The ultimate machining strategy with perfect adjustment of drills, milling cutters and parameters; this is necessary when machining medical screws made of titanium or stainless steel. Mikron Tool has developed a turnkey solution that simultaneously increases the processing speed of the hexagonal leaflet bone screws (better known as Torx) by 50%, while ensuring high profile accuracy and achieving almost burr-free results.

900 million medical screws are produced every year, showing an increasing trend. Every second gained in this process is important. Most of these screws are produced on a lathe, the threads are machined on the main shaft, and the hexalobal Torx sleeve is machined on the counter spindle. This particular operation is usually slower and defines the cycle time. This is an opportunity to improve efficiency and only takes a few seconds. Nevertheless, it saves time and money for manufacturers.

Mikron Tool provides a turnkey solution through Hexalobe-Program, which is not only based on cutting tools, but also provides a software package with the best machining strategy and correct parameters. Because during the development of the program, it is obvious that good results can only be obtained when all factors are in place.

The two materials used in 90% of medical screws today are titanium and stainless steel. When it comes to their behavior during chip removal, they are completely different. Therefore, in the development process, it is obvious that tools and appropriate strategies need to be differentiated. Especially titanium has high elasticity and requires stronger cutting force, which in turn will cause higher stress on the cutting edge. This meant that different carbides were required from the beginning of development.  

For the processing process, two materials must be distinguished. Although helical interpolation and side milling are possible using titanium alloys, Mikron Tool only recommends side milling when using stainless steel. Although this particular process is a bit time-consuming.

At the beginning of development, a well-defined strategy is as important as cutting tools. Because when choosing tools, you must know how these tools are applied.

Different methods were considered from the beginning. It turns out that it is particularly efficient to use a minimum of different tools, namely step drilling-milling a hexagonal pyramid-and then deburring.

Other strategies, such as pre-drilling and centering of six outer diameters and subsequent milling and deburring are a challenge, especially when using titanium. Facts have proved that this is a difficult task for micro-drills.

Among the selected strategies, the milling process is the most time-consuming operation and can be done by helical interpolation (maximum pitch 0.8 xd) or side milling (maximum step 0.5 xd).

Each program has its advantages and disadvantages: When using helical interpolation, the milling cutter generates a slight lateral load (Fx) because of the vertical stress (Fz) at the same time.

The higher stresses on the cutting edge corners must be considered, so they are geometrically strengthened. Generally, this results in faster and smoother operation. Using side milling (or wall milling), the total depth can be reached in several steps because only two axes are used in each step. Even if the radial stress on the tool is higher, the step distance of the entire operation remains the same.    

The four processes (pre-drilling-chamfering-milling-deburring) need to be completed in three processes with two tools. First of all, the step drill is used to machine the center hole and 120 degree chamfer, which immediately has an excellent surface quality of Ra 0.2 um and Rz 0.8 um. After milling the final shape, the tool will be used again in the last process to achieve an almost burr-free surface and excellent surface quality. Milling cutters with 3 or 4 grooves (depending on the diameter) are available in two standard lengths, so different Torx shapes can be machined according to the screw type. Its high rigidity allows machining with high feed and large steps, while still ensuring the necessary six-lobe contour accuracy. The surface quality is provided by a special geometric shape, which can also work in high steps, so the operation can be completed in a few steps.

In addition to efficient machining, which means shorter cycle times and longer tool life, the development of new tools is also focused on achieving the highest quality. Strict tolerances must be maintained to ensure the contour accuracy and wall verticality of the hexalobal sleeve, from the first screw produced to the last screw.

Obviously, surface quality and burr conditions are also an important topic.

Alberto Gotti, the head of development, confirmed that there is no "perfect solution for all situations."

Although this turnkey program including cutting tools, strategies and parameter recommendations provides excellent support to the end user, he still faces the challenge of finding the "his" balance between the most effective machining, high precision and quality.

The first experience of using the new CrazyDrill and CrazyMill Hexalobe tools on DMG Mori’s 32/8 lathe shows that compared with other available tools on the market, the cycle time can be reduced by 50% to 60%, and it has the ability to achieve rapid feed (per edge 0.03 mm/revolution) and high surface quality (Ra = 0.2 microns).

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