Torsion Box for Sawstop extension table

[Tom Dovan]

Hello,

I am new to the forum and this is my first post. I am installing an Incra PRL-2 router lift in my 52" Sawstop extension wing. To help with deflection I am planning on building a torsion box. The center of the lift plate will be offset 7" from the edge of the extension wing (adjacent to the cast iron sawstop table)

The box dimensions will be 27" x 29" and will have an opening in it to fit the router and lift (the lift plate will sit in a recessed cutout in the sawstop extension wing).

The torsion box will be attached to the under surface of the extension wing.

Any suggestions on the thickness of the skins and the internal web? I was planning on 1/2" plywood skins and web, with the webs being on approx. 5" centers. I'm wondering how much the opening for the router will weaken the torsion box and if I should use 3/4 plywood instead.

Thanks in advance for any help!

Tom

[Bryan Lisowski]

I would be concerned about the thickness of the Torsion box being able to work with the router lift. If the top is too thick, will the lift be able to get the bit above the table height? Might be better to laminate 2 pieces of 3/4 MDF and then put laminate on both sides.

[Tom Dovan]

I was planning on routing out a recess in the sawstop extension wing for the router plate, so the lift plate will sit flush with the sawstop extension wing. I would build a cut-out in the torsion box to allow room for the router lift and the router. My concern was - will this cutout weaken the torsion box siginificantly and thus should I use 3/4" plywood vs 1/2".

Thanks

[Howard Rosenberg]

It's not the faces of a torsion box that maintain its flatness and rigidity. It's the height of the ribs and their spacing.

You didn't say how high (thick) you'll make the torsion box.

Make the ribs as tall as you can. Can you go 3.5"? Or even better, 4"? Then you can use 1/2" plywood for the top face with no prob. But I'd still use 3/4" plywood for the ribs.

And even though it's more work and uses more materials, go to 4" spacing for the ribs and you can easily miss the small area in the bottom skin for the router with no deflection.

Howard

[Tom Dovan]

I can definitely go 3.5-4" on the ribs and decrease the spacing I'm trying to get this done with one sheet of plywood so if I use 3/4" for the ribs I'll use the 3/4 for the skins also. I assume that would be more rigid than two 3/4" MDF boards laminated together?

Thanks

Tom

[Ruperto Mendiones]

What about using resin-treated corrugated cardboard core? Using 1/4" one-sided melamine on both faces, a 4' section, supported at the ends on sawhorses, accommodated 250 lb load at the center.
The corrugated core saves a lot of labor cutting ribs. See Fine Woodworking May 2011, issue 219.

[Robert Engel]

Yes, a couple sheets of good quality ply or MDF laminated will work fine.
IMO there's no need for a torsion box on a top this size.

I made my router table (24x30) out of a double thickness ply with double laminate on both sides and its as dead flat as can be.

My advice is save the effort for something like an assembly table.

[Tom Dovan]

If I use double thickness 3/4 plywood, do I need to add a formica type laminate? This will sit under the sawstop wing which is already double laminated. I'm using it as a support. Sort of a buttress for the sawstop extension wing. I was going to attach it with screws from underneath. Two sheets of plywood would be a lot easier!

Thanks for all the replies!

Tom