Combination Wire and Flat Rolling Mills
Q: I’m thinking of getting a rolling mill, but I was wondering if I should get the kind with grooved rollers, or just flat. What are those grooves really good for—making square wire, or half-round stock, or what?
A: Depending on the mill, some of which don’t have the half-round grooves, yes. The most commonly used are the “square” grooves, but these don’t produce a true square. Their purpose is not to provide a final shape, but to start with a larger piece, for example a long narrow ingot such as might be produced with a wire ingot mold or the wire portion of a combination ingot mold, and break those down into a narrower long wire blank that can then be finally shaped with drawplates, usually to round wire, but also to other cross sections as desired. The shape they produce is a square with cut corners. It’s not quite octagonal. The reason is that the flats at the corners allow the product of one set of grooves to fit into the next smaller grooves after being rotated 90 degrees, without producing a flange on the edge.
You can, of course, use that semi-square wire as is, if it suits your needs, but usually it’s a starting point. Often, for example, one might then run it through the flat mill to produce rectangular stock, if one is not drawing it into a wire with draw plates. The half-round grooves are less useful, since they’re not such a generic shape But they’re useful for producing half round stock, such as for ring shanks, without so much filing, or the need to draw it down. Drawing wire with narrower, sharper edges, like shallower half round, or triangle, etc, is more difficult than drawing round wire, and of course one must have the drawplates too.
Probably the most common use for the half-round grooves is for ring shank stock, but of course you can use it for anything you might want half-round wire for…most jewelers work with both sheet metal and wire. Having both capabilities allows you to make both. Plus, making wire is generally easier than making sheet metal, since in some metals (silver especially) getting defect-free ingots for sheet metal is trickier than for wire, where minor defects don’t produce blisters that ruin the final stock.
The combo mills have limited width available for sheet, so you make only smaller pieces. If you work with larger-sized jewelry pieces, that’s a limitation worth considering. Many commercial “bench” jewelers, however, rarely need the larger size sheet. Similarly, the wire grooves on a combo mill don’t start with as large a size. So you need to start with a smaller-diameter wire ingot. That can be a bit more difficult to pour, and limits the amount of wire you can make in a single length. Whether this is a problem depends on the type of work you do. As said before, you can make almost any type of wire you wish because the mill itself doesn’t usually give you your final wire product (unlike with sheet metal, where it does). You will still need drawplates and the means to draw wire. But you need the mill first, since you cannot start with an ingot and directly draw wire from it.
The flat-only mills simply give you more width to work with. That allows you to make wider stock if you need it. Plus, without the grooves machined into the rollers, the rolls are a bit stronger, flexing less under load. That can give you more accurate sheet thickness, especially when rolling very thin stock, as well as often allowing you to start with a slightly thicker ingot. Having the ability to make your own wire is very useful. Among other things, the cost to buy wire already made incurs higher manufacturing costs from the metals suppliers than does sheet metal, so there are savings involved. And you can use even fairly small amounts of scrap to produce usable amounts of wire.
But if you can afford them, two mills are much better than one combo unit (or one of the double units with a single mill having two sets of rolls, one over the other. But the cost for two mills is of course, double that of one, so the compromise of a combo mill gives you much of the basic capability with just the cost of one mill. Half-round grooves, or the capacity some mills have of adding side rolls for half-round, triangle, or patterned rolls, give you added capability for shapes that can be more difficult to produce without them. Not everyone uses or needs those functions. The flat rolls and basic wire rolls offer by far the most useful capabilities.
by Peter W. Rowe M.F.A., G.G.