Stick-slip motion control of a wood-stick tool for lapping a led lens mold

Friction known as friction heat, friction noise, friction wear and friction force and so on is an undesirable physical phenomenon in almost machine systems. In this chapter, however, a friction force generated by a stick-slip motion is introduced to improve the lapping quality of small LED lens molds. Here, the lapping means the finishin or polishing of metallic molds by using diamond paste. A novel desktop orthogonal-type robot, which has abilities of compliant motion and stick-slip motion, is firs presented for lapping small metallic molds with a curved surface such as a LED lens mold. The robot consists of three single-axis devices with a high position resolution of 1 µm built in Cartesian-space. A thin wood stick tool is attached to the tip of the z-axis. The tool tip has a small ball-end shape with a diameter of 1 mm. The control system is composed of a force feedback loop, position feedback loop and position feedforward loop. The force feedback loop controls the polishing force consisting of contact force in normal direction and kinetic friction forces in tangent direction. It is assumed that the kinetic friction forces are generated by Coulomb friction and viscous friction. The position feedback loop controls the position in pick feed direction, e.g., z-direction. The position feedforward loop leads the tool tip along a desired trajectory called cutter location data (CL data). The CL data forming a spiral path are generated from the main-processor of a CAD/CAM system. The proposed robot realizes a compliant motion required for the surface following control along a spiral path. The surface following control is the basic and fundamental strategy for simply constructing an automatic lapping system. In order to improve the lapping performance, a small stick-slip motion control strategy is further added to the control system. The small stick-slip motion is orthogonally generated to the tool moving direction. Generally, the stick-slip motion is an undesirable phenomenon and should be eliminated in precision machineries. However, the proposed robot employs a small stick-slip motion, so that the polishing energy can be f nely changed to partially improve the lapping quality. The effectiveness of the robot is examined through an actual lapping test of a LED lens mold with a diameter of 4 mm.