Case Studies

Vane Tip Grinding on
Overhauled Helicopter Engine

Client: Major Aircraft Engine Manufacturer

Background:
An aircraft gas turbine engine typically goes through numerous rebuilding cycles in its lifetime. The engines are constructed of difficult to machine high temperature alloys such as Inconel and Hastelloy. Machining problems are further compounded by the thin cross-sections, irregular shapes and complex fixturing needed to hold the parts. Finish grinding of these alloys is used for the close tolerances required.

Manufacturing Challenge:
The engines case has a series of thin airfoils called stator vanes mounted to the inside of the engine case. These are replaced periodically and must be internally ground to a close tolerance for the specific rotor clearance. Each of the 5 stages has a different angle that must be dressed on to the grinding wheel. The parts had traditionally been ground using a manual vertical boring mill retrofitted with an internal grinding spindle. The parts would be mounted, indicated true and then each stage was carefully ground. After grinding the part had to be removed and set into a special gage for measurement. If any stages were incorrect the part had to be remounted and indicated and then re-ground. The typical engine would take 8 hours to complete. If any stages were ground oversize the vanes had to be scrapped and replaced and the process repeated. In addition to the long cycle time the company experienced a costly high scrap rate.

Solution:
Everite provided a Model HG-16 CNC Internal Electrochemical Grinder fitted with a custom designed laser measurement system and tooling to hold the engine cases. The operator would mount and indicate the engine case. A grinding wheel with the correct angle would be mounted and the cycle would begin. The CNC would direct the grinding wheel to the motorized wheel dresser to true the wheel to the exact angle needed. The wheel would then grind the appropriate stage using one roughing pass and one finish pass. The machine would retract the wheel and then stop the cycle to allow the operator to extend the Laser gage. The CNC would then move the Laser into position, rotate the engine and measure the diameter of each of the vanes. The measurements were recorded to an Excel spreadsheet on a PC. The process was repeated for each of the 5 stages and then a full measurement report was issued from the PC. Any discrepant dimensions are corrected before the engine is removed from the machine saving the time to mount and re-align it. The typical cycle time for the Everite machine was 2 hours per engine, a saving of 6 hours per engine and a significant reduction in scrap.
 

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