SANDING AND POLISHING ROBOT SYSTEMS
Sanding and Polishing Robot Systems
If you are taking time from your busy day to read this paper, you probably already know most of the reasons for automating sanding and polishing processes. Labor in most areas of the United States is difficult to find and retain, and manual sanding and polishing requires technique, can be dirty, and is difficult to ergonomically perform. Automated sanding and polishing of metal substrates is decades old technology, and robots have polished aluminum engine castings for at least twenty years. This document primarily focuses on both preparing substrates for a finishing process and repairing finished substrates from a process miscue. Sanding and polishing robot systems operate on four simple premises. The tool must remain perpendicular to the surface, move at a set speed, at a set pressure, and in a set pattern. Orchestrating the robot cell to repeatedly sand or polish products each and every manufacturing day draws from mechanical engineer, control technologies, physics, and likely some trial and error.
Sanding and polishing robot cells when compared to manual sanding and polishing are highly productive. When programmed correctly the robot cell can reduce refinishing costs due to worker processing errors, out produce the worker by several fold, use fewer consumable materials, and largely eliminate worker injury costs due to repetitious joint movement associated with sanding and polishing tools.
Sandpaper is abrasive material on coated paper. The grit number associated with the paper determines the coarseness. Lower grit number paper cuts material quicker than higher grit number paper. Grits around 400 produce a smooth enough surface to apply automotive quality finishes without apparent surface scratches.
Dry sandpaper is by far the preferred technology. The paper is offered from 12 to 2500 grit. Twelve grit will quickly remove volumes of plastic, fillers or wood. Less aggressive sandpaper is used to shape and smooth components to the desired form. Fine sandpaper is used to prepare the surface for the finish coat.
Sandpaper is sold to use with or without a flushing liquid. Wet sandpaper uses water for fine finish coat surface preparations with grit numbers at and above 400. The water helps keep the grit from plugging by washing away paint particles. The water both increases sanding productivity and sandpaper life. Water also reduces floating sanding dust and finishing particulate contamination. The water does sometimes hide defects. The downside to wet sanding: process issues when masking tape and paper is required, possible water contamination in the finishing process, mess from the water and removed substrate, and worker turnover from the wet and cold conditions.
Sanding and Polishing Spindles
Production manual sanding for finishing is mostly via random orbital machines powered by air. The pictures below are an electric rotational sander with disk tool changer equipped on sanding and polishing robots. The sander assembly provides a highly controllable RPM and a reliable method for robot disk pad exchange of either sandpaper renewal or disk pad diameter.
Disk Engaged Disk Disengaged
Sanding disk or polishing pad maintenance is mostly semi-automatic. The robot cell incorporates sanding and or polishing rotating tool trees. Ready-for-use disks and or polishing pads are positioned inside the cell and spent disks or pads rotated outside the robot zone for manual service. Several different pad sizes are available: large pads for open areas, small pads for tight areas or small spot sanding. The tree is designed to organize the disks and pads according to frequency of use. Sensors in the tree indicates proper placement and notifies the operator the need for tool service.
Polishing Compound Dispensing
Polishing compound, like sandpaper, contains grit. Polishing grit is very fine and suspended in polishing liquid. The robot is fitting with a polish liquid applicator which is usually a spraying device and polish metering pump. The robot is programmed to automatically apply the polish then use the appropriate polishing pad to treat the surface.
Robot Force Sensing
Maintaining tool speed and repeating the same path perfectly describes a robot function. The repeatable pressure requirement makes the robot selection more unique. The sanding and polishing tool needs to press against the surface perpendicularly and with even pressure. Conventional robot controls do not offer a push pressure setting in the parameters. Responding to the need, several companies have developed and now manufacture devices to add the ability of programmable pressure.
Through-the arm force control is accomplished by mounting a six-axis force/torque transducer to the robot wrist plate and then attaching a tool to the transducer as shown in Figure 1. To apply a given force, the robot pushes the tool against the part surface and a force/torque transducer outputs a corresponding electronic signal. This output signal indicates the magnitude of the forces along the X, Y and Z axes and their associated torques about those axes relative to the robot wrist plate. The robot controller receives the force/torque transducer output signal and compares it to the user-requested force to determine if a force error exists. Invariably some force error will exist and the robot arm must change position relative to the part surface to apply more or less force. This compensating motion brings the force error to zero and maintains the user requested force. A complication exists in that any force correcting motion must also be performed in conjunction with the user prescribed path over the part during processing. This means that, by using the force/torque transducer feedback to position the robot arm during motion, the transducer and the robot are acting together, becoming a coupled system.
Edwin A. Erlbacker PhD
PushCorp, Garland, TX
Spindle Assembly with Force Sensor and Sanding Disk Tool
Force sensing is one half of the constant pressure solution. Fixturing the part is equally important. The force sensor cannot properly output a pressure signal to the robot control if the part flexes away from the sanding or polishing tool. If the part twists in the fixture the sensor may create an oscillation in the robot motor servo control loop. Dr. Erlbacker recommends some compliance in the sanding disk pad to dampen the feedback signal. Correct part fixture design is optimally 90% experience and 10% luck. Solid fixtures may partially block access to the sanding or polishing tool. Weight and ergonomics also compromise fixture design. Most sanding and polishing robot cells probably operate on an array of parts. Heavy fixtures are not ergonomic. Trade off on fixture designs may take several iterations.
In some applications the robot will not be able to reach all the process areas. Modern robot controls employ software and hardware capable of adding and controlling additional servo axis. Using the additional servo capabilities, cell integrators can move the fixture and part under program control, enlarging the robot reach capability. Additional axis control is most common with robots with limited reach.
Pre-finish sanding is a prime candidate for robotic sanding. Sanding substrates prior to finishing, shapes and smooths the surface. Often the sanding procedure requires several passes over the substrate, each pass reducing the sandpaper grit until the surface smoothness is adequate for the desired coating results. Automation can reliably remove material faster with less chance of error. Fiberglass, SMC, and wood products require significant surface sanding during the manufacturing process. Fiberglass and SMC substrates are used for lower production automobiles, motorcycles, and off-road vehicles often finished with exotic colors and clear coats. Pre-finish processing of the vehicle components requires several sanding steps to level, taper and smooth. Each step requiring the proper equipment, learned technique, and processes followed. All difficult manual process steps to manage the level necessary for the product.
Post Finish Sanding
Sanding after finishing is either a requirement to add additional finish layers like two-tones or a repair function from a misstep in the coating process. Scuffing intermediate coats for adhesion is necessary, but often a detriment to the product’s final quality. Sanding creates dirt. Failure to remove the dirt generates more finish defects. Inter-coat adhesion requires all surfaces to be sanded. Repeatable manual sanding is difficult to maintain. Missed surfaces produce adhesion failures which show up later in the product life generating unwanted warranty costs and customer dissatisfaction. Post finishing sanding most often is the first step in a finish repair process where a defect, usually a dirt spec, is gently removed using a very fine grit sandpaper followed by a polishing process to remove the fine sanding scratches. Dirt removal post finishing sanding, with the intention of later polishing, is a learned technique. Improper force on the sandpaper removes too much of the paint layer requiring the product to be completely recoated. Robotic polishing reduces the need for trained operators and improves yield.
Sanding associated with the finishing process is a necessary evil. Almost always the substrate requiring the sanding process is an engineering cost/performance trade off, and the paint department stuck with coating the product with a less than perfect solution. Production costs due to sanding are substantial. Manufacturers should always strive to find substrate choices that reduce or eliminate sanding.
Robot Cell Safety Systems
Fixtures hold the part. The cell holds the fixture. An operator tends the cell by loading the part onto the fixture. A safety cell system is required to allow the operator to enter the robot zone and service the fixture. Several scenarios are possible. Often it is advantageous to keep the robot continuously sanding or polishing. Either the robot or the fixture can be automatically moved, clearing a zone for operator tending. Some robot controls offer software safety limits, or a simple lockout/tagout method can provide tender safety.
Robot Transport-Moves the Robot Between Two Fixture Rotators
Instructing the sanding and polishing robot cell what work to perform is accomplished by different levels of technology. If the entire product surface is sanded or polished the Operator Interface is a touch screen with product buttons. If the system is spot sanding and polishing, the interface is required to “Pick the Spot” on the screen. A touch screen interface will require the ability to easily add parts and spots. Affordable vision for defect identification is still in dream stage. It is possible to use a fluorescent pen to circle areas and a vision system to identify the spot. The robot must possess all needed programmed movements for each spot circle. Software for “self-programming” where a scanner or 3D CAD part data is used to generate the robot movements, continues to improve.
“Pick a spot” System User Interface
Beyond the technology discussed in this paper, all types of ancillary devices may be required to support the cell. Depending on the part(s) configuration(s) some manual sanding or polishing may be required. Justification is labor, quality, and consumables. Sandpaper is process sensitive. The correct pressure, speed, and path will cut many times faster, with cell users estimating 5 times faster than manual. Similarly, sanding consumables are reduced due to the robot ability to keep the sanding tool flat and the path repeatable. Many polishing processes are clear coat dirt defect repairs. Excessive polishing can burn through the clear coat requiring a repaint or scraping the part. Polishing compound that is manually applied can be a process mess. Polish applied via the robot system is more controlled, saving material and reducing housekeeping.
Like all automation, successful sanding and polishing robot cells are a collaboration of planning, engineering, execution, and personnel. Benefits from successful implementation are generous. Sanders become operators and ergonomic injuries associated with repetition are reduced. The return on investment meets most management guidelines and customers receive more consistent results.