Characteristics of cutting forces and chip formation in machining of titanium alloys

1. Characteristics of cutting forces and chip formation in machining of titanium alloys Authors: S. Sun, M. Brandt, M.S. Dargusch October 5, 2010 Presented by: Chris Vidmar

2. Introduction • Titanium alloys are seeing increasing demands due to superior properties such as • Excellent strength-to-weight ratio • Strong corrosion resistance • Retains high strength at high temperature • Classified as hard to machine • Low thermal conductivity • High chemical reactivity • Low modulus of elasticity

3. Introduction (cont.) • High-cost and time consuming process is driving research efforts to understand the cutting process and chip formation. • Segmented chip formation is due to localized shearing, which results in cyclic forces, causing chatter and limiting material removal rate • An understanding of these dynamic cutting forces will lead to increased understanding of chip formation and tool wear.

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5. Materials and experimental procedures • Ti6Al4V bar with a diameter of 60 mm • 3.5 hp Hafco Metal Master lathe (Model AL540) by dry machining with a CNMX1204A2-SMH13A-type tool supplied by Sandvik • 3-component force sensor (PCB Model 260A01) with an upper frequency limit of 90 kHz • feed force (FX), thrust force (FY) and cutting force (FZ),

6. Results • Three sections: • Influence of feed rate • Effect of cutting speed • Characteristics of the cyclic force

7. Influence of feed rate • Severe tool vibration at feeds less than 0.122 mm • Cutting forces increase with increasing feed (exception between 0.122 and 0.149 due to high tool vibration) • Tool vibration constant at 260 Hz, independent of feed • Increasing force amplitude, drop after 0.122 mm feed • Vibration can be eliminated by changing tool entry angle or increasing feed rate

8. Effect of cutting speed • Force frequency increases linearly with cutting speed • Amplitude variation decreases with increasing cutting speed, except for where the frequencies were multiples of 260 Hz (the intrinsic harmonic frequency of the cutting) • Due to increasing temperature, which reduces modulus of elasticity

9. Effect of cutting speed (cont.) • Average cutting forced increased up to 21 m/min due to strain hardening • Decreased dramatically from 21 to 57 m/min (attributed to thermal softening) • Small increase from 57 to 75 • Constant from 75 to 113 followed by gradual decrease • Due to dramatic increase in strength with strain rate (makes increasing cutting speed difficult) • Force increased linearly with depth, frequency remained constant

10. Effect of cutting speed (cont.) • Continuous chip formation and static cutting forces possible at low cutting speeds in certain sections due to inhomogeneous structure • Static cutting forces reduce and disappear at 75 m/min • Cyclic force dominates above 75 m/min resulting in purely segmented chips

11. Characteristics of the cyclic force • Chip segmentation frequency and cyclic force frequency show very good correlation • Cyclic force is the result of chip segmentation • Cyclic frequency is directly proportional to cutting speed and indirectly proportional to feed rate • Amplitude increase linearly with depth of cut and is inversely proportional to cutting speed • Equations don’t always apply

12. Conclusions • Both segmented and continuous chips possible at low cutting speeds • Maximum cyclic force always 1.18 times higher than static force regardless of depth • Segmented chips only above 75 m/min • Cyclic force directly proportional to cutting speed and indirectly proportional feet rate • Amplitude increases with increasing depth and feed rate and decreases with speeds from 67 m/min except when the cyclic force frequency matched the machine harmonic frequency • Force decreases with cutting sped due to thermal hardening, except from 10 to 21 and 57 to 75 attributed to two phases of strain rate hardening • Authors suggest that a new physical model be developed to explain segmented chip formation

13. Useful? • Effective for industries involved in mass production of titanium parts (aerospace) • Data can be useful in maximizing machining efficiency of titanium by minimizing forces and maximizing speeds to produce products quicker at lower costs • Reducing vibrations can improve surface finish and increase tool life