Adhesive workholding allows turbine blades to be ground in one hit

Traditionally, due to clamp interference, four operations are required on conventional CNC grinders to machine the fir-tree and wedge face on the root as well as the shroud end features at the tip. The one-hit manufacturing process is not only faster, but also eliminates work-in-progress and the risk of introducing inaccuracy due to repeated refixturing.

The technique is being developed in the UK by NCMT, which sells Makino’s Viper grinding platforms across Europe. Since May 2015, the company has also been European agent for the patented Blue Photon photo-activated adhesive workholding system from the USA.

Developed at The Pennsylvania State University to fixture delicate and complex parts of any shape without distortion for tight-tolerance machining and inspection, it involves applying spots of adhesive between 0.5mm and 3.0mm thick that are cured for 30 to 60 seconds by ultraviolet light via fibre optic cables.

The machined workpiece can be easily removed by rotating the gripper inserts to shear the adhesive joint. The residual adhesive is removed with hot water or by application of another heat source, which optimally should be between 60°C and 80°C.

NCMT’s research department has exploited Blue Photon’s versatility by designing a turbine blade fixture. It incorporates four gripper inserts that, once adhesive has been applied and cured, hold the blade securely by one side of the aerofoil. The fixture is mounted on the table of a Makino iGrinder using a zero-point clamping system to ensure a high degree of repeatability.

In tests, it was confirmed that the clamping force produced by the fixture could easily withstand the requirements of machining. The material removal rate actually exceeded that achievable when the blade was mechanically clamped, as the pressure had to be limited to avoid component distortion and loss of accuracy.

www.ncmt.co.uk