Hard Milling vs. wire EDMing of steel molds

Say you own a small mold shop and quoted a job for a medical customer looking to produce implantable microclips for blood vessel repair. The job requires a number of eight-cavity molds—enough work to keep your sinker EDMs busy for weeks. Just then, your sales rep walks in with the bad news: a startup shop down the street quoted the job for half of what you did by hard milling the mold cavities, circumventing EDMing altogether.

Hard milling is an established process, but can it really compete with EDMing? The answer is a resounding … sometimes. When it comes to micromoldmaking, EDMing is great at producing intricate shapes, deep cavities, large surface areas and square corners. But if you make a lot of molds with part geometries smaller than a pea, relatively shallow and no more than 10 times the diameter of your smallest cutting tool, consider hard milling.

Before hard milling micromolds, however, understand that the process typically requires high-speed spindles and special tools, according to John Bradford, micromachining R&D team leader at Makino Micromachining, Auburn Hills, Mich. Bradford and the R&D team spend their days finding better ways to perform both EDMing and high-speed machining.

“You need refined processes to successfully machine at the microscale,” said Bradford. “If a company has the right infrastructure, one geared toward hard milling, then that type of machining becomes fairly achievable, and your costs become less prohibitive.”

What are those costs? For starters, machine tools specially designed for this sort of work might cost up to $450,000. This might sound like a lot for a vertical machining center, but it is essential for micro-HSM. “You need a spindle that can control not only the tool tip location at all times, but also the vibration and runout of the cutting tool,” Bradford said. Controlling the 3-D location of the tool typically requires linear motors, glass-scale feedback and mechanical control resolution down to 10nm.

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