Productiveness Evaluation of a Machine Tool Manual Setup Compared with Automated CNC Machine

The automatic tool change of machine tools affects the productive efficiency in several ways such as starving time reduction, increase/decrease of production rate as well reliability and reduce the process related costs of manufacturing. Based on that background an analysis of experimental scenarios of manual tool change versus automatic tool change was made in order to compare and evaluate its related production rate.


I. INTRODUCTION
Short for "computer numerical control," CNC machining is a manufacturing process in which pre-programmed computer software dictates the movement of factory tools and machinery. The process can be used to control a range of complex machinery, from grinders and lathes to mills and routers. With CNC machining, threedimensional cutting tasks can be accomplished in a single set of prompts. CNC machines have been facing various applications where the automation is required. It can produce simple parts or complex parts through machining center computer integrated. Although the manufacturing processes require flexibility and complexity several attributes are achieved such as flexibility, accuracy, repeatability and consequently quality [1].
The improvements of machine tools in manufacturing processes had been developed to the purpose of increase productiveness without loss of quality and equipment downtime reduction due to human interface in the process. Setup time reduction have been used by manufacturers as a solution to increase productivity and reduce the related transformation costs. Nevertheless the setup time reduction is relevant due to three simple principles: (1) faster technologies of tool change reduce the probability of human error; (2) how lower is the setup time the production behavior becomes more dynamic; and (3) increase of machining saturation of the equipment [2]. Automatic tool change (ATC) integrated to the machining centers enables the reduction of non-productive time and allows tools availability to complex parts machining [3]. The purpose of this article is to evaluate the production rate considering the usage of the ATC concept by the comparison with manual tool change.

II. THEORETICAL FRAMEWORK
Tool change time represents the total time to perform the operations independently if manual, semi-automatic or automatic. Some examples of operations are the approach, adjust, corrections and offsets. Therefore the manual tool change time calculation is made by the sum of time operation tasks that do not generate chip [4]. Automatic tool change is defined as the minimum interval required Based on the machining cycle time is possible to establish the productiveness ratio σ in parts per hour under the perspective of tool change.
The combination of equations (9) and (10) generates to the following equation.

III. METHODOLOGY
The first step of methodology was simulate the tool change through a programmable device an then apply the theory mentioned before and its impact in the manufacturing productiveness. In order to simulate the application of CNC machine to the quick tool change automation the LEGO NXT 9797 kit and an educational programmable robot were used the reproduce the environment studied. To compare the tool change technology it was used a tool plan of a CNC tool with a tool change and machining time settle-up to drilling of Ø8 mm with drill change to Ø10 mm both in single step. The following bill of materials were used during the experiment:  01 commercial aluminum hub with 100 mm edge;  02 HSS steel drill bits Ø10 mm and Ø8 mm;  01 drilling machine with 60 mm maximum drilling depth; Parameters:  03 different operators;  02 holes and one drill change per operator;  03 measurements per operator, totaling 18 holes and 09 drill changes;  Pre-drill hole with Ø8 mm exchange for Ø10 mm drill;  Rotation at 1100 rpm;  02 chronometer.

Tab. 4. Machining parameters of FAMAR CNC machine
The data collected applied in the equation (12) established the productiveness ratio between the manual tool change with automatic tool change. These data are plotted in the following figure 1.

Fig. 1. Ratio between parts per hour (σ) with setup time ( )
V. CONCLUSION The ratio σ express the productiveness in terms of parts per hour under the manual setup perspective. It connects the manual setup with automatic setup in a way to extract the ratio between both environments there is a direct and proportional ratio between manual setup times with the need for automation of setup is verified by the results presented. By the curve behavior interpretation the increase of difference between manual and automatic setup time results in decrease of hourly productiveness rate. Thus based on the math presented in this paper it is possible to estimate productiveness margin for both systems.