A look at popular machining center spindle connections
A lot of attention is paid to the “business end” of CNC toolholders–the part that actually holds the tool. And rightly so: the ability of the holder to clamp a tool tightly and evenly, with minimal runout, at increasing spindle speeds, impacts productivity concerns ranging from speeds and feeds to tool and spindle life to part dimensional accuracy and surface finish.
But what about the other end of the toolholder–the spindle connection? It can also impact productivity, in ways both positive and negative.
If you’re equipping an existing machine, the connection is what it is. But, if you’re in the market for a new machining center, it may be worth considering what type of connection you wish to have based on the advantages and limitations of each technology. Here’s a look at the relative merits of popular spindle connections for machining center applications, as well as a sampling of new toolholding products for milling (see sidebar).
Over the years, there have been many spindle connection configurations developed for a variety of applications. But for machining centers, there are two systems commonly in use: V-taper and HSK.
Most solid V-taper holders have a collar or flange that allows the tool to be grasped by the machining center’s toolchange mechanism. Toolholders with a single collar are generally designated as NMTB.
Dual-flange holders are either V-flange or BT-flange, depending on the precise flange configuration. V-flange toolholders are often referred to as Caterpillar or simply CAT tooling, because the design was developed about 30 years ago by engineers at Caterpillar Tractor Co. working in conjunction with machine builders. The design eventually became a national standard, and the majority of toolholders currently in use in the US are CAT style. Japanese and European applications, on the other hand, may use BT-flange holders.
The variations in flange configuration mean that CAT and BT tools are not interchangeable; damage to the machining center’s ATC or tools flying around the shop floor could result from switching one type for the other.
But the spindle connections of both types of holders use the same taper–3.5″ in 12″. And, both use the same size definitions, ranging from 30-taper to 60-taper. Small 30-taper tools are most often used in drilling centers and other similar applications. Only the very largest machine tools would require 60-taper, which is relatively rare. The vast majority of machining centers that use solid V-taper tooling have either a 40- or 50-taper spindle.
Standard V-taper toolholders make contact with the spindle along a corresponding taper in the spindle body, and are held tightly against the spindle taper by a drawbar mechanism. Even when the holder is fully seated in the spindle, a small gap remains between the toolholder flange and the spindle face.
According to one toolholder manufacturer, solid V-taper tools are economical, reliable, and do the job for the majority of machining center applications. But, at higher spindle speeds–possibly as low as 10,000 – 12,000 rpm, and certainly at 20,000 rpm–the dimensional accuracy of V-taper tools may be reduced. This is due to centrifugal forces resulting from high speeds, which cause the size of the spindle mouth to increase slightly, or “bell mouth.” As the spindle mouth grows, it allows the drawbar to pull the tool farther up into the spindle, compromising Z-axis accuracy. If the holder pulls far enough into the spindle, it could even stick and damage the machine. The tool may also move off center, affecting accuracy in the X and Y axes as well.
The HSK spindle interface is growing in popularity, particularly in high-speed operations. HSK features a shorter taper design that speeds tool changes, but its main advantage is that it solves the problem of bell-mouthing at higher spindle speeds by using contact between both the spindle face and taper. HSK holders come in several sizes; HSK 63 and 100–the approximate equivalents of CAT 40 and 50– are the most commonly used.
The taper on HSK tools is not only shorter than V-taper tooling, it’s also hollow. In spindles designed for the HSK connection, the drawbar causes gripper fingers to wedge into the clamping position. The fingers pull the toolholder into the spindle, resulting in an interference fit in the spindle and forcing the toolholder flange against the spindle face. This in turn results in excellent radial and axial positioning.
The drawbar system in an HSK spindle provides a positive lock between the toolholder and spindle, and it does so without preloading the entire spindle. Centrifugal force resulting from spindle rotation actually increases the system’s gripping strength and rigidity.
According to one supplier, the HSK design increases static and dynamic rigidity by a factor of four or more over V-taper tooling. Face contact provides repeatable and constant Z-axis positioning, and runout is low due to the tight tolerances between the spindle and tool shank.
But those same tolerances can be an enemy if contamination is a problem. As one source for this article points out, contamination can make a long taper connection run badly, but it can cause an HSK spindle connection to fail.
This can happen because the HSK taper is hollow. Its dimensional compliance takes up any allowable tolerance in the connection. But, when contamination prevents the toolholder face from seating, the torque of machining can initiate cracking between the corner of the deep keyway and the hole in the taper. Eventually, the toolholder shank will fail, which can also damage the spindle.
Alternatives to both V-taper and HSK tooling attempt to combine the best features of both systems while minimizing their weaknesses.
An example that offers simultaneous contact with both the spindle face and taper is the BIG Plus system, offered in the US by BIG/Kaiser Precision Tooling Inc. (Elk Grove Village, IL). According to the company, this patented connection is the de-facto standard in Japan, licensed for use by that country’s largest machine builders.
The system can be used with spindles configured for V-taper tools, but achieves dual contact by eliminating the gap between the machine spindle face and the toolholder flange face. For example, BIG/Kaiser says, CAT 40 and 50 tooling will normally have a gap of roughly 0.125″ (3.2 mm) between the spindle face and the toolholder flange face. The BIG Plus connection reduces this gap to zero when toolholders designed specifically for the system are used.
Users can also continue to use conventional V-taper tooling in situations when higher rigidity or accuracy is not required. With standard tools, the gap between spindle face and holder flange face for CAT 40 – 50 tooling is 0.04 – 0.06″ (1 – 1.5 mm).
A proprietary system offered by Lyndex-Nikken (Mundelein, IL) also uses dual contact to improve cutting performance and lengthen tool life, the company says.
The 3Lock system uses internal expanding pressure to maintain correct taper-to-flange ratio and counteract centrifugal force produced by spindle rotation. The system compensates for bell mouthing at high speeds by preloading the toolholder taper against the spindle bore.
An added benefit of this construction is the dampening effect created by the spring and sleeve design. Toolholders consist of a solid conical core, like that of a traditional V-taper toolholder, surrounded by a cone-shaped shell. Featuring an axial slit, the shell can move independently of the solid core. A Belleville disk spring between the slit shell and the taper flange compresses when the holder is drawn up into the spindle bore, creating a preload on the shell. Thus, as the spindle bore begins to bell-mouth, the shell compensates and maintains full taper contact.
According to Lyndex-Nikken, the resulting stability under dynamic cutting conditions enables the toolholder to absorb vibrations and slight out-of-balance conditions, improving tool life and workpiece surface finishes. The system is compatible with standard V-taper tools.