Shop Doc – Justifying Use of a High-Speed-Spindle

Dear Shop Doc,

We have often heard the high speed machine spindle is expensive and has to be replaced at some point. Can you shed some light on the high speed spindle construction and service?

Speedster

Dear Speedster,

To understand the cost and justification of a High-Speed-Spindle, let’s look at the more common belt-driven spindle first.  A belt-driven spindle has the motor and spindle mounted separately, linked with a belt-pulley mechanism. With this simple and cost effective system, builders can also install pulley combinations that change ratios on the fly to boost both low end torque and high end rpm. However this time honored design runs into difficulties when rpm continues to push higher. Slipping, vibration, and noise from belt-pulley mechanism eventually become hard to control, so most builders cap belt-driven spindles around 12,000 to15,000 rpm. To answer the market’s demand for higher rpm, the industry’s solution is the Integral-Motor-Spindle (also known as a motorized spindle or built-in spindle).

Integral-Motor-Spindle has all three elements – motor, spindle and tooling – built into one single unit. Its motor winding surrounds the rotary shaft, completely eliminating the mechanical linkage, like belts, pulleys or gears. It can deliver low vibration speed all the way to 100,000 rpm and beyond.  But cramming all these elements into one tight unit makes an Integral-Motor-Spindle a more complex device that carries a higher price tag than that of a belt-driven spindle. Over the years, the Integral-Motor-Spindle has proven itself, becoming the spindle of choice for speed over 12,000 rpm. Practically all main-stream high-speed-spindles are Integral-Motor-Spindles. Due to its clean self-contained modular design, we have seen Integral-Motor-Spindles constantly extending their uses. They show up in some not so high-speed, heavy-duty 50-Taper CNC mills and high-end lathes and offer comparable, if not better, spindle life to that of a belt-driven spindle.

However, when it comes to High-Speed-Spindle life with speed over 20,000 rpm, there are some justified concerns. Our experience shows the spindle life is much more sensitive to how it is used, and the biggest culprit for premature failure is cutting heavier than the High-Speed-Spindle designed for.

High-Speed-Spindle advocates smaller tools with faster and lighter cuts (High-Speed-Machining method) not only because it works for many applications – like surfacing and hard milling – but also because of the spindle limitation. First of all, once spindle bearing DN factor (speed times bore) reaches a limit, increasing max speed (N) requires decreasing bearing ID (D),  which in turn constraints the tool holder size. Typically you will find HSK63 for 24,000 rpm, HSK50 for 36,000 rpm, HSK40 for 42,000 rpm and HSK32 for 60,000 rpm. When tool holder size is reduced, so is overall tooling rigidity. Secondly, motor size is often limited by the housing available for the spindle, and with no belt/gear ratio to amplify the torque, a High-Speed-Spindle can lack low end torque for heavy cutting.  When a programmer enjoys the high speed but is inconsiderate of the rigidity and torque the High-Speed-Spindle has sacrificed, and cuts too heavy from time to time, that would cause a shortened spindle life. That’s why proper programming training with the machine delivery is critical.

Regarding the pricy image of the High-Speed-Spindle, one observation we have is that it has less to do with spindle life and more to do with its crash-resistant ability. The High-Speed-Spindle is compact and complex, and like any device of this nature, it tends to be less forgiving of mistakes. A survivable or low-cost crash for a simple belt-driven spindle might not be the case for a High-Speed-Spindle.

From service point of view, one should not try to fix a High-Speed-Spindle on the field. It’s typically a cartridge design, so switch out entirely and ship to the factory for repair. For an end user, it is important to ask the machine sales person about the spindle service program in advance, and make sure the high- speed machine or spindle OEM has a repair program in the States instead of overseas.

Jesse Xi Chen
Compumachine Inc.