Edge-gluing flat panels
Tuesday 13 July 2010
If you have ever spent hours hand planing a glued-up panel because each board was one thirty-second to one sixteenth non-flush to the next, you will appreciate the need to improve your gluing technique. I have shown many students in my workshop that I can often achieve a total flatness within 0.003in or better straight out of the clamps for a panel of perhaps 16 x 32in. These days that rivals the flatness of cast-iron tops on tablesaws, especially those imported at unusually low prices.
Out-of-flatness of 0.003in or even 0.005in is easily remedied by hand plane, followed by finish sanding as necessary. The goal, then, when gluing up a panel, is not only to keep the boards as flush as possible at joint lines, but also to maintain general flatness across the panel.
Correct milling procedure with jointer and thicknesser goes a long way to achieving flat panels. In fact errors over the length of the panel are usually very small indeed if original milling went well. However, small mistakes in the angle at which you milled the edges of the boards can result in a panel that is badly cupped across its width.
Imagine an error of just one degree in each edge, but compounding in the same direction on every single edge in the glue-up, not that you would be quite that unlucky. Such a result would leave the panel badly cupped even over relatively short distances.
You may have learned to hand plane two mating edges at the same time with boards placed in a vice in a butterflied manner. This ensures mating edges add up to 180 degrees even if hand planed somewhat off 90 degrees to the faces. This does, indeed, work if your hand plane blade is wide enough to handle the job of two boards at once, and I use this theory to improve the edges of all my boards on the jointer just prior to a panel glue-up.
While I normally mill one edge on the jointer and the opposite through the thicknesser to ensure parallelism, this special technique involves using just the jointer for all edges.
Joint the right edge of the first board with its top face against the jointer fence, then joint the left edge of the second board with its bottom face against the fence. Following this method means that a one degree error in the jointer fence setting, off 90 degrees, is cancelled out, allowing the two boards to form 180 degrees when joined.
This technique means every second edge is jointed against the grain, but I make exceptionally fine cuts, perhaps as little as 0.004-0.005in, to prevent tearout.
This fine cut will really test the accuracy of your jointer because it cannot be made consistently without very good parallelism of your jointer beds.
Pencil marks made across the board edges in numerous places ensure you do not miss a spot when jointing. A small change of angle can force you to miss an entire strip near one corner when making such fine cuts, which you cannot tell by sound alone. The pencil marks tell the story, convincing you that a second pass is necessary.
During a panicked glue-up with so much to do in so little time, keeping all boards perfectly flush while securing the clamps is a very tall order.
For years now, I have used wooden cauls clamped across the faces of the panel to keep boards as close to flush as possible. Some duct tape on the cauls prevents them from being glued to the panel, as regular wood glue does not stick to plastic.
F-clamps sandwich the panel in between the pairs of cauls, removing one major headache from the task.
I place one set of cauls near the centre of the panel and another at the extreme ends of the panel, flush to the shortest board. Remember that boards are typically staggered at the ends, attempting to achieve the best possible grain match on the face. The panel is trimmed to final size only after the glue-up is complete.
Three pairs of cauls work well for panels perhaps 35-40in long. Any longer and I would introduce a fourth or fifth pair for increased accuracy.
Only after cauls have been secured tightly do I begin to add pipe clamps to pull the boards against each other. I find pipe clamps to be best for the job as they can exert a great deal of pressure with very little effort.
I start by securing one clamp at the middle of the panel, then secure the rest, moving outwards from the centre to allow gaps to be pushed towards the outside of the panel.
Placing outside clamps on first and the middle ones after is much like pushing an air bubble around underneath wallpaper. I suggest one clamp every 8in or less, assuming the boards are at least two-and-a-half to 3in wide.
I believe pipe clamp placement has a lot to do with the final flatness of your panel. Firstly, all clamps exert more pressure on the bar or pipe side compared to the open end where the jaws cannot open slightly under load.
For that reason, placing all clamps on one side of a panel will result in a curve.
I place one pipe on top, the next underneath and so on, alternating from top to bottom, with the threaded screws centred on the thickness of the stock.
This means varying the amounts of space between the pipes and the panel depending on the thickness of the panel. If a panel is so thick that the threaded screw of the clamp cannot even reach the centre of the panel thickness, this indicates that two clamps are needed at each clamping point. You can stagger the clamps if the panel is not thick enough for two clamps to fit one above the other.
3 tips for success
1. Use the butterfly technique to remill every edge on your jointer just prior to glue-up. This gives the glue a freshly milled surface to soak into, improves the straightness of each edge and, most importantly, ensures 180 degree meeting points from one board to the next
2. Use wooden cauls to keep boards flush during the glue-up. This prevents the boards from sliding around due to slippery glue surfaces
3. Use enough clamps to squeeze the boards together, alternate the clamps from side to side and centre their threaded screws on the thickness of the stock