Unusual patterns of upset rim stymie error investigators

Published : 02/18/12
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Before a blank can be considered ready for coining, it must pass through an upset mill. Here the blank is squeezed to a smaller diameter and is simultaneously provided with a low proto-rim at the outer margin of each face. In this way a blank becomes a planchet.

Upsetting or “rimming” provides many benefits, as listed here and with credit to Ken Potter and Sean Moffatt.

1. The reduced diameter allows the planchet to fit easily into the collar.

2. Upsetting smooths out the rough edge left by blanking.

3. The proto-rim makes it easier for the design rim to strike up.

4. Planchets pass through machinery faster and more smoothly than blanks, since only a narrow ring is in contact with any flat surface.

5. Planchets don’t have the same tendency to stick together that blanks have.

6. The proto-rim protects the interior of the planchet from abrasion.

The upset mill consists of a rotating drum and a fixed half-ring. Each element has a series of V-shaped grooves cut into it. The blank is fed into a facing pair of grooves and is propelled (rolled along) by the rotation of the drum. The distance between the deepest parts of the facing grooves shrinks as the blank moves along the half-ring. This gradually squeezes the blank to the diameter of a planchet.

The height of the proto-rim and the shape of the planchet’s edge will vary within and between denominations. In cents, dimes and higher denominations, the junction between the apex of the proto-rim and the most lateral point on the edge is beveled. The strip in the center of the edge is flat, however. In vertical cross-section the edge has the shape of a truncated triangle, or trapezoid. The shape of the trapezoid is determined by the shape of the grooves. Minor variation in the distance between the two grooves creates variation in the height of the proto-rim.

Planchets for 5-cent coins generally show a flatter edge and a lower proto-rim, with a very narrow bevel at best.

Some planchets show a pattern of upset that falls well beyond the normal range of variation. Our first example is an off-center 5-cent coin with a pronounced proto-rim, wide bevel and narrow, flattened center strip.

Far stranger is a 5-cent coin that previously appeared in the Jan. 23 “Collectors’ Clearinghouse” column. As shown in the close-up photo, the proto-rim is tall and wide with a sharp internal margin. A thin pleated apron extends in from the proto-rim. The gently sloping outer surface of the proto-rim almost looks machined. The edge of the planchet is strongly convex in cross-section.

Neither example bears any resemblance to a typical pattern of upset, illustrated here by a double-struck 5-cent coin. Here the edge is wide and flat, the beveled zone is very narrow, and the proto-rim is very low.

Atypical patterns of upset may reflect any of the following circumstances:

1. Improper machining of grooves.

2. Grooves with wear or damage.

3. Grooves with an experimental cross-sectional contour.

4. Use of an upset mill intended for another denomination, domestic or foreign.

5. Circumferential pre-strike damage that happens to resemble the effects of upsetting.

I suspect the 5-cent coin with the pleated apron falls into the last category.

The next coin up for consideration is an off-center zinc cent with just a dusting of copper plating distributed in a bulls-eye pattern. The edge is completely flat and vertical, and the proto-rim is tall, thin and sharp. It’s quite different from a second off-center cent which shows a typical pattern of upset.

The minimally plated zinc cent is rather similar to our last example, a presumed 5-cent planchet struck way off-center. The edge is flat and the proto-rim is thin and sharp. The surface of the planchet has an unusual bright gleam.

Other photos of oddly upset planchets can be found in the September/October 2005 Errorscope.

Editor's note: Several images and captions were mismatched in the Collectors' Clearinghouse column in the print and digital editions of the Feb. 27 issue of Coin World. The images and captions are properly matched here.

Coin World’s Collectors’ Clearinghouse department does not accept coins or other items for examination without prior permission from News Editor William T. Gibbs. Materials sent to Clearinghouse without prior permission will be returned unexamined. Please address all Clearinghouse inquiries to cweditor@coinworld.com or to 800-673-8311, Ext. 172.

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