About finish hard turning

The finish hard turning process is characterized by high flexibility combined with the ability to achieve sub-micron workpiece quality, similar to precision grinding. The advantages are in its potential for machining complex workpieces in a single set-up without the need for additional finishing operations.

Finish hard turning has established itself as a powerful alternative to grinding particularly for workpieces with complex shapes in a low- mid volume production environment.

Benefits of finish hard turning with a high precision turning lathe
– Complete machining of complex geometric forms combining ID and OD operations in one set-up.
– Potentially the highest possible workpiece quality obtainable
– High material removal rates contribute to reduction in cycle times
– Cost-saving: fewer machines required because of complete machining on one platform
– Simple 2- axis turning process with setup and changeover under 1 hour
– Eliminate the need for coolant in many applications

The hard turning process can be classified into three categories depending on accuracy requirements. The Mikroturn lathe is in the highest, possible category:

Finish hard turning / High precision turning
Finish hard turning aims to produce ready-to-use workpieces with very high accuracy tolerances, achieving dimensional and form tolerances of ≤ 2 microns and surface finishes of 0.1 – 0.4 microns (Ra).

Specially designed machines are absolutely required for high-precision hard turning, like the Hembrug Mikroturn hard turning lathes, in order to meet these tolerances.
Machining parameters: cutting speed 150-200 m/min, feedrate 0.01-0.1 mm, 0.02-0.3 mm 

Precision turning
Precision finish turning targets the production of ready-to-use components. However, when the turning operation produces lower workpart quality, from 5-12 microns, then a finishing operation will normally be required.
Machining parameters: cutting speed 100-200 m/min, feedrate 0.05-0.15 mm, cutting depth 0.1-0.5 mm

Rough turning
Rough turning is the process where steel components are initially machined after heat treat to relatively open tolerances. Many of the newer generation CNC lathes are capable of rough hard turning.
Machining parameters: cutting speed 50-150 m/min, feedrate 0.1-0.3 mm, cutting depth 0.5-3 mm

Workpieces and materials

A high precision finish hard turning machine can produce a wide variety of workpieces and material types. The most common workpiece and steel types:
Ball screw nuts
20CrMo5, 100Cr6, 12CrMo4
Bearing rings and rollers
100Cr6, 18CrNiMo7, 100crMnSi6
Hydraulic components
100Cr6, 18CrNi8, 9SMn28U
Drive shafts and gears
20MnCr55, 21NiCrMo2
Tool and mold making
HSS, D2, A2

What cutting inserts are suitable for hard turning?

The best cutting insert for these materials is CBN, cubic boron nitride. CBN is the second hardest cutting material after diamond and capable of turning hardened steel from 55 up to 70 HRC with a sufficiently long tool life. In addition, it is also heat resistant to about 1200 degrees. The temperature for hard fine machining ranges between 700 and 1100 degrees.

The cost per insert varies between 15 and 25 EURO. However, there are many types of inserts available and is dependent on workpiece characteristics and geometry. Tool life depends on several factors. Most insert suppliers estimate tool life over time in the cut, normally between 30 and 40 minutes. Hembrug on the other hand calculates with a standard cutting length of 6-8 kilometers. If the process is optimized it can even reach up to 10 km or more.

The use of coolant

Finish hard turning offers the possibility to not use coolant with workpieces where cutting times are shorter. Coolant, however, can play an important role in process stability. One of the advantages of finish turning over grinding is that the majority of heat produced is removed in the chip. But, some heat does enter the part and chilled coolant can play a roll in maintaining stable part temperature. This is especially beneficial when micron level quality must be achieved. Additionally, coolant can enhance chip removal and potentially increase tool life. Thus, when to use coolant or not is application dependent

Is hard turning for my company?

The suitability of finish hard turning is application dependent and does not offer the same benefits for everyone. Please contact us, we will be happy to advise you based on your workpieces and what problems you may be experiencing. We can complete a machine proposal for your specific application. Sales@hembrug.com / +31(0)23-5124900.

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