Wood-fired!

Base and Lid - Part 4 of the Wedding Present

April 17th, 2008

This continues the tips and techniques for the document box known here as the Wedding Present. This post deals with the bottom panel and the lid.

The base

Before the carcass was glued-up a slot was cut on the lower inside of each side piece (taking care to stop at the tails).

Slots: The lower slot is the groove to take the ‘tongue’ of the base panel. (The other slot is for a narrow rail that will support the gondola.)

The base is a floating panel. This allows the base to expand and contract across its width with changes in humidity. The panel was prepared by edge-joining two pieces and thicknessing to 6mm. A rebate was cut on each edge to match the slot cut in the sides of the box. A very slight bevel was put on the edge that will be visible from the bottom.

Base panel: A rebate is cut to leave a tongue that is a firm fit in the thickness of the slot. The depth of tongue leaves 1mm each side for expansion across the grain. (The panel is shown upside down - the tongue is flush with the top surface.)

This diagram shows the arrangement. The orientation is chosen to give a continuous closed surface inside the box. The underneath will show the narrow gap between the base and the sides of the box that allows for expansion of the base. The small bevel relieves the sharp edge that would be visible.

The base panel was sanded and fully finished (with nitrocellulose lacquer) and fitted before the final glue-up of the carcass.

The lid

The lid is a floating panel in a narrow frame. The frame is mitred at the corners and strengthened by floating tenons. The same tongue and groove method was employed to hold the panel as for the bottom.

The mortice for the floating tenon is made with a slot cutter mounted in a router table. Marks on the fence tell me where to start and stop the cuts.

The frame was made before the bevels were put on the outside edges so that the maximum cramping area was available. The chevron-shaped floating tenons are visible in the following photo.

Clamping a frame

The setup used to glue the corners of the lid frame. Note two of the chevron-shaped floating tenons in the middle.

I use this method for nearly all frame and box construction where the corners are mitred. A scrap of wood (usually MDF) is glued to the outside faces of the members so that good cramping pressure can be securely applied.

The brief account of the process is:

1. cut 45° triangular blocks of the same thickness as the elements
2. glue the long side (hypotenuse) of the blocks to the frame element - mark or estimate the centre of the joint and place the blocks so that the clamping pressure can be applied directly across that centre
3. glue up and cramp the joint
4. saw away most of each cramping block when the joint is dry - taking care not to touch the frame
5. plane the outside of the joint to remove the remaining waste material

Plane the remains of the cramping block from the lid.

Now this seems like a lot of effort. However weighed against the risk of not getting a good joint in a frame or box I believe it is well justified. (Anyone who has tried to fudge the cramping only to have it fall apart in the middle of glue-up knows that this is a good bet.) I have used this technique on dozens of projects. It is the best way to guarantee a minimal glue-line and joint longevity.

I usually use yellow glue for this. Some people apply only a dab and don’t rub the components together in the hope that the block will snap off afterwards but this carries its own risks. I have also tried using double-sided tape, but the risk is still there and has certainly bitten me. My advice is to glue it on strongly so you’re not afraid to use plenty of cramping pressure and just plane it off afterwards!

I always use tape to mask the faces in the inside corner of the joint to aid in cleaning up the excess glue (as described here).

After two corners have been glued and cleaned, the finished panel is inserted and the last two corners are glued at the same time.

Gluing the last two corners of the lid with the finished panel in place

The completed box lid.

The lid after bevelling and finishing.

Part 1 The present
Part 2 The wood
Part 3 The dovetails

Entry Filed under: Wood