Wide belt sander - Sanding a thick top to remove bow?

[andrew whicker]

Hi all,

I have a big table top: 2.5" thick, 40" wide, 8 ft long

It ended the glue up with a bit of a bow (about 1/8" low in the middle of the top facing side). Is it possible to take this bow out with a wide belt sander? I've heard people use these as planers, but that would mean that internal rollers would press the piece flat. Are wide belt sanders as 'strong' as planers? Will the bow be exactly the same when it leaves the belt sander?

The woodshop I go to from time to time uses a big TimeSaver. I can drive over and ask, just thought I would get the gist from the interwebs first since I'm at work.

cheers,

[Mike Hollingsworth]

I do it all the time at a smaller scale with my 20" WBS.

[Patrick Varley]

Is it truly a bow? In other words, is the opposite side convex? If it is, it means you're going to need to flatten both sides. When you do this in a planer, the way to do it is to build a planer sled so that you can shim the top against a flat reference surface (the sled) and prevent it from deflecting as it rolls through. Once you've "jointed" the once face you can flip it and run it through without the sled to plane the opposite face flat and parallel.

I suppose you could do this for such a large table top, but I think the easier option is probably a router sled and some rails. The video below gives you the details if you aren't familiar:

[J.R. Rutter]

The hold down rollers won't be strong enough to flatten out a 2.5" thick top. The sanding drum pressure can though, so I would plan on some light passes with coarse grit to get things started.

[andrew whicker]

Awesome! Thanks.

[Floyd Mah]

Can you cut the table in half and re-glue it after flipping one of the halves end for end? I ran into this problem after I did the glue-up for my workbench. A huge bow. Luckily, either surface was good for the top, so I cut it and did a flip. This kept me from losing too much thickness when I flattened it. Also, if the tops are chosen specifically to be the top, you can also cut it in half and plane/joint the gluing surfaces to get a flat surface. These are options if you didn't want to lose thickness by planing.

[Peter Christensen]

I’m only guessing since I don’t have wide belt experience just a small drum sander. I would get a 3/4 or 1” sheet of MDF and put shims wherever needed under the slab and run that through the sander. When the first side is done run the second side without the MDF.

Bring lots of spinach.

[andrew whicker]

I'll have to take some photos, but I'm thinking of re-gluing anyway. The top is three pieces. The two outside pieces are live edges and so they can't move. The middle piece can. They were all big slab pieces before I cut them down.

I accidentally glued all the pieces with the growth rings in the same direction, if you know what I mean. The piece could warp significantly. So maybe best bet is to flip the middle piece end for end anyway. The bow immediately after gluing is the same bow of the growth rings, so it could get worse.

I do live in Utah, so I think that gives me a bit of a break on the humidity factor.

[andrew whicker]

Here are the three pieces. The bow is such that the middle is lowest. I haven't removed the glue from the bottom yet. I'm assuming it's a proper bow.
0625181527.jpg

[Wade Lippman]

I've never done this, but...

I would would put a 1/8" shim under the center of the bow and then sand it. The shim should keep the board from being forced flat and just sand the top off. Then flip it over and sand off the ends.

Well, it ought to work.

[Martin Wasner]

Throw it on a cnc and hit it with a fly cutter.

It won't come out totally in a widebelt. Mostly, but not completely.

Plus feeding that thing through a widebelt twenty plus times is painful. It probably weighs over 200#

[Kevin Jenness]

Nomenclature quibble: a curve across the grain is cupping; a curve along the length is a bow (or crook, if the edge is curved).

Is the cup due to inaccurate edge jointing (are the individual pieces flat?) or to change in moisture content? If the planks are convex on the sap side, they are gaining moisture. This seems unlikely given your location, but possible depending on how they were dried and stored. There are some major end checks in your photo that indicate a possible drying problem. Have you checked the MC?

[eugene thomas]

I would do the shims trick get one side flat then flip and sand the other side.

[Leo Graywacz]

I've never done this, but...

I would would put a 1/8" shim under the center of the bow and then sand it. The shim should keep the board from being forced flat and just sand the top off. Then flip it over and sand off the ends.

Well, it ought to work.

I do this all the time. I do it with a sander or a planer. You still do light passes. You can tape the shim down, but that has a chance to move. Or use 2P-10 or other quick glue to spot tack the shim down. Just be aware that the bow may not be equal going down the board. You may have to taper the shim. Use a straight edge to determine you have the shim to the proper thickness to get the top flat.

[Jim Andrew]

I just have a small open end widebelt sander, but am amazed at how much it flattens panels. The shim does seem like a good idea.