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I Forge Iron

StahlUndEisen

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  1. Guys, In my day job I'm a metallurgical engineer at a hot strip mill that rolls everything from steel to cobalt-based super alloys, so I've been banging my head against rolling problems for years. I'd like to say something about roll size. Both your rolling force and torque are proportional to the contact area between the roll and the metal. So, the larger the roll, the more torque and force are required. You wouldn't believe how fast power requirements scale up with roll size! For instance, a flat rolling mill with 36" diameter rolls may need 15,000 HP at 60RPM. (And still stall.) Now, one other thing. If you are using a McDonald-type roll gap adjustment with the foot pedal, that's all the protection your mill needs. If you have any other type of roll gap adjustment, I'd definitely design a shear pin or something like that into the mill drive. It's kind of tough to imagine a cobble could get real bad on a McDonald mill, but then again, it's better not to take chances because cobbles can break things that you would never think could be broken. Also, larger rolls equal larger heat sinks, so your work piece will lose heat that much faster, and making small billets, that can mean extra heats real quick. Now, there's also another factor you might want to include in your equation. That's roll bite angle. For the most part, it's hard to feed steel into a mill if the roll bite angle gets bigger than 15 degrees. Advantage: big rolls means it's easier to take a bigger bite. However, you've got two ways around this problem. One is you could knurl your work rolls, or welding beads all the way around the roll would accomplish the same thing. That extra texture on the roll face will draw the work piece in even if the usual roll bite angle limit is exceeded. The other way is just to taper the end of your work piece. Once the tapered end feeds into the mill, the friction will pull the thicker part in. In the case of a McDonald-type mill, if you're trying to take a draught that's too large for your roll size (not likely), you could slightly fuller the part of the billet right by the handle, and bite down on that part. I think the McDonald mill is a great practical design for a mill that one man can operate efficiently.
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