Tolerances · 5 min read
Mold base tolerances: what to put on the drawing
A base can be machined perfectly and still be rejected if the drawing never said which dimensions mattered. This is the short list we ask about.
Plenty of drawings arrive with a general tolerance note and nothing else. That is workable for most of the plate, but it leaves the features that actually decide whether the mold assembles without a tolerance of their own — and those are the ones that cause disputes at inspection.
The features that need their own callout. In practice it comes down to four groups: the mold pockets, the guide-pillar and guide-bush bores, the shut-off faces, and plate parallelism. Get those right and the mold goes together; get them marginal and every downstream step inherits the problem.
What we hold as standard. On fitting and ground features — the four groups above — we work to ±0.02 mm. Ground working faces are flat and parallel to 0.02 mm / 100 mm, datum faces are square to 0.02 mm / 100 mm, guide-pillar bore position is held to ±0.02 mm, and ground surfaces finish to Ra 0.8 µm. On dimensions that are not a fit — clearance edges, non-mating edges, nominal sizes — the general tolerance is 0.3 mm, and there is no reason to tighten it.
Above 600 mm, talk to us. Thermal movement and handling make a blanket figure misleading on large plates, so the tolerance on a big base is agreed case by case with the drawing in front of us. We confirm the figure we will hold before the quotation is issued, so the number in the quote is the number we inspect against.
Tell us the datum. If your design measures from a particular edge or face, mark it. Half of the tolerance questions we get trace back to two parties measuring from different references, not to the machining itself.
Say which faces get used as a fit and which do not. A plate quoted at fitting tolerance throughout costs more than it needs to. Marking the functional faces usually reduces the price without touching the function.
Inspection travels with the shipment. Critical features are measured on a coordinate measuring machine before packing, and the records can be sent with the base. If your incoming inspection measures from the same datum, the numbers match.
The practical version of all this: send the drawing, mark the functional faces and the datum, and state any tolerance tighter than ±0.02 mm. Everything else we can hold as standard, and the quotation will say so.
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