Machining or manufacturing performance boosts the competitiveness of machine shops. A company is said to have attained high machining performance if it achieves indicators like operating efficiency, waste reduction, customer satisfaction, product quality, employee motivation, etc. This means that efficiency alone can be used as an indicator of performance, as it relates to the number of parts produced. In this regard, and all other factors held constant, optimizing manufacturing or machining performance entails improving efficiency. Accordingly, this article delves into the efficiency metrics in CNC operations that, when optimized, increase the output of machined parts.
Table of Contents
Understanding Efficiency in CNC Machining
Efficiency in CNC machining simply refers to the number of parts a CNC machine can produce within a given timeframe, often in an hour. Efficiency offers several benefits, including less time spent at the machine. Other benefits include better quality parts and cost savings due to the ability to work on a large number of parts in a short time.
The quest for improved efficiency began in the mid-1980s. This was just a few years after the development of CNC and computer-aided manufacturing (CAM) in the 1970s. But already, the positive impact of efficient machining had become apparent. The 1980s saw the development of multitasking CNC machines. This resulted from better and more powerful chips that increased processing and calculation speeds. It also became easier for operators to create programs thanks to fully functional graphic programming and machine controllers with graphical programming interfaces.
Advancements in the CNC and CAM spaces became commonplace in the 1990s and 2000s. For instance, during the last decade of the 20th century, PC-based CAM software had become so evolved that operators no longer had to write programs using the machines’ controls. Instead, they wrote the programs on their computers at the office desks. By the 2000s, automated CAM software was doing much of the programming.
Similar advancements were also witnessed in the world of CNC machining. Today, machine control systems collect data, monitor and alter machining parameters, and automate various manufacturing processes. They also precisely control machinery movements, tool changes and adjustments, and production sequences. As a result, they ensure consistency in product quality, reduced human intervention, and greater efficiency. As CNC machining technology evolves, CNC machines can accomplish more operations in one setup, taking up less time.
Core Efficiency Metrics in CNC Operations
A common practice in many shop floors is the reliance on CNC machine control systems to collect performance-related information during machining operations. Sensors fitted into the machines facilitate continuous data collection on various aspects of operations, from production-related characteristics and performance metrics to machine health and efficiency metrics. For instance, these sensors measure parameters like vibration, temperature, and motor current. They also monitor machine components like tool heads, bearings, spindles, motors, and actuators, just to mention a few.
Analyzed data helps with predictive maintenance of CNC machines, enabling control systems to detect irregularities or signs of wear. In addition, the embedded machine control systems use this data to automatically manage and modify process parameters, thereby maintaining accurate operating conditions throughout the metal fabrication process. They rely on this data to optimize machining efficiency. This brings us to the core efficiency metrics in CNC operations collected by machine control systems:
1. Cutting Speed
Also known as the surface speed, the cutting speed takes on many definitions depending on the type of cutter. The surface speed of a drill, for example, refers to the speed in meters per minute or feet per minute of the drill rim. The cutting speed of a sawing machine refers to the blade’s velocity, expressed in meters per minute or feet per minute. It is worth noting that the cutting speed depends on the hardness, machinability, and structure of the material being machined.
A low cutting speed translates to low efficiency. Conversely, when the cutting speed is too high, the blade or drill dulls or wears too quickly, prompting regular replacements and regrinding of cutters damaging the workpiece. If the cutter is not changed, it can burn up due to excessive fiction, ruining the workpiece. Extremely high cutting speeds also affect machine efficiency.
2. Feed Rate
Feed rate refers to the speed at which the cutter travels along a distance known as feed before reaching the workpiece. It is measured in millimeters per minute (mpm) or inches per minute (ipm). Feed rate directly affects the productivity and efficiency of a CNC machine. For instance, a high feed rate could damage work tools and the workpiece, requiring corrective measures that could take unnecessarily long to complete. This way, a high feed rate would negatively impact quality. Thus, the feed rate must fall within the optimum ballpark for the best results.
The feed rate is independent of the spindle speed (the RPM of the spindle). Feed rate depends on a number of factors, including:
- Availability of lubrication
- Depth and width of the cut/hole (which affects the ability to remove chips out of the hole)
- Workpiece material type and its strength and material uniformity
- Cutter parameters like the size of the cutter, cutter material, cutter sharpness, and type of cutter
- Expected finish
- Required accuracy or tolerance of the hole
- Power, rigidity, and strength of the machine/setup
3. Material Removal Rate (MRR)
One of the efficiency metrics in CNC operations, material removal rate (MRR) is the volume or weight of material removed from the surface of a material within a given period, usually per minute. A high MRR indicates high efficiency, while a low MRR signals low efficiency. The MRR depends on the combined effect of the feed rate, cutting speed, and depth of cut. For instance, slow cutting speeds result in inefficient MRR because of tool breakage, poor finish, and slow production, while optimum cutting speeds cause efficient MRR.
4. Tool Wear and Tool Life
Tool wear is one of the efficiency metrics in CNC. It is a crucial predictor of the useful life of tools. In addition, it affects the quality of machined parts. For instance, tool wear causes a loss in the dimensional accuracy of finished products, decreases the surface integrity, amplifies unwanted vibrations in the tool and workpiece (chatter), and damages the workpiece. These outcomes often affect efficiency.
You can measure tool wear either indirectly or directly. The former approach involves the estimation of the signals emanating from various signals, such as current consumption, vibration, feed forces, acoustics, and surface texture. The latter involves using a calibrated tool to take measurements over the tool wear zones.
The CNC machine control system can be configured to detect a decrease in the tool life, at which point it can adjust parameters that prolong tool life. For instance, it can adjust the cutting speed, feed rate, and volume of lubricant discharged.
5. Revolutions Per Minute (RPM)
Revolutions per minute or RPM is the spindle speed. It is defined as the number of times per minute the spindle goes around the longitudinal (spindle) axis. RPM affects the surface speed. Larger drills require slower RPMs than smaller drills to achieve the same surface speed at their rim. For instance, drills smaller than 3 mm or 1/8 inches in diameter require a high RPM of 12,000 or more for efficient cutting and to avoid breakage. A larger drill will require less RPM.
In addition to the cutter size, factors like the type of alloy being machined, its hardness, and the operation being performed also affect the RPM. This means there is an optimum surface speed for each material being drilled or cut. Within the context of drilling, five different approaches have been advanced to determine the right RPM for a drill, including:
- Built-in RPM calculations in CAM software
- Dedicated calculator app
- Operator experience
- Standard formula (RPM equals four multiplied by the surface speed divided by the diameter of the drill)
- RPM chart
Advanced Metrics for Deeper Insights
1. Rejection Rate and Part Quality Rate
The rejection rate is the number of parts fabricated within a given period that do not conform to the design specifications and must, therefore, be discarded. In this regard, rejections simply signify that time was lost when creating the defective part. A high rejection rate indicates failures at different stages of the metal fabrication or machining process. Put simply, it negatively affects the efficiency of the CNC operation.
On the opposite end of the spectrum is the part quality rate. The part quality rate relates to the number of quality parts produced within a given timeframe. A high part quality rate indicates the high efficiency of the CNC machining process.
2. Overall Equipment Efficiency
Overall equipment efficiency (OEE) is a measure of machine performance. It is affected by problematic events such as damage to machine components. OEE is expressed as a percentage. It is calculated by multiplying various variables, including the availability of the machine, part quality rate, and performance efficiency. To assess the availability of the machine, you have to consider the setup time, tool change cycle time, breakdown time, maintenance downtime, and work time.
3. Power or Energy Consumption
Power or energy consumption is one of the advanced efficiency metrics in CNC machining. It is an excellent indicator of a CNC machine’s uptime. The longer a machine runs, the higher the energy consumption. In the same vein, a higher machine’s uptime leads to the production of more units, hence higher efficiency. If the energy consumption is low, that typically implies that the machine ran for a short time and, as a result, produced fewer units. Put simply, low energy consumption signals less efficiency.
4. Cost per Part
Cost per part is calculated by dividing the total production costs by the number of parts machined. A lower cost per part indicates that more parts have been produced, signaling higher efficiency. Conversely, a higher cost per part signifies that few parts have been produced, translating to lower efficiency.
Other Parameters Affecting Efficiency in CNC Machining
Besides the core efficiency metrics in CNC machining, other factors or parameters can impact the efficiency of your CNC machining operations. They include:
1. Part-to-Part and New Setup Turnaround Time
Whether you want to make a one-off part or several parts, you have to set up each individually. For added accuracy and, by extension, to boost efficiency, you have to undertake probing in CNC machines, either manually or automatically. Therefore, within this context, the turnaround time refers to the time it takes to secure a part using chucks or change parts. The turnaround time usually depends on the type of CNC machine. Machines that support automatic part changes generally have a shorter turnaround time than manual machines.
Another factor affecting the turnaround time is the work-holding method (e.g., jawed or collet chucks). For instance, collet chucks have a faster turnaround time than jawed chucks. On the other hand, mandrels have a moderately slow to very slow turnaround time, while face plate setups are very slow.
2. Inefficient CNC Programs
Naturally, an inefficient CNC program translates to inefficient machining. Such a program can contain instructions defining inefficient and unnecessary toolpaths that lose valuable machining time. For this reason, it is crucial to write programs that define better and dynamic toolpaths.
These toolpaths are bound to provide new and efficient ways of machining materials. Coupled with the fact that modern machines can perform high-speed machining, these toolpaths greatly enhance efficiency. It is worth pointing out that the skill to write such CNC programs comes with experience. So, if you are a beginner, you can use CAM software to generate the programs.
3. Ongoing Maintenance Downtime
CNC machines require regular maintenance. Typically, operators refill lube and coolant levels, replace drill cutters, and service motors, just to mention a few maintenance activities. While these tasks do not ordinarily involve machining, they must occur to ensure the longevity of the expensive machines.
In some cases, some of these tasks can be undertaken as the machine is running. But in other cases, operators have to switch off the machine to carry out maintenance. This is known as maintenance downtime. It affects efficiency or efficiency metrics in CNC machining because it reduces the number of parts a machine can make within a given fixed timeframe.
4. Tool Changing Cycle Time
Modern CNC machines can store, sort, and automatically change the machining tools. Some can store at least 16 to 18 cutting tools and are configured with conveyor systems or tool carriage extensions to hold the tools and facilitate tool changes.
However, despite the existence of such features, tool changes are not instantaneous. The machine has to stop machining, retract the tool, change it, and then move the tool to the workpiece to continue machining. The time difference between the sequential cuts with two different tools is known as the chip-to-chip or tool-to-tool cycle time. It goes without saying that a long cycle time affects efficiency.
5. Hybrid Manufacturing
Hybrid manufacturing combines two or more manufacturing processes, each using a distinct technology or active energy source. These processes, technologies, and energy sources interact and influence each other, either sequentially or simultaneously, in a controlled manner within the same machine. This interaction promotes faster material removal (i.e., shortens the machining time), enhancing efficiency. Similarly, hybrid micro-machining achieves efficiency in more or less the same manner.
6. Retract Height and Rapid Travel
The retract height, R, is the safest distance from a workpiece at which the tool is positioned. On the other hand, rapid travel (or simply rapid) is the fastest speed a CNC machine can produce to move the tool from the retract height to the workpiece for machining or from the workpiece to the retract height for a tool change.
The rapid and retract height impact efficiency since shortening the retract height reduces travel distance. While this helps save only a few seconds or milliseconds, the time savings often compound. It is, however, worth pointing out that a short retract height can cause safety issues. So, only shorten this distance if you are an experienced operator and if your shop’s policy allows.
Illustration Showing Retract Height (R) (source)
In addition to the above, other factors affecting the efficiency of CNC machines include:
- Machine control systems’ user experience (user-friendliness)
- Faster acceleration in CNC waterjet machines.
Improving Efficiency through Technology
Technology is increasingly finding its way into manufacturing plants and machine shops as companies and professionals look for ways to improve efficiency and productivity and reduce costs. This is part of a concerted effort to embrace and implement Industry 4.0, a paradigm characterized by integrating smart digital technologies into industrial and manufacturing processes.
The list is endless, from model-based enterprises (MBEs), which rely on 3D CAD models to manage and organize business processes, to the digital thread, digital twins, and more. Companies also use cloud computing, edge computing, the Internet of Things (IoT), artificial intelligence, robotics, automation, and much more to improve efficiency. The following technologies can be used to improve the efficiency of CNC machining:
1. Sensors
Sensors collect and monitor crucial manufacturing and machining data. Sensors measure crucial operational parameters like temperature, vibration, humidity, and pressure. These parameters are reliable indicators for issues that can impact efficiency. For instance, vibration or chatter can indicate excessive RPM speed, poor holding method, high depth of cut, and more. Elevated temperatures coupled with low humidity can indicate a lack of lubrication.
2. CAM Software
CAM software can improve CNC machining efficiency by ensuring precise, accurate, and fast machining. For instance, this software can quickly identify and create drill points around the cross-section of the digital model of a part. The software can also create cut sequences that remove excess material before actual machining commences. Moreover, CAM software can calculate the requisite drill RPM, which promotes efficiency.
In addition to these capabilities, advanced CAM software can regenerate or reprocess a CNC program once generated to create more efficient arcs that fit irregular shapes. This way, CAM software enables operators to machine creative new shapes that would be impossible to make otherwise. CAM software also makes it easier and less costly to use CNC machines.
3. Robotics and Automation
The availability of skilled labor is one of the challenges plaguing manufacturers in today’s fast-paced world. And with machinists and operators constantly needing to learn new skills to keep up with emerging trends, human resources can be a thorn in the flesh for manufacturers. This is despite the fact that such companies must continuously increase overall efficiency. Fortunately, companies can solve the human resources challenges by embracing automation. Automation maximizes machining by eliminating human-induced delays.
Today, some modern CNC machines, like Okuma’s ARMROID and STANDROID, have built-in robots packaged within the machine. This robot performs in-machine cleaning, part loading and unloading, chip removal, and chatter suppression. To further boost automation, you can configure your CNC machines with automatic pallet changers and gantry loaders.
4. Artificial Intelligence
There are plenty of different ways artificial intelligence (AI) can be applied to enhance the efficiency of CNC operations. For instance, data collected by the various sensors can be analyzed to establish trends that can accurately predict events. It is this exact approach that makes predictive maintenance possible. Machine learning algorithms use the data collected to build models that can predict future breakdowns. Predictive maintenance helps avoid unexpected breakdowns, which could result in prolonged downtime, adversely affecting efficiency.
The second application of AI is in CAM software. For instance, CAM Assist, a plug-in for Autodesk’s Fusion 360 software, uses AI inference techniques and computational optimization to establish the toolset and manufacturing strategy needed to create a part. It also uses this technology to determine the optimum feed rate and cutting speed. Reports show that the plug-in can reduce by 80% the time it takes to create a CNC machine program. In a typical machine shop or manufacturing company, such a reduction enhances efficiency and saves time. Other software like GibbCAM, Cimatron, and SigmaNEST are also using AI to boost the efficiency and productivity of the programming process.
Strategies for Continuous Improvement
1. Real-time Monitoring
Real-time monitoring refers to the practice of continuously receiving up-to-date data on CNC machines, machining processes, and systems with no delay between data collection and analysis. Access to real-time data enables machine operators, manufacturers, and shop owners to identify and respond swiftly to any emerging issue that may affect efficiency. This way, real-time monitoring reduces downtime. And given that CNC machines must have an uptime of 90-95% to pay for themselves, the wages of operators, overhead costs, and profits, real-time monitoring has a financial benefit, too.
2. Lean Manufacturing
Lean manufacturing focuses on quality control to eliminate waste and enhance process and product quality. It is intended to satisfy customer demand, meaning products are only produced if and when needed. As a result, companies that adopt lean manufacturing principles use less material and inventory, require less investment, and take up less space. Still, additional benefits abound. For instance, companies implementing lean principles have reported a 90% reduction in cycle time, a 50% increase in productivity, and an 80% improvement in quality.
3. Training
Training is a proven strategy to equip your employees, colleagues, and machine operators with the knowledge and skills to easily identify and eliminate wastefulness and boost efficiency. For instance, the training programs could discuss important efficiency metrics in CNC, including materials’ best cutting speed and feed rate. They could also detail how MRR, tool wear, cost per part, part quality rate, and other efficiency metrics in CNC boost quality. It is worth pointing out that the knowledge gained from training programs will likely also enhance employees’ productivity.
4. Updating and Using High-Quality CAM Software
Always use high-quality, up-to-date CAM software. This is because such software will expectedly have advanced features that enhance the efficiency of CNC operations. Moreover, up-to-date software includes newly introduced features, some geared towards greater efficiency.
5. Regular Maintenance
Regular maintenance of CNC machines boosts uptime. It enhances the machines’ work time, ensuring they produce parts longer, thus improving efficiency. Regular maintenance achieves this by maintaining the machine in top-notch condition and preventing breakdowns that could lead to prolonged downtimes.
Conclusion
Efficiency is a much sought-after outcome for manufacturing companies. It helps them reduce costs, optimize energy consumption, and enhance productivity. Efficiency in CNC machining refers to the number of parts or units produced within a given period. It depends on several factors, including tool changing cycle times, part-to-part turnaround time, usage of hybrid manufacturing methods, the rapid and retract height, efficiency of the CNC programs, and ongoing maintenance downtime.
While you can measure efficiency directly by counting the number of parts produced in a given time unit, you can also use a number of efficiency metrics in CNC. These include the cutting speed, tool wear and tool life, feed and feed rate, material removal rate, and revolutions per minute. You can also use advanced efficiency metrics in CNC machining, including the overall equipment effectiveness, rejection rate and part quality rate, cost per part, and energy consumption. If, by using these metrics, you establish that your CNC machines’ efficiency is wanting, you could integrate technologies like sensors, AI, automation, and advanced CAM software to improve efficiency. You could also implement strategies like real-time monitoring, regular maintenance, training, and lean manufacturing to further boost the efficiency of your CNC machining operations.
