Floating Panel

Two frame-and-panel doors on a joiner's bench: as moisture changes, the panel left free in its grooves slides and survives, while its glued twin cracks as it dries.

Floating panel: free to swell and shrink across the grain inside its groovesGlued twin: fixed at both edges, so it splits as it driesTurn the knob from oven-dry to soaked. The floating panel widens and narrows across its grain: watch the gaps at both ends of the groove where the near rail is cut through, and the strip of tongue between the field and each stile. Its length along the grain stays put. The glued twin is held at both edges, so once it dries below the moisture it was glued in at, it splits down a grain line from its end. Between oven-dry and soaked, flat-sawn stock moves roughly 5–10% across the grain and quarter-sawn about half as much; the readout scales that range to the knob. That range is far wider than the seasonal swing, which is several millimetres across a wide panel. Tap the glue brush (or press G) to glue in a new twin at the present moisture. Illustrative, not from the guide: the movement is drawn four times larger than life, with grooves cut deep enough to take it; the 250 mm panel; both panels fitted at mid-range; and the twin splitting at 1% restrained shrinkage. The knob and the hair hygrometer are a bench-model moisture control and instrument.

What you're looking at

A frame-and-panel door is the classic way to make doors, chests and wall panelling: a wide solid board (the panel) sits in a groove cut round the inside of a frame of stiles and rails. Two small doors lie on the bench; the one at the back left has a floating panel, and its twin at the front right has the same panel glued fast into its stiles. Each door's near rail, along its front edge, is cut through lengthwise so you can see the groove and the panel's thin edge (its tongue) sitting in it. At the right stands a hair hygrometer (a moisture gauge) with two small end-grain sample blocks and a peg in front of them, and the moisture knob on the stand's front corner; the glue pot and brush stand by the twin at the front left, and the panels, crack, peg and gauge needle are the parts that move.

What it means

Wood swells and shrinks with moisture for its whole life: roughly 5 to 10% across the grain between soaked and oven-dry, and almost nothing along it. Over one summer and winter that means a wide panel changes width by several millimetres, and the guide warns that a panel fixed rigidly at both edges will split. Letting it float in grooves solves that completely, which is why every good door and chest is built this way. Quarter-sawn boards, cut so the growth rings run edge to edge, move about half as much as flat-sawn ones and stay flat, so they are the choice for anything that must stay true.

Try this

  1. Drag the moisture knob right or up to wet the wood, left or down to dry it; with the figure focused, use the Left and Right arrow keys (Shift for bigger steps), Home for oven-dry and End for soaked. The gauge needle swings, the floating panel widens or narrows, and the gaps at both ends of its groove close or open in the cut-away rail, while the panel's length stays put; the readout's percentages climb or fall. Wood moves across the grain, not along it, and the groove gives that movement somewhere to go.
  2. Press G for a fresh glued twin (it starts out split when the page opens), then press End to soak the wood and Home to dry it to oven-dry, watching the twin at the front right. Swelling, the twin is held tight and the readout says twin tight; once it dries well below the moisture it was glued in at, it splits down a grain line from its end and the readout says twin split. A panel held at both edges cannot shrink freely, so the shrinkage tears it apart, just as the guide's failure table says.
  3. Tap the peg in front of the sample blocks on the stand, or press Q. The peg moves to the other sample block, the panels' face pattern changes between arched flat-sawn figure and straight quarter-sawn stripes, the floating panel's movement roughly halves or doubles, and the readout switches between flat-sawn and quarter-sawn. Quarter-sawn stock moves about half as much as flat-sawn, so it pushes and pulls on its frame less.
  4. Tap the glue brush standing in the pot, or press G. The twin is replaced by a whole, uncracked panel glued in at the present moisture, and the readout shows twin holding. Whether a glued panel cracks depends on how much drier it gets than the day it was glued, so a new twin starts from zero stress.

Reading the readout

The first field says which stock is chosen, flat-sawn or quarter-sawn, and the second says how far the moisture knob is set from oven-dry (0%) to soaked (100%). After "across" comes how much the panel has swelled across its grain since oven-dry, as the guide's range scaled to the knob setting (for example +2.5–5.0% halfway for flat-sawn), with ±0% at oven-dry and <0.5% just above it. The last field is the glued twin's state: holding (glued and whole, not wetter than when it was glued), tight (swollen and pressed hard into its frame) or split (it has cracked down a grain line).

What the model uses

  • Wood moves a lot across the grain, roughly 5–10% between soaking wet and oven dry, somewhat less radially, and almost nothing along the grain.
  • A wide flat panel will change width by several millimetres between summer and winter; fixed rigidly at both edges it will split (failure table: panel splits down the middle; fix: frame and panel, let it float).
  • The frame and panel, a solid panel held in grooves in a frame and free to float, solves the problem.
  • Quarter-sawn boards move about half as much as flat-sawn and stay flat.
  • Illustrative, not from the guide: the model moves the wood by the middle of the guide's range (7.5% flat-sawn, 3.75% quarter-sawn) and the drawing shows that movement four times larger, with grooves deep enough to take it; a 250 mm panel; both panels fitted at mid-range (a twin glued in by the reader is fitted at the moisture of the moment); the glued twin splits at 1% restrained shrinkage and the split never closes below a hairline.

The figure moves the panels linearly with moisture at one illustrative rate, drawn four times larger than life; it ignores cupping, the slow pace at which moisture reaches the middle of a board, and glue creep, and lets a split close back to a hairline as the twin swells. This is a code-drawn explanatory model for orientation, not an equipment drawing or a safety procedure; follow the guide's warnings.