Fasteners and Finishes- Part IV

The plating source is often the blame for many automated assembly problems. The manufacturer requests a coating or plating of a certain type from the finisher. When the manufacturer receives the final product he runs off with it, only to call back in a few days to say that the plating is clogging up the assembly process. When the problem involves recalls and expensive warranty costs, the manufacturer may want a percentage paid by the finisher. This month's column talks about the interfaces of finishes with automated assembly tooling, processes and materials.

Automated processes are of two types. The high-tech version is completely controlled by robotic function. The other involves feeder tubes and fancy tools, but has a human operator somewhere in the process. Robotic installation requires that a shipment of material be 100 pct free of foreign material. This is one of the major complaints that assemblers have about finishers in general. Mixed parts occur because some stick in the barrels and are dragged over, stock is mixed in the plant by accident, and sometimes they come mixed to the finisher from the heat treater. Receiving inspection should be a basic requirement of all plating operations.

Robots function tirelessly, but they have an I.Q. of about 15. Installation of a bolt without threads or the wrong size will be attempted. If the part doesn't go in the hole, the robot stops and the system goes down. Mixed stock will not function in automated systems. If the problem is excessive (beyond the point where a simple sorting could separate the parts), special high-speed sorting can be performed. Some companies guarantee 100-pct no-mix conformance, but it costs. The additional charges may have to be passed on to the manufacturer. A small percentage of mixes are generally not a problem unless the assembly process is fully robotic.

While some complaints may be forthcoming, human operators will just toss aside incorrect and damaged parts. The major concern is where the parts jam feed lines and bowls. Then the downtime can be as much as 30 pct, and the manufacturer will come looking for compensation.

The type of finish applied often causes feed and assembly problems. Oil-based coatings tend to coat the plastic feed tubes and clog the system, stopping the flow of parts and the line process. While one common plant solution is to increase the feed pressure, the cure is worse than the problem. The parts tend to break loose suddenly and shoot out the feed tubes at high velocity.

Many new coatings are metallic-based paints. The flaking off of pieces in feed hoppers causes a poor finish on the part, leading to early corrosion. It also causes feed tube and assembly problems with the plating dust. In many areas, the airborne metallic particles are not allowed by OSHA and local environmental regulations. One study on the amount of time that parts spend in a vibratory-type feed hopper showed that not all the parts were fed out after 30 minutes. The last part in several cases took about 45 minutes to clear the bowl. That is a lot of time to be banging around. In addition to flaking and dusting, the likelihood of parts being nicked or otherwise damaged is great. Soft platings are especially prone to damage.

If the part has a recessed head drive, is of a small diameter with fine pitch threads and/or has an integrated washer, it is not a candidate for the thick organics and metallic coatings so popular today. Numerous companies running production coatings today will quote exceptions to the specification if asked to run this type coating on a "problem" part. Sorting for defects associated with these concern areas will use up as much as 35 pct of available labor.

It is a good practice to ask how the parts are going to be used when they arrive for plating. Uses that have potential for poor assembly and fit or will be hopper fed should be discussed with the customer to see if an alternate coating would be acceptable. Parts that are slated for robotic assembly should have a rider attached to the order requesting full sorting and 100 pct no-mix requirements, with an additional charge for this process.