A CNC machine tool breakdown is expensive and disruptive, but it is at least visible. Arguably, a bigger problem is gradual deterioration that reduces accuracy, wears moving parts and, ultimately, shortens the life of the machine.
These problems and costs are avoided through a program of effective planned CNC maintenance. This blog covers what that should look like and the benefits to be expected. First, let’s look at an overview of what makes CNC machines so complicated (and so useful to manufacturers).
Understanding CNC machines
Almost all machining of wood, metal and composites is carried out on CNC machine tools. CNC is an acronym for Computer Numerical Control, which refers to automation of the machine movements. Here we’ll discuss what CNC machines are and the various types deployed in manufacturing.
What are CNC machines?
Machine tools comprise a spindle and several motion axes. Together these move the workpiece against the cutting tool, or vice versa, to cut away material. In a CNC machine, the axes are powered by motors with encoders to report their speed and direction, and each axis has an optical scale for position and travel measurement.
A computer controller mounted on the machine turns motors on and off and sets their speeds and movements in accordance with instructions provided in a part program. The controller can be programmed at the machine, although in most plants programming is done offline using Computer-Aided Manufacturing (CAM) software.
An electrical cabinet stands next to the machine. This houses the motor drives, relays and other devices used to manage machine operation.
Types of CNC machines
The most widely used types are milling machines and lathes. Milling machines (“mills”) hold a rotating cutting tool in a spindle that may be mounted either horizontally or vertically. The workpiece is clamped to a table, which three or more motion axes move past the cutter.
In a lathe, the workpiece is held in a chuck or collet and the cutting tool is moved axially by two axes. More sophisticated versions have multiple tools in a drum or carousel.
Other types of CNC machines use a gantry structure where the cutting spindle moves in X and Y directions over a rectangular work area. This approach is used for laser cutting machines, waterjet cutters and routers. In fabrication, bending machines are typically equipped with CNC.
The maintenance needs of all these CNC machines are similar. Dirt must be excluded from slideways, bearings and encoders, and lubricating oil and filters must be periodically replaced. Neglecting these aspects will accelerate wear.
Overheating of either the electrical cabinet or the spindle can cause a variety of problems, including failure of key components. To address this, replace filters regularly to ensure good airflow and ensure lubricating systems are working correctly.
Batteries used as backup power for encoders and other components with memories can fail if not replaced periodically. If this happens the machine can lose its home positions, which will leave it unable to run. In some cases, a service rep visit will be needed.
CNC machine preventive maintenance checklists
Regular planned CNC machine maintenance reduces the risk of accuracy problems and extends life. It’s often easiest to schedule this on a calendar basis. Adopting an hours-run basis can reduce the total amount of maintenance work done, but requires setting up appropriate monitors and alerts. (This is where the time between inspections and checks is related to how much the machine has been used.)
A useful tool for ensuring that inspections and servicing are not overlooked is the CNC preventive maintenance checklist. Specific details will vary by type of machine, but the following is broadly applicable.
Daily maintenance
1. Clean machine surfaces and the workspace.
2. Check and refill lubricant levels.
3. Inspect tool condition and sharpness.
4. Remove chips and debris from the machine. (Brushing is preferable as blowing can push chips under wipers that should keep dirt away from critical surfaces.)
Weekly maintenance
1. Lubricate moving parts and guideways.
2. Check and tighten loose bolts and screws.
3. Inspect coolant levels and top up if necessary.
4. Check belts and pulleys for wear and tension.
5. Inspect wipers for damage and replace as necessary.
Monthly maintenance
1. Inspect electrical connections and wiring. (Thermal imaging is useful for finding hotspots that indicate high resistance, as caused by poor quality connections.)
2. Check alignment and calibration of the machine. (A dial indicator can verify alignment of the axes and check for backlash. A ballbar is useful for checking the accuracy of circular interpolation. More extensive checks require specialist equipment: consider engaging an industrial calibration service.)
3. Replace worn or damaged parts.
4. Perform a detailed inspection of the entire machine to check for damage, wear or other deterioration.
Quarterly maintenance
1. Inspect and clean filters and screens.
2. Check the hydraulic system and top up fluids.
3. Inspect the spindle and chuck for wear. Vibration and surface finish problems often indicator wear in spindle components.
4. Update maintenance records and schedule.
Annual maintenance
1. Perform a thorough inspection and overhaul.
2. Calibrate all measurement systems.
3. Check and replace critical components as needed.
4. Review and update the maintenance schedule.
Benefits of preventive maintenance for CNC machines
Performing CNC machine preventive maintenance requires a significant time commitment. For this reason, it helps to understand the benefits. These are:
- Longer machine life: With less wear the machine will continue performing to the required standard for longer without needing replacement of major components.
- Performance and accuracy maintained: With less mechanical wear, a machine will continue to deliver the accuracy and repeatability needed, preventing quality problems such as geometric errors and poor surface finish.
- Reduced downtime and costs: Preventive maintenance reduces unexpected breakdowns, supporting higher OEE and lower repair costs.
Best practices for CNC preventive maintenance
For anyone looking to implement a program of CNC machine maintenance or optimize an existing program, here are some best practices to follow:
- Develop a maintenance schedule: Start with recommendations from the manufacturer, then refine this based on your experience and suggestions from the CNC machine operator and the maintenance team. (Your requirements will vary based on the amount and type of machining done on your equipment.)
- Adopt predictive maintenance practices: Use data from existing machine sensors to identify trends that allow failure prediction and add sensors and inspections for additional data.
- Use maintenance management software: A computerized maintenance management system (CMMS) simplifies scheduling and planning of inspections and checks, including CNC machine mechanical maintenance. Links to spare parts inventories support this work.
- Train and involve staff: Your CNC machines are significant capital assets. Ensure operators and maintenance technicians understand the preventive maintenance procedures they should follow to look after them.
Complex machines need specialized skills
CNC machine tools are significant revenue and profit generators, but it is important to minimize downtime and prolong their life as much as possible. The tips, checklists and best practices discussed here will help with both.
Given the complexity of CNC machines, it’s often worth partnering with an experienced preventive maintenance services provider. The specialists at ATS are very familiar with all types of CNC machines and can help with both immediate needs and implementing effective preventive maintenance. Contact us to learn more.
