• 1-888-TEC-EASE
  • Outside USA: 1-716-785-6015
Free Tec-Ease Mug GD&T Textbooks GD&T Training GD&T Public Seminars GD&T Computer Based Training GD&T Reference Material
+/- Tolerances Just Don’t Fly!
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

This is a Throttle Pulley Sector used to guide throttle cables around a 90° angle on a domestic airliner. The two counterbores shown in the sectional view receive bearings which determine location and orientation of the part in the next assembly. The bearing bores and the 7.064 radius are machined in different machines and fixtures. There is nothing on the original drawing relating the 7.064 radius to the axis of the counterbores. When the eccentricity of this radius to the bearing bores is too great, the throttle cable can "walk" off the Sector past the Guide Pins. When this happens, an engine will shut down without warning. At assembly, the critical gap is inspected with a NO-GO gage. If the gap is too large, and the NO-GO gage "goes", the Throttle Pulley Sector Assembly (with the ball bearings) is scrapped and another assembly is tried!

The second drawing illustrates the use of profile of a surface and datum referencing to solve the problem. This approach captures the design intent.

Those of you who continue to resist using geometric tolerancing can fly the plane that was built with ± tolerances. I will fly much safer with GD&T!

By the way, this profile toleranced shape is easily inspected with a simple fixture, an optical comparator and an overlay. No need for a CMM.

Back to Tips Tip added May 2013