Adapting CAM strategies to the ‘through-the-looking-glass’ world of micromachining
At first he was disoriented. After working in the world of traditional manufacturing for 20 years, all of the things Alistair Purdon thought he knew about feeds, speeds and step-overs no longer seemed to apply.
He had just taken a job with a small company, Micro Precision Parts Manufacturing, Qualicum Beach, Vancouver, B.C., which specializes in micromanufacturing. Purdon’s job was to transfer his knowledge of CNC manufacturing disciplines to a company whose president, Steve Cotton, started out as a certified Rolex watch repairman. But first, Purdon would have to get his bearings.
“The variables had changed and that baffled me a little bit,” he said. “I thought I would take all the techniques and rules of thumb I had learned using CNC equipment to manufacture mid-sized parts and the same proportions to making really small parts. It just didn’t work. It took me a few months to get used to the differences.”
What are some of the frequently encountered micromachining problems that can cause even experienced machinists like Purdon to lose their bearings, and what are some CAM-related strategies that can get them on the right path?
Steve Cotton, president of MPPM, working on a micropart at his Vancouver,
B.C., shop. Courtesy of CNC Software.
Nonlinear scaling
Perhaps the most disconcerting thing about dipping into the world of micromachining, as Purdon discovered, is that the adjustment of parameters to varying materials, geometries and part sizes is no longer a linear process. You simply can’t interpolate feeds, speeds and DOCs from what you are accustomed to for cutting mid-sized parts. It rarely works.
The best approach will sometimes be the exact opposite of what you might expect. For example, many machinists turn to climb milling for finer finishes. On the microlevel, climb milling tends to grab the material too much. Conventional milling seems to work better.
Micropart manufacturers are being asked to machine all sorts of exotic materials, including nonmetals. Cotton said some of the materials he has machined recently include titanium, Radel thermoplastic and D-2 alloy, as well as unique blends of materials so proprietary that his customers refuse to identify them. “They don’t want to risk losing the competitive advantage of machining a unique material, so they ship me the stuff and give me a generic description of the cutting properties,” Cotton said. “ ‘It cuts something like titanium,’ they might tell me. Frequently, it does not cut anything like that.”
Cutting hard materials with 0.5mm × 20mm diamond tools that are spinning at 150,000 rpm can be very challenging for the micromachining shop owner because if he’s not careful, the tools will break. The cost of breaking a diamond cutting tool the size of a sewing needle may be $600, not to mention the cost of damaging the part. Even worse, the machine operator may not even be aware that the tool has broken for many hours, because the sound of tools cutting microparts is not audible above the spindle’s whir.
Fifty of these medical parts are so small they
could fit on a penny. MPPM used a computer-
simulated manufacturing environment to devise
an integrated toolpath and workholding solution
that allows the parts to be manufactured 20 at
a time. Courtesy of CNC Software.
Premature tool wear can also be a big problem. Cotton experienced that when he first cut medical parts made of Radel, which is desirable for medical parts because of its ability to withstand high sterilization temperatures. What Cotton did not know is that Radel is a highly abrasive ceramic material that wore out his cutters after making just a handful of parts.
Workholding is always an issue, but can be more difficult with parts so small you can place 50 of them on the face of a penny. Even getting the workpieces in and out of the fixtures is a struggle. If you drop them, you may not find them again.
When tiny parts are made to tight tolerances, stacking errors that are
inconsequential in mid-size part manufacturing are magnified substantially in the micro world. As a result, even tolerance stacks within the CAM software itself must be taken into consideration to ensure specifications can be reasonably met.
Reorientation with CAM
As with all other forms of machining, an experienced programmer is an essential ingredient for establishing safe and efficient micromanufacturing processes. Beyond that, a high-quality CAM system provides the programmer with a set of tools to establish toolpaths for machining faster with fewer mistakes. The CAM system should also provide tools to capture and replicate the programmer’s hard-won knowledge consistently for future use with similar projects. Some of the most relevant CAM tools for micromachining include the following:
Integrated workholding. CAD capabilities within the CAM system allow programmers to design a holding mechanism around the actual parts and replicate the individual part-holding solution within multipart fixtures. This is particularly important when making microparts because they are frequently manufactured dozens at a time in a single fixture. The fixture must provide access to the tool in a confined area and facilitate flipping and removing parts without losing or damaging them.
Computer simulation. Once the programmer has created the manufacturing process, he can observe it in a computer-simulated environment on the CAM system to verify stock removal as well as tool and holder clearance. He can also watch the tool move through cutting sequences and calculate machining cycles. These verification capabilities may be even more important for machine shops that do a great deal of prototyping work. When the emphasis is on fast turnarounds, shops cannot afford the luxury of developing a safe and efficient manufacturing process at the machine.
Tiny toolpaths. Perhaps the hottest area of innovation among CAM vendors is the creation of new toolpaths that provide special benefits in specific manufacturing situations. While most CAM vendors offer toolpaths with similar descriptions, there may be important differences in the degree of control the user can exert within the toolpath. For example, some CAM programs offer many ways to control step-over—such as straight, smooth, retract, at-an-angle, arc-on and arc-off. This gives users the flexibility they need to deal with unanticipated problems created by the challenging geometries and materials frequently found in micromachining applications.
High-speed toolpaths. Toolpaths designed initially but not exclusively for high-speed machining are ideal for micropart manufacturing because they make very smooth transitions in and out of the cut to minimize stress on the tool, while letting the speed of the tool do most of the work. When the part has some oddly shaped areas that make unexpectedly heavy cuts unavoidable (either they are too deep or require too much tool engagement), advanced CAM software has functions that allow users to make fine, manual alterations to the toolpaths in these areas of the program and protect the tool.
Roughing. Ultrasmooth toolpaths are particularly important for roughing phases where large amounts of stock are aggressively removed. Things frequently go wrong when roughing, resulting in the accidental burying of a tool. As a result, the CAM system’s toolpaths must be flexible enough to provide reliable and safe roughing options. Trochoidal milling is a roughing option designed to save tools. Instead of going directly into a heavy cut, the roughing toolpath can be preset to “nibble” into the heavy cut when the cut exceeds a predetermined step-over limit (for example, 10 percent). Instead of burying the tool, the program channels the tool through the area by making very small cutting loops (trochoidal moves) until the step-over is back below the limit.
Fine finishing. Smooth toolpath transitions are important for finishing because they eliminate or reduce tedious and expensive manual finishing operations done under a jeweler’s loupe. Some effective finishing toolpaths for micromanufacturing include spirals, scallops, paths with proprietary blending algorithms and paths filtered to optimize the density of G-code points seen by the controller to smooth out its acceleration and deceleration.
This titanium aneurysm clip was machined on a Haas 4-axis mill using
Mastercam’s scallop toolpath to minimize subsequent hand-finishing operations.
For parts with complex shapes, a scallop toolpath smoothes the cutting transitions
by using an algorithm that measures the step-over from the previous step-over
directly from the surface of the part (as opposed to projecting it down the
Z-axis of the machine). Tolerances are maintained to within 20µm.
Courtesy of Haas Automation.
CAM value-added
Advanced CAM systems provide users with several other benefits integral to micromanufacturing success, including the following:
Libraries. A CAM micromachining system requires a robust database that includes information on toolpaths, materials, tools and workholders. This allows the user to rapidly apply successful manufacturing strategies from previous jobs. This proprietary knowledge base can enhance the shop’s capabilities and improve profitability, so backing up this resource and making it redundant are recommended.
Ancillary features. As micromachining requirements become better understood, some CAM vendors have been quick to add functions such as:
- automatic collision detection filtering capability,
- facilities for entering compensation factors for tool runout and thermal compensation, and
- routines that automatically check for tool breakage and wear.
Support team. Micromanufacturers often find themselves in uncharted waters, so trusted vendors of CNC equipment, tools, accessories and CAD and CAM systems should be part of the support team whenever feasible.
To formalize this advisory relationship, Cotton purchased a maintenance license for his CAM system. When he runs into an unexpected problem, his dealer logs into the shop’s programming environment remotely and they work out the solution together, sharing the same computer screen. When he is busy with prototyping work, Cotton uses this service several times a month.
Development. Like micromanufacturing itself, CAM systems are works in progress. Vendors have many initiatives underway to better understand the challenges and provide more and better solutions. Establishing an ongoing relationship with a CAM vendor will not only supply the answers needed today, but lead to more effective micro capabilities tomorrow. µ
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About the author: David Conigliaro is product manager, Mastercam Mill for CNC Software Inc., Tolland, Conn. Email:
Dave.Conigliaro@mastercam.com. Phone: (860) 875-5006.
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