Micromilling 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.

High-speed milling a multi-cavity micromold. Photo courtesy Makino.

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.

High spindle speeds are important, yet probably not as high as you might think. For example, Makino’s iQ300 has a maximum speed of 45,000 rpm, but Bradford explained that running the spindle wide-open is pointless unless appropriate feed rates can be maintained. “We establish a target speed and then the machine tool dynamically adjusts the feed rates depending on the geometric data it’s encountering as the cutter is moving.”

A rule of thumb when determining chip load per tooth is that 1 to 5 percent of cutter diameter should be engaged during cutting, but several factors may reduce this value, including workpiece hardness and material, part geometry and the aspect ratio. The latter is critical when deciding between EDMing and hard milling. “EDMing clobbers hard milling in aspect ratios above 8-to-1,” Bradford said. In short, if you need to run a 0.002"-dia. endmill, you’d better reconsider micro-HSM if you’re going much deeper than 0.016".

A mold insert for miniature LEDs. Photo courtesy Makino.

A 0.002" endmill is half the diameter of a human hair. How do you even measure something that small? According to Bradford, traditional laser measurement systems cannot be used because the accuracy of a laser is relative to the tool diameter that you’re measuring. As the tool diameter decreases, the percentage of error grows, with as much as 50 percent to 60 percent error on the very smallest tools.

When measuring microtools to ensure 100 percent accuracy before micro-HSM, Gisbert Ledvon, business development manager at GF AgieCharmilles LLC, Lincolnshire, Ill., said, “Laser measurement is not accurate enough. This is why we developed our Intelligent Tool Measurement system, where we use a camera (optical system) to look at the cutting tool and compare it to the CAD file of the cutting tool itself.”

According to Ledvon, GF AgieCharmilles used the system while manufacturing a 40-cavity LED micromold. Using its Mikron HSM 400ULP machine, AgieCharmilles was able to hold positional tolerances of ±1.5µm.

Microlution Inc., Chicago, also focuses on building machines for producing microparts. “From our inception in 2005, we saw a need for a specialized type of milling machine geared to small parts,” said Andy Philips, company president.

In addition to linear motors, glass scales and a small work cube, Microlution employs 160,000-rpm spindles, granite supporting structures and high-speed machine controllers.

Maintaining extremely consistent cutter engagement is the most important element when performing micro-HSM, said Philips. “Every time that tool comes around, you want it to engage the exact same amount of material,” he said. “This is more important when hard milling because there’s a much narrower range of criteria for successful cutter engagement. If you’re cutting aluminum, there’s a wide range of what the tool is going to be able to cut. But with hardened materials, if the feed rate is too low, you rub the tool and generate heat. And too much feed rate or a too-deep DOC leads to inconsistent surface quality and tool breakage.”

All this talk of tool engagement, aspect ratios and DOCs might sound like black magic, but Philips said there’s definitely a science to it. There are various cutting strategies, tool geometries, coolants and tool engagement parameters—all with different variables. Fundamentally, it’s the same process as at the macroscale, but some of the factors unimportant at the macroscale become crucial at the microscale.

Why not sinker EDM? “There are some things you can only do with an EDM, such as small-diameter deep holes and deep pockets,” said Philips. “But if you’re talking about geometries where there are no cutting limitations—in other words, either technology can produce the part—then hard milling will win hands down.” µ

About the author: Kip Hanson is a manufacturing consultant and freelance writer. Telephone: (520) 548-7328. E-mail: khanson@jwr.com.

Toolpaths: precision is key when making micromolds

When cutting tools are the size of a human hair or smaller and move a few microns at a time, they must have a precise toolpath. Cimatron Technologies Inc. develops software for designing and manufacturing mold and die tooling, and has been exploring “smaller” opportunities.

CimatronE Micro Milling software generates NC toolpaths for a micromold. Image courtesy Cimatron.

“We saw a huge opportunity for the micromilling industry, and have partnered with several manufacturers of micromilling machines to address this,” said Ralph Picklo, vice president of sales for Cimatron, Novi, Mich. He explained that the biggest issue with micromilling is achieving tight tolerances.

“With these small tools, you need to have very discrete tool control and be able to cut the way that’s best for the machine as well as the cutting tool,” he said. This requires toolpaths that follow the part geometry, eliminate sudden moves, provide smooth corner transitions and accurately calculate remaining stock. “You need smooth toolpaths and a constant chip load. If chip load varies, you’ll snap the tool.”

According to Picklo, you can’t take just any CAM software and start programming for micromilling, because they typically aren’t suitable for the tolerance requirements. “Many standard CAM packages have stock models with ±0.004" of accuracy. You can’t do micromilling with that. If you think about it, you might be working with a 0.006"-dia. cutter, so you’d better know where every couple tenths of stock is. The more zeros after the decimal point, the better.”

—K. Hanson