Drawer unit drawing conflict: dimension chain and recovery
A workshop is making 10 kitchen drawer units. The carcass is 500 mm wide externally, built from 18 mm melamine-faced chipboard. Each unit has two drawers side by side, with 3 mm gaps at the left edge, between the two drawer fronts, and at the right edge. The drawer runners need 12.5 mm clearance on each side of the drawer box. The CAD drawing specifies drawer fronts of 230 mm and drawer boxes of 200 mm. The first unit is assembled and the two fronts overlap in the centre by 2.5 mm; the drawer boxes fit but sit slightly off-centre. The workshop has already cut all 20 fronts to 230 mm and all 20 boxes to 200 mm. The client wants the units delivered in two weeks. The CAD operator says the drawing is correct because it was exported from a 3D model.
1. Show by calculation whether the 230 mm drawer front can be correct. Identify the dimension that should be changed.
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Internal width = 500 − (2 × 18) = 464 mm. Three 3 mm gaps take 9 mm, so the two fronts must total 464 − 9 = 455 mm. Each front = 455/2 = 227.5 mm. Two 230 mm fronts total 460 mm; adding the 36 mm sides and 9 mm gaps gives 505 mm, which is 5 mm wider than the carcass. In the assembled unit that 5 mm shows as a 2.5 mm overlap at the centre because the fronts are pushed together. The 230 mm dimension is therefore wrong. The 200 mm drawer box can be correct if the runner clearance is checked, but the 230 mm front cannot be. The CAD operator's claim that the 3D model is correct is not evidence; the model may have been built with the same error, or the drawing may have been dimensioned from the model without checking the chain. The controlling dimension should be the 500 mm carcass and the 3 mm gaps; the front should be 227.5 mm.
2. Calculate the actual clearance each side of a 200 mm drawer box and explain one consequence of the off-centre fit.
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With a 227.5 mm front, a 200 mm drawer box leaves 27.5 mm total clearance, or 13.75 mm each side. The runner needs 12.5 mm each side, so there is 1.25 mm extra each side. That is not a failure, but it means the box can slide 1.25 mm off-centre unless the runner is packed or the box is aligned to the front. If the fitter centres the 200 mm box in the 227.5 mm opening, the front will be centred, but the runner screws may be at the limit of their slots. A 1.25 mm packer behind each runner would take up the extra and keep the box centred. The consequence of ignoring it is a visible uneven gap at the front and a drawer that rubs on one side. The 200 mm box is usable, but the drawing should show the packer or specify a 202.5 mm box if available.
3. Recommend a recovery plan for the 20 already-cut fronts, with a drawing-control change to prevent recurrence.
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Recovery: the 20 fronts are 230 mm, 2.5 mm too wide each. Trim 2.5 mm from the inner edge of each front, re-edge with matching melamine edge banding, and re-check the 3 mm centre gap. Trimming 2.5 mm from one edge keeps the outer edge and the grain/colour consistent. If the fronts are already edge-banded on both edges, trim the inner edge and re-band only that edge. Do not simply widen the carcass: the units are already built and the client expects 500 mm. For the 200 mm boxes, fit a 1.25 mm packer each side or move the runner fixing by 1.25 mm. Drawing control: make the 500 mm carcass, 18 mm sides and 3 mm gaps the driving dimensions; add a dimension chain showing 227.5 mm fronts; have a second person check the chain before release; and issue a revision with a change note. The CAD model should be updated from the corrected drawing, not the other way round.