CNC Lathe Advances Breed Novel Cutting Tool Designs

Y-axis travels and secondary spindles have engendered some new approaches to familiar tasks.

In the metal cutting realm, a “new” cutting tool usually means something very much like an old cutting tool. Sometimes it’s of a familiar style but with genuine improvement in the cutting geometry or material, but more often it’s an addition to an existing tool line but in a different diameter or length. Revealed much less often are cutting tools that are of an entirely new design and based on a novel approach to the cutting task.

Two recent tools fit that criteria. Designed for different tasks by different companies, what they have in common are methods of taking advantage of the flexibility of CNC lathes in novel ways.

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A Turning Tool in a Milling Spindle

FreeTurn is a turning tool from Ceratizit (ceratizit.com). It takes advantage of the capabilities of combination turning/milling centers to enable traditional turning operations—roughing, finishing, contour turning, face turning and longitudinal turning—to be done, without pause, with a single tool. This saves lots of time and money.

“Imagine a workpiece that needs to be manufactured with multiple contours. In most cases it’s necessary for each contour to have its own tool and insert. This means a pre-defined approach angle with a unique chip breaker and corner radius,” says Ceratizit turning product manager Dirk Martin. So, to go from roughing to finishing requires separate tools and inserts; likewise, to go from a contour of one angle to a contour with a different angle. Those tool costs add up.

On top of that, even with the speed of modern tool-changing technology, every second that it takes to change a tool—every second, that is, that metal isn’t being cut—reduces throughput, which also increases costs.

What the engineers at Ceratizit realized was that there’s no need for the tool to be limited to a single angle, nor to a single cutting edge if there’s a milling spindle available as is the case with mill-turn machines. In Ceratizit’s High Dynamic Turning or HDT system, the milling spindle holds a specialized tool, the FreeTurn, which is used not for milling but for the turning operation. The use of the milling spindle lets the tool approach and point of contact to be varied, and because it is able to rotate—or index—it allows the multi-sided cutting insert, which features edges with different properties--angle points, corner radii, surface finishes and so on—to be deployed as needed during the operation.

“Instead of the classic, static position of the insert in the holder, the milling spindle is now used to produce the corresponding approach angle to the workpiece,” according to Martin. “The use of the spindle drive creates a degree of freedom of 360 degrees without the risk of collision, providing flexibility.” The angle of approach can also be changed any time—even while cutting. This not only enables flexible machining of almost every workpiece contour, but also adjusting to optimize chip breaking, feed rates and tool life.

The upshot is a single tool that can conceivably replace several tools, resulting in savings in tool change times, tool magazine loadouts, and tools themselves.

Parting in Y

Another tool designed to leverage improvements in CNC lathes is the CoroCut QD parting tool from Sandvik Coromant (sandvik.coromant.com). Parting is the task of using a blade-like tool to sever a finished workpiece from the bar stock it was made from.

Traditionally, the parting tool needs to be moved into the workpiece using the lathe’s cross-slide; it moves in the X-axis. This directs the vector of cutting forces sharply across the cross-section of the tool, resulting in high loads, making the tool susceptible to deformation that can damage the workpiece at worst and result in a shorter tool life at best. Parting blade designs have tried to compensate for this by increasing the tool height relative to the width.

The CoroCut QD instead takes advantage of newer CNC lathe designs that allow tool movement up and down along the Y axis.

This means the top face of the tool’s insert is placed parallel to the end of the blade, as if rotating the insert seat 90 degrees counterclockwise. The 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 method eliminates critical stresses typical to conventional blades and increases the blade stiffness by six times compared with standard designs. Or put another way, the extent of deformations was as low as one-sixth of the deformations generated in standard blades.

The CoroCut QD can part diameters up to 7 inches. The blades come in standard widths of 3 and 4 mm, 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.