BTM Ébéniste common mistakes
BTM Ébéniste · Understand the traps and how to avoid them.
Reading a cote off a plan without checking whether the drawing is to scale, then cutting pieces to the measured value instead of the dimensioned value.
Candidates treat the printed plan as a measuring surface because it looks precise, and they forget that plans are dimensioned drawings where the written cote governs; photocopies and PDF prints also distort scale.
What to do instead
Always read the chiffres inscrits, never measure the paper. Before cutting, list every cote in a tableau de débit and mark those that are 'hors échelle'. Use a scale rule only when the plan explicitly states an échelle and no cote is given, and note the calculation on the plan.
Confusing vue de face, vue de dessus and coupe in a multi-view drawing, placing a mortise or a moulure on the wrong face.
The candidate recognises the overall shape but not the projection convention (premier dièdre / projection européenne) or the hatching convention for sections, so they map features to the wrong view.
What to do instead
Before any layout, identify the three views, the sens de projection and the hachures of coupes on the plan. Practise redrawing simple assemblies in three views with the hachures correct. On the atelier drawing, annotate each face (A, B, C) and carry that annotation through to the débit.
Choosing a panel (contreplaqué, MDF, aggloméré) or a solid-wood species for a piece where movement, humidity or load makes that choice wrong, e.g. fixing a wide solid-wood table top rigidly or using MDF in a humid bathroom.
Candidates memorise 'good/bad' material labels instead of reasoning from the properties: retrait/gonflement, reprise d'humidité, résistance à la flexion, tenue des fixations. They also ignore the sens du fil and the plis du contreplaqué.
What to do instead
For each element, state the constraint first (humidité, charge, mouvement, finition), then select the material and justify it in one sentence. Learn the typical values: taux d'humidité d'équilibre du bois, retrait différentiel tangentiel/radial, sens des plis. Design wide solid tops with fixation allowing movement (traverses, pattes, trous oblongs).
Not allowing for movement and machining allowance in the débit: cutting pieces to final dimension immediately, or cutting a tenon to exact length without a surépaisseur.
Candidates want to save time and skip the distinction between débit brut, débit corroyé and dimension finie; they also forget that rabotage and ponçage remove material.
What to do instead
Write a débit with three columns (brut, corroyé, fini) and add the standard surépaisseurs for rabotage and tronçonnage. Cut oversize, plane to the corroyé dimension, then machine to the fini dimension after checking the bois est stabilisé. Verify each piece with the rule and the équerre before usinage.
Machining an assembly (mortise, tenon, rainure) without a dry-fit and without checking the sens du fil, so the tenon short-grain breaks or the assembly is loose/tight on one side.
Candidates focus on the tool setting and forget the assembly logic: reference faces, sens du fil, and the need to trace the same face marks (faces de référence) across all pieces.
What to do instead
Mark the face de référence and the face de chant on every piece before usinage, and keep the marks visible. Machine test pieces first, do a montage à blanc of the whole frame, and check équerrage with the diagonals. Adjust the tenon thickness to the mortise, never the reverse.
Designing or building a joint that is structurally wrong for its function, e.g. a simple butt joint for a load-bearing frame, a mortise-and-tenon without a cheville in a stressed rail, or a dovetail oriented the wrong way.
Candidates learn joint names by rote and cannot match the joint to the direction of the load and the sens du fil; they also skip the question of whether the joint will be glued, pinned or demountable.
What to do instead
For each joint, state the effort it resists (traction, cisaillement, arrachement) and the reason for the choice. Know the standard families: tenon-mortaise, mi-bois, queue d'aronde, lamelle/biscuit, tourillon, enture. Draw the joint at 1:1 before machining it and check the grain direction on the drawing.
Assembling and gluing a whole ouvrage without a montage à blanc, without checking équerrage and without planning the order of assembly, so the frame racks or a piece is glued in the wrong sequence.
Time pressure in the workshop and the belief that a dry fit is optional; candidates also underestimate open time of the colle and the need for serre-joints in sufficient number.
What to do instead
Always do a complete montage à blanc with the serre-joints in place, check the diagonals and the aplombs, then number the pieces. Write the ordre de montage before gluing, prepare the serre-joints and cales, and respect the temps ouvert of the colle. Check équerrage again immediately after serrage.
Applying a finish without preparing the surface correctly: skipping the dégraissage, raising the grain, or applying the first coat too heavily on an absorbent wood, leading to patches and raised fibres.
Candidates treat finishing as a cosmetic last step and do not link it to the preparation sequence (ponçage progressif, dépoussiérage, mouillage/égrenage) or to the product's fiche technique.
What to do instead
Follow the sequence: ponçage progressif (grain croissant), dépoussiérage, mouillage et égrenage for solid wood, then the finish per the fiche technique (dilution, temps entre couches, conditions de température et d'humidité). Test on an offcut of the same wood before the piece.
Restoring an old piece by sanding it down to bare wood and refinishing it uniformly, erasing patine and original finish, or using an inappropriate product on an unknown finish.
Candidates equate 'restoration' with 'renovation' and do not first identify the existing finish (vernis, cire, gomme-laque, peinture) or the value of the patine.
What to do instead
Start with identification tests on a hidden area (solvent test, observation under light) and record the état before touching anything. Clean before refinishing; conserve patine where possible. Use reversible products, work on a small hidden zone first, and document every intervention.
Working with machines (scie circulaire, toupie, défonceuse) without the guards, push sticks or correct blade height, or using a machine while wearing loose clothing or gloves near rotating parts.
Candidates learn the machine's function but not its risk analysis; habit and haste in the atelier normalise bypassing protectors, and the EPI are seen as an annoyance.
What to do instead
Apply the principle: protecteur en place, poussoir for short pieces, hauteur de lame réglée, pièce bridée. Know the risks per machine (recul de la pièce, projection, bruit, poussière) and the associated EPI (lunettes, protection auditive, masque, chaussures). Never wear gloves or loose sleeves near rotating tools, and never remove a protecteur to save time.
Treating the atelier as a fixed layout and ignoring the circulation of pieces, the stockage of chutes and the evacuation of poussières, so the workspace becomes a hazard by mid-project.
Candidates plan the work but not the space; they underestimate how offcuts, machines and drying pieces compete for the same surface, and they do not connect poussière de bois to explosion and health risks.
What to do instead
Plan the implantation before starting: zones of débit, usinage, assemblage, finition, stockage, with clear circulation. Keep chutes in a dedicated container, connect machines to aspiration, and clean dust daily. Reserve a separate zone for finishing (ventilation, no ignition source) and for drying glued pieces.
Managing a chantier without a written planning or a validated métré, discovering on site that a cote, a passage or a fixation point does not match the plan.
Candidates rely on the plan alone and skip the relevé de cotes sur site; they also do not formalise the échange with the client (validation, devis, délais) and improvise when a surprise appears.
What to do instead
Do a relevé de cotes on site before fabrication, note passages, obstacles and points de fixation, and compare with the plan. Keep a written planning (approvisionnement, fabrication, pose), a devis validé and a trace of client decisions. On site, protect the existing, plan the manutention, and report any écart in writing before modifying the ouvrage.