Factors You Must Consider When Purchasing a New VMC
In a typical production machine shop, the processing capabilities required can vary greatly between applications: some value power and torque over rpm, others value spindle speed more than torque or power. To ensure the highest level of productivity in all cases, all of these factors must be evaluated when considering a new investment in a vertical machining center.
With over 200 different manufacturers of vertical machining centers (VMCs) on the market today, selecting the right VMC for a production environment can be a challenging and stressful endeavor. Each machine is designed and built differently, with unique features and characteristics. The key is determining how each impacts performance.
In order to cut through the clutter to find the right solution, manufacturers must first learn how to effectively evaluate machine performance by understanding key differentiating characteristics. Several factors to consider during evaluation are parasitic times, cutting times, thermal control, machine construction and production support features.
Today the average non-cut, parasitic time of a typical production machine shop consumes approximately 30 percent of the machining process. Part designs continue to grow in complexity, with a higher volume and diversity of closely spaced three-dimensional features. In order to minimize parasitic time and optimize production capabilities, evaluation of axis motion and tool-change technologies are critical in the selection of a production VMC.
There are substantial differences in the axis motions of modern VMCs, even when evaluating just the top tier of machine tool manufacturers. We conducted a recent comparison test to evaluate the differences in axis rapid traverse rates of several leading VMCs that demonstrated variations as high as 41 percent. A similar evaluation was given to axis acceleration and deceleration rates, resulting in a 45 percent difference between averaged rates. The Z-axis acceleration and deceleration rates in particular, which are crucial in drilling, pocket milling and tapping operations, resulted in performance variations as high as 75 percent.
When each machine was tested over a 10 in rapid traverse, the vertical machining center with the highest levels of rapid traverse and acceleration and deceleration rates outperformed all other machines by completing the 10 in movement in 0.51 seconds – a 17.6 percent improvement over other market-leading machines.
The second key factor equally as crucial to parasitic time is tool change. When considering a VMC, manufacturers should examine the full tool change time from the moment the spindle stops to leave the work zone to the point at which it returns to the desired position and speed. While spec comparisons between leading VMCs might indicate minimal differences in tool-to-tool time, a true comparison that includes chip-to-chip time reveals variations as high as 18.4 percent.
By combining the 17.6 percent difference in axis motion performance and 18.4 percent variation in tool-change performance, the total average parasitic time saved is 18 percent. As such, today’s average production machine shop could reduce its parasitic time from 30 percent to just 12 percent. For shops that run 2,000 hours per year, this could mean a saving of 108 hours per year for each shift. When applied to an average shop rate of $50 per hour, the annual parasitic time saved could be as much as $5,400 per shift.
In essence, the differences in axis motion and tool-change performance of production VMCs can have a significant impact on a manufacturer’s profitability.
Alternative to parasitic time is the actual time a vertical machining center spends cutting. With parasitic time consuming 30 percent of the machining process, the remaining 70 percent of the machine process is spent in the cut. Cutting times are generally examined by spindle capability, in which there are three primary factors to evaluate: power, torque and speed.
The horsepower of a spindle is critical in achieving high metal-removal rates. Based on comparison tests we have administered, considerable differences in spindle horsepower can be observed even in some of the leading VMCs. Of the machines evaluated, horsepower specifications varied as much as 67 percent, or approximately two-thirds. Obviously, a noticeable difference can be seen in the roughing capabilities of each machine.
Spindle torque is a necessary complement to horsepower in the machining of hardened materials, providing additional strength to overcome aggressive machining forces. Vertical machining centers with low torque capability may hinder a manufacturer’s ability to accurately face mill and end mill at the low rpms required for cast iron steel and other similar materials. Among some of the market-leading vertical machining centers, we have observed varying specifications as diverse as 73 percent. Without careful evaluation of both torque and horsepower, manufacturers may find themselves limited in the types of materials they can efficiently process.
The last factor to consider in evaluating spindle capability is speed. In comparing several leading VMCs, spindle speed specifications ranged anywhere from 7,500 rpm to 14,000 rpm – nearly 90 percent variation. This disparaging difference can pose a significant impact on a manufacturer’s versatility and productivity. By using machines with a wide range of speed and high horsepower, manufacturers can significantly improve their versatility and productivity for overall reductions in cutting time.
By combining these comparison results to evaluate spindle power and torque capabilities over the full spectrum of available speeds, it becomes clear that these characteristics work hand in hand. Vertical machining centers with high horsepower and wide-ranging spindle speeds offered anywhere from 1.5 times to 2.3 times the power capability of other market-leading machines, depending on the desired rpm settings. Similarly, machines offering the highest levels of torque and wide-ranging spindles provide torque capabilities anywhere from 1.6 to 3.6 times that of other market-leading VMCs.
With this level of power and torque capability, manufacturers can expect to save as much as 30 percent to 50 percent on their cutting times which, at a 2,000 hour per year shop rate, could mean anywhere from 420 hours to 700 hours of time saved. By projecting these figures into a $50 per hour average shop rate, the cost saving could be as high as $35,000 per year when running a single shift.
In a typical production machine shop, the type of processing capabilities required can vary greatly between applications, demonstrating the importance of each spindle characteristic. For example, a manufacturer may run cast iron or steel one day, valuing the power and torque over rpm. However, the next day, this same manufacturer could run brass, or drill small holes in aluminum, making spindle speed more valuable than torque or power. To ensure the highest level of productivity in all applications, manufacturers considering new investments in VMCs must evaluate all factors with equal importance.
Another key factor to evaluate while selecting a vertical machining center is thermal control. Modern thermal-control technologies have made significant advancements in both the productivity and accuracy of machining centers. Some key thermal-control characteristics to consider are spindle warm-up times, spindle growth and temperatures of linear axes.
When evaluating differences in spindle warm-up times of several leading VMCs, one vertical machining center in particular required a 30 minute warm-up cycle after 24 hours of idle time and a 10 minute warm-up cycle after eight hours of idle time. This scenario would mean that over the course of a 52 week period, manufacturers that run one shift a day with weekend closures could be spending nearly 68 hours waiting on spindle warm-up cycles (26 hours per year for Monday warm-up cycles and 41.7 hours per year for other daily warm-up cycles). At a shop rate of $50 per hour, these manufacturers could be losing as much as $3,385 per year in productivity.
Another thermal-control consideration that should be accounted for is spindle thermal growth. While high levels of speed, power and torque are desirable for productivity, these qualities also generate a great deal of heat, resulting in thermal growth and potential part inaccuracies. In order to obtain these features while retaining quality, a vertical machining center must feature a variety of modern spindle-cooling technologies. Even in many of the leading VMCs on the market, spindle chillers and oilmatic units are not included as standard features. Without these technologies, manufacturers run a higher risk of thermal growth, which becomes critical in boring operations and other features where Z-axis depths are critical.
Linear Axis Temperature
In addition to controlling thermal growth of the spindle, manufacturers should consider temperature-control features of the linear axes to maintain accuracy and part quality. For instance, passing coolant through not only the bearing package but also the ballscrews themselves can allow for greater accuracy control over time. In addition, running these components at lower temperatures can extend their life span and overall machine reliability.
Additional means for evaluating the accuracy of a vertical machining center would lie in the construction of the machine. Manufacturers should be wary of thin frames, structures with overhangs and unsupported components or motion. Machines with these components can face distortion, deflection, inaccuracy and vibration dampening when machining. Manufacturers should also pay attention to the weight and mass of the machine and how the machine is arranged to provide stiffness, rigidity and ultimately accuracy. If a vertical machining center is not rigid and stiff, the X-, Y- and Z-axis thrust can suffer, resulting in lower positioning accuracy and repeatability.
The last key factors for manufacturers to evaluate before investing in a vertical machining center are general production support features, which encompass chip and coolant management, ergonomics and ease of operation.
Chip and Coolant Management
Chip and coolant management systems are an essential feature to any vertical machining center, ensuring effective and efficient removal of chips from the work zone for improved production accuracy.
The most common means for managing chips within the work zone of a vertical machining center is a variety of coolant systems, including flood, through-spindle, flush and chip wash. While each of the market-leading machines included these coolant systems, a significant difference in coolant tank capacity ranged anywhere from 55 gal to 230 gal. For manufacturers that require simultaneous operation of all coolant systems, a 55 gal capacity tank could run the risk of using up all of the coolant, leaving an empty tank. This type of basic design flaw is something manufacturers should be cautious of, even among the leading VMCs on the market today.
Another chip and coolant management feature to evaluate is the availability of a lift-up chip conveyor. Astoundingly, the majority of market-leading machines do not offer lift-up chip conveyors as a standard feature. For a manufacturer evaluating VMCs, this feature could make the price tag more appealing; however, the loss in productivity resulting from operators stopping the machine to manage chips can result in an even higher cost over time. Features such as this should be considered standard for shops that expect to be competitive globally.
Ergonomics is another critical production support feature to consider, dictating the speed and ease at which operators can access parts and tools. While most modern VMCs now follow the same “C” frame construction for easy access to the work zone, many machines on the market still do not include a tool magazine. In this situation, operators have to load tools through the spindle, meaning anytime a tool needs to be changed, updated or maintained, the manufacturer is experiencing extended downtime. Consequently, all tool maintenance becomes parasitic time. With a tool magazine, spindle downtime is minimized and operators are free to set up the following job for higher efficiency.
Ease of Operation (EOO)
The ease of operation for a VMC relies primarily on its control capabilities. Many modern market-leading machines now use a menu-driven touch-screen control panel. This type of design is intuitive and easy for operators to find and retrieve the information they need.
The most effective control systems are configured for a production environment. As previously mentioned, a typical production machine shop runs a wide variety of part programs. The ideal machine control for a production vertical machining center takes this into consideration, offering extensive part program and tool data storage. By cataloging tool magazine data, manufacturers can make informed tooling adjustments on the fly for greater efficiency. In addition, an effective control system can offer more than the standard six-coordinate work system, providing the flexibility to use multiple fixtures and vices on the table. Other desirable machine control support capabilities include Ethernet, DNC, PCMCIA and RS-232-C.
THE IDEAL PRODUCTION SOLUTION
By evaluating VMCs based on the factors discussed above, manufacturers can ensure the ideal solution for their production environment. Each key factor holds a tremendous impact on productivity, quality, efficiency and ultimately profitability. No two VMCs uphold the same standards, even among market leading machines. In order for manufacturers to see through the clutter, it’s important to keep in mind the factors that will make the biggest impact on their ability to compete globally.