A: That’s pretty much normal behavior for small rolling mills, which is what virtually any hand rolling mill is. For one thing, we generally are rolling metal that is not exactly uniform in its structure and internal stresses, whether from the prior passes through the mill or defects or inclusions in the metal. But whatever the reason, the compression of the metal may not be exactly uniform from one side to the other, so the metal bends as it comes out of the mill. In particular, with newly-annealed sheet, the compressive force on the metal is greatest right at the surface, which elongates a bit more than the center. You can easily see this at the ends, where the center appears depressed compared to the edges or middle of a wire. So the surface ends up more work-hardened than the interior.
With the next pass, this results in more of the elongation and compression of the metal happening closer to the center, which is still softer. This progresses until the metal becomes uniformly compressed and work-hardened throughout its thickness. In practice, when rolling a sheet, for example, right after annealing, you’ll see it curve or get wavy as it leaves the mill, and this increases but changes a bit as you continue with further passes. But about the time the internal stresses in the metal become uniform, it pretty much straightens out again. not usually to perfectly flat, but a lot closer than it had been before. This is a good clue to tell you it’s now time to anneal again, since going further can overstress the metal and start it cracking.
When you buy commercially prepared sheet metal, it’s flat in part because the roll diameters used are a lot larger than the small rolling mills we use. And generally those rolling mills are more rigid and highly finished, so the variance in the metal is less, thus the variance in stresses in the metal also is less. And such metal usually is furnace-annealed, which also contributes to more uniform structure. But basically, the effect you’re seeing is normal. If you need it to be flat, anneal the metal once it’s the thickness you wish, then gently pound it flat with a rawhide, rubber, or plastic mallet on a good flat steel block. I like the lead-weighted dead-blow type of mallets best for this sort of thing. Wire is easily straightened this way; sheet takes more work and equipment. Anneal wire after you’ve drawn it to size. Put one end in a vise, grab the other end with pliers or draw tongs, and pull enough so you feel the wire slightly “give”, stretching ever so little. That’s all it takes. Your wire will now be dead straight.
by Peter W. Rowe M.F.A., G.G.