Fast Tool Servos for Precision Machining of Irregularly Shaped Surfaces Graduate Students: James Simonelli, Ryan Beach; Principal Investigator: Tsu‐Chin Tsao 

Hydraulic Piezoelectric Actuator Stroke Amplifier 

Underlying Principles • Like traditional hydraulic systems, this system uses

fluid to transmit force and displacement between an input and output plate

• Instead of trading displacement for force multiplication, the design uses a piezoelectric actuator to provide large-force, but small-stroke motion to the large-area plate to amplify the stroke at the smaller-area output plate.

• Fluid chamber behaves as a very stiff spring that also multiplies displacement.

Overview Fast Tool Servos are turning tools capable of high-frequency motion not typically possible with conventional computer numeric control (CNC) tooling. Typical applications are turning of non-circular parts [1,2,3] and chatter suppression [4,5]. This particular design uses a piezoelectric actuator (PEA) to provide the base motion and a hydraulic amplifier design to increase the stroke of the device from the PEA stroke of 60 microns to the design stroke of one millimeter, a factor of approximately 16.

Design and Modeling

References [1] Krishnamoorthy, K.; Chi-Ying Lin; Tsu-Chin Tsao, "Design and control of a dual stage fast tool servo for precision machining," Control Applications, 2004. Proceedings of the 2004 IEEE International Conference on , vol.1, no., pp.742,747 Vol.1, 2-4 Sept. 2004 [2] D. Tran, Daniel B. DeBra, Design of a Fast Short-Stroke Hydraulic Actuator, CIRP Annals - Manufacturing Technology, Volume 43, Issue 1, 1994, Pages 469-472 [3] Haifeng Wang, Shuyan Yang, Design and control of a fast tool servo used in noncircular piston turning process, Mechanical Systems and Signal Processing, Volume 36, Issue 1, March 2013, Pages 87-94 [4] Pratt, J.R. and Nayfeh, A.H. (1999) Design and Modeling for Chatter Control. Nonlinear Dynamics, 19, 49-69 [5] J.B. Mann, Y. Guo, C. Saldana, W.D. Compton, S. Chandrasekar, Enhancing material removal processes using modulation-assisted machining, Tribology International, Volume 44, Issue 10, September 2011, Pages 1225-1235

Conclusions

Fig. 2: Current Amplifier Design (Mass-Spring-Damper Model Overlaid)

Fig. 3: Entire Device (including packaging), Exploded View.

Fig.1: Hydraulic Amplifier Schematic

• A metal bellows and membrane was used instead of a plunger/wiper design to withstand the large chamber pressures necessitated by the high design return spring stiffness.

• Limiting factor for device bandwidth is the return spring; its stiffness affects the location of the first resonant peak.

• Return spring stiffness directly affects peak chamber pressure and input force requirements, which are both limited by physical constraints.

• System can be modeled as a mass-spring-damper system with the hydraulic chamber described by a first-order differential equation by adding chamber pressure as a state.

• To ensure that the PEA is always applying force in the same direction and that the chamber is always pressurized, both the actuator and the tool carriage are preloaded.

This Fast Tool Servo design makes it possible to turn non-circular profiles with a total runout of one millimeter or less, and can be used to suppress chatter while machining traditional parts, allowing manufacturers to cut faster, decreasing per-part cycle times. The ability to produce parts with irregular geometry on a traditional platform like a lathe, as well as the ability to increase cutting speeds past the current limits imposed by tool chatter will allow this tool to increase production efficiency.

Fig. 4: System Bode Plot for Various Return Spring Constants

Fast Tool Servo For Non‐Circular Boring 

Overview The fast tool servo (FTS) is a custom-designed tool holder used to cut piston pin holes. Piston pin holes have a unique non-circular geometry, they have an oval hole with a trumpet-shaped profile. This geometry is difficult to create because the work piece must remain stationary. The FTS mounts to a lathe and is designed to accept multiple sizes of boring bars. It uses a piezoelectric actuator to actuate the boring bar about a flexure.

Conclusions The design of this FTS provides a new method of machining piston pin holes using a lathe. The final results of this tool will allow cutting variations of up to 60 microns. The precision of the tip displacement is in the sub-micron range at a frequency of 200Hz. The final pin hole size will be 0.8in along the major axis.

Design and Modeling • Equations for the total tool travel, Δy

relating to the piezoelectric actuator’s free travel range Δp.

∆y= l1

l2

kp

kf / l12 + kp ∆p

Fig. 5: FTS Component Diagram

Fig. 6: FTS Picture Assembled on Lathe

Fig. 7: Block Diagram of Components and Electronics Fig. 8: Cross-Section of Device Showing Free Body Diagram

Fig. 9: Entire Device Exploded View