119 CHAPTER 6 EXPERIMENTAL VALIDATION AND RESULTS AND DISCUSSIONS 6.1 CNC INTRODUCTION The CNC systems were first commercially introduced around 1970, and they applied the soft-wired controller approach to good advantage. One standard computer control could be adapted to various machine tools by programming control functions into the computer memory for particular machine. Being less expensive, the machine tool builders design the CNC control panel as an integral part of the machine tool rather than as a separate stand-alone cabinet. This reduces the floor space requirements for the machine. The very large scale integrated (VLSI) circuits used in these controllers are advantageous to the machine tool designer and to the machine user. 6.1.1 CNC machine Production equipment with computer numerical control is a major component of CAD/CAM technology. For flexible automation on the shopfloor, CNC machines plays major role. This technology is applied for large scale in industries of material processing equipment. For manufacturing a component, the CAD/CAM process generates a numerical control program which can run on the CNC machine. The integration of CNC machines in computer integrated machines (CIM) technology is the today s concept of many industries.
120 6.1.2 CNC turning centre These are the other types of popular CNC machines and are generally classified as: Vertical machine Horizontal machine Classification of Horizontal machine Horizontal machines are classified into: Chucking machines Shaft machines Universal machines Some of the latest CNC lathes are twin spindle machines on which the first and second operations can be performed on the two spindles in succession. The component is removed from the first spindle in first operation and loaded into the second spindle for the second operation automatically. A typical configuration of a turning centre is shown in Figure 6.1. Figure 6.1 Typical configuration of a turning centre
121 Chucking machines: Chucking machines usually have shorter beds and a single saddle with single drum type turret which accommodates tools or two independents saddles with turret. Many of the present day chucks offer optional swing-in-tailstock to facilitate shaft work. During normal working as a chucker, the tailstock is swung away from the work area. Shaft lathes: Shaft lathes are intended mainly used between centre works. They have hydraulic of pneumatic tailstock and roller steadies for supporting the work piece. Tooling is mainly for external working. Universal lathes: Universal lathes are suitable for both chucking and for bar work. Four axis machines have two turrets each mounted on an independent slide facilitates simultaneous machining with two tools. 6.1.3 Work holding system To locate and hold the work piece, in CNC machine tool, work holding system is used. The work holding devices used in CNC machines are: Fixtures Hydraulic chucks Collect attachment Bar holding system Steadies Mostly fixtures are used as a work holding device in CNC machines. The following are the features for CNC machine tools: Fixture length compensation. Quick loading and unloading of fixtures on pallets. Better accuracy of the fixture Rigidity to withstand cutting force.
122 6.1.4 Work holding devices In order to keep setup time to minimum, the work holding devices should be accurate, easy and quick to operate, ensure rigidity against heavy cuts. Commonly used work holding devices are Collet chucks Jaw chucks Arbors Fixtures Collet chucks: Collet chucks are widely used to clamp barstock in machining on lathes. Their main feature is a collet which is a steel sleeve with spline tapered portion forming jaw. Jaw chucks: Jaw chucks are used to hold irregular shaped individual work piece. Various jaws employed for work holding are: Three jaw self-centering chuck Four jaw independent chuck Two jaw box chuck Power chucks Arbors: Arbors may be some times used for holding small pieces having accurately drilling holes. Threaded type and expanding type arbors are in use. Fixtures: Special fixtures may be employed to hold jobs which cannot otherwise be held in collets, jaw and arbors.
123 6.1.5 Tooling system Tooling system for NC is designed to eliminate operator error and maximize productive machine hour. It can be done in one or more of the following ways: Using quick-change tool holders Automatic tool selection Changing tool automatically for sequence operations Permitting accurate presetting of tools outside the machine. Facilitating tool selection and tool changing through the numerical control program. While tooling for NC machine might appear to be specialized, the actual components and principles involved have much in common with what could be considered proper practice for conventional machine tools. Numerical control merely hastens the general acceptance of more advanced tooling concepts. 6.1.6 CNC controller CNC system also contains other features associated with the computer of the CNC controller. These include a keyboard to aid in tape editing and a cathode-ray tube (CRT) screen on which messages are displayed for the operator. The availability of MCU feature is important since they contribute to the satisfactory performance of the entire CNC system.
124 6.1.7 Part Program The part program is a set of instructions proposed to get the machined part staring with the desired blank and the NC machine tool. Each line of instruction is capable of specifying dimensional and non-dimensional data and is written in a specific format. This format is known as NC block. Figure 6.2 shows the part program procedure. Part programming Manual programming Computer aided programming Tape of Disk Tape reader Microcomputer Memory CNC Machine tool Figure 6.2 Layout of part program procedure This work is carried out by a programmer. He prepares the planning sheet and writes the instructions in a coded form which is acceptable to the controller of machine tool is represented by Hitomi (1979). Methods of creating part programming: The following are various methods of creating part programming Manual part programming
125 Computer-assisted part programming (CAD/CAM based programming system) Manual data input Computer automated part programming 6.1.8 Programming procedure Basically, we can write program in two ways. They are Absolute programming Incremental programming The computer will however be informed to interpret properly The G codes for the above two types are G90 - absolute programming G91 - incremental programming For example in absolute programming, the distance at my point at any instant will be measured from the origin (X=0, Y=0). Whereas in incremental programming, the instant point will be noted as (X=0, Y=0). Further measurement will be made from the particular point only. The following steps should be kept in mind while writing the program Fixation of coordinate system Reference of G and M codes Dimensions of work and tools. Locating the fixture and machine table Speed and feed according to the work and tool material
126 6.1.9 Part Program Format and Symbols The part program format is N4/G2/X43/Y43/F03/S200/M03. From the above format, we should understand the following N indicate the block number which has the number 1 to 9999 G denotes the preparatory functions having two digits 00 to 99 X and Y co-ordinates may have upto seven digits each like 1234567. F indicates the feed given. S indicates the speed of work (or) spindle M denotes miscellaneous functions Enter or End of the block (for each line it should be given) 6.2 MACHINE SPECIFICATION Technical Specifications Table 6.1 Technical specification of CNC machine Sl no Specifications Unit 16 TC 1 Swing over Bed mm 400 2 Turning Dia mm 225 3 Turning Length mm 300 4 Spindle Speed rpm 50-3500 5 Spindle Motor kw 7.5 6 Z axis Stroke mm 325 7 X axis Stroke mm 125 8 Tools in Turret numbers 8 9 Rapid Traverse m / min 20
127 6.3 SPECIFICATION FOR OUR EXAMPLE PROBLEM Table 6.2 Specification to example problem Parameter Signs and units Values Length of the workpiece L (mm) 100 Diameter of the workpiece D i, (mm) 40 Nose radius of the cutter r e (mm) 1.2 Direct labor cost plus overhead k o (Rs./min) 0.5 Cutting edge cost lc, (Rs./cutting edge) 1.0 Tool change time t e (min/cutting edge) 0.5 Preparation time t p (min/piece) 0.25 Tool advance/return time h 2 (min) 0.3 Maximum cutting speed V max (rpm) 3500 Minimum cutting speed V min (rpm) 50 Maximum feed rate f n. (mm/rev) 0.4 Minimum feed rate f n. (mm/rev) 0.01 Maximum depth of cut for finishing d s (mm) 1.5 Minimum depth of cut for finishing d s min.. (mm) 0.3 Maximum depth of cut for roughing d r.. (mm) 1.5 Minimum depth of cut for roughing d r, (mm) 0.3 Tool replacement time T (min) 0.5 Maximum cutting force F n. (kgf) 1960 Maximum cutting power P m. (kw) 7.5 Machine tool efficiency 0.85 Constants and exponents in cutting force and power equations k, =1058, = 0.75, v = 0.95
128 6.4 NC CODE N1 G28U0W0 G0T0101 G97S1500M3 G0X50.0Z20.0M07 Z0.7 G1X-1.0F0.15 G0W0.5X50. Z0.2 G1X-1.0F0.15 G0X47.0W1.0 G1Z-50.3F.18 X48.0 G0Z1.0 X45.0 G1Z-50.3 U0.5 G0Z1.0 X43.0 G1Z-50.3 U0.5
129 G0Z1.0 X41.0 G1Z-50.3 U0.5 G0Z1.0 X39.0 G1Z-50.3 U5.0 G0Z1.0 X37.0 G1Z-14.55 U0.5 G0Z1.0 X35.0 G1Z-14.55 U0.5 G0Z1.0 X30.0 G1Z0.2 X33.0Z-1.3 G1Z-14.55 X37.0 X38.5W-0.75 Z-50.3
130 X25.5 X26.5F.1 G03X19.5Z-47.172R6.F.08 G1Z-41.123F1. 6.5 TEST EXAMPLE Figure 6.3 Test component Material - EN8 Tool tip - CNMG 6.6 EXPERIMENTAL VALIDATION In the industry where this work is carried out, the current practice for rough and finish turning operation is given. For rough turning, the cutting speed, feed rate and depth of cut are taken as 1500rpm, 0.2 mm/rev, 0.752 mm respectively. For these operating parameters, the time for machining is 2.875 min and the tool wear measured is 0.703µ.
131 For finish turning, the cutting speed, feed rate and depth of cut are taken as 2000 rpm, 0.13 mm/rev, 0.2 mm respectively. For these operating parameters, the time for machining is 0.646 min and the tool wear measured is 0.079 µ. So the total time taken to complete the job is 3.521 min and tool wear measured is 0.782µ. 6.6.1 Experimental validation for GA - OT Table 6.3 Experimental validation for GA - OT Operation Cutting Speed (rpm) Current Practice Feed rate (mm/rev) Depth of cut (mm) Machining time (min) Proposed by Genetic Algorithm (GA) Cutting Speed (rpm) Feed rate (mm/rev) Depth of cut (mm) Machining time (min) Time saved (min) Rough Turning Finishing Turning 1500 0.2 0.752 2.875 1254.762 0.375 0.51 2.725 2000 0.13 0.2 0.646 1254.762 0.375 0.51 0.522 0.274 Total Machining Time 3.521 3.247 With the implementation of Genetic Algorithm (GA) (Chapter 4), the optimum cutting parameters resulted is given in Table 4.14. Using the optimal cutting parameter for the test component, the actual time taken to complete the job is 3.247 min. The results of current practice mentioned and that of GA are compared in the Table 6.3. From this, the minimum time is obtained using GA and the time saved is 0.274 min/piece.
132 6.6.2 Experimental validation for GA TW Table 6.4 Experimental validation for GA - TW Operation Rough Turning Finishing Turning Cutting Speed (rpm) Current Practice Feed rate (mm/rev) Depth of cut (mm) Tool wear (µ) Proposed by Genetic Algorithm (GA) Cutting Speed (rpm) Feed rate (mm/rev) Depth of cut (mm) Tool wear (µ) 1500 0.2 0.752 0.703 1254.762 0.375 0.51 0.612 2000 0.13 0.2 0.079 1254.762 0.375 0.51 0.059 Total tool wear 0.782 0.671 Tool wear (µ) reduced 0.111 With the implementation of Genetic Algorithm (GA) (Chapter 4), the optimum cutting parameters resulted is given in Table 4.14. The results of current practice mentioned and that of GA are compared in the Table 6.4. From this, the minimum tool wear measured in GA is 0.671 µ and the tool wear reduced to 0.111 µ. 6.6.3 Experimental validation for PSO OT Table 6.5 Experimental validation for PSO - OT Operation Cutting Speed (rpm) Current Practice Feed rate (mm/rev) Depth of cut (mm) Machining time (min) Proposed by Particle Swarm Optimization (PSO) Feed Depth Machining rate of cut time (min) (mm/rev) (mm) Cutting Speed (rpm) Rough Turning 1500 0.2 0.752 2.875 2788 0.233 0.776 2.613 Finishing Turning 2000 0.13 0.2 0.646 2788 0.233 0.776 0.511 Total machining time 3.521 3.124 Time saved (min) 0.397 With the implementation of Particle Swarm Optimization (PSO) (Chapter 5), the optimum cutting parameters resulted is given in Table 5.14. Using the optimal cutting parameter for the test component, the actual time taken to complete the job is 3.124 min.
133 The results of current practice mentioned and that of PSO are compared in the Table 6.5. From this, the minimum time is obtained using PSO and the time saved is 0.397 min/piece. 6.6.4 Experimental validation for PSO TW Table 6.6 Experimental validation for PSO - TW Operation Rough Turning Finishing Turning Cutting Speed (rpm) Current Practice Feed Depth of rate cut (mm) (mm/rev) Tool wear (µ) Proposed by Particle Swarm Optimization (PSO) Cutting Speed (rpm) Feed rate (mm/rev) Depth of cut (mm) Tool Wear (µ) 1500 0.2 0.752 0.703 2788 0.233 0.776 0.605 2000 0.13 0.2 0.079 2788 0.233 0.776 0.088 Total tool wear 0.782 0.693 Tool Wear (µ) reduced 0.089 With the implementation of Particle Swarm Optimization (PSO) (Chapter 5), the optimum cutting parameters resulted is given in Table 5.14. The results of current practice mentioned and that of PSO are compared in the Table 6.6. From this, the minimum tool wear measured in PSO is 0.693 µ and the tool wear reduced to 0.089 µ. 6.7 RESULTS AND DISCUSSIONS In a CNC turning centre, optimization procedure using Genetic Algorithm and Particle Swarm Optimization is developed for turning, facing and undercutting processes. Implementation of GA and PSO algorithms are carried out using C ++ language. The optimum values of decision variables such as cutting speed and feed for minimum time are found out for rough turning and finish turning.
134 Using CNC machine the operation is carried out for the test component in current practice. In actual practice the cutting process are carried out by NC code. The cutting speed, feed rate and depth of cut are noted and tabulated in Table 6.3. The actual machining time is to complete the job is 3.521 min. The number of roughing passes is 6 and one finish pass to complete the job. The tool wear is measured to finish the job is 0.782 µ and tabulated in Table 6.4. In actual practice the cost is calculated Rs. 180/hr. Cost per each component is Rs. 10.563. The Genetic Algorithm (GA) concept is applied with optimal cutting parameter for the test component, the actual time taken to complete the job is 3.247 min and tabulated in Table 6.3. The tool wear is measured to finish the job is 0.671µ and tabulated in Table 6.4. The cost per each component is calculated as Rs. 9.741. PSO algorithm is applied with optimal cutting parameter for the test component, the actual time taken to complete the job is 3.124 min. The comparative statement between the current practice and the PSO concept is mentioned in the Table 6.5. The tool wear is measured to finish the job is 0.693µ and tabulated in Table 6.6. The cost per component is calculated as Rs. 9.372.