laminating sheets of plywood for workbench

[Zahid Naqvi]

I recently discovered that my work bench top had a bit of sag in the middle. Since it was MDF I ripped it off and am in the process of replacing it with plywood. I've got two sheets of 1/2" and one sheet of 3/4" ply, incase you are wondering the bench top is relatively small 24"X48" (that's all I can fit in my garage). The 1/2" sheets are slightly bent (borg ply :mad: ) but the 3/4" is quite flat. So how do I glue these together and keep 'em flat during the process. I do have 1.5" oak skirting that I plan to add on the front and back. I can joint and clamp this along the edges during glueup.
I thought about stacking them on the floor and putting weight on top, but floors are seldom flat themselves. The problem is with this being plywood I can't flatten it later using a handplane.

Any ideas :confused:

[skip coyne]

I would build a 2x4 frame , well cross braced the size of the bench top then screw the plywood to it .

[Jamie Buxton]

Do you have a jointer, or a jointer plane? If so....

Joint some lumber (maybe 2x4s) so you know they are straight. Screw them to the edges of the 3/4 ply. Leave the heads in the 2x4 so they aren't buried in subsequent steps. Put the assembly down on the shop floor, 2x4s down. The edges are straight, but you need to remove any twist. The old mason's crossed strings method works well to observe twist. Put shims under the corners to adjust. When you get the 3/4 ply flat and unwarped, laminate on the other plywood.

[Jamie Buxton]

Mason's crossed strings....

Run strings from corner to corner, to form a big X. At each corner, put a shim so that the string are not touching the plywood. Make the shims on the upper string one string-diameter thicker than the shims on the other string. If the surface is flat, the strings will just kiss. If you're being really careful, use monofilament line. It has a nice well-defined diameter.

Tension the strings well. In this horizontal application, a sag in one string will throw off the measurement. Actually, you only need to put the same amount of tension in both strings. I pluck the strings and listen to the pitch. If they're the same (and the strings are made from the same material), they have the same tension.

[Zahid Naqvi]

Mason's cross strings :confused:
I may need a visual aid for this.
I do have a jointer, and as stated in my original post I have 6/4 red Oak that I am planning to attach to the top as skirting on the front and back. Perhaps I can use this instead of the 2X4's

[James Carmichael]

You could always rip the plywood into 2-3" widths, lay them up on edge, and put a hardboard top on it. The only problem would be flattening. You could find someone with a big enough drum sander (like a cabinet shop). This would probably yield a more stable top.

[Matt Benton]

I just built a torsion box base for a tool stand I am putting together.

The base is 24" X 72" 3/4 mdf on a 2X4 frame. I can tell you it is as flat as possible and extremely rigid.

[Chris Friesen]

I just built a torsion box base for a tool stand I am putting together.... I can tell you it is as flat as possible and extremely rigid.

This doesn't help if you don't have a flat surface on which to *build* the torsion box...

[Zahid Naqvi]

Since the torsion box has never been mentioned as an option for a workbench top I never really considered it. I wonder how it would perform over a long period after a few years of wear and tear.

[Matt Benton]

Chris,

I have two tables, 30" X 50" each. Laid the mdf across both tables and shimmed the legs of one until the surface was flat.

Zahid,

The torsion box is just structural. If you're concerned about wear, use an oak skirt on the sides and hardboard on top, and replace the hardboard as needed.

[Jim Thiel]

This doesn't help if you don't have a flat surface on which to *build* the torsion box...

The idea is to made a flat surface to made a flat surface. a pair of shop benches, some shims, and a bit of time and you can make a flat location for building flat tables.

Zahid, was the MDF supported over the 4' length? How many plys and how great was the deflection?

I would bet that a torsion box would work well, I just prefer the mass of a solid top.

Jim

[John Bush]

Hi Zahid,
I used two layers of 3/4 ply screwed together with a masonite top and solid edge trim. I made sure the base was true, screwed down the first layer of ply, making sure it was flat , then screwed on the next layer. I then screwed on the masonite so I can replace it as it wears. Glueing together wasn't necessary and made it easy to contol the flex of the (cheap) ply. Much easier than glueing and I can undo it prn. Good luck,John.

[Lee Schierer]

Wow, pretty elaborate suggestions for making a flat bench top.

For most purposes, I would think you could find a 2' x 4' area of the floor in the garage or house that is flat enough. Use a 2 foot level (four footer if you have one) and check a few areas of the garage floor for flatness or use a floor in the house. When you locate a suitable area, glue up the pieces and if they are cupped, put the cups face to face and put a lot of weight on them. Once the glue dries they should stay pretty flat.

If you own a pick up, mini van or SUV the bed is probably flat enough too.

[Jim Thiel]

...flat enough.

Flat enough??? Heresy!!! :)

Any opportunity to needlessly complicate an otherwise simple operation is an opportunity taken!!

Besides, I was offering a chance to build some benches for building a flat spot for building a benchtop. :)

Jim

[Greg Funk]

Mason's cross strings :confused:
I may need a visual aid for this.

Here are some excellent instructions for building a torsion box including 4 methods (including crossed strings) to ensure a flat top:
http://www.geocities.com/bawanewsletter/jun02/torsionbox1.pdf

[Russ Filtz]

That's a great tutorial! Would have taken me awhile to figure out the "X" method. The only caveat I would add is to make sure the tension in the two legs of string are at least close to the same. If one has a lot more tension than the other it will sag less and possibly mess up the flatness. It's probably not that critical as long as the tension is decent enough so they appear flat.

[Ron Robinson]

I used a solid core door for one of my benches. So far it has been very stable.

Ron

[Zahid Naqvi]

Here are some excellent instructions for building a torsion box including 4 methods (including crossed strings) to ensure a flat top:
http://www.geocities.com/bawanewsletter/jun02/torsionbox1.pdf

Thanks Greg, just what I needed to get through my thick skull :D

[Zahid Naqvi]

I used a solid core door for one of my benches. So far it has been very stable.

Ron

I would too if I had space for a 4'X8' table, I have space for only 1/4 of that.

[Zahid Naqvi]

Zahid, was the MDF supported over the 4' length? How many plys and how great was the deflection?

Jim

Jim, the MDF was sitting on top of a 2X4 frame. The sag was about 1/16" but that is enough to throw off any flattening effort from a jointer plane. I later found out the M&T between the front and back rails and legs was not really alligned. Actually the top of the legs was sticking up about 1/16" on one side. I have that corrected now and am using the same base. In a way I do have a flat reference surface except for the void in the middle

[Zahid Naqvi]

I think I'll just dry clamp everything and stack it on top of the frame and use the cross strings approach to determine how flat or unflat the top is. I can easily plane or shim the frame to make appropriate adjustments. Since the top has to sit on this frame I think it makes sense to build it on top of it as well.

Thanks for all the wonderful ideas guys.

[John Lucas]

... The 1/2" sheets are slightly bent (borg ply :mad: ) but the 3/4" is quite flat. So how do I glue these together and keep 'em flat during the process. I do have 1.5" oak skirting that I plan to add on the front and back. I can joint and clamp this along the edges during glueup.
Any ideas :confused:

Zahid, some great ideas above but I suggest the very simplest. The table size is 2 x 4 so use you table saw surface as the "flat" even if it is smaller. You are going to be clamping the outside area probably better than the inner so configure your layers so that the bend is up on the outside. Place one 1/2" down on the TS, the 3/4" and then the other bent 1/2 on top. Use 2 x 4's as cauls whan you clamp the top bent one. Use the Titebond cold press glue if you can get it to get a little more leeway for gluing up. Leave it over night.
When unclamped, it should be relatively flat. I would not apply the outer edge yet. I would use some steel corner stock. It is readily avail at the home centers and in the 1" variety will be pretty flat and hold that. Use that around the edges. DOnt use screws but nuts and bolts. Let your outside oak cover that.

[glenn bradley]

My TS was the flatest surface in my shop. When I laminated my benchtop, I covered the TS with wax paper and used it as a reference surface. As I could not clamp across the surface, I used weights to apply pressure.