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A new dimension in parting off

Technology 2017-08-17 Turkka Kulmala Borgs

Challenge: Conventional parting blades suffer from high vibration-inducing loads across their weakest section. Solution: Rethinking of the tool design shifts the resultant load to the strongest section and dramatically reduces deflection and noise.

​In conventional lathe-type machine tools the capability of feed motion in the XZ plane is an inherent limitation. In a parting operation, this inevitably directs the resultant vector of cutting forces sharply across the cross-section of the tool, resulting in high loads and susceptibility to deformation. The classical parting blade design compensates for this with a large height relative to the width.

Sandvik Coromant engineers now have a remedy that utilizes the modern machines’ capability to feed the tool in the Y direction. The new concept places the top face of the insert parallel to the end of the blade, as if rotating the insert seat 90° antitclockwise. The new blade cuts its way into the workpiece with essentially its front end, which roughly aligns the resultant vector of cutting forces with the longitudinal axis of the blade. FEM analysis confirms that this eliminates critical stresses typical to conventional blades and increases the blade stiffness by six times compared with standard designs. Put another way, the extent of deformations was as low as one-sixth of the deformations generated in standard blades.

In terms of application, the new Y-axis parting method is astonishingly simple. Similarly to conventional blades, the cutting edge is positioned as close to the centre line of the workpiece as possible. This is accomplished automatically as a side product of the tool length measurement, which the new tool requires for each set-up. This also verifies the correct centre height.

​Two machine tools types in particular benefit from the new concept: multitask machines and turning centres.

The domain of turning centres is mass production, typically from bar stock of 65 millimetres in diameter, where the biggest benefits of Y-axis parting are improved productivity and surface quality. The opportunities for quality optimization are very interesting because parting off is typically the last stage for a component. Thus, any work stages after parting off are preferably eliminated. Machining economics can be further improved by reducing the parting width.

​For multitask machines, Y-axis parting blades primarily offer increased accessibility and capability for larger diameters. A pre-test confirmed a 50 percent increase in the overhang when parting off a conventional 120-millimetre diameter bar at the maximum feed capacity of the insert. A 300 percent productivity increase was achieved with no process security complications. In a customer test case, Y-axis parting successfully replaced band sawing for a 180-millimetre diameter Inconel bar, resulting in significant productivity improvement due to dramatically shorter machining times.

Y-axis parting requires minimal changes in a typical production setting. The increased maximum cutting diameter of 180 millimetres can obviously alter set-ups, but in other respects very little needs to be changed. The CoroCut QD blades come in standard widths of 3 and 4 millimetres, fit in regular holders and revolvers and use standard inserts. The changes are mainly in pre-setting, programming and slightly different measurements due to compensation in the Y direction.

Test case
The Sandvik Coromant Production unit in Renningen, Germany, is currently using Y-axis parting in production of CoroChuck 930 and other basic holders. Expectations have been exceeded in all diameters and materials, and the new solution is being escalated into all applicable machines and bar diameters up to 102 millimetres.
“With the rapid sales increase of CoroChuck 930 we really need to ramp up our production,” says Mattias Brandt, team leader CNC programming. Adds CNC programmer Stefan Frick, “We have been able to cut our cycle times by up to 15 seconds only by changing the parting-off cycle to utilize the Y-axis, and at the same time we also get a much more robust process.”

Machine: DMG GMX400
Material: Alloy steel X40 and 16Mn
Component: CoroChuck 930
Cutting data: vc 120 m/min, fn 0.35 mm/rev
Results: Machining time reduction of five to 15 sec/part, thanks to substantially higher feed rate, increased from 0.10 mm/rev to 0.35 mm/rev.



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