Surface quality: Mirror-finishing or polishing?

In the example, test pieces were inserts for a sheet metal forming tool made of cold work steel with a hardness of 48 HRC. Two pieces were milled from solid material in a Röders RXP 601 DSH 5-axis milling center using different CAM strategies and appropriate milling tools.

For the first piece, tools and milling strategies in-line with current practices in mold manufacturing were selected. This is referred to as the "classic" approach. In contrast, the possibilities offered by efficient CAM strategies, modern 5-axis HSC milling and new tool designs were fully exploited for the second part.

However, this "modern" approach requires a significantly greater programming effort, especially for the finishing step. The objective in both cases was to achieve the optimal result in geometric accuracy and surface quality.

Finishing and surface quality"Unfortunately, finishing strategies rarely focus on surface quality," said Fuest. The selected cutter is often too large, leaving more residual material that has to be removed in time-consuming, subsequent operations. Although the difference between the lateral stepover of a D16 and a D12 ball cutter is minimal, significantly more time than previously saved by the larger stepover is needed to machine the residual material areas afterward. In practice, a part is often first machined in 3-axis Z-constant mode and then in 3-axis axis-parallel mode to minimize this residual material. Although this can be quickly and easily programmed, it takes significantly more processing time. Similarly, 5-axis avoidance or collision-avoidance options are rarely implemented because of the necessary programming effort.

5-axis milling is indispensable if better surface qualities and especially high-gloss surfaces are to be achieved without manual reworking. In this case, the milling path has to follow the surface and the tool must always engage at an angle to avoid a center cut.

Required: A milling center that is both robust and precise"High cost effectiveness requires the performance of all operations in the same setup on the same machine," says Dr. Oliver Gossel, Head of Sales at Röders GmbH in Soltau, Germany. "In addition to the necessary roughing performance, the machine must be especially able to achieve the required accuracy and surface quality." The 5-axis Röders RXP 601 DSH used for the tests is extremely robust and can handle both roughing and finishing. The HSC spindle reaches speeds of up to 30,000 rpm, and the gantry design and rotary axes with high-load counter bearings ensure maximum rigidity. For optimal precision, all axes use frictionless direct drives supported by frictionless weight compensation in the Z-axis.

Another special feature of this machine plays a specific role in the directional changes that are critical for precision machining. The high "jerk" (i.e., change in acceleration with time) of the drives enables desired feed rate to be maintained for as long as possible, even on highly curved surfaces. Sophisticated temperature management also plays a special role. Because heat is the enemy of precision, all the key system components have their own temperature control.

The temperature of the temperature control medium circulating in this is kept stable with a precision of +/- 0.1 Kelvin. An outstanding unique selling point of Röders systems is the in-house-developed control system.

A block processing time of less than 0.1 millisecond and a look-ahead of more than 10,000 blocks yield optimal accuracy and surface quality. Especially noteworthy is the exceptionally high control speed with a clock frequency of 32 kHz in all axes. This means the toolpath is corrected every 0.03 milliseconds.