Measurement uncertainty and choice of measuring equipment
ISO 286 and the system that ensures the parts fit
Two tool parts that are drawn to fit together must also fit together when milled turned and ground finished — whether made same day or year apart. That is the purpose of GPS: Geometrical Product Specifications the international system of standards that defines how dimensions tolerances and surface requirements must be specified and measured so a drawing can be read same by all
§ISO 286: the fit system
ISO 286 is the part of the GPS system that establishes tolerances and fits for cylindrical dimensions such as holes and shafts. The system works with IT tolerance grades (from IT01 to IT18), where a low number means a tight tolerance and a high number a coarser tolerance, combined with a letter that indicates where the tolerance zone lies relative to the nominal dimension – for example H for a hole where the lower tolerance is zero.
§Hole and axle system
In practice, the hole system is usually used, where the hole tolerance is kept fixed (typically H), while the shaft tolerance is varied to achieve the desired fit — loose, sliding or interference. It is simpler to design tools after, because you only need one set of standard reamers or drills for the holes, while the shafts can be turned or ground to the desired fit.
| IT grade | Typical application |
|---|---|
| IT01-IT4 | Very tight tolerances — measuring tools, precision fits in moulds |
| IT5-IT9 | Common machine parts and fit surfaces in tools |
| IT10-IT13 | Coarser dimensions on parts without critical fitting |
| IT14-IT18 | Free dimensions and non-functional dimensions |
§Måleværktøj i værkstedet
- 01Sliding gauge — fast control measurement, typically with readings down to 0.02-0.05 mm
- 02Micrometer — more precise measurement of individual dimensions, typically down to 0.001 mm
- 03Indicator clock — checks roundness, flatness and small deviations
- 043D measuring machine (CMM) — measures complex geometries and documents the result automatically
§Fits: loose, tight and press
A loose fit provides air between parts so they can move freely relative to each other — important in movable ejector systems. A press fit on the other hand holds parts tightly together without additional fastening as when a guide bushing is pressed into a mold plate. The choice of fit is as much a design decision as a pure number.
§Documentation of measurement control
Measured results should be documented so it can later be proven that a part actually met the required tolerances when approved — particularly important if the tool is delivered to a customer with their own quality requirements or is part of a quality management system.