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All Products/ Turning/ Hard Turning/ Hard Turning Information/
What is Hard Turning?
What hardness ranges are appropriate for Hard Turning?
What should the part length-to-diameter ratio be?
What will help ensure successful hard turning?
What are some of the advantages of hard turning?
When are CBN cutting inserts the appropriate choice?
When are Ceramic inserts a good choice?
When are Cemet inserts the right choice?
Are natural and synthetic diamond inserts good choice for hard turning?
Can Hard Turning eliminate Grinding?
Why are Hardinge lathes ideal for Hard Turning?
What is Hard Turning? What Hardness ranges are appropriate for Hard Turning?
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 approach to machining hardened steel depends on the degree of hardness and its depth (if case hardened). 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.
What should the part length-to-diameter ratio be? An acceptable part length-to-diameter ratio is up to 4:1 for unsupported workpieces, and up to 8:1 for parts with tailstock support. A harder part and tighter tolerance requirements may change these guidelines.
What will help ensure successful hard turning?
Machining success depends largely on component rigidity, the geometry to be turned, lathe rigidity, and vibration damping characteristics.
What are some of the advantages of hard turning?
Advantages of machining hardened steel include:
- Single-point turning of complex contours can be accomplished, eliminating costly form wheels
- Multiple operations can be machined with one set-up, resulting in less part handling and less opportunity for part damage
- Low micro-inch finishes can be achieved: 4 to 16 micro
- Reduced machine tool cost, adding up to greater productivity, better production control, shorter throughput and greater profits
What materials are appropriate for hard turning?
When are CBN cutting inserts the appropriate choice?
If the hardness ranges between 50-68 Rc and the depth of hardness is greater than the depth of material to be removed, then Cubic Boron Nitride (CBN) is the best medium. CBN will give good tool life and wear properties. Surface finishes of 11-15 micro-inches can be achieved and maintained. ISO inserts are available with multiple grades to suit different machining requirements. Insert hardness and, therefore, wear rate are traded for toughness and ability to withstand shock loading from interrupted cuts, i.e., keyways.
- Hardest (Suitable for plain diameters only)
- Medium (Suitable for moderate interruption)
- Toughest (Suitable for machining gear o/d)
Typical cutting data for CBN:
- 315 - 335 feet per minute (96-102 m/min)
- .008" - .010" (0.2-0.25 mm) depth of cut
- .004" IPR (0.1 mm/rev) feedrate
The size of the insert nose radius determines the surface finish achieved within the limits of machine and component rigidity. CBN is undoubtedly the best option, provided the workpiece material is uniformly hard. It will work on steel with a hardness above 50Rc.
When are Ceramic inserts a good choice?
Ceramics in hardened steel turning have applications and are very economically priced compared to other types of inserts. The nature of the material necessitates the use of blunt edge geometry, which inevitably increases cutting forces and reduces surface finish potential.
Typical cutting data for steel at 60 Rc:
- 315 feet per minute (96 m/min)
- .005 " (0.127 mm) depth of cut
- .005" IPR (0.127 mm/rev) feedrate
Cost per edge can be low, but failures on ceramic inserts can be catastrophic, in which case all edges may not be usable. Cermet inserts are available in most ISO shapes and nose radii.
When are Cermet inserts the right choice?
Cermet (solid titanium carbide) inserts for hardened steel turning applications have some benefits over CBN and ceramics under particular conditions. If the application involves turning through a hard case into a soft core, then cermet will respond better than CBN. It does not have the wear resistance of CBN, but the tool tip will wear proportionally under most circumstances rather than fail due to breakage.
Cutting data:
- 315 feet per minute (96 m/min)
- .005" (0.127 mm) depth of cut
- .004" IPR (0.101 mm/rev) feedrate
Cost per edge is equivalent to the latest type of multi-coated carbides. Cermet inserts are available in most ISO shapes and nose radii.
Are natural and synthetic diamond inserts good choices for hard turning?
Natural diamond and synthetic diamond are the least preferred options for operations involving machining of hardened steel. They have high costs per edge and poor reliability in terms of interrupted cuts and wear mode. Brazing cannot always be guaranteed and re-grinding is not always possible. Diamond also interacts chemically with steels, and can cause failure.
Can Hard Turning Eliminate Grinding?
Grinding does--and likely always will--have a place in manufacturing, as all components can not be hard turned due to tolerance requirements and the surface integrity of the part.
However, Hard Turning does offer distinct advantages, including:
- Price. Compared to grinding machines, lathes are relatively inexpensive to purchase. Grinding machines are also more expensive to operate due to setup and cycle times. Higher level grinders can perform multiple operations, but cost more and require more support equipment, such as balancers and dressers.
- Versatility. You can "soft turn" and Hard Turn on the same machine tool.
- Metal Removal. You can achieve up to 4-1 and 6-1 higher metal removal rates.
- Flexibility. Changes are easier if the part configuration changes. And lathes can handle small lot sizes and complex shapes much more efficiently than a grinding machine.
- Environmental Issues. Lathes produce chips, which are less costly to dispose of than the swarf produced by grinding machines.
Why are Hardinge lathes ideal for Hard Turning?
Success in Hard Turning is defined by a number of factors:
- Machine rigidity
- Workholding Rigidity
- Good vibration damping characteristics
- Rigid tool location
- Component part rigidity
- Rigid cutting tools and advanced insert materials
Hardinge lathes meet these requirements with the following key features:
- Machine rigidity: Heavy-duty linear guideways on Hardinge QUEST lathes are typically 40-60% heavier-duty units than those fitted to most competitive machine tools of the same size. The low-stress drive systems on QUEST machines have larger axis motors and shorter pitch ballscrews than most competitive machines, making for high dynamic stiffness--critical for successful hard turning.
- The patented Hardinge spindle design on Hardinge QUEST and ELITE series lathes ensures that the workpiece is seated as close as possible to the spindle bearings for maximum rigidity and accuracy. This greatly improves the Hard turning process compared to competitive workholding solutions.
- In addition, Hardinge is the largest manufacturer of lathe spindle tooling systems in the world. This ensures that the workholding systems made for our machines are designed specifically for our machines.
- QUEST lathes are built on a rigid cast iron base with HARCRETE Polymer Composite reinforcement. Vibration damping is critical to the successful application of hard turning. HARCRETE is located in key positions of the base to assure maximum rigidity. This unique design can help you save 30% or more on cutting tool expenses, and reduce vibration at the spindle by 60%.
- All external tools on QUEST machines equipped with Hardinge turrets mount directly into the tool slots provided in the turret body. The close proximity of the cutting tool to the coupling reduces the risk of vibration, thus improving the Hard Turning process.
- QUEST lathes feature digital glass scales incorporated for both the X and Z axes. The spindle motor and collet closer assemblies are dynamically balanced. And X and Z axis error compensation is performed to fine tune positioning and compensate offset at the tool tip.
- In short, Hardinge machines offer the ideal machining process from the standpoint of part cost, capability, and accuracy.
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