Automating an Encouraging Bottom Line
by Joe Saad, saadj@acmemfg.com

It's an odd but impressive sight. A heavy-payload, six-axis robot, equipped with twin spindles, holding two large, cast aluminum, aftermarket wheels that weigh 75 pounds each. The two wheels are positioned simultaneously by the robot at one set of aggressive "cut buff" wheels, and then moved to another set of final coloring buff wheels, housed in an enclosed work cell (see Figure 1). The process finishes all exposed surfaces in a single part setup and gripping. Seven axes of motion (including synchronized rotation of the two automotive wheels) are utilized. The cell processes cast forged aluminum and copper plated wheels, in three shifts, with high repeatability and surprising aggressive throughput.

Figure 1: Two aluminum wheels are positioned simultaneously by the robot at one set of cut buff wheels.

This application is a good example of the field-proven robotic automation available today for buffing and polishing operations.

Meeting Cost Reduction Goals
For the scores of decorative or functional metal parts that require grinding, polishing, satin finishing, buffing, or deburring, surface finish quality imperatives are a given. Tolerances, in many cases, are tight and non-negotiable. The good news is that the basic, top-tier benefit of robotic finishing just keeps getting better. It's the consistent, process repeatability that you can achieve part to part, day to day, cell to cell, and plant to plant, that makes it immediately valuable.

Elimination of manual processing of heavy or hard-to-handle parts is also a major factor in many situations. The ongoing pressure to minimize capital equipment expenditures, as well as tooling and operating media costs, is a double-edged sword for many manufacturers because they realize that it takes investment in automation and tooling to help reduce operating media costs.
Productivity-impacting innovations to robotic automation fall into four categories: finishing media, robotic cell configuration, part-handling automation, and operational efficiency.

Reducing Media Costs
Buffs, compounds, abrasive belts, wheels, and brushes are the consumable "tooling" (media) in a finishing operation, and they represent a significant opportunity for cost reduction. For starters, switching from manual to robotic automation finishing alone can give you two to four times longer media life, as a result of contact efficiency (time on the part), constant work pressure, cycle time repeatability, part path programming capability, and media life management.

There have been recent innovations to finishing media that further the cause, with structured abrasives leading the development. Ordered rows of microscopic, precisely shaped "pyramids" now replace the randomly shaped mineral particles on traditional abrasive belts. The technology is called mircroreplication, and it creates structured coated abrasive belts that have proven to reduce grinding and finishing costs by extending the life of the belt.

Fine belt finishes can be achieved, with greater uniformity and improved surface quality. The process can often be simplified, with several polishing operations eliminated. These new abrasives are used for grinding, polishing, superfinishing, and deburring.

Flexible, automatic force control for finishing heads has been refined to the point where compliance (response and reaction) can be amazingly optimized to suit the operation. If one surface area needs more pressure, it gets it; a few seconds later, pressure is lessened as the robot repositions the part to another surface. Meanwhile, speeds and feed rate are adjusted.

Most importantly, critical size tolerances and finishes can be achieved via extremely light float pressures, down to two or three pounds of force while maintaining proper response.

Automatic media wear compensation software ensures consistent performance throughout the life of the media by adjusting cell process parameters to match the performance and lifespan of the media. Media life management software maintains media wear factors, such as belt life, and increases the SFPM (surface-feet-per-minute) cutting rate and the cutting force to compensate as the belt wears. It can also automatically compensate by positioning the buff wheel closer to the work as the buff wheel wears out.

Configuration Creativity Yields High Production Rates
There is a perception that robotic cells have traditionally provided lower throughput levels than dedicated finishing systems, but this productivity concern is not totally accurate.

To get optimized processing efficiency, integrators have capitalized on the fact that there are several robot configurations to consider. The robot can take the part to the finishing heads, or it can take the finishing tools to the part. In the majority of applications, the part is held by the robot end-of-arm tooling and moved around a stationary finishing wheel or belt. When very large parts are being processed, such as truck step bars or aircraft skin, buffing wheels and motors are mounted right to the robot and moved to a stationary part in a fixture. Automotive wheels can go either way, depending on the production rates required.

Figure 2: Production rates are increased when finished heads are stacked to create a four-head configuration.

Stacking the finishing heads to create a four-head configuration (see Figure 2) can increase production rates since all required operations can be accomplished within a single, continuous cycle and a single set-up and part gripping (with no between-process part handling).

Grinding or deburring, for instance, can be combined with sequential surface finishing operations, from polishing to buffing. Belt and wheel stations can be arranged in the same cell, with wheels mounted either vertically or horizontally.
Multiple robots in a single cell can perform the same process simultaneously. They can also be configured to hand off parts to each other for a sequential process, where each robot does half the process. This works well when a regripping of the part is required to expose all surface areas.

The range of robotic scenarios is still growing. Six-axis robots can now reliably run in continuous production metal finishing applications with payloads of up to 450 kg.

Automation Increases Uptime and Cell Throughput
Part handling and queuing options for robotic finishing cells are providing reliable, unattended processing time of two or three hours or more. For small to medium size parts that are fixtured in trays, simple dual-drawer arrangements allow one tray to be loaded with parts by an operator while the robot unloads, processes, and loads parts from another tray in a second drawer adjacent to the first. The drawers pull open directly from the cell enclosure (see Figure 3). The use of two drawers creates a queued part reserve for continuous, uninterrupted production.

Figure 3: Drawers that pull out of the cell enclosure hold fixtured parts in trays. Shown here is a builder's hardware being polished.

Vision systems are being integrated into robotic finishing cells to identify parts and verify positioning to ensure parts are properly loaded prior to pick-up by the robot. The latest "visionary" quest involves the development of laser/vision applications to provide 100% dimensional part inspection.

Potentially, this capability will provide closed-loop feedback to the robot for modifying process parameters such as force control and contact time on the finishing belt or wheel.

For applications where the finishing tool is held by the robot (with a stationary workpiece), automated in-process tool changeover has cut downtime significantly and made multi-tool processes, using various media, possible in terms of reducing cycle time and increasing overall productivity. Small batch finishing by automated means has become feasible. Typically, end-of-arm tooling is held in fixtures and accessed by the robot(s).

Slot Machines/Plating and Polishing Job Shop

The parts: Housing components for gaming machines. The objective: Small batch (job shop) productivity via labor and time savings. The robotic operation: Robot takes part to four heads with various coated and structured abrasive media for edge grinding and surface finishing prior to chrome plating. Buffing is eliminated.

Medical Implants

The parts: Orthopedic implants including knees, hip balls and shells, tibial trays, cups, and plates. Materials: Chromium cobalt, zirconium, titanium, and stainless steel. The objectives: Reduce media usage by more than 50%; reduce labor content per parts; achieve faster changeover. The robotic operation: Powerful controls with media life and process management capability, combined with modular tooling, allow complete finishing on families of parts with minimized media usage.

Motorcycle Parts

The parts: Motorcycle aftermarket (from gear shifts to air cleaner covers). The objective: Reduce scrap and labor-intensive finishing operations on complicated part design. The robotic operation: Abrasive belts and wheels with non-woven media, plus buffing wheels, and automatic force-controlled heads Parts are finished prior to chrome plating at a production rate of 15 to 120 parts per hour (depending on part complexity). Part queuing allows one hour or more of unattended operations.

Controls and Programming Innovations
In addition to the goal of gaining more robotic cell utilization, shorter lead times are routinely desired to meet competitive pressures, whether the manufacturer is an OEM or a job shop. Off-line programming and simulation of a robotic finishing operation, even for complex parts and shapes, speeds up the process and increases utilization.

First developed by Acme in 2003, off-line programming software is provided for every Acme robotic system.

Instead of using teach-pendant programs to walk the robot through various paths in slow motion, the entire finishing program can be created and verified away from the actual cell. Robot movement and part manipulation are determined by powerful software that incorporates process parameters such as spindle speed, force, and SFPM that are entered using menu-driven, graphical screens. An animated simulation reveals robot reach and space issues, so they can be quickly addressed. Applications can be validated in a virtual environment without the cost of an actual robot and/or tooling. It also allows the user to efficiently create and compare different layouts and component selections, to determine the combination that will afford maximum flexibility and productivity.

For existing cells, off-line programming eliminates the need to take the cell out of production to create additional part programs, or to modify existing part programs. Another benefit is part process "movies" (.AVI files) can be output for presentation or proposal uses. Finally, off-line programming makes a great training tool.

The evolution of controls and related software, coupled with Ethernet/modem integration, has positively impacted many aspects of automated finishing, from changeover during small batch processing to remote part editing and troubleshooting. PC-based operator interfaces drive on-line diagnostics and documentation. Virtually unlimited part programs can be stored and accessed quickly at the cell.

Advanced process monitoring includes process management systems that monitor spindle motor load, part temperature, robot load, cycle time, and media speed. Individual cycles, from part to part, can be compared. Deviations from the optimum cycle can be monitored.

Consult a Robotic System Integrator
When some or all of these technologies are embraced, payback on robotic finishing systems is in the range of six months to one year. Consultation from a systems integrator that combines robotic programming capability with finishing media expertise is highly recommended when considering process options and the feasibility of automation.

Even when the part and/or finishing operation is complex, chances are it can be done, and it can save time and money.