July 2020 - 5: Tools that give an extra lift
Based in Hohenlinden near Munich, Herrmann CNC-Drehtechnik GmbH has made a name for itself as a preferred supplier in the aerospace industry. Numerous aerospace OEMs rely on the precision and quality of the Bavarian machining specialist. To ensure the required quality, the machining experts at Herrmann use precision tools from Paul Horn GmbH.
\u201cWhen it comes to machining high-tech materials, we need suitable tooling solutions that can withstand high loads and live up to our quality standards,\u201d says Production Manager Stefan Schubel. He uses grooving tools from Horn to make pedestal bearings from tempered steel.
There is a common misconception when describing the extendable parts on a wing as \u2018landing flaps are there for landing\u2019. The correct name is a \u2018high-lift device\u2019 and these devices are required during take-off and landing to increase lift. Otherwise, the wing area would not be able to provide enough lift at low take-off and landing speeds. They also increase the drag when landing, as otherwise the speed would be too high and the wheel brakes could be overloaded. The pilot can extend and retract the slats (on the leading edge of the wing) and the flaps (on the trailing edge of the wing) to precisely control the lift. The slats and flaps are moved by means of a mechanism inside the wing and this is where the pedestal bearing from Hohenlinden comes in. The bearing transfers the torque of the linkage and compensates for axial play employing a spline. The pedestal bearing also serves as a flexible coupling between the shafts.
Working with exotic materials on a day-to-day basis\u201cHorn is constantly developing useful improvements to its tool coatings and cutting geometries. As a supplier to the aerospace industry, we spend every day working with special materials that are not always easy to machine,\u201d explains Schubel. When it comes to grooving the hub and shaft of the pedestal bearing, Schubel and his team use tools from Horn.
The bearing hub is made from 1.4548 (X5CrNiCuNb17-4-4), tough, high-strength steel often used in aerospace applications due to its high corrosion resistance and excellent mechanical properties. The machining experts at Herrmann CNC-Drehtechnik use the trochoidal grooving method to produce the large, deep recess in the workpiece. A full radius grooving insert from Horn\u2019s S229 System with a radius of 2mm is used for roughing. For the substrate, Horn application engineer Korbinian Niedermeier recommended the AS66 grade. \u201cIn conjunction with Horn\u2019s coating, this substrate is ideal for machining materials of this kind,\u201d says Niedermeier.
Trochoidal turningTrochoidal turning is ideal for turning deep, wide recesses where a high chip volume needs to be generated. \u201cWe have been actively using this method for around two years. We use the ESPRIT CAM system, as this software was one of the first to offer the option of trochoidal grooving,\u201d says Schubel.
The grooving process starts with a 30mm wide and 15mm deep (incremental) recess processed via trochoidal roughing with the full radius of the indexable insert with a cutting speed of Vc=140m/min and a cutting depth of Ap=1mm. The feed rate is set at Fn = 0.25 mm/rev and the finishing allowance is 0.2mm. Finishing is carried out on two sides with a 3mm wide insert with a corner radius of 0.2 mm that is selected from the S229 system. The total production time for the recess is under two minutes. \u201cThanks to this grooving method, we have no problems with long chips when working with a material that is difficult to machine,\u201d says Schubel.
The shaft of the pedestal bearing is made from SAE 51431 (1.4044), tempered to 1,350\u20131,500N/mm2. \u201cIt is often difficult to get hold of this special-grade steel. We purchase the material directly from the manufacturer in France, so we don\u2019t order lots of different diameter variants. The smaller quantities of the components we are producing justify the high metal removal seen in some cases. However, we wouldn\u2019t turn a 26.4mm shaft from a 55mm diameter bar,\u201d explains the production manager. An insert from Horn\u2019s 223 System is used for producing the 5mm wide recesses. In this case, each cutting edge has a life of over 300 workpieces. A Supermini 105 insert is used for internal profiling of the hole and the 45-degree chamfer. Finishing of the axial recess is also carried out using a Supermini. \u201cThese tools allow us to achieve the required surface qualities without any problems,\u201d says Schubel.
Focus on qualityDuring production, the focus is always on the quality of the components. To meet the strict requirements for aerospace certification according to DIN EN: 9100, it is essential to have a clean and tidy working environment, highly qualified personnel and a quality assurance process featuring high-tech measuring machines. However, official certification inspections are not the end of the story because Herrmann\u2019s customers also visit the site to carry out regular audits. \u201cCleanliness, precision and constant quality control are an absolute must when it comes to our everyday work\u201d, says Schubel.
The company produces every class of component for the aerospace industry, from Class 3 to Class 1. The most critical of these is the Class 1 component, the failure of which seriously jeopardises the safety of the aircraft. When a Class 2 component fails, it seriously impairs the ability of the aircraft to function but landing is still possible. The failure of a Class 3 component has no impact on functionality.
Originally founded in a large garage back in 1986, Herrmann CNC-Drehtechnik GmbH now has 7,000 square metres of combined production and office space in Hohenlinden, near Munich. This Bavarian company, which employs 100 people, specialises in the manufacture of aerospace components. Thanks to its expertise in the machining of critical components and hard-to-cut materials, the company has established a very good reputation with numerous large aerospace corporations, gaining \u2018preferred supplier\u2019 status.
The partnership between Herrmann and Horn spans more than 25 years. \u201cWorking with the technical advisers and Horn\u2019s internal sales team is always a pleasant and productive experience. We look forward to continuing to use Horn tools and hope that Horn will continue to provide the same level of reliability and delivery performance in the future,\u201d says Schubel.