Thermal Compensation: Sound Science in Milling
The growing Six Sigma movement and other trends in manufacturing have made the heat generated by machining centers more of a concern than before. The prevalence of tight quality control practices is forcing supply chains in many industries to hold tolerances tighter than those specified on the print.
"People use to be happy making parts to print tolerance," said Greg Hyatt, manager of the product and process development group at Makino Inc. (Mason, OH). "Now those tolerances meet no one's expectations. Manufacturers are pursuing statistically capable processes and often tighter tolerances on the same print."
To satisfy Six Sigma requirements, operations performed on machining centers and milling machines must perform to within ten-thousandths of an inch, even though a print might specify only to within thousandths. The situation becomes more extreme when print tolerances are tighter. Consequently, the temperatures of the workpiece and the machine's internal components are affecting the machine's ability to hold tolerances more significantly than in the past.
Complicating matters are the competitive pressures to mill faster. During the past few years, builders have responded by pushing machining centers to cut at higher speeds and feeds. A 1-G acceleration and 3,000-ipm rapid traverse rate are now common in high-speed machines. Soon 2 G and 4,000 ipm could become routine. The problem is that these high speeds and accelerations require drive motors with greater power. Not only do these motors consume more energy and generate more heat, but the friction from the faster feeds also creates more heat in bearings, ballscrews and ways.
To control the generation and flow of heat that would otherwise degrade accuracy, most builders have resorted to a number of tactics, starting with studying the thermodynamics of machines through finite element analysis (FEA). The designs are adjusted to keep heat to a minimum and permit the machine to grow in controlled and predictable ways. Many also have used the knowledge gained from FEA studies and capability measurements to write software for helping the machine compensate for thermal errors.
Some builders attribute a machine's micron-level accuracy and repeatability to thermal compensation software. They also credit the software with productivity advantages in the mornings when operators first start the machines. The software can mitigate, or even eliminate, errors from the large amount of thermal growth that occurs until the machine reaches a steady state. Then, as the day progresses, it can make further adjustments whenever usage and work cycles change.
Such software packages are usually transparent to the operator. Because of the computing power available today, they can run unseen behind the scenes inside the machine's computer numerical control (CNC), collecting feedback and using algorithms to correct the tool's position automatically. The integration of such software is so tight, even on high-speed machines, that it is as vital to a machine's infrastructure as the casting. In fact, many experts believe that internal software compensating for thermal and other errors has grown to become the third leg of the machining technology stool that has previously rested solely on machine design and cutting tools.
A number of approaches
Thermal compensation software comes in two basic versions, one that acts upon temperature measurements directly and another that responds to the current drawn by motors or other measurements corresponding to the machine's activity. Activity-based software was instrumental in the ability of OKK USA Corp. (Glendale Heights, IL) to offer the die-and-mold industry its MCV 1260 vertical machining center, a C-frame machine with 49 inches of Y-axis travel. Length of the Y-axis's reach has been a constraint for this class of machine in the past because it amplifies the effect of temperature fluctuations and causes the machining head to grow away from the column. Compensation software corrects for this growth and removes the constraint.
OKK's Soft Scale software compensates for thermal differences in two ways, said Bob Meier, an OKK spokesman. Soft Scale II adjusts the table based on the difference between temperature measurements from sensors in the spindle head and at the base of the machine. To correct for thermal expansion along the ballscrews, Soft Scale I tracks the activity of the servodrives. Compensation increases as the machine becomes more active and as the ballscrews' nuts travel farther from their anchors. Activity increases friction along each screw and the heat generated inside its motors. Greater distances be-tween the anchors and nuts means more material is susceptible to expansion.
The software and an internal cooling system have allowed OKK's engineers to create a large machine that is accurate and repeatable to 10 microns along the Y axis. Meier said that this rating is good at all times, including warm-up periods at the beginning of the day. He said that repeatability improves once the machine reaches a steady state.
ATAC10 software, artificial intelligence thermal distortion accuracy control, used by Hitachi Seiki USA Inc. (Congers, NY) approaches the problem differently. Not only does it correct for a "composite" or overall error instead of correcting for the growth of specific components, but it also reacts to direct temperature measurements instead of the machine's activity. Engineers in Japan created error maps for the machines and described them with multivariable empirical formulas. Algorithms inside the software compute Y- and Z-axis offsets based on how much the temperatures at as many as six locations differ from ambient temperature, as measured by a sensor either in the bed or under it. The algorithms account for both long, continuous cuts and interruptions from lunch breaks, tool changes and setups.
Chiron America Inc. (Charlotte, NC) also has adopted the approach of measuring temperature directly and correcting for composite errors. After 6 years of performing FEAs and conducting dynamic tests, researchers at the University of Acken (Germany) and at Chiron's facility in Germany constructed a computer model of the machines and developed a software package that compensates for thermal growth. Called Thermocontrol, the software receives and stores temperature feedback through a network of five temperature sensors mounted inside the machine at the base, column, headstock, and upper and lower spindle bearings.
The temperature sensors monitor the hot spots that the researchers identified and modeled. "When the sealed spindle warms up, it grows and radiates heat through the headstock, causing it to expand too," said Norm Holtzhauer, project engineering manager at Chiron. "A lot of hot chips can raise the coolant temperature up to 15 F, which can heat up the machine's base."
G codes embedded in part programs tell the CNC to read the five temperatures and their histories, and compute the appropriate position offset for each axis. Holtzhauer said that the average shop could expect 10-micron accuracy, whether the machine is cold or warm. The caveat is that the ambient temperature must fluctuate less than 15 F. With correct tuning of the algorithms and active control of external and internal temperatures, Thermocontrol can hold tolerances within a few microns.
Is it cheating?
Despite the success that many builders claim for thermal compensation, the practice is controversial in some quarters. A number of builders berate it as largely ineffective, referring to it as electronic trickery or likening it to a Band-Aid covering symptoms of a larger problem. The argument centers on the nonlinearity of thermal errors and the complexity of the resulting distortion. They believe that the problem is too complex to model accurately and that allowing the errors to exist affects the alignment of the machine.
Another problem they note is that most machines do not have enough CNC- controlled axes to correct the errors, which can include twisting of the bed and column. "Any compensation software would have to correct position in more than three axes," said Hyatt. "In addition to coordinating the X, Y and Z axes, it would need to account for changes in at least two rotary axes, meaning that the machine would need at least five programmable axes to provide enough degrees of freedom to correct the errors."
Instead of relying on thermal-compensation software, Makino and other like-minded builders stress a good, symmetrical design and temperature control. They eliminate as many of the hot spots from the design as possible, and they cool the hot spots with chilled glycol or oil. At a recent trade show, for example, a number of builders introduced machines that send chilled fluids through hollow ballscrews and spindle shafts, and a few even circulated fluid through the casting. New designs at the show also exploited symmetry to ensure that any growth is orthogonal to avoid any twisting.
Although Makino eschews software for correcting thermal errors, it deploys some for aggressive control over spindle temperature. Rather than switching the builder's chiller on or off, software regulates the mixing of oil from two reservoirs, one at ambient temperature and one overchilled, to get the temperature necessary to maintain constant temperature at any moment. The software computes the required cooling duty based on temperature and spindle speed. "Depending on the conditions of the cut, it calculates how many kilowatts that the spindle will draw," said Hyatt. "Knowing the efficiency of the motor, it projects the heat generated by the spindle motor."
Although builders using thermal compensation packages agree that minimizing thermal error from the outset is critical, their response to accusations of electronic trickery is to point at the results. "Operators are not concerned with what is occurring inside the machine," said Yusuf Venjara, general manager of engineering at Hitachi Seiki. "They simply want their tools to go to the points in space that they specify. Our software can do that within 10 microns." Because the validity of any science lies with the repeatability of the results, Venjara's claim confirms that thermal compensation on machining centers and milling machines can be a sound method for controlling accuracy within six sigmas.