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Dec. 1, 1999
With input from one of its best customers, an EDM manufacturer improves on the design for its new-generation wire machine.

With input from one of its best customers, an EDM manufacturer improves on the design for its new-generation wire machine.

Some design changes on the Charmilles Robofil 2030SI EDM are the result of input from Hutchinson Technologies Inc.

A general view of Hutchinson's feed line and forming system for the suspension itself.

Here, operators build suspension assemblies at Hutchinson.

Using parts made on a Charmilles EDM, this unit bends the arm to hold the microscopic clearance between the disk and head.

Before building its next-generation wire-EDM, Charmilles Technologies partnered with Hutchinson Technology Inc. for suggestions as to what changes or features were needed. Why ask them? The Hutchinson, Minn., shop EDMs parts with tolerances as tight as 25 to 50 millionths of an inch, and Charmilles felt this more than qualified the shop's input for improving machine accuracy, stability, and repeatability. As a result of the collaboration, Charmilles developed the Robofil 2030SI wire EDM with upgrades to the machine's Z column, wire threader, and generator.

Hutchinson makes suspension-drive components for computers — the reader head and its associated arm located above the hard drive disk. Reader heads actually fly, similar to an airplane wing, over the disk at an altitude of only 2 millionths of an inch. So part tolerances, especially for wall straightness, are crucial. For example, on a 1-in. punch, the shop holds a wall straightness tolerance down around 25 millionths of an inch. Hutchinson's first suggestion to Charmilles was for a more stable column to better these tolerances.

"To achieve that kind of straightness," says Richard Wolf, district manager at Charmilles, "requires drastic steps by the machine builder to squeeze out every few millionths more of machine accuracy." Charmilles got these few millionths more of straightness by chilling the 2030's entire Z column with cold water.

According to Wolf, it wasn't enough to just chill the surrounding areas near the head, tooling, and workpiece. Heat can cause movement, and on a column-design machine that works in millionths of an inch, that movement is magnified, says Wolf.

Hutchinson's second suggestion was for more consistent automatic wire threading. The 2030 rethreads wire in the gap, even with fine diameter wire, says Wolf. For example, the new machine can automatically thread a 0.004-in.-dia. wire through a 0.006-in. hole. Also for better wire threading, the 2030's lower arm is more stabilized and the guide system has no built-in clearance.

In addition, Charmilles improved how the machine stretches and straightens the wire for threading. Using air, the 2030 takes out curls and shapes the wire to the form of a hypodermic needle, says Wolf, making automatic threading more consistent and reliable.

Hutchinson has eight Charmilles EDMs, six of which have been updated with the new SI generator developed specifically for carbide. "The system," says Wolf, "eliminates micro cracking, the effects of electrolysis, corrosion of the water, and thermal changes." Basically, it creates a cooler running spark.

"With the SI system, we can more precisely control the recast layer, resulting in less jig grinding and other secondary surface operations," comments Ken Van Antwerpen, tooling manufacturing manager at Hutchinson. This kind of control, adds Van Antwerpen, is imperative when working with parts measuring 12, 10, or even 8 millionths of an inch.

Hutchinson EDMs tooling for forming its parts, which originate from material as thin as 0.0007 to 0.004-in. This gage-quality tooling actually manufactures product, and the process is done in a dry condition. "We walk a fine red line doing this process," says Van Antwerpen, "because oxide buildup and any interfacial boundary wear would clog the gap, tolerance, or clearance." This means tooling would have to be readjusted or repolished. With the surface control system on the Charmilles EDMs, Hutchinson produces finishes that help deter these conditions.

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