CONSTRUCTION
MACHINERY

CIVILEQUIP BUYING GUIDE

Put the tolerance on the drawing before you ask for a price

How general tolerances, geometric tolerances and casting tolerance grades change a machined or cast part quote, and what a dimensional report should prove.

By · Updated

Untoleranced dimensions are still a requirement

A Chinese job shop will quote from whatever the drawing says. If a dimension has no tolerance and the title block names no general-tolerance standard, the supplier chooses one, usually its own habit. ISO 2768-1 exists to stop that guessing: it gives four general-tolerance classes for linear and angular dimensions of machined or sheet-metal parts, so one note in the title block covers every dimension that has no individual tolerance.

That note matters most on custom work. The Wenlin machining profile and its CNC component example show a supplier that builds to the buyer's drawing rather than a catalogue, so the drawing is the whole specification. The same applies to formed sheet metal: a replacement equipment cover from Future Machinery can be bent correctly and still miss the mounting holes if the hole pattern is left to the general tolerance by accident.

Mark the features that decide fit

General tolerances suit non-critical features. Bores, shafts, sealing faces and bolt patterns need their own limits, and often a geometric tolerance. ISO 1101 defines the symbol language for form, orientation, location and run-out, and it is what a supplier's inspector reads when deciding whether a part passes.

Bearing seats show why. An EPEN multilayer bushing or a Mingxin graphite-plugged JDB bushing only works with the housing bore and shaft it was selected for, so the installed fit belongs on your drawing, not only in the bearing catalogue. A ceramic self-aligning bearing raises the same question about clearance. Gearbox interfaces are similar: the shaft, flange and spigot of an HMRV worm gearbox are exactly what Unimotics asks the buyer to fix before a repeat order.

Castings need a different tolerance scheme

Do not put a machining tolerance on an as-cast surface. ISO 8062-3:2023 gives general dimensional and geometrical tolerances and machining allowance grades for castings, for all cast metals and processes, with the grade depending on how the part is made. A drawing for a cast part should therefore say which surfaces stay as cast, which are machined, and how much stock is left for machining.

The Henao construction casting, the JW0020 casting and the Muyang bracket are all made to drawing; each profile asks whether you are buying a rough casting or a finished part. For high-pressure die casting, the tool itself carries the geometry, which is why the Zonzao reducer-housing die case is a design case rather than a stock part. Our separate guide on choosing a casting route explains how process and tolerance grade go together.

What a dimensional report should prove

A dimensional report is useful only if it can be traced back to your drawing. Ask for: the drawing number and revision measured against; every ballooned dimension with nominal, limit and measured value; the instrument used for each feature; and the sample identity. NIST describes metrological traceability as an unbroken chain of calibrations to stated references; for a buyer, the practical form is a calibration record for the gauge or CMM that produced the critical numbers.

Measurement also matters for production equipment. Hushengyuan integrates pressing with measurement, and Hangan builds test stations; in both, the display resolution of an instrument is not its accuracy, and the acceptance test should state which one is meant. When the drawing, the tolerance note and the report agree, comparing quotes for construction components becomes a comparison of the same part.

Sources

  1. ISO 2768-1:1989 General tolerances — Part 1
    Four general-tolerance classes for linear and angular dimensions of machined or sheet-metal parts; ISO lists it as current and due to be replaced.
  2. ISO 1101:2017 Geometrical tolerancing
    Symbol language for form, orientation, location and run-out tolerances.
  3. ISO 8062-3:2023 Tolerances and machining allowances for castings
    General dimensional and geometrical tolerances and machining allowance grades for castings in all metals and processes.
  4. NIST Policy on Metrological Traceability
    Traceability as an unbroken chain of calibrations to specified references.

Explore related company and product information

Prepare your sourcing request