19-11-2012, 05:03 PM
ASSIGNMENT 1 Report on a CNC machining centre and its application
CNC machining.pdf (Size: 1.67 MB / Downloads: 108)
INTRODUCTION TO CNC MACHINES
Controlling a machine tool by means of prepared program, which consists of blocks, or series of commands/numbers, is known as numerical control. Numerical Control [NC] for machine tools was introduced in 1950 by Prof. John T Parsons. The first CNC machine was built at the Massachusetts institute of technology [MIT] in 1953 by joint efforts of US Air force, MIT and the Parsons Corporation.
The CNC machine basically entails three main regions:-
• The control system
• The drives (driving elements) and
• The machine tool
Three axis CNC machine:
1. In 3-axis CNC vertical machine, the working table moves along x- and y-axis, and the tool along zaxis.
2. In machining, tool orientation is fixed, either in vertical or horizontal direction.
3. If all surfaces to be machined are accessible by the cutter in one setup, a 3-axis CNC machine is
used
Arrow Series 2 vertical machining centres
The above mentioned have been fitted with an array of productivity options by manufacturer Cincinnati Machine, high-speed spindle is available on Arrows equipped with the Siemens Acramatic control. The motor-driven spindle with HSK 63 tooling provides excellent balance and vibration-free performance. Generous torque capacity ensures high performance in semi finishing hardened steels and machining aerospace materials that involve high metal-removal rates.
Specifications: Arrow 2 VMC has User-friendly interface design; customized GE Fanuc 18i-M CNC control. This control employs the latest Pentium technology to support future upgrades and enhanced with graphical simulation.
The x-y-z travel of Arrow Series 2 centres provides a work envelope as large as 3,048 x 762 x 770 mm for work pieces weighing up to 3,000 kg. Linear-scale feedback standard on the x- and y-axes maintains positioning accuracy and repeatability of ±3 μm and ±1 μm, respectively, across the entire travel range. Below is shown look of machine. [19]
Repeatability:
Definition 1: It is irreproducible errors in bearing or leads screws and is the limit of accuracy attainable in a particular machine.
While calculating the accuracy component will be considered correct if its dimensions lie anywhere within tolerance band. If a certain slide position is commanded many times in succession, there will be a drift, or scatter in the positions actually taken up by slides .This scatter is a measure of repeatability of system.
Definition 2: Repeatability is the ability of a motion system to reliably achieve a commanded position over many attempts. Manufacturers often specify unidirectional repeatability. This is the ability to repeat a motion increment in one direction only. This specification side-step issue of backlash and hysteresis, and therefore is less meaningful for many real world applications where reversal of motion direction is common.
Spindle:
Rotating spindle: all work or tool carrying spindles rotating aped are subjected to deflection and thrust forces depending on nature of work being performed. To increase stability and minimize torsional strain on the spindles they are designed to be short and stiff as possible, and the final drive to the spindle is located as near to front bearing as possible. It is basically categorized into two types they are Brush type and Brushless spindles or AC spindles, Rotational movements are controlled by the spindles by circular bearings. [5]