Flattening Oversize Table Tops

      If a table top is too big to fit through the wide-belt or drum sander, how do you get it flat? Here's an extended discussion with several valuable suggestions. June 29, 2008

I need some advice for making table tops. I've done a few in the past and they were not too complicated. Good clean, square edges, biscuit joints, clamp them and go. My dilemma is the initial sanding to level the top. I have an 18" open end drum sander, so on most projects, two passes through and I'm pretty much there (a pass on each side).

However, I have one in-house now that will be a 5' round and the only thing I can come up with to do the initial leveling is using a belt sander. I shiver at the thought of it. In a normal top shop what would be the tool of choice for this?

Forum Responses
(Architectural Woodworking Forum)
From contributor P:
I make at least one table top a week and have done many that size and larger. Don't use the biscuits, glue up the top and set in parallel jaw clamps. Tighten the clamps to just when the glue starts squeezing out and go along each seam with the handle end of a dead-blow hammer tapping the seams level with each other. Tighten the clamps a little more and go through them again, making sure they're flat. Then tighten them down to whatever pressure you feel is appropriate and re-check them again.

I don't have a wide-belt or a drum sander and have yet to need one using this method. A good r/o sander is all you need to flatten the top. It's important to use parallel jaw clamps, and good ones. Pipe clamps and bar clamps are okay, but much more apt to bow the top, even if you do stagger top and bottom.

From the original questioner:
Thanks Contributor P. I have a habit of over thinking some projects which also leads to overkill on the construction. This will be a whole lot simpler and easier to manage then my original idea.

From Gene Wengert, forum technical advisor:
As a point of information, biscuits do not add substantial strength to an edge-to-edge glue joint. They are useful for positioning and that is it.

From contributor V:
I use to use an inline air sander like the body shops use with 60 grit to flatten and level the top then finish out like normal with random orbit.

As for the drum sander you can glue up in sections that will fit, sand and the join the sections together. I have a 51 inch drum and a 37" wide belt now and sometimes still run into that problem.

From the original questioner:
Funny but in the past I've questioned the biscuit for strength, mostly asking suppliers, which might be my biggest source for misinformation. The answer I get is that it increases surface area for glue joints and the tenon aspect gains a stronger joint. Yet, when I glue up without them I'll break the grain before breaking the joint. So I agree 100% with you.

From Gene Wengert, forum technical advisor:
Thanks for your agreement. Figure it this way - a 1" x 30" edge-to-edge joint is 15 square inches. A biscuit, 1/4" thick x 2" in length would add about 1/2 extra square inch but will subtract 1/2" square inch from the edge-to-edge area. If the biscuit was 4" wide, requiring a 2" deep slot on both solid wood pieces, will you get any more strength from the joint than if the biscuit was only 1/4" wide?

If you think about it, the fact that the biscuit is held tightly in the slot just means that it is acting like the wood that it replaced. Having said all this, if we consider a joint with end grain (such as a 45 degree miter joint), then we have a weak joint initially as end grain does not glue well, so tenons, dowels, etc. will help.

From contributor T:
I agree with Contributor P. I don't have those big-buck clamps though so I just use bar clamps over and under with the glue-up on lifts which I shim parallel with each other. To flatten, I use a 4" belt sander at two diagonals, then straight across the grain and only after there are no ledges do I sand with the grain and go through the grits.

From contributor N:
We glue up our larger panels by first surfacing the wood so all of the stock is the same thickness and running a glue joint bit to help with alignment. You can find them at Rocker, Woodcraft or just about any store that sells router bits. This does not necessarily add any extra strength or gluing surface area but does help for nice, even alignment across every board without really expensive clamps. Careful setup will allow you to go right to RO sanding after it's set.

From contributor R:
I've been in the business 35 years and would never waste the time with biscuits and especially that glue joint bit. Butt joints are plenty strong. Curved cauls to line the pieces up and a ROS will do the job. I'm still a big fan of a stroke sander over a drum sander and even a widebelt for shops like yours with door, panel and table top making. A small widebelt isn't as versatile as a stroke sander in my experience.

From contributor J:
Biscuits are a waste of time on edge/face glue ups. They may help some on end grain glue ups but that would be it. Also the comment on using the dead blow hammer to even things up is probably not the best idea. Once the joint is tight it should not be bothered with until it is dry. The staves need to be aligned best as possible before the clamps are tightened. We are strictly an edge glue panel manufacturer and I am always on my guys about beating staves flat as it is not a good practice. It will weaken the final joint.

From Gene Wengert, forum technical advisor:
I strongly support Contributor Js comments about flattening after pressure. In fact, if your edges are not 90 degrees to the faces, you can expect open joints. So, rather than hammer, spend time getting proper edges. If you are ripping cupped lumber, the edges will not be square. This is why rough lumber is planed first. This planing develops a flat reference surface for ripping. Planing at this point need not be 100% clean faces however; just enough to get a mostly clean face so that the piece will rip without moving.

From contributor F:
I agree that hammering joints could be bad but it isn't totally out of the question. It depends on the timing and temperature, etc. When the glue is still extremely wet you can adjust a joint any way you want. Like Dr. Wengert says, flatten your stock on the face first, then work the edges straight and square.

When this is done, you can adjust the face alignment with finger pressure as you incrementally tighten the clamps.

I find that using very light pressure adjustments on the clamps is the key to keeping the boards from slipping out of alignment. Tighten each clamp a tiny bit at a time and make adjustments of board face alignment in between tightening.

From contributor M:
I make curved wooden tops that are too big and heavy for stroke sanders and wide belt sanders. I was puzzling over how to flatten my first prototype one day (15' long) while I simultaneously had a flooring crew work on a new floor next to my shop. I borrowed their edging sander which has a 7" disc and wheels to keep it flat. Once you get the hang of it, it does a wonderful job of flattening and it's fast. Just a little hard on the back. They rent for about 40 dollars a day at any tool rental. I think it was a Clark model. I finish sanded with a 6" Random Orbital sander which also went fast because the flooring sander did a good job.

From contributor C:
One note of caution to the hammering crowd - forcing a warped board to lie flat in the clamps will result in an unstable panel. After ripping, jointing, and rough planing my stock, I let it sit at least overnight before I go to the final glue jointing. Any piece that moves doesn't make the grade.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor I:
Your table top is your problem. If you are going to be making table tops then go out and buy the right tools. A 50" double drum sander, 15"planer, 12" jointer, and a good table saw with the right blades will do the trick. You have a perfect fit and no problems.

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