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Horn Blog

New coating increases grooving productivity

July 2021
Author: paulhorn
Company: Hartmetall-Werkzeugfabrik Paul Horn GmbH
New coating increases grooving productivity

For contract manufacturers, continuously reviewing their production processes is essential to maintaining their competitiveness. Such optimisation can achieve impressive results when manufacturers of cutting tools apply their expertise and incorporate advanced machining strategies.

Successful suppliers of turned parts utilise the latest production equipment and technologies and impress their customers with top performance in terms of their production methods and cost-effectiveness. By consistently implementing this strategy, in just a few years TecVo Zerspanungstechnik has transformed into an in-demand supplier of turned parts up to 380 mm (14.961") in diameter. This company, which is based in Bühl on the edge of the Black Forest, focuses on supplying components for the hydraulics, fittings and construction industries as well as for rail vehicles and mechanical engineering. The service based around the core business of turning ranges from customer consultation during product development to additional machining tasks such as milling, surface finishing, precision machining and heat treatment, sometimes in collaboration with predominantly local companies.

Rz values for internal grooves require new machining strategies

The large proportion of turned parts with many kinds of internal grooves is constantly presenting Production Manager Sven Vollmer and his team with new challenges. An example of this is the introduction of various recesses with an average roughness of Rz≤ 6.3 µm (248.0315 µin) in three geometrically similar turned parts. To fulfil orders for batches of 50 to 200 pieces, a future-proof solution was required to machine the workpieces reliably and cost effectively within the given time frame.

The first tests with carbide grooving inserts from HORN and other tool suppliers did not reliably produce the Rz specifications, which resulted in some of the workpieces needing to be polished manually. The next trial used ground cermet indexable inserts which produced significantly better results, but they were still unsatisfactory in terms of the tool life and surface quality. However, the tools recommended by Thomas Schnurr, Technical Consultant at HORN, encouraged the machining specialists to continue down this path and in the next trial they used type 229 cermet indexable inserts with the new EG3 coating.

Tried-and-tested coatings for diverse applications

A coating of a few thousandths of a millimetre in thickness decisively influences tool wear along with its associated effects on the machine, energy consumption, cutting fluids and consumables. To adapt this coating to the HORN product range in line with application requirements, the company is continually investing in coating processes such as PVD sputter technology and the new coating technology HiPIMS (high-power impulse magnetron sputtering). HiPIMS produces an even more homogeneous and significantly more durable coating with a hardness and toughness that demonstrate its strengths, particularly when it comes to steel machining and machining small and miniature parts. This technology made it possible to coat various grooving and milling tools with the new EG3 and EG5 titanium aluminium nitride (TiAlN) coatings developed by HORN. EG3 is predominantly used for the Supermini boring tools (hole diameters ≥ 0.2 mm (0.00787")) and ground indexable inserts. Both tool types are particularly well suited to the EG3 coating thanks to their smooth, highly adhesive surface. The EG5 coating is primarily used on indexable inserts with an edge radius of 0.01 (0.000394") – 0.03 mm (0.00118") used for circular interpolation milling.

Although they have different thicknesses, both coatings have a very dense structure with a particularly smooth top layer and improved adhesion properties. This balances the relationship between the coating adhesion and the residual stress and ensures exceptional hardness of the cutting edge. A gold-coloured layer provides a finish, making it easier to detect wear.

A variety of workpieces requires operation-specific tools

The new EG3 coating now had to demonstrate its strengths to TecVo on three different workpieces. The type 213 toolholder developed for grooving and longitudinal turning could be used for all three workpieces so was the perfect tool carrier. The short version of the holder (length 150 mm (5.906"), shank diameter 32 mm (1.259")) is suitable for machining holes from 38 mm (1.496"). Depending on the cutting insert, groove depths of up to 15 mm (0.591") can be achieved at retraction distances of up to 110 mm (4.331"). The tool system excels at this overhang with a concentricity of 0.05 mm (0.00196"). 0.02 mm (0.000787") is guaranteed for retraction depths of up to 90 mm (3.543").

The holder clamps a double-edged cermet indexable insert coated with EG3 from the S229 product range. With a 3 mm (0.118") cutting width, it enables groove depths of up to 7.5 mm (0.295"). Its free cutting geometry with a small cutting edge radius of 0.01 mm (0.000394") without chip formation has a very stable wedge angle. Its interrupted cutting in high-strength steels is an impressive feature.

Over the course of the tests, inserts with the chipbreaker geometries .10. and .20. were used without chip formation. Geometry .10. has a rounded cutting surface towards the rear and geometry .20. has a straight cutting surface towards the rear.

Identical cutting parameters reduce programming effort

Production Manager Sven Vollmer and Thomas Schnurr evaluated the performance of the cutting inserts step by step, starting with workpiece no. 1 made of C45E. Three 15.1 mm (0.594") wide internal grooves starting from an initial diameter of 81.3 mm (3.201") up to a final diameter of 85H8 mm had to be machined into this part.

The cutting speed of vc = 250 m/min (820.21 ft/min), the feed rate of f = 0.08 mm (0.00315) and the cutting depth of ap = 0.2 mm (0.00787") as well as an emulsion coolant of six percent were suitable parameters for machining  without vibration or interruptions despite the overhang of 70 mm (2.756") – 80 mm (3.149"). The specified roughness Rz ≤ 6.3 μm was reliably maintained and the cutting insert achieved a tool life of 50 workpieces with an operating time of 57 minutes per cutting edge. As the first signs of wear on the corner radius became apparent in this application, Sven Vollmer decided to change the insert to ensure the reliability of the production process. He was very satisfied with the result as he could use the uncoated cermet insert to machine up to 15 workpieces.

New coating excels with a tool life that is up to 3 times longer

Encouraging results were also obtained with workpiece no. 2 made of C45E when grooving three 8.2 mm
(0.323") wide internal grooves with an initial diameter of 72.0 mm (2.835") and a final diameter of 82.2 mm (3.236"). With the geometry .20. and the parameters tested with workpiece no. 1, 70 parts could be reliably grooved to the desired accuracy – the figure was max. 25 parts for the uncoated insert. The machining time per workpiece was 1.2 minutes. Again, the reason for insert replacement when grooving this workpiece was apparent wear on the corner radius and on the cutting edge.

Final confirmation of the chosen strategy was provided by the workpiece no. 3 made of the material S355J2G3 (St52-3N) with three plunge cuts. With the cutting parameters that had already proved effective on the previous parts and the chip shape geometry .10., 44 workpieces (25 with the uncoated cutting insert) were reliably machined to the desired accuracy. The operating time per cutting edge was 40 minutes. The cutting insert was changed at this quantity to prevent tolerance deviations in the surface finish.

Objective achieved: Reliable, ultra-smooth Rz values for increased tool life

The tests with the EG3 coating also impressed Managing Director Tina Vollmer. Like Production Manager Sven Vollmer, she also sees great potential in these grooving tools. This is why, after the tests had been completed, TecVo now uses the geometry .20. with the coating EG3 for all workpieces of this order package – there are currently eight pieces with up to six plunge cuts each. The significantly longer tool lives compared with the previously used cutting inserts and adherence to the specified roughness values were crucial factors in this decision. The S229 cutting inserts produced ultra-smooth surfaces for almost all workpieces until shortly before the end of their tool life with Rz values well below the specifications. The universal geometry .20. has also really proved its worth with other orders. These product features are extremely important for safeguarding the future of the service provider, which is constantly needing to meet individual customer requirements. This is firstly because the responsible HORN Technical Consultant is on hand as a highly competent expert in machining and process optimisation, and secondly because HORN supplies standard and special tools in a very short time frame thanks to its in-house production – from carbide powders to ready-to-use inserts – which can ultimately be a decisive competitive advantage for a service provider.

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