Experimental investigation of safety coefficients used in the calculation of clamping forces on a fixture

The aim of fixtures is to fix the workpiece to the machine tools and provide secure clamping at the suitable position during operation. A safety coefficient (K) is used in theoretical calculations for secure clamping. Incorrect selection of K leads to unsafe clamping of the workpiece and inaccurate selection of fixture parts. The exact value of K should be used in manufacturing processes for more secure clamping and proper selection of fixture parts. In this study, design and manufacture of a three dimensional dynamometer were performed to determine exact value of K experimentally. Three strain gages with 20 kN, three insulated strain gage input module, a terminal board, a data acquisition card for transferring signals to the computer and a software to record and evaluate the data were used in the system. Experiments were carried out by clamping of samples at one point in three planes with 45º and limiting all degree of freedoms. vii Machining tests were carried out on a vertical CNC machining centre at five cutting speeds, three feed rates and three depths of cuts. Coolant was not used during the tests. The workpiece material was AISI 1040 steel. At 0,08-0,1 and 0,15 mm/tooth feed rates and 0,5 mm depth of cut, the highest value of K was found to be 1,1245. When the depth of cut was increased to 1 and 2 mm, the highest values of K were found to be 1,2259 and 1,3339, respectively. In addition, a model was developed through multiple regression analysis of the obtained results. Coefficient of determination of the developed model was found to be R2=0,91. Key Words : Fixtures, clamping force, clamping safety coefficient, dynamometer, force measurement