The objective of this feasibility study was to integrate a robotic system to sand the exteriors of welded steel safes of different sizes.
Four Project Phases:
This Study Determined:
The Study Did Not Determine:
The fixturing required for part location in the final production setup was not included in this study. Fixturing for the individual safes was the responsibility of the customer.
Equipment Used In the Feasibility Study
The equipment used for the feasibility study consisted of reconditioned components from the robotics integrator's stock. This kept study costs to a minimum while continuing to demonstrate proof of concept.
Evaluating the Safes
Upon arrival, the safes were thoroughly inspected for surface contour and texture. A slight elevation was noted at the edges of all sides between the face and the corner. The corners were seen as a challenge to excessive material removal. The sides of all the safes were slightly warped. The robotics integrator determined that a tool compliance might be required because of this feature.
The doors were slightly recessed from the door frame. Because of this recess, the edges and the corners of the doors could be difficult to access with the circumferential sanding head. The hinges for the doors protruded out from the face of the safe. The robotics integrator realized the challenge of sanding the entire hinge with a large diameter sanding device. Manual sanding would be required either before or after the robotic sanding process in certain areas that were not reached by the automated sanding tool. The safe surface texture was very smooth and evenly distributed - a feature that promised an even finish at the end of the process.
The sanding head was parallel to the floor during the first trial. The sanding head was programmed to start at one edge and manipulated with a constant force to the opposing edge. This was done with 220 grit sand paper at a travel speed of 175cm/min. The sanding head was placed completely flat with a maximum surface contact between the sand paper and the safe. Several passes exhibited an even, smooth surface; however, there were a few passes that removed excessive material and exposed bare metal. Excess was removed near the edges of the safe where the elevation differential had been noted.
Adjustments Following First Trial
Based on the first trial results, the robotics integrator determined that a higher grit sanding paper placed at a different angle would eliminate excessive material removal. The results improved however there were still a few areas where excessive material was being removed near the edges.
Robot Programming Adjustments
Because the results were not completely satisfactory, the robotics integrator changed the robot programming. Instead of keeping a constant force throughout the entire stroke, the path was modified so that the edges were eliminated from the horizontal movements. The edges were sanded separately using one vertical stroke with the sanding head. The third trial yielded better results than the second trial. No bare metal was exposed. An entire safe was sanded for the customer to examine, using this approach.
Sanding Tool Adjustments
The final feasibility trial used an alternate sanding device. The two motives for using this were reduced cycle time and z-compliance for the sanding face. The sanding device that was chosen had a larger sanding surface. It also used a foam buffer beneath the sand paper. The larger size reduced the cycle time because it took care of the surface with fewer passes. The foam buffer handled irregularities in the safe contour. Fig. IV depicts the sanding device mounted on the Motoman SK16 MRC. This trial had a positive outcome. The foam buffer provided sufficient compliance for all surfaces and cycle time was reduced.
Recommended Robotic Equipment
The feasibility study determined appropriate robot/peripherals for quotation. The Motoman SK16 MRC used in the feasibility study proved to be too small to reach the sides and top of the largest safe if used independently. However, using multiple Motoman SK16 MRC robots would provide an envelope capable of reaching an entire safe sitting in a stationary position. A quote was offered for this option.
If a single robot was desired, it would be necessary to select a large reach robot such as the Motoman UP130 XRC. In addition to a single robot, it would also require the use of a positioning device to present the safe to the robot at various positions; this option will also be quoted.
Recommended Sanding Equipment
As for the sanding equipment, the robotics integrator recommended a rotational or orbital sanding device with a minimum 6" diameter sanding face. The unit could be either pneumatically or servo controlled. The sand paper should be 220 grit to 320 grit to provide a smooth finish and reduce the possibility of removing material.
A passive compliance device is also recommended. This will provide the capability of removing an even amount of material on all surfaces regardless of any contour changes. The compliance device must have the ability to react to change in material height while adapting to irregularities to the contour of the parts. This equipment was explored and presented in the final quotation to the customer.