England, Gordon, (Independent Metallurgist and Consultant to the Thermal Spray Coating Industry) Surface Engineering Forum, “Hardness Scale Conversion Table” http://www.gordonengland.co.uk/hardness/hardness_conversion_3c.htm 9/1/2015. The test procedure involves pressing a 1/16″ metal ball into the coin material with a known amount of pressure and measuring the depth of penetration. (Latrobe, 52100), Alloy L6 is a versatile oil-hardening tool steel that is characterized by good toughness and is suitable for tools, dies and machine parts. Although there is no metal flow in the design bordering the crater, there is also no trace of a pressure ridge, indicating that the defect was present before the strike. If it indents ½ way (a full hemisphere), then it leaves a mark with a diameter = the diameter of the sphere. The depth of penetration of the indenter must be less than 10% of the metal thickness, and the reading must be taken at least two indenter diameters from an edge.” (Materion Corporation, 9/4/2015). This hardness range is approximately 33 to 58 Rockwell C hardness. The Rockwell Scale was invented in 1908 by two men, Hugh Rockwell and Stanley Rockwell (no relation). The level of hardness of these steel alloys requires testing with the Rockwell C Scale. (Concurrent Technologies Corporation, Page 42) The actual value on this 2000-D was measured at 59.1. http://customer.cartech.com/assets/documents/datasheets/LSS_L6.pdf. Annealed silver steel has a hardness of 27 on the Rockwell scale of hardness (RHC). But when we are dealing with Jewelry, most people are more familiar with the … I know the odds are low that a Rockwell Test Mark will make it through the minting process. When did the Ancient Egyptians start believing in the afterlife? If an unstruck planchet is too hard the image pressed on the coin will not be clear. It would be even more improbable for all these things to occur twice on the same coin as the result of random post-mint contact, as appears on this 1995-D. On the other hand, Rockwell Testing Protocol does suggest that an object be tested at least twice so the results can be averaged. The 1 mm diameter depression left by a BB strike would be almost flat! The Rockwell hardness test continues to be applied as a tool for Chord Calculation Site https://www.easycalculation.com/area/chord-length-circle.php. In addition, a BB would most likely leave a pressure ridge around the circumference of the depression. “Rockwell test marks applied to planchets before the strike are recognized among some U.S. coins. • show the same depth as a Rockwell 15T test mark AND This might cause a certain confusion, because sometimes a hardness of the HRC scale is ordered, although the small dimensions of the sample make tests with a total test load of 1471 N impossible. Fern, Ronn and Potter, Ken, “Another Error Coin Pioneer Passes … Lonesome John Devine 1933-2013”, CoinWeek July 30, 2013, http://www.coinweek.com/coins/error-coins/another-error-coin-pioneer-passes-lonesome-john-devine-1933-2013/ 9/14/2015. This is enough indentation to leave rather steep, upward sloping, sides to the mark. (Figure 10), Acknowledgements Thanks to Mark Obstalecki, Numismatist, for Testing with the Rockwell F Scale. The other cent with indentations is a 1995-D Lincoln Cent with 2 indentations of the same size: These indentations seem to show the correct shape for a hardness test mark. 122-123.). Answer Rockwell C is a method to measure the hardness of a metal object. I have attempted to take these ideas seriously and have come to the realization that a critical piece of information in considering each possibility is the nature of the geometry of the tips of these instruments. “…the Rockwell test makes two depth measurements. Any mark on a coin thought to have originated from a Rockwell Test on a Die (where the mark on the coin would be raised) or Master Hub (where the mark on the coin would be an indentation), would need to exhibit the same shape as in this cross sectional view of the spheroconical diamond tip indenter rather than the hemispherical shape of a Rockwell 15T Test Mark. The steel alloys are Alloy 52100 for small diameter coins and Alloy L6 for quarter on up. This same source shows an image (shown below with permission) of an alleged Rockwell Test Mark in planchet on a 1971-S Lincoln Cent. He said that a Rockwell test mark on a zinc planchet will still be visible after a coin is struck on that planchet. It should be noticed that the diameter of the indentation on the John Devine authenticated coin is slightly less than that on the Rockwell B Scale indentation. Hardness should be checked on cross sections when nonuniform cold work causes high surface hardness. But the smooth texture of the pit and its central location provide a strong argument in favor of a Rockwell test mark.” (Diamond, Pages 17 – 18), “Lonesome” John Devine, who died in 2013, was a highly respected error coin pioneer. The act of submitting material shall constitute an expressed warranty by the contributor that the material is original; if not, the source and permission must be provided. Several measurements should be taken to avoid a false reading caused by microstructural inconsistencies such as hard beryllide particles or grain boundaries. Notes on the Vickers Hardness of particular elements: Silicon: converted from Mohs scale Germanium: converted from Mohs scale Arsenic: converted from Brinell scale Ruthenium: converted from Brinell scale Osmium: converted from Brinell scale Up to date, curated data provided by Mathematica's ElementData function from Wolfram Research, Inc. I know of no way to calculate the probability of all those things occurring as a result of random post-mint damage – but I think it would be astronomical. Errorscope 2020 But, of course, the reality is much more complex, because metal samples vary radically. Another type of Rockwell Test Mark which may be found on Lincoln cents is a raised bump. Even though the indentation on the 2000-D cent appears to be the correct size and shape, a detailed examination appears to show a pressure ridge around the circumference. For a BB to leave a 1 mm diameter indentation, the depth of the indentation must be only about 1/20 of the diameter of the BB – which would leave an extremely shallow slope to the sides of the mark. As was observed in the 1971-S cent authenticated by John Devine, the diameter of the indentation may be slightly smaller than the indentation on the reference coin, due to some “closing up” during the strike. Its relatively high nickel content gives it greater impact toughness. It is equal to the magnitude of the force exerted by 1 kilogram of mass in the average magnitude of gravity on earth). The pit is oval, rather than circular, but this can be attributed to distortion produced by the strike. Rockwell B Scale: Preliminary Force = 98.07N (10 kgf) Total Force = 980.7N (100kgf) How long will the footprints on the moon last? After a set period of time, the depth of the indentation is measured for a second time. I am not aware of any stylus which meets these criteria, but perhaps there is one. The composition of the Lincoln Cent (except for 1943 when the cent was zinc coated steel) up to 1962 was 95% copper and 5% tin and zinc. Usually the entire process is carried out by automatic machinery. Diamond is the hardest known material. The caliber of a normal BB gun is 4.5 mm. A number in parentheses indicates the year of last reapproval. Since beginning this research project, I have encountered numerous reactions proposing that some other object caused these indentations. This alloy from 1982 to the present is actually Zinc Alloy 190 electroplated with 8 microns of copper. The most we can say is that a particular indentation meets the expectations of a Rockwell Test Mark. The Rockwell hardness test measures hardness in the simplest way possible: by pressing an indenter into the surface of the material with a specific load and then measuring how far the indentor was able to penetrate. There are other Rockwell Scale Protocols for harder materials where either a different sized ball or a spheroconical diamond is used to make the indentation. Silver – 2.5 – 3; Aluminum – 2.5 – 3; Copper – 3; Brass – 3; Bronze – 3; Nickel – 4; Platinum – 4 – 4.5; Steel – 4 – 4.5; Iron – 4.5; Palladium – 4.75; Rhodium – 6; Titanium – 6; Hardened Steel – 7 – 8; Tungsten – 7.5; Tungsten Carbide – 8.5 – 9; Metal Hardness is usually defined by the Rockwell Hardness Test. (Jardin Zinc Sales 8/14/2015). The ball (say) is brought in contact with the material to be tested. (Harper and Miller, Page 62.) Imagine a sphere which makes an indentation on a blank. Hardness level (Mohs scale) Common Alloys Pros Cons; 24k yellow gold: 2.5 brilliant luster: … One is a 2000-D Lincoln Memorial Cent with one indentation on the reverse (Figure 5). Dusharme, Dirk, Quality Digest, “Hardness Testing: Absolutely Not Absolute” http://www.qualitydigest.com/april98/html/hardness.html 9/2/2015. However, holding a 1/16″ ball bearing in my hand impresses upon me how tiny such a tip must be. The Rockwell scale is a hardness scale based on indentation hardness of a material. These same images without overlays may be seen in Figure 4. The Rockwell hardness is calculated from the difference between the depth of the indentation after the application of the total force and its initial depth under the preliminary force.
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