A new direction for turning

Manufacturing Engineering, Apr 2002 by Huddle, David

Plunge turning can be a cost-effective grinding alternative

Actually, the concept of plunge turning is not a new idea, but it's being applied using newly developed materials. Plunge turning is a highly productive form of hard turning that uses an insert's entire cutting edge or a portion of the cutting edge to create an orthogonal cut. This relatively new process has been proven to reduce machining time by as much as 90%, producing parts with comparable surface integrity to those that are ground.

In conventional turning, which generates a continuous groove, surface finish is largely determined by insert nose radius, feed rates, cutting speeds, and depth of cut. By contrast, the surface quality of plunge-turned components mainly depends on the quality of the cutting edge. As a result, plunge-turned components display low residual stress in the axial direction, and sealing properties equal to or better than ground surfaces.

Using a solid polycrystalline cubic boron nitride (PCBN) insert, stress patterns remain constant, even when large batches of parts are machined. In a test we performed that examined the physical properties of 250 plunge-turned parts, residual stress was significantly lower than the stresses caused by grinding.

Part quality is additionally affected by the distribution of residual stress in the tangential direction. In another test of 250 parts that we conducted, both grinding and plunging produced tensile residual stress at the surface, but the thickness of the affected zone was significantly thinner on plunge-turned components.

Plunge turning was also found to reduce the occurrence of white layers, because it reduces flank wear on the tool edge due to shorter cutting time per machined surface. Less flank wear also generates lower cutting forces and less friction, which results in less heat exposure to the surface of the workpiece.

The most critical properties of ground or hard-- turned components are dimension (diameter), roundness, straightness, bearing area, surface roughness, and residual stresses. For the first four properties, plunging can easily achieve the following tolerances for a gear wheel or bearing ring.

*Diameter +/-0.0006" (0.015 mm)

* Roundness +/-0.00015" (0.004 mm)

* Straightness +/-0.00015" (0.004 mm)

* Bearing Area 90% deep (0.00008" 0.002 mm)

And since plunge turning effectively removes metal by "peeling" the softened chip from the workpiece, coolant is not generally recommended. This helps to keep costs down while eliminating environmental issues surrounding coolant use. Dry machining also reduces the time and money spent on government-regulated chip disposal and reclamation processes.

There are many potential applications for plunge turning. The vast majority of hard components used in the automotive industry are machined to final geometrical form after hardening. Currently, grinding is the predominant method for finishing these parts, which include bearings, gears, shafts, and pinions. However, thanks to improvements in machine tool rigidity and the development of PCBN cutting tools, hard and plunge turning are gaining recognition as cost-effective alternatives to grinding.

Hard turning is performed on materials in the R, 45-70 range using a variety of tipped or solid cutting inserts. Since its introduction in the mid-1980s, the process has dramatically increased in popularity, and sales of PCBN cutting tools now exceed $250 million annually. Clearly, more and more manufacturers are recognizing the advantages of hard or plunge turning. But due to the cost of PCBN tools, perhaps up to 20 times more than conventional tools, many continue to view it as an expensive process.

Is plunge turning with PCBN really more expensive? While such tools can cost up to 10 to 20 times more than conventional tools, many studies by industrial research organizations or academic institutions have shown them to be 10 to 300 times more effective in terms of overall productivity and tool life. In part, these findings are based on a tool cost-per-parts analysis. For a better understanding of the economic benefits of hard turning, it helps to consider a few factors that are sometimes overlooked by the accounting department. These include tool change time, setup time, cycle time, machine maintenance, part quality, and original machine cost.

Part of the cost-effectiveness of plunge turning may be attributed to the machine tool itself. A grinder is a much larger investment than a CNC lathe, which is typically one half to one third of a grinder's cost. Also, CNC lathes are much more flexible in terms of machining capabilities. Tool changes can be made in less than two minutes, without the production time losses necessary for a wheel change. This flexibility allows fast, cost-effective production of small batches of parts.

Low maintenance is also a benefit, as worn PCBN tools may be quickly removed and replaced with new inserts, and do not require truing or dressing to maintain the cutting profile. CNC lathes also take up less floor space than grinders, do not require flume systems, and, in many hard turning applications, do not even require coolant.