A REVIEW OF DIE CURVATURE AT THE US MINT SYNOPSIS Throughout a significant portion of the history of coinage die design at the US Mint, the die radius (a measure of die curvature) best suited for a coin was ideally determined by the disposition of the design and the area of the coin. It appears that, in most cases the relief of the coin design artwork was the primary determining factor. Die curvature was significantly convex enough where its presence was frequently visible on a struck coin, at least just inside the rim. In more recent years, the Mint, in a persistent effort to reduce coinage costs, began to implement matching die curvatures and planchet profiles and adding an element of exponential die curvature, rather than spherical die curvature, to the die design, at least on some issues. This entailed giving the approved design of the artwork on the coin a lower profile without affecting the appearance. This results in an overall “flatter” die crown, with less stamping force being required which extends die life and reduces costs. This paper may be viewed online without downloading at this link: https://www.coincommunity.com/pdf/A-Review-of-Die-Curvature-at-the-Us-Min.pdf
I'm going to mark this one for a Saturday morning read. Supposed to be rainy and stormy, so this will be perfect with a cup of coffee.
An explanation of the difference between “Spherical” and Exponential Curvature: This image is an imaginary cross section of the possible curvatures of the crown/basin of a Die/Coin. Imagine the placement and orientation of the Die/Coin on each curve where the center is placed at the “flattest” point of each curve and follow the shape of the curve to the rim/edge/die gutter. Placement on the “Spherical” results in the same curvature at each placement point from the center to the rim/edge/die gutter. However, placement on the Exponential Curve produces a different curvature from each placement point to the rim/edge/die gutter, growing steeper towards the periphery. The “steepness” varies as the point of placement changes. The area bracketed by Red is the most likely the general area covered by the Exponential Curvatures being explored by the Mint.
My impression of it was that the die would be an exponential curves both directions, something like this (but flatter in the middle). But this subject is way above my pay grade.
This varied widely from series to series. Some series have a shallow bowl shape (like the Morgan dollar), and some of them had much deeper curvatures (like the St. Gaudens). Some of them even had different curvatures in different parts of the fields (like the Buffalo nickel). Modern coins have an incredibly shallow curve, which tends to prolong die life.
I have the books Roger Burdette wrote which describe in detail the processes, but I don't have access to them right now. However, he mentioned it in a post on the NGC forums a few years ago, in a discussion on basining dies and how Prooflike coins were created: Link: https://boards.ngccoin.com/topic/270582-cause-of-pl-coins/
Thanks for the link! I see the discussion about different radii, and took it to mean different from each other rather than internally different. I do see his reference to coins without a defined radius, and will follow up on that, too. I would like to find the reference to Buffalos' having different curvatures in different part of the fields! I will see what I can find in Roger's book (I only have one!). Appreciate the reference!
