Dimensional inspection of investment castings

Measuring equipment

1 coordinate measuring machine bridge and probe head · 2 probe on an as-cast feature · 3 casting located on its datum targets

Hand instruments. Calipers, micrometers, height gauges, bore gauges, thread and plug gauges and radius gauges are used for individual dimensions and in-process checks.
Fixed gauges and checking fixtures. Go/no-go gauges and fixtures that locate the casting on its datum targets allow rapid checks of critical features in series production.
Coordinate measuring machines (CMM). Tactile CMMs measure sizes, positions and geometrical tolerances in the datum system of the drawing and are the reference method for first article inspection.
Optical 3D scanning. Structured-light or laser scanners capture the complete surface and compare it with the CAD model as a colour deviation map. This is particularly informative for castings because it shows distortion, shrinkage patterns and local deviations at a glance.
Form and profile measurement. Roundness and form testers, contour and roughness instruments are used on machined functional features.
Computed tomography can measure internal features that are otherwise inaccessible, in addition to revealing internal defects.

Alignment and datums

How the part is aligned decides the result. A casting measured in a best-fit alignment may appear in tolerance although it cannot be clamped and machined correctly, and the reverse. Good practice is:

to measure from the datum targets defined on the drawing in accordance with ISO 5459;
to use the same targets in the machining fixture;
to report the alignment method on the measurement report;
to use best-fit comparison as an additional diagnostic for distortion, not as a substitute.

As-cast features

Cast surfaces have texture and small form deviations, so a size measured at one cross-section may differ from the next. Reports should therefore state how a feature was evaluated, for example the number of probing points or the evaluation of a scanned surface. Features with draft, radii and gate or parting witnesses need agreed measuring positions.

First article and series inspection

For initial samples, every drawing characteristic is measured on an agreed number of parts and reported with nominal value, tolerance, actual value and deviation. The results are used to correct the wax die where necessary. In series, a control plan defines which characteristics are measured, with which method and at what frequency; critical dimensions may be monitored statistically.

Measurement uncertainty and conditions

Measuring equipment is calibrated with traceability to national standards. Measurement is carried out at controlled temperature for tight tolerances, and the uncertainty of the method has to be small in relation to the tolerance. The condition in which the part is measured, for example after heat treatment and blasting but before coating, should correspond to the condition defined on the drawing.