Exploring high speed cutting for ultra-hard materials

Related Vendors

DIPROFIL Diamantprodukter AB

Ramada Acos

Union Tool Europe SA

Creating the right climate

In May 2013, air conditioning was installed in the milling department, with an accuracy of ±1C. Such conditions are found in rooms for high-accuracy measuring machines. In addition, the shop had the opportunity to test Union Tool’s UDC end mills with a new coating. The first test was on a customer part of fine-grain carbide with 9% cobalt and a hardness of 91.5 HRa. Machinists were said to be nervous when they started the test due to concerns of damaging the spindle. “But then, while our Kern was buzzing as usual like a flying bee, we knew that it was working,” Bräm said. After the first test, the process had produced chips, very tiny ones 0.25mm long and 0.003mm thick, but nevertheless clearly visible under the microscope.

Ready for the second test

The second test series focussed on a defined part made of a normal-grain carbide with 10% cobalt (typical grain size 2µm and hardness 89.9 HRa). The shop wanted to machine a shape extremely difficult for alternative machining technologies such as grinding, erosion or laser: cavities with different depths, free-standing shapes and an important type of material removal. Shop manager Bräm said the special feature of the defined form was that the company needed to mill the internal shape into the solid material in 3-axis mode, that is to say they had to plunge vertically.

Since the shop wanted a real endurance test, it selected a slightly special procedure: roughing and finishing the contour with the same tool, a 2mm ball-nose cutter. The defined test geometry included the following points: surface quality, contour accuracy, machining time and milling tool wear.

Finding strengths, weaknesses

The points of contour accuracy and edge strength were the most interesting for the machining specialists, who assumed these would be the weak points of HSC carbide milling. The question was: What would be the effect of machining of large and small internal radii, and especially of sharp outside edges on quality?

Following the success of the first test, the second test was expected to show top results. The shop managers said the Z-axis of the milling machine was standing as if “nailed in place”, and the end mill displayed extremely stable behavior. The company managed to detect the minimal change of the surface of the ball-nose cutter with its analysis equipment (See results in photo gallery).

“Everything ran so smoothly, it felt almost spooky.” Bräm said. The tool worked without building up pressure, constantly producing chips, and the machine moved with no vibration from the tool. Here, programming with the CAM system helped. The results were measured after milling. Suitable polishing methods provided surface qualities of Ra 0.05µm, and the milled surfaces were said to behave in a totally different way during polishing than eroded surfaces. There is thus grounds to presume the milled surface has the same hardness as the base material. With erosion, the process can lead to a surface layer that needs removal by polishing to reach the basic material with the corresponding hardness. This can be a further advantage of HSC milling of carbide compared with electro-erosion machining: less polishing and, therefore, higher contour accuracy. For suitable geometries, this can lead to significantly reduced machining time.

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