Production Time Savers
It's not just fast spindles
By Robert B. Aronson
Out-of-cut time is the enemy of productivity. A number of events have to occur before and after a tool reaches the workpiece. These time-consuming events are critical.
Most manufacturing operations require a variety of tools that are generally supplied by a tool magazine. "There is a lot of consideration given to magazine operations," says Tim Jones, product line manager for Makino (Mason, OH). "The first is the number of directions. A single-direction motion does not supply tools as quickly as does a magazine with bidirectional motion.
"The next consideration is fixed address [the tool goes back to the same area it originally came from] versus random address [a tool goes back to where the next tool comes from]. There are significant cycle time implications for the two.
"Another time saver is parallel processing," says Jones. "Instead of having a series of sequential operations, you execute a number of concurrent actions. For example, instead of executing the following--stop spindle, stop coolant, open auto tool change door in sequence, etc.--all operations can be performed simultaneously. This can save several seconds on each tool change, over a long production run using a large number of tools. Parallel processing can increase manufacturing capacity by weeks or even months on an annual basis.
"Many high-speed machining centers have not allowed for ease of automation issues such as thermal growth and vibration," according to Mal Sudhakar, vice president, Mikron Corp. (Holliston, MA). "This has inhibited users from operating in a truly unmanned mode."
Mikron offers a high-speed machining center designed for automation that addresses these problems. One model, their HSM400, has a table chuck with a palletizing system and a seven-position pallet changer as standard features.
To address specific issues of high speed machining process, the machines have several hardware and software modules, called "smart machine" modules that improve process reliability.
One such module, the Advanced Process System, has a built-in vibration sensor in the spindle together with a spindle diagnostic module. The sensor measures the vibration levels at the bearings during the cutting process and the spindle diagnostic module records the information. The vibration levels can also be displayed on the monitor of the CNC control. Limit values can be set relating to the vibration level and if the values reach certain levels a warning can be issued or at higher levels the cutting process can be stopped preventing damage to the machine and workpiece.
A new module labeled Intelligent Thermal Control addresses the issue of thermal growth. It has built-in intelligence on the thermal behavior of the machine and compensates automatically for thermal drift for all operating conditions. With this module, a warm-up phase or pre-heat cycles are no longer necessary.
Robots to the rescue. Because wide variation in parts is now the rule in many manufacturing operations, fixed automation is being replaced by more flexible techniques. Part handling has evolved as one of the major robotic applications. According to Richard Johnson, general manager, materials handling, Fanuc Robotics (Rochester Hills, MI), the robot's precision and repeatability can have major advantages over manual part handling.
Some of the more impressive claims for increased capacity and productivity come from operations that have added robots that offer consistency, reliability, and accuracy. Robots can provide six degrees of freedom (X,Y,Z axes plus roll, pitch, and yaw), which in many cases matches human dexterity. "Anything your wrist can do, a robot can do," says Jack Justice, market segment manager for general robotics, Motoman (Carrollton, OH).
Several factors have made robots more attractive. First, costs have dropped dramatically, programming is simpler, vision systems are less costly and complex. "About 10% of the material-handling robots we sell today use vision," says Fanuc's Johnson. "It's easier to use vision than to pay the cost of positioning the parts."
At the same time, robot reliability has increased greatly. "Not too many years ago, the mean time between failures [MTBF] was 6000 to 8000 hr," says Justice. "Now it's more than 60,000 hr."
Some machine tool builders have opted for built-in robots. These are usually small, robust units incorporated into the work area that can perform in that hostile environment. According to Fanuc's Johnson: "We have a specialty robot that can be built into the machine tool's work area and can operate with the wrist submerged in coolant and has the ability to withstand coolant pressures of 2850 psi [20.6 MPa]."
Overhead gantry robots can have an advantage in multimachine operations. Fanuc Robotics offers a "top loader" system in which a six-axis robot moves overhead on a rail down the center of a cell and feeds and removes parts using a double-ended "hand." One carries the raw part and the other the recently finished part. This configuration leaves the floor space open for maintenance and greatly enhances the robot's work envelope.
Linear motors and ballscrews are the major means of positioning for machine tools. Linear motors have picked up a share of the market, but are still a fairly specialized application.
The iron-core or single-side linear motor is the design most commonly used with machine tools. Essentially, the design consists of a moving forcer and a stationary permanent magnet track. It is generally lower-cost and has a higher force density than other designs.
"The linear motor has its greatest advantage when both speed and precision are needed," says Michael Backman, vice president, Rockwell Automation, Anorad Div. (Shirley, NY). "High-precision, high-speed, extreme-throughput applications are ideal. It also offers high dynamic stiffness, good acceleraton and deceleration, and requires very little maintenance. Linear motors offer the advantages of less wear than ballscrews and therefore provide greater long-term accuracy. Backlash is also eliminated because it is a direct drive.
The linear motor can move microns or meters at full force instantly. No gearing is necessary. To multiply force, a ballscrew can use a gearbox. With linear motors, each motor delivers a specific force. To achieve higher forces, designers can choose a larger motor, use multiple forcers on the same track, or use multiple forcers on multiple tracks.
At the same time, ballscrew design has not been stagnant. There have been advances in kinematics, allowing greater acceleration. Positioning systems that move both the part and the tool simultaneously, rather than move only the spindle or the worktable, are now available.
Another advance in positioning is the direct drive-torque motor. The table itself becomes the stator driven by the magnetic field. There are fewer components, so acceleration and speed are greater and there is almost no backlash. According to Joseph Biondo of Bosch Rexroth (Hoffman Estates, IL), torque motors are available with speeds to 1200 rpm and torque to 4700 Nm.
After years of research, Cincinnati Lamb (Hebron, KY) is offering its first machine tool using linear motors. It's the HyperMach-RT, a five-axis profiling machine for the aerospace market.
This interest in applying linear motors to aerospace applications is a response to that industry's growing interest in the functionality of the faster equipment and downstream benefits realized by manufacturing fewer, more complex parts. "They are going after a more 'automotive' style of productivity to replace their more traditional 'hand crafted' construction methods," according to Randy Von Moll, manager, product development. The product is a fresh design that takes advantage of the linear motor's capabilities, chiefly increased acc and dec. We significantly reduced weight, cutting down moving mass to one fourth that of the previous generation of similar machines, but increasing acceleration by an order of magnitude. At the same time the structure was optimized to have the highest possible dynamic stiffness using the least amount of material."
Rotary tables can greatly increase productivity, either as an addition to a new machine or retrofitted to an existing setup. For example, a rotary table can enhance the capabilities of a three-axis machining center to four or five-axis capabilities. Rotation speed, positioning accuracy, and ease of loading and unloading are important features.
"Our table's gear systems don't use a bronze alloy," explains Preben Hansen of Lyndex-Nikken, Inc. (Mundelein, IL). "We have a hard-carbide worm screw for long-term reliability and accuracy. Our Nikken-designed ion-nitrided worm wheels and carbide worm screws reduce friction and wear by up to eight times. Reduction of wear translates into reduced maintenance, and therefore more uptime and productivity.
"In general, table speeds are increasing as are accuracy requirements. Our latest design has a top speed of 133 rpm and a positioning accuracy of 15 arc-sec." Often these tables are used as a fourth or fifth axis, so the drives and controls have to be able to tie into the machine tool controls, he adds.
"Our units can be independently programmed when needed," says Hansen. "This is because operators sometimes want the flexibility of using a single table at several machines, or because the controls of an older machine are not upgradeable to run an additional axis."
Tool changing can have a major influence on out-of-cut time. Often, backup tool storage is needed when the operation requires more tools than a standard magazine can provide. One such tool storage and retrieval system, offered by Bertsche Engineering (Buffalo Grove, IL) supplies from 60 to 300 tools. The tools are stored in vertical racks organized as an X-Y matrix serviced by a programmed tool transporter or "rabbit" that picks tools from the rack and carries them to and from the machine magazine.
"The tool transporter is programmed to fetch and store tools in the most efficient way as a background task from the tool storage racks," explains company president Rich Bertsche. "Selection time may run from 20 to 60 seconds, depending largely on the size of the storage system. Usually this type of storage becomes practical when more than 60 tools are needed."
Part holders have had to adapt to the demand for high-volume production of smaller parts. Kurt Mfg. (Minneapolis) offers a "serrated tower" system with clamps that fit into serrations on the tower face. With this unit, positioning is more accurate, clamping forces can be higher, and part density is greater.
"Custom designed workholding is a growing trend," says manager Steve Kane. "More customers are asking for workholding that is dedicated to a family of parts because it cuts costs by reducing part handling involving less efficient workholding devices. To support this custom designed workholding trend, Kurt offers a 'CarvLock' product line with holding elements that can be easily machined into special configurations to hold specialized shapes. These custom elements then can be configured easily into the Kurt line of vises and towers, including the serrated versions, for use on virtually any machining center thereby greatly increasing productivity."
Production automation is not limited to elements built into the machine tool. Manufacturers that are working to automate the one-off and low volume operations may take advantage of specialized off-machine automation.
The systems offered by Erowa (Arlington Heights, IL) are an example. Although their units eliminate some manual operations, the overall effect is to save jobs through increased accuracy and productivity. Their installations specialize in automating one-off or short-production-run operations providing for the complete automated manufacture of a part with accuracy of 0.002 mm.
According to Tom Watkins, VP engineering, their greatest success has been in the die/mold and aerospace industries. In a typical process a setup operator positions the workpiece on a pallet and associates the pallet with the predefined workpiece machining program via an identification chip. The pallet is then measured in a CMM or other measuring system to obtain the X, Y, Z and C axis offsets which in turn are associated via the chip.
In operation, the handling device initializes the storage magazine by reading the chips and the job management database is then updated with the information. From there, the software controls the process and priority of the workflow, initializes DNC downloading of the part programs / offsets and controls the handling device. Robots position the workpiece on the table, reposition the workpiece after each machining operation, and make tool changes.
Erowa system robots can work with parts weighting up to 200 kg in a work envelope about 320-mm square.
Where Are the Problems?
Liberty Precision Industries (Rochester, NY) is an integrator, a company that does turnkey projects for manufacturers. They are a little different in that they not only put together systems from existing components, but they can, when necessary, build their own. And this includes entire machine tools. In evaluating client problems, company CEO Douglas Woods noted these factors as the ones that most commonly disrupt productivity.
Tooling. The user modifies optimal tooling in favor of lower-cost equipment, or does not take advantage of the latest technology and match tooling to particular manufacturing requirements.
Chips. Through poor handling procedures, they are allowed to build up in critical areas that lead to delays, inefficient cutting, mis-clamping and intermittent automation faults.
Metrology. There are faults in all aspects of this process. Measurements are not made. The wrong measurements are made. Improper instruments are used. Results are improperly interpreted.
Training. Operators and/or maintenance personnel are not familiar with the equipment. Often problems can be easily solved when personnel know the equipment well, or when the latest user-friendly, simplified controls are used.
Automation. Part handling, when operating as designed, is a great production-time saver. However, initial setup can be complex, and the potential for error is high, particularly with systems that require coordination of a number of events. Users should be prepared to take the time and effort needed to set up such systems properly.
This article was first published in the January 2004 edition of Manufacturing Engineering magazine.
Published Date : 1/1/2004