Producers of machined components and manufactured goods are continually challenged to reduce cost, improve quality and minimize setup times in order to remain competitive. Frequently the answer is found with new technology solutions. Such is the case with grinding where the traditional operations involve expensive machinery and generally have long manufacturing cycles, costly support equipment, and lengthy setup times. The newer solution is a hard turning process, which is best performed with appropriately configured turning centers or lathes.

What is it?

Hard turning is defined as the process of single point cutting of part pieces that have hardness values over 45 Rc. Typically, however, hard turned part pieces will be found to lie within the range of 58-68 Rc. The hard turning process is similar enough to conventional “soft” turning that the introduction of this process into the normal factory environment can happen with relatively small operational changes when the proper elements have been addressed.

Hard turning is best accomplished with cutting inserts made from either CBN (Cubic Boron Nitride), Cermet or Ceramic. Since hard turning is single point cutting, a significant benefit of this process is the capability to produce contours and to generate complex forms with the inherent motion capability of modern machine tools. High quality hard turning applications do require a properly configured machine tool and the appropriate tooling. For many applications, CBN tooling will be the most dominant choice. However, Ceramic and Cermet also have roles with this process.

The range of applications for hard turning can vary widely, where at one end of the process spectrum hard turning serves as a grinding replacement process, and can also be quite effective for pre-grind preparation processes. The attractiveness of the process lies in the performance numbers. A properly configured hard turning cell would typically demonstrate the following:

• Surface finishes of 0.00011" (.003 mm)
  
• Roundness values of .000009" (.00025 mm)
  
• Size control ranges of .00020" (.005mm)
  
• Production rates of 4- 6 over comparable grinding operations

Hard turning is a technology-driven process that requires certain performance features of the machine tool, workholding, process and the tooling.

How does it apply?

Hard turning can be certainly considered for most pre-grind applications, which are followed by an abbreviated grinding cycle. In some cases the hard turned surface may complete the operation and will completely eliminate the grinding cycle.

Hard turning is an important technology because all manufacturers are continually seeking ways to manufacture their parts with lower cost, higher quality, rapid setups, lower investment, smaller tooling inventory and the elimination of non-valued added activities. The migration of processing from grinders to lathes can satisfy each and every one of these goals.

If one were to list the current applications of hard turning it would certainly be a voluminous document. On a daily basis, parts are being hard turned in the following industry segments; automotive, bearing, marine, punch and die, mold, hydraulics and pneumatics, machine tool and aerospace. While these industries are representative, this list is certainly not conclusive and new applications and industry segments are constantly being added.

Commonly processed hard turned materials would include:

• Steel Alloys such as
  • Bearing steels
  • Hot and cold-work tool steels
  • High speed steels
  • Die steels
  • Case hardened steels
• Unique hard materials and aircraft types that fall within the hardness range

What are the benefits?

With hard turning, you'll reduce your costs in many ways:

• "Soft turn" and hard turn on the same machine
• Smaller floor space requirement
• Lower overall investment
• Metal removal rates of 4-6 times greater
• Can turn complex contours
• Multiple operations in a single setup
• Low micro finishes
• Easier configuration changes
• Lower cost tooling inventory
• Higher metal removal rates
• Easier waste management (chips vs. "swarf")

What are the concerns?

Hard turning is a viable process that has real and measurable economic and quality benefits. This is particularly true with a machine tool that has a high level of dynamic stiffness and the necessary accuracy performance. The more demanding the application in terms of finish, roundness and size control, the more emphasis must be placed upon the characteristics of the machine tool.

From the process standpoint there are several areas of consideration. With the correctly chosen cutting tools, the hard turning process can support either coolant cutting or dry cutting. If the processing choice is to cut dry then the temperature of the chips and the workpiece need to be taken into account from both a safety and operational standpoint. Other considerations for dry cutting would include:

• Consider the workpiece temp when gaging
• Expect higher tool temperatures and lower tool life
• Surface finish is generally not as good as cuts made with coolant
• Protection from high temperature chips is required
• Correctly chosen tool material i.e., not Ceramic
• The chip salvage may be more cost effective
  

Some applications may be sensitive to the surface condition caused by "white layer" formation (see "limitations", below).

The current tooling technology allows the user to be able to choose between “wet or dry” operations. Wet operations refer to processes under flood or high-pressure with a water-soluble coolant.

The decision to produce under wet or dry conditions is normally made at the individual factory level. Some facilities have a local philosophy or mandate regarding the preference to operate one way or the other and fortunately, either forms of hard turning can be accommodated. There are several key items when choosing to operate wet and the first of these is the type of fluid to be used. Generally, straight oils should be avoided because of the inherent fire hazard. This is particularly true if during a cut the coolant flow is disrupted and the unquenched, high temperature chips contact the oil. Under these conditions, oils with a low flash point could start and sustain a fire.

Another point for wet operations is the importance to properly direct the coolant flow by applying fluid to both the top and the bottom of the tool tip simultaneously. Generated chip strings will frequently shield the coolant from the tool until the chip breaks away. The result is thermal shock and a process of degradation of the cutting edge. Anticipate this when establishing the coolant nozzle locations from a slight sideward vantage point. High-pressure coolant at pressures of approximately 68-95 atmospheres seems to be beneficial in keeping the chips small and manageable and in making the overall process more robust. As previously stated, the shorter chip results in a reduced amount of coolant blockage and less thermal shock to the cutting edge. Another variable in coolant cutting operations that can easily sabotage a fine-tuned process is an improper coolant mixture. Concentration, cleanliness and pH levels cannot be ignored for a proper application.

The one possible exception to coolant cutting is on interrupted surfaces, which seem to perform better in a dry environment. Logically, this is due to the higher degree of thermal shock caused during the interruption when the coolant has a better access to the tool tip and then immediately is followed by a re-entry into the workpiece and the severe temperatures.

Factors that influence the hard turning process

Like all new manufacturing process, hard turning is technology-driven. The factors that influence the hard turning process can be categorized into the main topics of machine construction, process, workholding, materials and heat treat and tooling.

Part Materials applicable to Hard Turning

The typical materials which are routinely hard turned include those of the following broad category descriptions:

• Steel alloys,
• Bearing steels
• Hot and cold work tool steels
• High speed steels
• Die Steels
• Case hardened steels
• Waspoloy, Stellite and other aerospace alloys
• Nitrited irons and hard chrome coatings
• Heat treatable powered metallurgy

Why is an understanding of material science critical?

Certainly, an understanding of material science is vital to the heat treat operator so that the correct process and hardness range is accomplished. Material not properly drawn back might crack prematurely because of the high hardness. The best hard turning results will be achieved when the hardness range is as small as possible (a spread of less then 2 point is ideal), and the case depth is maintained consistently.

The key to success is optimizing the hard turning process to reduce overall cost(s)--purity of Material, Tool life, surface finish, and accuracy.

Reduced part cost due to better tool life

The range of tool life for a well-tuned process, which uses CBN, can be multiple 100’s of part pieces. This is of course based upon the machine, the application, the metal removal amount, the required tolerance level and the surface finish requirements.

Limitations

Tooling

• When considering the tooling material, it's important to understand the application and critical attributes such as size and finish requirements. The typical brazed tip CBN insert has a cost structure 3-4 times that of carbide. Ceramic, on the other hand, has a cost structure more similar to carbide but would not be used for applications which had a tolerance range smaller then .001” (.02540mm). Parts requiring a greater accuracy would logically use CBN.
• Ceramic does not perform well in the presence of high thermal shocks, so it is not generally a good candidate for coolant cutting.

White Layer

Hard turned surfaces frequently experience "white layer" formation, which appears as a white layer at the surface of the material under metallographic examination. This layer depth can vary greatly but for general discussions it is in the area of 1 micro-meter (.0000040”/.00010mm) thick.

The white layer cannot be seen visually but requires a metallographic examination to detect its presence. According to Griffiths (1987) white layer can be caused by either 1) severe plastic deformation that causes rapid grain refinement or 2) phase transformations as a result of rapid heating and quenching. White layer formation is not limited to hard turning operations but is also routinely found in grinding applications. White layer is not desirable in products which have high contact stresses and where fatigue failures can occur.

Machine Process Capability

Rigidity is critical for successful hard turning: the rigidity of tooling, workholding, and the machine tool itself are all crucial elements that will affect your ability to successfully hard turn. Hard turning is a technology-driven process, dependent upon:

• Machine technology
• Process technology
• Materials and tooling technology
• Workholding technology

When considering hard turning, the question is not “can it be done” (because many machine tools can hard turn), but “how well can it be done?” Success in hard turning is largely a measure of the machine construction and design along with the workholding and tool holding. The level of rigidity and damping in a hard turning application cannot be minimized.

That's where Hardinge has a competitive advantage that translates to a competitive edge for you.

In the area of hard turning, it's well-established that the presence of vibration is not desirable from multiple standpoints. A machine which has improved damping will demonstrate improvements in lowering the amplitude of vibration and the time to decay, all while maintaining static stiffness. The real and measurable results are longer tool life, better surface finishes, improved accuracy, increased productivity and higher overall part quality. System rigidity is of utmost importance.

Machine dynamic stiffness (MDS) MDS is one of the most important attributes for hard turning. MDS a ratio of the force to displacement at the exciting frequency...a function of the static stiffness and the system damping. When the static stiffness cannot be increased, then it's necessary to increase the damping to increase the MDS. The upper bound in part surface finish quality is determined by MDS, as is the upper bound in tool life.

The benefits of high dynamic stiffness include:

• Lower operating vibration
• Substantially improved tool life
• Substantially improved part quality
• Higher through-put
• Less machining parameter adjustments