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Paul Horn's new cutting material improved tool life

Paul Horn's new cutting material, improved the previous tool lives

Machining Composites As Fine As Paul Horn

Using this new cutting material, immediately improved the previous tool life of between 750 and 1,000 parts to a reliable 3,000 parts.

When machining composites, the challenges stem from the individual properties of the highly resilient fibres made from carbon, aramid or glass that are bonded with elastomers, thermoplastics or thermosetting plastics. Extremely hard and abrasive fibres are combined with softer plastics, especially those that are heat-sensitive. Depending on the position of the fibres, composites have either isotropic or anisotropic properties. 

There is a diverse range of materials due to the different orientations of the fibre layers, different percentages of plastics used and often as a result of combining composites with sheet metal. The requirements for machining are clean, tear-free surfaces without any protruding fibres or delamination.

This leads to conflicting requirements for the tool cutting edge such as light and cool cutting coupled with a highly stable, long tool life and smooth tool surfaces that prevent adhesion of the material.

Difference Between PCD And CVD

“When designing tools for composite machining, the key is to combine the various tool design options in the best way possible. Just a few of the parameters that can be used to define the optimum tools are, the axis intersection angle of the cutting edges on the top layers for compression cuts, cut divisions to reduce the cutting pressure, pre-machining by scoring the top fibres and coordinated rake and clearance angles,” analyses Markus Kannwischer, Technical Director and Member of the Management Board at Paul Horn GmbH in Tübingen. 

Once the macro geometric parameters have been determined, the appropriate grade can be selected. Certain options have to be discounted due to the macro geometry and in the case of very small tools, solid carbide offers considerably more design freedom than PCD-tipped or CVD-tipped milling cutters, for example.

The substrate should be sufficiently tough to be able to absorb the dynamic cutting forces generated when machining the fibres. To improve service life, the tools are often diamond-coated. This makes it possible to create a tough cutting edge with a very hard surface layer (hardness 10,000 HV5) as well as a high level of sharpness.

This ensures process reliability and long tool life. However, the process of seeding the diamond grains will only work if the cobalt content of the carbide is not too high. In the case of PCD, hard individual diamond grains are bonded with tough cobalt or other elements.

The composite material is electrically conductive and can be machined and shaped by electrical discharge machining. CVD consists exclusively of diamond and is mainly laser-cut or ground.

This makes CVD much harder than PCD. Conversely, PCD is tougher than CVD, which means it is better protected against edge fracture. As PCD is made from individual diamond grains, the properties of the PCD can be more finely matched to the individual application by using different grain sizes on the cutting edge.

Consequently, the parameter window within which CVD can be applied is significantly smaller than for PCD. However, if the correct parameters are selected, CVD tools have a much longer service life than PCD-tipped tools.

Researched And Tested

The top priority during machining tests is safety. In addition to the usual safety precautions for such tests, attention must also be paid to dust extraction. The maximum allowable concentrations (MAC) must be observed.

“When carrying out machining trials at our test centre, we rely on special extraction equipment. We use all kinds of measuring systems, such as cutting force measuring instruments, high-speed cameras and surface measurement equipment. Often customers’ workpieces are flat parts that require a corresponding workholding. We conduct the functional testing in-house and then perform the final tool life tests in conjunction with customers on their machines. We collaborate with institutes to carry out fundamental studies and further series of tests. The results are always evaluated by the customer, as everyone assesses the requirements for a cutting edge or for a milled surface differently,” states Kannwischer.

In the field of cutting tools, bore machining is the biggest market.

“Here at HORN, we focus predominantly on the trimming and production of pockets and openings, in other words: on milling tool production,” explains Kannwischer.

When it comes to diamond coatings for solid carbide tools, new multilayer diamond coatings are ensuring a high level of wear resistance while at the same time providing a sharp cutting edge. The combination of tougher carbides and good layer adhesion is increasing the range of applications for diamond-coated solid carbide tools.

In the area of CVD cutting edges, knowledge has increased considerably over recent years as far as the correct application parameters are concerned, with the result that huge leaps in performance have been achieved here as well. In conjunction with the latest laser technology for producing sharp and precise cutting edges, this has broadened the range of CVD applications significantly.

Customer Success Story: Deutsche Technoplast

One example is a switch or clamping cam used to transport bottles in a bottling plant by Deutsche Technoplast, which is based in the German town of Wörth an der Donau. 18 different variants of this switch cam in various lengths are injection-moulded and machined in quantities of approximately 250,000 each year.

The material is polyether ether ketone (PEEK), of which 30 to 50 percent is carbon fibres. It is a plastic boasting high mechanical and thermal resistance, which has a highly abrasive effect on the tool’s cutting edge during machining. Available in left or right versions, the different switch cams have a bearing surface roughly in the middle with a diameter of 20, 24 or 30 mm depending on their size and tolerance of e8.

The switch cams are supported by a component made from food-safe stainless steel. Arranged 60 times in a row, the cams bear the weight of the filled glass bottles in the bottling machine. Each of them can withstand 20 million load cycles over a calculated tool life of four years.

As an alternative to PCD, the Horn field sales representative recommended a CCGT insert with soldered CVD diamond cutting edge for the standard holder with a standard interface that had been used up to this point. This CVD diamond cutting edge has a positive cutting angle and a cutting edge radius of 0.4 mm. Using this new cutting material, immediately improved the previous tool life of between 750 and 1,000 parts to a reliable 3,000 parts, while also enhancing the consistency of the surface quality throughout the tool life.



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