1999 SBA Peculiarity

[cpm9ball]

I have one uncirculated roll of each mintmark that I removed from paper wrappers to put into (square) tubes. The Philadelphia coins fit easily with a little space at the top of the tube, but the Denver coins extend beyond the lip of the tube so that the cap does not lock into place. I measured the thickness of one of each mintmark, and I found that the "P" was 1.96mm in thickness and the "D" was 2.03mm in thickness.

 

Is this within normal tolerances?

 

Chris

[Conder101]

The thickness of a coin at the rim is a function of the force of the strike. Surprisingly the way it works is the greater the striking force the thicker the edge of the coin is. Since the edges are thicker and when you stack the coins they rest on their edges. The mint that is using a higher striking pressure results in a taller stack.

 

As far as I know the mint has no tolerance numbers for the thickness of the edge of the coin. I would think that if you were to measure the thickness of the edge on a proof coin you would get a pretty good idea of what the maximum thickness would be.

[physics-fan3.14]

The thickness of a coin at the rim is a function of the force of the strike. Surprisingly the way it works is the greater the striking force the thicker the edge of the coin is. Since the edges are thicker and when you stack the coins they rest on their edges. The mint that is using a higher striking pressure results in a taller stack.

 

As far as I know the mint has no tolerance numbers for the thickness of the edge of the coin. I would think that if you were to measure the thickness of the edge on a proof coin you would get a pretty good idea of what the maximum thickness would be.

 

Very interesting indeed. So that implies that Denver is using a higher striking force, which would explain why '99 D's have a much higher frequency of Full Talon coins. The 99 P in FT is reportedly somewhat difficult to find.

 

BTW, Chris, what are you using to measure the thickness? I want to get a set of vernier calipers, but haven't seen a good set I like yet. Is yours digital? Does it have rubber tips? (Most of the calipers I've seen are metal, I would be very afraid of scratching my coins). Give me the skinny on calipers, guys!

[Oldtrader3]

Rubber tips would not measure accurately. You can buy calipers with hard plastic jaws which probably would not not scratch coin surfaces. The plastic calipers will not measure as accurately as the metal ones though. A ball tip micrometer might work better than calipers and would be less likely to scratch surfaces.

[cpm9ball]

BTW, Chris, what are you using to measure the thickness? I want to get a set of vernier calipers, but haven't seen a good set I like yet. Is yours digital? Does it have rubber tips? (Most of the calipers I've seen are metal, I would be very afraid of scratching my coins). Give me the skinny on calipers, guys!

 

I ordered the electronic digital caliper from the ANA Store. You can set it for inches or millimeters, and it measures to 6" (150mm). It has metal tips so you just have to be very careful.

 

Chris

 

 

[cpm9ball]

I would think that if you were to measure the thickness of the edge on a proof coin you would get a pretty good idea of what the maximum thickness would be.

 

Interesting! I just measured one of my 1999 SBA Proof Dollars, and it was 1.94mm, less than both of the uncirculated coins. Is it possible that the roll of planchet metal was thicker than it was supposed to be? Unfortunately, I don't have a scale to weigh them.

 

Chris

[RWB]

Producing coin of consistent thickness has been an issue at the US Mints since the late 19th century. (Due mostly to the introduction of coin-operated amusement and vending machines.) Eventually, the engraver and coiner worked out an ad hoc standard and tolerance. (See Renaissance of American Coinage 1905-1908) This remained a problem with the new Lincoln cents. While working out the silver subsidiary coin designs in 1916, thickness – particularly uneven thickness at the rim – was a major design problem.

 

Investigation showed that thickness was affected by the distribution of elements in the design and the relationship between the height and upset angle of the planchet, and the die/collar dimensions. Simply put, the strike had to form the rim first, which then acted like a dam to fix thickness and bounce a shock wave back to the central part of the coin. Annealing of blanks was also a factor with softer blanks producing more consistent results.

 

I’m unclear on how directly this century-old information relates to modern minting equipment. However, it suggests that thickness is not governed only by strike pressure.

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