CNC Machine Tools
The special design features of CNC machine tools have resulted in use of higher cutting speeds and feeds, leading to considerable saving in the cycle time. To fully exploit the higher metal removal rates of the CNC machines, the tooling used should be able to withstand the higher cutting forces in the process and help to reduce the down time to a minimum possible.
A CNC machine tool is a computer-controlled machine that performs a sequence of operations on a workpiece. The three most common types of CNC machine tools are turning centers, machining centers, and grinding machines.
The tooling used on CNC machines should be:
(a) Rigid to withstand high metal removal rates
(b) Capable of being pre-set and re-set in the shortest possible time to keep the down time to minimum.
Accurate enough to produce repetitive accuracy on the job. In conventional machines, the cutting tool cuts metal for about 25% of the total machining time whereas the CNC machine tools are expected to cut metal for 70 to 80% of the time. Since CNC machines are very costly, the down time on these machines has to be reduced to a minimum. The tooling for CNC machine tools includes the cutting tools, and tool and work holding device.
Automatic Tool Changers
The CNC machines are intended to perform a number of operations in a single setting of the workpiece. To decrease the down-time in change-over from one operation then to the next, the CNC machines are furnished with automatic tool change facility. The tool is automatically selected and changed based on the tool control function (T-word) in the part program.
Turning centres are available with the tool turret containing 6 to 12 tools. As the tool change command is acknowledged by the control system, the tool turret moves to a fixed tool change position and the required tool comes to the cutting position.
On the machining centres, automatic tool changers (ATC) are provided to reduce the idle time between changes over from one operation to another. The ATC consists of a tool magazine for storing the tools and a tool change unit for transferring the tool from top magazine to spindle. The tool previously fixed on the spindle is removed and replaced in the tool magazine. The initial position of various tools is fed to the control system, which then keeps updating the data regarding tool number fitted in the particular pocket in the tool magazine. Tool magazines with up to 60 tools are quite common in India. The tool change cycle consists of two parts:
(i) Tool Selection Cycle
The tool for successive operation is selected during the previous machining operation. The selected tool comes to the tool change position, whenever the tool selection command is received by the system.
(ii) Tool Transfer Cycle
In this part of the tool change cycle, the tool which is lying selected in the magazine, is transferred to the spindle and the tool which is in the spindle is transferred to the magazine. Before the tool transfer takes place, the spindle is turned off and moves to the tool change position, so that the tools will not hit the workpiece.
To further reduce the non-productive time, the CNC machines are provided with automatic pallet change systems. Twin-pallet CNC machines are very common but machines with up to 5 pallets are also being used. The multi-pallet system enables the operator to load the workpiece on one pallet while machining of workpiece on second pallet is going on. This helps in rapid change of workpiece, thereby reducing the idle time of the machine.