Quality 101: Speak a Common Language
Geometric Dimensioning and Tolerancing (GD&T) is the language of communicating dimensional information on a product part print or process drawing. Manufacturers find it valuable to precisely communicate functional product characteristics, manufacturing process controls and gaging measurement plans.
GD&T is used in many industrial applications where assembly processes or complex component geometry is present. Most notable, are applications in transportation, such as aerospace, automotive and shipbuilding; consumer appliances; electronics packaging; mining; and food processing. GD&T is highly symbolic and standardized in documents, including ASME Y14.5M-1994 and ISO 1101-1983 (E).
GD&T methods of communicating are increasingly important as the world's economy moves toward globalization. Far-flung supplier lines in recent years have resulted in hidden costs for assemblers. Someone may reason that a product design is thoroughly validated when it has been established in a controlled manufacturing process.
Theoretically, the design could be migrated to another supplier either for increased volume or lower cost.
Consider the assembly of four components-a rotating shaft, a chain sprocket, a plate and a block assembly. To assemble, the rotating shaft is first inserted into the block assembly. Next the plate is slipped over the end of the shaft and fastened to the face of the block. Pushing the shaft against the plate, the sprocket is fastened to the end of the shaft. When the assembly is complete, the plate is captured between a shoulder on the shaft and the sprocket. The intended arrangement leaves a 0.25-millimeter design gap between the sprocket, plate and shaft shoulder so that the shaft is free to rotate.
In this plant, however, some of the sprockets were scoring the plate and restricting shaft rotation. Obviously the sprocket-to-plate design gap became critical for its function in the assembly. This event was mystifying since there were no recent changes, nor were any components out-of-spec. The only correlating factor was the new supplier for block assemblies.
In reviewing the design GD&T, a profile of surface callout on the block face:
| 0.3 |A-B|E specified the allowed variation in surface location relative to its datums, the lower bore in the block. This means the face should not exceed +/- 0.15 millimeter from its basic dimension when the block is fixtured with an expanding arbor in the lower bore A-B, and slid against a locating pad on datum feature E. The profile of surface callout was presented alone on the block part print; it did not have other refining GD&T callouts like perpendicularity or flatness.
The profile callout primarily is used to specify the allowable variation in location, but in the absence of other callouts, it also allows orientation or tilt, and form-flatness-to vary within the same tolerance zone.
The new supplier process, however, was using up a substantial portion of the tolerance as orientation or tilt variation. Components from both processes passed inspection for profile, but the second had an inherent tilt. The tilt on the block face would cause the plate to tilt reducing the design gap to the sprocket.
The fix was to add a perpendicularity callout to the existing profile on the block face. The perpendicularity refines the tolerance zone of the profile to limit the orientation or tilt variation relative to the upper shaft bore: ⊥| 0.15 |C-D. The effect of profile and perpendicularity together means the face should be held square or perpendicular within +/-0.075 millimeter relative to the upper bore C-D, and should be held within +/-0.15 millimeter of its basic dimension relative to the lower bore A-B and face E.
GD&T helped the manufacturers in this example fix their interference conditions that appeared during final assembly after switching suppliers. GD&T helps all organizations around the world speak a common language.