CNC routing technology has dramatically increased the productivity of even the smallest shops by consolidating the capabilities of many traditional machines into one very precise and efficient machine. Modern CNC routers feature heavy machine bases and large spindles for optimized rigidity, precision and speed. Interchangeable chucks [tool holders] and onboard tool magazines allow CNCs to handle a variety of activities including routing, sawing, boring, mortising and profiling; and while the interchangeable chuck has greatly improved the flexibility and productivity of these machines, it is also the machine's Achilles heel. Even precision cone chucks, such as the very popular HSK-63F version, introduce a point of weakness and tool run-out tolerances which adversely affect cut quality and machine performance. Machine operators are often quick to blame poor performance on the cutting tools, and inadvertently overlook less obvious problems with the tool holding system.
Basic collet chucks are the most popular tool holding system due to their flexibility and ability to secure a variety of shank sizes via a simple collet change. However, the flexibility offered by the use of collets also introduces multiple run-out tolerances, which can greatly diminish the advantages of this system. In addition to the tolerance between the spindle end and the chuck body, tolerances between the collet and chuck, collet and tool shank, and collet nut and collet body can greatly increase tool run-out and dramatically reduce machine performance.
Collet chucks also require frequent cleaning and maintenance, which is frequently overlooked. Chucks, collets and collet nuts should be thoroughly cleaned at every tool change, and special care must be taken to clean all mating and threaded surfaces. Failure to thoroughly clean these components can result in exaggerated run-out [eccentricity] and balance issues.
The interchangeable collets, which offer increased flexibility, are also the primary problem with this system. Failure to follow proper tool colleting procedures will result in deformed collets, which reduces tool concentricity and holding power. These "flexible" collets should also be discarded after 400-600 hours of use, but this recommendation is often disregarded due to cost considerations. Collet chuck manufacturers recommend the use of torque wrenches to avoid under- or over-tightening of collet nuts. Many users consider torque wrenches an unnecessary expense, but not using a torque wrench can lead to immediate damage to the collets, collet nuts or chucks and may create unsafe operating circumstances.
Several premium non-collet systems are available as an alternative to the problematic collet-based systems. These systems exert direct pressure between the inside of the chuck and the tool shank—either through hydraulic or mechanical expansion and contraction. The primary obstacles of these premium systems are initial cost and limited ability to hold a variety of shank sizes within the same chuck.
Hydraulic chucks [hydro chucks] have long been considered the most precise chuck system available. While hydraulic forces may initially offer the best tool concentricity, this precision can diminish when the tool is subjected to typical cutting forces. Hydro chucks utilize sealed grease or oil-filled cavities and a thin flexible inner wall to uniformly and precisely grip the tool shank. The oil-filled cavities are pressurized with a high-pressure grease gun or with screws which constrict the size of the sealed cavity when tightened.
The disadvantages of hydro chucks begin with their substantial initial cost which is typically two to three times that of standard collet chucks. The secondary problem involves the flexible internal pressure wall, which can deform if the tool shank is improperly inserted. The flexible pressure wall may also permit the tool shank to deflect inside the chuck, under cutting pressure, which will reduce cut quality and machine performance. Hydro chucks are also shank diameter specific and require relatively expensive reducing sleeves to adapt smaller diameter shanks. Finally, the chucks must be maintained and the pressure seals replaced periodically to prevent loss of pressure and system failure.
The most recent addition to the high-precision chuck solution involves shrink fitting tools into solid chucks. Heat-shrink fit technology is based on the thermal expansion properties of metal and utilizes electromagnetic induction heat to rapidly heat chucks for setting and removing cutting tools. The solid steel alloy chucks have a smaller diameter tool receiver than the shank of the cutting tool. Induction heat is used to rapidly expand the receiving end of the chuck (three to four seconds), so a cutting tool shank can be inserted. As the chuck cools, it uniformly shrinks around the shank, essentially creating a one-piece chuck and cutting tool. The induction heating system allows for rapid re-heating of the chuck receiver and easy removal of the dull tool. The controlled heat provided by induction heating allows a heat shrink chuck to be reused thousands of times without tolerance or metallurgical changes to the metal.
By eliminating collets and moving/mechanical parts which induce excess tolerances, heat-shrink technology optimizes the concentricity and balance qualities of the tool, allowing the tool and machine to perform at their most efficient levels. The solid steel chuck design offers the strongest gripping power and torque transfer value of any standard system and prevents tool deflection or slipping common to lesser chuck options. Advantages to the user include:
• Maximized tool life
• Reduced tool changes
• Fastest operating speeds
• Increased machine output
• Perfected cut quality
• Reduced rework and scrap loss
• Fast tool changes
• Increased operator safety
• Reduced maintenance and replacement costs
Heat shrink systems such as Leitz ThermoGrip System have become the standard solution in the metalworking industry and are quickly becoming the preferred system for machining wood- and plastic-based products. The price of the chucks is comparable to that of standard collet chucks, and the cost of the induction heating systems can easily be justified against increased machine performance and output.
Source: Leitz Tooling Systems. Visit LeitzTooling.com or call (800) 253-6070.
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