Figure 7. RANCOR drill chuck and dr ill bit Figure 8. Side view of the guide wheel roller,  clearly showing the Nedox coating, and the  two curved contact surfaces for maintaining  near line contact with the bit shank RANCOR’s final chuck design is comprised of six guide wheel rollers arranged in 3 pairs, 120 degrees  apart from each other to constrain the drill bit. This configuration was chosen as a superior weight reduction design as opposed to a more conventional eight roller set in a 90-degree separated orientation. Utilizing guide wheels in this chuck is a key feat ure for enabling successful ejection of the drill bit under  high loading conditions. The design also helps guide the drill bit insertion and minimizes percussive  energy loss by allowing a small amount of axial motion during drilling. As shown in Figure 8, the surface  of the guide wheel rollers is curved in two positions to help maintain line contact with the bit shank and  still provide a means for transmitting torque to the bit. Maintaining the four surface contacts on each wheel set ensures that the auger is constrained in that plane and only allows axial motion from the drill bit. The drawback from this design is the relative size of the chuck required to house the rollers. However, this design still enables a maximum drill angle relative to the normal vector to the rock to be as large as 26 degrees and still allow for a 6-cm-long core to be captured before the chuck touches the rock. This  allows for significant margin in the angular positioning accuracy of a robotic arm relative to the local surface normal of the rock. The geometry of the chuck housing was optimized to minimize weight. CNC milling was required to  fabricate the chuck housing out of a single piece of 6061 aluminum. Material for the guide wheels were  selected as 455 stainless steel heat treated to condition H900 and plated with a Nedox SF-2 coating to  reduce the rolling friction and increase the surface hardness. Steel pins were pressed through the bearings in the guide wheel rollers. Post-assembly, dimension checks were performed by inserting the  drill bit augers into the chuck. Any tolerance corrections were then made by removing small amounts of material from the hexagonal faces of the bit shank. Interfacing the auger with the rest of the drill housing  required a durable bearing interface. Sealed Silverthin JSA020 four point contact bearings were chosen  for this purpose. Matched in pairs, these bearings are capable of sustaining thrust loads up to 3527 Nunder dynamic conditions and 7615 N under static conditions. Life expectancy of these bearings under  expected loads show that these bearings should out last other bearings and components of the drill system. Overall, the functionality of the chuck and its minimal weight were favorable over other design  alternatives designed to the same requirements. This chuck design has proven to be a reliable and robust  component of the drill. Mass of the chuck was calculated to be 0.444 kg, approximately 15% of the overall  drill weight. Driving the chuck is the main drive motor for the drill, a Maxon 22-mm size brushless motor. In an effort to  minimize weight and simplify operation, coupling the percussion mechanism with the auger rotation  221