Coin diameters and thicknesses as of Aug. 1881 0

[RWB]

This excerpt is a little unusual in that it gives not only coin diameters but their thicknesses. Note the use of twentieths and thousandths of an inch in measurements.

 

[Conder101]

The thickness figures are interesting, but do they relate to the blank thickness, planchet rim thickness, or maximum thickness post strike (assuming measured at the rim which should be the thickest part of the coin to protect the devices. I assume maximum because finished rim height is a function of the malleability of the metal and the force of the strike.)

[Just Bob]

Quote

The diameters of United States coins are not fixed by law.

I assume this is no longer the case, and at some point, laws standardizing coin diameters were passed.

[DWLange]

I don't believe there are any laws regarding diameters. What is specified are the weight and fineness alone, and the diameter is determined by the Mint itself based on the need to match existing coins and by the quality of strike that results. This latter feature was tested with several of the new coin issues of the late 1830s. The Reeded Edge Halves had multiple differences in diameter during 1836-37, and the Coronet Half Eagles of 1840 issues are known with two collar diameters. When the Shield Nickel was replaced by the Liberty Head in 1883, the diameter of this denomination was raised a bit for better striking and less die stress, but this was done without Congressional action. The larger diameter survives to the present day.

[RWB]

23 hours ago, Conder101 said:

The thickness figures are interesting, but do they relate to the blank thickness, planchet rim thickness, or maximum thickness post strike (assuming measured at the rim which should be the thickest part of the coin to protect the devices. I assume maximum because finished rim height is a function of the malleability of the metal and the force of the strike.)

Conder101: Coin thickness and diameter.

Coin diameter is not legally specified except for the small dollar coin. Diameters became "fixed" when coin counting and dispensing machines came into use. A similar commercial pressure, along with physical characteristics of alloys, led to thickness at the rim of modern coins. Members are referred to discussions between businesses and the Mint Bureau regarding Buffalo nickel designs (Renaissance of American Coinage 1909-1915); Brandt Automatic Cashier Co., AT&T (Renaissance of American Coinage 1916-1921; David W Lange -The Complete Guide to Mercury Dimes ) and the Inco & Gould private pattern pieces of the 1960s and 70s (Private Pattern and Related Pieces: International Nickel & Gould Incorporated).

RE: Coin changes in 1820s-1830s. Multiple changes were tested and put into production for some denominations during this period. The goal was to produce coins of more uniform appearance. These would be, it was hoped, more likely to remain in domestic circulation as full-value pieces and not chopped up as were Spanish and Mexican silver issues. Minting difficulties arose because the old screw presses ("fly press") did not have (or maintain) sufficiently tight mechanical tolerances to allow full advantage of closed collars. Additionally, the Mint did not yet have a modern reducing lathe. This, when combined with uncertain steel quality, inconsistent die hardening and tempering, and other layers of physical/mechanical mismatches, limited improved coin production until 1837 and later. Peale's suite of improved minting equipment was then able to strike coins of greatly improved uniformity, consistency, commercial acceptance and political presentation.

[Conder101]

At some point they did decide to specify the diameters in the law.  Interestingly the small dollar has the diameter specified but the weight is not specified.

US code  title 31  section 5112

(a)The Secretary of the Treasury may mint and issue only the following coins:

(1) a dollar coin that is 1.043 inches in diameter.

(2) a half dollar coin that is 1.205 inches in diameter and weighs 11.34 grams.

(3) a quarter dollar coin that is 0.955 inch in diameter and weighs 5.67 grams.

(4) a dime coin that is 0.705 inch in diameter and weighs 2.268 grams.

(5) a 5-cent coin that is 0.835 inch in diameter and weighs 5 grams.

(6) except as provided under subsection (c) of this section, a one-cent coin that is 0.75 inch in diameter and weighs 3.11 grams.

 

On 10/23/2019 at 5:29 PM, RWB said:

A similar commercial pressure, along with physical characteristics of alloys, led to thickness at the rim of modern coins.

Yet the thickness at the rim of the coin DOES vary.  So if the specified thickness a maximum thickness as struck?

[RWB]

Well, the letter is from 1881 - before most vending machines (or "slot machines" as they were called) were available. The coin-operated, vending industry, and automated coin counting were the primary movers in tighter control of coin dimensions - especially rim height. Even a slight fin could cause a coin to stick in a pay telephone slot and the customer was out his money and the 'phone had to be repaired.

[numisport]

What do you think the diameter measurement in twentieths of an inch came from rather than thousandths ? Since I use sixteenths when measuring lengths [or even millimeters I suppose] where did use of twentieths evolve from ?

[RWB]

Not sure. Mint letters mention 32nds, 16ths and 10ths most frequently. Maybe 20ths just made it easier to express in fractions. What is especially odd is that measurements were given in fractions rather than decimals. The Mint Bureau had switched from fractions to decimals in the 1830s - although medals continued to be commonly measured in multiples of 32nds.