Constructional Details of CNC Machines

The basic design of a conventional machine tool is not suitable for CNC machines. Many design changes are required for ... thumbnail 1 summary
The basic design of a conventional machine tool is not suitable for CNC machines. Many design changes are required for CNC machines as compared to their conventional counterparts, due to a number of additional requirements which CNC machines are expected to meet. The manual hand wheel controls in the conventional machines are replaced by axis drive motors in CNC machines. If the axis drive motors have to operate against heavy loads due to friction at the sliding surfaces or due to inertia of moving components due to some other factors, the motors will have to develop high power output which, in turn will ask for large size motors. Higher cutting speeds and feed and improved tooling used in CNC machines subject the machine tool to high multidirectional forces. Also the set- up time and change- over time between the jobs are considerably reduced in CNC machines and most of the time is spent in actually cutting the material higher percentage of cutting time will result in faster wear of slide ways, guide ways, lead screw and gears, etc. In addition safety of the operator working on the machining is very important. In order to take care of above and many other factors, there is a need for special consideration to be given to the design of CNC machine tools in the following areas:

(a) Machine Structure. The design and the construction of CNC machines should be such that it meets the following main objectives:

(i) High Precision and Repeatability. Accuracy or precision is the ability of a machine to produce desired dimensions. Repeatability is the ability to produce the same part for the same dimensions every time. Accuracy of a machine depends mainly on its control resolution which is the minimum distance between two points which the machine can differentiate. Repeatability is affected by mechanical errors such as backlash in the machine. The ability of Machine Control Unit (MCU) to differentiate between closely spaced points is a function of factors such as the controller's bit storage capacity, drive motor and the type of feedback sensor. Illustration of High Precision and Repeatability figure below illustrates accuracy and repeatability as a function of control resolution and mechanical errors.
illustration of high precision and repeatability
(ii) Reliability. It depends on the speed and load during operation as well as on the quality of the machine structure and control system.

(iii) Efficiency. The machine structure should be such that it should support all the machining operation stresses and strains with the capability of performing all type of operations in specified time.

(b) Slide ways. The design of slide ways in CNC machine tools should be:

(i) Reduced friction
(ii) Reduced Wear
(iii) Satisfy the requirements of movement of the slides
(iv) Improve smoothness of the drive
Note. To meet these requirements in CNC machine tools slide ways, the techniques used include hydrostatic slide ways linear bearings with balls, rollers or needles and surface coatings.

(c) Spindle Mounting. In CNC machines, large variation in cutting speed is required. The cutting speed may vary from 10 metres per minute to 1000 meters per minute or more. The cutting speeds are provided by rotation of the main spindle with the help of an electrical motor. The infinitely variable speed system used in CNC machines are employed with either electric motors (A.C. or D.C.) or fluid motors.
spindle mounting

(d) Drive Units. Drive motors are required to drive the main spindle (Spindle drive) and to drive the saddles or carriage (Axis drive). In addition there may be some motors in CNC machine for services such as coolant pumps, swarf removal, etc.

(e) Elements of motion transmission and positional slides. The conventional machines use Lead screw for motion transmission purposes. The Lead screw with acme threads is not suitable for CNC machines due to higher friction between lead screw and the nut and poor power transmission efficiency  and  inaccuracy  due  to  backlash.  These  problems  have  been overcome with the use of recirculating ball screw and nut arrangement. The advantage of using ball screw and nut assembly is:

(i) High efficiency
(ii) Reversibility
(iii) Wear and life
(iv) No stick slip

(f) Location of transducers. In CNC machines the control of all machine function is totally transferred to a computerized control system. The control unit should be situated so that it is convenient for the operator to operate the machine from the central place. The facilities which a control unit should offer are:

(i) Indicate the current status and position of various machine tool features and give feed back
(ii) Allow manual or semi-manual control machine tool elements.
(iii) Enable machine tool to be programmed.

(g) Swarf removal. CNC machines are designed to work at optimum cutting condition with the improved cutting tools on a continuous operation basis. Since the cutting time is much more in CNC machines, the volume of swarf generated is also more.  Unless  the swarf  is  quickly and  efficiently removed from the cutting zone,  it  can  affect  the  cutting process  and  the quality  of  the  finished  product.  Also the swarf cannot be allowed to accumulate at the machine tool because it may hamper the access to the machine tool. In addition some auxiliary functions like automatic component loading or automatic tool change may also be affected by accumulation of swarf. To avoid these problems, an efficient swarf control system should be provided with the CNC machine tools with some mechanism to remove the swarf from the cutter and cutting zone and for the disposal of swarf from the machine tool area itself.
linear conveyer for swarf removal
(h) Safety of operator: Safety of operator is very important aspect which cannot be overlooked. To ensure safe working conditions the CNC machine tools are provided with metallic or plastic guards. Where it is not possible to provide effective guards, proximity protection is provided by perimeter guards, pressure mats or light barriers.

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