Application of High Speed Machining | Industrial Engineering

The manufacturing time of a component is sum of setup time, tool changes, work holding, and machining. Automation has resulted in minimisation of first three components. Further economy is now possible only by reducing machining time for which high speed machining is used. The most optimum speed for most materials is higher than 30,000 m/min as compared to 50 to 500 m/min in conventional machining.

High speed machining results in higher stock removal, improved surface quality, better workpiece accuracy, lower parts cost, and improvement in machinability.

Reduction in manufacturing times and manufacturing costs is achieved by:

i. Increasing the chip removal rate.

ii. Increasing the feed rates tenfold.

iii. Reducing cutting forces by 30 per cent with very low reaction forces on the parts, allowing thin wall parts to be machined with higher accuracy.

iv. Decreasing part temperature.

v. Improving surface finish.

vi. Lowering tool wear.

High speed/high precision machining has become a reality with the advent of new CNC systems, progress in the field of bearing design for high speed applications, automatic tool changing, tool retention devices, tool materials, high structural rigidity with a high level of damping.

High speed machining finds applications in die and mould manufacturing with a view to produce surfaces which are very close to the required final shape accuracy and the surface quality, aerospace engineering, production of critical thin walled components prone to heat distortion as well as production of precision parts.

Since high speed machining produces very high surface quality, subsequent finishing operations can be eliminated wholly or in part.

High-speed machining can be used to machine parts that require the removal of significant amounts of material and to machine long, thin webs. The need to reduce costs and to increase productivity has created new interest in high ­speed machining. The development of tougher, more refractory tool materials and of high-speed machining spindles has contributed to growth in high speed machining.

In high speed machining it is important to study the separation process and the frictional condition along the tool rake face in order to predict the cutting forces, and the stress distribution along both the deformed and un-deformed chips.

In high speed machining, on increasing the cutting velocity, the cutting force, through force, and specific cutting energy decrease due to softening of the cut material.

The progressive development of primary shear zone can be observed. The primary zone is a zone where primary shear action occurs, and it extends from the tool tip to the free surface where the chip starts to curl. The stress is concentrated at the ends of the shear zone (the part of the chip near the tool tip, and the free side of the chip); in which the development of stress distribution may lead to initial failure predicted to begin at the two zones.

Requirements of High Speed Machining:

To cope with the high feeds and speeds required for high speed machining, the machine must have high structural rigidity with a high level of damping, and be relatively light to move fast. The structure should be strong enough to withstand the high rates of cutting. High dynamic performance, i.e., high spindle speeds, axis feed rates and axis acceleration/deceleration rates are essential.

Slide base should have a low coefficient of friction. The machining area should be completely enclosed with shrouding strong enough to resist impact from a tool breaking at high speeds. Spindles must be precise, rigid and reliable at 8000 to 30,000 rpm or more for long periods with minimal growth from heat.

Dynamical balanced high speed spindles with high concentricity, constant torque and sensors to detect overheating are some of the requirements. Spindle rotation error must be controlled to minimise tool eccentricity.

Bearings are the most critical components of high speed spindles, determining their life and load capacity. There are four types of bearings for high speed spindles viz. magnetic, hydrostatic, air and rolling elements.

Magnetic bearings have the lowest coefficient of friction and allow very high speeds relative to the main load capacity. They are used in highly specialised machines used for ultra-precision and super finishing operations. The nearly infinite life of magnetic bearings is limited by their relatively small overload capacity and the possibility of a power failure or electronic malfunction.

Spindles with hydrostatic bearings have good damping properties and are well suited for high performance and ultra-precision machining. In such applications, they can have a life of 20,000 hours or more. They are however, restricted in speed due to frictional heat and the related power loss.

Air bearings are commonly used in applications with relatively low loads, such as high speed printed circuit board drilling and small diameter grinding and composite routing, with turn rates going from 10,000 to more than 200,000 rpm.

They have minimum burn out and are suitable for ultra-precision machining. In the high performance machine tool market they are limited by small overload capacities. Once the load capacity of the air film is exceeded, even slightly, the film collapses and the bearing makes contact, causing catastrophic failure.

Angular contact ball bearings are the most commonly used bearings for high-speed spindles. They can be used at high speeds or high loads and usually have good overload tolerances. Ceramic bearings are sometimes used to minimise the heat generation in the spindle and provide stabilised machining.

Cooling of the spindle should be provided so as to remove any heat generated in the bearings and to maintain the temperature of the spindle equal to that of the machine, minimising expansion.

Rigidity of spindle is important to ensure accurate tool paths and avoiding chatter and poor surface finishes. Spindle rigidity is determined by the number, arrangement and stiffness of the bearing and by the shaft stiffness.

High-speed spindles must be dynamically balanced to avoid detrimental dynamic forces, balancing the rotating assembly, including the tool holder and tool.

The thermal distortion due to heat generated by high speeds is kept at minimum to maintain the accuracies of the machine.

At speeds of 500 m/min and above the nature of the chip formation is completely modified. Both tool and workpiece material in the cutting zone remains completely cold, with all the heat being carried away by the chips. This results in lowering temperatures of the part and lower residual stresses.

Since the cutting edge is cool, cratering is no longer an observed phenomenon. The tool life is also strongly influenced by the cutting tool material. The higher peripheral speeds together with polycrystalline cubic boron nitride cutting tool enable machining of pre-heat treated mould and die-steels of high hardness maintaining surface finish values approaching those of a ground surface.