5. Machine Tool Dynamics _ Economics.pptx

this is a machine tool essential. to fully understand machine tool dynamics and their movement along.

1. Machine Tool Dynamics, High-Speed Machining and Economics of Machine Tools 1 Mo n da y , F e b r u a r y 6 , 2023

2. 2 During cutting, high forces are generated and the energy is dissipated through plastic deformation and friction, leading to deflections of the structural components of the system and to vibrations (relative motion between the tool and workpiece). These vibrations should be minimized because they degrade machining accuracy and the machined surface texture. Machining Dynamics Machining Dynamics is the science of vibration and frequencies created at the tool point during machining. Monday, February 6, 2023

3. Machining Dynamics Machining Dynamics includes the following Tool Vibrations Cutting Forces Natural Frequencies The vibration causes rapid wear of machine parts and unwanted vibrations may cause loosening of parts from the machine. Excessive vibration may cause operational inefficiency , product- quality problems , and increase the cost of manufacturing . 3 Mostly in machine tools during the cutting process, time-dependent (dynamic) forces also come into play and produce fairly rapidly varying deflections in the machine-work-tool system. To achieve higher accuracy and productivity vibration in machine tool must be controlled. For analysis of dynamic behaviour of machine tool rigidity and stability are two important characteristics Monday, February 6, 2023

4. 4 Monday, February 6, 2023 Effect of Vibration Effect of vibrations on machine tool: The machine tool is made of various parts and when vibrations are produced, they also start vibrating at same frequency. If this frequency approaches the natural frequency of vibration of that part then amplitude of vibrations will be very excessive and the part may break even. Effects of vibrations on work-piece: Due to presence of vibrations the surface finish obtained will be very poor. Due to vibrations, the dimensional accuracy of the job is also affected. Effect of vibrations on tool-life: As the tool-life is a function of the cutting variables only, the tool-life is greatly affected by presence of vibrations in machine tools. It is found out that the tool life is decreased by about 70—80% of the normal value if vibrations are present. Effect of vibrations on cutting conditions: By presence of vibrations in machine tools, the chip thickness as removed by the cutting tool does not remain constant. Vibration also results chatter in tool, variable penetration rate and cutting speed.

5. 5 Parameters Affecting Machining Vibration Monday, February 6, 2023

6. 6 Monday, February 6, 2023 Sources of Vibration in Machine Tool Inhomogeneous work piece material Hard spots or a crust in work piece leads to free vibration . Discontinuous chip removal results in fluctuation of the cutting thrust. The breaking away of a built-up edge from the tool face also imparts impulses to the cutting tool. Variation in chip cross section Variation in the cross-sectional area of chip is due to the shape of the machined surface or due to the configuration of tool. Pulses imparted to the tool and job. Pulses have shallow fronts for turning of eccentric parts and steep fronts for slotted parts and for milling/broaching. Bouncing of the cutting tool on machined surface can be minimized by closing the recess with a plug or with filler .

7. 7 Monday, February 6, 2023 Sources of Vibration in Machine Tool Disturbances in the work piece and tool drives Forced vibration induced by rotation of unbalanced member affects the surface finish & tool life. This can be eliminated by careful balancing or by self-centering. Rotating components should be placed in position where effect of unbalance is less. Electric motors produce both rectilinear and torsional vibration. R ec tili near v i bra ti ons are due t o a non-un i form a i r gap be t ween t he st a t or & ro t o r , asymmetry of windings, unbalance, bearing irregularities, misalignment with driven shaft. Torsional vibration is due to various electrical irregularities. Vibration transmitted from the environment Shock and vibration generated in presses, machine tools, internal-combustion engines, compressors etc., are transmitted through the foundation to other machines, which they may set into forced vibration. Vibration transmitted through the floor may be reduced by vibration isolation.

8. 8 Vibration control in machine tool The vibration behaviour of a machine tool can be improved by a reduction of the Intensity of the sources of vibration by Enhancement of the effective static stiffness and damping. Appropriate choice of cutting regimes, tool design and work-piece design. Both parameters are critical for accuracy of machine tools, stiffness by reducing structural deformations from the cutting forces and damping by accelerating the decay of transient vibrations. Changing the cutting conditions e.g. by decreasing the feed rate and depth of cut and increasing the rake angle. Changing the tool geometry and the method of clamping the work piece. By changing the characteristics of the vibratory system MR fluid-controlled boring bar with chatter suppression Monday, February 6, 2023 Damped vibration absorber on a horizontal milling machine Damping of turning tool

9. 9 Types of Machine Tool Vibrations Free Vibration Free (Random or transient) vibration is caused by shock (or impulsive) loading of the machine tool, e.g. when the tool strikes a hard grain in the work-piece, suddenly enters into the work-piece or when a certain impulse gets transmitted through the machine foundation. In such cases, a relatively large force acts for a relatively small time on the elastic structure of the machine tool. As a consequence, it starts vibrating. Related to the natural characteristic vibration of the system. Needs an initial impact, but amplitude decays with time. Monday, February 6, 2023

10. 10 Types of Machine Tool Vibrations Forced Vibration During the operation of the machine tool, periodic forces are brought into play by such forcing factors as unbalanced rotating members, bearing imperfections, mountings defects or misalignments in machine members. Also external vibration may be transmitted to the machine tool via foundation. In all such cases, the machine itself starts vibrating at the frequency of the forcing factors. Dependent on an external exciting force (i.e. an unbalanced shaft). Frequency and amplitude of motion are dependent on characteristics of the external source. Monday, February 6, 2023

11. 11 Types of Machine Tool Vibrations Self-Induced Vibration Such vibration is caused, built up and sustained by the interaction or coupling inherent between the cutting process and vibratory system comprising the machine, tool and work- piece form a closed loop. Occurs when a steady input of energy is Monday, February 6, 2023 frequency i ncrea s es conver t ed i n t o m ove m ent t hat has and with characteristics of a m p lit ude. A m p lit ude time.

12. 12 Single-degree-of-freedom system. A machine structure that is subjected to a periodic force will vibrate at the forcing frequency. Several basic results can be illustrated by first considering a singledegree-of-freedom system. For the one-degree-of-freedom system, the equation of motion is Monday, February 6, 2023

13. 13 Single-degree-of-freedom system. … … … … … … . (1) Equation 1 represents a motion of angular frequency ω f at an amplitude given by: Resonance occurs when ω f equals ω n and the amplitude at resonance is F /2ξω n . Monday, February 6, 2023

14. 14 Single-degree-of-freedom system. It can be seen from these results that to minimize the amplitude of vibration of a Monday, February 6, 2023 mass-spring t he da m p i ng be as l a r ge as damped system, should poss i b l e frequency of t he and t he na t ural system should be significantly less than the frequency of the disturbing force.

15. 15 Chatter Chatter is a highly complex phenomenon which can be characterized by unstable, chaotic motions of the machining system and by strong fluctuation of the cutting forces, which tend to amplify and reinforce the phenomenon. Chatter can occur in different metal removal processes: milling, turning, drilling, boring, broaching and grinding. Tt can cause abnormal tool wear or tool breakage, damage of both tooling structure and s p i nd l e bear i ngs . M oreove r , i t can s er i ou sl y a f fect t he s urface roughne s s and t he dimensional accuracy of the workpiece. Monday, February 6, 2023

16. 16 Effect of Chatter Poor surface quality Unacceptable inaccuracy Excessive noise Disproportionate tool wear Machine tool damage Reduced material removal rate (MRR) Increased costs in terms of production time Waste of materials and energy Environmental impact in terms of materials and energy Costs of recycling, reprocessing or dumping non-acceptable final parts to recycling points Monday, February 6, 2023

17. Classification of Chatter Chatter is usually classified as primary or secondary. Primary chatter typically occurs at low spindle rotational speeds, i.e. when the ratio of the spindle revolution period to the chatter vibration period is high. Primary chatter is caused by physical mechanisms such as friction between the tool and the chip/workpiece on contact surfaces, stress distribution on the normal rake face, thermoplastic behaviour of the chip material , mode-coupling and others. The friction between the tool clearance face and the workpiece surface may vary significantly during the process, due to the waviness of the machined surface generated in the previous cut. Secondary chatter occurs at higher revolutions per minute rpm. 17 Monday, February 6, 2023 Secondary chatter, mostly present at higher spindle rotational speeds, is caused by regenerative effect , which is an unstable behaviour of the uncut chip thickness due to a combination of vibrations with the waviness produced by the previous tooth passage .

18. 18 Diagram of regenerative chatter due to the cutting of an undulated surface . Monday, February 6, 2023 Regenerative Chatter The regenerative chatter is one of the most common of dangerous phenomena and belongs to the class of self- excited vibration . Any fluctuation of cutting force induces a time-varying displacement of the cutting tool and consequently a non-constant depth of cut, which in turns yields a variation of the cutting force. As a result, chatter vibration is generated, and thus a wavy workpiece surface is produced.

19. 19 Monday, February 6, 2023 H i gh-Sp ee d M ac h i n i ng ( H SM)

20. 20 High-speed machining (HSM) High-speed machining (HSM) is an advanced and emerging machining technique employed universally to machine complex parts with high productivity, improved quality, sustainability, and economy. Initially, HSM was developed to machine missile and aircraft components made up of Aluminum and its alloys. Defining factors to define HSM: Cutting speed magnitude, Spindle speed (rotating tool), Working material, Dynamic behaviour, DN number (product of the spindle diameter in mm and spindle speed in rpm). Monday, February 6, 2023

21. 21 where F is cutting force and v is the cutting High-speed machining (HSM) For HSM, and conventional machining, velocity. Another accurate and more popular definition of HSM is DN number—the diameter of spindle (mm) multiplied by maximum spindle speed (rpm). For HSM, the typical range of DN number is between 1.5 million and 2 million. The usual spindle speeds vary between 8,000 and 120,000 rpm, even though some spindles are designed to rotate at 170,000 rpm. Monday, February 6, 2023

22. 22 General properties of High-Speed Machining Monday, February 6, 2023

23. 23 Monday, February 6, 2023 Advantages and salient features of HSM Higher productivity and metal removal rate H S M can m ach i ne s urfaces with exce ll ent d im en si onal accuracy and e limi na t e B UE formation with high precision. Excellent surface finish and quality. It can generate burr-free surfaces HSM eliminates the unnecessary steps such as asembly, finishing, and polishing operation. Microscale and precision manufacturing. Eco-friendly process: HSM eliminates the need for cutting fluids and prefers dry cutting. Low cutting forces and stress-free components. Improved tool life. Application of HSM H S M i s w i de l y u s ed i n aero s pace produc ti on, d i e and m o l d m anufac t ur i ng, au t o m o ti ve industries, micromachining, precision components, optic industries, and household appliances

24. 24 Selection Conditions for HSM Monday, February 6, 2023

25. 25 Difference between HSM and Conventional Machining Monday, February 6, 2023

26. 26 Monday, February 6, 2023 Economics of Metal Cutting Operations and Optimisation

27. 27 Economics of Machining Basically overall or total machining time (T m ) is the summation of three different time elements closely associated with the machining or metal cutting process. These three elements include— actual cutting time (T c ), total tool changing time (T ct ) and other handling or idle time (T i ). Beside these three time elements, cost of cutting tool is also required to incorporate for any optimization. Monday, February 6, 2023

28. 28 What is actual cutting time (T c )? If, L is the total length of cut (mm), N is the spindle speed (rpm) and s is the feed rate (mm/rev), then estimated uninterrupted cutting or machining time can be expressed as: Increase in cutting speed and feed rate will result in reduction of actual cutting time as material removal rate (MRR) will increase. Hence, cost associated with cutting time will decrease if speed or feed is increased. Monday, February 6, 2023

29. 29 What is tool changing time (T ct )? Every time a tool has certain life within which it can perform satisfactorily; and thus replacement or resharpening is required to perform once tool life exceeds. Mathematically, tool changing time (T ct ) can be expressed as follows. Basically it is the multiplication of tool changing time for one tool change (TCT) and the number of times such tool changing is desired within the specified cutting time (T c ) TL=tool life Monday, February 6, 2023

30. 30 What is Part Handling Time or Idle Time (T i )? It takes care of loading and unloading of job and is independent of the cutting velocity or feed rate employed during machining. It is frequently termed as idle time as machine remains idle during loading and unloading. Monday, February 6, 2023

31. 31 Tooling Cost If higher speed or feed is employed, the result will be faster tool wear and reduced tool life, which will ultimately multiply expenditure as more tools are required for cutting same length. Mathematically, tooling cost can be determined by the formula: price of individual cutting tool (K 2 ) Monday, February 6, 2023

32. 32 Overall Machining Time and Cost Monday, February 6, 2023