JC Wood Technology common mistakes
JC Wood Technology · Understand the traps and how to avoid them.
Designing a solid timber tabletop with a fixed, fully glued connection to its rails, justifying it with 'the timber is kiln dried to 12%, so it is stable and will not move'.
Candidates memorise a single nominal moisture content figure and treat it as a guarantee of dimensional stability. They do not connect equilibrium moisture content (EMC) to the service environment, and they confuse moisture content expressed on a wet basis with the dry-basis calculation, so movement is never quantified.
What to do instead
Calculate expected movement for the species and grain direction using the dry-basis moisture content formula and published shrinkage figures, then design a allowance of roughly 3–4 mm per 300 mm of width. Fix tops with buttons or slotted holes and glue only a central zone, leaving the rest free to move.
Grading a board with a large, dark-rimmed knot as 'sound' because it feels firm in the face, then using that area in a structural rail or a load-bearing leg.
Defects are learned as a visual vocabulary rather than as cause-and-effect. Candidates recognise a knot but do not distinguish a live knot from a dead knot with a separated growth ring, and they do not link defect position to the tension edge or to grade rules.
What to do instead
Probe suspect knots and look for a continuous dark ring or movement under pressure. Classify knots as live or dead and reject dead knots, checks and shakes in tension zones. State the structural consequence, not just the defect name, when selecting material.
Sharpening a chisel freehand by lifting the handle slightly on each stroke, then testing sharpness by slicing paper rather than by paring end grain.
Candidates chase a wire edge without controlling the bevel angle. Lifting the handle rounds the bevel, and paper cutting rewards a thin, polished edge rather than a geometrically correct one, so the fault is hidden.
What to do instead
Use a honing guide set to the existing bevel angle and flatten the back on the same stone until the scratch pattern is uniform. Raise a burr on the bevel, remove it from the back, and verify by paring a clean shaving from end grain and by checking the bevel for rock with a straightedge.
Crosscutting a long board on the table saw by pressing it against the rip fence and feeding it through with no stop block.
The rip fence is the most visible reference surface, so it is used as a length stop. This traps the offcut between the fence and the blade, and the rising rear teeth lift and throw the workpiece.
What to do instead
Crosscut with a mitre gauge or a crosscut sled, never against the rip fence alone. When a length stop is needed, clamp a stop block to the fence ahead of the blade so the offcut is free. Keep the riving knife fitted and stand clear of the kickback path.
Cutting a mortise-and-tenon joint with the tenon equal to half the rail thickness, or omitting a haunch where a through mortise enters a stile.
The one-third rule is remembered as a slogan but not as a ratio tied to the rail thickness and to the remaining cheek material. Haunching is treated as decoration rather than as a way to resist racking and to protect the mortise wall.
What to do instead
Size the tenon at about one-third of the rail thickness, centre it in the leg, and keep cheeks thick enough to resist splitting. Add a haunch to through and wedged tenons, cut shoulders square and test the fit dry before glue-up.
Tightening every clamp on a PVA edge joint as hard as possible and judging success by the amount of squeeze-out, believing more pressure means a stronger joint.
Squeeze-out is visible and is read as evidence of a good bond. In practice, excessive pressure on a close-fitting joint forces adhesive out of the glueline and starves it, and uneven clamping pressure bows the assembly.
What to do instead
Apply moderate, even pressure just sufficient for a continuous bead of squeeze-out along the joint, using cauls to distribute load. Prepare joint faces flat and slightly absorbent, size end grain where required, and check the glueline after curing rather than adding clamp force.
Applying a water-based polyurethane over a 'sanding sealer' without checking its composition, then blaming the finish when it beads or peels.
Product names are treated as generic categories. Sealers containing wax or stearate, and waxed shellac, repel water-based finishes, and the incompatibility only appears after the topcoat has dried.
What to do instead
Read the technical data sheet and confirm the sealer is compatible and dewaxed. Run a test panel through the full schedule, including sanding between coats, and use dewaxed shellac as a barrier coat where an unknown finish must be covered.
Placing the plan view below the front elevation on a drawing that is meant to be third-angle, or mixing first-angle and third-angle layouts on the same sheet.
The two conventions are learned as mirror images but not as a rule about where the object sits relative to the planes. Candidates copy a familiar layout without checking the projection symbol.
What to do instead
Decide the convention before drawing and place the projection symbol on the sheet. Unfold the viewing box consistently: in third angle the view seen from above goes above the elevation, and in first angle it goes below. Check every auxiliary view against the same rule.
Dry-sweeping MDF and fine sanding dust, or using a domestic vacuum and a disposable dust mask as the only controls, because the visible sweepings have been cleared away.
Respirable dust is invisible, so a clean-looking bench is mistaken for a safe one. Domestic vacuums and basic masks are assumed to be equivalent to local exhaust ventilation, and the hazard from formaldehyde-containing resin binders in MDF is overlooked.
What to do instead
Capture dust at source with an M-class or H-class extractor connected to the tool, and never dry-sweep or use compressed air. Wet-wipe surfaces, wear fit-tested RPE appropriate to the exposure, and follow the safety data sheet for board products.
Writing 'FSC-certified timber' into a specification because a supplier's website displays the FSC logo, without checking the claim type or the chain-of-custody code.
The logo is treated as proof of the individual product's status. Candidates do not distinguish FSC 100%, FSC Mix and FSC Recycled claims, and they do not know that a certificate covers a supplier's system rather than every item in the yard.
What to do instead
Verify the supplier's FSC licence code and the claim type on the invoice, and ask for chain-of-custody documentation for the specific batch. State the claim precisely, for example 'FSC Mix Credit', and record the evidence with the specification.