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High Speed Spindle Design
and Construction
William Popoli President IBAG North America |
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Introduction
Although these criteria may seem obvious, for the spindle designer, they represent a wide array of needs that are quite difficult to meet and optimize, in one design. As we will discover, many of the criteria are contradictory to one another, and eventually a compromise must be chosen to provide the best design. The machine tool, also, will present design constraints to the spindle. The amount of available space in the head, cost considerations, complexity and market demands will affect the ultimate spindle design. Cost will have a significant impact on the final spindle design. A very sophisticated and capable spindle design will not be acceptable on a low-cost machine tool. Consequently, an advanced machine tool design can justify the higher cost of a more capable and complex spindle package. In fact, a fast and accurate machine tool will demand a reliable high-speed spindle system. This paper will give a brief overview of the major components required to comprise a high speed milling spindle design. Emphasis will be on commercially available components, that are available at reasonable cost, and most commonly used today on existing machine tools. Future trends will also be mentioned. In addition to the high speed spindle system design, maintenance and reliability issues will also be discussed. High Seed Spindle
Design: Major Component List
Each of these components will be discussed, with emphasis on selection criteria and effectiveness for a given machine tool specification. The machine tool we will assume is a modern CNC machining center with automatic tool changing ability (ATC). Spindle Style: Belt-Driven
or Integral Motor-Spindle Belt-Driven Spindle
Design Power and rotation are supplied to this spindle by an external motor. The motor is mounted adjacent to the spindle, and the torque is transmitted to the spindle shaft by means of a cogged or V-belt. The power, torque and speed of the spindle will therefore depend upon the characteristics of the driving motor, and the belt ratio used between the motor and the spindle. The principal advantages of the belt-driven spindle design are as follows:
However, there are also some limitations of a belt-driven spindle design, particularly when a high speed spindle is required:
Summary Integral Motor-Spindle
Design In general, a complete motor-spindle is comprised of the spindle shaft, including motor element, and tooling system. The spindle shaft is held in position by a set of high precision bearings. The bearings require a lubrication method, such as grease or oil. The spindle shaft then will rotate up to the maximum speed, and exhibit the power characteristics of the motor type that is used. The selection of a particular component will, of course, depend upon the requirements of the machine tool. Also, compromises must be made in order to provide the best combination of speed, power, stiffness and load capacity. The following sections will describe in more detail the design and selection criteria used for the major components of a high speed motor-spindle. Spindle Motor Power
and Torque Spindle motors will generally provide constant torque up to the base speed, and constant horsepower after the base speed As power is a function of speed multiplied by torque, the following curves are typical. Conventional spindle heads multiply available torque by using mechanical components such as gears and pulleys. Motor-spindles, however, must rely upon a single motor characteristic to provide the power and speed needed for machining across the full range of operation. The result is generally that the spindles are designed and intended to be used at or near full speed. Below this, as power falls off, little heavy machining is feasible. Integral AC induction motors are typically three phase,
requiring a special electronic drive to provide the electrical power
source. The drive is a high frequency type, and provides a variable
voltage and variable frequency to the spindle motor. The speed of an
AC motor is determined by the following formula: Speed (RPM) = (Frequency in Hz x 120) / (# of motor poles) This would dictate that a two pole spindle motor, having a top speed of 30,000 RPM, would require a drive with the capability to provide full motor voltage at an output frequency of 500 Rz. If this motor was a four pole type, then a maximum frequency of 1000 Hz would be required. Very high speed drives utilize an open-loop concept, providing voltage and current to the motor without any real-time feedback to close the velocity or position loop. Many drives, however, do use magnetic or optical feedback to the spindle drive. This is used to regulate speed, provide programmable positioning of the spindle shaft, and in some cases rigid tapping. Orientation is required for many tooling systems for ATC operation. DC brushless and Flux vector are examples of closed loop systems. Spindle Air Seals
and Labyrinth Designs To protect against this condition, spindle designers utilize some type of seal to prevent contamination from entering the spindle. The most simple type is a positive air over-pressure. Compressed air is directed into the spindle housing, at low pressure. The air feeds outward to the front and rear of the spindle, providing a low flow of air. This flow prevents contamination from entering into the spindle. This is particularly important for motor-spindles, due to a "chimney affect" which often occurs. As a motor-spindle operates, losses in the rotor will produce heat. As the only contact between the rotor and the spindle housing is through the bearings, the shaft will increase in temperature. When the spindle is stopped, the hot rotor will heat the adjacent air, which will rise. This movement of air, as in a chimney, will draw outside air into the spindle, often bringing contamination with it. This can be very damaging if the material being cut is graphite or carbon. A positive air over-pressure will protect the spindle from this effect. One of the most vulnerable areas in a spindle is near the spindle nose. In this area, the front bearings are very close to the machining area, and subjected to the coolant splashing and chips. Therefore, it is important to provide an extra measure of care to protect the sensitive spindle bearings. Contact seals are not feasible, due to the high speeds. Instead, labyrinth seals are used A labyrinth seal is a non-contact sealing system comprised of a fixed and rotating part. Both parts have channels and grooves machined into them, so they fit together to form a series of passageways between the spindle bearing and the outside air. It is very difficult for a particle of dirt of coolant liquid to pass through a labyrinth seal. Labyrinth seals, used in conjunction with positive air over-pressure, provides very good protection for a high speed spindle. Tool Retention System This report will not attempt to compare the tooling styles, however, 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. Spindle Housing The primary function of a spindle housing is to locate the bearings. High precision bearings, being run at top dN values, must be located exactly in terms of geometry and size. In addition, the housing will provide the lubrication, air seal, cooling water or oil, and other utilities required by the spindle. If the spindle utilizes oil lubrication, the housing will include drilled passages to deliver the oil or oil mist to each bearing, and out of the bearing to a return line. A cooling liquid is often used to remove heat produced by the spindle motor stator, as this heat would affect the size and accuracy of the spindle as a complete unit. The spindle housing is typically connected to the machine tool by means of a flange or attaching bracket. Care should be taken when handling any precision spindle. Crashes, dents, and other damage can affect the accuracy and bearing life. Conclusions As we have seen, bearings will impact a spindle design to the greatest degree. High speed spindle designs most often run bearings systems up to the limit, in order to be the most productive. And, as integral motors are limited in maximum torque available, higher speeds will yield higher power. To reach these speeds, and maintain a reasonable life, precision bearings must be used, along with complex bearing lubrication systems. Oil jet or mist systems not only boost the speed of the bearings, they also provide cooling and cleaning functions as well. Maintenance is critical to the performance of precision bearing systems. Positive over-pressure and labyrinth air seals also should be used to protect the bearing environment. In addition to the bearings, the spindle shaft design must be capable of providing a strong motor, suitable tooling retention system, and stiffness without developing bending problems. And, all rotating components must operate in a balanced condition. The spindle housing must support and locate the bearings accurately, and provide the utilities needed by the spindle system. It must be robust and stiff, as the housing transfers all forces from the spindle to the machine tool. In general, a high speed spindle design will be the result of many compromises. Bearing size and type will dictate maximum speeds possible. Increasing pre-loads and additional tandem bearings will increase stiffness, but speed will be sacrificed High power motors will not fit into the design envelope, and more complex drive systems are required. Higher speeds require higher precision tooling systems, better balance, and cleanliness to obtain the desired results. Shop discipline must be strict. Operators should be well trained and encouraged to learn more about the machine tool. Future Trends
As unattainable as these qualities may sound, they will be fulfilled in the future. High speeds can be accomplished through the use of magnetic or fluid bearings. These non-contact bearing systems will exhibit no mechanical wear, so their life will be very long. Electronic sensors will monitor all aspects of the spindle operation, including cutting loads. Imbalance can be compensated for as the spindle runs. Diagnostic information can be relayed to the CNC for action. Superconducting materials and new motor technologies will provide compact, high power motor system's that produce little heat. Thermal affects on the spindle shaft can be compensated for electronically. References The Barden Corporation SNFA Route de la Glane 143/B SKF Specialty Products Kluber Tribologyr William Popoli
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Re-posted with permission. |
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