EDM rings the changes

New industries, such as mobile phone manufacturing, have created a need for more specialised machine tools. Douglas Friedli looks at how electric discharge machining is anwering the call

The UK machine tool industry has been going through something of a rough patch recently, as manufacturers have cut investment in the face of recession. So when the managing director of a machine tool company says: `Things just keep getting better and better,’ it is worthwhile finding out who and why.

This particular optimist is Glynn Fletcher, who heads Swiss manufacturer Agie’s UK operation. And the reason for his optimism is a product that appears to have found its time – the electric discharge machine, which uses electric current to burn through conductive material (see `What is EDM’, page 20).

Fletcher estimates Agie’s year-on-year sales are up 15%. Machine tool distributors estimate sales of EDM in the UK have risen 50-100% over the past five years, with similar growth predicted for the next five.

The reasons for this impressive growth appear to be twofold. First, some of EDM’s traditional mould and die-making markets are undergoing a revival. Second, several new markets are being opened up in industries where the sort of precision and streamlined processes that EDMallows are important.

The mobile phone industry has been one of the biggest manufacturing success stories of recent years and has opened up a huge new market for EDM suppliers, as mouldmakers try to meet the very precise specifications set by handset manufacturers.

Walsall-based Ryford Plastics has just installed six new Agie EDM machines – four die-sink and two wire erosion – to cope with demand from mobile phone giant Motorola. The product specification is extremely precise, covering everything from the grippable texture of the outside of the phone to the intricate detailing inside.

In this industry, standardisation is the watchword. `Cavities must have exactly the same surface texture, whether they are made in the US, Germany or the UK,’ says Stan Parker, Ryford’s tool room manager. `The only way you can get that texture is through spark erosion.’

At the moment, the typical lead time for a mobile phone die is 9-10 weeks. Reducing this time is vital for an industry which expects products to have a maximum life of 12-18 months. Ryford believes its investment in new machines, which can work unaided 24 hours a day, will allow it to reduce lead time to eight weeks. If nothing else, speeding up the process might make life more interesting. `Watching spark erosion is about as exciting as watching paint dry,’ says Parker.

Most EDM sales are to the tool, mould and die sector. As well as being used to manufacture very precise and intricate plastic products, EDM is also useful to toolmakers who work with hard materials. Because the process relies on current rather the hardness of a cutting surface, materials like polycrystalline diamond can be cut relatively easily.

Beyond these traditional markets, something else is happening. General engineering companies are starting to use wire-erosion EDM as a substitute for milling, grinding and turning.

Its selling points include precision, including the ability to cut sharp corners, and a reduction in the number of stages required to make each part. These are all advantages which have made the UK’s Formula One industry an enthusiastic early adopter of the process.

Dave Hawke, machine shop manager at McLaren International, says: `One particular benefit of EDM is the lack of pressure during machining compared to traditional methods such as milling. Flimsy parts do not require expensive fixturing as wire erosion produces no side loads.’

Other industries starting to use EDM more include aerospace, where turbine blade manufacturing benefits from the its ability to turn a lump of metal into an almost finished part.

It is also being used for smaller products for which the length of time the process takes does not matter as much as meeting exact specifications. Examples include tooth implants and metal connectors.

If EDM does have a problem it is with the speed of the process, which is slower in terms of metal removal than conventional machining. But manufacturers point out that EDM requires far less preparation and finishing, because an almost-finished product can be cut directly from raw material.

According to Bernard Hom, head of Liverpool University’s department of industrial studies, the only cloud on the horizon for EDM is high-speed machining. `HSM is becoming more economical for forging dies and shallow cuts, but there are some things HSM simply cannot do, for example cutting deep slots and intricate features, because of the problem of tool vibration.’

Hom sees EDM competing with the threat from HSM by becoming faster and easier to control. `Two weeks ago I was in the US, where a group is developing new electrode materials such as zirconium alloy,’ he says. `These could increase removal rates by 50% or more.’

The efficiency of the process is also being improved by more sophisticated controls, for example by the increased use of fuzzy logic. This adjusts the process to changing local conditions such as temperature or air pressure.

Another problem which has held back sales in the past is electrolysis, but EDM manufacturers have now worked out how to deal with it. The problem arises from the electric current interacting with particles in the dielectric fluid surrounding the process. This leads to increased wear on the working parts of the machine, and poor surface integrity of the part being made.

However, reversing the polarity of the current thousands of times a second gets around the problem by preventing any long-term build-up of contaminants.

Increasing sales have given manufacturers confidence that EDM’s time has come. If manufacturers can keep opening up new markets, there is a good chance that their optimism will be justified.

What is EDM?

Electric discharge machining works on the principle that an electric current can be used to burn off tiny quantities from a lump of conductive material.

Working at a rate of several millions of sparks per second, these tiny quantities cumulatively form an accurate cut or impression. There are two distinct methods and in both, the electrode and the part being made are surrounded by a non-conducting dielectric fluid.

In the first, wire erosion EDM, current is applied to an ultra-thin wire, often made of brass or copper, which slices through a conductor like a cheese wire through a lump of cheddar. New wire is fed through constantly, allowing intricate profiles to be cut to tolerances of around 2-3 microns.

With the second method, die sink, sinker or ram EDM, current is applied to an electrode, generally a shaped piece of graphite, copper or copper alloy, which sparks and burns off metal. The electrode is often the inverted shape of the object it is to create, so an electrode to produce a mobile phone mould will look like a mobile phone.