Gilles' Outlet

April 9, 2007

Gluing Panels

Filed under: Woodworking — Gilles @ 5:42 am

The edges of two 3/4” x 6 poplar boards are machined on a router table with a glue joint bit. The boards are fitted and glued together to create a 3/4” x 12 panel. After drying, the panel is sanded smooth. 


Skill Level: 2-3 (Basic – Intermediate)

Time Taken: About Two Hours

In furniture making, it is common to need solid wood panels wider than available stock. Doors, table tops or wooden cutting boards are canonical examples.

There are various techniques to construct a wider panel out of stock. One can plane boards first and then use a a jointer to ensure edges are flat, square and true. Boards can then be glued butted one to another and held with clamps as glue dries. This creates strong joints. Biscuit jointery produces even stronger joints.

Schematically, the strength of a glued joint increases with the glued surface of the joint. In this article, a glue joint router bit is used to produce a very strong glued panel.

Left: the two poplar boards which will be glued together. I selected boards with similar grain pattern and marked the best looking face "UP".

To select boards for a glued panel, I usually put them side by side on a flat surface and pair the ones which have flush (or mostly flush) edges. Selecting straight boards with square, true edges is critical to final visual aspect of the glued panel.

In this case, my boards matched almost perfectly.

Right: the glue joint router bit. It came as part of a six piece raised panel and drawer set I purchased from MLCS Woodworking.

It creates an edge which offers as much as 50% more glue surface than a simple butt joint.

MLCS Woodworking’s web site offers a free 33 pages documents which explained that:

"The height of the bit is adjusted so that the center of the joint on the cutter is centered on the thickness of the wood".

I centered what I thought was the center of the bit on the stock and cut a few joints with very little success. Once jointed, boards were off by as much as 1/8”. In addition to that, you need a flat edge for every test cut. A previously cut edge cannot be reused. 

Obviously, setup instructions were not helping me much. I guess glue joint bits are not popular because few seem to be able to set them up quickly and accurately.

Eventually, I figured out the right way to set up the bit.

This picture shows a magnified view of the the cuting edge (red outline on the left). The tooth-shaped portion of the cutting edge (marked by the green arrow) is not centered on the bit vertically.

The actual center of the bit is at the middle of the upper angled edge of that tooth-like projection (marked by the burgundy line labelled "center line").

So the center of the stock must be aligned with the center of the angled edge (aka "center line" on the diagram). In practice, this is not as easy as it sounds because this angled area is tiny. Clearly, the setup of the bit depends on the thickness of the stock

When adjusting the height, remember that if the bit is off center, by x”, the joint will not be flush on both sides by x / 2 ” so you want to lower or raise the bit by small increments. 

It takes a fair amount of time and patience to adjust this bit properly the first time. When I reached a satisfactory setup, I cut a template in 3/4” stock and saved it for future use.

I can now use the template to quickly adjust the bit’s height. I simply lay the template on the table and raise or lower the bit so the template fits exactly in the profile of the bit.

Left: a piece of scrap wood of the same thickness of the stock (3/4”) was clamped at the end of the board. This helps preventing wood chipping at the end of the board.

It also provides additional support when routing at the end of the workpiece. The glue joint bit I am using is not equipped with a bearing. Without this support, the end of the workpiece would sink in the bit, creating an uneven joint.

Right (re-enactment): boards were routed using the clamp as a push handle. The featherboard (blue piece of plastic left of my hand) should be in contact with the workpiece.

In order for the tongue on one board to fit in the groove on the other board, one of the workpiece must be flipped over. I routed one board with the "UP" sign down (on the table) and the other with the "UP" sign facing up so when assembled both faces marked "UP" would end up on the same side.

Joints were lightly sanded with 400 grit sand paper to eliminate all wood fibers which could prevent the joint from fitting perfectly.

Any wood dust was removed with compressed air.

Left: glue was applied on one side of the joint. The glue was spread evenly on the joint using a clean plumber’s flux brush. Boards were assembled and clamped in place.

A wet rag was used to remove as much excess gue as possible. Glue oozing out of a joint is a good sign: it indicates the glue filled the joint. 

Right: the clamped panel. While clamps need to maintain pieces tigthly, too much pressure is not recommended. 

I let the glue dry overnight. The next day, I removed all clamps and used a power finish sander to smooth the glued edges with 120 grit sand paper. I also sanded off all traces of dried glue on the panel. Dried glue prevents finish to stick on the wood and this creates pretty ugly stains on the wood.

In my experience, even the most perfectly adjusted joint requires some (minimal) sanding.

I then used 400 grit sand paper to give the panel a mirror like finish.

Tools Used:

  • Table Mounted Router
  • Glue Joint Bit
  • Speed Square
  • Power Sander
  • Clamps
  • Plumber’s Flux Brush 
  • Basic Carpentry Tools

Materials Used:

  • Poplar 3/4 X 6 – Two boards of 17”  long
  • Carpenter’s Yellow Glue
  • Sand Paper: 120 and 400 grit

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