Di-Spark – Precision EDM – Machine tool aspects of precision EDM machining
Continuing our series of electrical discharge machining articles (extracts from our EDM manufacturing guide, ‘The EDM Handbook’), this snippet examines the machine tool aspects of running a successful precision EDM machining company.
Di-Spark: The Precision EDM Machining Company
11. Precision EDM: MACHINE TOOL ASPECTS
EDM machine tools continue to evolve as builders strive for improvements. Computer-designed high quality castings usually make up the backbone of most machines however one builder has departed from the traditional design, and instead of using castings, constructs their machine frame out of an inert, cement-like material. Both wire and spark erosion machines use various frame designs, typically C-Frames for smaller machines and bridge or gantry styles for larger models.
Whether movement is either done by the worktable or by the gantry, most CNC machines are controlled through precision-ground re-circulating ball screws, totally eliminating errors due to backlash. Movements are usually over linear-motion slide-ways for minimal inertia resistance. This is very important to help achieve the fine finishes and high accuracy demanded today.
Precision EDM spark erosion machine tool sizes range from small, bench-top models to huge machines capable of sparking motor car bumper and dashboard moulds tools. Wire machines likewise run from small machines with a working envelope of 150 mm x 275 mm x 150 mm to units capable of machining 600 mm x 1000 mm x 300 mm. Specially modified machines have successfully wire eroded parts over 500 mm tall. Consider the degree of technology and sensitivity required to monitor and control a moving work-piece weighing over two tonnes as it is being precisely cut by a brass wire only 0.35 mm diameter!
Some wire machines cut only in the X and Y axes and can only produce two-dimensional or straight-walled parts. Other machines have four axes – the traditional X and Y axes with the addition of U and V axes. This allows the machine to cut tapered shapes, for example, the angular relief required on press and mould tools.
Most of us have seen the unique wire eroded sample parts displayed at machine tool exhibitions and sales demonstrations. These typically demonstrate the machine’s ability to produce a transitional aperture formed by a profile at the top surface of the work-piece and an independent profile at the bottom surface of the work-piece.
Tapering capabilities usually average from 10 to 15 degrees, which will encompass the majority of taper applications, but some machines specialise in steep taper cutting. This is usually done with the U and V axes incorporated within articulated heads that can swivel to angles of 30 degrees or more, keeping the wire guides tangent to the travel of the wire and also incorporating submerged flushing conditions.
Some machines can drive fifth and sixth axes. The fifth axis being a programmable Z axis, giving the machine the ability to raise and lower the upper guide across an uneven work-piece. This keeps the guides and nozzles as close to the work-piece as possible for better accuracy and flushing. The sixth axis can be an auxiliary device such as a rotary table. This offers the creative and imaginative designer and engineer almost unlimited capability to produce impossible parts.
All manual spark erosion machines use the three primary axes X, Y and Z. CNC units can control these axes simultaneously with the addition of an integrated C axis. This feature allows the machine to perform indexing and helical machining as in sparking threads or cutting helical forms in hardened work-pieces. Additional axes can be added using auxiliary devices such as rotary tables placed on the worktable.
The continuing development of the power supply (or generator) is a major reason for the rapid advancement in cutting speeds.
It is interesting to look at the different methods and approaches taken by the machine tool builders in the ever-increasing quest for improved metal removal.
Improvements in the wire electrode, such as coated wires, have allowed the full potential of power supplies to be used.
Spark erosion electrode technology has continued to improve and electrode materials like graphite have led to an increase in metal removal rates. The development of high-density isotropic graphite is replacing copper electrodes. They offer reduced electrode wear and can be easier to fabricate into the desired shape, thereby reducing EDM operating costs.
In precision EDM, the amount of power or amperage is relative to the size of the generator. Some wire erosion machine tool suppliers provide 40 amps generators. Another supplier uses less than 20 amps in conjunction with modified spark waveforms to accomplish the same amount of work. Spark erosion machines have generators varying from 30 amps to 128 amps. All are successful.
Some of the modern high-performance wire EDM machines are successfully cutting over 300 square millimetres per minute in tool steels and even higher cutting speeds have been achieved in aluminium. Who would have imagined that cutting speeds of wire EDM would be over 30 times faster than they were just 15 years ago?
These significant increases in cutting speeds have allowed wire EDM to encroach upon traditional milling applications. Not only can wire EDM compare to conventional machining on a metal removal basis but also running costs are lower and there is often no requirement for subsequent operations such as stress relieving or de-burring.
There are many variables to contend with in high-speed wire EDM cutting such as sharp corners, changing sections and entry and exit cuts. These require adaptive controls with the ability to look ahead many blocks of program code and to adapt the current and/or the frequency of the spark to allow accurate machining without breaking the wire whilst maintaining the required geometric profile. The faster the cutting speeds and greater the accuracy, the faster the computer needs to be able to identify a situation and effect change to stabilise the machining.
