Machine Selection Based on Capability, Functionality, Design and Construction
Choosing the right machine for the right application is always a workout at best. And to choose a machine that can also produce the more complex parts that may present themselves down the road beyond its initial need is an important part of the machine selection process. To further complicate the process, does one choose a “standard”, “performance” or “high-performance” machine level (or platform) that pushes the envelope of capability and functionality? Many machine tool builders offer machines in some or all of the “levels” (refer to figure 1). Typically as capability and functionality increase, the levels of performance expand as well. But along with increased capability and functionality found with high-performance machines comes a higher price tag. So the trade off is zeroing in on the many variables, including budget, workflow, manpower availability and expertise to name a few.
Machines at the “standard” level include features for doing prototype work or small-to-medium sized production (refer to “standard” part in figure 2)…primarily basic 2-axis lathe parts. At the “performance” level, machines take on increased capability and functionality engineered for medium-to-full scale production environments. This will include live tooling, C-axis, sub spindle, etc. Material hardness, surface finish and possibly quantity are more critical considerations. The “high-performance” level is when additional axes (Y-axis), twin spindles, automation, material hardness, quantity and possibly grinding quality tolerances and finishes are required. Machines in this category can be distinguished by the components that are engineered into the machine structure.
Let’s now take a look at machine capability and functionality. And for the focus of this article we will concentrate on turning centers, although conceptually the article applies to other machine tool types (VMCs, grinding machines, etc.) as well.
Capability is the ability to achieve a desired level of performance based on accuracy, productivity and reliability.
• Accuracy—dimensional, shape (form) and surface finish.
• Productivity—Output (metal removal rate) and ability to cut different materials, irregular shapes with different types of stationary and live tools.
• Reliability—MTBF (mean time between failure), MTTR (mean time to repair) and durability (ability to keep initial level of productivity and accuracy).
Drivers include material hardness, surface finish, tolerance and throughput; factors include casting design and material, spindle and drive system, ball screws, and way system. Two major machine factors are defined (in general) by a machine tool structure that is assembled from several major structural and kinematical components. Structural elements include machine base, headstock, carriage and cross slide (Z- and X-axes), turret body, top plate, sub spindle body, etc. Kinematical elements include spindles, spindle drives, X- and Z-axis drive systems, guides ways, ball screws, support bearings and drives.
With Ra being the most common measure of surface finish (figure 3), the level of finish machined on a specific product is based on material Rc hardness (figure 4), desired throughput (figure 5) and machine stiffness. If the manufacturer only needs to produce a few parts and the material is not exotic, a low Ra is possible even with “standard” level machines. However, more commonly a low Ra will require a higher level of capability in order to machine consistent parts in low, medium or large quantities.
Any machine may be capable of producing accurate parts to a tight tolerance with enough human intervention. But once the quantity goes up and the demand for accuracy and repeatability remain tight, the move to more capability with reduced human intervention will attain a higher level of accuracy. Key factors with part tolerance include accuracy and repeatability, CPK/PPK requirements and throughput level. Note that when it comes to accuracy all standards are not the same. It is very important to confirm what standard or guideline is being used for fair comparison between different manufacturers. For example, are machine builders using ISO (International Standards Organization) “standards”, VDI (European) “guidelines”, JIS (Japanese Industrial Standards) or NMTBA (National Machine Tool Builders Association) “definition”? This is essential data when developing a comparative spreadsheet. JIS, for example, are far simpler and less rigorous than the other three methods.
All three “levels” of machine performance are capable of being in a production environment. The key factors of throughput, beyond the quantity of parts, are level of part complexity, overall part tolerance and surface finish (cylindricity may also be a factor), and type and hardness of material. The point being that if the part is basic and the tolerance is wide open, then a volume of parts on a “standard” level machine may be possible. If the quantity is high and the tolerance tight, then a “high-performance” machine is the better choice.
This term relates to the multitude of operations that can be performed at one machine level (platform) on any part in a single set-up. Functionality drivers include part complexity and manufacturing strategy. Factors include number of axes, additional process functions, automation and in-process controls. So what does this mean? If a complex part (figure 6) with various feature orientations requiring close geometric tolerances needs to be produced in one set-up, then the machine needs to be configured with multi-tasking capabilities.
Once the machine capability and functionality needs are determined, the next area of concern is machine selection based on performance (material hardness, surface finish requirements or possibly quantity). Three part illustrations are shown in figure 2, each requiring machining operations from simple to complex…or ”performance” to “high-performance.”
General (typical) guidelines for material selection include:
“Standard” level - in the 0 to 35 Rc range: Cast iron, cast steel, brass bronze, aluminum, and coppers
“Performance” level - typically up to 45 Rc range: All the above plus, stainless steels,
“High-performance” level - typically up to 70 Rc range: All the above plus titanium alloys, magnesium alloys, exotic alloys and heat-resistant (super) alloys, such as Hastelloy, Inconnel and Nimonic and Waspaloy
Metals listed at the “high-performance” level may be run on “standard” level machines with less complex part configuration requirements, but having the best match-up of machine performance properties (i.e. speeds, feeds, spindle and axis drive and motor power ratings, etc.) will determine if the part can be produced effectively.
Each level has distinct design characteristics for machine components to achieve desired results. This includes machine stiffness, damping characteristics, base type and weight, castings and materials, ball screws (diameter size, type and classification), linear guides (size, type and classification), motors and drives (size, type and classification), overall system compliance, and control features (functions and type).
Design life of these components is equally important to consider. A new machine that holds very good accuracies for the first few years of operation can deteriorate quickly based on the level of components used. On the other hand, a machine designed and built with consideration of the size, type and classification of all critical components will maintain a high level of accuracy far longer.
Thermal design considerations vary between machine levels. In the case of the “high-precision” Hardinge RS-series turning center (figure 7), continuous airflow in and around the thermally symmetrical headstock frame (figure 8) affords optimum thermal stability for increased part accuracy. The symmetrical dissipation of heat minimizes the transfer of heat generated by the spindle bearings into the cast iron machine structure. This design allows the spindle centerline to remain in a fixed location, unlike “standard” and “performance” level machine spindles that may migrate vertically as a result of thermal growth. Other key areas of heat generation are in the axis way systems. “Standard” level way systems can experience heat buildup especially under heavy load conditions. Whereas, heavy-duty, high-accuracy linear guide ways and ball screws not only allow large load ratings, but greater positioning accuracy and less thermal growth. And to compensate for ball screw growth, high-performance machines typically incorporate linear glass scales. Thermal considerations are also important as they relate to turrets, carriage design, material selection, etc. for each level. Plus, keep in mind the machine environment. Temperature swings can adversely affect part accuracy. Coolant temperature, for example, can be maintained when using a coolant chiller, thus minimizing the impact heat can have on maintaining a high level of part accuracy.
By now you’re probably asking yourself what makes a “high-performance” turning center a high performer? It’s having the latest technological components designed into and built into the machine. This way the entire structure becomes a single harmonious entity. Plus including other factors that few builders offer, such as a collet-ready spindle for direct use of a collet (no spindle adaptation chuck), allows the workpiece to be gripped and machined as close to the spindle bearings as possible (figures 9 and 10). In fact, gripping a part with a collet is the preferred method for high-precision machining. This ability further enhances the tolerance and surface finish capability and allows effective turning of hardened metals, further reducing or eliminating costly grinding operations. One such example of a “high-performance” machine is the Hardinge Super-Precision RS MSY multi-tasking turning center (figure 11). The machine includes “high-performance” features, such as linear scales, unique ESA precision tooling and top plate system, Harcrete polymer composite filled in strategic areas of the cast iron base, high grade linear guides, ball screws and guide trucks. Plus the collet-ready spindle is driven by a perfectly matched motor and drive system for optimum speed and torque. High-pressure and through-spindle coolant capabilities help during the machining process to keep chips flowing away from the machining area, while maintaining proper cutting temperature.
Design of RS-series machines also included the latest software design platform and FEA (finite element analysis) techniques to accurately depict the structural deflection, stress levels, thermal response and vibration response of the assembled components and the assembled machine. Extreme-case loadings were also used to verify adverse machining conditions. Every machine produced receives laser calibration to the X and Z-axis to ensure positioning accuracy and straightness. Accuracy certification is included as assurance of machine accuracy. Size repeatability, surface finish quality and thermal stability are a hallmark for Hardinge RS-series turning centers with maintained accuracy over long runs.
Inherent design features for SPC control include the polymer composite reinforced cast iron base and its vibration damping capability, spindle tooling mounted directly in the spindle as described earlier, and ball screw pitch, diameters and motors matched for speed and longevity. High-accuracy linear guideways, ball screws and encoders, provide the ability to machine superior surface finishes, maintain tighter control over workpiece diameter/tolerance variation, and have superior roundness capabilities on a production basis. Benefits include lower costs through scrap reduction, high SPC capability, minimal downtime, longer tool life, less operator intervention and higher profits.
Moving up to high-performance
So when is a good time to consider moving to a high-performance turning center? There’s a host of factors affecting this decision in addition to the aforementioned, including availability of machinist (reduced number of machinists running multiple machines), expectation of machining complex parts having tight feature orientations, staying ahead in the technology curve for competitive quoting, materials to be machined, to name a few. Another way to look at it…the higher technological features a machine has to offer, the longer the competitive timeline. Faster spindle speeds, faster traverse rates, higher quality ball screws, environmentally-friendly lubrications all allow the user to stay ahead of the curve rather than using “standard” level machines built with less stringent standards, components, capability and functionality.
Once you have decided to look at high-performance machines, you may find the price will make you gun shy even though the machine may be the very one you know your operation needs to be truly competitive. And by then you will have had cost concerns like: “I’ve lost control of my outsourced parts and every day it’s costing me money,” or “My maintenance costs including downtime and repair parts are killing me on my older machines,” or “I can’t find or keep good help for whatever reason.” And the list of questions may go on and on.
Then try the 5-year 2% rule to turn problems into opportunities. Say you find a machine with just the right capability and functionality, or in other words, it has all the bells and whistles you’re looking for. Then take a look at the total cost of the machine leased over a 5-year term. The formula: $205,000 total machine price x 2% = $4100 monthly payment; or $4100 divided by 20 working days = $205 per day; or $205 per day divided by 8 hours = $26 per hour.
Another way to see the affordability with some compromise would be to formulate starting with the machine base price and add the option costs one at a time. For example: $120,000 for the base machine x 2% = $2400 monthly payment; divided by 20 days = $120 per day; or $15 per hour. Then do the same formula with the options (use Live Tooling and C-axis as example): $17,000 x 2% = $340 monthly payment; or $17 per day; or $2.13 per hour. Now you’re at $17.13 per hour. So on and so forth do the math for other options until you get to that price comfort zone with the needed capability and functionality.
Actual costs and rates do vary, but this is a good rule-of-thumb calculator. And if you put down some cash with the lease, then that will get your payments down even further.