Metal removal rates are faster today than ever before. What was regarded high speed yesterday is considered conventional today. But high speed is a relative term and thus a general definition of high speed is hard to pin down. What is very fast for one industry segment seems hostile to another. Modern machine tool makers commonly sell spindles capable of speeds up to 40,000 rpm. However, traditionally regrindable solid speed steel cutters - around two inches in diameter - are generally limited to just 10,000 rpm. Thus, high speed machining (HSM) has no set definition or absolute parameters, but one workable definition is machining with spindle speeds of 20,000 rpm or more.
Today, HSM represents high productivity machining - an integrated process that slashes part cycle time in hardened steels as well as soft aluminium. It can use high spindle speed and/or high workpiece feed to make fast, light cuts. Advanced indexable and solid cutting tools, and modern tool holders have broadened applications for HSM. To exploit such advances and maximize productivity, HSM shops must, however, build close working partnership with knowledgeable machine, software and tool suppliers.
Delivering the best
Technological innovations coupled with market demands are driving shops to faster metal cutting rates These include better and more capable machine tools and CNC processors that allow the machine to accurately cut at increasingly higher speeds and feeds. Commercial considerations are also driving shops towards higher rates of productivity.
Choosing the cutting tool:
The HSM process demands an innovative approach to indexable cutting tools. Selection of right cutting tool plays a major role in HSM, as it determines, just how fast you can cut. Many machining centres today run at speeds beyond the capacity of tooling without premature failure or excessive wear.
Just as the process makes demands on the indexable cutting tools, higher cutting speeds and forces require new cutting tool materials. The heat and pressure encountered in hardened alloys at higher speeds and feed rates cause rapid failures in conventional inserts and thus call for specialised materials.
The right material:
A cutting tool material has specific attributes that make it usable in a metal cutting application. In general, two performance criteria are used to determine the applicability of a material. These are toughness or resistance to fracture (ductility) and thermal hardness (resistance to heat).
A myriad combination of coatings, substrates and base materials can be created to deliver specific proportions of toughness and thermal hardness to fit various applications. Some of the common cutting tool materials used for HSM are high-speed steel (HSS), tungsten carbide (uncoated and coated), cermets, ceramics, polycrystalline diamond (PCD) and cubic boron nitride (CBN). Starting with HSS and progressing to diamond and CBN coatings, a scale can be built progressively from best toughness characteristics to best thermal hardness. HSS can take a pounding, but no heat. Ceramics and diamond can take heat, but have less resistance to shock.
Holding it right
HSM is a sophisticated technology, where everything has to be done since high speed tends to magnify imperfection.
It is very important to hold the workpiece securely and allow for easy holding of subsequent parts. The fixture should support the workpiece on a solid base and have enough mass to help damp the vibrations induced by the cutter. Fixtures for high speed machining need not be overly complex but should follow good shop practice. For example, a good vise is adequate if it supports the workpiece securely. Positive stops should be used to prevent torquing or movement of the workpiece in response to cutter motion.
Successful high speed machining is dependent on static and dynamic rigidity among the many components that bring together the tool and the workpiece.
In the system-comprising spindle, tool holder and cutting tool, the tool holder is the link that has maximum effect on overall concentricity and balance. There are two systems of tool holders: steep taper V-flange and hollow shank taper (HSK). The difference between these two systems is the manner in which they are seated in the machine tool spindle bore.
At high speeds, centrifugal force is strong enough to enlarge the spindle bore slightly. This can cause the V-flange holder to be drawn up into the spindle, as its contact with the spindle bore is only in the axial plane, leading to a stuck tool. The HSK holder is designed to provide a simultaneous or two- plane fit on both the spindle face and the spindle tape. The face contact prevents the tool from moving up the bore. This also increases the contact area giving better rigidity, which enables users to cut with longer tools and to use side-cutting milling tools more aggressively. For high speed machining applications, the trend definitely points toward simultaneous fit, spindle-tool interface as a requirement.
How the tool is gripped in the tool holder is another consideration for HSM. Some tool holders are not symmetrical. For HSM, it is advisable to build tools using a holder and cutter combination that is symmetrical. Shrink fit, which uses a heated bore that expands for the cutter and a then clamp as it cools, is a popular choice for HSM. The working of hydraulic tool holders is the same as shrink Fit in that they support the tool shank completely. Regardless of the tool holder - cutter combination, symmetry is the key consideration.
The right angle
Speed of the cutter has major influence in the generation of heat at the cutting edge of the tool. Maintaining a high chip load or feed can dissipate this heat. The chip load of a cutter is influenced by the rake angle of the cutter edge. Rake angles vary from positive through neutral to negative. Positive rake angles present a sharper, but weaker, edge to the workpiece. Positive rake tools tend to pull the workpiece toward them during the cut. These tools also tend to push chips up and away from the cutting zone. Negative scrape tools have a much stronger leading edge and tend to push against the work piece in the direction of the cutter feed. These tools cut less freely than positive rakes and therefore, consume more horsepower to cut.
High speed tooling geometry, in general, mirrors the geometry of conventional machining. The trend in HSM is towards a positive lead angle tooling. This lead angle effect allows greater ipt, by lifting the chip, while maintaining the same chip thickness. This greater feed rate results in higher speed machining. The goal should be the formation of a sufficiently thick chip, which acts as a heat sink.
A high-speed spindle is a critical component of a high speed machining process. The CNC, tool, machining centre and other process components are all optimized to use higher spindle speed productively. A high-speed spindle presents a trade-off between cutting force and cutting speed.
Generally, these spindles have direct-drive motors, which means the motor must fit inside the spindle housing. Also, high-speed spindle bearings, trade stiffness for speed. This is one more reason why HSM generally employs light depths of cut.
This is the product of bearing diameter in mm (D) x top speed in rpm (N).
The DN number captures the trade-off between a spindle bearings stiffness and speed. Though bigger bearings are stiffer, smaller bearings can be used at higher speeds more effectively. Hybrid ball bearings, today, are capable of a maximum DN number of about 2 million. For performance beyond 2 million, a non-contact bearing design may be required.
Hybrid ball bearings:
Hybrid ball bearings can replace the all-steel ball bearings in high-speed spindles. The lighter and stiffer ceramic balls in these bearings deflect less from centrifugal force. This improves efficiency and quiets vibrations. Ceramic balls also deliver longer life.
Some of the limitations of hydraulic ball bearings can be overcome by neither using nor contact bearings. But many of these non-contact spindle designs are still being tested. The three non-contact bearing types are: hydrostatic bearings, where fluid such as water, supports the spindle shaft: air bearings, where air pressure supports the spindle shaft; and magnetic bearings, where spindle the shaft is supported by a dynamic magnetic field.
Retrofitting a faster spindle to conventional machining centre allows a shop to realize some of HSMs benefits.
The machines existing spindle can be replaced with a high-speed one.