September 2007 Edition

MACHINING Rxs  FROM DR. J

Breaking the Code

Spindle specifications aren't mystery numbers. Understanding them can mean better machine shopping and cutting.

The heart of the machine tool is the spindle. Its capabilities define the performance of a machine tool. Often, spindle specification is expressed as 15hp S3-30%, 60min or 20KW S6-5min. What do these figures mean?

These terms represent the relationship between power, torque, and speed when choosing a spindle for an application. For example, is a spindle rated for 20hp with 1,500-rpm base speed better than a 10hp spindle with a base speed of 500 rpm for cutting titanium?

A typical power-torque-rpm relationship for a spindle driven by an AC induction motor and vector drive is shown below.

 

 

Note the following points:

• Between zero and base speed, the power increases in a linear fashion, while the torque is a constant. This is the rated torque of the spindle and is available at the base speed of the machine. Note that with a vector drive, full-rated torque is available from zero speed, while motors driven by an inverter will have zero torque at zero speed.
• Between a base speed and the maximum speed of the motor, the power is a constant, while the torque decreases — remember that power is proportional to torque times the speed. This is called the rated power of the spindle.
• For some motors, the rated power may decrease beyond a certain rpm (point C).

It is important machine tool users understand the power-torque-rpm curve since the cutting processes have to be designed so required power/torque at a speed is less than the available power/torque. When the process exceeds the available power and torque, it overheats the motor eventually burning it out.

Based on different duty cycle ratings, rated power and torques have different values. It is important to distinguish the different ratings so the application is designed for a machine's capabilities. The International Electrotechnical Commission — IEC — has released a duty cycle rating standard.

Every motor has a continuous-duty rating for power and torque — an S1 rating. S1 is based on reaching thermal equilibrium on a sufficiently long duration. Since most motors in machine tools — axes or spindle motors — are used in a non-continuous duty cycle, the power/torque capability is higher than the continuous duty cycle, since the motor heats up differently. A standard specification — IEC 34 — describes the capability of a motor based on a set duty cycle. The ratings are S1 through S9.

The four ratings for machine tool spindles are S1, S3, S6, and peak load ratings:

• S1 — Continuous duty rating: Constant load with duration long enough for motor to reach thermal equilibrium.
• S3 — Intermittent periodic duty type without starting: A sequence of similar duty cycles at constant load separated by no-load — zero spindle speed — conditions.
• S6 — Continuous operation — periodic duty type: A sequence of similar duty cycles at constant load separated by no-load — but continuously running — condition.

S3 and S6 ratings are expressed as power available for a given percentage of load period in a given cycle duration. When no cycle time duration is specified, a 10-minute time applies as default. For example

• 15hp S3-30%, 60min — S3 rating for the spindle is 15hp available when the spindle is under constant load for 18 minutes — 30 percent of the 60-minute cycle.
• 10KW S6-60% — S6 rating of the spindle is 10 kW when the spindle is used under a constant load for 6 minutes — 60 percent of a 10-minute cycle.
• Peak load rating — The instantaneous power available for a very short burst, such as entering a cut, or for accelerating the spindle to speed.

Knowing machines' ratings offers two advantages. First, it allows a one-to-one comparison when considering a machine purchase. Second, it lets the design of the cutting process' required power/torque match the machine's rpm availability.

If you have a technology topic that you'd like me to address, send me an e-mail in care of pnofel@nelsonpub.com.

Have a machining question or dilemma?

The Doctor is in. If you have a topic you'd like the good doctor to address, send it to Dr. J at pnofel@nelsonpub.com.

Jairam Manjunathaiah, Ph.D., — Dr. J — serves as vice president of engineering for MAG Fadal, a division of MAG-IAS. He leads the new product development, product maintenance, and engineering team.

What do you think?
Will the information in this article increase efficiency or save time, money, or effort? Let us know by e-mail from our website at www.ModernApplicationsNews.com or e-mail the editor at pnofel@nelsonpub.com.