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Tooling and application strategies for parting-off and grooving
10/29/2013 10:00:00 AM

Fagersta, October 2013 – The basic goals of parting-off and grooving processes are the same as those for general turning operations: achieving the desired shape of the part being machined, meeting specifications for accuracy and maximising productivity. In addition, however, parting-off and grooving applications are distinguished by specific issues in regard to tool strength and rigidity as well as chip control. Tool manufacturers employ innovative tooling designs as well as advanced coolant delivery strategies to meet the special requirements of parting-off and grooving processes. 

Tooling for parting-off and grooving
Like general turning operations, parting-off and grooving processes involve rotating a workpiece and cutting it with a stationary tool. The first consideration is configuring a cutting tool system that will produce the desired part shape. As a result, the designs of parting-off and grooving tool systems vary according to the size and depth of the features being machined. For deep grooving and parting-off operations on large parts, as well as for shallow grooving and cut off on smaller parts, tool manufacturers, for instance, will offer on-edge or tangential-configuration parting-off and grooving inserts that are clamped directly in a holder.

One example is Seco Tools new line of star-shaped tangential design inserts with four cutting edges called the X4 series. Available in cutting widths from 0.5 mm – 3 mm (0.02" – 0.12"), the inserts are engineered to minimise material consumption in parting-off and enable precise grooving and copying of small and medium-sized complex parts. Depending on the cutting edge width, the tool’s maximum cutting depth ranges from 2.6mm- 6.5mm (0.10" – 0.26") and can cut off maximum bar diameters from 5.2 to 13 mm (0.20” to 0.52”). The inserts’ tangential design directs cutting forces into the holder to maximise rigidity, stability and productivity. 

Following determination of the basic insert shape, choice of the lead angle of the cutting edge is a key factor in parting-off and grooving effectiveness. A zero-degree lead angle tool provides perpendicular alignment to the workpiece and transfers cutting forces directly into the toolholder, which enhances accuracy, tool life and surface finish. As parting-off is completed, however, a zero-degree tool leaves a small knob or pip at the centre of the bar being cut off. If the pip is undesirable, using a tool with a slight lead angle will cut off the pip as it passes through the centre of the part. Lead angle tools also are less prone to generate burrs in some workpiece materials.

After the basic tool configuration is set, workpiece material characteristics generally dictate the insert grade that will machine a part most efficiently. A tough workpiece material or interrupted cut will indicate use of an insert grade biased towards strength and impact resistance, while a particularly abrasive workpiece will require an insert grade engineered for wear resistance. Tool coatings such as Seco’s Duratomic® CVD aluminum-oxide coating are used to fine-tune tool characteristics for specific workpiece materials and toolpaths.

Application recommendations
There are certain specific setup recommendations for parting-off and grooving tools. Care should be taken to mount the tool so the blade is truly perpendicular to the axis of the workpiece. This minimises axial forces on the tool and will prevent rubbing on the sides of the insert. Regarding tool position, the centre height of the cutting edge should be as close to the centre of the workpiece as possible, within +/- 0.1 mm/ 0.004", again to avoid putting excessive pressure on the tool and possibly decreasing tool life.

Cutting parameters for parting-off and grooving tools differ somewhat from those employed in general turning. If spindle speed is constant, the cutting speed of the parting-off tool decreases to zero when it reaches the centre of the bar. The slowing speed puts heavy stress on the tool and can lead to built up edge. As a result, feed rate should be reduced up to 75 percent as the tool reaches the centre of the part. In addition, cutting speed can be adjusted to minimise vibration. The inserts used in parting-off and grooving operations usually are narrow, which can tend to produce instability in the cut. Therefore, holding the insert in the shortest blade possible and clamping it in the largest tool shank that will not interfere with the workpiece will also help control vibration. Ensuring rigidity of the machine tool itself, a necessity in any machining operation, will also serve to dampen unwanted vibration

Chip control challenges
The limited space in the cutting zone characteristic of parting-off and grooving operations creates challenges regarding control of the chips produced in machining. Especially in the parting-off process, the cutting tool is surrounded on both sides by workpiece material while in the cut, restricting the chips’ path of escape. Finally, depending on the workpiece material, the thin chips generated in parting-off and grooving operations tend not to break. An uncontrolled continuous chip can jam in the cut, mar the workpiece, and endanger the operator. In addition, chip control problems will preclude untended or “lights out” operation.

Many tools for parting-off and grooving feature cutting edge geometries with features engineered to bend the chip and break it if possible. Seco’s example is its MC chip breaking geometry. If surface finish and other considerations permit, a pause in tool feed – known as dwelling – during the cut can help break chips. Another method for chip control is application of coolant, which can flush away chips that otherwise might clog the cutting zone. However, traditional flood coolant usually has insufficient pressure to reach the cutting zone in parting-off and grooving applications. Additionally, it is difficult to position flood coolant nozzles for optimum placement of the coolant stream. Finally, the relatively weak flow of flood coolant may turn to steam in the cutting zone and actually form an insulating barrier that can contain, instead of dissipate, the heat generated in the cutting process.

An alternative to flood coolant is coolant applied at high pressure and as close to the cutting edge as possible. Today’s machine tool coolant pumps generally provide coolant at pressures between 20 bar (290 psi) and 70 bar (1,015 psi). Seco’s coolant delivery tooling system, for instance, offers the versatility to operate from low pressures with some productivity impact at around 5 bar (72 psi) to high pressures at 70 bar (1,015 psi), as well as extended capacity of 275 bar (4,351 psi).

For maximum effectiveness, high-pressure coolant must be delivered in a targeted fashion, as close to the cutting zone as possible. Tool manufacturers have developed a number of high-pressure coolant delivery systems. A popular method involves routing the coolant through the cutting insert. Seco has determined, however, that the most effective coolant flow generates a “wedge” between the insert rake cutting zone and the chip, lifting the chip and breaking it off. It is apparent that when coolant is channeled through cutting inserts, it is difficult to direct the stream into the optimum direction to create the wedge. It is not enough to get the coolant in the neighbourhood of the cutting zone; to act as a wedge, the stream must be positioned closer to and directed towards the cutting edge.

Consequently, Seco developed a coolant delivery system called Jetstream Tooling® that directs high-pressure coolant through inducers located in the toolholders themselves. The small-diameter apertures of the inducers generate an acute, high-velocity stream of coolant that can penetrate and lubricate the high-friction zone between the workpiece and tool’s cutting edge. Recently, in an innovation aimed at controlling chips in difficult operations, the company has incorporated what it calls Jetstream

Tooling®Duo technology in its X4 parting-off and grooving toolholders. This method delivers coolant from two outlets. In addition to upper jets that are directed to the optimal point of the rake face, the new Duo technology uses an additional coolant jet to flush the clearance surface. The cutting edge receives high-pressure coolant from two opposite directions – above and below – maximising control of chip flow as well as cooling the cutting zone.

Specialised applications
Chip control is especially essential when processing hard-to-machine workpiece materials such as titanium alloys and stainless steel. These materials provide high strength and high resistance to heat and wear, and are often used in high-value parts in the aerospace, power generation and medical industries. But the very characteristics that make these materials excellent for use in critical applications decreases their machinability. Chip breaking depends on chips absorbing and being softened by the heat generated in cutting, but titanium alloys, for example, is a poor conductor of heat that produces tough chips that are hard to break.

Sharp tools with high positive rake can cut materials like titanium alloys productively. However, to control chips and maximise productivity, high-pressure coolant delivery tooling is often needed. The combination of targeted coolant flow and the wedge effect between the insert rake face and chip results in chips that break into smaller, more easily managed pieces.

Conclusion
Parting-off and grooving processes make up an important subset of turning operations. They also provide a number of singular challenges. The restricted cutting zone characteristic of these processes requires careful consideration of basic tool shape, geometry, and insert materials, as well as setup details and cutting parameters. Chip control, a concern in any machining operation, becomes more critical when space is tight and narrow cuts produce chips that are thin and hard to break. Tool manufacturers have developed chip control geometries that can help solve this problem, and cutting strategies such as feed pauses can contribute as well. High-pressure coolant provided in a carefully-targeted fashion can be an excellent way to control chips. Because uncontrolled chips require constant operator oversight, a major benefit of consistent chip control is gaining the ability to perform lights out/untended or minimally tended machining operations. The coolant also provides the same advantages it offers in other machining operations, including longer tool life and/or the ability to increase cutting parameters. Together, the tools, techniques, and innovation of today’s parting-off and grooving tooling enable users to maximise productivity in this specialised but important group of machining processes.

 

By:

Carlos Bueno-Martinez, Global Product Manager Parting-off & Grooving, Seco Tools

 

Seco Tools is a leading manufacturer of high performance metal cutting tools. Seco’s product range includes a complete programme of tools and inserts for turning, milling, drilling, reaming and boring as well as complementary tool holding systems. With more than 25,000 standard products, Seco is a complete solutions provider for the metal cutting industry and equips machine tools from the spindle down to the cutting edge.

 The company is headquartered in Fagersta, Sweden and represented in more than 50 countries worldwide with 40 subsidiaries, distributors and channel partners.