Machining techniques using micro tools

Parts are now getting smaller and smaller, and they include more and more parts to provide more functionality. Making small, complex parts requires micro-tools that are non-standard and extremely deformable and breakable. Tools for micromachining do not react to the cutting environment like large tools. There are a number of microfabrication problems that affect the basic mechanisms of the process, which are fundamentally different from those that exist when machining with large gauge tools. These problems include changes that occur during chip formation, changes in cutting forces, abnormal vibrations and instabilities in the process, and the formation and characteristics of the machined surface.

Micro-parts are used in a wide range of applications in the aerospace, automotive, biomedical, electronics, information technology, optical and telecommunications industries. In order to reduce costs, most of the miniature parts are made with molds. Moldmakers face a variety of challenges, ranging from rare materials to special mold coatings, milling parts with 0.002 inch (50 μm) diameter tools, EDM machining with 0.00078 inch (20 μm) wire, and "Submarine" level accuracy, etc. Many parts are now designed with features that are less than 100μm in size - just slightly larger than the human hair.

Micro-parts are usually machined with a mold that must be micromilled to achieve the required accuracy.
(Photo courtesy of Cimatron)

For such small parts and feature sizes, precision has a new meaning. For example, a tolerance of ±0.0002 inches (±5 μm) is certainly quite different for parts that are 0.2 inches in size, as opposed to parts that are 0.002 inches in size.

Cimatron Co., Ltd., based in Givat Shmuel, Israel, is a CAD/CAM software supplier, says Hari Sridharan, engineering director. “In order to achieve the quality and precision required in microfabrication, while meeting economic and commercial constraints, it must also be Optimized and synchronized throughout the manufacturing chain.” As simple and moderately complex mold production is transferred to countries with low labor costs, moldmakers in the US and Europe are turning to more advanced technologies, such as micro molds and Micro-milling to maintain your competitive momentum.

CimatronE software features built-in high-speed cutting strategies optimized for micro-milling surfaces and other specialized machining functions.

Small parts, big challenges <br>The main challenges in this area include direct milling of molds consisting of micro-components and the manufacture of EDM micro-electrodes. Challenges associated with micro-milling include the use of micro-tools as small as 100 μm or less, spindle speeds up to 150,000 rpm and surface roughness of 0.2 μm (0.0000078 inches). At the same time, it is often unrealistic to polish fine parts with fine structures, so micro-milling requires no polishing.

Why micro-tools are deformed and broken Compared to conventional milling, the tool is more susceptible to deformation and breakage during micro-milling. Ken Yap, director of Suwa Precision Engineering Co., Ltd. in Singapore, believes that there are three main reasons why micromachining tools are more susceptible to breakage.

First, as the metal is removed during processing, as the chip thickness decreases, the required specific energy is greatly increased. This means that for micromachining, with a small depth of cut, as the chip becomes thinner, the resistance of the tip of the microtool is greater than in conventional machining. It is as if the workpiece material is getting harder and harder in microfabrication. This resistance is large enough to exceed the bending strength limit of the tool tip even before any significant wear of the tool and may result in breakage of the tool tip. One way to prevent this is to ensure that the chip thickness is less than the blade tip radius.

Second, the sharp increase in cutting force and stress caused by chip clogging during micro-milling may also cause damage to the tip. In most micro-milling operations using a micro-tip with two cutting edges, each cutting edge removes chips from the cutting zone at half turn. However, if chip clogging occurs, the cutting force and stress rise beyond the tip bending strength limit during several tool rotations, and the tool will be damaged. For this reason, some users prefer to use high speed steel knives because they are softer than carbide tools and can better tolerate the presence of chip clogging.

The third reason is that the tool tip loses its own edge due to the thickening of the cutting edge and cannot be processed efficiently. As the workpiece begins to impact the top of the tip, the tip is slightly deformed. The increase in tool distortion and the stress caused by the milling per revolution eventually cause the tool tip to break. This process is also known as complete damage.

Due to these phenomena, most of the machines used for micro-milling are equipped with sensors (for measuring the force acting on the tool tip) and advanced CAM software (predicting the chip load throughout the micromachining process).

The Alpha 2 Plus micro drill from Titex Precision Tools features external cooling with optimized chip flute and tip geometry.
The new product range is available in 80 sizes ranging from 0.5mm (0.019685") to 2.9mm (0.1141732").

CAD/CAM Requirements <br>In order to achieve the micro-size and the extreme precision required for micro-machining, this can not be achieved by simply reducing the size of the milling machine, tool holder and tool. Again, software must be tuned and optimized for subtle processes. Forming and modifying geometry with the right precision, flatness, and continuity is just the starting point for a micropart CAD solution. The CAM system must be able to handle tight tolerances and super finishing. Due to the inability of the operator to intervene to prevent tool breakage, the software must accurately consider the chip load throughout the machining process.

Cimatron has introduced an NC software solution for micro-milling – available with the CimatronE 7.0 product package. When the solution was released in 2005, the company said the software was the first commercial solution for micro-milling. The software is a product of Cimatron's participation in a micro-milling research project, sponsored by the European Community and hosted by the Fraunhofer Institute for Production Technology (IPT) in Aachen, Germany.

Since manual polishing is not an option for micro-milling, Cimatron software supports high-speed milling without polishing. To avoid the risk of tool breakage, reduce air cuts, maintain tool load and extend tool life, the software automatically records the details of the remaining blanks, even at the micro component level. The software supports 5-axis tilting so that deeper areas can be machined with taper tools. With the ability to tilt the tool from the material, it is possible to machine with a shorter tool. However, since continuous 5-axis milling is currently not as precise as 3-axis milling, machine specifications and actual performance must be carefully verified when performing fine milling with continuous 5-axis machining. Other features of the software include an optimized milling strategy and built-in CAD with precise (0.001 μm) surface generation and modification tools.