http://www.ncwoodworker.net/pp/data/500/medium/P5220035.JPG

http://www.ncwoodworker.net/pp/data/500/medium/P5200028.JPG

For a general purpose table with similar demands but without any thickness limitations, look at this 8' X 18" X 3" torsion box table (bridge!). It was made with THIN 3/16" hardboard (one side fuzzy) using yellow glue only. In the photo it is loaded with over 300 lbs. of bricks (there is another row behind the front bricks) yet is deflecting less than 1/2" (measured) in the center!! I am sure it would hold much more. I originally planned to load it to failure but I didn't have enough bricks, plus I figured I could use it as a scaffold.

http://www.ncwoodworker.net/pp/data/500/medium/IMG_2579.jpg

There is an excellent torsion box article coming out in the next (Dec/Jan) issue of American Woodworker Magazine due out in a few weeks.

[Scott Hildenbrand]

Thanks for the input, Alan.. Appreciate it greatly.

I've run across your shop several times online while searching for this or that.. Top notch and beautiful work.. LOVE the whole table saw setup. You've done a ton that makes so much [common] sense to me.. Saves me the trouble of thinking it up myself, for sure.. ;)

Ok.. So let me understand.. The back outfeed section is only partial torsion box? So where the miter slots are is a more solid surface than the 1/2" skin?

While stewing over all this, I'd decided to go with miter slots but not just on one plane.. What I want is to me able to rotate the table long ways in case I'm cutting LONG items and want them fully supported. So my miter slots in the outfeed table will most likely intersect # on both planes.

Anyway.. And to everyone.. Thanks for all the information, tips, opinions and whatnot.

[Alan Schaffter]

Thanks for the input, Alan.. Appreciate it greatly.

I've run across your shop several times online while searching for this or that.. Top notch and beautiful work.. LOVE the whole table saw setup. You've done a ton that makes so much [common] sense to me.. Saves me the trouble of thinking it up myself, for sure.. ;)

Ok.. So let me understand.. The back outfeed section is only partial torsion box? So where the miter slots are is a more solid surface than the 1/2" skin?

I glued a rectangular backer of 1/2" MDF to the bottom of the top skin under the miter slots.


While stewing over all this, I'd decided to go with miter slots but not just on one plane.. What I want is to me able to rotate the table long ways in case I'm cutting LONG items and want them fully supported. So my miter slots in the outfeed table will most likely intersect # on both planes.

I see no problem with that. A backer like I used should do fine


Anyway.. And to everyone.. Thanks for all the information, tips, opinions and whatnot.

[Rick Lizek]

http://books.google.com/books?id=5xYBcTalTWQC&printsec=frontcover&dq=ken+horner#v=onepage&q=&f=false
See page 207 on torsion boxes. His books are well worth it. Hope that helps.

[Alan Schaffter]

http://books.google.com/books?id=5xYBcTalTWQC&printsec=frontcover&dq=ken+horner#v=onepage&q=&f=false
See page 207 on torsion boxes. His books are well worth it. Hope that helps.

Horner's book has an excellent chapter on torsion boxes. I found a few problems with it however. He uses a lot of math which I don't think many woodworkers will want to mess with, since most of it leads to nowhere except comparative analysis since he doesn't include much in the way of materials data.

He stresses the importance of the skins and their contribution to the stiffness of the torsion box, but misses the point because he talks about skin thickness instead of the main skin characteristic needed- strength along (parallel to) the skin's surface- its resistance to stretching and compression- and that skin thickness (along with web spacing) mainly affects the torsion box's ability to resist point loads- which may or may not be important in a particular application.

Though he provides a number of "rules of thumb", neither he nor any other source I have read gives a nice, neat cook book recipe for making a torsion box. While I agree with his rule of thumb concerning web spacing, his article provides no foundation for his data. Unfortunately, the sad fact is that you need to go way beyond the math and engineering he provides to accurately design a torsion box.

Another good source of torsion box info is "Design and Fabrication of Plywood Stressed-Skin Panels (http://gp.com/BUILD/DocumentViewer.aspx?repository=BP&elementid=3815)," by the Engineered Wood Association. Stressed-skin panels are torsion boxes.

One truly telling statement they make in the "Design" section is that "stressed skin panels are designed by a "cut and try" method. A trial section must first be assumed then checked for its ability to do the job intended." The "Timber Designers Manual" also has similar info. You will easily see any rule of thumb would not be credible. The spacing of the short webs CAN be much greater as far as preventing bending, but now you get into problems with local surface deformation.

My best advice is experiment- start with a light weight torsion box made from 3/16" - 1/4" hardboard. If you plan to do some heavy pounding- increase the thickness of the top skin, and or reduce the spacing of the web. Note, also, if you cover your top with laminate you not only increase the surface durability and make it so glue pops off easily, you also increase the strength and rigidity of the top skin, locally, and the overall stiffness of the torsion box.

[Sean Nagle]

I was giving more thought to what kind of base would go well with a torsion box tabletop for an outfeed table. One of the storage requirements I have is to conveniently store four tablesaw sleds. They have never had a good home in my shop. A thought occurred to me that a wire shelf base would provide a "baker's rack" sort of arrangement that would be perfect for sleds and extra fences for a tablesaw. I checked online and found that some very sizable wire racks are available (for my application a 36" x 48" rack is perfect). Another benefit is that the shelves are adjustable. This would go perfectly with a torsion box tabletop since it wouldn't require any stretchers or skirt. The shelving is a bit pricey though.

http://www.globalindustrial.com/gcs/prod/30006563/i/productInfo.web
http://shelving.com/Merchant2/merchant.mvc?page=WSS/PROD/WS3654/WS364854-S

[Alan Schaffter]

I was giving more thought to what kind of base would go well with a torsion box tabletop for an outfeed table. One of the storage requirements I have is to conveniently store four tablesaw sleds. They have never had a good home in my shop. A thought occurred to me that a wire shelf base would provide a "baker's rack" sort of arrangement that would be perfect for sleds and extra fences for a tablesaw. I checked online and found that some very sizable wire racks are available (for my application a 36" x 48" rack is perfect). Another benefit is that the shelves are adjustable. This would go perfectly with a torsion box tabletop since it wouldn't require any stretchers or skirt. The shelving is a bit pricey though.

http://www.globalindustrial.com/gcs/prod/30006563/i/productInfo.web
http://shelving.com/Merchant2/merchant.mvc?page=WSS/PROD/WS3654/WS364854-S

If you build the torsion box the way I built my assembly table, by incorporating reinforced leg sockets into the web you don't need aprons or stretchers either. While not quite as stable as a table with aprons and stretchers, it is pretty darn stable.

http://www.ncwoodworker.net/pp/data/500/medium/P5010049.JPG

If you locate the leg sockets at the corners you can use bolts to attach the legs so they are removable.

[Sean Nagle]

Alan, thanks for the tip. That will surely help maximize the storage space beneath the table.

A question on the torsion box top. How would this sandwich work out?

1/2" or 3/4" melamine (replaceable top)
1/4" MDF or plywood
2" webbing
1/2" MDF or plywood

[Alan Schaffter]

Alan, thanks for the tip. That will surely help maximize the storage space beneath the table.

A question on the torsion box top. How would this sandwich work out?

1/2" or 3/4" melamine (replaceable top)
1/4" MDF or plywood
2" webbing
1/2" MDF or plywood

If you are just using this as an outfeed table which doesn't get much abuse, I would go with 1/4" or at most a 1/2" MDF top (if needing some resistance to point loads). Cover it with laminate for a more durable surface. Use an open web or 1/4" bottom skin. Depending on the size you need, you can sometimes get really good prices at the big box stores on laminate with damaged edges. If you really want a replaceable top just use a layer of 3/16 or 1/4" tempered hardboard (masonite) or coated hardboard.

3/4" melamine (which has particle core) is VERY heavy and not very strong. I have never seen it coated just one side. Double-sided is not suitable for a top skin since the glue bond between the ribs and skin will be very weak. Remember, also, if you go with a sacrificial top layer over the top skin, you need to attach it somehow so it stays stationary and flat. The only damage I have on my outfeed table is scratches from the end of my Unifence. With care or a nylon glide I could have prevented that, but oh, well.

I don't like ply for torsion box skins because it rarely comes flat from the supplier, especially the stuff from big box stores. Also, the thickness of much of cheap ply today is not consistent. You want something that will lay flat without much effort or else the top could warp or you will need a lot of weight to do your glue up.

FYI, if you want a really a flat table, you must have a flat construction platform. Follow the David Marks method or the similar procedure I used in my AWW article.

[Bob Carreiro]

[QUOTE=The strength of an I-beam comes from the flanges ability to resist tension and compression loads - the only purpose of the web is to hold the two flanges apart and to keep them from slipping relative to each other.]

Hi Mike,
The strength of a beam comes from the web (the taller the web, the stronger the beam) and flanges prevent the web from buckling to either side while under load.
Bob

[Josh Satterlee]

Hey Creekers. What about using 1/4" melamine skins top AND bottom, 1/2" ribs (to ease finding ribs with the screws/nails), then fill the voids with rigid foam insulation. Time consuming, but seems like that would resist the errant hammer blow, and still be light as heck. And the melamine would work great for glue-ups and as outfeed...

To address the clamping issue, you could fill the edge webs with wood (even 2x scraps). The avg throat capacity is only around 2", so it wouldn't take much edge material.

Maybe we should have a Torsion Box competition:

Assemble a 2' x 4' torsion box, support at each far end with sawhorses, and see how much weight it can support in the center. The drop a 16 oz hammer 10 times and resist damage. Then we find some cost vs. weight vs. strength calculation and award a winner. I bet if it was light enough to be moved by LOML, that would be worth extra points.

[Alan Schaffter]

[QUOTE]
Hi Mike,
The strength of a beam comes from the web (the taller the web, the stronger the beam) and flanges prevent the web from buckling to either side while under load.
Bob

A bit of semantics- within certain limitations, it is not the web material but the height of the web that has the most effect on the rigidity of a torsion box. Increasing the height of the web by 25% will almost double the strength, doubling the web height will result in an 8 fold improvement (third power).

But, dimensions being equal, the skins (their ability to avoid stretching and compressing along their surface) contribute more to the strength of a torsion box than the web. Horner's book shows the math to illustrate this. A rather exagerated example is you can make a strong torsion box with a cardboard web, but not with skins made from the same cardboard.

[Scott Vigder]

Celebrating it's third birthday, this torsion table is still the

http://www.sawmillcreek.org/showthread.php?p=456279#poststop


go-to glue-up, measuring, and most stable surface in the shop. Still no need to flip it over yet!

[Floyd Mah]

I can't resist joining this small discussion. I've recently built a small outfeed table, but my requirements were for just a small panel to catch material that would otherwise fall off the back of the saw. I essentially built a hollow core door and attached it to the back of the saw. My materials were 3/16" hardboard for the skins, 3/4" pine for the sides and the web was constructed out of several sheets of corrugated cardboard that I bandsawed into strips. Thickness was about 1.25 inches when done.

While the materials list would make this seem fairly flimsy, when glued up the panel was very stiff. The webbing was made up by gluing strips of cardboard into a grid. (As it turned out, I had to resaw the webbing to final thickness when I had already made up the web). The cardboard webbing occupied about 50% of the space inside the frame and was stretched to fit the grid before the final glue-up. Glue was used liberally. Because of the density of the cardboard web and plentiful gluing points, I think that I achieved a better rigidity than if I had used a wooden grid as in most of the examples I've seen on the other posts.

Anyway, cut up some corrugated cardboard and see how incompressible it can be. It's basically a spare parts type of torsion box material. The hollow core door people are on the right track.

[rick carpenter]

As far as thickness, I stated 2" tops, but forgot I'd planned to do the 1/4" hardboard replaceable topper.

Now, lightness of the top is really a null point. I'm NOT building a plane, where torsion boxes were originally developed for. It NEEDS to be able to take a few swift hits of a hammer. So that begs the question.... IF I do a 1/4" ply skin on the bottom AND top, will the 1/4" ply + 1/4" hardboard be enough mass to be able to take a beating?

Like I said, it will be a "do as much as possible" table handling outfeed, glueup and quite likely some hand tool work. It may end up having recesses routed in the top to allow for a bench clamp to be used. I've got a Kregg Bench Klamp standing by in fact.


You're probably asking too much of a .25 ply top here with any kind of beating on it and with the clamp recessed in. I suggest you use a .5 ply top, but I think I have a better clamping idea for you. I have an 18" x 5' torsion box, 1x2 poplar webbing skinned with .5 baltic ply, for a portable miter saw base mounted on one end. I use the other end for a worktop and can pound on it without fear. In the areas where I drilled holes for mounting the saw base I glued in two 2" blocks between the skins so I can really torque the base into place without any give in the skin between webs. I'd suggest you do the same and use cheapo Jorgenson 12" clamps. You could do this in several places for different clamping needs, and if you don't do too many you won't compromise the skins. I have no problem taking my box out to a worksite, it's light yet heavy enough, takes a beating, and it's sturdy on sawhorses -- so yours mounted for an outfeed, even with .25 ply on the bottom, should be more than fine.

Not that money grows in trees, but this is cheap enough that experimentation in different dimensions could be done without spending too much.