This post covers the last detail of the work order.
The diameter of this head is 90 inches, measured on the outside (OD – outside diameter). The circumference, also measured on the outside (OC – outside circumference), is 23 feet and 6 and three-quarter inches. To begin flanging a head, you measure the diameter of the dished head you begin with (or the circle of steel you begin with if it is a flange-only), then subtract from that the diameter of the finished head, then divide that by two. That is how much metal you place behind the icr roll. Say the dished head is 96 inches. The finished head will be 90 inches. So subtract this second number from the first to get 6 inches. Which you divide by 2. That means you begin flanging this head with 3 inches of metal behind the icr roll. Simple, right?
For a head three-eighths inch thick or thicker it would be good to do this. But a thinner head will wrinkle if you try to take too much metal behind the roll at one time. And you don’t want to risk making a flange-only small, so you would take a little less than this. Unless there is a tight minimum thickness, then you might risk it because you couldn’t flange the head too long for fear of thinning it out. And if it’s aluminum that you need to heat up, you don’t want to put it into size because the head will shrink once it cools off. As you can see, there are a lot of variables to consider.
So you make a pass and wrap some metal around the icr roll. Then you stop to check the diameter. You keep doing this until you get close. Then you start checking the circumference. We are allowed to be plus or minus one-eighth inch on the circumference. So on this head the final circumference can be from 23 feet, 6 and five-eighths inches to 23 feet, 6 and seven-eighths inches.
To measure a circumference on the outside, you clip a metal tape to the head and spin it around until you come back to the clip. Measuring a circumference on the inside is a bit trickier. We use a two-foot wheel marked in one-eighth inch increments. Not only do you have to keep a steady hand to get an accurate measurement, but also count the revolutions as you spin the head.
Nothing is worse than making a head way small. A little small is all right. Sometimes you even make a head a little small on purpose, so that when you pull it back out into size you also pull the crush-up out of the radius. But if you make a head way small you’ll have a job making it look right after pulling it back out. What you end up doing is practically pulling it back out to the dish you began with, then rolling out all the humps you put in the head by doing this. Very tedious.
That’s why I’ve got marks all over the machine I usually operate. I measure the dished head I begin with, figure how much metal I should put behind the icr roll to start with, then make a mark on my machine. If this turns out to be the right place to start from in forming this head, then I’ll start all the other pieces on this order from this mark. Not only that, but I’ve now got marks all over my machine, so when I measure and position a head in what should be the right place to begin flanging, if I’m near a mark, I figure that mark is there for a reason, and it’s probably there from the last time I flanged a head this size, so I’ll nudge the head up or back a little so I’m on that mark. That usually works out well for me.
You have to pay special attention to those two little letters OD and ID. You would not believe how many times I’ve confused them. If you make a head ID that is supposed to be OD, that’s no problem. If a 90 inch ID head is 90 inches OD, that means you are still big and merely have to make the head a little smaller. A cinch. But if you make a head OD that is supposed to be ID, that’s when you cry. Say the head is a half-inch thick. When it is 90 inches measured on the outside, that means it will be 89 inches measured on the inside. Which means you are an inch small in diameter! Way small.
You also have to count carefully when using the wheel to measure an inside circumference. Eight one-eighth inch marks to an inch, twenty-four inches to two feet. And we also measure sixteenth inches, so you have to count the spaces between the marks, too. You have the same problem with a tape measuring the OD, but a tape spells out exactly what you’ve got, whereas with a wheel you have to keep track of how many revolutions there have been as you spin the head, with each revolution being two feet. The circumference on this head is 26 feet, 6 and three-quarter inches. If you form it to 26 feet, 6 and eleven-sixteenths inches, you are in size, since that is less than an eighth small. But to read that sixteenth you have to read the spaces between the marks.
Another aggravation is actually putting the head into size. Say you get close, about a quarter of an inch of being within tolerance. So you take a little metal behind the icr roll, and nothing happens. Same size. So you try it with a little more side roll pressure. Nothing happens again. So you try taking a little more metal behind the icr roll. Whoa, suddenly you are a half-inch small. It seems like the metal defies the laws of physics sometimes. Or you get close to putting the head into size, then you make it a little small. Then you pull it out, to find you are now a little big. So you push the metal back in, and guess what, you are a little small again. And on and on. Excruciating.
All this may sound confusing. But I’ve measured about a million heads with tapes and wheels, it’s now second nature. And besides, you always check the diameter against the circumference. They have to agree. If they don’t, then you’ve measured something wrong. We used to have inspectors check our heads after we finished and took them out of the machine. But now we flanger operators do the final inspection and fill out the inspection reports ourselves. Inspectors are needlessly nit-picky. We know what the heads are supposed to look like. Used to be if something was close to being right, the inspector would take the inspection sheet to a supervisor and show him what was wrong with the head. If it wasn’t too bad the supervisor would sign for it. Now we sign for it. Each flanger operator has a designated metal stamp letter that will identify heads he runs. Mine is ‘L’. So if you ever see a tank head with an ‘L’ stamped on the edge, you know it is perfect. Because I formed it.