The Need for Balanced Toolholders
Universal Engineering has been producing dynamically balanced precision toolholders and complete spindle assemblies for over two decades. It has only been in recent years, with the advent of new high-speed machines and spindles that are capable of running up to 10,000 and even 50,000 RPM, that the performance of these balanced designs could be proven, tested and evaluated.
The need for balanced toolholders has always existed for applications with spindle speeds in excess of 5,000 RPM. Unfortunately, the majority of users opt not to apply balanced designs either because of their unfamiliarity with the benefits of the holders, the age of the equipment, or the moderate premium cost that is typically associated with these designs. Currently there are no actual standards for this type of tooling so speeds and tooling requirements rest with the user. Although some recently introduced standard high-speed machining center models have helped to provide some data for toolholder standardization in terms of tooling size, the majority of toolholder systems are made-to-order.
Balancing the Toolholders
Balanced toolholders are processed over special precision balancing equipment that utilizes either drilling or grinding to perform the dynamic balancing operation. For end mill holders, drilling is normally performed. For other types of symmetrical holders peripheral grinding is used. The specifications for this balancing are typically based on general machinery vibration data similar to that developed by International Research and Development Corporation. The lower the velocity (in./sec.) of the out-of-balance condition, hence vibration, the better the balance. Universal Engineering has standardized on what is believed to be the most stringent specifications for these high-speed toolholders with a 0.0392-in./sec.*
As stringent as these balancing specifications appear, and as precise as balancing can be performed with the equipment available to toolholder manufacturers, the spindle manufacturer will most likely argue the interpretation of what precision balancing is needed and what is going to be needed from the toolholder standpoint.
Tool Shank Configurations
The Universal line of dynamically balanced toolholders is currently manufactured in V-Flange, BT, NMM and Kwik-Switch/Pawl-Lock tool shank configurations in either solid or collet chuck designs.
The collet chuck presents additional concerns as the growth during rotation effects an element like a slotted collet. For that reason, it is imperative that the collet be as accurate as possible in terms of diameter and concentricity specifications. Runout data where the tool is used, and not at the collet face, is valuable information.
Pawl-Lock Design
The front locking patented Kwik-Switch/Pawl-Lock nut design is the basic industry standard for manual tool changing machines and more recently when auxiliary high-speed spindles are used. When spindle design does not permit a drawbar to be used because of bearing size and spindle construction, as is the case with many high-speed designs, the tool must be securely and confidently held in a different manner. The Pawl-Lock is one system that eliminates the possibility of the tool loosening and throwing tools. In this design, a serrated locking ring in the nut is contacted by a spring tensioned pawl with serrations in one end that engage the ring in a ratchet effect as it is tightened; securely locking the nut into position. A special wrench is used to depress the pawl to allow loosening.
Conclusion
As the emergence of high-speed machining continues to
expand, and the cooperation between the machine tool builder, spindle manufacturer and
toolholder maker becomes more defined in terms of requirements and needs the results
should come quickly. New improvements in controls to more precisely produce constant
high-speed feed rates is coming, as well as interest in balancing of the cutting tools
themselves, particularly milling cutters. As is normally the case, the toolholder will
continue to be one of the vital links.
Universal Engineering / DeVlieg® Tooling Systems / Tool Preset Service & Calibration
Last updated 02/10/2007