Computerized Numerical Control Machines

CNC Machines are Computerized Numerical Control Machines. They are available in single or multi-head and multi-axis control. They are used for large scale complex and intricate machining jobs. They are versatile enough to do several machining operations of complex nature, such as turning, milling, drilling, boring, thread-cutting, complex profile cutting, etc. by using special CNC program.

One CNC machine with an operator or without operator has the capacity not only to perform the work of about ten conventional machines but also it can turn out most accurate machining operations with amazing speed. Lot of time is saved in the form of set-up timings, taking measurements of the job in progress, loading the jobs, changing tools, etc which are inevitable while working with conventional machines.

In spite of so many advantages, the machine suffers from the disadvantage of its high cost and expensive installation. But the disadvantages pale into insignificance, when compared with its advantages and when there is sufficient work load that warrants its acquisition. The high production level and less number of operatives offset the disadvantages and convert the possession of the CNC machine to a profitable proposition.

The production targets can be met easily by installing a CNC machine and thereby you can attract more customers, besides giving them satisfaction in sticking to the delivery schedules. Besides, the quality of work generated by the CNC machine enhances customer satisfaction, as can be gathered from their feedback and positive response in the form of increased orders.

Now, considering the high cost of the machine, you cannot afford to keep it idling for prolonged period. The break-down period should be kept to minimum. The productive hours from the machine should be kept up as much as possible to recoup the cost of the asset and generate profits. This requires a sound maintenance strategy.

Before the procurement of the machine is initiated, technicians or artisan staff and engineers should be earmarked for training in the operation and maintenance of the CNC machines being procured. So, this aspect should be included in the draft contract for the procurement of CNC machines.

Then, proper place should be earmarked in the factory for installation of CNC machines. The following precautions should be observed in this regard:

1. There should be a separate glass enclosure with air-conditioning facility for the CNC machines or pressurized chamber.
2. There should be separate electrical grounding for each CNC machine.
3. It is preferable not to have welding sets operating close by. The return currents from the welding sets could be harmful for the working of the CNC machines. If the operation of a welding set in their vicinity becomes unavoidable, then high conductivity cable should be used for carrying the return current.
4. There should not be high power forging machines in the proximity, as the impact of vibrations will have adverse impact on the working of the CNC machines.
5. There should not be high voltage fluctuations in the power supply system. Each CNC machine should be equipped with an Isolation Transformer and high-speed voltage stabilizer to take care of voltage fluctuations, electrical noise and electrical surges.
6. There should be a separate feeder for the power supply of CNC machines. If the budget permits, UPS may also be installed.
7. There should be a well equipped CNC laboratory (if one does not already exist) which can efficiently cater to the Mechanical and Electrical maintenance needs. The laboratory should be adequately equipped with digital oscilloscopes, IC testers, logic probes, programming kits, hydraulic trainer kit, micro-processor training kit, pressure gauge testing and calibration equipment, temperature-cum humidity checking meters and well equipped library with modern racks consisting of indexing facility so as to keep the CNC manuals, drawings and CNC literature.
8. The CNC laboratory should be air-conditioned and provided with protective gear at each work table to inhibit electrostatic charges.

The maintenance of CNC machines requires special culture. Each machine should be provided with a history card, which should contain the following information:

1. The name of the CNC machine, the date of its installation, the address and the

Telephone numbers of the manufacturer. The details of website of the manufacturer and the contact details of the agent may also be recorded.

1. Dynamic data like the hours of idling of the machine (with date and time) and

the causes thereof, the nature of breakdown and action taken to set right the

the defect, the components replaced etc.

1. Circuit modifications made from time to time as per requirement, or

modifications made to the system or circuitry as per requirements with date.

The history sheets of CNC machines should be studied and analyzed at regular intervals for taking prompt action.

The CNC machines should be kept absolutely clean and free from the metallic dust and scrap generated by following an efficient system of daily cleaning and disposal. The metallic dust and scrap are most harmful for the working of these machines. They can gather on the PCBs and cause damage to the machine due to short circuit. The metallic dust and scrap pose potential threat to the working of limit switches, proximity switches and micro-switches.

The grounding of the machine should be checked regularly. The ground wires may get snapped. So their continuity should be checked in the periodical maintenance checks. The functioning of the voltage stabilizer, refrigerating system, isolating transformer, etc should be checked periodically. When the refrigerant is to be replaced, it should be done by the specified type of refrigerant as indicated on the name-plate of the equipment. The hydraulic oil, which is required to be replenished, should be of specified grade and type. Otherwise, the system will not function properly. All these aspects should be covered in the maintenance check lists.

Protective devices in the control panel should be checked regularly. In the event of failure of motors, the causes of failure should be investigated. Periodical checking of the bearings, the lubrication system, limit switches and protective relays will avert the incidence of many of the failures. In the event of failure of axis motors, the functioning of the overload relay and the single-phase preventer should be checked first before restoring the repaired motor. Otherwise, there is imminent danger of the repaired motor failing immediately after starting or after a few minutes of operation.

Though PCB failures of a good CNC system are a rare occurrence, the replacement of PCBs in the event of their failure is a very expensive proposition. The cost of the PCBs is exorbitantly high. Hence, card level repairing is indispensable for the maintenance team. For this purpose, the CNC laboratory must be well equipped with requisite diagnostic tools, like the Storage Oscilloscope, the micro-processor based IC tester which contains adequate library of the ICs and microprocessors, logic probes and PCB repairing kits. Staff should be well qualified and trained for carrying out repairs at card level. The manufacturers of the CNC system offer training facilities, which should be availed. In addition, there are also good training institutions where tailor made courses are conducted. Besides, in-field training should also be imparted to the staff by instructors and veteran staff.

The importance of maintaining inventory of essential spares needs to be emphasized here. Generally, some important spares are supplied with the machine, which should be replenished as per need. Most of the CNC systems become obsolete in a very short time, due to strident advancements of technology. As such, the lead time in procuring the spares is very high. CNC machines cannot be allowed to be kept idle for long periods for want of a spare component. A lot of planning based on experience goes into the maintaining of stores of spares. Too many spares add to the overhead costs of the machine maintenance, while inadequate number of spares adversely affects the productive hours of the machine. Striking the correct balance is definitely a skillful job.

If the same type of CNC system is chosen for a majority of the fleet of CNC machines in a factory, the saving in the cost of maintaining the spares gets drastically reduced. The idle time of the CNC machines will also become negligible, due to efficient spares maintenance. Such a situation will permit keeping of some spare PCBs, linear scales, encoders, hydraulic valves, etc in hand. Procurement of such expensive spares will not be economical if the fleet of CNC machines contains various types of CNC control systems.

A CNC machine is a good asset, but it becomes a great burden if it cannot be maintained in working condition or if it cannot be fed with enough loads. It, therefore, warrants thorough ground work and infrastructure before welcoming one such machine into your factory premises.