Many folks already know the answer to this, but, I imagine there are also a number who don't. If you are trying to form or roll a ring out of flat bar or round stock, where do you come up with the flat length to start out with? Most have heard the terms, I.D. or O.D. when referring to the outside diameter or the inside diameter, centerline dimension isn't so common. Centerline dimension, however, is the one that counts when you are trying to come up with the flat layout length for material prior to making it circular. You can either take the finished I.D. of the round part and add one material thickness to it, or take the finished O.D. of the round part and subtract one material thickness from it, in either case you will end up with the same answer. Once this dimension is known, multiply this number times pie, the product from this equation will yield the circumference or flat length of the material prior to rolling or forming.
     Depending on the process used to roll or form the material from this point, you may need to add a little bit extra to each end of the part to allow for the flat spots that are encountered by using forming rolls or press brake dies. This is the case when the part needs to be extremely round without any flat spots. Generally the ends of the material will need to be rolled or formed to the appropriate size prior to the completion of the rounding out process and then the excess material is trimmed off before completion of the rolling or forming. Thus it will be flat with the ends turned up at the appropriate radius, prior to completion. If slip rolls are used for the forming, the part can then be rolled until the ends meet, slipped off the machine pulled together (they will naturally spring apart a bit) and then tacked and prepared for welding if it is required. If the part is to be done on a pressbrake, it will need to be done in two halves as the parts will hit the ram of the machine and not allow a full circle to be formed (if you bend off the end of the machine you can possibly do a complete round part). Whether done in one piece or two, you will need to allow additional material on each end of the part to be trimmed off after radiusing the ends and before completion of the forming process. Also, if done in two halves you will need to start forming at one end and proceeding to the middle, then turn the part around and form from the other end towards the middle to avoid interference with the ram on the machine. I know that I have simplified this description a bunch, as there are many other variables that come into play most of the time. A little food for thought and a subject for comment. Regards, aevald