High-speed spindle for CNC machining center

This is done by incorporating common tooling systems, such as CAT, BT and ISO styles. DIN and ISO tooling standard has been developed with particular application for high speed, known as HSK.

 

The CAT, BT, and ISO standards are questionable as tooling choices for very high-speed. As these tooling standards were developed prior to high speed cutting, the tolerances allowed do not always match the strict requirements of high speed machining. If one of these styles is used, accuracy, cleanliness, and most importantly balance are very critical issues to consider.

 

The spindle must provide a means to locate and clamp the toolholder. This is accomplished by machining a taper in one end of the spindle, manufactured to match the appropriate taper angle and diameter required by that tooling specification. In addition, a clamping mechanism must be provided to hold the toolholder in the taper during machining operations. This device, a drawbar, must provide sufficient pulling force to overcome all forces created by cutting that would tend to pull the tool out of the spindle. The most common technique used in drawbar construction is to stack belleville washers to create a long tension ring. The end of the drawbar grips the toolholder retention knob, and holds the toolholder in position in the taper. When a tool change must occur, a hydraulic or pneumatic cylinder compresses the drawbar, and the toolholder is released.

 

With regard to spindle design, the drawbar presents some challenges. A drawbar is a movable device, and with each actuation the springs may end up in slightly different locations. This can create a balance problem, which could cause unwanted vibration at high speeds. To overcome this, drawbar components are manufactured to close tolerances, and guide bushings are used internally.

 

Also, as speeds increase, the holding force required also increases. It is not practical to increase the holding force by simply increasing the number of washers, as this would require that the spindle shaft be longer (remember bending modes?). It is also not always practical to increase the diameter of the washers, as this may require the shaft to be larger (larger bearings, lower speed!).

 

To satisfy the holding force requirement, mechanical locking systems are sometimes used. The drawbar uses belleville washers to pull the toolholder into the taper. Once seated, however, a mechanical locking system then is actuated. The locking components may be small balls or cams. After the locking mechanism is in place, all cutting forces are directed against the solid steel shaft, not against the belleville washers. This system provides very high holding force and rigidity, which is critical to the high speed cutting process.

 

As the spindle will be used with an ATC magazine, it is necessary to have electronic sensors or switches built-in to indicate to the control logic when a tool is clamped, unclamped, or missing. These signals must be derived from monitoring the position of the drawbar.