Rio Grande was founded more than 60 years ago by my father, Saul Bell. A jeweler for more than 80 years, Saul was involved in every imaginable aspect of the jewelry industry, including manufacturing, diamond-setting, watch-making, wholesaling and retailing. He loved to share his vast knowledge of the jewelry-making process. My brothers, sisters and I still benefit daily from his knowledge and wisdom, and I hope to pass on some of what I have learned from him and from other master jewelers through this series of blog posts.
Ever wonder why your order for however-many ounces of wire is always a little over or under what you ordered? The simple answer is: tolerances.
Tolerances are developed to fit manufacturing capabilities and, while it is possible—even standard—to have tight (close) tolerances, a tolerance is, by definition, a variance between the intended and the actual. Let’s take a look at the two common forms of fabricated metals: sheet and wire.
When sheet is rolled, it begins with a thick plate and is rolled down to size. The tolerances are generally not quite as tight on sheet as they are on wire because the rolls in a rolling mill are a bit more difficult to control than the hole in a wire-drawing die.
The mechanical action of rolling sheet through a mill to reduce its thickness generates a substantial amount of heat. This heat causes the mill’s rolls to expand, reducing the gap between the rolls. At the same time, the sheet material itself also heats up and expands, so when it cools down to room temperature, it will be a little thinner. The mill operator can adjust for this expansion by opening the gap between the rolls slightly as the heat builds and, if you take a long coil of sheet stock and carefully measure every foot from one end to the other, you will find just where the adjustments were made. The tolerance is usually expressed as a plus or minus amount allowable. I don’t know of any standard for precious metal in the United States, but there is one in Germany. For example, the tolerance on a 6″-wide sheet between 0.50mm and 0.80mm thick is + or – 0.04mm; that means that a sheet presented as 0.5mm thick can be anywhere from 0.46mm to 0.54mm thick and still be within tolerance.
When wire is made, it is drawn through a carbide or diamond die with a precision hole in it. Because the mass of the die and the wire is much less than that of a rolling mill and sheet material, heat is less of a problem and the die is usually flooded with a lubricant that also acts as a coolant. Because of these factors, the tolerance on wire can be held much tighter than is possible on sheet.
A wire between 0.50mm and 1.00mm thick has a tolerance 0.012mm, and a wire between 2.00mm and 4.00mm thick has a tolerance 0.03mm—the tolerance more than doubles between these two wire sizes. As a general rule, the thicker the material, the wider the tolerance will be; that’s because tolerance is a percentage of the thickness.
In the case of a special-shape wire such as bead wire, the wire is hammered into shape and the metal that is displaced in the area where the wire is narrowed is pushed (or ‘upset’) into the center of the bead, where the wire is thicker. “Upsetting” is the term for increasing the cross section of a piece of metal by means of hammering it. This is a volumetric movement of metal from one place to another. In the manufacturing process, this is much harder to control than simply rolling sheet or drawing wire.
Several factor affect the manufacturing process, including the beginning wire diameter (remember it has a +/– tolerance), the temper of the wire (which also has a tolerance), the heat generated in the dies, the lubricants used, and how precisely the dies were set in the first place. To put this in perspective, 0.1mm is equal to .003937″, which is about the thickness of a single human hair, so applying a tolerance of +/– .005″ is not unjustified for a piece of metal that was beaten into shape with a hammer. Processes involving metal upset are among the most difficult forging operations; holding as close as .005″ is reasonable for these processes, just as holding a +/– tolerance of 0.00012″ is reasonable for drawn round wire of similar size.
And this is all happening at the manufacturing end. When these products come to Rio Grande, we check that the 18-gauge wire (and all the other fabrication metals) we order actually is within our tolerance of 0.0005″ for that metal in that gauge size. Here is how our product manager for fabrication metals puts it:
Although 0.0005″ is a tight tolerance, as a percentage of the mass, it can vary quite a bit. Consider a heavier-gauge sheet such as 12-gauge. This 12-gauge sheet should be .081″ thick, so we give our mill a range of .0805″ to .0815″. As a percentage of variance, this is very slight. Now consider a very fine-gauge sheet, say 30-gauge; 30-gauge sheet should be .010″ thick. So, we would accept a range of .0095″–.0105″. This is a more significant percentage of variance.
This variance on very thin gauges can sometimes make for surprising differences in terms of the number of feet to an ounce. For example, 30-gauge fine silver wire is listed as having 100 feet to a troy ounce, based on the .010″ diameter. Mathematically, .0095″ is 95% of .010″. So it follows that if a given lot of 30-gauge wire is made to the .0095″ diameter, there will be 5% more running feet to the troy ounce. So one troy ounce might be as much as 105 feet, or conversely, as little as 95 feet if the wire was made a bit heavy (.0105″).
In our precious metals stock room, customer orders are weighed and measured. We accept orders for precious metals in a variety of ways. Many customers enjoy ordering wire by the foot or inch, and ordering sheet by the exact dimensions they need. Some customers, though, simply order by the troy ounce (ozt.) or pennyweight (dwt.). Regardless of how the order was given, the final tally is computed by weight. A tolerance of +/–10% applies to all orders. A one-ounce order will weigh no less than 0.90 ozt. and no more than 1.10 ozt.
I hope this helps answer the question we started with and that it will be a little easier to order the fabrication metal you need in your business. Let me know if you have any questions; I’m happy to answer any you may have.