While I worked for Panasonic Factory Automation, I installed several MVIIc chip shooters. These machines placed about 10 surface mount chips per second. If the placement rate dropped below about 99.99 percent, it was time to stop and fix the problem. I had installed and verified the calibration of a machine in a new manufacturing area at Celestica, then a subsidiary of IBM. During the acceptance testing, the chips were slightly skewed, but within specifications. The machine didn’t fail, but it wasn’t as good as we expected it to be.
The calibration involved the theta 1, 2, and 3 axes. The procedure was to install the large camera jig to a head on the turret, manually index it to the theta 1, and engage the axis. Then we would carefully index the turret backward to the placement position and use the X-Y table and a dial indicator to measure the runout. To do theta 2 and 3, the turret was indexed backward to the axis, engaged, indexed forward, and measured. All three theta axes were less than the specified runout spread. I think it was 0.1mm across 1 inch, but the chips were still skewed.
I checked and rechecked the theta axes calibration. A senior technician also checked the calibration and found nothing wrong. The manufacturing area was not in production yet, so the problem was not urgent. The fall-back position was that the chips skewing were still within specifications, so we could get along with the skew, but it wasn’t ideal.
The problem bothered me. I continued to wonder why the calibration was good while the placement was skewed. Eventually, I reviewed the placement sequence of operations step-by-step in my mind. The theta 1 engages the nozzle and rotates it to the pickup orientation. The machine indexes to pick up, picks a chip, and then indexes to theta 2, machine vision. Then it takes a picture of the chip on the nozzle. The machine then indexes to theta 3 and corrects angle variations detected at theta 2. The chip is then placed.
The machine then selects the nozzle for the next chip placement and indexes to theta 1. The nozzle brake at theta 1 contacts the nozzle to create drag to ensure engagement. But what was the theta 1 brake actually doing? The exact indexing angle of the theta 1 nozzle brake engagement and disengagement was not in the technical manual. I carefully indexed the turret manually a little at a time, walking from the front to the back, and observing the theta 1 nozzle brake. Lo and behold, the brake was contacting the nozzle during the turret indexing. I adjusted the theta 1 brake to clear the nozzle during indexing, and the skewing went away.
This entry was submitted by Glenn Aitchison and edited by Rob Spiegel.
Glenn Aitchison’s first field service job was in 1987. Since then, he has worked with automotive robotics and other industrial automation and machinery. He has received his Certificates of Qualification as an industrial electrician and as an industrial mechanic (Millwright).
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
I agree, Rob. You have to give credit to Glenn, especially considering that he said: "the chips were slightly skewed, but within specifications." In other words, he chased down the solution to a misplacement of 1/250th of an inch, even though it was already in spec.
The best example I've heard of this, Island-Al is from a process plant. Apparently they laid off so many workers that there was nobody left who knew how to turn off the plant. That may be an urban myth, but even so it makes a point.
Good observation, Chuck. That shows just how precise it needs to be in order for it to be "right." So you have to hand it to this Sherlock Ohms to correct a situation that was so very close and "within spec."
The loss of experienced engineers is most tragic indeed. My boss says (thinks) we can get replacements easily. I have seen examples of these replacements firsthand and have yet to be impressed. One can hire poor engineers and technicians for ten cents a pound, and they are overpriced. Great ones are cheap at one hundred dollars an ounce, but are difficult to find.
In my experience engineers and techies are one of two flavors. The first are hourly employees who think only of the paycheck and watch the clock. Most belong to a union but I have no problem with that. The second are the guys who think only of the problem(s) at hand and are somewhat mystified that somebody keeps putting money in their checking account. In the late 60's accountants in the company I worked for kept calling me because I seldom cashed my paychecks. Direct deposit later fixed this problem. Last Sunday I received a PDF manual on some equipment we were having problems with. This vendor went into work to find, and send me, the documentation. He is one of the second flavors.
Parado was exactly correct with the 20-80 rule where 20 percent of the people perform 80 percent of the work. In the long run the 20 percent earn much more money but usually find little time to spend it. Of that original 20 percent, twenty percent of them are the true Golden Child ones.
That was a while ago, but I think the brake pad was a rubber piece on a pivoting arm. The brake does not hold the rotational orientation of the nozzle - inherent friction in the assembly does that. The brake holds the nozzle still from spinning until the theta 1 meshes. There is very little friction wear in the braking action. The theta engagement is a steel V-block that nests into a matching V-slot in the steel nozzle. The wear that does happen is the V-shape of the block, from the vertical engagement about 10 times per second.
Talk about getting things just right: It's instructive to note that placement of the surface mount chips was within 0.1 mm across one inch. Since a millimeter is 1/25th of an inch, it means that these chips were being placed with 1/250th of an inch...and this was unsatisfactory.
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