Our investigation into costs and manufacturing lead-times directed us to PRECISION HARD TURNING as an alternative to grinding (a long machining process) for a wide range of rotating parts after heat treatment.
Compared with cylindrical grinding, precision hard turning enables the manufacturing process to be rationalised with shorter cycles and the performance of several steps in one single operation, it increases productivity, contributes to quality and avoids the need for cutting oils.
We have wide experience of batch production that enables us to find solutions for many situations, even for especially difficult steels and for hard turning. Up to 70 Hrc: dimensional accuracy 5 microns, geometry within a micron, surface condition less than 0,2 micron Ra.
Example of particularly suitable applications : Complex geometry parts, forming tools, rollers, Gears, drive shafts, with or without keyways, Bearing bushes, roll bands, Dies, stamps, for blanking and deep drawing, etc.
...and even for small batches and individual parts.
Precision hard turning replaces grinding
It does not change the metallurgical properties of the part, as the danger of overheating is practically absent with good machining conditions.
With hard turning the programmed tool machines the part directly. This is a great advantage when machining parts with a complex profile when geometrical homogeneity is difficult to achieve by grinding.
Complete machining is often possible without repositioning the work piece.
Machining and implementation are quicker even for single parts.
Less dangerous for the environment as most of the machining is done dry.
The hard turning technique
To be competitive under hard turning the turning machine must have high static and dynamic stability. In terms of precision, a lathe of this type is comparable to a top-of-the-range grinder (motorised spindle) It is essential that the whole of the machining system be stable. This applies to work piece clamping and the tool holder. The hard turning process features small cutting depths and feed and high specific cutting loads.
By the very principles of the cutting, which are quite different from those of normal turning of non-heat treated materials, very high temperatures (500° to 1500° depending on the situation) occur during working in the contact areas between the cutting edge and the work piece. This results in a softening of the machined material in the tool contact area.
The first factor is the thermal conductivity of the cutting material. For finishing work a cutting material with a low thermal conductivity is the first choice to be made. During the cutting process the cutting temperature is transferred to the shear zone. This improves the cutting process.
The second factor is the wear resistance of the cutting material. This is a determining factor when machining highly abrasive materials like Z 160 CDV 12. In the case of impact work, the toughness and the stability of the cutting edge geometry are determining factors.
Cutting materials
The high temperatures and the cutting forces specific to hard turning in the contact zones demand special qualities from hard turning tools. Because of their capacity to retain their hardness and consequently wear resistance when hot, ceramics and CBN’s are the obvious cutting materials for hard turning. If we forget the high cost of CBN’s in relation to composite ceramics, lCBN is the best cutting material for hard turning. Its difference lies in the fact that the usual opposition between toughness and hardness does not apply in the same proportions in this material. CBN is relatively unaffected by thermal and mechanical shocks. These are good conditions for non-continuous-cut machining. This allows a greater machined length, both in continuous and non-continuous cutting.
CBN tips are made from polycrystalline cubic boron nitride powder (CBN), sintered at high pressure and high temperature onto a substrate of tungsten carbide. During this sintering the cobalt from the tungsten carbide rises into the interface with the CBN and the cobalt catalyst of the CBN descends to the tungsten carbide substrate, thus forming an interface that forms a bond between the substrate and the CBN layer.
Composite ceramics and CBN’s are complementary in their respective applications. CBN’s are preferably used for non-continuous cut hard turning and machining very hard cylinders. Composite ceramics are more suitable for continuous cut precision hard turning with a constant machining allowance. In addition they are more economical. This is an important factor in a cost-benefit calculation.
Cutting edge geometry
Cutting edge geometry has a great influence on the performance of ceramics and CBN’s. A bevel gives better control of the cutting forces and strengthens the cutting edge.
Results
Made possible by the arrival of very hard tools resulting from the latest technologies, the choice of precision hard turning required years of research and development in terms of the equipment, process and tools.
Equipment : Static and dynamic stability - absorption of vibrations - high precision - high speed motorised spindle.
Process : Part handling – clamping stiffness - importance of pre-turning – specific features of the programming -form correction programming – machining parameters – elimination of spraying.
Tools : Selection and preparation of tool holders, tip testing, suitable materials/grades/suppliers for the machined materials/conditions, study of their toughness, study and improvement in their service life, cost-benefit calculation.
Today we routinely machine high alloy steels heat treated to 70 Hrc, both inside and outside to dimensional accuracy of 5 microns, a surface condition of less than 0.2 microns Ra and geometry within a micron, and all this even with impact machining. In the same way “hard threading”, after heat treatment, enables us to obtain perfect geometry and surface condition quality providing optimum centring.
The cycle time is greatly reduced, up to 4 times less for Z160 rollers heat treated to 62 Hrc. This modern machining process provides certain quality with shorter production lead-times and more competitive costs.
Complex geometry parts, forming tools, rollers,
