Has anyone tried a torsion box for a router table?

[David Rose]

I am still struggling with how to get me plywood laminated router table flat. Removing the plastic laminate and adhesive sounds like it could be a disastor. If not I could then plane it and install another or the same piece of plastic. Since the insert rings are the low point, it should work. Then it may sag again. The sag is front to rear AND side to side for at least .030".

A thought that hit me today was maybe using a torsion box. This top needs to be relatively light to hinge the way it does for bit changing. Two 3/4" pieces of plywood are about the limit. That's what I have now. With close 2" or so grid spacing would 1/4" tempered hardboard or MDF be thick enough? I can't find 3/8 locally and 1/2 would start approaching the weight of the plywood I think. Besides thickness for the router screw attachment, what would be the drawbacks to the thinner stuff? Oh, and the router will be attached to the top layer so there will be a 6" or greater circular section missing in the bottom layer. Is this even a feasible idea? I have never seen it done or heard of it.

David

[Allan Johanson]

Hi David,

Being a relative newbie and all I guess I've really missed something about your router table top problem. If it's sagging and you can't reinforce the bottom because the reinforcements would get in the way, why not make a new top from Baltic Birch plywood?

Use two layers of 3/4" ply with some plastic laminate top and bottom. The hardwood ply should be very sag resistant. Bring your straightedge to the store when shopping for the wood so you can be sure you buy flat stock. I've seen some bad stuff out there.

Hey - if your new top is very heavy making lifting it difficult, maybe you could attach some gas struts like what you find on a car trunk. Then you'd have to coolest router table around. :cool:

Sorry I've missed something. I hope yours gets fixed soon.

Allan

[David Rose]

Hi Allan,

This table was made from two layers of Baltic Birch 3/4" ply. Both pieces had a slight crown so I turned them in and clamped the edges. I edge banded with oak and laminated top and bottom with plastic laminate and backer board. I thought the base supported it pretty well. It was pretty flat for awhile. It has been in use for about 2 years and sometime in there it started sagging. :( I shimmed some spots a few days ago on top of the drawer partitions to see if any of it would pull out. It didn't. I suspect it would take two years... again. Then of course the area around the router would not be raised. It would take a major system of support ribs to get the whole thing. Not only is there sag side to side but front to rear. It isn't too heavy to lift, though the tip sounds cool, but it is about at the limit of weight that I want for it. Thus the torsion box idea.

Thanks for the "well wishes". I'll figure out something.

David


Hi David,

Being a relative newbie and all I guess I've really missed something about your router table top problem. If it's sagging and you can't reinforce the bottom because the reinforcements would get in the way, why not make a new top from Baltic Birch plywood?

Use two layers of 3/4" ply with some plastic laminate top and bottom. The hardwood ply should be very sag resistant. Bring your straightedge to the store when shopping for the wood so you can be sure you buy flat stock. I've seen some bad stuff out there.

Hey - if your new top is very heavy making lifting it difficult, maybe you could attach some gas struts like what you find on a car trunk. Then you'd have to coolest router table around. :cool:

Sorry I've missed something. I hope yours gets fixed soon.

Allan

[Dominic Greco]

When I was building my router table, I used (2) plys of 3/4" MDF which I edge banded with walnut and then applied laminte. Since this was the first thing I built, I set it aside to straddle between(2) sawhorses. Bad move on my part since the top sagged over the next two weeks.

I got some great advice from a friend who suggested that I get (2) 1 x 1 x 1/8" thick angles and lag bolt them to the underside (along the along the length). I identified the high spots and then used brass shims and/or washers between the angle and the table. The lag bolts were 1 1/4" long and pulled that tabletop back into alignment in no time flat.

In order to get the table to sit nicely on my newly constructed carcss (alal Router Magic by Bill Hylton), I constructed a frame around the angles enclosing it in a sort of torsion box. It made the top a bit heavier, but who cares?

I've attached a pic of the final assembly. Please forgive the shoddy quality of the picture. this was before I got a digital camera.

Good luck.

Dominic Greco

[Ken Salisbury]

Well - if it isn't "The Dominic of Greco"

Where have you been ?

[Paul Dwight]

I built a torsion box for the top of my router table. I ruined a couple of work pieces when the top of my previous router table sagged without me noticing (until too late!), and I was determined that would NEVER happen again. It has worked. The torsion box top is still as flat as the day I built it, near as I can tell.

My table design is much different than yours, however, and my torsion box is much too thick for your application. Mine's about 4" thick, as I recall. (It's not light, either -- built entirely from 1/2" baltic birch ply.)


I might offer a couple of suggestions, though. First, the "hole" in my torsion box through which the router fits is not square. I shaped and sized the opening so the router I use (Ryobi RE600) would just fit through. This way, the support of the torsion box runs as close to the router as is physically possible.

Second, my router is fastened to a 3/8" thick phenolic plate which is mortised into the top of the torsion box. I was afraid that if I hung the router from the 1/2" baltic birch that forms the top of the torsion box, it would eventually sag. I don't worry about that with the phenolic plate. My plate is removable so I can change bits, but there's no reason you couldn't glue, screw and/or bolt the plate in place. I bought the phenolic sheet from Woodcraft (they sell melamine-coated phenolic sheets for making zero-clearance inserts for the table saw).

If the cost is not an obstacle, and easier alternative would be to buy a solid phenolic table top from Marc Sommerfeld. I just checked that website (www.sommerfeldtools.com) and the 3/4" thick phenolic table top is apparently still available -- part no. 999.501.09. It aint cheap, though -- $199 plus shipping.

Hope some of this helps. -- Paul

[Dominic Greco]

Well - if it isn't "The Dominic of Greco"

Where have you been ?

Hi Ken,
Been working my tail off here at the old salt mines. I've not done too much in the way of woodworking. Other than roughing out some LARGE bowls.

Well, gotta get back to it.

See ya,
Dominic

[Jamie Buxton]

David --
Yes, a torsion box works well in this application. If you're needing flatness at the .030" level, the challenge in making the box will be making it flat. You build a torsion box in stages -- first the bottom skin, then the spacer grid, and then the top skin. Before you glue the top skin on, the box is still kinda floppy. You can warp it and twist it. After you put the second skin on, the box will not move. Any non-flatness present before the second skin went on is forever built-in. Okay, I guess you can plane it out, but it is much easier to build it right. The easiest way to build the thing is to find yourself a gluing table which is as flat as you want the router table to be. Perhaps your tablesaw table is flat enough. Then you just set the box on the gluing table and weight the heck out of it while the glue dries on the top skin.
Hogging a big hole out of the bottom has some effect on the stiffness, but not much. As long as you do have most of the bottom surface covered by the bottom skin, you'll be okay. You might add some extra pieces of spacer grid material to surround the hole fairly closely.
As to whether 1/4" material is good enough for the spacer grid, yes, that will do. Me, I like plywood rather than MDF or tempered hardboard. It is stiffer and doesn't have the threat of cold-creep. I've built spacer grids from 1/8" ply, and they've worked fine.
To make the spacer grid, here's how I do it. All the slats run edge-to-edge. At each intersection, cut a slot halfway through each slat so that they just slot together. If the slot were exact, you might call it a lap joint. However, the slot really is just a clearance slot, and is not a joint. I clamp the slats together and gang-cut the slots with the the tablesaw. A good thing about using 1/8" ply for the grid is that you can cut the slots in the slats with a standard saw blade. It cuts a kerf that is about 1/8" thick, and the nominal 1/8" ply is actually more like .1" thick, so it works out well.
If the spacer grid is 2" on center, 1/4" ply will be good enough for the skins.
One more thought.. Before you glue that second skin on, glue blocking in the interior for screws or what ever you're going to fasten to the torsion box. Then you'll have nice hefty screw holes -- not just the thin skin or grid material.

Jamie

[David Rose]

Hey Dom! It's great to see you wading with us!

First let me say that I built the same setup without an insert plate and fastened the Dewalt 625 to about 1/2" of the top layer and used Rousseau (sp?) rings. My intent, which has worked well, was to raise the top to change bits (from below) and make a coarse setting of the bit. The plug for the router is inside the cabinet too, so that makes things easy. I really like the hinged top for this with this router.

If I cover up much of my bit access area it will make things tight. And they aren't "loose" now. :rolleyes: I guess I could go with a plate and pull the router. I'm getting to be an "older dog" who is trying to learn new tricks, but I don't really like that one unless absolutely necessary. I haven't tried a "bent wrench", but with inserts in place I don't see how they would work.

I've thought about inverting the angle iron and inletting it part way into the bottom of the top. I hate to break the integrity of the backer on the bottom and part of the core.

By the way, for some reason the sag is fairly gradual and runs all the way across and front to back. The "front to back" part is worse for many things like sliding dovetails with a sled. There is little sag, oddly, behind the router. I think that 15x15" unsupported area is just a little much.

Obviously, I'm going to have to make some compromises to get things working. Oh, well...

No apology needed on the pics. They show the idea perfectly.

David


When I was building my router table, I used (2) plys of 3/4" MDF which I edge banded with walnut and then applied laminte. Since this was the first thing I built, I set it aside to straddle between(2) sawhorses. Bad move on my part since the top sagged over the next two weeks.

I got some great advice from a friend who suggested that I get (2) 1 x 1 x 1/8" thick angles and lag bolt them to the underside (along the along the length). I identified the high spots and then used brass shims and/or washers between the angle and the table. The lag bolts were 1 1/4" long and pulled that tabletop back into alignment in no time flat.

In order to get the table to sit nicely on my newly constructed carcss (alal Router Magic by Bill Hylton), I constructed a frame around the angles enclosing it in a sort of torsion box. It made the top a bit heavier, but who cares?

I've attached a pic of the final assembly. Please forgive the shoddy quality of the picture. this was before I got a digital camera.

Good luck.

Dominic Greco

[David Rose]

Paul, I have been looking at the Sommerfeld table. I think it is 24x32 which is a little small for my cabinet, fence and some table specific fixtures. Also, my support bracing is a little weak with a 15x15" unsupported area. It might sag. The money is getting to be less of an issue since I ruined a fixture I was building. I wouldn't dare try rails and styles with it now, and I have some to do.

It just registered on my oldish, slowish brain that the torsion box is going to block router access for bit changes (tilted top) worse than the braces Dominic suggested. :mad:

I'll try a version of Dom's work then call Mr. Sommerfeld and see what he can do.

David

[David Rose]

Jamie, thanks for the torsion box lessons and further info. I built one for a chop saw table that worked out great. But I know only enough about them to get in trouble. They are pretty fascinating to me. It's kind of funny as I've always thought hollow core doors seemed like junk. ;) Opinions can change as needs do, huh?

David



David --
Yes, a torsion box works well in this application. If you're needing flatness at the .030" level, the challenge in making the box will be making it flat. You build a torsion box in stages -- first the bottom skin, then the spacer grid, and then the top skin. Before you glue the top skin on, the box is still kinda floppy. You can warp it and twist it. After you put the second skin on, the box will not move. Any non-flatness present before the second skin went on is forever built-in. Okay, I guess you can plane it out, but it is much easier to build it right. The easiest way to build the thing is to find yourself a gluing table which is as flat as you want the router table to be. Perhaps your tablesaw table is flat enough. Then you just set the box on the gluing table and weight the heck out of it while the glue dries on the top skin.
Hogging a big hole out of the bottom has some effect on the stiffness, but not much. As long as you do have most of the bottom surface covered by the bottom skin, you'll be okay. You might add some extra pieces of spacer grid material to surround the hole fairly closely.
As to whether 1/4" material is good enough for the spacer grid, yes, that will do. Me, I like plywood rather than MDF or tempered hardboard. It is stiffer and doesn't have the threat of cold-creep. I've built spacer grids from 1/8" ply, and they've worked fine.
To make the spacer grid, here's how I do it. All the slats run edge-to-edge. At each intersection, cut a slot halfway through each slat so that they just slot together. If the slot were exact, you might call it a lap joint. However, the slot really is just a clearance slot, and is not a joint. I clamp the slats together and gang-cut the slots with the the tablesaw. A good thing about using 1/8" ply for the grid is that you can cut the slots in the slats with a standard saw blade. It cuts a kerf that is about 1/8" thick, and the nominal 1/8" ply is actually more like .1" thick, so it works out well.
If the spacer grid is 2" on center, 1/4" ply will be good enough for the skins.
One more thought.. Before you glue that second skin on, glue blocking in the interior for screws or what ever you're going to fasten to the torsion box. Then you'll have nice hefty screw holes -- not just the thin skin or grid material.

Jamie

[Jamie Buxton]

It's kind of funny as I've always thought hollow core doors seemed like junk. ;) Opinions can change as needs do, huh?

David

Yeah, that's true. But if you want to upgrade your mental model, torsion-box technology is used all over the place, not just hollow-core doors. For instance, airplane wings are built with ribs, a spar, and a skin; that's a torsion box. Sporting equipment like snowboards or skis are often some version of a torsion-box. High-performance sailboat hulls are often torsion-box (they call it honeycomb after the appearance of the spacer material). And the surfboard is really a torsion-box. It uses styrofoam for the spacer, and a fiberglass skin.