GF Machining Solutions, the EDM, Milling and Laser Ablation machine tool specialist and automation systems solutions provider, has supplied The Open University (OU) with a new state-of-the-art wire EDM machine.
The machine, an AgieCharmilles CUT 1000, was installed in the OU’s Department of Engineering & Innovation in its Materials Engineering R & D workshop facility in February 2015, and is being used as an integral part of a residual stress measurement service (called the Contour Method) provided by The Open University to UK and internationally-based companies and research institutions and organisations.
NB: Residual stresses are the stresses present in a part free from any external load, and are generated by manufacturing processes that include forging, welding, casting, rolling etc.). Residual stresses often play a significant role in many material failures. They can “add to” the operational in- service stresses and can create degradation leading to crack initiation, growth and ultimately premature failure.
Because of their important contribution to failure and their almost universal presence, knowledge of residual stress is crucial to understanding, analysing and predicting the life of any engineered part or engineering structure. As a consequence, residual stress research and development is particularly relevant for the aerospace, power generation (including Nuclear) and the rail sector - to name but a few.
The Contour Method
For those not ‘in the know’, the Contour Method is a proven destructive measurement technique for mapping residual stresses in manufactured components. The method consists of a number of phases: 1) cutting a sample under investigation into two parts; 2) measuring the distortion on the newly cut surfaces caused by stress relaxation, and 3) processing the measured surface deformations, and 4) calculating the stresses present before the cutting operation by analytically ‘forcing the new cut surface back flat’ using finite element modelling.
The cutting operation in Phase 1 is performed by a wire EDM machine because the wire EDM process, unlike conventional machining methods, is relatively stress free when used in skim-cut mode, doesn’t lead to work hardening and can be relied upon to make a straight precise cut.
Wire EDM machining – but not as you know it
In more ‘traditional’ applications, wire EDM is used to machine parts with complex shapes and usually entails making an initial rough cut using high-voltage pulses to achieve high material removal rates. The rough cut is then followed by a number of trim or skim cuts to machine the finished part to the precision and high-quality surface finishes required.
However, the employment of a number of cutting ‘passes’, creates problems for the Contour Method.
1. Firstly the rough cut creates a large recast layer which introduces its own stresses and can alter the deformation of the cut surfaces;
2. Subsequent ‘trim’ cuts remove the surface profile created by the initial rough cut – making meaningful measurement impossible.
As a consequence, the Contour Method (to be truly effective) requires only one pass (often with a larger depth of cut than normal) to be made, and that the surfaces cut have a good surface finish/integrity.
These requirements create challenges for a wire EDM machine operating within its usual and manufacturer’s recommended parameters and can lead to - wire pinching/breakage; wire entry and exit issues that could result in surface waviness and surface bowing or flared edges.
Choosing the right wire EDM machine
The CUT 1000 is equipped with high-performance digital Clean-Cut generator technology (which virtually eliminates the creation of a recast layer), and features a wire guide system capable of handling wires with diameters ranging from 0.20mm to 0.02mm; a thermally-stable and rigid machine design that helps increase accuracy and delivers improved and consistent surface finishes; and a range of SMART technology software modules (including 3D set-up) for increased flexibility and process reliability.
The Open University conducted a number of ‘witnessed and controlled’ contour cuts on welded test components using a CUT 1000 machine at Microtec EDM Ltd. The improved quality of the contour cut surfaces using fine wire EDM prompted the Open University to invest in it own CUT 1000 machine.
Following its installation in February the machine has been put through its paces.
To increase the CUT 1000’s flexibility and usefulness so that it can deliver exceptional surface finishes irrespective of the material being machined or the depth of cut, controlled experiments where the machine’s software and generator settings are altered and reconfigured, have been the order of the day.
This iterative process of pushing the machine to its limits and recording the results is on-going.