Back to the work order.
This line tells me what kind, or style, of head I’ll be working on. Standard is the simplest kind we make. The F&D merely stands for ‘flanged and dished’. A standard usually has a sharp corner (a small icr), and usually has a square cut, our simplest machining operation. It’s our el cheapo. We hold it to the same standards as an ASME head, but we aren’t required to.
An ASME head is held to a strict code. The diameter can only be one percent out of round. The straight flange can only be toed in or out two and a half degrees. The length of the straight flange can only be a quarter-inch more or less than what is required. The overall height can only be one and a quarter per cent of the diameter deep or five-eighths per cent of the diameter shallow than what is required. The icr can be slightly larger than required, but not smaller. There can be no more than fifteen per cent thin out in the thickness. Bevels must be plus or minus two and a half degrees than what is required. Tapers are plus a sixteenth inch and minus nothing in thickness. Tapers and bore-ups can be longer than required, but not shorter.
This might not sound very strict. But remember the size of the heads we work on. We are not fashioning small precision components. Customers can ask for tighter tolerances, and they frequently do. But by code this is all we are required to maintain.
An elliptical head is a semi-dome shaped head in which the overall height is a quarter of the diameter.
These three styles – standard, ASME, and elliptical – are the most common heads we make. But life would get boring if this was all there was.
Hemispheres are pressed segments welded together to form a head in which the diameter and overall height are equal. We can nudge these into size and machine the edge. The important thing is not to distort the hemisphere shape.
Flange-only is a flat circle of steel which we turn up the edge. The important thing is to maintain the flatness. Which can be very difficult to do if the heads are very big, or if they are made up of several pieces welded together, or if they are very thin. Or if you make them small. You can not make a flange-only small. It warps up when you pull it back out. They also warp up if you use too much pressure with the side rolls, or not enough pressure with the side rolls, or try to flange them too quickly, or try to bend too much metal at one time. Any of these problems can cause the flat plate to end up looking like a taco shell. On one order I had ten ninety-inch diameter quarter inch thick aluminum flange-only heads. That was fun. Also, some large thin flange-only heads look terrible while in the machine, sagging every which way, but once you place them on the floor they will flatten out nicely. Most of the time.
Flared and dished heads look like flying saucers. Instead of bending the edge vertical like on nearly all heads, I bend it horizontal. I need to use a special forming roll for this, so these heads involve a lot of set-up to run. They aren’t especially difficult, as long as you figure how much metal to turn up. The customer requires the flared edge to be a certain length. If you don’t bend enough metal at first and have to bend more, it ends up looking like crap. The flares can be long but not short, so I always bend more than I think I need, to be safe.
Dished heads only get machined on the edge. We do two different kinds of detail on these heads. A radial cut maintains the slope of the edge as it is after being pressed. A vertical cut is a vertical ninety degree cut.
A reverse head is flanged upside-down. The pressed head is loaded into the machine with the inside radius down instead of up. So we have to bend the metal entirely around the icr roll. This puts a strain on the machine, but it is not terribly difficult. But because you are bending so much metal in a way the machine wasn’t designed to bend it, it is easy to dig ridges and gouges into the head. Which isn’t good.
A cone resembles an ice cream cone, the pointed kind, that can be ten feet or more deep. We form an inside corner at the top of the cone and flange it into size. The most difficult thing is not to break the cone off the steel plate it is welded to at the bottom. If you use too much pressure, or don’t keep the head level, or flange the cone so long it gets hard, you can break the welds. Then you have to stop and take it to a welder to weld it to the plate again. They usually cuss about having to do this. They try not to weld the plate on too securely because once we are done with it they have to cut the plate off. But if they don’t weld the plate on good enough, the welds break and they have to do it over. It’s a judgement call for them. Other than not breaking the welds holding the cone to the plate, you try not to deform the cone shape.
Customers are always coming up with ingenious absurdities for us. There was this one order which called for a head to have two icr’s. Two corners. Near the edge, where it’s supposed to be, and one about half-way to the center. That proved impossible, and the customer gave up on it. Also one customer has us cut all the straight flange and half the icr off. Another customer wants the flange flared to a certain angle, instead of being straight. They never cease to come up with new ways to torment us.