How to Pull Off Tough Machining Jobs Without Pulling Out

The requirements of heavy duty machining are a daily challenge in the aerospace and energy producing industries. Innovative materials, such as various titanium alloys, are not only light but also very rigid, corrosion resistant and difficult to machine. This doesn’t only affect the machine concepts and processes, but also the cutting tools and tool holders that are being used.

Many workpieces are made from a solid block: during this milling process, up to 90 percent of the material is being removed. In order to optimize the process economically as well as qualitatively and in order to achieve a high metal removal rate, high torques and feed rates with low rpm are chosen. But during this High Performance Cutting operation (HPC), high pulling forces occur. In combination with high cutting forces and aggressive feed rates, a flexing movement of the tool in the tool holder is created which in the end increases the risk of tool pull out. This especially affects all of the tool holder designs that provide accurate clamping and a high run-out accuracy, such as shrink, hydraulic or milling chucks.

To answer this challenge, the Safe-Lock™ pull out protection system from Haimer GmbH (Igenhausen, Germany) emerged: spiral-shaped grooves are ground into the cutting tool shank and have the same angle like that in the tool. In combination with special EDM form-closed drive keys in the tool holder (shrink fit chucks, collet chucks or also hydraulic chucks), these grooves prevent the cutting tool from twisting or being pulled out of the chuck during extreme machining, which causes very high costs when producing expensive work pieces. The combination of pull out protection and high runout accuracy leads to less vibration and very efficient metal removal rates. This makes it possible to significantly increase metal removal rates by increasing the cutting depth and the feeds. In addition, tool wear will be reduced.

This pull out protection system has become widespread within the aerospace industry. “Through the introduction of this system and the shrinking technology from Haimer, we can guarantee process reliability even with milling challenging high temperature materials,” explained Alexander Steurer, a senior manager for NC – Programming Stator Components at MTU Aero Engines AG (Munich, Germany). “This is a prerequisite to guarantee smooth processing during manufacturing of frames and castings, given our high degree of automation.“

The benefits of less than 3 µm runout that this symmetrical pull out protection system design provides, coupled with optimum balance and the possibility for easy length presetting, were substantial reasons for MTU to switch to this system instead of continuing to use Whistle Notch or Weldon tooling systems. While these other systems do actually prevent tool pull out, both are unsymmetrical by design, resulting in insufficient runout and balance accuracy. But the combination of pull-out protection and highest concentricity in the Safe-Lock system leads to low vibration and, as a result, a very stable machining process. Through increased cutting depths and feeds, MTU increased their metal removal rates significantly. And with the improved runout accuracy of the Haimer shrink fit chucks, they improved their tool life by up to 50 percent.

The Safe-Lock system has also found enthusiastic followers in the fiercely competitive automotive industry. “Perfect quality and delivery reliability are the basic requirements in order to quote in our industry,” noted Daniel Rautenbach, the managing director of CNC machining specialist Glätzer GmbH (Solingen, Germany). “Pricing is highly competitive. For this reason, in our business, the difference between profit and loss comes down to process efficiency, meaning quality without compromise is a must.”

During one of their biggest machining projects, Glätzer operations manager Ingo Schulten became aware of this pull out protection system and started using it in the specific machining application of a part for a pneumatically operated truck disk-brake that consisted of Type EN-GJS-800-2 spheroidal-graphite cast iron. In order to mill concave contours, the contact between cutting tool and the workpiece isn’t just punctual, it actually covers between 30 percent or 40 percent of the tool. “The extremely high engagement and cutting forces cause the tool to want to pull out from the holder,” explained Schulten. “The Weldon Chucks that we were using ensured that the cutting tool stayed in the holder, but the side lock screw prevented the tool from achieving good runout accuracy. Tool life was very unstable and even led to tool breakage.”

The milling tests with Safe-Lock convinced Schulten and the other employees at Glätzer: “To me, the switch to this system seemed obvious, like using an electric starter instead of a crank to start a car,” smiled Schulten. “The cutting data improved significantly. The tool life consistently increased by 40 percent.”

This pull out protection system is also becoming increasingly popular during HSC machining applications with high-helix end mills and in trochoidal milling operations, where the cutting speed and axial depth of cut can be increased through software support and productivity is significantly improved. Milling operations are carried out three times faster with deeper depths of cut, even in applications with hard and difficult to machine materials. However, this also increases the danger of tool pull out. Even though only a thin chip is usually removed during trochoidal milling operations, the entire length of the cutting tool edge is often used during the process, resulting in higher axial forces that force the operator to pay attention to safe cutting tool clamping.

A shrink fit chuck with the Safe-Lock system can be an ideal system in this application because it offers more security than the Weldon system, is easier to install, and can be clamped very precisely. The balancing and runout characteristics of the shrinking technology, in combination with the clamping safety of the pull out protection system, permit the possibility of greater productivity through faster permissible speeds and increased tool life, all with complete tool security assurance.

Haimer USA, LLC, 134 E Hill Street, Villa Park, IL 60181, 630-833-1500, Fax: 630-833-1507, www.haimer-usa.com.