Tip Grinder Boots Aero Performance

With the widespread application of the jet turbine to civil aviation, more emphasis has been placed on improving operating efficiency.

One means of achieving this is to minimize and closely control the running clearance between the compressor rotor and its housing. Optimal running clearances also bring other advantages like noise reduction. For military engines, it means increased thrust for improved mission performance.

The compressor rotor consists of a drum with up to 16 stages of aerofoil shaped blades spaced along its length. Each stage contains dozens of blades. When the drum is rotated, the action of the blades, by virtue of their profile and orientation, draws in air and compresses it prior to combustion in the after chambers.

Speedy automation

The blades are not rigidly attached to the rotor, but fit loosely into a type of dovetail slot known as a fir tree form. When the engine is rotating at working speed, typically 7,000 the blades are retained in a stable attitude against the dovetail sides by centrifugal force. The small amount of play in the joint is essential to accommodate thermal and mechanical forces acting on the blades.

To control the clearance between the blade tips and outer casing, the tips are ground in situ on the rotor, both for new engines and those requiring refurbishment. Because of the damage and wear caused by the ingestion of foreign bodies, such as stones and airborne dust, engines are normally overhauled at between 5,000 and 10,000 flying hours.

Until about 1980, the grinding was usually carried out by slowly rotating the workpiece on a universal type of machine and then deburring each blade by hand - a laborious process that could take several days.

In the early 1980s, the Butler Newall Machine Tool Company of Halifax, northern England, developed an automated machine tool specifically for high-speed grinding of compressor and turbine blade tips. During machining the workpiece is run at about 2,000 rpm so as to simulate operating conditions and thus present the blades in their correct working attitude. It is this feature in particular that contributes to the improvement in accuracy and productivity of the new process.

Work speed ranges

Known as the blade tip grinder (BTG), the machine has a six-axis computer numerical control (CNC) system permitting a high level of automated operation, reducing the whole grinding and deburring sequence down to a few hours.

A stiff cast iron bed carries a sliding rectangular work-table upon which the workpiece is mounted on work spindles, and rotated by a 55 kWDC motor-driven headstock with work speeds ranging from 100 to 4,200 rpm. The standard machine can accept rotors up to 1 m dia and 1,5 m in length. A smaller, higher speed version, designed for machining helicopter and tank rotors, is also available.

The cross bed carries the compound slide that provides the in-feed for the wheelhead. The grinding wheel is powered by an 11 kW AC motor and can be run at up to 800 rpm.

At certain rotor stages, the blade tips are angled to the axis of the rotor by up to 13° and, in order to grind them, the wheel lie ad has to be swung over to the appropriate angle by means of an auxiliary CNC slide and pivot bearing. The pivot has been arranged so that the grinding wheel periphery lies exactly over its centre, an important feature that simplifies setting and programming.

Deburring is carried out under automatic control using an air-driven abrasive impregnated nylon brush which can be rotated in either direction to dress both leading and trailing edge burrs.

KEYS TO FINAL TESTS

Unit one 1)A, C, D; 2) B; 3) C; 4) C; 5) E; 6) D; 7) B; 8) A; 9) materials; 10) economists; 11) Master’s … Doctor’s; 12) scientist; 13) The engineer; 14) energy; 15) progress; 16) B; 17) C; 18) D; 19) A; 20) D.

Unit two 1) B, C, E; 2) B; 3) C; 4) D; 5) A; 6) F; 7) C; 8) E; 9) climb milling; 10) drawing; 11) reservoir; 12) column; 13) intermediate; 14) wear; 15) friction; 16) B; 17) D; 18) A; 19) B; 20) A.

Unit three 1) A, D, E; 2) C; 3) B; 4) C; 5) D; 6) F; 7) B; 8) A; 9) chuck; 10) motion; 11) diameter; 12) bed; 13) headstock; 14) slide; 15) overhaul 16) semiautomatic; 17) C; 18) A; 19) C; 20) D.

Unit four 1) B, C, E; 2) C; 3) C; 4) F; 5) C; 6) A; 7) E; 8) B; 9) application; 10) holes; 11) workpieces; 12) base; 13) cutting fluid; 14) radial drills; 15) travels; 16) B; 17) D; 18) a; 19) C; 20) B.

Unit five 1) B, C, D; 2) B; 3) C; 4) F; 5) C; 6) A; 7) E; 8) B; 9) abrasive; 10) operation(s); 11) operations; 12) plain; 13) angle; 14) diameter; 15) horizontal; 16) C; 17) A; 18) C; 19) B; 20) A.