We are very tolerant, up to 0.1mm…
How tolerant are you? Now we’ve mentioned this before, and ruffled a few feathers in the process! When it comes to pieces fitting together 0.1mm is all you’re allowed if you want something to look as it should.
But, I hear you cry, wood moves! All you need is a temperature change or a change in humidity. Never mind whether anyone actually turns on their central heating or air conditioning!
Yes, wood does move, but only materially across the width of the grain. The grain swells with humidity and shrinks in width with dry heat. What it doesn’t do is change length. Not much anyway. Yes, we design in an allowance of up to 1-2% for changes in dimension across the width of a piece. But we can still absolutely rely on stable lengths to the extent that we can and should finish certain components, surfaces and projects to within 0.1mm.
For example, when you have a mortice and tenon joint, the shoulders around the tenon should fit flat and snug against the morticed component. The surface of the morticed pieced should be absolutely flat. Straight-edge flat, unsanded, straight-off-the-plane-flat, not duffed over. Trickier to do, but no less important, the tenon shoulders also have to be perfectly flat, ideally straight off a table saw, but often the result of careful handwork using a sharp shoulder plane.
When edge jointing a dining table top, the jointed edges come together under clamping and when they do, there needs to be a nice, tight, even joint line. Ideally invisible, but no wider than a hair’s width.
Now you see it…
The thing is, you can’t see a joint with a 0.0mm gap. A 0.1mm gap will be filled with glue and require great scrutiny to see, but a 0.2mm gap is a great yawning divide. It is a gaping chasm, a… OK, it isn’t really these things, but I’ll tell you what it is. It is visible.
A simple thing you can do is get some feeler gauges and go round your own furniture to see if you can squeeze a 0.2mm feeler into any of the joints. You may be surprised.
The third example is possibly the most confounding. A piston fit draw will go from too tight to just right within a 0.1mm shaving, and from just right to the tiniest bit rattly if you take it a further 0.1mm.
Any competent cabinetmaker will have at their disposal a number of reliable sharp tools. Whether it’s a plane, chisel or scalpel, all can make a cut, slice or shaving of 0.05mm or less, so what’s the problem!
Until next time,