Vacuum panel flatness

[william watts]

On the shop tools form I saw Glenn Bradly's out feed table/storage unit he made, and decided I need one similar. I don't think I can build a flat torsion box or find plywood flat enough for the table top. The question is if I laminate ply wood, maybe 3 pieces of 1/2 in. ply, with masonite on both sides in a vacuum bag press will that ensure flatness? Pressure will be equal on all sides so there shouldn't be any curve or distortion to it. I have done several smaller panels for cabinet doors and veneer work all came out flat. Size will be about 30x 48. This project is down the road aways, I am installing a dust collection system and a new table saw in 1/2 my 2 care garage.:)

Sorry, meant to post in the general wood working form. Moderators are welcome to move it

[Gerry Grzadzinski]

In a bag, there's no guarantee that anything will come out flat.
But if all the panels are flat going in, there's a fairly good chance that they'll stay flat, as long as there's even pressure in the bag. The question is, how flat are they to start with?

[Tim Janssen]

Could you not use MDF rather then plywood. It is very flat. I used 3 layers of MDF to make my workbench and used masonite as a sacrificial top. The top is perfectly flat.
Cheers.

Tim

[william watts]

Thanks for taking the time to reply. Gerry I see how equal pressure on all sides won't change anything, I really thought I was on to something. Tim's suggestion is reliable and is what I will do. The problem that I have is making the under support flat enough for even support thru the entire surface.

[John W Johnson]

Thanks for taking the time to reply. Gerry I see how equal pressure on all sides won't change anything, I really thought I was on to something. Tim's suggestion is reliable and is what I will do. The problem that I have is making the under support flat enough for even support thru the entire surface.

First you need to cut your skins to the size of the box. I used 1/2" MDF and my tops are still flat after four years. Next you cut and plane your web pieces.

To get your level, flat base select a spot you can complete the project without moving anything. Start with a saw horse base that is at least as wide as your box. Level them side to side, use wood door shims under each leg. Then plane 2-3-4 quartersawn looking studs flat and equal width (the number depends on the width of your box, I used 3 for 24" & 28" wide tops). They should be = the length of your box. Level them end to end and side to side on top of the saw horses, again using wedges. Then put one of the skins on top of the 2 x4s. You should now have a level, flat base to start building your webbing. Glue and tack the interior pieces together, then glue the top to the webbing and hold it in place with brads. Flip the assembly over and repeat, glue and tack on the skin that started as your base. You need to work pretty fast, those 2x4s won't retain their flatness very long. I made two torsion boxes and had to re-plane mine when they sat overnight to make the second box.

[glenn bradley]

Thanks for taking the time to reply. Gerry I see how equal pressure on all sides won't change anything, I really thought I was on to something. Tim's suggestion is reliable and is what I will do. The problem that I have is making the under support flat enough for even support thru the entire surface.

It is a challenge to make a flat laminated item when you lack a flat surface big enough to reference from. In my build (I don't know if I posted those pics) I used layers of tape to shim some areas of the surface I was using in order to get my first layer dead flat. A row of jointed board across saw horses is another method folks use to do torsion boxes and this would apply to large laminations as well. The kitchen table shimmed with strips of tape could also serve since this is not a very messy glue up.

I used weights to "clamp" each layer individually as I have had problems in the past with clamps and cauls distorting my assembly of such things. The method I used was to cut the top to exact size, place it face down on my surface and shim to flat. Then add the next layer about 1/8" over-sized in each direction and weight it down. Once dried overnight, I routed the oversized edge flush. I then did the same with the tempered hardboard; over-sized, routes flush.