Recently I worked on a project to assemble sheet metal parts with rivets. Several noteworthy aspects of this project come to mind. The first being the fact thay because people can put the parts together doesn't mean that they can be asseembled automatically.
Stamped parts that are punched and THEN formed can only be located from the stamped details; the profile or outer edges are subject to the variances of material thickness as well as hardness (springback). The customer thought that because workers could locate the parts and line up the holes for riveting, fixturing and automation could do the same but with a real reduction in labor cost: until we reviewed the part tolerances.
The tool maker had negotiated tolerances that were easy for him to meet, but in a review of the two making holes and their respective mating surfces, there was +/- over a half a hole diameter in the tolerance. So, if you locate the part, even on punched features, the holes only line up about half of the time. Okay, let's locate the mating parts when they are loaded by placing thme over pins that retract as the rivet is installed into the hole. THis worked for two of the holes, but not for all the holes. There was no way to locate the two pivot points from the inside of the parts without a piece of "loose" tooling. This was unacceptable to the customer.
The parts had an assembled tolerance of +/- 0.4 degrees and the customer required a Cpk of 1.333 on the assembled critical dimensions. The downtime requirement was very low and the requirement for efficiency was 99%+ at a rate of 360 parts per hour. The budget was tight and finally the recommendation to the customer was to load/unload the parts with operators. Install the easy rivets over retracting pins and then install the rivets in the positions with location problems manually. Inspection was to be automated.
The system generated an overall throughput increase of about 20% and had a cash payback of about 14 month based on labor savings alone. The customer was not please with the solution, and cancelled the contract. Sometimes you just have to walk away from a project if it is really not possible.
I would like to hear from any on you who have installed Pop Rivets into blind hole with no pins to line up the holes. In this case because of the way the parts are formed, all the rivets had to be install from the beakout side ir sharp edge of the stamping making getting into the hole even tougher. Do any of you out there have similar experiences that you'll share?
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