[Alan Richter]

Opinions vary about the need for CAD and CAM software specifically designed for micromanufacturing, but those designing and making microparts and microfeatures on larger parts agree that knowing what to do with the software you have is essential.

“The thing about software as a general concept is that [workpiece] size doesn’t matter, in theory,” said Hanan Fishman, president of Delcam’s PartMaker Inc. division, a Fort Washington, Pa., CAM software provider. “But in reality size does matter. I suspect more engineering and CAM systems are specifically designed for the midrange-sized parts.”

Fishman noted that PartMaker CAM software is a good choice for micromachining because the software developer has worked with many customers who do Swiss-style machining of tiny parts. That experience has enabled the company to provide its customers the flexibility to adjust machining parameters for microscale requirements and the ability to visualize the machining process.

“The biggest issue in the microworld is visualization,” he said. “The value of visualization goes up when the naked eye’s ability to visualize it occurring in the machine goes down.”

Similar to big parts, additional processing is required for microparts—but with very different numbers. “When you’re working in the microworld, your biggest concern is with the accuracy of the significant digits to the right of the decimal point,” Fishman said.

The number of digits to the right of the decimal point is greater for microparts than those to the left for macroscale ones based in part on the machine tools geared for each, according to Hari Sridharan, vice president of engineering for Cimatron Technologies Inc., Novi, Mich. “Milling machines for general-purpose applications are coordinated for four and sometimes five digits,” he said, referring to the English measurement system. “Whereas in micromilling, you probably have to go to six- or seven-digit accuracies.”

That requires CAM software to input and output data to the same level of precision. “It’s all well and good to say the software can input to six decimal places of precision, but if it can’t output any more than three, what’s the point, and vice versa,” Fishman said.

Geared for micro

Sridharan added that to achieve the accuracy level required for micromilling, the CAM software also needs to be geared for micromilling. To that end, Cimatron offers the CimatronE Micro Milling CAM package, which incorporates a unique algorithm and corresponding set of NC tools that enable effective scaling of jobs to the micro environment, according to the company. That environment includes machining demands such as generating 0.5mm or narrower rib widths, achieving precision of 5μm or higher and imparting surface finishes of 0.2μm Ra or finer, according to Cimatron.

“The basic requirement for surface finish is how accurately you can calculate the toolpath,” Sridharan said, “and our micromilling algorithm allows us to calculate the toolpath much more accurately than conventional methods.”

In addition, many CAM systems use a triangulated mesh for mathematical calculations of the geometry, and Sridharan noted that the meshing techniques are slightly different for microparts compared to larger ones. That means the triangulated mesh calibrations performed for tiny parts and features are sometimes not accurate enough, according to Sridharan. “CimatronE has special algorithms built in so it can deal with these small parts.”

Others have found that post-processor codes created for their specific machines have overcome any problems that could be encountered when micromachining. That was the case for Micro Precision Parts Manufacturing Ltd., Qualicum Beach, British Columbia, Canada. Steve Cotton, the company’s owner and president, noted that MPPM uses Mastercam CAM software and initially experienced some issues related to resolution of the cuts. “Some of our step-overs are 0.02mm, so we had to fine-tune and tweak the resolution,” he said.

That required CNC Software Inc., Tolland, Conn., the developer of Mastercam, to write post-processors applicable to MPPM’s machines and the size of parts and features being produced, such as a 0.065"-dia. part with complex surfaces on the ends and another one with a 1/16" radius. “Most machines can’t interpolate that because their settings and resolution are not fine enough,” Cotton said. “We developed [the post-processors] with Mastercam over 3 years.” Since then, no CAM issues have arisen.

Manufacturability

Being able to machine even the most-complex micropart still requires that a part design can physically be machined. “We’ve had the odd occasion where we said to the designer, ‘You’ve drawn this and it looks real pretty but to actually get a machine to manufacture that you need to adjust this and this and this,’ ” Cotton said. “But, by working with designers and with Mastercam, we’ve been able to produce just about everything that’s been put in front of us.” He noted that most of the designers MPPM works with use SolidWorks CAD software from Dassault Systèmes SolidWorks Corp., Concord, Mass.

That’s partly because the software is able to design part geometries at a very small level, according to Shaun Murphy, manager of CAD product management for SolidWorks. He explained that solid modeling kernels for CAD have a specific work-sphere size. “Due to the resolution that’s built inside the kernel, you have a volume of space available to design between,” Murphy said. “If you start with micro design features and stay in that rough ballpark, you can design fine.”

However, problems can occur when machining microfeatures on macro-scale components because the resolution sphere would be exceeded.

According to Sridharan, those problems can be avoided by machining the large part sections with Cimatron’s regular algorithms and then using the CAM software’s micromilling algorithm when applying small tools to create microfeatures and achieve the requisite tight tolerances and fine surface finishes.

“The switch between the two is seamless and in the same interface, but it’s not automatic,” he said. “You have to select the area you want to do micromilling, and you have to select the area that you want to do regular machining.”

Incorporating design validation in the early conceptual phase before the model is fully defined—rather than as a post-process activity—is the latest trend in the design of microscale parts, Murphy added. “Before, it was let’s design it, let’s build it and then let’s test it to see if it’ll actually work,” he said. “But with design, the longer you wait in the cycle, the less capability you have of being able to affect change.”

According to Murphy, validating a design early in the process is effective because the finite-element model that’s created for validation is usually a simplified version of the realistic design model, reducing the complexity and time required for making the finite-element calculation. For example, holes that don’t impact a part’s structural integrity can be removed without affecting the result of that calculation. “The software doesn’t really care about them,” he said.

However, the designer has to be experienced enough to know which holes will affect part integrity and which ones won’t. “Some of the modern [design software] tools do make it easier to apply these techniques earlier in the cycle,” Murphy said, “but there still has to be a level of knowledge about what you’re doing or you can shoot yourself in the foot. For example, the etch holes in a gyroscope not only affect the resonance frequency of the device but are also required for gas damping calculations.”

Micromachining wisdom

Software is also no substitute for experience when selecting micromachining parameters. “It’s very difficult to select the machining parameters, but we do it based on our knowledge,” said Dr. Simon S. Park, professor at the Mechanical and Manufacturing Department in the Schulich School of Engineering at the University of Calgary, Alberta, and a team leader of the university’s Micro Engineering Dynamics Automation Lab (MEDAL). “When I set the rpm of the spindle speed, I set it in such a way that it will not chatter. And when I set the feed rate and depth of cut, I do it in such a way that it does not violate the tolerance level for tool deflection.”

He noted that MEDAL does use a simulation program developed in-house for simulating cutting forces, as well as Esprit Mold CAM software from DP Technology Corp., Camarillo, Calif., primarily for toolpath generation, and Moldflow simulation software from Moldflow Corp., Framingham, Mass., for design validation and optimization.

“We use the software packages to provide a rough guideline for the parameters and then we tweak them,” Park said. “The biggest challenge at the CAD/CAM end in defining parts is that you have to consider the cutting forces to compensate for deflection of the cutting tools and their paths.” MEDAL manually compensates for that deflection, and its knowledge base enables the lab to achieve the required part accuracy.

Park added that the biggest challenge when micromolding is overcoming premature solidification of the plastic being injected into a mold because the material cools quickly when passing through a small channel with its high area-to-volume ratio. “Instead of the material filling properly, it solidifies prematurely and you have an incomplete fit,” Park said.

Accumold LLC, Ankeny, Iowa, a production plastic-injection molding house that also makes tooling, doesn’t use design validation and optimization software because it doesn’t provide what’s needed for molding microsized parts, said Aaron Johnson, the company’s marketing manager. “From what we know about that type of software, there are limitations because it might say you can’t flow plastic in that spot and we say, ‘Oh, yeah!?’ Making incorrect assumptions is a high potential in something of that nature.”

Johnson added that Accumold uses CAD and CAM programs for its tool building processes and hasn’t experienced any problems even for the tightest tolerance and finest surface finish requirements. “Our analysis is all done out of experience and internal training,” he said. “We have not found a software package that says, ‘Here’s how you set up your machine to run this part or, yes, this will mold or, no, it won’t mold. It is more brain power than it is computer power.”

Brad Holtz, president and CEO of Cyon Research Corp., a Bethesda, Md., provider of analysis and consulting services for engineering technology markets, concurred that the user’s knowledge base is more important than the software tools when designing and manufacturing microscale parts.
That requires having the knowledge to apply the tool set rather than having a tool set that’s designed specifically for micromachining. “If you’re somebody who knows what’s happening at the microscale, you’re not going to be having any software issues, but you have to be explicit,” Holtz said. “It’s a user-based function as opposed to having somebody who designs widgets and needs to design something at the micro-
scale. He’s not going to know what he’s getting himself into.” µ

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About the author: Alan Richter is senior editor of MICROmanufacturing. Telephone: (847) 714-0175. E-mail: alanr@jwr.com.



PartMaker CAM software simulates complex micromachining operations to check for collisions and other errors. Shown is the process for machining an angulated dental abutment. Photo courtesy Partmaker.





The CimatronE Micro Milling CAM software constantly and automatically records remaining stock to help prevent delicate microtools from breaking. A 0.1mm-dia. tool was applied to produce this electrode using CimatronE Micro Milling CAM software (inset), which offers adaptive machining strategies such as adaptive feed control to maintain constant tool load. Photos courtesy Cimatron Technology.



The biggest issue in the microworld is visualization. The value of visualization goes up when the naked eye’s ability to visualize it occurring in the machine goes down. Photo courtesy Cimatron Technology.




An example of a microtool applied at the University of Calgary’s Micro Engineering Dynamics Automation Laboratory. Photo courtesy University of Calgary.

Toolpath generation for efficient micromachining


Cutting tools as small as 50μm in diameter are required for machining microscale parts and features, which means the tools are quite fragile. According to Dr. Simon Park of the University of Calgary’s Micro Engineering Dynamics Automation Laboratory, applying 1 newton of force—an extremely small force—at the tip of such a small tool can deflect the tip up to 4μm. He added that when machining with microtools, both rubbing and cutting occurs, with chip thickness equal to a tool’s edge radius, which might range from 0.5μm to 4μm, depending on whether it’s coated or not. Unlike macroscale machining where shearing is the main metal-removal activity, micromachining is more like grinding.

“It’s a plowing effect and that causes an increase in cutting force and generates poor surface finishes,” Park said.

Grinding, however, is required when making ceramic microcomponents, said Steve Cotton of Micro Precision Parts Manufacturing Ltd. “We say machining, but it’s actually a precision grinding process,” he said. “Holding precision with grinding tools is a real art.” Those tools are typically diamond microtools that cost $800 or more. “And you can burn a tool in an hour.”

To achieve maximum tool life and productivity when micromachining, a CAM software package that’s able to generate accurate toolpaths is required. One such program is CimatronE Micro Milling, which calculates the remaining stock that has to be removed on a workpiece, according to Hari Sridharan of Cimatron Technologies Inc. “The toolpaths cannot be accurate unless you have technology to accurately understand the knowledge of stock remaining,” he said.

In addition to following a toolpath that requires too much material for a tool to remove without slowing down to avoid breakage, it’s important that air cutting—the time the tool is not in the cut—is reduced as much as possible to enhance productivity. Sridharan said Cimatron’s preview technology allows a user to see the outcome of a toolpath before it’s generated. That saves the end user time by enabling him to make the right decisions about toolpath strategies before simulation.

—A. Richter




The design for a MEMS (microelectricalmechanical systems) gyroscope device produced using SolidWorks CAD software. Photo courtesy SolidWorks.



Accumold molds a range of microscale components. Photo courtesy Accumold.



Contributors


Accumold LLC
(515) 964-5741
www.accu-mold.com

Cimatron Technologies Inc.
(248) 596-9700
www.cimatrontech.com

Cyon Research Corp.
(301) 365-9085
www.cyonresearch.com

Dassault Systèmes
SolidWorks Corp.
(800) 693-9000
www.solidworks.com

Micro Engineering Dynamics
Automation Lab
University of Calgary
(403) 220-6959
www.enme.ucalgary.ca/research/medal

Micro Precision Parts
Manufacturing Ltd.
(250) 752-5401
www.precisionmicromachining.com

PartMaker Inc.
(215) 643-5077
www.partmaker.com