Hard turning of materials with hardnesses ranging from 45 to 68 in the Rockwell range using a variety of inserts or integral turning inserts. Since its introduction in the mid-1980s, the popularity of this process has increased dramatically, and the sales of PCBN tools are now more than $2.5 billion annually. It is clear that more and more manufacturers are recognizing the advantages of hard turning. But because of the cost of PCBN tools, many people still see it as an expensive process.
Most of the hard parts used in the automotive industry are machined to the final geometry after hardening. Grinding is currently the primary method for these parts, including bearings, gears, shafts and pinions. However, thanks to the improvement in machine tool rigidity and the development of polycrystalline cubic boron nitride (PCBN) tools, hard turning is becoming popular as an economical alternative to grinding.
Is it more expensive to use PCBN for hard turning?
When PCBN tools cost 10 to 20 times more than conventional tools, research shows that they are 10 to 300 times more efficient in terms of overall productivity and tool life. To some extent, these findings are based on tool cost analysis for each part allocation. In order to better understand the economics of hard turning, it is helpful to consider some factors that are sometimes overlooked by the accounting department. These factors include tool change time, adjustment time, machining time, machine maintenance, part quality, and machine original cost.
The economics of hard turning may be partly due to the machine itself. Grinders are much more expensive than CNC lathes, which are typically between one-third and one-half the cost of a grinding machine. Also, CNC lathes are more flexible in terms of processing capabilities. The tool change can be completed in less than two minutes without the loss of production time necessary to replace the wheel. These flexibility allow for fast, economical production of small batch parts.
Low maintenance is also an advantage, as worn PCBN tools can be quickly removed and replaced with new ones without the need for corrections or corrections to maintain the cutting profile. CNC lathes take up less space than grinders, do not require a tank system, and do not even require coolant in many hard turning applications.
Because hard turning effectively removes metal by 'peeling' softened chips from the workpiece, coolant is generally not recommended. This helps reduce costs while eliminating environmental damage caused by the use of coolant. Dry processing also reduces the time and money spent on government-controlled chip handling and recycling.
Get a good surface finish with less processing time
While it is known that grinding achieves a good surface finish at relatively high feed rates, hard turning with PCBN inserts results in an equivalent or better surface finish at significantly higher metal removal rates. Although the process has a small depth of cut and a small feed composition, the reduction in traditional hard turning time is estimated to be as high as 60%, and up to 90% for the latest plunge turning technology.
Studies have shown that hard turning achieves a better surface finish than grinding by using the correct tool nose radius, feed rate or new wiper blade technology combination. In fact, hard turning can be achieved with a simpler mounting than a grinding adjustment. However, there is still much debate surrounding the comprehensive surface integrity of hard-car parts.
Hard turning affects the surface microstructure by creating a residual stress pattern and a hard surface area, also known as a whitening layer. These thin rehardened layers are typically excessive tempering zones immediately below the albino layer. Due to the modification of the microstructure of the material, the rehardened layer is white under an optical microscope and the tempering zone appears dark.
Studies have confirmed the presence of a whitening layer on both hard and ground surfaces. Although they are usually associated with surface residual tensile stress, the whitening layer may also predict residual compressive stress. In any case, the causes of the whitening layers and their lack of knowledge of the processed workpieces.
Some studies suggest using a cutting fluid to help eliminate the whitening layer, while others show no effect on the coolant. There is also some evidence that tool conditions affect the formation of albino layers. In general, new tools tend to produce non-destructive surfaces, and the increase in whitening layers increases as tool wear increases. This may be caused by an increase in the flank wear caused by the heat generated by the friction between the tool and the workpiece, or a higher plastic deformation caused by the increase in friction.
New tools and technologies reduce machining time by up to 90%
In recent years, the 'unknown' surface integrity of hard-car parts has also caused some reluctance to turn hard turning as a key surface finish. However, recent developments in tool technology are likely to challenge these concepts. In particular, new integral PCBN inserts and new plug-in technology have been proven to achieve comparable or better precision than conventional grinding processes.
One of the recent developments is the Xiangshan High CBN100 and CBN300, which are the most economical options for hard-car finishing. The integral inserts CBN100 and CBN300, specially designed for hardened steel, offer the same number of cutting edges as conventional insert or carbide inserts, plus a long cutting edge. Due to their overall structure, the new blades offer significantly higher wear resistance and lower cost per edge. In a study comparing Seco's integral inserts with insert inserts from five other different tool manufacturers, the cost per blade cost ranged from 25% to 208%.
As an added advantage, the CBN100 and CBN300 also make it possible to insert a car. Inserting is a high-productivity form of hard turning that uses the entire cutting edge or part of the cutting edge to create right-angle cutting. This relatively new process has been proven to reduce processing time by up to 90%, and the surface integrity of the parts produced is the same as those of those ground.
The most critical characteristics of a ground or hard-turned part are size (diameter), roundness, straightness, load-bearing area, surface roughness and residual stress. For the first four characteristics, the following accuracy can be easily obtained for a large gear or bearing ring.
Diameter +/- 0.0006 in. (0.015 mm)
Roundness +/- 0.00015 in. (0.004 mm)
Straightness +/-0.00015in>(0.004mm)
The bearing area is 90% deep (0.00008in. 0.002mm)
In conventional turning that produces continuous tracks, the surface finish is determined primarily by the tool nose radius, feed rate, cutting speed, and depth of cut. In contrast, the surface quality of the inserted parts depends mainly on the quality of the cutting edge. As a result, the inserted parts show low residual stresses in the axial direction, and the sealing characteristics are the same or better than the ground surfaces.
These stress modes are kept constant using integral PCBN inserts, even in large batches. In a test that examined the physical characteristics of 250 insert parts, the residual stress was found to be significantly lower than the stress caused by grinding. Part quality is also affected by the tangential residual stress distribution. In the 250 parts tested separately, both grinding and inserting produced residual tensile stress on the surface, but the thickness of the affected area on the inserted parts was obviously thinner.
It has also been found that stowage reduces the occurrence of a whitening layer because it reduces the flank wear on the cutting edge due to the shorter cutting time of each machined surface. This also results in lower cutting forces and less friction, resulting in less heat transfer to the workpiece surface.
Although hard turning is often seen as much more expensive than traditional finishing methods, recent developments in cutting tools and techniques are gradually changing these perceptions. With the answer to questions about the ability to meet surface finish and integrity requirements, hard turning finishing will be more widely recognized as an economical alternative to grinding. Greater flexibility, faster tool change and longer tool life have a role in reducing machining costs, increasing productivity and better part quality, far exceeding the initial cost of PCBN tools.
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