Why are there variances in sheet-good thicknesses?
Why don't your project plans provide dimensions that reflect the actual thickness of plywood? I built a lingerie chest from WOOD® plans, but when I cut tongues on the 3⁄4 "-thick plywood dust panels—cutting away 1⁄4 " from each face as stated in the plan—the tongues were too thin for the grooves in the sides.
—Jerry McCullough, Athens, Alabama
Sorry for the confusion, Jerry. But because sheet goods differ in thickness by type, brand, and region, we can't precisely spell out the thicknesses of project parts made from those products. So we identify these parts by their nominal thicknesses (1⁄4 ", 1⁄2 ", or, in this case, 3⁄4 "). Unfortunately, most plywoods measure 1⁄64 –1⁄32 " less than the stated thicknesses. Some products, such as medium-density fiberboard (MDF) or medium-density overlay (MDO), typically are still made to precise stated thickesses. (See the photos above for three different materials labeled 3⁄4 " [0.75] thick.)
So, how can you avoid problems with variances in sheet-good thicknesses? When cutting joinery or interior parts on any project, always cut those parts to fit your project. To help with this, make test cuts in scrap material of the same thickness to dial in machine setups (tenons to fit in mortises, tongues in grooves, as well as dovetails and box joints) before cutting your actual workpieces to size.