Heidenhain controls and scales for turbine EDM

The use of Heidenhain's TNC 416 contouring control system and linear and rotary encoders on a customised five-axis electro-discharge machine (EDM) for turbine blade bleed holes.

The use of Heidenhain's TNC 416 contouring control system and linear and rotary encoders on a customised five-axis electro-discharge machine (EDM) developed by Sparcatron has allowed significant in-roads to be made in the production of critical turbine blade bleed holes.

With six Sparcatron Watersparc machines already installed on the aerospace customer's site, the combination of high accuracy positioning and process control has eliminated tooling changeover for different shaped holes.

This has been achieved through the combination of the Sparcatron machine and Heidenhain's TNC control and encoder technologies which has transformed the EDM drilling process more into one of milling, eliminating the need for a range of specially shaped EDM electrodes.

The five-axis Sparcatron machines were developed by the Stonehouse, Gloucestershire based electro-discharge machine builder in conjunction with Heidenhain (GB) of Burgess Hill in West Sussex.

Since installation, the customer maintains the technical superiority of the process means it can perform the same job as fast as a laser with virtually no recast layer and at a fraction of the cost.

Modern turbine blades are cast with internal cooling passages created from ceramic cores.

When the cores are removed, the apertures are welded and very fine bleed holes have to be drilled through the advanced Nickel-based super alloy of the blade and the much harder weld material.

In practice, only EDM and laser based production processes have proven to be capable of achieving the desired results.

For the turbine blade to function, clean and accurate holes are essential to introduce cooling air at a 'mere' 7000C while the blades are operating in an atmosphere of 1500C, which is close to the melting point of the turbine blade material.

However, even in this difficult environment, if a blade is correctly manufactured it will provide seven or eight years' satisfactory service.

According to design criteria, the holes either take the form of a circle or a variety of shapes and sizes from around 0.3 mm to 3 mm diameter.

To add to the complexity of the component, they can be required in various orientations on the same blade.

The development, by Sparcatron of the Watersparc high speed EDM, has created a cost-effective process to meet the specification of the blade design.

As the name implies, Watersparc uses de-ionised water at a pressure of 1,000 psi, rather than oil as the dielectric.

Water also reduces the likelihood of safety and fire hazards, as well as any process effects on the recast layer of the component material.

However, because water has a greater resistance than oil, it requires a correspondingly higher capacitance.

So, although machining can be three times faster, a larger spark is required from the EDM generator which also has to be controlled at higher processing speeds.

This makes the role of the control system so much more critical.

One advantage of the Watersparc machine is that the process does not require the use of specially-shaped electrodes so there is no need for multiple set-ups to accommodate the different holes.

The machine is able to generate all the sizes, shapes and angular positions utilising the capability of the Heidenhain five-axis TNC 416 control with just a single hollow brass electrode, through which the dielectric is pumped.

The five-axis operation of the control transforms the drilling process into one of contour milling.

The electrode is preset to rotate at 200 revs/min which not only creates a self-cleaning motion, it reduces any likelihood of tip wear.

With the rotating electrode design, the original brief was ach ieved.

However, the Sparcatron can also be used with rotation switched-out in order to perform plunge shaping.

Because of the levels of hole complexity, Sparcatron also selected Heidenhain linear and rotary encoders on all machine axes to ensure precise closed-loop positioning.

A member of the customer's Turbine Facility's technical team, recalled how the first Sparcatron machine was designed and built in just three and half months.

He praised the effort put in saying: "Although they are a small company, they were well able to respond very quickly." His original brief was also quite open in that what they wanted was to be able to drill almost any shape and they selected the Heidenhain control since it offered the greatest development potential coupled with good on-going technical support.

The Sparcatron is able to machine four turbine blades at a time.

These are mounted via the 'fir-tree' location into wire-cut clamps on the machine bed which is designed to accept a wide range of blade sizes.

A novel use of the electrode allows it to double as a touch probe to provide workpiece datum positioning to be fed back to the TNC control.

The required accuracy is high - on one set of blades, for example, the blade tip needs to be drilled with a set of nine holes of 0.3 mm diameter.

These are arranged at varying angles across the blade, to a positional tolerance of 0.25 mm and a tolerance on the hole diameter of ? 0.05 mm.

To achieve and repeat these tolerances, the use of Heidenhain linear encoders has been vital, giving an actual, rather than a hypothetical, position.

The linear scales are also 'self-correcting' both for wear and to counter any thermal effects.

Currently, there are up to 30 programs held in the TNC 416 control memory.

This, in conjunction with the elimination of tooling, makes switching between part numbers simple and quick.

Cutting four blades at a time at a rate of four set ups an hour, the production requirement is now fully in step with the rest of the new facility.

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